Additive Effect of Iron and Copper Sulfates in Heteropolyacid Solution As a Cathode Reaction Mediator on the Performance of Redox Flow PEFCs
Authors: Yamamoto Naoki , Eguchi Koichi 
Published: December 2020

Since the Industrial Revolution, we have relied heavily on fossil resources. Even today, they are indispensable for our cultural activities, but environmental problems such as global warming and soil pollution caused by their combustion have been concerned. These issues have grown an interest in alternative fuels. In this situation, hydrogen has been regarded as promising next-generation energy sources in terms of energy content and environmental load. Moreover, fuel cells, which directly convert the chemical energy of hydrogen into electrical energy, are cleaner and more efficient than conventional power generation devices.
Polymer electrolyte fuel cells (PEFCs) have been already commercialized as power sources for automobiles due to low operating temperature and high current density. However, the carbon-supported platinum or platinum-based alloy is widely applied as the cathode. Therefore, these fuel cells are very costly and required to reduce the platinum usage for more widespread application of PEFCs.
We proposed a redox flow PEFC as one of the solutions to this problem [1]. The redox flow PEFC employing the heteropolyacid solution of H6PV3Mo9O40 as a redox mediator for the cathode reaction was constructed. In this system, the cathode is free from noble metals for electrocatalyst. The electrochemical reduction of heteropolyanion over the carbon cathode and the subsequent re-oxidation of the partially reduced heteropolyanion by exposure to the dissolved oxygen in the regenerator are important processes for the continuous power generation. Especially, the improvement of re-oxidation rate contributes to a stable operation with higher current density. Therefore, the development of catalyst to promote this process will lead to the improved performance.
For the heteropolytungstates of H3PW12O40 and H5SiVW11O40, it has been reported that the re-oxidation of their reduced anion species are efficiently catalyzed by a combination of copper ion and iron ion [2].The catalytic pathway consists of a reversible reaction between Cu(II) and Fe(II), followed by fast oxidation of reduced heteropolyanion and Cu(I) by Fe(III) and O2, respectively, to regenerate Fe(II) and Cu(II). In this study, therefore, the influence of the addition of iron and copper sulfates to the H6PV3Mo9O40 solution on the cell performance was evaluated. As a result, the obtained current density and the re-oxidation rate were improved by the addition. Also, in the discharge operation at a constant current the terminal voltage remained higher in using the solution with these sulfates than without additives. Accordingly, the addition of iron and copper sulfates to H6PV3Mo9O40 solution contributed to the improvement of the performance of the redox flow PEFCs.
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[1] T. Matsui, E. Morikawa, S. Nakada, T. Okanishi, H. Muroyama, Y. Hirao, T. Takahashi, K. Eguchi. ACS Appl. Mater. Inter.18119-18125 (2016).
[2] M. Kim, M. Chamack, Y. V. Geletii, C. L. Hill, Inorg. Chem. 311-318 (2018)