Effect of Iron Doping on Oxygen Reduction Activity of Zirconium Oxides Containing Carbon and Nitrogen Using Pyrazine Carboxylic Acid As Platinum Alternative Cathodes for PEFC

Ishihara, Akimitsu

The use of carbon-supported platinum catalysts as cathode of present polymer electrolyte fuel cells (PEFCs) have some problems such as high cost and low amount of resources of platinum. Therefore, we have attempted to develop substitute of platinum based on group 4 transition metal oxides. We found that stable oxygen vacancies on the surface and lattice distortion by doping foreign elements of these metal oxides could act as active sites of oxygen reduction reaction (ORR) [1]. However, although zirconium oxides show some ORR activity, the onset potential of the ORR still remains low. Then, we focused on the improvement of the ORR activity by an addition of iron ions to zirconium oxides according to the research of P. Madkikar et. al. [2]. In this study, we used pyrazine carboxylic acid as ligand of metal ions to prepare zirconium-iron complexes including carbon and nitrogen as precursors. The precursors were heat-treated under low oxygen pressure to obtain the iron-added zirconium oxide-based catalysts, which are designated as ZrO_x-Fe. We investigated the correlation between the ORR activity and physical characteristics such as crystalline phase.

ZrO_x-Fe catalysts were synthesized according to the previous patent [3]. We prepared zirconium complexes solutions by mixing zirconium butoxide and acetic acid in acetylacetone as a solvent. On the other hand, we also prepared iron complexes solutions by mixing iron acetic acid and pyrazine carboxylic acid in mixed solvents of water, ethanol and acetic acid. The amount of the iron addition was changed in atomic ratios of 0, 0.004, 0.01, 0.1, and 0.2 against Zr atoms. The iron atomic ratio of 0.004 was selected according to P. Madkikar’s results, because Fe atomic ratio of 0.0036 caused the highest ORR activity in the report [2]. After mixing of two solutions, the solutions were stirring several days, and the precursors were obtained by evaporation and vacuum drying. These precursors were heat-treated under argon including 2 % hydrogen and 0.05 % oxygen for low oxygen pressure oxidation, and finally ZrO_x-Fe catalysts were obtained. To investigate the effect of Fe addition to zirconium oxides, we also synthesized iron-based catalyst without Zr in the same way. Electrochemical measurements were conducted in the solution of 0.5 M H₂SO₄ at 30 ^oC. In this study, we used the ORR onset potential as the parameter of the ORR activity, which was defined as the potential when the ORR current (i_ORR) was 0.01 ampere per a mass of the whole catalysts.

Among the ZrO_x-Fe catalysts prepared in this study, ZrO_x-Fe with the Fe atomic ratio of 0.1 showed the highest E_ORR, which was designated as ZrO_x-Fe(0.1). Therefore, we compared with ZrO_x and FeO_x prepared by the same way as well as conventional catalysts. Fig. 1 shows the ORR polarization curves of the ZrO_x, ZrO_x / CNT [1], ZrO_x-Fe / KB [2], FeO_x and ZrO_x-Fe(0.1) catalysts. As shown in Fig.1, the ZrO_x-Fe(0.1) catalyst had highest ORR onset potential among these catalysts. Comparing with ZrO_x, FeO_x and ZrO_x-Fe(0.1), the Fe addition into zirconium oxide was essential to increase the ORR onset potential. In addition, the onset potential of the ZrO_x-Fe(0.1) was higher than that of the ZrO_x-Fe / KB [2] and our previous ZrO_x / CNT [1]. The onset potential of the ZrO_x-Fe(0.1) was exceeded 0.9 V that we have never been able to reach with zirconium oxides. Therefore, the addition of iron to zirconium oxides improved drastically the quality of the active sites of the ORR.

Next, we examined crystalline phase of all ZrO_x-Fe catalysts with XRD. All ZrO_x-Fe catalysts were consisted of ZrO₂ monoclinic or tetragonal phase. We calculated monoclinic rate in each catalyst using the equation in the literature [4]. The change of atomic ratio of Fe caused the different monoclinic rate. A positive correlation between the ORR onset potentials and the monoclinic rates of the ZrO_x-Fe was obtained. Thus, the ORR onset potential increased with increasing the monoclinic rate in the catalysts, suggesting that the monoclinic phase formed the active sites of the ORR.

Acknowledgement

The authors thank the financial support of the New Energy and Industrial Technology Development Organization (NEDO), JSPS grants-in-aid for scientific research, and ENEOS Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation.

References

(1) A. Ishihara et. al., Int. J. Hydro. Energy, 45, 5438 (2019).

(2) P. Madkikar et. al., J. Electrochem. Soc., 166, F3032 (2019).

(3) Showa Denko Corp., R. Monden et. al., Oxygen reduction catalysts, the methods of these, and polymer electrolyte fuel cells, JP 2013-240785, 2013-12-05.

(4) H. Toraya et. al., J. Am. Ceram. Soc., 67, C119 (1984).