Indirect Fuel Cell Based on a Redox-Flow Battery Toward the Reduction of Platinum Usage

Abstract

An indirect fuel cell [1-3], which is composed of a redox-flow battery (RFB) and a chemical reactor to regenerate (oxidize) the catholyte by oxygen and another reactor to regenerate (reduce) the anolyte by a fuel, is constructed as an alternative to a PEFC. We have already proposed a new design for the RFB [4] for our indirect fuel cell, which has a gas phase of hydrogen to separate the catholyte and anolyte to eliminate the crossover of ionic species. In this work, to clarify the possibility to use an indirect fuel cell to reduce the usage of noble-metal catalysts, a non-noble-metal catalyst and a mixture of two molecular catalysts were used in the catholyte and anolyte regenerators, respectively. In the catholyte regenerator, a carbonized hemoglobin powder was used as a solid catalyst. This material has been proved to be effective as an electrocatalyst for oxygen reduction reaction (ORR) in a PEFC [5]. In this work, it was used after immobilization on a polymer sheet support. In the anolyte regenerator, a rhodium complex was used for hydrogen oxidation and another rhodium complex for carbon monoxide oxidation. Although these two complexes contain a noble metal, they are soluble in the electrolyte and used as homogeneous catalysts. Since the active site of such a complex molecular catalyst is a single metal atom, the usage of metal can be reduced compared with a metallic platinum catalyst. This work demonstrates a steady-state operation of the above-mentioned indirect fuel cell by feeding of oxygen and CO-containing hydrogen to the catholyte and anolyte regenerators, respectively. This work was supported by JSPS KAKENHI Grant Number 25289363. [1] A.M. Posner, Fuel, 34 (1955) 330. [2] S. Ashimura, Y. Miyake, Denki Kagaku, 31 (1963) 598. [3] D.-G. Oei, J. Appl. Electrochem. 12 (1982) 41. [4] Z. Siroma, S. Yamazaki, N. Fujiwara, M. Asahi, T. Nagai, T. Ioroi, J. Power Sources, 242 (2013) 106. [5] J. Maruyama, I. Abe, Chem. Mater., 18 (2006) 1303.