In recent research, an anion exchange membrane fuel cell (AEMFC) using hydrazine hydrate liquid fuel¹has been considered to be one of the promising candidates to breakthrough resource problem with non-platinum group metal (non-PGM) catalysts. Among non-PGM catalyst for oxygen reduction reaction (ORR), materials based on transition metal-nitrogen-carbon networks (M-N-C) demonstrate the high catalytic activity. In AEMFC with M-N-C catalysts, chemical reactions in cathode take place as shown in following equations (1) and (2), respectively.

Chemical reaction in cathode

O₂ + H₂O + 2e^- → HO₂^- + OH^-                                              (1)

HO₂^- + H₂O + 2e^- → 3OH^-                                                  (2)

                   HO₂^- generated in reaction (1) is known to form radicals (ex. ·OH) and reduce the durability of membrane / ionomer, so HO₂^- reduction in cathode is required for popularization of AEMFC. In this study, we focus on analysis for HO₂^- generation mechanism of iron-nitrogen-carbon (Fe-N-C) catalyst.

                   Fe-N-C catalysts were prepared by sacrificial support method². The Prepared Fe-N-C catalyst composes Fe-N-C and Fe metal particle (see the high resolution TEM image shown in Fig. 1). In order to evaluate relationship between structure of Fe-N-C catalysts and HO₂^|generation, synchrotron characterization; X-ray absorption fine structure (XAFS) and rotating ring disk electrode (RRDE) have been done for 3 samples (catalyst A-C) prepared under different conditions (materials of precursors, acid treatment etc.).

                   Fig. 2 shows Fourier-transforms of the Fe K-edge extended x-ray absorption fine structure (ex-situ EXAFS) spectra. As synthesized Fe catalysts have two peaks. The first nearest neighbor peak of Fe around 1.3 Å is assigned to Fe-N-C. The second peak around 2.2 Å is assigned to Fe-Fe originated from Fe metal particle.

                   Fig. 3 shows relationship between ratio of HO₂^- generation; P(HO₂^-) calculated from RRDE results and ratio of Fe metal / Fe-N-C evaluated from the peaks of EXAFS spectra. We confirm that P(HO₂^-) is in proportion to ratio of Fe metal / Fe-N-C. From these result, we consider that Fe-N-C enhances reaction (2) and reduces HO₂^-generation.

                   Additionally, in-situ XAFS study and density function theory (DFT) calculations are performed to understand the potential dependence of local atomic and electronic structure of Fe-N-C catalyst. The details of HO₂^-generation mechanism indicated from these results will be shown and discussed in the meeting.

References

[1] K. Asazawa, K. Yamada, A. Oka, M. Taniguchi, T. Kobayashi, Angew. Chem. Int. Ed., 46, 8024 (2007).

[2] A. Serov, M. H. Robson, B. Halevi, K. Artyushkova, and P. Atanassov, Electrochem. Commun., 22, 53 (2012).

Acknowledgements

                   This work was supported by CREST, JST. This work was performed at the BL14B2 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2013B1712 and 2014A1793). The computation was mainly carried out using the computer facilities at Research Institute for Information Technology, Kyushu University.