Effect of iron-carbide formation on the number of active sites in Fe–N–C catalysts for the oxygen reduction reaction in acidic media

Kramm, Ulrike I.; Herrmann-Geppert, Iris; Fiechter, S.; Zehl, Gerald; Zizak, Ivo; Dorbandt, Iris; Schmeißer, Dieter; Bogdanoff, Peter

doi:10.1039/c3ta13821f

Cited by 113 publications

(115 citation statements)

References 27 publications

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“…In addition to this, the use of sulfur strongly affected the morphology and performance of the catalysts [16,17]. Sulfur usually enables a higher porosity and lower graphitization [16,18], but more important it was found to prevent carbide formation that was assigned to active site destruction by works of us and of others [17,19,20]. That indeed excess iron was at the origin of active site destruction was further confirmed through an alternative strategy to prevent active site destruction [21].…”

Section: Resultsmentioning

confidence: 75%

“…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]. Sulfur usually enables a higher porosity and lower graphitization [16,18], but more important it was found to prevent carbide formation that was assigned to active site destruction by works of us and of others [17,19,20].…”

Section: Resultsmentioning

confidence: 99%

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Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

et al. 2018

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Abstract: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 H 2 SO 4 . In addition to this, N 2 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.

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Section: Resultsmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 99%

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

et al. 2018

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“…This is understandable since the metallic cobalt and iron crystalline structures known to catalyze graphitization are not detected by XRD in the pristine Fe-N-C and Co-N-C catalysts while chromium nitrides do not catalyze graphitization. 29,[43][44] The Fourier-transforms of the extended X-ray absorption fine structure (EXAFS) spectra of the Fe-N-C and Co-N-C catalysts are shown in Fig. 3a-3b (thick solid lines), while the corresponding X-ray absorption near edge structure (XANES) spectra are shown in Fig.…”

Section: Resultsmentioning

confidence: 99%

“…However, it is notoriously difficult to achieve a high density of Fe(Co)N x C y moieties in highly graphitic substrates via pyrolysis. 23,29 Mitigation of the degradation induced by high-voltage…”

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confidence: 99%

Degradation by Hydrogen Peroxide of Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction

Goellner

Armel

Zitolo

et al. 2015

J. Electrochem. Soc.

176

168

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Fe-N-C and Co-N-C materials are promising catalysts for reducing oxygen in fuel cells. The degradation of such catalysts induced by H 2 O 2 was investigated by contacting them ex situ with various amounts of H 2 O 2 . The degradation increased with increasing amounts of H 2 O 2 . The effect was most severe for Cr-N-C followed by Fe-N-C and last by Co-N-C. Treatment with H 2 O 2 leads to diminished oxygen reduction activity at high potential and/or reduced transport properties at high current density in fuel cell. From spectroscopic characterization, it was found that 66 and 80% of the CoN x C y and FeN x C y moieties present in pristine catalysts survived the extensive H 2 O 2 treatment, respectively. In parallel, the activity for oxygen reduction was divided by ca 6-10 for Fe-N-C and by ca 3 for Co-N-C. The results suggest that the main degradation mechanism in fuel cell for such catalysts is due to a chemical reaction with H 2 O 2 that is generated during operation. The super-proportional decrease of the oxygen reduction activity with loss of FeN x C y and CoN x C y moieties suggests either that only a small fraction of such moieties are initially located on the top surface, or that their turnover frequency for oxygen reduction was drastically reduced due to surface oxidation by H 2 O 2 .

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“…Too large Fe or Co contents lead to the formation of highly graphitized carbon structures during pyrolysis [13,17]. In such graphitized structures, the number of MeNxCy active sites is low, which leads to a low ORR activity [18,19]. In order to maximize the activity, it is necessary to reach a high specific area, and especially a high microporous area [7,[20][21][22].…”

Section: Introductionmentioning

confidence: 99%

Effect of ZIF-8 Crystal Size on the O2 Electro-Reduction Performance of Pyrolyzed Fe–N–C Catalysts

2015

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Abstract:The effect of ZIF-8 crystal size on the morphology and performance of Fe-N-C catalysts synthesized via the pyrolysis of a ferrous salt, phenanthroline and the metal-organic framework ZIF-8 is investigated in detail. Various ZIF-8 samples with average crystal size ranging from 100 to 1600 nm were prepared. The process parameters allowing a templating effect after argon pyrolysis were investigated. It is shown that the milling speed, used to prepare catalyst precursors, and the heating mode, used for pyrolysis, are critical factors for templating nano-ZIFs into nano-sized Fe-N-C particles with open porosity. Templating could be achieved when combining a reduced milling speed with a ramped heating mode. For templated Fe-N-C materials, the performance and activity improved with decreased ZIF-8 crystal size. With the Fe-N-C catalyst templated from the smallest ZIF-8 crystals, the current densities in H2/O2 polymer electrolyte fuel cell at 0.5 V reached ca. 900 mA cm −2 , compared to only ca. 450 mA cm −2 with our previous approach. This templating process opens the path to a morphological control of Fe-N-C catalysts derived from metal-organic frameworks which, when combined with the versatility of the coordination chemistry of such materials, offers a platform for the rational design of optimized Metal-N-C catalysts.

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Effect of iron-carbide formation on the number of active sites in Fe–N–C catalysts for the oxygen reduction reaction in acidic media

Abstract: This work presents two strategies on how the disintegration of FeN4 sites by iron carbide formation can be avoided.

Cited by 113 publications

References 27 publications

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

Influence of the Structure-Forming Agent on the Performance of Fe-N-C Catalysts

Degradation by Hydrogen Peroxide of Metal-Nitrogen-Carbon Catalysts for Oxygen Reduction

Effect of ZIF-8 Crystal Size on the O2 Electro-Reduction Performance of Pyrolyzed Fe–N–C Catalysts

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