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Influence of the Molecular and Mesoscopic Structure on the Electrocatalytic Activity of Pyrolysed CoTMPP in the Oxygen Reduction
Abstract and Figures
Novel oxygen reduction electrocatalysts are obtained by thermo lysis of chelates such as cobalt tetramethoxy phenyl porphyrin CoTMPP in presence of the foaming agent FeC2O4. EXAFS spectra point to the presence of molecular Fe N4 and Co N4 centres embedded in a graphite like matrix. The foaming agent provokes the generation of a porous material 800 m2 g exhibiting a high fraction of mesopores 5 to 30 nm . Adding elemental sulphur to the precursors CoTMPP and FeC2O4, the catalytic activity in ORR of the final catalyst significantly im proves due to an increase of mesopores and the formation of extended graphene layers in a glassy carbon type material. The added sulphur not only fa cili tates the removal of inorganic by products formed in the process of pyrolysis, but also provides an appropriate environment for cataly tic centres so that the formation of undesired H2O2 is signifi cantly decreased. In RDE measurements a catalytic activity in ORR equal to commercial platinum catalysts has been achieved
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... To overcome this limitation, the first method that enabled the preparation of Me-N-C catalysts without additional carbon support, is the oxalate-supported pyrolysis of porphyrins [12]. In this approach oxalate works as a structure-forming agent (SFA) as the final catalyst resembles its morphology [13][14][15]. In addition to this, the use of sulfur strongly affected the morphology and performance of the catalysts [16,17]. ...
In this work, the influence of the structure-forming agent on the composition, morphology and oxygen reduction reaction (ORR) activity of Fe-N-C catalysts was investigated. As structure-forming agents (SFAs), dicyandiamide (DCDA) (nitrogen source) or oxalic acid (oxygen source) or mixtures thereof were used. For characterization, cyclic voltammetry and rotating disc electrode (RDE) experiments were performed in 0.1 M H2SO4. In addition to this, N2 sorption measurements and Raman spectroscopy were performed for the structural, and elemental analysis for chemical characterization. The role of metal, nitrogen and carbon sources within the synthesis of Fe-N-C catalysts has been pointed out before. Here, we show that the optimum in terms of ORR activity is achieved if both N-and O-containing SFAs are used in almost similar fractions. All catalysts display a redox couple, where its position depends on the fractions of SFAs. The SFA has also a strong impact on the morphology: Catalysts that were prepared with a larger fraction of N-containing SFA revealed a higher order in graphitization, indicated by bands in the 2nd order range of the Raman spectra. Nevertheless, the optimum in terms of ORR activity is obtained for the catalyst with highest D/G band ratio. Therefore, the results indicate that the presence of an additional oxygen-containing SFA is beneficial within the preparation.
... To overcome this limitation, the first method that enabled the preparation of Me-N-C catalysts without additional carbon support, is the oxalate-supported pyrolysis of porphyrins developed at the Hahn-Meitner-Institute in Berlin (now Helmholtz-Zentrum Berlin) [12]. In this approach oxalate works as a structure forming agent (SFA) as the final catalyst resembles its morphology [13][14][15]. In addition to this, the use of sulfur strongly affected the morphology and performance of the catalysts [16][17]. ...
In this work the influence of the structure forming agent on the composition, morphology and oxygen reduction reaction (ORR) activity of Fe-N-C catalysts was investigated. As structure forming agent (SFA), dicyandiamide (DCDA) (nitrogen source) or oxalic acid (oxygen source) or mixtures thereof were used. For characterization, cyclic voltammetry and rotating disc electrode (RDE) experiments were performed in 0.1 M H2SO4. In addition to this, N2 sorption measurements and Raman spectroscopy were performed for the structural characterization. The role of metal, nitrogen and carbon sources within the synthesis of Fe-N-C catalysts has been pointed out before. Here, we show that the optimum in terms of ORR activity is achieved if both N- and O-containing SFAs are used in almost similar fractions. All catalysts display a redox couple, whereat its position depends on the fractions of SFAs. The SFA has also a strong impact on the morphology: Catalysts that were prepared with a larger fraction of N-containing SFA revealed a higher order in graphitization, indicated by bands in the 2nd order range of the Raman spectra. Nevertheless, the optimum in terms of ORR activity is obtained for the catalyst with highest D/G band ratio. Therefore, the results indicate that the presence of an additional oxygen-containing SFA is beneficial within the preparation.
More efficient energy conversion systems may help to reduce the use of fossil fuels and the emission of greenhouse gases. Polymer Electrolyte Membrane Fuel Cells are such devices which are of particular interest for automotive applications. Unfortunately, cost issues are still limiting the application of this technology in a highly competitive market. An important part of this is the inherently high cost of platinum which is commonly used as electrocatalyst. But recently, the replacement of platinum by NNMC has come into the focus of reach. The most promising approaches – comprising the use of crystalline phases and catalysts with molecular active centers – are described in this entry; limitations of both classes of materials are discussed.
Fe-based electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) were produced using a sputter deposition process involving a carbon paper (CP) and a NH3 heat treatment environment. The Fe-N-CP sample reacted with ammonia for 10 min at 950 degrees C displayed a greater current density than the samples reacted with ammonia for other reaction times. The impedance of the samples treated for 10 min with ammonia shows the highest value, which means much higher ligand formation and lower electric conductivity, which are consistent with the results of cyclic voltammetry (CV). This is directly due to the formation of ligands between the Fe and CP used when exposed to a nitrogen environment. While the current density of the Fe-based electrocatalysts under review did not exceed that of standard Pt/C electrocatalysts, the results herein suggest that nonprecious metal electrodes may be a viable alternative in PEMFCs. (C) 2013 The Japan Society of Applied Physics
In 2007, the field of oxygen reduction reaction (ORR) by porphyrins was partially reviewed by Y. Kiros. The new development reported under the current review is the effect of surface state (composition) on porphyrin’s activity toward oxygen reduction in fuel cells, and the highlight of the analogue of fuel cell cathode activity and the oxygen reduction activity in nature. This field also needs a fresh look vs. its overall importance. The porphyrins themselves are moderate oxygen reduction catalysts, and their activity needs to be enhanced. In perspectives of fuel cells, the methods that are practiced and reviewed here are the use of carbon and gold support, pyrolysis and co-deposition with transition metal oxides. Reasons for the porphyrin enhancements and catalysis have been scrutinized. Although great progress has been achieved, there are still some challenges ahead (concerning the ORR mechanism on porphyrins and the activity and stability enhancement for porphyrins to meet practical applications). Therefore, exploring ORR mechanisms on porphyrin, the continuation of the characterization efforts and enhancing porphyrins catalytic performance (a paper in preparation from this group), as well achieving better catalyst durability, are major focuses for fuel cell research in the future.
Non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) are an active subject of recent research on proton exchange membrane fuel cells. In this study, we report a new approach to preparation of self-supported and nano-structured NPMCs using pre-prepared polyaniline (PANI) nanofibers as both nitrogen and carbon precursors. The synthesized NPMCs possess nanoworm structures preserved from the PANI precursor and exhibit high onset potential of 0.905 V vs. RHE and selective activity of nearly four-electron ORR pathways. A significant enhancement in the intrinsic activity and onset potential for the ORR is observed when the Fe content in the precursor is increased from 0 to 3.0 wt.%, while further addition to 10.0 wt.% results in a decrease in the catalytic activity.
Four series of carbon blacks, with various disordered carbon content, have been fabricated at the Sid Richardson Carbon Corporation and were used as supports to obtain heat-treated Fe/N/C electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs). All catalysts were very active and differed by about one order of magnitude in their electrocatalytic activity. Two pristine carbon blacks were then selected to determine, by Raman spectroscopy and X-ray diffraction, which structural parameters of the pristine carbons are important in electrocatalysis. It was found that W-D (the full width at half-maximum of the D band in the Raman spectrum) is indicative of the disordered phase content in the pristine carbon black. The pristine carbon black having the largest W-D also yielded the best ORR electrocatalytic activity. L-a and L-c are structural parameters representing the lateral (L-a) and vertical (L-c) extensions of the graphitic crystallites. They do not change with the catalytic activity, at least not with a change of one order of magnitude of the catalytic site density. Only the distance between the graphene layers in the graphitic crystallites (d(002)) changes with the electrocatalytic activity, but only marginally. This is, however, believed to be an indirect effect related to the gasification of disordered carbon found between the graphitic crystallites in pristine carbon black. (c) 2007 The Electrochemical Society.
Active Fe-based electrocatalysts were prepared using ballmilled graphite powder as a carbon support. The best performing catalysts
were achieved by acid-washing, iron-loading, and pyrolyzing the ballmilled graphite powders. Only
of ballmilling was required to produce optimal catalytic activity. High-energy ballmilling of pristine graphite powder under
nitrogen was shown to reduce crystallite size, increase nitrogen content, increase surface area, increase degree of disorder,
and inevitably introduce metallic impurities. Acid-washing treatment of ballmilled graphite powders reduced, but did not completely
eliminate, metallic impurities. Iron enrichment and pyrolysis of acid-washed, ballmilled graphite powder was shown to increase
catalytic activity, have little effect on crystallite size, increase surface area, and decrease degree of disorder. It was
found that catalytic activity increases as crystallite size decreases, degree of disorder and nitrogen content increase, and
micropore specific surface area increases. Fuel cell test results have shown that the order of increasing maximum power density
follows the order of increasing catalytic activity. Interestingly, the optimal crystallite size parameter
and maximum activity for catalysts made with either ballmilled graphite powder or carbon black is almost the same.
Dramatic advances in the understanding of x-ray absorption fine structure (XAFS) have been made over the past few decades, which have led ultimately to a highly quantitative theory. This review covers these developments from a unified multiple-scattering viewpoint. The authors focus on extended x-ray absorption fine structure (EXAFS) well above an x-ray edge, and, to a lesser extent, on x-ray absorption near-edge structure (XANES) closer to an edge. The discussion includes both formal considerations, derived from a many-electron formulation, and practical computational methods based on independent-electron models, with many-body effects lumped into various inelastic losses and energy shifts. The main conceptual issues in XAFS theory are identified and their relative importance is assessed; these include the convergence of the multiple-scattering expansion, curved-wave effects, the scattering potential, inelastic losses, self-energy shifts, and vibrations and structural disorder. The advantages and limitations of current computational approaches are addressed, with particular regard to quantitative experimental comparisons.
IFEFFIT, an interactive program and scriptable library of XAFS algorithms is presented. The core algorithms of AUTOBK and FEFFIT have been combined with general data manipulation and interactive graphics into a single package. IFEFFIT comes with a command-line program that can be run either interactively or in batch-mode. It also provides a library of functions that can be used easily from C or Fortran, as well as high level scripting languages such as Tcl, Perl and Python. Using this library, a Graphical User Interface for rapid 'online' data analysis is demonstrated. IFEFFIT is freely available with an Open Source license. Outside use, development, and contributions are encouraged.
The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.
A number of cobalt and iron porphyrins heat treated at temperatures as low as 400°C exhibit substantial electrocatalytic activity for O2 reduction and reasonable stability in alkaline electrolytes. The effects of this heat treatment on the structure and overall properties of these materials, however, are not well understood. Differential thermogravimetry analyses have shown that in the case of the Co and Fe-μ-oxo forms of tetra methoxyphenyl porphyrin (TMPP) and the metal-free form as well, the onset for partial decomposition occurs at temperatures of about 400–500°C for the macrocycles either as crystals or dispersed on high area Vulcan XC-72. The results obtained with several pyrolysis—mass spectrometric techniques have indicated that the fraction of volatile nitrogen to non-nitrogen containing species generated during the heat treatment is much higher for the metal-free than for the iron-μ-oxo or CoTMPP. Microanalysis also confirms that with the Fe-μ-oxo and CoTMPP part of the nitrogen is retained. Possible models for the nature of the active sites are discussed.
Tetracarboxylic cobalt phthalocyanine (CoPcTc) has been adsorbed on carbon black (C). The resulting CoPcTc/C has been heat-treated in Ar at various temperatures ranging from 100 to 1100 °C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. Heattreated CoPcTc/C materials have been characterized by TGA, DSC, bulk elemental analyses, XRD, XPS and ToF SIMS. Their electrocatalytic properties have been evaluated by rde and gde measurements. The highest activity is found for CoPcTc/C heat-treated between 500 and 700 °C. In this temperature range, the catalytic site can be traced back either to the intact polymer (<600 °C) or to phthalocyanine fragments still containing Co, even as CoN4 chelates. However, short term life tests on the initially most active catalysts indicate that these catalysts are not stable compared to those obtained after pyrolysis of CoPcTc/C at 900 °C. The active site of the latter catalysts is related to inorganic cobalt present as metal and oxides. TEM reveals that inorganic cobalt is surrounded by a protecting graphite shell rendering it chemically stable in acidic media.
The four models proposed up till now to explain the beneficial effect of a heat treatment on the O2-reduction activity of transition-metal chelate/carbon catalysts are discussed in the light of available evidence. Two recent experiments designed to discriminate between the various possibilities are treated in some detail. It is concluded that the continued existence of the MeN4 moiety of the chelate is the structural feature which is associated with the high activity of heat-treated materials.
- A Shukla
- R K Raman
- N A Choudry
- K R Priolkar
- P R Sarode
- S Emura
- R Kumashiro
A. K Shukla, R. K. Raman, N. A. Choudry, K. R. Priolkar, P. R. Sarode, S. Emura
and R. Kumashiro, Journal of Electroanalytical Chemistry, 563, 181 (2004).
- U A Paulus
- A Wokaun
- G G Scherer
- T J Schmidt
- V Stamenkovic
- N M Markovic
- P N Ross
U.A. Paulus, A. Wokaun, G. G. Scherer, T. J. Schmidt, V. Stamenkovic, N.M.
Markovic and P.N. Ross, Electrochimica Acta, 47, 3787 (2002).
- M Bron
- P Bogdanoff
- S Fiechter
- I Dorbandt
- H Schulenburg
- H Tributsch
M. Bron, P. Bogdanoff, S. Fiechter, I. Dorbandt, H. Schulenburg and H. Tributsch,
Journal of Electroanalytical Chemistry, 500, 510 (2001).
- H Schulenburg
- M Hilgendorff
- I Dorbandt
- J Radnik
- P Bogdanoff
- S Fiechter
- M Bron
- H Tributsch
H. Schulenburg, M. Hilgendorff, I. Dorbandt, J. Radnik, P. Bogdanoff, S. Fiechter,
M. Bron and H. Tributsch, Journal of Power Sources, 155, 47 (2006).