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Influence of the Metal Center in M–N–C Catalysts on the CO 2 Reduction Reaction on Gas Diffusion Electrodes
Abstract
In this work, the influences of various transition metal ions as active sites in high purity metal- and nitrogen-doped carbon catalysts (in short M−N−C), where M: Mn³⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, or Sn⁴⁺ in the catalyst powders, were systematically investigated for the electrochemical reduction of CO2 in the aqueous electrolyte. The almost exclusive presence of isolated M−N4 centers as catalytic sites was determined by X-ray photoelectron spectroscopy (XPS). The catalysts were electrochemically investigated in a gas diffusion electrode arrangement in bypass mode coupled in-line to a mass spectrometer. This allowed for the nearly simultaneous detection of products and current densities in linear sweep voltammetry experiments, from which potential-dependent specific production rates and faradaic efficiencies could be derived. Postmortem XPS analyses were performed after various stages of operation on the Cu−N−C catalyst, which was the only catalyst to produce hydrocarbons (CH4 and C2H4) in significant amounts. The data provided insights into the potential-induced electronic changes of the Cu−N−C catal st occurrin under operatin conditions. Our work further experimentall revealed the high affinity of M−N−C catalysts to convert CO2 to industrially relevant carbonaceous raw materials, while effectively suppressing the competing hydrogen evolution reaction. These results led to a better understanding of the role of the active sites, especially the central metal ion, in M−N−C and could contribute significantly to the improvement of selectivities and activities for the CO2RR in this catalyst class through tailor-made optimization strategies.
... The nitrogen chemical potential (l N ) is treated as a parameter for the use of different nitrogen precursors in experiments. 11,[46][47][48][49][50][51] l N is referenced to NH 3 , N 2 , and H 2 at 1 bar and can be expressed as ...
... At this condition, we include 0.1 M H 2 CO 3 solution as the typical electrolyte used in the experiments. 50,53,58 In contrast to the ORR conditions, the anion from most of the considered electrolytes, except from 0.1 M H 2 CO 3 , do not bind with the single metal site. Thus, most considered electrolytes possibly have little impact on catalytic activity and stability of the MN 4 site under the CO 2 RR. ...
... Furthermore, a series of M/N/C catalysts (M ¼ Mn 3þ , Fe 3þ , Co 2þ , Ni 2þ , Cu 2þ , Zn 2þ , and Sn 2þ ) have been synthesized and evaluated for electrochemical CO 2 reduction in aqueous electrolyte (0.5 M KHCO 3 and 0.5 M K 2 SO 4 ) by Paul et al. 50 They have found that the Cu/N/C catalyst can produce hydrocarbon products in significant amounts which have also been demonstrated to correlate with a partial reduction of Cu 2þ to Cu 0 accompanied by a partial demetallation of the CuN 4 site. According to our calculation at CO 2 RR condition in 0.1 M H 2 CO 3 þ CO, both the CuN 4 site and the MnN 4 site on both bulk graphene and the graphene edge have a relatively higher thermodynamic driving force than other metal elements. ...
A single metal site incorporated in N-doped carbon (M/N/C) is a promising electrocatalyst. Here, we perform a computation investigation of the effect of electrolyte anion adsorption on the activity and stability of single-atom catalysts (MN 4 ) with M as transition metal and p-block metal. The MN 4 site on two different graphene structures (bulk graphene and graphene edge) is studied under electrochemical conditions for the oxygen reduction reaction (ORR) and the CO 2 reduction reaction (CO 2 RR). Because of the two-dimensional nature of the catalyst, reaction intermediates and electrolyte ions can interact with both sides of the single-atom catalyst. As a result, the electrolyte anions compete with water and adsorbate on the single metal site, in some cases either poisoning or modifying the catalyst activity and thermodynamic stability. We find most electrolyte anions adsorbs on the single metal site under ORR conditions but not at the lower potentials for the CO 2 RR. Still, the adsorption of water and gas molecules can occur under CO 2 RR conditions. For example, under ORR conditions, the thermodynamic driving force of the *SO 4 -FeN 4 site in the 0.1 M H 2 SO 4 solution is about 0.47–0.56 eV lower than the *O-FeN 4 site in water, depending on the local carbon structure. Additionally, the stabilization by electrolyte anions depends on the nature of the metal atom. Our study demonstrates the important role of electrolytes and the coordination environment for the activity and stability of the M/N/C catalyst.
... 5 Besides, Fe-N-C catalysts might also include Fe-based particles such as metallic Fe, Fe carbide or oxides. 6 The analysis of the structure of Fe atoms in Fe-N-C materials is often carried out using Mössbauer (MS) and/or X-ray absorption spectroscopies, where other complementary techniques such as electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy are used for further characterization of the material both ex situ 7,8 and in situ/operando. [9][10][11] In general, the research related to Fe-N-C catalysts is focused either on the correlations between catalyst activity and its structure defined ex situ, or on the analysis of catalyst behavior under reaction conditions, i.e. in situ/operando. ...
... The catalysts analyzed in this work have been obtained via different preparations: Fe0.5 is a metal-organic framework (MOF)-derived material prepared via ramp pyrolysis synthesis (labeled as Fe0.5RP in previous publications) 19 ; C-PPy, is a polypyrrole-derived isotopically enriched 57 Fe-N-C; 20 additionally a non-pyrolysed carbon-supported porphyrin Porph/Cprec and a porphyrin-based catalyst Porph/Ccat are included, both related to FeTMPPCl (5,10,15,20tetrakis(4-methoxyphenyl)-21H,23H-porphine iron (III) chloride) supported on carbon. 8 More details on the preparation procedures can be found in the Supporting Information. On the basis of previous data of low-and room-temperature Mössbauer spectroscopy, one can describe the materials with respect to their Fe-related compositions as follows: Porph/Cprec contains only porphyrin-like FeN4 environments with an axial (chlorine) ligand, 8 both Fe0.5 5 and Porph/Ccat might be dominated by FeNxCy moieties, whereas C-PPy represents a mixture of molecular FeNxCy sites and crystalline or amorphous side phases. ...
... 8 More details on the preparation procedures can be found in the Supporting Information. On the basis of previous data of low-and room-temperature Mössbauer spectroscopy, one can describe the materials with respect to their Fe-related compositions as follows: Porph/Cprec contains only porphyrin-like FeN4 environments with an axial (chlorine) ligand, 8 both Fe0.5 5 and Porph/Ccat might be dominated by FeNxCy moieties, whereas C-PPy represents a mixture of molecular FeNxCy sites and crystalline or amorphous side phases. 8,10 The Fe Kβ HERFD XANES and XES spectra recorded on the catalysts powders are shown in Figure 1 and Figure S1. ...
Evaluation of the electrocatalyst performance data includes an electrode preparation step. Herein, we compare the structural composition of Fe−N−C materials, used to electrocatalyze the oxygen reduction reaction in proton-exchange membrane fuel cells, before and after catalyst layer preparation. The effects of this step on the electronic structure and local coordination of Fe were investigated by X-ray absorption (XAS) and emission spectroscopies (XES), for Fe−N−C materials prepared via different synthetic routes. This work underlines the importance of determining the Fe−N−C catalyst structure in the prepared electrode for further studies of the structure−activity−stability correlations.
... 28,29 Due to their high unsaturated coordination, maximum atom utilization, distinctive electronic structure, and capacity to suppress HER, they have drawn expressive attention in CO 2 RR. 30 Covalent triazine frameworks (CTFs), 31 of the typical COF 32 family, composed of strong covalent bonds, are another potential catalyst-supporting materials with similar properties. 33 Unlike MOFs, CTFs, and COFs can be synthesized with various heteroatoms (i.e., N, S, etc.) and conductive features. ...
... This result suggests that the Sn(IV) single-atom species is most probably the dominant catalytic site for the electrochemical CO 2 reduction to formic acid. 43 Meanwhile, Sn(II) may be a potential active site for the electroreduction of CO 2 to CO. ...
Sn-based materials have been demonstrated as promising catalysts for the selective electrochemical CO2 reduction reaction (CO2RR). However, the detailed structures of catalytic intermediates and the key surface species remain to be identified. In this work, a series of single-Sn-atom catalysts with well-defined structures is developed as model systems to explore their electrochemical reactivity toward CO2RR. The selectivity and activity of CO2 reduction to formic acid on Sn-single-atom sites are shown to be correlated with Sn(IV)-N4 moieties axially coordinated with oxygen (O-Sn-N4), reaching an optimal HCOOH Faradaic efficiency of 89.4% with a partial current density (jHCOOH) of 74.8 mA·cm-2 at -1.0 V vs reversible hydrogen electrode (RHE). Employing a combination of operando X-ray absorption spectroscopy, attenuated total reflectance surface-enhanced infrared absorption spectroscopy, Raman spectroscopy, and 119Sn Mössbauer spectroscopy, surface-bound bidentate tin carbonate species are captured during CO2RR. Moreover, the electronic and coordination structures of the single-Sn-atom species under reaction conditions are determined. Density functional theory (DFT) calculations further support the preferred formation of Sn-O-CO2 species over the O-Sn-N4 sites, which effectively modulates the adsorption configuration of the reactive intermediates and lowers the energy barrier for the hydrogenation of *OCHO species, as compared to the preferred formation of *COOH species over the Sn-N4 sites, thereby greatly facilitating CO2-to-HCOOH conversion.
... Atomically dispersed metal-nitrogen-carbon (M-NC) materials have emerged as promising electrocatalysts for the conversion of CO 2 into CO with many advantages such as low cost, high electrical conductivity, large available surface area, and tunable porous structure/ morphology. 7,[19][20][21][22][23] More importantly, their surface properties can be extensively tuned by mediating the coordination microenvironment of atomic metal species. [24][25][26][27][28][29] It is well known that the atomic non-precious transition metals (TMs) in a TM-N 4 coordination structure show reasonably high activity and Faraday efficiency (FE) in converting CO 2 into CO, 30 as demonstrated by various nonprecious TMs including Ni, 31 Co, 32 Fe, 25 Mn, 33 Cu, 34,35 Zn, 36 and so forth. ...
Design of supportive atomic sites with a controllably adjusted coordinating environment is essential to advancing the reduction of CO2 to value‐added fuels and chemicals and to achieving carbon neutralization. Herein, atomic Ni (Zn) sites that are uniquely coordinated with ternary Zn (Ni)/N/O ligands were successfully decorated on formamide‐derived porous carbon nanomaterials, possibly forming an atomic structure of Ni(N2O1)‐Zn(N2O1), as studied by combining X‐ray photoelectron spectroscopy and X‐ray absorption spectroscopy. With the mediation of additional O coordination, the Ni–Zn dual site induces significantly decreased desorption of molecular CO. The NiZn‐NC decorated with rich Ni(N2O1)‐Zn(N2O1) sites remarkably gained >97% CO Faraday efficiency over a wide potential range of ‒0.8 to ‒1.1 V (relative to reversible hydrogen electrode). Density functional theory computations suggest that the N/O dual coordination effectively modulates the electronic structure of the Ni–Zn duplex and optimizes the adsorption and conversion properties of CO2 and subsequent intermediates. Different from the conventional pathway of using Ni as the active site in the Ni–Zn duplex, it is found that the Ni‐neighboring Zn sites in the Ni(N2O1)‐Zn(N2O1) coordination showed much lower energy barriers of the CO2 protonation step and the subsequent dehydroxylation step. The binary Ni–Zn dual site with a delicately engineered coordination structure of Ni(N2O1)‐Zn(N2O1) shows CO Faradic efficiency of >97% over a wide potential range of ‒0.8 to ‒1.1 V. The N/O dual coordination optimizes the adsorption and conversion properties of CO2, rendering the Zn site in ternary O/N/Ni coordination as an active site for efficient CO2 reduction as it promotes the CO2 protonation and subsequent dehydroxylation with much lower energy barriers.
... Recently, as alternatives, earth-abundant transition metal-based single-atom catalysts (SACs) with atomically anchored metal atoms in the form of M-N 4 on N-doped carbon substrates (also known as M-N-C) have attracted great interest for CO 2 RR due to their maximized atomic utilization efficiency, tunable electronic properties, and distinctive catalytic characteristics [11][12][13][14][15]. Among them, the Fe-N-C catalysts are expected to exhibit high CO 2 RR performance at lower overpotentials, which can be comparable to or even better than noble metal-based electrocatalysts [16][17][18][19][20][21]. For example, Ye et al. [21] prepared a Fe-N-C catalyst with highly exposed Fe-N sites (C-AFCZIF-8), which displayed high Faradaic efficiency (FE) for CO generation (FE CO ) at low overpotentials (e.g., 89.1% at −0.33 V), outperforming the nanostructured Pd catalyst. ...
Transition metal-based single-atom catalysts (TM-SACs) are promising alternatives to Au- and Ag-based electrocatalysts for CO production through CO 2 reduction reaction. However, developing TM-SACs with high activity and selectivity at low overpotentials is challenging. Herein, a novel Fe-based SAC with Si doping (Fe-N-C-Si) was prepared, which shows a record-high electrocatalytic performance toward the CO 2 -to-CO conversion with exceptional current density (>350.0 mA cm ⁻² ) and ~100% Faradaic efficiency (FE) at the overpotential of <400 mV, far superior to the reported Fe-based SACs. Further assembling Fe-N-C-Si as the cathode in a rechargeable Zn-CO 2 battery delivers an outstanding performance with a maximal power density of 2.44 mW cm ⁻² at an output voltage of 0.30 V, as well as high cycling stability and FE (>90%) for CO production. Experimental combined with theoretical analysis unraveled that the nearby Si dopants in the form of Si-C/N bonds modulate the electronic structure of the atomic Fe sites in Fe-N-C-Si to markedly accelerate the key pathway involving *CO intermediate desorption, inhibiting the poisoning of the Fe sites under high CO coverage and thus boosting the CO 2 RR performance. This work provides an efficient strategy to tune the adsorption/desorption behaviors of intermediates on single-atom sites to improve their electrocatalytic performance.
... [9][10][11][12][13] Particularly, M-N-C catalysts have been reported as active materials to promote CO 2 RR. [14][15][16] For instance, Wang et al. synthesized a series of Ga SACs in different coordination environments (with P, S, and N atom doping). Through the CO 2 RR performance test, they found that the Ga-N 3 S-PC catalyst differs from the traditional bulk gallium metal and Ga-N 4 catalysts which exhibits >90% FE of the CO product. ...
Efficient catalysts are highly desirable for the selective electrochemical CO2 reduction reaction (CO2RR). Ni single-atom catalysts are known as promising CO2RR catalysts, while Ni NPs are expected to catalyze the competing HER. In this work, we have modified the Ni NPs by encapsulating them into porous Ni-N-C nanosheets (Ni@Ni-N-C), to boost the synergy between Ni NPs and dispersed Ni-N species towards CO2RR. The CO faradaic efficiency (FECO) reached 96.4% at -0.9 V and retained over 90% in a wide potential window. More importantly, FECO values of over 94% have been obtained from -50 to -170 mA cm-2 with a peak FECO of 99% in a flow cell. Our work demonstrates that the surface modification of Ni NPs can inhibit the unexpected HER and activate the surface sites, offering a practical design strategy for CO2RR catalysts.
... In the field of electrochemistry, metal and nitrogen codoped carbons (MÀ NÀ C) have attracted the interest of the community due to their application in many relevant energy conversion processes, such as oxygen reduction reaction (ORR), [1] CO 2 reduction reaction (CO 2 RR) [2] and ammonia electrosynthesis. [3] In order to replace expensive precious group metals (PGM) electrocatalysts, MÀ NÀ C need to meet their activity and stability, or at least compensate with their reduced cost. ...
M−N−C electrocatalysts are considered pivotal to replace expensive precious group metal‐based materials in electrocatalytic conversions. However, their development is hampered by the limited availability of methods for the evaluation of the intrinsic activity of different active sites, like pyrrolic FeN4 sites within Fe−N−Cs. Currently, new synthetic procedures based on active‐site imprinting followed by an ion exchange reaction, e.g. Zn‐to‐Fe, are producing single‐site M−N−Cs with outstanding activity. Based on the same replacement principle, we employed a conservative iron extraction to partially remove the Fe ions from the N4 cavities in Fe−N−Cs. Having catalysts with the same morphological properties and Fe ligation that differ solely in Fe content allows for the facile determination of the decrease in density of active sites and their turn‐over frequency. In this way, insight into the specific activity of M−N−Cs is obtained and for single‐site catalysts the intrinsic activity of the site is accessible. This new approach surpasses limitations of methods that rely on probe molecules and, together with those techniques, offers a novel tool to unfold the complexity of Fe−N−C catalyst and M−N−Cs in general.
... them natural candidates for investigating the CO2RR reactivity of tin in Sn-N-C. Recent work from Zu et al.[168] and Paul et al.[169] focused on the design and utilization of Sn-N-C and reported faradaic efficiency toward formate generation up to 75 %, owing to the stabilization of the CO2 •intermediates on the Sn-Nx moieties. Finally, achieving C2+ products on non-NP based electrocatalysts requires the adsorption of COads onto neighbouring sites to alloy the formation of the C-C bond, and, therefore, the design of sites alloying such conformation, e.g.dual-atoms sites (Fe2-Nx, Fe-Co-Nx, etc.). ...
While supported metal nanoparticles cannot achieve full electrochemical utilization of metal atoms, catalysts featuring single‐metal atom sites may offer this possibility, along with advantages in selectivity. However, the passage from nanometric to atomic dimension is not without consequences. It first raises the question of efficient and robust synthesis methods, and underlines the need of cutting‐edge characterization techniques that can target single‐metal atoms. These analytical tools are also pivotal to gain insights into the structure of the active sites, and establish atomic structure–catalytic activity–selectivity–stability relationships. Herein, we illustrate these topics for electrocatalysis, with a particular focus on metal–nitrogen–carbon single‐metal atom catalysts, for which a fantastic leap forward has been achieved in the last 15 years, triggered by the growing interest in sustainable energy storage and conversion systems.
... Recently, transition metal and nitrogen co-doped carbon (M-N-C) materials have been considered as promising candidates due to the advantages of low cost, high specific surface area, tunable structures and unique electronic structures. [10][11][12][13][14] Various M-N-C materials have been employed as electrocatalysts in CO 2 RR. Experimentally, the undercoordinated M-N x (x = 2-4) sites in heat-treated Ni-N/C, Co-N/C and Cu-N/C catalysts can effectively increase the CO 2 adsorption and thereby enhancing CO 2 RR activity. ...
Electrocatalytic CO 2 reduction reaction (CO 2 RR) is a very prospective strategy to reduce CO 2 to valuable fuels and chemical products, thereby alleviating the growing energy crisis and greenhouse effect. In this study, CO 2 RR mechanisms on M 3 (TABTO) 2 (M = Sc-Cu, Y-Mo and Ru-Rh, TABTO = 1,3,5-triamino-2,4,6-benzenetriol) are investigated by means of density functional method. The results show that the studied catalysts are stable thermodynamically. Co 3 (TABTO) 2 exhibits the best catalytic performance for the formation of CH 3 OH with the same overpotential of 0.41 V both in the gas phase and in solution. For Fe 3 (TABTO) 2 , however, the product is HCOOH with the overpotential of 0.29 V in the gas phase and 0.70 V in solution. For Ru 3 (TABTO) 2 to produce CH 4 , solvent effect reduces the overpotential significantly from 0.97 V in the gas phase to 0.54 V in solution, making it to be a promising CO 2 RR catalyst. Moreover, the improvement of CO 2 RR catalyst activity can be achieved by the axial oxygen modification in M 3 (TABTO) 2 (M = Sc, Y and V). A good relationship between d band center and overpotential is observed, which might provide us with a new direction to design the promising catalyst.
... For reduction of the highly stable molecules like CO 2 and N 2 , doping transition metal atoms or decorating with dispersed metal atoms or nanoparticles in the carbon materials would be necessary. The transition metal-nitrogen-carbon (M-N-C) materials and metalorganic frameworks (MOFs) are demonstrated with superior activity for electrochemical CO 2 RR, which have been described in many literature [46][47][48][49] . Besides, 2D porous carbon nitride and boron carbon nitrides are potential photocatalysts owing to their suitable band gap and outstanding light harvesting capability. ...
Activation of p -block elements to replace the rare and precious transition metals for renewable energy applications is highly desirable. In this review, we go over recent experimental and theoretical progress on the low-dimensional non-metal materials for clean energy production, including carbon, silicon, oxide, boron, and phosphorus-based nanostructures, with the p -block elements serving as active sites. We aim to elucidate the mechanism for triggering activity in different kinds of non-metal systems, and extract general principles for controlling the p -orbital-mediated reactivity from a theoretical point of view. The perspectives and challenges for developing high-efficiency non-metal catalysts are provided in the end.
... 34 In addition, atomically dispersed metal sites on carbon, especially those with M-N-C coordination, have demonstrated excellent catalytic activities both experimentally and theoretically. 44,45 Catalysts with combined atomic sites, nanoclusters or even metallic particles have also been regarded as M-N-C catalysts, indicating the feasibility of multiple sites or clusters acting as a secondary active center. However, these synergistic or cooperative active sites consisting of multiple metal sites are not fully understood. ...
The development of cost-effectiveness, high-performance catalysts at the atomic level has become a challenging issue for large-scale applications of renewable clean energy conversion. Featured with adjustable structure characteristics and maximum...
The use of precious metal electrocatalysts in clean electrochemical energy conversion and storage applications is widespread, but the sustainability of these materials, in terms of their availability and cost, is constrained. In this research, iron triad-based bimetallic nitrogen-doped carbon (M–N–C) materials were investigated as potential bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synthesis of bimetallic FeCo–N–C, CoNi–N–C, and FeNi–N–C catalysts involved a precisely optimized carbonization process of their respective metal–organic precursors. Comprehensive structural analysis was undertaken to elucidate the morphology of the prepared M–N–C materials, while their electrocatalytic performance was assessed through cyclic voltammetry and rotating disk electrode measurements in a 0.1 M KOH solution. All bimetallic catalyst materials demonstrated impressive bifunctional electrocatalytic performance in both the ORR and the OER. However, the FeNi–N–C catalyst proved notably more stable, particularly in the OER conditions. Employed as a bifunctional catalyst for ORR/OER within a customized zinc–air battery, FeNi–N–C exhibited a remarkable discharge–charge voltage gap of only 0.86 V, alongside a peak power density of 60 mW cm–2. The outstanding stability of FeNi–N–C, operational for about 55 h at 2 mA cm–2, highlights its robustness for prolonged application.
Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI, the coordination...
Electrochemical CO2 reduction reaction (CO2RR) to CO is closely correlated with appropriate sorption of *COOH and *CO species toward the electrode surface, and the proton transfer process that often competes...
Electrochemical CO2 reduction is a sustainable approach in green chemistry that enables the production of valuable chemicals and fuels while mitigating the environmental impact associated with CO2 emissions. Despite its several advantages, this technology suffers from an intrinsically low CO2 solubility in aqueous solutions, resulting in a lower local CO2 concentration near the electrode, which yields lower current densities and restricts product selectivity. Gas diffusion electrodes (GDEs), particularly those with tubular architectures, can solve these issues by increasing the local CO2 concentration and triple-phase interface, providing abundant electroactive sites to achieve superior reaction rates. In this study, robust and self-supported Cu flow-through gas diffusion electrodes (FTGDEs) were synthesized for efficient formate production via electrochemical CO2 reduction. They were further compared with traditional Cu electrodes, and it was found that higher local CO2 concentration due to improved mass transfer, the abundant surface area available for the generation of the triple-phase interface, and the porous structure of Cu FTGDEs enabled high formate Faradaic efficiency (76%) and current density (265 mA¸cm−2) at–0.9 V vs. reversible hydrogen electrode (RHE) in 0.5 mol·L−1 KHCO3. The combined phase inversion and calcination process of the Cu FTGDEs helped maintain a stable operation for several hours. The catalytic performance of the Cu FTGDEs was further investigated in a non-gas diffusion configuration to demonstrate the impact of local gas concentration on the activity and performance of electrochemical CO2 reduction. This study demonstrates the potential of flow-through gas-diffusion electrodes to enhance reaction kinetics for the highly efficient and selective reduction of CO2, offering promising applications in sustainable electrochemical processes.
The transition metal–nitrogen–carbon (M─N─C) with MNx sites has shown great potential in CO2 electroreduction (CO2RR) for producing high value‐added C1 products. However, a comprehensive and profound understanding of the intrinsic relationship between the density of metal single atoms and the CO2RR performance is still lacking. Herein, a series of Ni single‐atom catalysts is deliberately designed and prepared, anchored on layered N‐doped graphene‐like carbon (x Ni1@NG‐900, where x represents the Ni loading, 900 refers to the temperature). By modulating the precursor, the density of Ni single atoms (DNi) can be finely tuned from 0.01 to 1.19 atoms nm⁻². The CO2RR results demonstrate that the CO faradaic efficiency (FECO) predominantly increases from 13.4% to 96.2% as the DNi increased from 0 to 0.068 atoms nm⁻². Then the FECO showed a slow increase from 96.2% to 98.2% at −0.82 V versus reversible hydrogen electrode (RHE) when DNi increased from 0.068 to 1.19 atoms nm⁻². The theoretical calculations are in good agreement with experimental results, indicating a trade‐off relationship between DNi and CO2RR performance. These findings reveal the crucial role of the density of Ni single atoms in determining the CO2RR performance of M─N─C catalysts.
Degradation of single-site model Fe–N–C powder tracked using cryo-Mössbauer spectroscopy suggests a simple mechanism comprising the oxidation of Fe( ii ) to Fe( iii ) followed by precipitation of iron oxide. Curiously, the characteristic doublet D2 is converted into D1 upon exposure to air.
Electrocatalysts are crucial to drive the electrochemical carbon dioxide reduction reaction (CO2RR) which lower the energy barrier, tune the intricate reaction pathways and suppress competitive side‐reaction. Beyond the efficient active sites and advantageous local environment, a rapid mass transfer ability is also crucial for the catalyst design. However, it is rare that research has been done to investigate in detail the mass transfer process in CO2RR, and expose the underlying relationship between mass transfer and final performance. Here, a single‐atom Fe‐N‐C catalyst is shown with a highly ordered porous substrate containing hierarchical micropores, mesopores, and macropores. Such a delicate porous structure significantly facilitates the mass transfer process toward the isolated Fe sites, achieving excellent CO2RR performance, especially in the limited mass transfer region in a H‐cell with a maximum CO partial current density of ‐19 mA cm⁻². Operando electrochemical impedance spectroscopy and relevant distributed relaxation times analysis reveal the rapidly decreased mass transfer resistance with the increase of reduction potential. The Lattice Boltzmann method with Discrete Element method (LBM‐DEM) simulations are further performed to exhibit the origin of enhanced CO2RR performance from the facilitated gas diffusion process.
Immobilized molecular catalysts (IMCs) with well-defined active sites and tunable coordination environments are promising candidates for catalyzing the electrochemical CO2 reduction reaction (CO2RR) with high activity and selectivity. With substantial progress in catalyst development, IMCs are being transitioned from batch cells (e.g., H-cells) where activity is limited by CO2 solubility in aqueous electrolyte, to flow electrolyzers equipped with gas diffusion electrodes (GDEs) that can achieve commercially relevant CO2RR current densities. This transition is challenged by the drastic differences in the microenvironment (e.g., local pH, CO2 concentration, GDE wetting) between batch cells and flow electrolyzers, and a poor understanding of the implications of these microenvironment changes on CO2RR performance of IMCs. In this perspective, we highlight recent studies that probe the IMC-microenvironment interactions in GDE configurations, and we suggest strategies to understand better microenvironment effects using electrochemical measurements, in situ spectroelectrochemical measurements, and post-mortem catalyst characterization.
The electroreduction of CO2 (CO2RR) is a sustainable approach to mitigate the increased global CO2 emissions and further produce valuable chemicals. Electrocatalysts are crucial to lower the energy barrier, tune the intricate reaction pathways and suppress the competitive side-reactions. In this feature article, we give a brief overview of our journey in the design of efficient catalysts for the CO2RR. From bulk metals to nanoparticles to single-atom catalysts (SACs), we summarize our progress in the design of efficient metal nanoparticles by porosity engineering, defect engineering and alloy engineering, and developing single-atom catalysts with advanced metal sites, coordination environments, substrates and synthesis routes. We highlight the importance of reaction environments and provide an ionic liquid nanoconfinement strategy for local environment modification. In the end, we provide our views and perspectives for the future direction of the CO2RR towards commercialization.
For a future hydrogen economy, non‐precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal‐nitrogen‐carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN4) show promising performance, but an optimization rooted in structure‐property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well‐defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN4 centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [FeII(P⋅)]⁻ radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.
The electrocatalytic CO2 reduction is a promising strategy for CO2 resource utilization, and copper-based catalysts are well-known to be positive for converting CO2 to ethylene. However, the ethylene selectivity and catalyst persistence still need to be improved. Herein, a series of Metal- and nitrogen-doped carbon composite electrocatalysts with different ingredient ratios were constructed by pyrolysis of Cu(NO3)2 and polyvinyl pyrrolidone mixed precursor for CO2 reduction. Among these composite catalysts, the excessive Cu content caused an increase in the size of Cu nanoparticles and the surface coverage of carbon support, resulting in a highly competitive H2 evolution and a quite low CO2RR activity. Among the series of the catalysts with different ingredient ratios, Cu-10/NC with suitable catalyst formulation exhibited smaller Cu nanoparticle size and lower surface coverage, resulting in higher CO2 activity and conversion. Besides, there were more Cu⁺ existed in Cu-10/NC, which could be reduced to low coordination Cu under the electrochemical reduction process, resulting in enhanced *CO binding energy and stabilized *CO intermediates. Combining the high contents of pyrrolic-N and Cu-N doped in Cu-10/NC, which could lower the C-C dimerization activations barriers and facilitated C–C coupling reactions on Cu surface, all of these features contributed that Cu-10/NC was the optimal catalyst for CO2RR to ethylene. The FEC2H4 of Cu-10/NC reached 37% at −1.2 V (vs. RHE) and the electrocatalytic performance remained stable over 7 h. These findings provide new references to develop high-efficiency CO2 reduction electrocatalysts.
Transition metal and nitrogen doped carbon catalysts (MNC) are effective in electrochemical reduction of CO2 to CO with a high selectivity. However, scalable and cost-effective synthesis of active metal-nitrogen catalysts is yet to be developed. Herein, we report a simple and sustainable method that utilizes commercial carbon nanotubes (CNTs) to adsorb a pharmaceutical waste, sulfamethoxazole (SMX), followed by moderate pyrolysis to prepare an efficient MNC catalyst. The intrinsic metal impurities from CNTs are essential to form active metal sites, and it requires significantly less nitrogen precursor than methods using most widely nitrogen precursors such as melamine and urea. The CNT-SMX catalyst delivers high CO2RR performance with 91.5 % CO Faradaic efficiency and 14 mA/cm² CO partial current density at −0.76 V vs RHE in a traditional H-Cell. The catalyst is also efficient in a scalable flow cell, exhibiting 97.5 % CO selectivity at 300 mA/cm², plus stable CO2RR performance for more than 24 h at 100 mA/cm². The scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS) analyses confirm the existence of single atomic sites primarily in the form of Fe-N bonds that are active sites for CO2RR. Density functional theory (DFT) calculations suggest a synergy between the single atomic FeNC sites and Ni nanoparticles embedded in the CNTs, which enhances CO production rate and selectivity by lowering the desorption energy of *CO intermediate. To the best of our knowledge, the results in this work are among the top performing carbon-based catalysts. Furthermore, catalysts developed in this work are synthesized at a moderate temperature without pre-oxidation or post-acid-washing and utilize cheap or waste materials, presenting a simple, sustainable, and cost-effective way to synthesize highly active catalysts.
M‐N‐C electrocatalysts are considered pivotal to replace expensive precious group metal‐based materials in electrocatalytic conversions. However, their development is hampered by the limited availability of methods for the evaluation of the intrinsic activity of different active sites, like pyrrolic Fe‐N 4 sites within Fe‐N‐Cs. Currently, new synthetic procedures based on active‐site imprinting followed by an ion exchange reaction, e.g. Zn‐to‐Fe, are producing single‐site M‐N‐Cs with outstanding activity. Based on the same replacement principle, we employed a conservative iron extraction to partially remove the Fe ions from the N 4 cavities in Fe‐N‐Cs. Having catalysts with the same morphological properties and Fe ligation that differ solely in Fe content allows for the facile determination of the reduced density of active site and their turn‐over frequency. In this way, insight into the specific activity of M‐N‐Cs is obtained and for single‐site catalysts the intrinsic activity of the site is accessible. This new approach surpasses limitations of methods that rely on probe molecules and, together with those techniques, offers a novel tool to unfold the complexity of Fe‐N‐C catalyst and M‐N‐Cs in general.
For large-scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal- and nitrogen-doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the active site is an iron ion coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando 57Fe Mössbauer spectroscopy and quantum chemistry. A Mössbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mössbauer signatures (D2, D3a, D3b), implying a direct structural correspondence. Pyrrolic N-coordination, i.e., FeN4C12, is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coordination. These findings thus overcome the previously conflicting structural assignments for the active site and, moreover, identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.
The reduction of CO2 into valuable chemicals through electrocatalytic techniques provides a potential strategy to alleviate energy crises and environmental pollution. In this study, 2D M3(THT)2 (M = Fe, Co, Ni, Ru, Rh, Pd, THT = 2,3,6,7,10,11-triphenylenehexathiolate) framework as CO2 reduction reaction (CO2RR) electrocatalysts was investigated by using the density functional method. The results showed that the studied catalysts are stable both thermodynamically and electrochemically. Rh3(THT)2 exhibits the best catalytic performance to produce CH4 with the overpotential of 0.61 V in the gas phase and 0.63 V in solution. The inactive hydrogen evolution reaction in Rh3(THT)2 would favour CO2RR. For Fe3(THT)2 and Ru3(THT)2, the main product is CO. However, the strong CO adsorption on the catalyst surface can lead to catalyst to be poisoned, which makes Fe3(THT)2 and Ru3(THT)2 to be poor CO2RR catalysts. We anticipate that this work may provide a new avenue for the development of high-performance 2D metal-organic framework-based electrocatalysts.
The challenge of activating inert C-H bonds motivates a study of catalysts that draws from what can be accomplished by natural enzymes and translates these advantageous features into transition-metal complex (TMC) and material mimics. Inert C-H bond activation reactivity has been observed in a diverse number of predominantly iron-containing enzymes from the heme-P450s to nonheme iron α-ketoglutarate-dependent enzymes and methane monooxygenases. Computational studies have played a key role in correlating active-site variables, such as the primary coordination sphere, oxidation state, and spin state, to reactivity. TMCs, zeolites, metal-organic frameworks (MOFs), and single-atom catalysts (SACs) are synthetic inorganic materials that have been designed to incorporate Fe active sites in analogy to single sites in enzymes. In these systems, computational studies have been essential in supporting spectroscopic assignments and quantifying the effects of the metal-local environment on C-H bond reactivity. High-throughput virtual screening tools that have been widely used for bulk metal catalysis do not readily extend to the single-site inorganic catalysts where metal-ligand bonding and localized d-electrons govern reaction energetics. These localized d-electrons can also necessitate wave function theory calculations when density functional theory (DFT) is not sufficiently accurate. Where sufficient computational or experimental data can be gathered, machine learning has helped uncover more general design rules for reactivity or stability. As we continue to investigate metalloprotein active sites, we gain insights that enable us to design stable, active, and selective single-site catalysts.
Electrochemical reduction of carbon dioxide (CO2RR) into fuels and valuable chemicals is a promising approach to reduce CO2 emissions and alleviate global warming. Metal-nitrogen-carbon (M-N-C) based single-atom catalysts have exhibited as promising catalysts for driving this reaction with excellent performance. Most current works are based on screening the active metals and tuning the metal-nitrogen coordination structure. However, the importance of carbon substrates is often neglected. Here, we summarize recent achievements in the design and engineering of porosity of carbon structures for this class of catalyst. Four positive effects of the porous morphology are highlighted for M-N-C materials in CO2RR. Finally, current challenges and future perspectives for engineering porosity for M-N-C catalysts are provided.
Achieving efficient efficiency and selectivity for the electroreduction of CO2 to value‐added feedstocks has been challenging, due to the thermodynamic stability of CO2 molecules and the competing hydrogen evolution reaction. Herein, a dual‐single‐atom catalyst consisting of atomically dispersed CuN4 and NiN4 bimetal sites is synthesized with electrospun carbon nanofibers (CuNi‐DSA/CNFs). Theoretical and experimental studies reveal the strong electron interactions induced by the electronegativity offset between the Cu and Ni atoms. The delicately averaged and compensated electronic structures result in an offset effect that optimizes the adsorption strength of the *COOH intermediate and boosts the CO2 reduction reaction (CO2RR) kinetics, notably promoting the intrinsic activity and selectivity of the catalyst. The CuNi‐DSA/CNFs catalyst exhibits an outstanding FECO of 99.6% across a broad potential window of −0.78– −1.18 V (vs the reversible hydrogen electrode), a high turnover frequency of 2870 h–1, and excellent durability (25 h). Furthermore, an aqueous Zn‐CO2 battery for CO2 power conversion is constructed. This atomic‐level electronegativity offset of the dual‐atom structures provides an appealing direction to develop advanced electrocatalysts for the CO2RR.
MIL-100 is a class of trimesate MOFs that has been widely studied as candidates for various energy related applications. Understanding of the MOF’s pyrolysis is important as it determines the...
To advance the widespread implementation of electrochemical energy storage and conversion technologies, the development of inexpensive electrocatalysts is imperative. In this context, Fe/N/C‐materials represent a promising alternative to the costly noble metals currently used to catalyze the oxygen reduction reaction (ORR), and also display encouraging activities for the reduction of CO2. Nevertheless, the application of these materials in commercial devices requires further improvements in their performance and stability that are currently hindered by a lack of understanding of the nature of their active sites and the associated catalytic mechanisms. With this motivation, herein the authors exploit the high sensitivity of modulation excitation X‐ray absorption spectroscopy toward species undergoing potential‐induced changes to elucidate the operando local geometry of the active sites in two sorts of Fe/N/C‐catalysts. While the ligand environment of a part of both materials’ sites appears to change from six‐/five‐ to fourfold coordination upon potential decrease, they differ substantially when it comes to the geometry of the coordination sphere, with the more ORR‐active material undergoing more pronounced restructuring. Furthermore, these time‐resolved spectroscopic measurements yield unprecedented insights into the kinetics of Fe‐based molecular sites’ structural reorganization, identifying the oxidation of iron as a rate‐limiting process for the less ORR‐active catalyst.
Atomic catalysts supported on optimal supports exhibit an ideal strategy to maximize the utilization of active atoms for improving the catalytic efficiency and reducing the cost of catalysts. Among these atomic catalysts, double atom catalysts (DAC), and triple atom catalysts (TACs) are emerging materials, which represent the most basic active sites of the bridge and hollow sites and show high activity and high selectivity. In this chapter, the synthesis routes, characterization techniques, and important applications of DACs and TACs are summarized. Moreover, this chapter outlines the opportunities and challenges in developing DACs and TACs, which provide a comprehensive and distinct understanding of DACs and TACs and inspire further research in the field of catalysis.
Indium single-atom catalysts display large total current density (38.94 to 81.08 mA cm-2) over an extensive potential window (-0.91 to -1.41 V vs. RHE) for electrocatalytic CO2 reduction to formate...
Recently, Li-CO2 battery has gradually become a research hotspot due to its high discharge capacity, energy density and environmental benefits. However, it has been an important problem for researchers because of its slow decomposition kinetics and difficult to generalize to practical application. Herein, we prepared copper polyphthalocyanine-carbon nanotubes composites (CuPPc-CNTs) by solvothermal in-situ polymerization of copper phthalocyanine on the surface of carbon nanotubes as cathode for reversible Li-CO2 batteries, which exhibits a high discharge capacity of 18,652.7 mAh·g−1 at current density of 100 mA·g−1, 1.64 V polarization at 1,000 mA·g−1, and a stable cycles number of 160 is close to 1,630 h of charge-discharge process at 200 mA·g−1. Copper polyphthalocyanine has highly efficient copper single-atom catalytic sites with excellent CO2 adsorption and activation, while carbon nanotubes provide a conductive network. The synergistic effect of the two compounds enables it to have excellent catalytic activity. The density functional theory (DFT) calculation proved that the addition of copper polyphthalocyanine significantly improved the CO2 adsorption and activation process. This study provides an opportunity for the research of covalent organic polymers (COPs) single-atom catalyst in Li-CO2 battery field.
Although stainless steel is a promising candidate for oxygen evolution reaction (OER) electrodes, chalcogenization is typically necessary to avoid surface passivation. Herein, we modify the surface of SUS304 by selenization under mild conditions. The optimal selenization temperature (500 °C) is determined by analyzing the surface morphology and elemental distribution. The electrode composition and the role of Se in improving OER activity are clarified using X-ray photoelectron spectroscopy depth profiling. The electrode selenized at 500 °C is rich in oxygen vacancies and had a high Ni content after electrochemical pre-activation. Moreover, the overpotential is only 284.3 mV at 10 mA cm⁻² and no potential degradation occurred over 160 h, indicating excellent stability under alkaline conditions. Further, high stability is achieved during CO2 reduction in a real water matrix. These results provide new insights for modifying commercial stainless-steel electrodes to maximize OER activity for alkaline water splitting and neutral CO2 electrolysis.
Ir modification of FeNC catalysts improves the durability of the catalysts, but causes electronic changes that are disadvantageous for the activity.
This contribution reports the discovery and analysis of a p-block Sn-based catalyst for the electroreduction of molecular oxygen in acidic conditions at fuel cell cathodes; the catalyst is free of platinum-group metals and contains single-metal-atom actives sites coordinated by nitrogen. The prepared SnNC catalysts meet and exceed state-of-the-art FeNC catalysts in terms of intrinsic catalytic turn-over frequency and hydrogen–air fuel cell power density. The SnNC-NH3 catalysts displayed a 40–50% higher current density than FeNC-NH3 at cell voltages below 0.7 V. Additional benefits include a highly favourable selectivity for the four-electron reduction pathway and a Fenton-inactive character of Sn. A range of analytical techniques combined with density functional theory calculations indicate that stannic Sn(iv)Nx single-metal sites with moderate oxygen chemisorption properties and low pyridinic N coordination numbers act as catalytically active moieties. The superior proton-exchange membrane fuel cell performance of SnNC cathode catalysts under realistic, hydrogen–air fuel cell conditions, particularly after NH3 activation treatment, makes them a promising alternative to today’s state-of-the-art Fe-based catalysts. For oxygen reduction and hydrogen oxidation reactions, proton-exchange membrane fuel cells typically rely on precious-metal-based catalysts. A p-block single-metal-site tin/nitrogen-doped carbon is shown to exhibit promising electrocatalytic and fuel cell performance.
In this work, Fe‐N‐C catalysts are prepared from surface functionalized carbon nanotubes in combination with iron acetate and phenanthroline. An improved performance and structural composition are obtained by surface functionalization of the CNTs with indazole or pyridine. Catalyst composition and morphology are characterized by transmission electron microscopy, N2 sorption, photoelectron spectroscopy and ⁵⁷Fe transmission Mössbauer Spectroscopy. Whereas, activity and selectivity towards oxygen reduction reaction are determined from rotating ring disc electrode (RRDE) experiments. The durability and stability are evaluated by accelerated stress tests (0.0 to 1.2 V) and differential electrochemical mass spectroscopy (DEMS), respectively. It is shown that surface functionalization with indazole enables the direct attachment of FeN4 centers to carbon nanotubes so that no impurity species were detected and a high activity is achieved, that can be attributed to an improved turn‐over frequency and higher mass‐based site density. Even more striking is the excellent durability and stability of the realized catalyst. While these trends are well pronounced in RRDE and DEMS, challenges in the preparation of membrane electrode assemblies make the trend not as obvious in fuel cells.
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Scenarios used by the Intergovernmental Panel on Climate Change (IPCC) are central to climate science and policy. Recent studies find that observed trends and International Energy Agency (IEA) projections of global CO2 emissions have diverged from emission scenario outlooks widely employed in climate research. Here, we quantify the bases for this divergence, focusing on Kaya Identity factors: population, per-capita GDP, energy intensity (energy consumption/GDP), and carbon intensity (CO2 emissions/energy consumption). We compare 2005-2017 observations and IEA projections to 2040 of these variables, to “baseline” scenario projections from the IPCC’s Fifth Assessment Report (AR5), and from the Shared Socioeconomic Pathways (SSPs) used in the upcoming Sixth Assessment Report (AR6). We find that the historical divergence of observed CO2 emissions from baseline scenario projections can be explained largely by slower-than-projected per-capita GDP growth—predating the COVID-19 crisis. We also find carbon intensity divergence from baselines in IEA’s projections to 2040. IEA projects less coal energy expansion than the baseline scenarios, with divergence expected to continue to 2100. Future economic growth is uncertain, but we show that past divergence from observations makes it unlikely that per-capita GDP growth will catch up to baselines before mid-century. Some experts hypothesize high enough economic growth rates to allow per-capita GDP growth to catch up to or exceed baseline scenarios by 2100. However, we argue that this magnitude of catch-up may be unlikely, in light of: headwinds such as aging and debt, the likelihood of unanticipated economic crises, the fact that past economic forecasts have tended to over-project, the aftermath of the current pandemic, and economic impacts of climate change unaccounted-for in the baseline scenarios. Our analyses inform the rapidly evolving discussions on climate and development futures, and on uses of scenarios in climate science and policy.
Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO2 reduction (CO2R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.
Understanding Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR) under ambient conditions is both fundamentally interesting and technologically important for selective CO2RR to hydrocarbons. Current Cu-catalysts studied for the CO2RR can show high activity, but tend to yield a mixture of different hydrocarbons, posing a serious challenge on using any of these catalysts for selective CO2RR. Here, we report a new perovskite-type copper(I) nitride (Cu3N) nanocube (NC) catalyst for selective CO2RR. The 25 nm Cu3N NCs show high CO2RR selectivity and stability to ethylene (C2H4) at -1.6 V (vs reversible hydrogen electrode (RHE)) with the Faradaic efficiency of 60%, mass activity of 34 A/g and C2H4/CH4 molar ratio > 2000. More detailed electrochemical, X-ray photon spectroscopy, and density functional theory calculation suggest that the high CO2RR selectivity is likely a result of (100) Cu(I) stabilization by the Cu3N structure, which favors CO-CHO coupling on the (100) Cu3N surface, leading to selective formation of C2H4. Our study presents a good example of utilizing metal nitrides as highly efficient nanocatalysts for selective CO2RR to hydrocarbons that will be important for sustainable chemistry/energy applications.
Unlike energy efficiency and selectivity challenges, the kinetic effects of impure or intentionally mixed CO2 feeds on the catalytic reactivity of the direct electrochemical CO2 reduction reaction (CO2RR) have been poorly studied. Given that industrial CO2 feeds are often contaminated with CO, a closer investigation of the CO2RR under CO2/CO co-feed conditions is warranted. Here, we report mechanistic insights into the CO2RR reactivity of CO2/CO co-feeds on Cu-based nanocatalysts. Kinetic isotope-labelling experiments—performed in an operando differential electrochemical mass spectrometry capillary flow cell with millisecond time resolution—showed an unexpected enhanced production of C2H4, with a yield increase of almost 50%, from a cross-coupled ¹²CO2–¹³CO reactive pathway. The results suggest the absence of site competition between CO2 and CO molecules on the reactive surface at the reactant-specific sites. The practical significance of sustained local interfacial CO partial pressures under CO2 depletion is demonstrated by metallic/non-metallic Cu/Ni–N-doped carbon tandem catalysts. Our findings show the mechanistic origin of improved C2 product formation under co-feeding, but also highlight technological opportunities of impure CO2/CO process feeds for H2O/CO2 co-electrolysers.
Production of multicarbon products (C2+) from CO2 electroreduction reaction (CO2RR) is highly desirable for storing renewable energy and reducing carbon emission. The electrochemical synthesis of CO2RR catalysts that are highly selective for C2+ products via electrolyte‐driven nanostructuring is presented. Nanostructured Cu catalysts synthesized in the presence of specific anions selectively convert CO2 into ethylene and multicarbon alcohols in aqueous 0.1 m KHCO3 solution, with the iodine‐modified catalyst displaying the highest Faradaic efficiency of 80 % and a partial geometric current density of ca. 31.2 mA cm⁻² for C2+ products at −0.9 V vs. RHE. Operando X‐ray absorption spectroscopy and quasi in situ X‐ray photoelectron spectroscopy measurements revealed that the high C2+ selectivity of these nanostructured Cu catalysts can be attributed to the highly roughened surface morphology induced by the synthesis, presence of subsurface oxygen and Cu⁺ species, and the adsorbed halides.
Developing highly efficient electrocatalysts based on cheap and earth-abundant metals for CO2 reduction is of great importance. Here we demonstrate that the electrocatalytic activity of manganese-based heterogeneous catalyst can be significantly improved through halogen and nitrogen dual-coordination to modulate the electronic structure of manganese atom. Such an electrocatalyst for CO2 reduction exhibits a maximum CO faradaic efficiency of 97% and high current density of ~10 mA cm-2 at a low overpotential of 0.49 V. Moreover, the turnover frequency can reach 38347 h-1 at overpotential of 0.49 V, which is the highest among the reported heterogeneous electrocatalysts for CO2 reduction. In situ X-ray absorption experiment and density-functional theory calculation reveal the modified electronic structure of the active manganese site, on which the free energy barrier for intermediate formation is greatly reduced, thus resulting in a great improvement of CO2 reduction performance.
Electrocatalytic CO 2 reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric CO 2 . The field has advanced immensely in recent years, taking significant strides towards commercial realization. Catalyst innovations have played a pivotal role in these advances, with a steady stream of new catalysts providing gains in CO 2 conversion efficiencies and selectivities of both C1 and C2 products. Comparatively few of these catalysts have been tested at commercially-relevant current densities (∼200 mA cm ⁻² ) due to transport limitations in traditional testing configurations and a research focus on fundamental catalyst kinetics, which are measured at substantially lower current densities. A catalyst's selectivity and activity, however, have been shown to be highly sensitive to the local reaction environment, which changes drastically as a function of reaction rate. As a consequence of this, the surface properties of many CO 2 reduction catalysts risk being optimized for the wrong operating conditions. The goal of this perspective is to communicate the substantial impact of reaction rate on catalytic behaviour and the operation of gas-diffusion layers for the CO 2 reduction reaction. In brief, this work motivates high current density catalyst testing as a necessary step to properly evaluate materials for electrochemical CO 2 reduction, and to accelerate the technology toward its envisioned application of neutralizing CO 2 emissions on a global scale.
We report novel structure-activity relationships and explore the chemical state and structure of catalytically active sites under operando conditions during the electrochemical CO2 reduction reaction (CO2RR) catalyzed by a series of porous iron-nitrogen-carbon (FeNC) catalysts. The FeNC catalysts were synthesized from different nitrogen precursors and, as a result of this, exhibited quite distinct physical properties, such as BET surface areas and distinct chemical N-functionalities in varying ratios. The chemical diversity of the FeNC catalysts was harnessed to set up correlations between the catalytic CO2RR activity and their chemical nitrogen-functionalities, which provided a deeper understanding between catalyst chemistry and function. XPS measurements revealed a dominant role of porphyrin-like Fe-N
x
motifs and pyridinic nitrogen species in catalyzing the overall reaction process. Operando EXAFS measurements revealed an unexpected change in the Fe oxidation state and associated coordination from Fe2+ to Fe1+. This redox change coincides with the onset of catalytic CH4 production around -0.9 VRHE. The ability of the solid state coordinative Fe1+-N
x
moiety to form hydrocarbons from CO2 is remarkable, as it represents the solid-state analogue to molecular Fe1+ coordination compounds with the same catalytic capability under homogeneous catalytic environments. This finding highlights a conceptual bridge between heterogeneous and homogenous catalysis and contributes significantly to our fundamental understanding of the FeNC catalyst function in the CO2RR.
Restructuring-induced catalytic activity is an intriguing phenomenon of fundamental importance to rational design of high-performance catalyst materials. We study three copper-complex materials for electrocatalytic carbon dioxide reduction. Among them, the copper(II) phthalocyanine exhibits by far the highest activity for yielding methane with a Faradaic efficiency of 66% and a partial current density of 13 mA cm-2 at the potential of - 1.06 V versus the reversible hydrogen electrode. Utilizing in-situ and operando X-ray absorption spectroscopy, we find that under the working conditions copper(II) phthalocyanine undergoes reversible structural and oxidation state changes to form ~ 2 nm metallic copper clusters, which catalyzes the carbon dioxide-to-methane conversion. Density functional calculations rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion-ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions.
Direct electrochemical reduction of CO2 to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO2-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO2 to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M–Nx moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe–N–C and especially Ni–N–C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M–Nx moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomicscale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M–Nx moieties and it provides predictive guidelines for the rational design of selective carbon-based CO2 reduction catalysts.
Electrocatalytic conversion of carbon dioxide has gained much interest for the synthesis of value-added chemicals and solar fuels. Important issues such as high overpotentials and competition of hydrogen evolution still need to be overcome for deeper insight into the reaction mechanism in order to steer the selectivity towards specific products. Herein we report on several metalloprotoporphyrins immobilized on a pyrolytic graphite electrode for the selective reduction of carbon dioxide to formic acid. No formic acid is detected on Cr-, Mn-, Co- and Fe-protoporphyrins in perchloric acid of pH 3, while Ni-, Pd-, Cu- and Ga-protoporphyrins show only a little formic acid. Rh, In and Sn metal centers produce significant amounts of formic acid. However, the faradaic efficiency varies from 1% to 70% depending on the metal center, the pH of the electrolyte and the applied potential. The differentiation of the faradaic efficiency for formic acid on these metalloprotoporphyrins is strongly related to the activity of the porphyrin for the hydrogen evolution reaction. CO2 reduction on Rh-protoporphyrin is shown to be coupled strongly to the hydrogen evolution reaction, whilst on Sn- and In-protoporphyrin such strong coupling between the two reactions is absent. The activity for the hydrogen evolution increases in the order In < Sn < Rh metal centers, leading to faradaic efficiency for formic acid increasing in the order Rh < Sn < In metal centers. In-protoporphyrin is the most stable and shows a high faradaic efficiency of ca. 70%, at a pH of 9.6 and a potential of −1.9 V vs RHE. Experiments in bicarbonate electrolyte were performed in an attempt to qualitatively study the role of bicarbonate in formic acid formation.
The reaction mechanisms for the reduction of carbon dioxide to formaldehyde catalyzed by bis(tricyclopentylphosphine) metal complexes, [RuH2(H2)(PCyp3)2] (1Ru), [FeH2(H2)(PCyp3)2] (1Fe) and [OsH4(PCyp3)2] (1Os), were studied computationally by using the density functional theory (DFT). 1Ru is a recently reported highly efficient catalyst for this reaction. 1Fe and 1Os are two analogues of 1Ru with the Ru atom replaced by Fe and Os, respectively. The total free energy barriers of the reactions catalyzed by 1Ru, 1Fe and 1Os are 24.2, 24.0 and 29.0 kcal/mol, respectively. With a barrier close to the experimentally observed Ru complex, the newly proposed iron complex is a potential low-cost catalyst for the reduction of carbon dioxide to formaldehyde under mild conditions. The electronic structures of intermediates and transition states in these reactions were analyzed by using the natural bond orbital theory.
Electrolysis is a potential useful approach for converting carbon dioxide into chemicals and fuels. The most active electrocatalysts for efficiently mediating the carbon dioxide reduction reaction (CO2RR) have long been assumed to be solid silver, gold, or copper. However, there is an emerging body of data showing that molecular catalysts can operate at levels of performance commensurate with solid-state catalysts. These recent advances in deploying molecular catalysts present entirely new opportunities for understanding CO2RR in electrochemical reactors, and tailoring active sites for the selective formation of CO2RR products. This Perspective highlights the recent advances and the opportunities for the implementation of molecular electrocatalysts into carbon dioxide electrolyzers.
Developing affordable electrocatalysts to facilitate the reduction of carbon dioxide (CO2) to high-value products with high selectivity, efficiency, and large current densities, is a critical step for the production of liquid carbon-based fuels. In this work, we show that inexpensive post-transition metals (tin (Sn) and lead (Pb)) and their alloys (PbSn) are excellent cathode materials to reduce CO2 in an ionic liquid/acetonitrile/water electrolyte media. Electrochemical impedance spectroscopy (EIS) measurements show that the PbSn alloys exhibit lower charge-transfer resistance when compared to the pure metal electrodes, as supported by electronic structure calculations. Current densities as high as 60 mA/cm 2 are observed with optimal mixtures of ionic liquid, acetonitrile, and water. Reduction product analysis identifies carbon monoxide (CO) and formate (HCOO-) as primary reduced products, with higher selectivity towards formate. Faradaic efficiency for formate on pure Pb and pure Sn was determined to be 80 ± 4% and 86 ± 3% respectively. FE% improves as either Pb is incorporated into Sn or vice versa, and a maximum FE of 91 ± 3% for both 50 % and 40 % Pb composition.
Pyrolyzing Cu(II) precursors absorbed on carbon black gave an eletrocatalytic material for the electroreduction of CO2. Controlled potential electrolysis and GC analysis revealed an unexpected selectivity towards methane even in alkaline media, with a methane Faradaic efficiency as high as 42%, a partial current density of 100 mA/cm² (at −1 V vs. RHE) and a methane/ethylene ratio of 4:1. XPS, EXAFS, and EDX mapping results indicate a single-site Cu(I) center as the catalytically active site. The limited size of the active sites is believed to be crucial for the preferential formation of methane over ethylene.
The photoelectrocatalytic CO2 reduction into value-added carbon-based energetic molecules is a promising strategy to store solar energy into chemicals, which lessen the concentration of atmospheric contamination. In this study, the graphitic supported and multiple functionalized nanowire TiO2 semiconductor was firstly used to CO2 reduction with water under photoelectrocatalytic conditions. These photocathodes of R-TiO2@GS were designed to be functionalized by organic ligands that catch CO2 and control the C-C coupling as the Calvin cycle in natural plants. These new organic–inorganic composite electrodes were facile to be prepared and well characterized by using NMR, UV–vis, FTIR, PL, TRPL, EIS and XPS spectra; XRD patterns; SEM, TEM images. The transient absorption spectra of photocathodes demonstrate the efficiency of electron transfer between the Eosin Y and semiconductor, ensuring CO2 reduction. Their Mott–Schottky plots show that the flat band potentials are improved by organic ligands, favoring the ethanol production. The light quantum efficiency of the best photoelectrocatalytic cell of S-TiO2@GS│SCE│Pt reaches to 1.0 % that is 2 times better than natural plant. To better understand the working process of the photoelectrocatalytic cell, a plausible mechanism of CO2 reduction in water was carefully proposed as well.
An absolute metal-free in-plane heterosystem consisting of g-C3N4 as scaffold and embedded half-metallic C(CN)3 as cocatalysts was conceptually designed for photoconversion of CO2. The no-slot joint between half-metallic C(CN)3 and g-C3N4 through covalent bonding generates a unique two-dimensional, π-conjugated hybrid structure, allowing obstacle-free transferring of the photogenerated electrons in g-C3N4 into C(CN)3 via an intrinsic driving force. Our theoretical calculations together with the in-situ Fourier transform infrared spectra indicate that the most negative charge distribution and binding energy with CO2 for C(CN)3 allow outstanding capture and chemical activation capacity toward CO2, and subsequently enable the optimized heterosystem to exhibit highly efficient and selective photocatalytic reduction of CO2 into CO, 7.8 and 1.9 times those produced with pristine and Pt-modified g-C3N4, respectively. The no-slot joint of organic half-metal cocatalysts with g-C3N4 may open up new opportunities for metal-free, polymer-based photocatalytic systems for CO2 conversion and solar fuel generation.
It is generally believed that CO2 electroreduction to multi‐carbon products such as ethanol or ethylene may be catalyzed with significant yield only on metallic copper surfaces, implying large ensembles of copper atoms. Here, we report on an inexpensive Cu‐N‐C material prepared via a simple pyrolytic route that exclusively feature single copper atoms with a CuN4 coordination environment, atomically dispersed in a nitrogen‐doped conductive carbon matrix. This material achieves aqueous CO2 electroreduction to ethanol at a Faradaic yield of 55% under optimized conditions (electrolyte: 0.1 M CsHCO3 , potential: ‐1.2V vs. RHE and gas‐phase recycling set up), as well as CO electroreduction to C2‐products (ethanol and ethylene) with a Faradaic yield of 80%. During electrolysis the isolated sites transiently convert into metallic copper nanoparticles, as shown by operando XAS analysis, which are likely to be the catalytically active species. Remarkably, this process is reversible and the initial material is recovered intact after electrolysis.
Flowing CO 2 boosts a molecular catalyst
Molecular electrocatalysts for CO 2 reduction have often appeared to lack sufficient activity or stability for practical application. Ren et al. now show that design of the surrounding electrochemical cell can substantially boost both features. They directly exposed a known molecular catalyst, cobalt phthalocyanine, to gaseous CO 2 in a flow cell architecture, rather than an aqueous electrolyte. The configuration accommodated current densities exceeding 150 milliamperes per square centimeter, with longevity limited by local proton concentration rather than catalyst stability.
Science , this issue p. 367
Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M-N-C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M-N-C materials toward four-electron oxygen reduction reaction (ORR) to H2O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H2O2, a future green "dream" process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H2O2 production over a series of M-N-C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M-N
x
sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M-N-C catalysts on the electrocatalytic activity/selectivity for ORR (H2O2 and H2O products) and H2O2 reduction reaction (H2O2RR). Co-N-C catalyst was uncovered with outstanding H2O2 productivity considering its high ORR activity, highest H2O2 selectivity, and lowest H2O2RR activity. The activity-selectivity trend over M-N-C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co-N-C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H2O2 productivity over Co-N-C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide gcatalyst-1 h-1 at a current density of 50 mA cm-2.
The electrochemical CO2 reduction reaction (CO2RR) is a promising technology for converting waste CO2 into chemicals which could be used as feeds stock for the chemical industry or as synthetic fuels. The technological viability of this process, however, is contingent on finding affordable and efficient catalyst. Recently, carbon‐based solid catalyst materials containing small amounts of nitrogen and transition metals (MNC) have emerged as a selective and cost efficient alternative to noble metal catalysts for the direct electrochemical reduction of CO2 to CO. In addition, other products have also been reported, including formic acid and methane. In this perspective, we offer a focused discussion of recent advances in the field of MNC catalysts for the CO2RR. We discuss the different factors that influence the catalytic performance of MNC focusing on DFT‐guided experimental studies aiming to elucidate key experimental parameters and molecular descriptors that control the activity and selectivity of this class of materials. We close addressing the remaining challenges, and take a look forward into future studies.
We report an experimental-computational study of mechanistic reaction pathways during the electrochemical reduction of CO2 to CH4, catalyzed by solid-state, single-site Fe-N-C catalysts. Fe-N-C catalysts feature molecularly dispersed catalytically active Fe-N motifs and represent a type of non-Cu-based catalysts that yield “beyond CO” hydrocarbon products. The various multi-step mechanistic pathways toward hydrocarbons with these catalysts has never been studied before and is the focus of this study. A number of different reactant molecules with varying formal carbon redox states, more specifically CO2, CO, CH2O, CH3OH and formate were electrochemically converted at the Fe-N sites, yet only CO2, CO and CH2O could be protonated into methane. Also, we observed a distinctly different pH dependence of the catalytic CH4 evolution from CO and CH2O, suggesting differences in the proton participation of rate determining steps. In comparing the experimental observations with Density Functional Theory (DFT) -derived Free Energy Diagrams of reactive intermediates along the reaction coordinates, we unraveled the distinctly different dominant mechanistic pathways and roles of CO and CH2O along the catalytic CO2-to-CH4 cascade and their rate-determine-steps (RDS). We close with the first comprehensive reaction network of the CO2 electroreduction on a M-N-C catalyst. Our findings offer valuable insights in the catalysis of the CO2RR on single site Fe-N-C catalysts that may prove useful in developing efficient, non-Cu-based catalysts for direct electrochemical hydrocarbons production.
To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.
This review provides an overview of the literature regarding heterogeneous molecular catalysts for electrochemical CO2 reduction (ECR). Fundamental aspects of the science, including aggregation, electrochemical rate laws, and electrode-catalyst electronic coupling, are discussed to provide a solid foundation on which to design experiments and interpret results. Mechanistic aspects of ECR are presented based on electrokinetic and spectroscopic measurements as well as density functional theory (DFT) calculations. Consensus is improving for electrokinetic measurements, but the redox state of the metal center under reaction conditions and DFT reaction pathways lack agreement in the literature. Concerning the tunable aspects of the molecular catalyst, the impacts of the metal center, ligand substituents, and electrode support on the activity and selectivity toward ECR are presented with an emphasis on those studies that controlled for aggregation and minimized mass-transport limitations. Extended three-dimensional (3D) structures such as polymers, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) are discussed as highly tunable architectures that begin to mimic the catalytic pockets of enzyme active sites. To achieve the full potential of these catalysts, design principles must emerge based on a combination of deconvoluting measurements to extract intrinsic catalyst properties and more reliable theoretical calculations to predict reaction pathways.
The electrochemical CO2 reduction reaction (CO2RR) to pure CO streams in electrolyzer devices is poised to be the most likely process for near-term commercialization and deployment in the polymers industry....
Electrocatalytic CO2 reduction to CO emerges as a potential route of utilizing the excessively emitted CO2. Metal‐N‐C hybrid structures have shown unique activities, of which the active centers and reaction mechanisms, however, remain unclear due to the ambiguity in true atomic structures for prepared catalysts. Herein, combining density functional theory calculations and experimental studies, we explored the reaction mechanisms on well‐defined metal‐N4 sites by using metal phthalocyanines as model catalysts. Our theoretical calculations reveal that cobalt phthalocyanine exhibits the optimum activity for CO2 reduction to CO, because of the moderate *CO binding energy on the Co site which accommodates the *COOH formation and the *CO desorption. It is further confirmed by experimental studies, where cobalt phthalocyanine delivers the best performance, with a maximal CO Faradaic efficiency reaching 99%, and maintains the stable performance for over 60 hours.
Using concepts of biomimetic catalysis, a kind of tin porphyrin‐based porous aromatic framework ( SnPor@ PAF ) with broad and strong optical absorption in the visible light region was successfully synthesized and subsequently used in the aerobic oxidation of sulfides to sulfoxides under ambient conditions and visible light irradiation, in which exhibited enzyme‐like features of high efficiency and high selectivity. More interestingly, heterogeneous SnPor@PAF was naturally regarded as an intriguing and versatile photosensitizer for photocatalytic transformation and could be reused several times because of its robust and rigid porphyrin framework. As expected, their π‐conjugated structure characteristic in the molecular skeleton might facilitate the activation of molecular oxygen under mild reaction conditions and promoted the production of reactive oxygen species (singlet oxygen ( ¹ O 2 ) and superoxide radical anion (O 2 .− )), which would involve energy transfer and/or electron transfer process. Experimental investigations including emission quenching experiment, oxygen‐isotope labelling, typical inhibition experiments, classical fluorescence probe study, photo‐oxidation of α‐terpinene and in situ electron spin resonance, could provide a mechanistic insight into the photocatalytic reactions.
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Electrochemical reduction reaction of carbon dioxide (CO2RR) to carbon monoxide (CO) is the basis for the further synthesis of more complex carbon‐based fuels or attractive feedstock. Single‐atom catalysts have unique electronic and geometric structures with respect to their bulk counterparts, thus exhibiting unexpected catalytic activities. Herein we report a nitrogen‐anchored Zn single atom catalyst for CO formation from CO2RR with high catalytic activity (onset overpotential down to 24 mV), high selectivity (Faradaic efficiency for CO (FECO) up to 95% at ‐0.43 V), remarkable durability (>75 h without decay of FECO) and large turnover frequency (TOF, up to 9969 h‐1). Further experimental and DFT results indicate that the four‐nitrogen‐anchored Zn single atom (Zn‐N4) is the main active site for CO2RR with low free energy barrier for the formation of *COOH as the rate‐limiting step.
Understanding the surface reactivity of CO, which is a key intermediate during electrochemical CO2 reduction, is crucial for the development of catalysts that selectively target desired products for the conversion of CO2 to fuels and chemicals. In this study, a custom-designed electrochemical cell is utilized to investigate planar polycrystalline copper as an electrocatalyst for CO reduction under alkaline conditions. Seven major CO reduction products have been observed including various hydrocarbons and oxygenates which are also common CO2 reduction products, strongly indicating that CO is a key reaction intermediate for these further-reduced products. A comparison of CO and CO2 reduction demonstrates that there is a large decrease in the overpotential for C–C coupled products under CO reduction conditions. The effects of CO partial pressure and electrolyte pH are investigated; we conclude that the aforementioned large potential shift is primarily a pH effect. Thus, alkaline conditions can be used to increase the energy efficiency of CO and CO2 reduction to C–C coupled products, when these cathode reactions are coupled to the oxygen evolution reaction at the anode. Further analysis of the reaction products reveals common trends in selectivity that indicate both the production of oxygenates and C–C coupled products are favored at lower overpotentials. These selectivity trends are generalized by comparing the results on planar Cu to current state-of-the-art high-surface-area Cu catalysts, which are able to achieve high oxygenate selectivity by operating at the same geometric current density at lower overpotentials. Combined, these findings outline key principles for designing CO and CO2 electrolyzers that are able to produce valuable C–C coupled products with high energy efficiency.
The CO2 electrochemical reduction reaction (CO2RR) is a promising technology for converting CO2 into chemicals and fuels, using surplus electricity from renewable sources. The technological viability of this process, however, is contingent on finding affordable and efficient catalysts. A range of materials containing abundant elements, such as N, C, and non‐noble metals, ranging from well‐defined immobilized complexes to doped carbon materials have emerged as a promising alternative. One of the main products of the CO2RR is CO, which is produced on these catalysts with selectivities comparable to those of noble metal catalysts. Furthermore, other valuable products, such as formic acid, hydrocarbons, and alcohols, have also been reported. The factors that control the catalytic performance of these materials, however, are not yet fully understood. A review of recent work is presented on heterogeneous nitrogen‐containing carbon catalysts for the CO2RR. The synthesis and characterization of these materials as well as their electrocatalytic performance are discussed. Combined experimental and theoretical studies are included to bring insight on the active sites and the reaction mechanism. This knowledge is key for developing optimal catalyst materials that meet the requirement in terms of activity, selectivity, and stability needed for commercial applications. Materials containing abundant elements such as N, C, and non‐noble metals have recently emerged as a promising alternative to metal‐based catalysts for the CO2 reduction reaction. Such catalysts include immobilized complexes, metal organic frameworks, and doped carbon materials.
A general and simple solvent-free procedure using direct heating of a ball-milled mixture of L-histidine-Fe2O3-FeCl3 is developed for the synthesis of iron and nitrogen doped porous carbon electrocatalysts. Through adjustment of the reactant ratios and the pyrolysis temperature, a series of electrocatalysts are easily obtained with varying activities for electrochemical CO2 reduction reaction (CO2RR). The electrocatalyst synthesized from L-histidine-Fe2O3-FeCl3 at a 4:1:0.25 component ratio at 1000 oC exhibits the highest faradaic efficiency of 83% for CO2 to CO conversion at a small overpotential (360 mV) in aqueous media. The use of a number of characterization techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy, and nitrogen sorption experiments, reveals that both Fe2O3 and FeCl3 contribute to the iron doping and formation of porosity. As a result, they are both crucial to produce the optimal CO2RR electrocatalyst. Correlation of the CO2RR activity with the carbon structure suggests that the degree of graphitization of the carbon electrocatalysts plays an important role in their CO2RR performance.
Reduction of CO2 holds the key to solving two major challenges taunting the society-clean energy and clean environment. There is an urgent need for the development of efficient non-noble metal-based catalysts that can reduce CO2 selectively and efficiently. Unfortunately, activation and reduction of CO2 can only be achieved by highly reduced metal centers jeopardizing the energy efficiency of the process. A carbon monoxide dehydrogenase inspired Co complex bearing a dithiolato ligand can reduce CO2, in wet acetonitrile, to CO with ∼95% selectivity over a wide potential range and 1559 s-1 rate with a remarkably low overpotential of 70 mV. Unlike most of the transition-metal-based systems that require reduction of the metal to its formal zerovalent state for CO2 reduction, this catalyst can reduce CO2 in its formal +1 state making it substantially more energy efficient than any system known to show similar reactivity. While covalent donation from one thiolate increases electron density at the Co(I) center enabling it to activate CO2, protonation of the bound thiolate, in the presence of H2O as a proton source, plays a crucial role in lowering overpotential (thermodynamics) and ensuring facile proton transfer to the bound CO2 ensuring facile (kinetics) reactivity. A very covalent Co(III)-C bond in a Co(III)-COOH intermediate is at the heart of selective protonation of the oxygen atoms to result in CO as the exclusive product of the reduction.
Major efforts are devoted to find efficient and selective electrocatalysts for solar fuel generation, thus enabling accelerated development of such technologies. A series of electrocatalysts from the metal–nitrogen–carbon (M–N–C) family was synthesized by a sacrificial support method (SSM). The salts of Cu, Mo, Pr, and Ce were employed as metal-precursors while the N–C network was formed by the high temperature pyrolysis of aminoantipyrine (AAPyr) organic compound. The structural and chemical features of the obtained electrocatalysts were characterized by SEM, XRD, TEM, BET, and XPS methods. Electrochemical characterization was performed towards CO2 electroreduction (CO2ER) and Hydrogen Evolution Reactions (HER) in neutral media. The metal-free N-doped carbon had a notable activity towards CO2ER, with the predominant formation of carbon monoxide (CO). This was also confirmed by isotopic labeling experiments, to exclude the possible contribution of the electrocatalyst as a carbon source. Upon introducing the metals into the carbon structure, the electrocatalytic activity towards HER increased. Particularly impressive current densities were achieved for the Cu and Mo-containing electrodes, where the presence of metal–N bonds was identified. These structural motifs are likely to be the electrocatalytic centers in the structure for the HER, which will be further studied for other metal-containing nanocarbons in our laboratories.
Transition metal is known to influence electrochemical activities over transition metals (M) and nitrogen (N)-codoped carbon (M–N–C) catalysts. However, champion transition metals centers in M–N–C for catalyzing CO2 reduction reaction (CO2RR) remain unclear, hindering further catalyst development with enhanced performance. Herein, we report the investigation of effects of five transition metals (Cr, Mn, Fe, Co, Ni) on CO2RR activities and mechanisms using metal-doped nitrogenated carbon nanosheets as model catalysts fabricated via a novel space-confinement-assisted molecular-level complexing approach. Analyzing N 1s XPS spectra confirmed the formation of M−N complexes via the coordination of metals atoms with pyridinic N, which was identified as the active species in CO2RR. According to activity descriptors including overpotentials, Faradaic efficiency (FE) and Turnover Frequency (TOF) per metal site, we here established that Fe and Ni are more active than Co, Mn, and Cr in M–N–C for the reduction of CO2 to CO. The main role of Fe is to reduce overpotentials, exhibiting the lowest onset overpotential of 0.19 V to yield CO on Fe–N–C. Ni can drastically improve CO selectivity and reaction rates, yielding the highest CO Faradaic efficiency of 96%, partial current density of −8.2 mA cm⁻², and TOF of 1060 h⁻¹ at a moderate overpotential of 0.65 V. Mechanism explorations reveal that CO2RR on M–N–C (M = Fe, Cr, Mn) undergoes the formation of a *COOH intermediate as the rate-determining step, whereas M–N–C (M = Ni, Co) catalyzes CO2RR via the transfer of the first electron to form a *CO2[rad]− species. On the basis of the findings, we suggest doping Fe and/or Ni to design advanced M–N–C for CO2 electroreduction.
Metal-nitrogen-carbon (M-N-C) catalysts represent a potential means of reducing cathode catalyst costs in low temperature fuel cell cathodes. Knowledge-based improvements have been hampered by the difficulty to deconvolute active site density and intrinsic turn over frequency. In the present work, M-N-C catalysts with a variety of secondary nitrogen precursors are addressed. CO Chemisorption in combination with Mossbauer spectroscopy are utilized in order to unravel previously inac-cessible relations between active site density, turn-over-frequency, and active site utilization, which provide a more funda-mental description and understanding of the origin of the catalytic reactivity; they also provide guidelines for further im-provements. Secondary nitrogen precursors impact quantity, quality, dispersion, and utilization of active sites in distinct ways. Secondary nitrogen precursors with high nitrogen content and micropore etching capabilities are most effective in improving catalysts performance.
A major impediment of the electrocatalytic CO2 reduction reaction (CRR) is the lack of electrocatalysts with both high efficiency and good selectivity toward liquid fuels or other valuable chemicals. Effective strategies for the design of electrocatalysts are yet to be discovered to substitute the conventional trial-and-error approach. This work shows that a combination of the density functional theory (DFT) computation and experimental validation on molecular scaffolding to coordinate the metal active centers presents a new molecular-level strategy for the development of electrocatalysts with high CRR selectivity toward hydrocarbon/alcohol. Taking the most widely investigated Cu as a probe, our study reveals that the use of graphitic carbon nitride (g-C3N4) as a molecular scaffold allows for an appropriate modification of the electronic structure of Cu in the resultant Cu-C3N4 complex. As a result, adsorption behavior of some key reaction intermediates can be optimized on the Cu-C3N4 surface, which greatly benefits the activation of CO2 and leads to a more facile CO2 reduction to desired products as compared with those on the Cu(111) surface and other kinds of Cu complexes formed on nitrogen- doped carbons. Remarkably, different from the most studied elementary metal surfaces, an intramolecular synergistic catalysis with dual active centers was for the first time observed on the Cu-C3N4 complex model, which possesses unique capability for the generation of C2 products. A good agreement between electrochemical measurements and the DFT analysis of CRR has been achieved based on the newly designed and synthesized Cu-C3N4 electrocatalyst.
Utilizing solar energy to fix CO2 with water into chemical fuels and oxygen, a mimic process of photosynthesis in nature, is becoming increasingly important but still challenged by low selectivity and activity, especially in CO2 electrocatalytic reduction. Here, we report transition-metal atoms coordinated in a graphene shell as active centers for aqueous CO2 reduction to CO with high faradic efficiencies over 90% under significant currents up to ∼60 mA/mg. We employed three-dimensional atom probe tomography to directly identify the single Ni atomic sites in graphene vacancies. Theoretical simulations suggest that compared with metallic Ni, the Ni atomic sites present different electronic structures that facilitate CO2-to-CO conversion and suppress the competing hydrogen evolution reaction dramatically. Coupled with Li⁺-tuned Co3O4 oxygen evolution catalyst and powered by a triple-junction solar cell, our artificial photosynthesis system achieves a peak solar-to-CO efficiency of 12.7% by using earth-abundant transition-metal electrocatalysts in a pH-equal system.
Electrochemical reduction of carbon dioxide (CO2) to value-added carbon products is a promising approach to reduce the CO2 level and mitigate energy crisis. However, poor product selectivity is still a major obstacle to the development of CO2 reduction. Here we demonstrated exclusive Ni-N4 sites through a topo-chemical transformation strategy, bringing unprecedentedly high activity and selectivity for CO2 reduction. Topo-chemical transformation by carbon layer coating successfully ensures preservation of Ni-N4 structure to maximum extent and avoids the agglomeration of Ni atoms to particles, providing abundant active sites for the catalytic reaction. Ni-N4 structure exhibits excellent activity for electrochemical reduction of CO2 with particularly high selectivity, achieving high faradaic efficiency over 90% for CO in the potential range from −0.5 to −0.9 V and give a maximum faradaic efficiency of 99% at -0.81 V with a current density of 28.6 mA cm-2. We anticipate exclusive catalytic sites will shed new light on the design of high-efficiency electrocatalyst for CO2 reduction.
An electronic structure analysis of two nickel(II) tetrapyrrole complexes bearing β-alkylthio substituents, NiOMTP and NiOETPz, has been carried out through a combination of high-resolution XPS experiments and DFT calculations. The Ni 2p XPS spectra show a 0.5 eV shift to higher energy of the Ni 2p3/2 and Ni 2p1/2 binding energies on going from the porphyrin to the porphyrazine complex. This shift, which is well-reproduced by relativistic spin-orbit ZORA calculations, is indicative of a depletion of electron density on the central metal. Such a depletion of electron density is related to the macrocycle-induced changes in the metal-ligand interactions. In the porphyrazine complex both the ligand to metal σ donation and the metal to ligand π-back donation increase. The latter increases slightly more than the former, however, leading to a decrease of electron density on the central metal.
The electrochemical reduction of CO2 to useful molecules offers an elegant technological solution to current energy security and sustainability issues because it sequesters carbon from the atmosphere, provides an energy storage solution for intermittent renewable sources, and can be used to produce fuels and industrial chemicals. Nanostructured carbon materials have been extensively used to catalyse some key electrochemical processes because of their excellent electrical conductivity, chemical stability, and abundant active sites. This progress report focuses on nanostructured carbon materials, namely graphene materials, carbon nanotubes, porphyrin materials, nanodiamond, and glassy carbon, which have recently shown promise as high performing CO2 reduction electrocatalysts and supports. Along with discussion regarding materials synthesis, structural characterisation, and electrochemical performance characterisation techniques used, this report will discuss the findings of recent computational CO2RR studies which have been key to elucidating active sites and reaction mechanisms, and developing strategies to break conventional scaling relationships. Lastly, challenges and future perspective of these carbon-based materials for CO2 reduction applications will be given. Much work is still required to realise the commercial viability of the technology, but advanced experimental techniques coupled with theoretical calculations are expected to facilitate future development of the technology.
Rising levels of carbon dioxide (CO2) are of significant concern in modern society, as they lead to global warming and consequential environmental and societal changes. It is of importance to develop industries with a zero or negative CO2 footprint. Electrochemistry, where one of the reagents is electrons, is an environmentally clean technology that is capable of addressing the conversion of CO2 to value-added products. The key factor in the process is the use of catalytic electrode materials that lead to the desired reaction and product. Significant progress in this field has been achieved in the past two years. This review discusses the progress in the development of electrocatalysts for CO2 reduction achieved during this time period.
Currently, no catalysts are completely selective for the electrochemical CO2 Reduction Reaction (CO2RR). Based on trends in density functional theory calculations of reaction intermediates we find that the single metal site in a porphyrine-like structure has a simple advantage of limiting the competing Hydrogen Evolution Reaction (HER). The single metal site in a porphyrine-like structure requires an ontop site binding of hydrogen, compared to the hollow site binding of hydrogen on a metal catalyst surface. The difference in binding site structure gives a fundamental energy-shift in the scaling relation of ∼0.3 eV between the COOH* vs. H* intermediate (CO2RR vs. HER). As a result, porphyrine-like catalysts have the advantage over metal catalyst of suppressing HER and enhancing CO2RR selectivity.
Selective electrochemical reduction of CO2 into energy-dense organic compounds is a promising strategy for using CO2 as a carbon source. Herein, we investigate a series of iron-based catalysts synthesized by pyrolysis of Fe-, N- and C- containing precursors for the electroreduction of CO2 to CO in aqueous conditions and demonstrate that the selectivi-ty of these materials for CO2 reduction over proton reduction is governed by the ratio of isolated FeN4 sites vs. Fe-based nanoparticles. This ratio can be synthetically tuned to generate electrocatalysts producing controlled CO/H2 ratios. It notably allows preparing materials containing only FeN4 sites, which are able to selectively reduce CO2 to CO in aqueous solution with Faradaic yields over 90% and at low overpotential.
Electrochemical and photochemical reduction of CO2, or a smart combination of both, are appealing approaches for the storage of renewable, intermittent energies and may lead to the production of fuels and of value added chemicals. By using only earth abundant metal (Cu, Ni, Co, Mn, Fe) complexes, cheap electrodes and/or cheap sacrificial electron donors and visible light sensitizers, systems functioning with molecular catalysts have been recently designed, showing promising results in particular for the two electrons reduction of the carbon dioxide. By combining experimental and mechanistic studies, key parameters controlling the catalysis efficiency have been deciphered, opening the way to the design of future, more efficient and durable catalysts, as well as to the development of electrochemical or photo-electrochemical cells, all being key steps for the emergence of applied devices. The most recent advances related to these issues are discussed in this review.
Nickel-nitrogen-modified graphene (Ni-N-Gr) is fabricated and Ni-N coordination sites on Ni-N-Gr as active centers effectively reduce CO2 to CO. The faradaic efficiency for CO formation reaches 90% at -0.7 to -0.9 V versus RHE, and the turnover frequency for CO production comes up to ≈2700 h(-1) at -0.7 V versus RHE.
The selective electrocatalytic conversion of CO2 into useful products is a major challenge in facilitating a closed carbon cycle. Here, based on first-principles calculations combined with computational hydrogen electrode model, we report a curvature dependent selectivity of CO2 reduction on cobalt-porphyrin nanotubes which are thermodynamically stable displaying tunable geometric and electronic properties with tube radius. We have found that CO production is preferred on nanotubes with larger diameter, and the predicted current density from microkinetics is larger than that on Au, the best metal catalyst for CO production from CO2 electroreduction. In contrast, the highly curved nanotubes with small radii tend to further catalyze CO reduction to CH4 gas and the overpotential is much lower as compared with the cases on Cu surfaces. The selectivity together with the feasibility of synthesis makes cobalt-porphyrin nanotubes very promising for CO2 conversion.