Operand XAFS Study of Hydrazine Oxidation Catalysts for Use in Anion Exchange Membrane Fuel Cells

In recent research, direct hydrazine hydrate fuel cells (DHFCs) have been considered to be one of the promising candidates to breakthrough resource problem using anion exchange membrane (AEM) with non-platinum group metal (non-PGM) catalysts for both anode and cathode. However, the catalytic activity and selectivity of anode catalyst of DHFCs are required to improve for popularization of DHFCs. In DHFC, electrochemical reactions take place in the anode and the cathode as shown in following equations (1) and (2), respectively.

Electrochemical reaction of DHFC

Anode: N₂H₄ + 4OH⁻ → N₂ + 4H₂O + 4e⁻     (1)

Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻             (2)

Overall: N₂H₄ + O₂  → N₂ + 2H₂O                 (3)

Chemical reaction (decomposition) in anode

N₂H₄ → N₂ + 2H₂                                          (4)

3N₂H₄ → N₂ + 4NH₃                                      (5)

In the electrooxidation reaction of hydrazine, the most desirable reaction is the four-electron reaction given by equation (1). Co exhibits a higher rate of chemical reaction (decomposition) than that of Ni. The reaction selectivity of anode catalyst of DHFC affects to the enlargement of the fuel cell system, because generated ammonia is required to decompose harmless gas to discharge to the environment.

In this study, carbon supported Ni, Co, and Ni_0.87Zn_0.13 catalysts (Ni/C, Co/C, and Ni_0.87Zn_0.13/C) as an anode catalyst of DHFC were synthesized by impregnation/freeze-drying procedure. Operand X-ray absorption fine structure spectroscopy (XAFS) study was performed to understand the potential dependence of local atomic and electronic structure of hydrazine oxidation catalyst in an alkaline electrolyte. The potential dependence of local atomic and electronic structure of prepared catalysts in 1M KOH and 1M KOH + 0.1M hydrazine hydrate (HH) was investigated using operand XAFS at SPring-8 in Japan.

Fig. 1 shows the schematic view of the operand XAFS measurement system. In this study a half-cell was used. Working electrodes were prepared by the same method for MEA fabrication. Ni, Co, Ni_0.87Zn_0.13 catalysts were coated on carbon paper (CP, GDL 10AA, SGL Technologies GmbH). Hg/HgO (XR440, Radiometer) and Pt coil were used as reference and counter electrode, respectively. Fig. 2 shows X-ray absorption near edge structure (XANES) spectra for Ni-K and Co-K edge in prepared catalysts coated on CP in alkaline 1M KOH + 0.1M hydrazine hydrate (HH), together with reference materials. Metallic structure of Ni_0.87Zn_0.13/C compared with Ni/C was supposed due to electron donor from Zn to Ni, and this atomic structure of Ni_0.87Zn_0.13/C was stable during electrochemical reaction in alkaline solution. In 1M KOH + 0.1M HH, Co-K edge of Co/C was closely with Co-foil except for potential of 0.402 V due to reduction by hydrazine hydrate against oxidized Co. From the operand XAFS results, high catalytic activity of Ni_0.87Zn_0.13/C compared with Ni/C was caused that Zn has a larger ionization tendency than Ni so that Ni in Ni_0.87Zn_0.13/C hardly gets rusty due to the sacrifice anticorrosive action. Ammonia was generated in DHFC by the surface of metallic Co reduced by hydrazine.

Acknowledgements

This work was performed under the Shared Use Program of Japan Atomic Energy Agency (JAEA) Facilities. Authors are grateful to Dr. Y. Nishihata and Dr. D. Matsumura of JAEA for the synchrotron XAFS measurement of beam number of 2013B3616 at BL14B1 of SPring-8. We would like to thank Professor Dr. P. Atanassov and Associate Professor Dr. A. Serov of Univ. of New Mexico for providing hydrazine oxidation catalysts. We are grateful to Professor Dr. N. Mizuno and Associate Professor Dr. K. Yamaguchi of The Tokyo Univ. for their advice on catalyst research.