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

International audience7 8 Fe-N-C and CoN -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 CoN -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 characterisation, 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 CoN -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

Probing active sites in iron-based catalysts for oxygen electro-reduction: A temperature-dependent 57Fe Mössbauer spectroscopy study

International audienceTwo Fe-N-C electrocatalysts for oxygen reduction were studied by 57Fe Mössbauer-spectroscopy between 300 and 5 K. The first catalyst contains almost exclusively FeNxCy moieties while the second contains additional crystalline phases, i.e. metallic iron and iron carbide. The Mössbauer parameters of two quadrupole doublets named D1 and D2, attributed to low and medium spin FeN4/C moieties, respectively, do not change with temperature down to 5 K. This indicates that such moieties do not undergo phase transition or magnetic ordering, supporting the view that the active sites are localized on isolated iron atoms. At room temperature, the Lamb-Mössbauer factors of doublets D1 and D2 are 0.46 and 0.52, smaller than those of α-Fe (0.67) and γ-Fe (0.78). These values allow for the first time a precise Mössbauer quantification of Fe species in Fe-N-C catalysts. The ORR activity is best correlated with the absolute content of the FeN4/C moiety associated with doublet D1, assigned to a FeIIN4/C moiety in low-spin state. The ORR turnover frequency of such moieties is however known to depend on chemical and electronic properties of the carbon matrix, which will require additional descriptor(s) than the site density in order to precisely interpret the ORR activity of such materials

Effect of Furfuryl Alcohol on Metal Organic Framework-based Fe/N/C Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells

International audienceFe/N/C electrocatalysts for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) have been synthesized from iron acetate (FeIIAc), 1,10-phenanthroline (phen), furfuryl alcohol (FA) and a thermally-decomposable metal-organic framework (MOF). The catalyst precursors have been prepared according to two main synthesis schemes. In the first one, a nitrogen-doped carbon was first synthesized from the MOF impregnated with FA, and this carbon was subsequently used as a microporous support for FeIIAc and phen. In the second approach, the FA-impregnated MOF was used as a support for FeIIAc and phen. The catalyst precursors prepared from these two approaches were subjected to a first pyrolysis in Ar and to a second pyrolysis in NH3. The effect of the pyrolysis temperature in Ar and heating rate were investigated. The as-prepared electrocatalysts were characterized by transmission electron microscopy, N2 sorption analysis, as well as Mössbauer and X-ray absorption spectroscopies for the optimized catalysts. The electrochemical properties towards the ORR were investigated by rotating-disk electrode voltammetry and H2-O2 PEMFC tests

Oxygen reduction activities compared in rotating-disk electrode and proton exchange membrane fuel cells for highly active Fe-N-C catalysts

International audienceIn the past three years, two novel synthesis methods for non-precious metal catalysts resulting in a breakthrough of their activity and performance at the cathode of the proton-exchange membrane fuel cell (PEMFC) have been reported by the group of Prof. Dodelet. While the activity of these novel Fe-based catalysts for the oxygen reduction reaction is very high in PEMFC, our preliminary activity measurements with the rotating disk electrode (RDE) technique on one of them showed an activity being a factor 30-100 lower than the one measured in PEMFC at 80°C. The present work explains to a large extent this huge difference. Two Fe-N-C catalysts synthesized via our novel approaches and one Fe-N-C catalyst synthesized via our classical approach were investigated in RDE and PEMFC. In both systems, the effect of the ink formulation (Nafion-to-catalyst ratio) was investigated. Optimization of the RDE ink formulation explains a factor between 5 and 10 in the two-decade gap mentioned above. Then, the effect of temperature in the RDE system was investigated. An increase from 20 to 80°C was found to result in a theoretical maximum twofold increase in activity. However, in practice, decreased O2 solubility with increased temperature cancels this effect. After taking into account these two parameters, a difference in ORR activity between RDE and PEMFC of ca a factor five still remained for one of the two novel Fe-N-C catalysts investigated here. The lower initial activity measured in RDE for this catalyst is shown to be due to the fast adsorption of anions (HSO4-) from the liquid H2SO4 electrolyte on protonated nitrogen atoms (NH+) found on its surface. The phenomenon of anion adsorption and associated decreased ORR activity also applies to the other novel Fe-N-C catalyst, but is slower and does not immediately occur in RDE

Synergy between molybdenum nitride and gold leading to platinum-like activity for hydrogen evolution

International audienceReduced size and direct electrochemical H2 compression are two distinct advantages of electrolyzers based on the acid–polymer electrolyte membrane technology over those relying on alkaline electrolytes. However, recourse to catalysts based on the scarce platinum-group-metals has hitherto been the price to pay. While the transition metal sulfides and nitrides of group VI have recently shown interesting activities for H2 evolution, the remaining activity gap with Pt needs to be reduced. Platinum owes its high activity to its optimum metal–hydrogen bond strength for H2 evolution, which is a proven descriptor of the activity on single-component catalysts. Here, we unravel a major synergetic effect between gold and molybdenum nitride which multiplies the hydrogen evolution activity ca. 100 times over that of either gold or molybdenum nitride. This two-phase catalytic material, featuring both strong and weak metal–hydrogen bonds, overcomes the limitations described by Sabatier's principle for single-component catalysts

Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials

International audienceWhile platinum has hitherto been the element of choice for catalysing oxygen electroreduction in acidic polymer fuel cells, tremendous progress has been reported for pyrolysed Fe–N–C materials. However, the structure of their active sites has remained elusive, delaying further advance. Here, we synthesized Fe–N–C materials quasi-free of crystallographic iron structures after argon or ammonia pyrolysis. These materials exhibit nearly identical Mössbauer spectra and identical X-ray absorption near-edge spectroscopy (XANES) spectra, revealing the same Fe-centred moieties. However, the much higher activity and basicity of NH3-pyrolysed Fe–N–C materials demonstrates that the turnover frequency of Fe-centred moieties depends on the physico-chemical properties of the support. Following a thorough XANES analysis, the detailed structures of two FeN4 porphyrinic architectures with different O2 adsorption modes were then identified. These porphyrinic moieties are not easily integrated in graphene sheets, in contrast with Fe-centred moieties assumed hitherto for pyrolysed Fe–N–C materials. These new insights open the path to bottom-up synthesis approaches and studies on site–support interactions