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Metal-organic framework derived hierarchically porous nitrogen-doped carbon nanostructures as novel electrocatalyst for oxygen reduction reaction
Abstract
The hierarchically porous nitrogen-doped carbon materials, derived from nitrogen-containing isoreticular metal-organic framework-3 (IRMOF-3) through direct carbonization, exhibited excellent electrocatalytic activity in alkaline solution for oxygen reduction reaction (ORR). This high activity is attributed to the presence of high percentage of quaternary and pyridinic nitrogen, the high surface area as well as good conductivity. When IRMOF-3 was carbonized at 950 °C (CIRMOF-3-950), it showed four-electron reduction pathway for ORR and exhibited better stability (about 78.5% current density was maintained) than platinum/carbon (Pt/C) in the current durability test. In addition, CIRMOF-3-950 presented high selectivity to cathode reactions compared to commercial Pt/C.
... Qiao et al. proved that quaternary-N benefited the limiting current density while pyridinic-N could decrease the adsorption energy of oxygen on electrode surface resulting in a positive onset potential [17]. As for the electrochemical capacitors application of N-doped porous carbon, many researchers reached a consensus on the contribution of a nitrogen atom to pseudocapacitance, which could enhance the surface compatibility with aqueous electrolyte and ensure the complete utilization of the exposed surface for charge storage [18][19][20]. Therefore, various methods have been investigated to prepare nitrogen-doped porous carbons. ...
... Recently, metal-organic frameworks (MOFs) have emerged as a new family of templates or precursors to synthesize porous carbons [11,15,19,[27][28][29][30][31]. Especially, ZIF-8 with imidazole group has been carbonized directly to prepare N-doped porous carbons [14,[32][33][34][35][36][37][38][39][40][41][42][43], in view of its rich nitrogen (24.5%), high specific surface area (1560 m 2 g À1 ) and porosity (0.79 cm 3 g À1 ) [32]. ...
... The PC1000@C electrode delivers a large specific capacitance of 225 F g À1 at a current density of 0.5 A g À1 , which is comparable to those previously reported high capacitances of MOF-derived nanoporous carbon electrodes at the same current densities [4,[51][52][53][54]. Although the specific capacitances of PC1000@C gradually decrease with the growth of current densities, its capacitance retention ratio at a current density of 10 A g À1 is 62% comparable to the value of other carbon materials [19,20]. ...
A core-shell structure composite, zeolitic imidazolate framework @ cetyltrimethylammonium bromide (ZIF-8@CTAB) was synthesized by CTAB micelle controlling the growth of ZIF-8 in aqueous systems. Direct carbonization of ZIF-8@CTAB at a high temperature produced the nitrogen-doped hierarchically porous carbon (named as PC1000@C). In comparison with the carbonization product of pure ZIF-8 (named as PC1000), PC1000@C possesses the higher specific surface area and two-times larger total pore volume. The results from elemental analysis shows the higher N content in PC1000 sample, while X-ray photoelectron spectroscopy curve-fitting shows the higher quaternary-N content in PC1000@C sample. The hierarchical microporous/mesoporous structure, high surface area and favorable N species in PC1000@C play an active role in catalyzing oxygen reduction reaction (ORR). The specific capacitance of porous carbon was calculated from the galvanostatic-discharge curve. PC1000@C exhibits a large specific capacitance of 225 F·g−1 at a current density of 0.5 A·g−1 and still retains 92% of initial capacitance after 1000 galvanostatic charge-discharge cycles.
... However,a side from ZIF-8 and ZIF-67, there is ap lethora of MOFs whose derived carbonsh ave been tested as electrocatalysts for the energy-related reactions. These MOF templates usually consist of Co [203,[207][208][209][210][211] and Fe [212][213][214][215] cations, and amino-functionalized ligands, for example, benzoimidazole, [207 ] imidazole, [201 ] 2-aminoterephtalic acid (H 2 BDC-NH 2 ), [212,213,216] trimethylamine, [217] 2,2'-bipyridine-5,5'-dicarboxylate (BPDC), [199] 5-amino-1H-tetrazol( 5-AT), [208] etc. Amino-functionalized linkers are preferred in respect to their non-aminated analogousb ecause the presence of Na toms during the pyrolysis process promotes the doping of the emerging carbon resulting in heteroatom-doped carbons, such as N-, S-, B-doped, and dual-doped (e.g. S,N-doped)c arbon materials with inhomogeneous charge distribution. ...
... Other reportedm etal-free doped carbons have been prepared starting directly from Ncontaining MOFs. For instance, Fu et al. [216] pyrolyzed at 950 8C aZ n-2-aminoterephtalic acid (H 2 BDC-NH 2 )b ased MOF,k nown as IRMOF-3, to produce an N-doped carbonaceous electrocatalyst with high selectivity and stability for a4e À pathway ORR process. Quian et al. [201] prepared am etal-free dual-doped carbon from aZ n-, N-a nd B-containing MOF templates,o btaining ORR electrocatalysts with TSs of ca. ...
Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable processes for future technologies and the development of highly efficient and cost-effective materials is one of the current major challenges. This results from the current global energy crisis, reflected in the depletion of fossil fuels and growth of the environmental pollution, which has stimulated the development of novel renewable energy storage and conversion technologies. Currently, several electrocatalysts (ECs) have been proposed and among them are the polyoxometalates (POMs), the metal-organic frameworks (MOFs) and their based composites. In this review we overview the progress in POM & MOF-based composites as electrocatalysts with a focus in the energy-related reactions. The review starts with a brief introduction to the hydrogen- and oxygen-based energy-related electrochemical reactions. Then, we not only summarize the methodologies used in the preparation of POMs, MOFs and their based composites as we provide a broad overview of the application of all these electrocatalysts for the reactions relevant to renewable energy storage and conversion technologies. We believe that this review will be of great interest to all those working in these fields, especially those interested in the development of new electrocatalysts based on POM and MOFs as it gives an up to date revision of this type of materials for hydrogen- and oxygen-based energy-related electrochemical reaction.
... 8,9 Recently, porous carbon (pC) derived from metal−organic frameworks (MOFs) has emerged as an effective material for a variety of applications including capacitive deionization, 10 CO 2 adsorption, 11 fuel cells, 12 and electrochemical applications. 13 Several pCs have been reported originating from different MOF types, including MOF-5 (Zn 4 O(OOCC 6 H 4 COO) 3 ), 14 zeolitic imidazolate framework (ZIF-8), 10,13 isoreticular metal−organic framework-3 (IRMOF-3), 15 MOF-199, 16 etc. The pC material is typically obtained by thermal treatment under inert atmosphere forming an ordered and calibrated porous structure with high specific surface area and large pore volume. ...
... Systems with high activity can thus be derived by coupling the N-doping effect with high electroactive surface area. 15 Furthermore, because of their highly porous structure and high specific surface area, carbon materials are also known as adsorbents of high interest for wastewater treatment. For instance, Jiao et al. claimed that Co-doped hierarchical porous carbon (Co/HPC) derived from Co/ZIF-8 carbonization is an efficient adsorbent for the extraction of trizine herbicides from wastewater. ...
A novel material was fabricated by deposition of graphitized Nitrogen-doped Porous Carbon layer (NPC) on commercial Carbon Felt (CF). The NPC was obtained via Atomic Layer Deposition of zinc oxide (ZnO) and its subsequent solvothermal conversion to Zeolitic Imidazolate Framework (ZIF-8) followed by its carbonization under controlled atmosphere. Both physical and electrochemical properties have been evaluated by Scanning Electron Microscopy, X-Ray Diffraction, Energy-Dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, nitrogen sorption, contact angle and cyclic voltammetry measurements. The parameters affecting the growth of NPC, such as the amount of ZnO/ZIF-8 material before calcination and thermal treatment temperature have been investigated in details. The versatility of the as-prepared NPC/CF material was assessed by studying: i) its adsorption ability and/or ii) its behavior as cathode in Electro-Fenton process (EF) for the elimination of a model refractory pollutant (acid orange 7 (AO7)). Once used as adsorbent, the NPC/CF proved good adsorption capacity with 97 % color removal of initial 0.02 mM dye concentration after 30min. Moreover, the application of such novel cathode could also reduce the cost for EF technology by using lower energy consumption at 0.54 kWh g−1 TOC. The apparent rate constant (kapp~0.8 min-1) obtained for NPC/CF was more than 7 times higher compared to pristine CF commercial electrode, thus leading to more than 90% TOC removal in 8 h. In addition, high reaction efficiency and system durability were attributed to continuous regeneration of the NPC/CF sorption capacity upon total mineralization of the pollutants accumulated at the electrode surface. Results confirmed that the new NPC/CF material behaves as a highly active electrode with attractive adsorption efficiency and at the same time it possess an excellent electrochemical activity in Electro-Fenton (EF) oxidation process for the removal of persistent water pollutants.
... Both experimental observation and theoretical calculation have revealed that heteroatom-doped (e.g., N, P, S, and B) carbon materials are a class of promising metal-free electrocatalysts for the ORR [13][14][15][16][17][18][19]. The enhanced electrocatalytic activity of these heteroatom-doped carbon materials is derived from the changes in the charge and spin density of carbon atoms adjacent to the heteroatoms. ...
... The inset of Fig. 3(d) displays the Koutecky-Levich (K-L) plots of the N 1 S 1 -CNW-900 aerogels at various potentials; these were derived from the LSV curves. The good linearity of the plots suggests first-order reaction kinetics with respect to the concentration of dissolved oxygen [14,35]. According to the slopes of the plots, the average number of electrons transferred on the N 1 S 1 -CNW-900 electrode was ~4.3, which is similar to that on a Pt/C electrode (~4.1). ...
Heteroatom doping, precise composition control, and rational morphology design are efficient strategies for producing novel nanocatalysts for the oxygen reduction reaction (ORR) in fuel cells. Herein, a cost-effective approach to synthesize nitrogen- and sulfur-codoped carbon nanowire aerogels using a hard templating method is proposed. The aerogels prepared using a combination of hydrothermal treatment and carbonization exhibit good catalytic activity for the ORR in alkaline solution. At the optimal annealing temperature and mass ratio between the nitrogen and sulfur precursors, the resultant aerogels show comparable electrocatalytic activity to that of a commercial Pt/C catalyst for the ORR. Importantly, the optimized catalyst shows much better long-term stability and satisfactory tolerance for the methanol crossover effect. These codoped aerogels are expected to have potential applications in fuel cells.
... Carbon nanomaterials [20][21][22][23][24][25][26][27], particularly nitrogendoped carbon materials [28,29], have been recognized as promising candidates within precious-metal-free catalysts for ORR in various energy storage and conversion systems due to their excellent electrical conductivity, remarkable ORR catalytic activity, and great cycling stability. For instance, Wong et al reported that N-doped graphene exhibits improved electrocatalytic behavior for ORR compared to undoped graphene [30]. ...
... Additionally, it can be easily found that the carbonization temperature had a profound impact on the nitrogen content and types of nitrogen, as shown in figure 3(c) and table S2 (online supplementary information). The total nitrogen content decreased from 8.33% to 4.90% in atom-icpercentage as carbonization temperature increased from 600°C to 1000°C since C-N bonds were less stable at high temperatures compared to C-C bonds [21]. With the increase in pyrolysis temperature from 600 to 800°C, the percentage of pyrrolic-N decreased whereas the content of pyridinic-N and graphitic-N increased, implying a transformation from pyrrolic-N to pyridinic-N and graphitic-N. ...
The development of metal-free catalysts to replace the use of Pt has played an important role in relation to its application to fuel cells. We report N-doped carbon nanofibers as the catalyst of an oxygen reduction reaction, which were synthesized via carbonizing bacterial cellulose-polypyrrole composites. The as-prepared material exhibited remarkable catalytic activity toward the oxygen reduction reaction with comparable onset potential and the ability to limit the current density of commercial Pt/C catalysts in both alkaline and acid media due to the unique porous three-dimensional network structure and the doped nitrogen atoms. The effect of N functionalities on catalytic behavior was systematically investigated. The results demonstrated that pyridinic-N was the dominating factor for catalytic performance toward the oxygen reduction reaction. Additionally, N-doped carbon nanofibers also demonstrated excellent cycling stability (93.2% and 89.4% retention of current density after chronoamperometry 20 000 s in alkaline and media, respectively), obviously superior to Pt/C.
... [21][22][23] Besides the crucial electrocatalytical role of nitrogen atom, other factors including the specific surface area, pore volume and pore structure of carbon materials cannot be overlooked owing to their important contribution to the electrocatalytic process. 3,15,19,[24][25][26][27][28] As a class of regular crystalline porous materials, metal-organic frameworks (MOFs) emerge as a new family of templates or precursors to synthesize porous carbons with high specific surface area. 16,24,27,[29][30][31][32][33][34][35] Especially, ZIF-8 (one type of MOFs) with imidazole group has been carbonized directly to prepare N-doped porous carbons, 6,7,32,34,[36][37][38][39][40][41][42] in view of its rich nitrogen (24.5%), high specific surface area (1560 m 2 · g −1 ) and porosity (0.79 cm 3 · g −1 ). ...
... 3,15,19,[24][25][26][27][28] As a class of regular crystalline porous materials, metal-organic frameworks (MOFs) emerge as a new family of templates or precursors to synthesize porous carbons with high specific surface area. 16,24,27,[29][30][31][32][33][34][35] Especially, ZIF-8 (one type of MOFs) with imidazole group has been carbonized directly to prepare N-doped porous carbons, 6,7,32,34,[36][37][38][39][40][41][42] in view of its rich nitrogen (24.5%), high specific surface area (1560 m 2 · g −1 ) and porosity (0.79 cm 3 · g −1 ). 7 In addition, some ZIF-8 composites, such as furfuryl alcohol@ZIF-8, NH 4 OH@ZIF-8, 34 polystyrene@ZIF-8, 39 graphene oxide@ZIF-8, 6 ZIF-8@ZIF-67 41 and carbon nanotubes@ZIF-8, 38 have been synthesized as both precursors and templates to prepare N-doped porous carbon. ...
As novel N-doped carbon precursors, zeolitic imidazolate [email protected]/* */ pyrrolidone ([email protected]/* */) composites were designed and synthesized. Direct carbonization of [email protected]/* */ composites at a high temperature produced the nitrogen-doped porous carbon materials (named as CP1000). And the electrocatalytic abilities of the corresponding carbonized products toward oxygen reduction reaction in alkaline solution were investigated. In comparison with the carbonized product of pure ZIF-8 (named as C1000), CP1000 possesses the higher specific surface area and total pore volume. By adjusting the initial dosage of PVP, three kinds of CP1000 with different size and N content were prepared. It was found that the high specific surface area and favorable quaternary-N species in CP1000 sample played an active role in catalyzing oxygen reduction reaction (ORR).
... Additionally, the water activation method has been utilized to increase the electrochemical activity of GF. [79] rGO is a carbonaceous material that possesses high electronic mobility as well as thermal and electrochemical stability, making it an attractive candidate for catalytic reactions during VRFB operation and for ensuring extended cycle life of the catalyst. Moreover, N-doped reduced graphene oxide (N-rGO) has been found to have greater catalytic activity, [123] with the amount of N-doping having a significant impact on its performance, as suggested by Jin et al. [124] Using urea as the nitrogen source, Shi et al. [125] synthesized N-doped graphene, pyrolyzed at 900°C. The result exhibits good electrochemical performance and enhanced the electrochemical reversibility and catalytic activity for the VO 2 + /VO 2 + reaction. ...
The vanadium redox flow battery (VRFB) is a highly regarded technology for large-scale energy storage due to its outstanding features, such as scalability, efficiency, long lifespan, and site independence. This paper provides a comprehensive analysis of its performance in carbon-based electrodes, along with a comprehensive review of the system's principles and mechanisms. It discusses potential applications, recent industrial involvement, and economic factors associated with VRFB technology. The study also covers the latest advancements in VRFB electrodes, including electrode surface modification and electrocatalyst materials, and highlights their effects on the VRFB system's performance. Additionally, the potential of two-dimensional material MXene to enhance electrode performance is evaluated, and the author concludes that MXenes offer significant advantages for use in high-power VRFB at a low cost. Finally, the paper reviews the challenges and future development of VRFB technology.
... As can be seen in Fig. 7 (a), the ethanol oxidation current densities of p-CNO, ox-CNO, and NeCNO electrocatalysts are 3.3 mA cm À2 , 3.8 mA cm À2, and 4.2 mA cm À2 , respectively. It can be observed that the NeCNO electrocatalyst gives a better current density than the ox-CNO and p-CNO electrocatalysts and indicates a promoted electrochemical performance by the inclusion of N atoms on top of the onion structure [59]. Also, this high performance is ascribed to the high surface area, degree of graphitization and porous nature of NeCNO electrocatalyst [5]. ...
In this study, we present the synthesis of pristine carbon (p-CNO), nitrogen doped (N–CNO) and oxygen functionalized (ox-CNO) nano onions, using flame pyrolysis, chemical vapour deposition, and reflux methods, respectively. Pd/p-CNO, Pd/N–CNO and Pd/ox-CNO electrocatalysts are prepared using a simple and quick microwave-assisted synthesis method. The various CNO and Pd/CNO electrocatalysts are fully characterized and the FTIR and XPS results reveal that the synthesized CNOs contain oxygen and nitrogen functional groups that facilitates the attachment and dispersion of the Pd nanoparticles. Electrochemical tests show that the N–CNO and Pd/N–CNO electrocatalysts exhibit high current density (4.2 mA cm ⁻² and 17.4 mA cm ⁻²), long-term stability (1.2 mA cm ⁻² and 6.9 mA cm ⁻²), and fast electron transfer when compared to the equivalent pristine and oxidized catalysts (and their Pd counterparts), and a commercial Pd/C electrocatalyst, towards ethanol oxidation reactions in alkaline medium.
... In particular, Fe-, Co-, and Ni-based oxides have been recently reported to exhibit high catalytic activities and are stable electrocatalysts [19]. Accordingly, various alternative electrocatalysts, such as metal-nitrogen-doped carbon materials, which are found to be the most promising candidates for ORR, were explored [20,21]. Several reviews on such NPMCs have been reported, since the considerable activity of cobalt phthalocyanine towards ORR was described [22]. ...
In this study, a non-precious metal-based electrocatalyst consisting of nitrogen-doped iron-coated reduced graphene oxide (FeNG) on carbon xerogel towards oxygen reduction reaction (ORR) in alkaline media is reported. Herein, we describe a facile three-step synthesis route towards enhanced ORR activity. The effect of pyrolysis temperature and the resulting structural variations of the designated catalyst towards ORR were investigated. The as-synthesized carbon xerogel samples were reduced (rCX) and then pyrolyzed at different temperatures, viz., 700, 900, and 1100 °C, followed by the incorporation of FeNG, and their performance towards ORR was studied. The resultant rCXFeNG (reduced carbon xerogel-iron-nitrogen-doped graphene) catalyst pyrolyzed at an optimum temperature of 1100 °C (rCXFeNG-1100) showed enhanced electrocatalytic performance towards ORR and exhibited an onset potential of 0.84 V vs. RHE (reversible hydrogen electrode). Besides, it is remarkable that rCXFeNG-1100 delivers a limiting current density of 5.55 mA cm−2, which is fairly equivalent to that of the commercial Pt/C electrocatalyst. It is noteworthy that the rCXFeNG-1100 electrocatalyst showed a four-electron transfer pathway for ORR and showed better stability and improved durability outperforming the commercial Pt/C electrocatalyst. The present study opens up a promising approach for the design and fabrication of cost-effective non-precious ORR electrocatalysts for alkaline polymer electrolyte fuel cells.
... At the present, platinum and its alloys are considered to be the best ORR electrocatalysts; however, they are prone to methanol and carbon monoxide poisoning, and their scarcity and high costs have greatly hindered large-scale commercialization of the technologies [5][6][7][8][9][10]. In recent years, catalysts composed of porous carbon doped with nitrogen and non-precious transition metals (MNC) have attracted widespread attention due to their high electrocatalytic performance, good corrosion resistance, and low costs [11][12][13][14][15][16], mostly due to the formation of MN x coordination moieties [17][18][19][20]. ...
Design and engineering of low-cost, effective catalysts for oxygen reduction reaction (ORR) plays a critical role in the development of fuel cells and metal-air batteries. Herein, we describe a facile template-assisted strategy for the fabrication of hollow porous carbon spheres codoped with nitrogen and iron species (FeNC) for ORR electrocatalysis. The samples are synthesized via one-step pyrolysis of a core-shell precursor, which is prepared by in-situ growth of a Fe-doped zeolite imidazolate framework (ZIF) shell onto the surface of polystyrene nanoparticles. The obtained FeNC composites exhibit a unique hollow structure with a large surface area, hierarchical porosity, and abundant FeNx sites. Notably, the sample prepared at 950 °C (FeNC-950) exhibits the best ORR activity among the series in alkaline media (with an onset and half-wave potential at +0.94 and + 0.84 V vs. RHE, respectively), a performance on par with that of Pt/C and leading relevant catalysts reported in recent literature, where ORR proceeds mostly via the efficient four-electron reduction pathway. The FeNC-950 catalyst also displays superior stability and tolerance to methanol, as compared to commercial Pt/C. The results suggest that high-performance ORR catalysts can be derived by deliberate structural engineering of the metal-organic framework precursors.
... Therefore, it is very important to develop highefficiency non-precious metal catalysts for Zn-air batteries. In recent years, efforts have been made to develop alternative non-precious metal electrocatalysts, including transition metals [8][9][10][11] and heteroatom-doped carbon materials [12][13][14][15]. In addition to being the ORR electrocatalysts, carbon nanomaterials like carbon nanotubes [16], graphene [17], graphene oxide [18], porous carbon nanosheets/network [19,20], carbon nanofibers [21], and porous carbon materials [22][23][24] are extensively applied to be the carriers to immobilize the non-precious metal catalysts due to their excellent electron conductivity and large specific surface area. ...
A neutral zinc–air battery has unique characteristics. One of the key issues for its normal operation is how to ensure that the cathode catalyst can effectively electrocatalyze oxygen reduction reaction (ORR) in a neutral solution. Here, we report a simple preparation of CoNi-doped C–N/CNT nanocomposite catalysts (CoNi/C–N/CNT) by pyrolysis of a mixture of nickel/cobalt salt, carbon nanotube (CNT), dicyandiamide (DCD), and ethylenediamine. The prepared CoNi/C–N/CNT nanocatalysts are composed of CoNi nanoparticles, hollow tubular C–N nanocomposites obtained from CoNi salts and DCD, and CNT added. Among the synthesized catalysts, the Co3Ni2/C–N/CNT45 has a high and stable ORR current density of 6.2 mA cm⁻² in 0.5 mol L⁻¹ KNO3 solution. Neutral Zn–air battery with the carbon paper coated with the CoNi/C–N/CNT nanocomposite as an air electrode and metal Zn as an anode was assembled. For the catalyst Co3Ni2/C–N/CNT45, the neutral Zn–air battery in 0.5 mol L⁻¹ KNO3 solution presents an open-circuit voltage of 1.16 V and a maximum power density of 50.1 mW cm⁻². Moreover, the constant discharging current density of 100 and 150 mA cm⁻² can last respectively 88 and 28 h until complete consumption of the Zn foil anode.
... Although such nitrogen-carbon (N-C) catalysts with wide size distribution of pores suffer from poor activity and stability in acidic media they can activate oxygen in alkaline electrolytes. 93 Electro-catalytic properties of N-decorated porous carbons for ORR is a consequence of interactions between the electron pairs of nitrogen and delocalized p electrons of a carbon matrix. 94,95 The ZIF-8 has been frequently demonstrated to generate N-C catalysts with high surface area and well porosity, although it cannot afford M-N-C active sites for ORR. ...
Fuel cells and metal-air batteries have been comprehensively investigated in recent years because of their high energy capacity, good efficiency and environmental friendly nature. Slow kinetics of oxygen reduction reaction (ORR), one of the main processes in fuel cells and metal-air batteries, is improved with platinum catalysts that confine the prevalent utilization of such electrochemical devices with increasing worth for them. However, platinum catalysts after long time usage exhibit weak operations due to the crossover effect and agglomeration. Metal–organic frameworks (MOFs), the porous crystalline materials, consisting of metal centers coordinated to organic ligands, are appropriate catalysts due to their superior properties such as high surface area and carbon content, tunable pore size and diverse metal nodes. In this review, we summarize the recent progress in synthesis and design of MOF-derived ORR electrocatalysts in acidic and alkaline fuel cells. Our focus is on the different methods developed for improving the activity and stability of MOF based ORR electrocatalysts.
Graphical Abstract
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... e results reveal that the content of the nitrogen element increases with an increase in pyrolysis temperature. e N1s spectra of the N-rGO modified electrodes were performed in Figure 5. e N1s spectra could be deconvoluted into three different peaks, corresponding to pyridinic-N (397.8 eV), pyrrolic-N (398.5 eV), and graphitic-N (400.8 eV) [31,32]. e relative content of each nitrogen specials is listed in Table 3. ...
As one of the key factors that limit the development of vanadium redox flow battery (VRFB), the positive redox couple of VO ²⁺ /VO 2⁺ plays an important role on the overall performance of VRFB. To improve the kinetics of a positive reaction, a new designed nitrogen-doped reduced graphene oxide-modified graphite felt (N-rGO/GF) electrode was prepared by coupling the methods of freeze-drying and pyrolysis. The characteristics of the prepared electrode were measured by scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) analysis, Raman spectroscopy (Raman), X-ray diffraction (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge tests. By coupling the methods of freeze-drying and pyrolysis, the N-rGO can be evenly dispersed on the surface of GF electrode, resulting in an excellent catalytic activity. The results demonstrate that the proposed N-rGO/GF electrode with pyrolysis temperature of 900°C shows excellent electrochemical performance and significantly improves the catalytic activity and electrochemical reversibility for the positive VO ²⁺ /VO 2⁺ reaction, indicating that the proposed composite electrode has potential applications in the improvement of VRFB performance.
... V, indicating the first-order reaction kinetics during the ORR process [24,25]. In contrast, the linearity and parallelism of the K-L fitting plots for the other samples and CB is poorer than those of MOF(Co)/C(3:1)-500 and 20 wt% Pt/C, suggesting more complex reaction kinetics on them [26,27]. According to the slope of the K-L fitting plots, the transferred electron numbers (n) of ORR at various potentials can be calculated as shown in Fig. 3c. ...
... V, indicating the first-order reaction kinetics during the ORR process [24,25]. In contrast, the linearity and parallelism of the K-L fitting plots for the other samples and CB is poorer than those of MOF(Co)/C(3:1)-500 and 20 wt% Pt/C, suggesting more complex reaction kinetics on them [26,27]. According to the slope of the K-L fitting plots, the transferred electron numbers (n) of ORR at various potentials can be calculated as shown in Fig. 3c. ...
... To further explore the ORR mechanism, RRDE linear sweep voltammetry (LSV) was conducted at various rotating rates, and the corresponding results are depicted in Fig. 5b and S6. The current densities were significantly enhanced by increasing rotating rate for all samples, indicating a fast oxygen flux from the bulk solution to the electrode surface under high rotating rate [66]. The incorporation of nitrogen into AC could enhance the current densities at all rotating rate, although apparent discrepancy compared with Pt/C was still observed. ...
Despite the wide application of activated carbon (AC) as cathode electrocatalyst in microbial fuel cell (MFC), the enhancement of its catalytic activity is crucial to reduce its high loading on air-cathode. Herein, we synthesize nitrogen-doped activated carbon (NAC) by pyrolyzing phthalocyanine (Pc) adsorbed on AC to develop an efficient oxygen reduction reaction (ORR) electrocatalyst. The optimized mass ratio of AC to Pc improves the crystalline structure and porous structure of the NAC. Elemental analysis indicates that this material contains appropriate content of pyrrolic and pyridinic types of nitrogen and oxygen species. The NAC shows an ORR onset potential of 0.468 V (vs. Standard hydrogen electrode), an electron transfer number of 3.90, and high electrochemically accessible surface area, thereby illustrating enhanced electrocatalytic activity in the neutral medium relative to alkali-treated activated carbon (b-AC) and commercial platinum catalyst. Owing to the high activity, a small amount of NAC with a loading of 15 mg cm− 2 on the air-cathode of MFC is sufficient to achieve the maximum power density of 1026.07 ± 10.83 mW m− 2, which is higher than that of b-AC and comparable to platinum catalyst. The reduced loading of NAC indicates that the material can be used as cathode electrocatalyst for the ongoing effort to scale up MFC in the future.
Graphical abstract
... Recently, great efforts have been made in enhancing the performance of nonprecious metal electrocatalysts. [12][13][14][15][16][17][18] A specific family of metal/N-doped carbon hybrid materials that contain N-coordinated sites with a transition metal embedded in carbon matrix have been regarded as promising catalysts for various electrocatalytic reactions. [19][20][21][22][23][24] In particular, Co or Fe/N-doped carbon materials have been demonstrated as highly active nonprecious metal catalysts for ORR. ...
... In contrast, the doping of N into the AC using N-containing precursor, such as cyanamide with simple sintering is an alternative and simple way to achieve the desired target [57]. Reflux with nitric acid [131], transfer-doping with PANI [132] and the sintering of metal-organic frameworks (MOFs) [133], are alternative methodologies reported for production of the N-doped carbon materials as the ORR catalyst for MFC applications. ...
Microbial fuel cells (MFCs) are a promising green approach for wastewater treatment with the simultaneous advantage of energy production. Among the various limiting factors, the cathodic limitation, with respect to performance and cost, is one of the main obstacles to the practical applications of MFCs. Despite the high performance of platinum and other metal-based cathodes, their practical use is limited by their high cost, low stability, and environmental toxicity. Oxygen is the most favorable electron acceptor in the case of MFCs, which reduces to water through a complicated oxygen reduction reaction (ORR). Carbon-based ORR catalysts possessing high surface area and good electrical conductivity improve the ORR kinetics by lowering the cathodic overpotential. Recently, a range of carbon-based materials have attracted attention for their exceptional ORR catalytic activity and high stability. Doping the carbon texture with a heteroatom improved their ORR activity remarkably through the favorable adsorption of oxygen and weaker molecular bonding. This review provides better insight into ORR catalysis for MFCs and the properties, performance, and applicability of various metal-free carbon-based electrocatalysts in MFCs to find the most appropriate cathodic catalyst for the practical applications. The approaches for improvement, key challenges, and future opportunities in this field are also explored.
... Furthermore, Fig. 7(b) showed that all of catalysts suffered a sudden drop once upon methanol was added in the electrolyte, which were 5%, 7% and 25% for HCT@HPC@AgNPs, HPC@AgNPs and Pt/C, respectively, revealing HCT@HPC@AgNPs had better anti-toxic capacity to methanol than the rests. Besides, the degradation of activity can be attributed to the inactivation of AgNPs, since, methanol can dissociate to CO and H on a noble-Ag electrode, on which CO is strongly adsorbed and accordingly becomes a detrimental intermediate [48,49]. ...
Porous C/Ag nanohybrids were one type of emerging and promising alternative oxygen reduction catalysts
for Pt, which was crucial to energy conversion and storage devices. Herein, a template-free, sustainable
and in-situ method was employed for preparing N-doped hollow carbon tube @ hierarchically
porous carbon supporting homogeneous Ag nanoparticles (HCT@HPC@AgNPs) by one-pot carbonization
with the assist of “chelating effect”. It was very interesting that the “chelating effect” can inhibit the selfaggregation of AgNPs during its growing process, since Agþ can be well immobilized on precursor.
Moreover, during the carbonization process, an amazing N-doped hollow carbon tube @ porous carbon
was also achieved by the synergistic effect of thermal degradation, spontaneous bubble-template and
self-doping. The resultant HCT@HPC@AgNPs was investigated by physical characterizations and electrochemical tests. The results indicated that HCT@HPC@AgNPs delivered distinct electrocatalytic activity
towards oxygen reduction reaction (ORR), long-term durability and anti-toxic capacity in alkaline electrolyte,
which was comparable or even better than Pt/C. Thus, HCT@HPC@AgNPs was a promising ORR
catalyst for application in the field of energy and catalysis.
... For example, methanol dissociates to CO and H on a platinum electrode, on which CO is strongly adsorbed and accordingly becomes a detrimental intermediate. 29,43 From Fig. 7(b), both Fe 3 O 4 @N/Co-C and Pt/C suffer a sudden drop once methanol was added. However, the drop for Fe 3 O 4 @N/Co-C and Pt/C was 4.7% and 9.2%, respectively, conrming the better resistance to poisoning of Fe 3 O 4 @N/Co-C. ...
A “Spontaneous bubble-template” method is fascinating in that bubbles are formed in situ during material processing and employed as a template for fabricating unique structures, which has not been reported in material science. It is sustainable, green and efficient in that no extra additives or post-treatment are used. Herein, novel metal–polymeric framework derived hierarchically porous carbon/Fe3O4 nanohybrids are prepared using a “spontaneous bubble-template” method by one-step carbonization. During the carbonization process, N and Co are self-doped on porous carbon in which in situ grown nano Fe3O4 is embedded (Fe3O4@N/Co–C). The as-prepared Fe3O4@N/Co–C displays
a three-dimensional interpenetrating morphology (electrochemical active area: 729.89 m2 g�1) with
well-distributed Fe3O4 nanoparticles (20–50 nm) which are coated with a carbon layer (3–5 nm).
Fe3O4@N/Co–C exhibits remarkable oxygen reduction activity in biofuel cells with a distinct output
voltage (576 mV) and power density (918 mW m�2), which are 3.6% and 17.8% higher than those of Pt
(0.5 mg cm�2), respectively. Besides biofuel cells, Fe3O4@N/Co–C may also have the potential for
application in chemical fuel cells, since it demonstrates better oxygen reduction activity in
electrochemical measurements. Thus, with the virtues of its low-cost, facile synthesis and large-scale
preparation, Fe3O4@N/Co–C is a promising electrocatalyst for the oxygen reduction reaction and
application in biofuel cells.
... 19,20 Our group synthesized hierarchically porous nitrogen-doped carbon materials, derived from nitrogen-containing isoreticular metal-organic framework-3 through direct carbonization. 21 The surface area, porosity and the surface functionalities (doped nitrogen content and type) can be controlled by carbonization temperature. The optimized product at 950 1C exhibited excellent electrocatalytic activity for ORR, which is close to commercial Pt/C. ...
Developing a low cost, highly active, durable cathode towards an oxygen reduction reaction (ORR) is one of
the high-priority research directions for commercialization of low-temperature polymer electrolyte
membrane fuel cells (PEMFCs). However, the electrochemical performance of PEMFCs is still hindered by the
high cost and insufficient durability of the traditional Pt-based cathode catalysts. Under these circumstances,
the search for efficient alternatives to replace Pt for constructing highly efficient nonprecious metal catalysts
(NPMCs) has been growing intensively and has received great interest. Combining with the compositional
effects, the accurate design of NPMCs with 3D porous nanostructures plays a significant role in further
enhancing ORR performance. These 3D porous architectures are able to provide higher specific surface
areas and larger pore volumes, not only maximizing the availability of electron transfer within the
nanosized electrocatalyst surface area but also providing better mass transport of reactants to the
electrocatalyst. In this Tutorial Review, we focus on the rational design and synthesis of different 3D
porous carbon-based nanomaterials, such as heteroatom-doped carbon, metal–nitrogen–carbon nanostructures
and a series of carbon/nonprecious metal-based hybrids. More importantly, their enhanced
ORR performances are also demonstrated by virtue of their favorably porous morphologies and
compositional effects. Finally, the future trends and perspectives for the highly efficient porous NPMCs
regarding the material design are discussed, with an emphasis on substantial development of advanced
carbon-based NPMCs for ORR in the near future.
Hydrogen obtained through the electrolysis of water is a potential solution to the growing demand of human society for energy. In addition, water electrolysis generates less environmental pollution than fossil energy sources. However, the preparation of highly active and low-cost electrocatalysts remains a key challenge. Here, we report a facile and inexpensive method to prepare palladium nanoparticles (Pd NPs) supported on aminated (-NH2) metal-organic frameworks (MOF). The obtained electrocatalyst (Pd@Uio-66-NH2) exhibits excellent electrocatalytic performance for the hydrogen evolution reaction (HER), featuring an ultralow overpotential (34 mV at 10 mA cm-2), small Tafel slope (41 mV dec-1), and superior stability in acid electrolyte. Systematic characterization demonstrated that -NH2 can effectively stabilize palladium acetate as the Lewis base. Meanwhile, the strong interaction between the lone pair electrons and the d-orbital ensures that the Pd atoms are uniformly distributed in the MOF material, inhibiting the agglomeration of metal NPs in the reaction. This strategy provides an avenue to prepare inexpensive and highly active catalysts for HER in acidic media.
The tailorable properties and numerous diversified morphologies enable metal-organic frameworks (MOFs) to be utilized in a wide range of applications such as water splitting, energy storage, and fuel cells. In recent times, rational designing and synthetic techniques of various MOFs and their nanoparticle derivatives have brought a possibility to be served as an exceptional electrocatalyst for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). This approach results from the improved surface area, porosity, pore-volume, and tunable structure of MOF-based materials. This chapter presents the introduction of ORR, OER, and HER along with detailed sections about MOF-derived nonprecious metal-based electrocatalysts for each reaction. The structural and electrochemical characteristics for various synthetic schemes of MOF-derived materials and their comparability with platinum-based electrocatalysts have been discussed in detail. In the last sections, materials displaying multifunctionality have been addressed.
To improve the power generation of microbial fuel cell (MFC), the cathode is modified to increase its oxygen reduction reaction (ORR) activity by using Co/UiO-66, which derived from pyrolyzing the mixture of Co(NO3) 2 as the metal precursor incorporated with NH2-UiO-66. It was found that Co/UiO-66 (MOF-900) has been developed as a high-performance electrocatalyst for ORR at a pyrolysis temperature of 900 °C. Therefore, Co/UiO-66 should be a promising oxygen reduction catalyst for application in MFCs. This study provides technical and theoretical validation for the MFC performance improvement by ORR active MOF-derived catalysts modified cathodes.
Weak electrical conduction impedes the application of Metal-organic framework (MOF) in electrochemistry. In this work, palladium-based MOF (Pd/MOF) is synthesized via i) simple solvothermal process and ii) direct chemical reduction method. N2 adsorption-desorption isotherms and thermogravimetric analyzer show that MOF have large specific surface area and good thermal stability. The synthesized materials were tested in acidic condition for hydrogen evolution reaction (HER). Electrochemical test results show that Pd/MOF with tafel slope of 85 mV·dec⁻¹, onset overpotential of 105 mV and ideal stability has better performance than bulk MOF for HER. The enhanced performance can be ascribed to the large porosity and specific surface of MOF, and the synergistic effect between Pd and MOF toward H2 production.
Mesoporous carbon derived from pyrolysis of metal-organic framework (MOF) is advantageous owing to its high specific surface area, large pore volume, and versatility in both structure and composition. Heteroatom doping on mesoporous carbon by synthesizing with heteroatom containing ligands (pre-synthetic process) or incorporating heteroatom containing compounds during pyrolysis (post-doping) can further enhance its electrochemical properties. Although both methods have been applied to increase the doping content, the effects of pre-synthetic process post-doping have not yet been systematically studied. Herein, we have synthesized mesoporous carbon derived from sulfur containing MOF for the first time using 2,5-disulfanylterephthalic acid as a ligand, and added dopants (melamine for N-doping and thiourea for N,S-codoping). We systematically compared the performance of mesoporous carbon as cathodes for oxygen reduction reaction (ORR) and lithium-sulfur (Li-S) battery with previous studies, which used terephthalic acid and its analogues as pre-synthetic ligands and various dopants as post-doping sources. Our work showed the synergetic effect of heteroatoms from dual-doping process – especially, sulfur (S from pre-synthetic process and N,S from post-doping), which not only enhanced catalytic activity (limiting current density (JL) of -5.19 mA⋅cm⁻²), stability, and methanol tolerance as an ORR catalyst but also rendered superior stability of 85.2% over 100 cycles as a cathode of Li-S battery.
Metal-organic frameworks (MOFs) have emerged as promising materials in the areas of gas storage, magnetism, luminescence and catalysis owing to their superior properties of highly crystalline structures. However, MOFs’ stability to thermal or humidity is greatly less than carbons because they are constructed from assembly of ligands with metal ions or clusters by coordination bonds. Transforming MOFs into carbons is bringing a new potential for MOFs to reach industrialization, and carbons with controlled pore size and surface-doping are one of the most important porous materials. By selecting MOFs as a precursor or template, carbons with heteroatom-doping and well-developed pores can be achieved. In this review, we discussed state-of-art study progress on newly development of MOF-derived metal-free porous carbons. In particular, the potential use of metal-free carbons from environmental and energy perspectives like adsorption, supercapacitor and catalyst were analyzed in detail. Besides, an outlook for future sustainable development of MOF-derived porous carbons was also prospected.
Electrifying the petroleum‐based transportation system and transitioning to a carbon‐free energy cycle require the participation of fuel cell technology using hydrogen as the fuel source. The major technical obstacle, however, resides on the electrocatalysts that promote the efficiency of energy conversion at the electrodes. In particular, oxygen reduction reaction (ORR) at the cathode desperately needs a high‐performance catalyst to enhance the reaction kinetics and hence reduce the overall cost of fuel cell operation. Porous carbon as ORR catalyst or catalyst support has received enormous attentions due to its adjustable catalytic activity, structural tunability and low cost. Recently, metal‐organic frameworks (MOFs) and their derived carbon materials present attractive properties as ORR catalysts thanks to their tunable micro‐nano structure, surface area, porosity, and compatibility with a variety of metal catalysts. In this book chapter, we will cover the recent progress on the MOF derived porous carbon ORR catalysts and discuss the structural and compositional relationship with their catalytic performance. We hope the short walkthrough of the MOF based catalyst development can give the readers a general picture of the key parameters of constructing a high‐performance ORR catalyst and appreciate the outcomes of nanomaterials design brought to the development of advanced fuel cell technology.
This study synthesized the metal–organic framework-derived composite Fe−N−Co@C-800-acid-leaching (Fe−N−Co@C-800-AL) through co-doped iron and nitrogen atoms in a zeolitic imidazolate framework-67, followed by pyrolysis at 800°C and the AL process. Fe−N−Co@C-800-AL was highly active in an oxygen reduction reaction, during which the number of transferred electrons (3.986) was close to an ideal four-electron transfer. In addition, Fe−N−Co@C-800-AL showed no obvious degradation even after potential cycling of half-cell measurement (30,000 cycles). The prepared material exhibited a porous structure composed of nanoparticles (NPs) that were randomly distributed on poly−hydrocarbon structures with a Brunauer–Emmett–Teller surface area of 449.0 m2 g−1. X-ray photoelectron spectroscopy demonstrated that the synthesized Fe−N−Co@C-800-AL contained large amounts of pyridinic nitrogen and graphitic nitrogen, which could significantly enhance the activity of the oxygen reduction reaction. Furthermore, X-ray absorption spectroscopy revealed the existence of Co−Co and Fe−Fe and a lack of Co−Nx and Fe−Nx moieties, which means an oxygen reduction reaction may occur on the microstructures of N-doped carbon with wrapped metal NPs (Co or Fe). These findings revealed that Fe−N−Co@C-800-AL had a porous structure, high surface area, and the presence of functional nitrogen, thereby making it suitable for oxygen reduction reaction.
Developing low-cost and highly effective doped carbon catalysts for the oxygen reduction reaction remains an urgent requirement for fuel cell applications. Herein, we design a facile and effective preparation strategy for the fabrication of a 3D porous carbon network catalyst comprised of an ultrathin nanosheet and anchored with Fe 3 C nanoparticles. The catalyst was prepared using an iron–tannin framework coated over g-C 3 N 4 as precursor, and simultaneously with g-C 3 N 4 as a nitriding agent and structural/morphological template. Optimum catalyst exhibits excellent ORR performance and durability in an alkaline medium; the half-wave potential (+0.86 V vs. RHE) is 40 mV more positive than that of commercial Pt/C, and its current density at +0.9 V (vs. RHE) reaches −1.153 mA cm ⁻² , which is almost 2.42 times that of commercial Pt/C. Significantly, the catalyst also shows outstanding ORR performance in acidic conditions with a half-wave potential of +0.73 V (vs. RHE), comparable to that of Pt/C, and better long-term stability than Pt/C. Based on our characterization results, we ascribe the outstanding performance of catalyst to: the enhanced amount of Fe–N x active sites and active nitrogen species, including pyridinic-N and graphitic-N; Fe 3 C nanoparticles covered with ultrathin doped carbon layer; and the high surface area and porous structure.
To improve the power generation of microbial fuel cell (MFC), the cathode is modified to increase its oxygen reduction reaction (ORR) activity by using a Cu, N-incorporated carbon-based material as catalyst, which obtained from pyrolyzing ORR active Cu (II)-based metal organic framework (MOF; Cu-bipy-BTC, bipy = 2,2′-bipyridine, BTC = 1,3,5-tricarboxylate). MOF-800 (the product of pyrolyzing Cu-bipy-BTC at 800 °C) shows porous structure with micropores ranging from 0.5 to 1.3 nm and mesopores ranging from 27 to 46 nm. It also exhibits improved ORR electrocatalytic activity with a higher current density of −3.06 mA cm⁻² compared to Cu-bipy-BTC. Moreover, the charge transfer resistance of MOF-800 cathode (1.38 Ω) is much smaller than that of Cu-bipy-BTC cathode (176.8 Ω). A maximum power density of 326 ± 11 mW m⁻² is achieved by MOF-800-MFC, which is 2.6 times of that of Cu-bipy-BTC-MFC and comparable with Pt/C-MFC (402 ± 17 mW m⁻²). The results imply the enhancements of ORR catalytic activity and electrical conductivity of MOF-800 are due to the enhanced porous structure and abundant active sites (C–N, Cu–Nχ), which result in the improved power generation of MFC. This study provides technical and theoretical validation for the MFC performance improvement by ORR active MOF-derived catalysts modified cathodes.
The demand for economical, sustainable and highly efficient catalysts to replace the Pt-based catalysts for proper industrialization of oxygen reduction reaction (ORR) has gained tremendous scientific interest. Herein, we report a facile strategy to develop doped nitrogen-rich carbon nano-onion architectures from the renewable biological resource, collagen, for use as a metal-free ORR catalyst. The product contains an appreciably high percentage of nitrogen (7.5%) integrated into the carbon molecular skeleton. The materials exhibit outstanding ORR electrocatalytic activity with low onset potential, high current density, superior methanol crossover immunity and better durability than the benchmark Pt/C catalyst in alkaline medium. The ORR followed an efficient direct reduction pathway of 4-electron transfer kinetics. The performance analysis of different samples demonstrates that the porous graphitic carbon with more nitrogen content provides adequate active catalytic sites for ORR activity and the pyridinic nitrogen act as an effective promoter for ORR. The findings ascertain that renewable biomasses can be easily transformed into novel carbon nanostructures with excellent catalytic activity.
Highly active bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reactions (ORR) have attracted increasing attention in metal-air batteries and fuel cells. CoFe nanoalloy particles encapsulated in nitrogen-doped carbon and nitrogen-doped carbon nanotubes (CoFe@NC-NCNT-H) are synthesized by pyrolyzing a Prussian blue analogue precursor (i.e. Fe3[Co(CN)6]2) as low as 600 °C, and followed by HNO3 treatment. Such low temperature pyrolysis and HNO3 treatment affords the hybrid mesoporous material with a high level of nitrogen content (∼10%) and a relatively high specific surface area (∼210.5 m² g⁻¹), capable of providing active sites and mass transport channels. In alkaline solution, CoFe@NC-NCNT-H is highly active towards OER with a low onset potential (∼1.35 V) and a small overpotential (∼380 mV) to reach 10.0 mA cm⁻², comparable to the state-of-the-art RuO2. CoFe@NC-NCNT-H is also a good ORR catalyst, and more importantly it exhibits an improved stability compared to commercial Pt/C. CoFe@NC-NCNT-H displays promise as a bifunctional catalyst with an extremely low potential difference (∼0.87 V between ORR at −3.0 mA cm⁻² and OER at 10.0 mA cm⁻²), superior to commercial Pt/C and RuO2. The facilely prepared CoFe@NC-NCNT-H with high bifunctional performance and stability promises great potential for ORR and OER.
This work is investigating electrocatalytic performance of boron (B) in two diverse classes of graphene hybrid materials; i) the pillared graphene organic framework (GOF) which is a type of adsorbate-induced boron doped graphene and ii) the substitutionally boron doped graphene (i.e. integrated boron in disrupted graphene lattice (BG)). Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) are among the key tools to identify chemical states of doped boron as well as its surface composition. Electron transfer efficiency of the easily synthesised GOFs (with CBO2 active chemistries) is compared with commonly cited BGs (with distinctive CBO2/BC3 moieties) which are synthesised under more rigorous thermal conditions. The intriguing feature of swelled GOFs is due to their mild solvothermal synthesising condition while managing electrochemical oxygen reduction reaction (ORR) through a dominant 4e⁻ pathway (i.e. based on Rotating-Disk Electrode (RDE) results). GOF has feasibility for scalable production and a performance which is comparable to former BGs materials. The new GOF undergoes subsequent structural modifications via electrochemical polishing (i.e. chronoamperometry) to enhance its ORR efficiency and conductivity. Results indicate about a 30% boost in O2-reduction performance of electrochemically reduced GOF (E.r.GOF) compared to native graphene organic framework and also substantial improvement in its former onset-potential (approx. 100 mV).
Intermetallic nanocrystals are currently receiving extensive attentions due to their well-defined crystal structures, highly ordered atomic distribution and superior structural stability that endow them with optimized catalytic activities, stabilities and high selectivity as electrocatalysts for fuel cells. Here, for the first time, we reported the facile synthesis of intermetallic Pd3Pb nanowire networks (IM-Pd3Pb NNs) with a one-step wet-chemical strategy at a relatively low temperature (i.e. 170 °C) in 1 h. The as-prepared IM-Pd3Pb NNs exhibited superior bifunctional catalytic performances toward oxygen reduction reaction (ORR) and ethanol oxidation reaction (EtOR) compared to commercial Pt/C and Pd black, respectively. Significantly, IM-Pd3Pb NNs also showed excellent methanol- and CO-tolerant ability as ORR cathode and EtOR anode electrocatalysts, respectively. The electrochemical active surface area and mass activity of IM-Pd3Pb NNs are about 3.4 times and 2 times higher than Pd black toward EtOR, respectively. As the Pt-free bifunctional electrocatalysts, 3D IM-Pd3Pb architectures with exceptional catalytic performances hold great promise in various applications of energy conversion and storage devices, sensors, electronics, optics and so on.
Investigation of highly active and cost-efficient electrocatalysts for oxygen reduction reaction is of great importance in a wide range of clean energy devices, including fuel cells and metal-air batteries. Herein, the simultaneous formation of Co9S8 and N,S-codoped carbon was achieved in a dual templates system. First, Co(OH)2 nanosheets and tetraethyl orthosilicate were utilized to direct the formation of two-dimensional carbon precursors, which were then dispersed into thiourea solution. After subsequent pyrolysis and templates removal, N/S-codoped porous carbon sheets confined Co9S8 catalysts (Co9S8/NSC) were obtained. Owing to the morphological and compositional advantages as well as the synergistic effects, the resultant Co9S8/NSC catalysts with modified doping level and pyrolysis degree exhibit superior ORR catalytic activity and long-term stability compared with the state-of-the-art Pt/C catalyst in alkaline media. Remarkably, the as-prepared carbon composites also reveal exceptional tolerance of methanol, indicating their potential applications in fuel cells.
To explore efficient non-noble metal-based electrocatalysts for oxygen reduction reaction (ORR), herein we developed a facile bottom-up approach for the fabrication of a hollow porous carbon sphere codoped with ultra-small Co nanoparticles and uniform nitrogen distribution (Co-HNCS) via one-step pyrolysis of a core-shell type precursor composing of polystyrene (PS) core and bimetallic ZIF (zeolite imidazolate framework) shell. The bimetallic Co-Zn-ZIFs (BMZIFs) was selected as the sacrifice template due to not only its high nitrogen content and regular porosity but also the superiority that Zn species in BMZIFs can both spatially separate Co species to suppress the aggregation of utra-small Co NPs and be evaporated to afford extra pores during high-temperature pyrolysis. As expected, by adjusting the starting molar ratio of Zn to Co, we were able to prepare Co-HNCS-x (x represent the molar ratio of Co to total starting metal feeding) that exhibited unique hollow structure with large surface areas, enhanced mass transport, high porosities, tunable particle sizes and graphitization degrees, abundant highly active CoNx sites, and thus significantly improved ORR performance. Particularly, the optimal Co-HNCS-0.2 exhibited the remarkable ORR activity (the onset and half-wave potentials were 0.94 and 0.82 V vs. RHE, respectively) via an efficient four-electron-dominant ORR process in alkaline medium, which outperformed that of commercial Pt/C (20 wt%, the onset and half-wave potentials were 0.93 and 0.80 V vs. RHE, respectively) and most of previously reported Co-based catalysts. Moreover, it also displayed much superior stability and tolerance to methanol as compared to Pt/C, further highlighting the merit of this facile synthesis approach. Our findings might inspire new thoughts on the development of precious-metal-free, highly-efficient and cost-effective ORR electrocatalysts derived from MOF.
Electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries have been extensively studied in recent decades for their excellent conversion efficiency, high energy capacity, and low environmental impact. However, sluggish kinetics of the oxygen-related reactions at air cathodes, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are still worth improving. Noble metals such as platinum (Pt), iridium (Ir), ruthenium (Ru) and their oxides are considered as the benchmark ORR and OER electrocatalysts, but they are expensive and prone to be poisoned due to the fuel crossover effect, and may suffer from agglomeration and leaching after long-term usage. To mitigate these limits, it is highly desirable to design alternative ORR/OER electrocatalysts with prominent performance. Metal-organic frameworks (MOFs) are a class of porous crystalline materials consisting metal ions/clusters coordinated by organic ligands. Their crystalline structure, tunable pore size and high surface area afford them wide opportunities as catalytic materials. This Review covers MOF-derived ORR/OER catalysts in electrochemical energy conversion, with a focus on the different strategies of material design and preparation, such as composition control and nanostructure fabrication, to improve the activity and durability of MOF-derived electrocatalysts.
Emulsion-templated porous polymers (polyHIPEs) with highly interconnected voids that range from a few micrometers to hundreds of micrometers are typically synthesized within the external phases of high internal phase emulsions (HIPEs), emulsions containing more than 74% internal phase. Recent advances in emulsion-templated polymers include new developments in HIPE formation, polymerization chemistries, macromolecular structures, crosslinking strategies, porous architectures, and surface functionalization. This article focuses upon emulsion-templated polymers through the prism of the research and development work in our laboratory. The innovative emulsion-templated systems described include shape-memory polymers, encapsulation systems, hydrogels, and porous carbons. This article also briefly reviews recent work in the field and draws some conclusions regarding trends and future directions. The abundance of diverse and disparate research directions pursued under the banner of “emulsion templating” is indicative of its high degree of versatility. Novel families of porous polymers with unique properties can now be devised and designed through the advances described herein.
By virtue of diverse structures and tunable properties, metal-organic frameworks (MOFs) have presented extensive applications including gas capture, energy storage, and catalysis. Recently, synthesis of MOFs and their derived nanomaterials provide an opportunity to obtain competent oxygen reduction reaction (ORR) electrocatalysts due to their large surface area, controllable composition and pore structure. This review starts with the introduction of MOFs and current challenges of ORR, followed by the discussion of MOF-based non-precious metal nanocatalysts (metal-free and metal/metal oxide-based carbonaceous materials) and their application in ORR electrocatalysis. Current issues in MOF-derived ORR catalysts and some corresponding strategies in terms of composition and morphology to enhance their electrocatalytic performance are highlighted. In the last section, a perspective for future development of MOFs and their derivatives as catalysts for ORR is discussed.
A hierarchically porous carbon monolith with a density of 0.059 g cm‑3 (97 % porosity) was generated through the carbonization of an emulsion-templated monolith formed from a deep-eutectic polymer based on the polycondensation of 2,5-dihydroxy-1,4-benzoquinone with excess urea. The mechanical integrity and thermal stability of the monolith were successfully enhanced through a chain extension reaction with terephthaloyl chloride (TCL) that occurred during/following the formation of a high internal phase emulsion (HIPE). The bimodal, open-cell macroporous structure of the monolith consisted of many smaller voids with an average diameter of 15 µm and some larger voids with an average diameter of 49 µm. Carbonization of the monolith introduced microporosity and meso/macro-porosity, yielding a high specific surface area (812 m² g‑1, largely from micropores), a micropore volume of 0.266 cm³ g‑1 (an average diameter of 0.67 nm), and a meso/macro-pore volume of 0.238 cm³ g‑1 (an average diameter of 8.1 nm). The elemental composition of the chain-extended polymeric monolith was similar to that predicted from the HIPE components except for a relatively low nitrogen content which may indicate the loss of some urea groups during the chain extension reaction with TCL. The nitrogen-carbon bonds in the carbon monolith from the chain-extended polymer were around 47% pyridinic, 20% pyrrolic, and 33% graphitic. While chain-extension reduced the nitrogen content, the mechanical integrity and thermal stability were enhanced, which was key to generating a highly microporous carbon monolith with a hierarchical porous structure. The carbon monolith exhibited promising results for aqueous solution sorption applications, in both batch and flow modes, owing to its advantageous combination of properties.
The development of active, durable and low-cost catalysts to replace noble metal-based materials is highly desirable to promote the sluggish oxygen reduction reaction in fuel cells. Herein, nitrogen and fluorine-codoped three-dimensional carbon nanowire aerogels, composed of interconnected carbon nanowires were for the first time synthesized by hydrothermal carbonization process. Owing to their porous nanostructures and heteroatom-doping, the as-prepared carbon nanowire aerogels with optimized composition present excellent electrocatalytic activity that is comparable to commercial Pt/C. Remarkably, the aerogels also exhibit superior stability and methanol tolerance. This synthesis procedure paves a new way to design novel heteroatom-doped catalysts.
The development of vanadium redox flow battery is limited by the sluggish kinetics of the reaction, especially the cathodic VO2+/VO2+ redox couples. Therefore, it is vital to develop new electrocatalysts with enhanced activity to improve the battery performance. Herein, we synthesized the hydrogel precursor by a facile hydrothermal method. After the following carbonization, nitrogen-doped reduced graphene oxide/carbon nanotube composite was obtained. By virtue of the large surface area and good conductivity, which are ensured by the unique hybrid structure, as well as the proper nitrogen doping, the as-prepared composite presents enhanced catalytic performance toward the VO2+/VO2+ redox reaction. We also demonstrated the composite with carbon nanotube loading of 2 mg/mL exhibits the highest activity and remarkable stability in aqueous solution due to the strong synergy between reduced graphene oxide and carbon nanotubes, indicating that this composite might show promising applications in vanadium redox flow battery.
We introduced a pore-expansion strategy to transform metal-organic framework (MOF) into foam-like carbons with high surface area and hierarchical pores. A typical kind of MOF was chosen as the starting material. The catalytic carbon, which displayed a remarkably enlarged surface area and high pore volume, was prepared by a solvent-assistant-linker-exchange method and a following pore-expansion carbonization process. Interestingly, the catalytic carbon possessed a hierarchically porous structure in which micro-, meso- and macro-pores coexisted. Benefiting from the structural merits, it showed superior catalytic performance towards oxygen reduction reaction (ORR). This work emphasizes the importance of interconnecting structures with hierarchical pores towards ORR and opens a new avenue to fabricate advanced nanostructures from MOFs for enhanced electrochemical applications.
Over the past several years, metal-organic framework (MOF)-derived platinum group metal free (PGM-free) electrocatalysts have gained considerable attention due to their high efficiency and low cost as potential replacement for platinum in catalyzing oxygen reduction reaction (ORR). In this review, we summarize the recent advancements in design, synthesis and characterization of MOF-derived ORR catalysts and their performances in acidic and alkaline media. We also discuss the key challenges such as durability and activity enhancement critical in moving forward this emerging electrocatalyst science.
A nitrogen-doped activated carbon (NDAC) as a cathode catalyst in microbial fuel cells (MFCs) was synthesized by a microwave-assisted method using ammonium carbonate as a nitrogen source. The prepared NDAC showed a higher BET surface area of up to 1717.8 m² g⁻¹ and a total pore volume of 0.79 cm³ g⁻¹. X-ray photoelectron spectroscopic analysis demonstrated that N was successfully doped on the surface of AC in three species, corresponding to pyrrolic N, pyridinic N and pyridine-N-oxide. Compared with untreated AC, the NDAC exhibited better electrocatalytic activity for the oxygen reduction reaction in rotating disk electrode tests, with a current density of 12.4 mA cm⁻² at a set potential of -0.8 V (vs. SCE) (AC, 11.3 mA cm⁻²) and an electron transfer number of 3.14 (AC, n = 2.83). MFCs equipped with a NDAC cathode achieved a higher maximum power density of 471 ± 11 mW m⁻² when fed with domestic wastewater, which was 1.3 times higher than that of the AC cathode. It also displayed long-term operation stability when dealing with real wastewater, indicating a promising cathode catalyst for MFCs towards practical applications.
The N-doped porous carbon monoliths prepared by direct carbonization of IRMOF-3, through an in situ activation and self-templating process, were found to exhibit significantly enhanced performance for the selective adsorption of CO2 compared to pristine IRMOF-3. The transformation from the microporous structure to the meso-macroporous structure opens the pathway for CO2 to more easily access the nitrogen anchors.
In this study, the catalyst of reduced graphene oxide (rGO) supported Mn2O3 doped MnO for oxygen reduction reaction (ORR) has been successfully synthesized. The structures and morphology of the Mn2O3 doped MnO were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). XRD tests reveal that the Mn2O3 is successfully doped to MnO. The cyclic voltammetry, Tafel, linear scanning voltammetry and current-time chronoamperometric tests prove that the Mn2O3 doped MnO has a better catalytic performance and stronger stability than pure MnO for oxygen reduction. Both the rotating disc electrode and rotating ring disc electrode tests approve that the ORR happens mainly through 4-electron reaction mechanism on Mn2O3 doped MnO. The ORR happens mainly through 2-electron reaction mechanism with the catalysis of pure MnO. The coexistence of manganese ions with different valent promotes the catalytic performance of metal oxide for oxygen reduction.
Development of state-of-the-art electrocatalysts with inexpensive and commercially available materials to facilitate sluggish cathodic oxygen reduction reaction (ORR) is a key issue in the development of fuel cells and other electrochemical energy devices. Although great progress has been achieved in this area of research and development, there are still some challenges in both their ORR activity and stability. The emergence of graphene (GN) provides an excellent alternative to electrode materials and great efforts have been made to utilize GN-based nanomaterials as promising electrode materials for ORR due to the high electrical conductivity, large specific surface area, profuse interlayer structure and abounding functional groups involved. It should be noted that rational design of these GN-based nanomaterials with well-defined morphology also plays an important role in their electrochemical performance for ORR. Considerable attempts were achieved to construct a variety of heteroatom doped GN nanomaterials or GN-based nanocomposites, aiming at fully using their excellent properties in their application in ORR. In this critical review, in line with the material design and engineering, some recent advancements in the development of GN-based electrocatalysts for ORR in electrochemical energy devices (fuel cells and batteries) are then highlighted, including heteroatom-doped GN nanomaterials, GN-based nonprecious hybrid nanocomposites (GN/metal oxides, GN/N-M, GN/carbon nitride, etc.) and GN/noble metal nanocomposites.
Combined plasmonic nanocrystals and metal-organic framework thin-films are fabricated for sensing gases in the near-infrared range. This nanocomposite thin-film shows a highly sensitive response in near-infrared absorption, and propose that is caused by effectively pre-concentrating gas molecules in metal-organic framework pores causing close proximity to the electromagnetic fields at the plasmonic nanocrystals surface.
Human urine, otherwise potentially polluting waste, is an universal unused resource in organic form disposed by the human body. We present for the first time "proof of concept" of a convenient, perhaps economically beneficial, and innovative template-free route to synthesize highly porous carbon containing heteroatoms such as N, S, Si, and P from human urine waste as a single precursor for carbon and multiple heteroatoms. High porosity is created through removal of inherently-present salt particles in as-prepared "Urine Carbon" (URC), and multiple heteroatoms are naturally doped into the carbon, making it unnecessary to employ troublesome expensive pore-generating templates as well as extra costly heteroatom-containing organic precursors. Additionally, isolation of rock salts is an extra bonus of present work. The technique is simple, but successful, offering naturally doped conductive hierarchical porous URC, which leads to superior electrocatalytic ORR activity comparable to state of the art Pt/C catalyst along with much improved durability and methanol tolerance, demonstrating that the URC can be a promising alternative to costly Pt-based electrocatalyst for ORR. The ORR activity can be addressed in terms of heteroatom doping, surface properties and electrical conductivity of the carbon framework.
Efficient and low-cost electrocatalysts for the oxygen evolution reaction are essential components of renewable energy technologies, such as solar fuel synthesis and providing a hydrogen source for powering fuel cells. Here we report that the nitrogen-doped carbon materials function as the efficient oxygen evolution electrocatalysts. In alkaline media, the material generated a current density of 10 mA cm(-2) at the overpotential of 0.38 V, values that are comparable to those of iridium and cobalt oxide catalysts and are the best among the non-metal oxygen evolution electrocatalyst. The electrochemical and physical studies indicate that the high oxygen evolution activity of the nitrogen/carbon materials is from the pyridinic-nitrogen- or/and quaternary-nitrogen-related active sites. Our findings suggest that the non-metal catalysts will be a potential alternative to the use of transition metal-based oxygen evolution catalysts.
Polymer electrolyte membrane (PEM) fuel cells are attracting much attention as promising clean power sources and an alternative to conventional internal combustion engines, secondary batteries, and other power sources. Much effort from government laboratories, industry, and academia has been devoted to developing PEM fuel cells, and great advances have been achieved. Although prototype cars powered by fuel cells have been delivered, successful commercialization requires fuel cell electrocatalysts, which are crucial components at the heart of fuel cells, meet exacting performance targets. In this review, we present a brief overview of the recent progress in fuel cell electrocatalysts, which involves catalyst supports, Pt and Pt-based electrocatalysts, and non-Pt electrocatalysts.
Recent interest in electricity production using microbial fuel cells makes it important to better understand O2 reduction in neutral solutions with non-precious metal catalysts. Higher O2 reduction activity was obtained using iron phthalocyanine supported on Ketjen black carbon (FePc-KJB) than with a platinum
catalyst in neutral pH. At low overpotentials, a Tafel slope close to −0.06V/dec in both acid and neutral pH suggested that
the mechanism of O2 reduction on FePc is not changed with the change of pH, and the reaction is mainly controlled by FeII/FeIII redox couple. This behaviour gives us new insight into catalysis using FePc, and further supports the use of FePc as a promising
catalyst for the oxygen reduction applications in neutral media.
Network Approaches to Highly Porous Materials
Metal-organic frameworks (MOFs), in which inorganic centers are bridged by organic linkers, can achieve very high porosity for gas absorption. However, as the materials develop larger void spaces, there is also more room for growing interpenetrating networks—filling the open spaces not with gas molecules but with more MOFs. Furukawa et al. (p. 424 , published online 1 July) describe the synthesis of a MOF in which zinc centers are bridged with long, highly conjugated organic linkers, but in which the overall symmetry of the networks created prevents formation of interpenetrating networks. Extremely high surface areas and storage capacities for hydrogen, carbon dioxide, and methane were observed.
In the domain of health, one important challenge is the efficient delivery of drugs in the body using non-toxic nanocarriers. Most of the existing carrier materials show poor drug loading (usually less than 5 wt% of the transported drug versus the carrier material) and/or rapid release of the proportion of the drug that is simply adsorbed (or anchored) at the external surface of the nanocarrier. In this context, porous hybrid solids, with the ability to tune their structures and porosities for better drug interactions and high loadings, are well suited to serve as nanocarriers for delivery and imaging applications. Here we show that specific non-toxic porous iron(III)-based metal-organic frameworks with engineered cores and surfaces, as well as imaging properties, function as superior nanocarriers for efficient controlled delivery of challenging antitumoural and retroviral drugs (that is, busulfan, azidothymidine triphosphate, doxorubicin or cidofovir) against cancer and AIDS. In addition to their high loadings, they also potentially associate therapeutics and diagnostics, thus opening the way for theranostics, or personalized patient treatments.
The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen reduction reactions (ORRs) cannot be replaced by other efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells. In air-saturated 0.1 molar potassium hydroxide, we observed a steady-state output potential of -80 millivolts and a current density of 4.1 milliamps per square centimeter at -0.22 volts, compared with -85 millivolts and 1.1 milliamps per square centimeter at -0.20 volts for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
Fill 'em up: The metal carboxylates MIL‐100 and MIL‐101 act as porous matrices (see picture; MIL=Materials of Institut Lavoisier) for drug‐delivery systems using Ibuprofen as a model substrate. Very large amounts of the drug could be incorporated, up to an unprecedented capacity of 1.4 g of drug per gram of porous solid for MIL‐101, and the total release of Ibuprofen was achieved under physiological conditions in 3 (MIL‐100) and 6 days (MIL‐101).
Heteroatom (N or S)-doped graphene with high surface area is successfully synthesized via thermal reaction between graphene oxide and guest gases (NH3 or H2S) on the basis of ultrathin graphene oxide-porous silica sheets at high temperatures. It is found that both N and S-doping can occur at annealing temperatures from 500 to 1000 °C to form the different binding configurations at the edges or on the planes of the graphene, such as pyridinic-N, pyrrolic-N, and graphitic-N for N-doped graphene, thiophene-like S, and oxidized S for S-doped graphene. Moreover, the resulting N and S-doped graphene sheets exhibit good electrocatalytic activity, long durability, and high selectivity when they are employed as metal-free catalysts for oxygen reduction reactions. This approach may provide an efficient platform for the synthesis of a series of heteroatom-doped graphenes for different applications.
For the goal of practical industrial development of fuel cells, inexpensive, sustainable, and high performance electrocatalysts for oxygen reduction reactions (ORR) are highly desirable alternatives to platinum (Pt) and other rare materials. In this work, sustainable fluorine (F)-doped carbon blacks (CB-F) as metal-free, low-cost, and high-performance electrocatalysts for ORR were synthesized for the first time. The performance (electrocatalytic activity, long-term operation stability, and tolerance to poisons) of the best one (BP-18F, based on Black Pearls 2000 (BP)) is on the same level as Pt-based or other best non-Pt-based catalysts in alkaline medium. The maximum power density of alkaline direct methanol fuel cell with BP-18F as the cathode (3 mg/cm2) is 15.56 mW/cm2 at 60 °C, compared with a maximum of 9.44 mW/cm2 for commercial Pt/C (3 mgPt/cm2). All these results unambiguously demonstrate that these sustainable CB-F catalysts are the most promising alternatives to Pt in an alkaline fuel cell. Since sustainable carbon blacks are 10 000 times less expensive and much more abundant than Pt or other rare materials, these CB-F electrocatalysts possess the best price/performance ratio for ORR to date.
Micro/mesoporous nitrogen-doped carbon (m-NC) materials have been prepared by a simple one-step pyrolysis of dopamine in the presence of FeCl3 at 350-750 °C. X-ray photoelectron spectroscopy (XPS) N 1s spectra indicate three types of C-N bonding patterns: graphitic N, pyrrolic N and pyridinic N. The electrocatalytic properties towards oxygen reduction reaction were investigated in alkaline media. Results indicated that the m-NC-600 with the highest content of pyridinic N and graphitic N showed improved electrocatalytic activity for the oxygen reduction reaction (ORR) in terms of onset potential, number of electron transferred and limiting current density. Moreover, the durability in alkaline media of the as-prepared m-NC-600 catalyst was found to be superior to the commercial Pt/C catalyst.
A hierarchically structured nitrogen-doped porous carbon is prepared from a nitrogen-containing isoreticular metal-organic framework (IRMOF-3) using a self-sacrificial templating method. IRMOF-3 itself provides the carbon and nitrogen content as well as the porous structure. For high carbonization temperatures (950 C), the carbonized MOF required no further purification steps, thus eliminating the need for solvents or acid. Nitrogen content and surface area are easily controlled by the carbonization temperature. The nitrogen content decreases from 7 to 3.3 at% as carbonization temperature increases from 600 to 950 C. There is a distinct tradeoff between nitrogen content, porosity, and defects in the carbon structure. Carbonized IRMOFs are evaluated as supercapacitor electrodes. For a carbonization temperature of 950C, the nitrogen-doped porous carbon has an exceptionally high capacitance of 239 F g-1. In comparison, an analogous nitrogen-free carbon bears a low capacitance of 24 F g-1, demonstrating the importance of nitrogen dopants in the charge storage process. The route is scalable in that multi-gram quantities of nitrogen-doped porous carbons are easily produced.
We for the first time demonstrate a simple and green approach to heteroatom (N and S) co-doped hierarchically porous carbons (N–S-HC) with high surface area by using one organic ionic liquid as nitrogen, sulfur and carbon sources and the eutectic salt as templating. The resultant dual-doped N–S-HC catalysts exhibit significantly enhanced electrocatalytic activity, long-term operation stability, and tolerance to crossover effect compared to commercial Pt/C for oxygen reduction reactions (ORR) in alkaline environment. The excellent electrocatalytic performance may be attributed to the synergistic effects, which includes more catalytic sites for ORR provided by N–S heteroatom doping and high electron transfer rate provided by hierarchically porous structure. The DFT calculations reveal that the dual doping of S and N atoms lead to the redistribution of spin and charge densities, which may be responsible for the formation of a large number of carbon atom active sites. This newly developed approach may supply an efficient platform for the synthesis of a series of heteroatom doped carbon materials for fuel cells and other applications.
Oxygen Reduction Reactions (ORR) are one of the main factors of major potential loss in low temperature fuel cells, such as microbial fuel cells and proton exchange membrane fuel cells. Various studies in the past decade have focused on determining a method to reduce the over potential of ORR and to replace the conventional costly Pt catalyst in both types of fuel cells. This review outlines important classes of abiotic catalysts and biocatalysts as electrochemical oxygen reduction reaction catalysts in microbial fuel cells. It was shown that manganese oxide and metal macrocycle compounds are good candidates for Pt catalyst replacements due to their high catalytic activity. Moreover, nitrogen doped nanocarbon material and electroconductive polymers are proven to have electrocatalytic activity, but further optimization is required if they are to replace Pt catalysts. A more interesting alternative is the use of bacteria as a biocatalyst in biocathodes, where the ORR is facilitated by bacterial metabolism within the biofilm formed on the cathode. More fundamental work is needed to understand the factors affecting the performance of the biocathode in order to improve the performance of the microbial fuel cells.
There is a growing interest in oxygen electrode catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as they play a key role in a wide range of renewable energy technologies such as fuel cells, metal-air batteries, and water splitting. Nevertheless, the development of highly-active bifunctional catalysts at low cost for both ORR and OER still remains a huge challenge. Herein, we report a new N-doped graphene/single-walled carbon nanotube (SWCNT) hybrid (NGSH) material as an efficient noble-metal-free bifunctional electrocatalyst for both ORR and OER. NGSHs were fabricated by in situ doping during chemical vapor deposition growth on layered double hydroxide derived bifunctional catalysts. Our one-step approach not only provides simultaneous growth of graphene and SWCNTs, leading to the formation of three dimensional interconnected network, but also brings the intrinsic dispersion of graphene and carbon nanotubes and the dispersion of N-containing functional groups within a highly conductive scaffold. Thus, the NGSHs possess a large specific surface area of 812.9 m(2) g(-1) and high electrical conductivity of 53.8 S cm(-1) . Despite of relatively low nitrogen content (0.53 at%), the NGSHs demonstrate a high ORR activity, much superior to two constituent components and even comparable to the commercial 20 wt% Pt/C catalysts with much better durability and resistance to crossover effect. The same hybrid material also presents high catalytic activity towards OER, rendering them high-performance cheap catalysts for both ORR and OER. Our result opens up new avenues for energy conversion technologies based on earth-abundant, scalable, noble-metal-free catalysts.
The fuel cell (FC), as a clean and high-efficiency device, has drawn a great deal of attention in terms of both fundamentals and applications. However, the high cost and scarcity of the requisite platinum catalyst as well as a sluggish oxygen reduction reaction (ORR) at the cathode in FC have become the greatest barrier to large-scale industrial application of FC. The development of novel non-precious metal catalysts (NPMC) with excellent electrocatalytic performance has been viewed as an important strategy to promote the development of FC. Recent studies have proven that metal free carbon materials doped with heteroatom (e.g. N, B, P, S or Se) have also shown striking electrocatalytic performance for ORR and become an important category of potential candidates for replacing Pt-based catalysts. This review summarizes recent achievements in heteroatom doped carbon materials as ORR catalysts, and will be beneficial to future development of other novel low-cost NPMCs with high activities and long lifetimes for practical FC applications.
Manganese dioxide–graphene nanosheet (MnO2/GNS) hybrid is used as an alternative cathode catalyst for oxygen reduction reaction in air-cathode microbial fuel cell (MFC). The results show that the nanostructured MnO2/GNS composite exhibit an excellent catalytic activity for oxygen reduction reaction due to MnO2 nanoparticles closely anchored on the excellent conductive graphene nanosheets. As a result, MFC with MnO2/GNS as air cathode catalyst generates a maximum power density of 2083 mW m−2, which is higher than that of MFC with pure manganese dioxide catalyst (1470 mW m−2). Therefore, MnO2/GNS nanocomposite is an efficient and cost-effective cathode catalyst for practical MFC applications.
The extensive use of Pt and platinum group metals (PGM) as electrocatalysts poses a significant cost barrier for the commercialization of polymer electrolyte membrane fuel cells (PEMFC). Replacing Pt with non-PGM electrocatalysts is a long-term pursuit of the scientific community. In this study, non-PGM cathode electrocatalysts for PEMFC were prepared by pyrolyzing an iron imidazolate framework. The new catalyst demonstrated excellent activity towards oxygen reduction reaction in the acidic medium. The catalytic activity was further improved by mixing with a zinc imidazolate framework, ZIF-8. The membrane electrode assembly made of such catalyst as the cathode demonstrated an onset potential of 0.977 V and measured volumetric current density of 12 A cm−3 at 0.8 V in a single cell test.
Doping duo: Mesoporous graphene doped with both N and S atoms (N-S-G) was prepared in one step and studied as an electrochemical catalyst for the oxygen reduction reaction (ORR). The catalyst shows excellent ORR performance comparable to that of commercial Pt/C. The outstanding activity of N-S-G results from both the large number and the synergistic effect of the dopant heteroatoms.
Lead dioxide (PbO2) was compared to platinum (Pt) as a cathode catalyst in a double-cell microbial fuel cell (MFC) utilizing glucose as a substrate in the anode chamber. Four types of cathodes were tested in this study including two PbO2 cathodes fabricated using a titanium base with butanol or Nafion® binders and PbO2 paste, one Pt/carbon cathode fabricated using a titanium base with a carbon–Pt paste, and a commercially available Pt/carbon cathode made from carbon paper with Pt on one side. The power density and polarization curves were compared for each cathode and cost estimates were calculated. Results indicate the PbO2 cathodes produced between 2 and 4× more power than the Pt cathodes. Furthermore, the PbO2 cathodes produced between 2 and 17× more power per initial fabrication or purchase cost than the Pt cathodes. This study suggests that cathode designs that incorporate PbO2 instead of Pt could possibly improve the feasibility of scaling up MFC designs for real world applications by improving power generation and lowering production cost.
The electrocatalytic performance of iron phthalocyanine, FePc, dispersed on a high surface area carbon substrate (Vulcan XC 72) was investigated in acid medium. The polarisation curve of oxygen reduction on the FePc/C catalyst in a methanol containing O2-saturated electrolyte was compared to that obtained on a platinum catalyst. Results showed clearly that FePc/C catalyst is totally insensitive to the presence of methanol and becomes a better catalyst than the Pt/C catalyst for the oxidation reduction reaction (orr) at low overpotentials, indicating that FePc/C catalyst is a good alternative to platinum based catalysts as a cathode catalyst for low working temperatures fuel cells. The major problem of this catalyst in acid medium is its poor long-term stability, which was demonstrated by electrochemical measurements and characterized by in situ infrared reflectance spectroscopy experiments. The infrared spectra showed that FePc degradation occurred via the substitution of the central metal of the macrocycle by two protons, leading to the formation of a free base phthalocyanine which is known to be inactive for oxygen reduction catalysis. From this observation, it was possible to improve the FePc stability by using a gas diffusion electrode. Under these conditions, oxygen reduction was performed on a FePc/C electrode for at least 24 h with a constant current density of −25 mA cm−2 at room temperature. Finally, cyclic voltammetry experiments at different electrode rotation rates Ω led to determine the Koutecký-Levich plots from which it was shown that the kinetic parameters for the orr at FePc/C electrodes are similar to those obtained at platinum particles. The total number of exchanged electrons nt during the orr on the FePc/C catalyst was determined by rotating ring disc electrode experiments leading to nt close to 3.9–4.
A review of the synthesis, structure, and properties of metal–organic frameworks (MOFs) is presented, highlighting the important advances in their research over the past decade. This new class of porous materials is attracting attention due to demonstrations of their large pore sizes, high apparent surface areas, selective uptake of small molecules, and optical or magnetic responses to the inclusion of guests. More importantly, their synthesis from molecular building blocks holds the potential for directed tailoring of these properties.
We report the facile synthesis of carbon supported PtAu alloy nanoparticles with high electrocatalytic activity as anode catalysts for direct formic acid fuel cells (DFAFCs). PtAu alloy nanoparticles are prepared by co-reducing HAuCl4 and H2PtCl6 with NaBH4 in the presence of sodium citrate and then deposited on Vulcan XC-72R carbon support (PtAu/C). The obtained catalysts are characterized with X-ray diffraction (XRD) and transmission electron microscope (TEM), which reveal the formation of PtAu alloy nanoparticles with an average diameter of 4.6 nm. Electrochemical measurements show that PtAu/C has seven times higher catalytic activity towards formic acid oxidation than Pt/C. This significantly enhanced activity of PtAu/C catalyst can be attributed to noncontinuous Pt sites formed in the presence of the neighbored Au sites, which promotes direct oxidation of formic acid. (C) 2009 Elsevier B.V. All rights reserved.
Graphene quantum dots (GQDs) represent a new class of quantum dots with unique properties. Doping GQDs with heteroatoms provides an attractive means of effectively tuning their intrinsic properties and exploiting new phenomena for advanced device applications. Herein we report a simple electrochemical approach to luminescent and electrocatalytically active nitrogen-doped GQDs (N-GQDs) with oxygen-rich functional groups. Unlike their N-free counterparts, the newly produced N-GQDs with a N/C atomic ratio of ca. 4.3% emit blue luminescence and possess an electrocatalytic activity comparable to that of a commercially available Pt/C catalyst for the oxygen reduction reaction (ORR) in an alkaline medium. In addition to their use as metal-free ORR catalysts in fuel cells, the superior luminescence characteristic of N-GQDs allows them to be used for biomedical imaging and other optoelectronic applications.
A critical review of the literature on metal-organic frameworks (MOF) as chemical sensors is presented. Functional groups on the surface may nucleate MOF growth in a specific crystallographic direction, leading to preferentially oriented films. Lan and co-workers reported two fluorescent Zn-based MOFs capable of sensing nitro-containing molecules relevant to detection of explosives. Feng et al. expanded the concept of using MOF structure to tune luminescence by demonstrating that both dynamic structural changes and incorporation of extrinsic dopants within the MOF pores can be used to create intense new emission. Robinson and co-workers reported humidity detection over a very broad concentration range using SAWs coated with Cu-BTC. The MOF film was grown directly on the quartz of 96.5 MHz devices without an intervening SAM, using the layer-by-layer (LBL) growth method developed by Fischer et al.
H(2)-air polymer-electrolyte-membrane fuel cells are electrochemical power generators with potential vehicle propulsion applications. To help reduce their cost and encourage widespread use, research has focused on replacing the expensive Pt-based electrocatalysts in polymer-electrolyte-membrane fuel cells with a lower-cost alternative. Fe-based cathode catalysts are promising contenders, but their power density has been low compared with Pt-based cathodes, largely due to poor mass-transport properties. Here we report an iron-acetate/phenanthroline/zeolitic-imidazolate-framework-derived electrocatalyst with increased volumetric activity and enhanced mass-transport properties. The zeolitic-imidazolate-framework serves as a microporous host for phenanthroline and ferrous acetate to form a catalyst precursor that is subsequently heat treated. A cathode made with the best electrocatalyst from this work, tested in H(2)-O(2,) has a power density of 0.75 W cm(-2) at 0.6 V, a meaningful voltage for polymer-electrolyte-membrane fuel cells operation, comparable with that of a commercial Pt-based cathode tested under identical conditions.
The electronic and chemical properties of graphene can be modulated by chemical doping foreign atoms and functional moieties. The general approach to the synthesis of nitrogen-doped graphene (NG), such as chemical vapor deposition (CVD) performed in gas phases, requires transitional metal catalysts which could contaminate the resultant products and thus affect their properties. In this paper, we propose a facile, catalyst-free thermal annealing approach for large-scale synthesis of NG using low-cost industrial material melamine as the nitrogen source. This approach can completely avoid the contamination of transition metal catalysts, and thus the intrinsic catalytic performance of pure NGs can be investigated. Detailed X-ray photoelectron spectrum analysis of the resultant products shows that the atomic percentage of nitrogen in doped graphene samples can be adjusted up to 10.1%. Such a high doping level has not been reported previously. High-resolution N1s spectra reveal that the as-made NG mainly contains pyridine-like nitrogen atoms. Electrochemical characterizations clearly demonstrate excellent electrocatalytic activity of NG toward the oxygen reduction reaction (ORR) in alkaline electrolytes, which is independent of nitrogen doping level. The present catalyst-free approach opens up the possibility for the synthesis of NG in gram-scale for electronic devices and cathodic materials for fuel cells and biosensors.
Carbon-supported precious metal catalysts are widely used in heterogeneous catalysis and electrocatalysis, and enhancement of catalyst dispersion and stability by controlling the interfacial structure is highly desired. Here we report a new method to deposit metal oxides and metal nanoparticles on graphene and form stable metal-metal oxide-graphene triple junctions for electrocatalysis applications. We first synthesize indium tin oxide (ITO) nanocrystals directly on functionalized graphene sheets, forming an ITO-graphene hybrid. Platinum nanoparticles are then deposited, forming a unique triple-junction structure (Pt-ITO-graphene). Our experimental work and periodic density functional theory (DFT) calculations show that the supported Pt nanoparticles are more stable at the Pt-ITO-graphene triple junctions. Furthermore, DFT calculations suggest that the defects and functional groups on graphene also play an important role in stabilizing the catalysts. These new catalyst materials were tested for oxygen reduction for potential applications in polymer electrolyte membrane fuel cells, and they exhibited greatly enhanced stability and activity.
(Graph Presented) These materials are no dopes: Nitrogendoped ordered mesoporous graphitic arrays (NOMGAs) prepared by a metalfree procedure exhibited higher electrocatalytic activity than the commercially available Pt-C catalyst (see plot), excellent long-term stability, and resistance to crossover effects in the oxygen-reduction reaction (ORR). Graphite-like nitrogen atoms appear to be responsible for the excellent electrochemical performance in the ORR.
Nitrogen-doped graphene (N-graphene) was synthesized by chemical vapor deposition of methane in the presence of ammonia. The resultant N-graphene was demonstrated to act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction via a four-electron pathway in alkaline fuel cells. To the best of our knowledge, this is the first report on the use of graphene and its derivatives as metal-free catalysts for oxygen reduction. The important role of N-doping to oxygen reduction reaction (ORR) can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications, even new catalytic materials for applications beyond fuel cells.
The role of metal-organic frameworks (MOFs) in the field of catalysis is discussed, and special focus is placed on their assets and limits in light of current challenges in catalysis and green chemistry. Their structural and dynamic features are presented in terms of catalytic functions along with how MOFs can be designed to bridge the gap between zeolites and enzymes. The contributions of MOFs to the field of catalysis are comprehensively reviewed and a list of catalytic candidates is given. The subject is presented from a multidisciplinary point of view covering solid-state chemistry, materials science, and catalysis.
A critical review of the emerging field of MOF-based catalysis is presented. Discussed are examples of: (a) opportunistic catalysis with metal nodes, (b) designed catalysis with framework nodes, (c) catalysis by homogeneous catalysts incorporated as framework struts, (d) catalysis by MOF-encapsulated molecular species, (e) catalysis by metal-free organic struts or cavity modifiers, and (f) catalysis by MOF-encapsulated clusters (66 references).
Increased attention is being focused on metal-organic frameworks as candidates for hydrogen storage materials. This is a result of their many favorable attributes, such as high porosity, reproducible and facile syntheses, amenability to scale-up, and chemical modification for targeting desired properties. A discussion of several strategies aimed at improving hydrogen uptake in these materials is presented. These strategies include the optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of open metal sites and lighter metals.
Nondoped and nitrogen-doped (N-doped) carbon nanofiber (CNF) electrodes were prepared via a floating catalyst chemical vapor deposition (CVD) method using precursors consisting of ferrocene and either xylene or pyridine to control the nitrogen content. Structural and compositional differences between the nondoped and N-doped varieties were assessed using TEM, BET, Raman, TGA, and XPS. Electrochemical methods were used to study the influence of nitrogen doping on the oxygen reduction reaction (ORR). The N-doped CNF electrodes demonstrate significant catalytic activity toward oxygen reduction in aqueous KNO(3) solutions at neutral to basic pH. Electrochemical data are presented which indicate that the ORR proceeds by the peroxide pathway via two successive two-electron reductions. However, for N-doped CNF electrodes, the reduction process can be treated as a catalytic regenerative process where the intermediate hydroperoxide (HO(2)(-)) is chemically decomposed to regenerate oxygen, 2HO(2)(-) <==> O(2) + 2OH(-). The proposed electrocatalysis mechanisms for ORR at both nondoped and N-doped varieties are supported by electrochemical simulations and by measured difference in hydroperoxide decomposition rate constants. Remarkably, approximately 100 fold enhancement for hydroperoxide decomposition is observed for N-doped CNFs, with rates comparable to the best known peroxide decomposition catalysts. Collectively the data indicate that exposed edge plane defects and nitrogen doping are important factors for influencing adsorption of reactive intermediates (i.e., superoxide, hydroperoxide) and for enhancing electrocatalysis for the ORR at nanostructured carbon electrodes.
(Figure Presented) Fill 'em up: The metal carboxylates MIL-100 and MIL-101 act as porous matrices (see picture; MIL = Materials of Institut Lavoisier) for drug-delivery systems using Ibuprofen as a model substrate. Very large amounts of the drug could be incorporated, up to an unprecedented capacity of 1.4 g of drug per gram of porous solid for MIL-101, and the total release of Ibuprofen was achieved under physiological conditions in 3 (MIL-100) and 6 days (MIL-101).
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Chronoamperometric responses of CIRMOF-3-950 and commercial Pt/C electrodes at À0.3 V vsAgCl in O 2 -saturated 0.1 M KOH solution at a rotation rate of 200 rpm. CV curves of CIRMOF-3-950 (b) and Pt/C (c) electrodes in O 2 -saturated 0.1 M KOH solution before and after the addition of methanol
- Fig
Fig. 5. (a) Chronoamperometric responses of CIRMOF-3-950 and commercial Pt/C electrodes at À0.3 V vs. Ag/AgCl in O 2 -saturated 0.1 M KOH solution at a rotation rate of
200 rpm. CV curves of CIRMOF-3-950 (b) and Pt/C (c) electrodes in O 2 -saturated 0.1 M KOH solution before and after the addition of methanol.
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