Highly stable membrane–electrode assembly using ether-linkage-free spirobifluorene-based aromatic polyelectrolytes for direct formate solid alkaline fuel cells

Abstract

The low stability of membrane–electrode assemblies (MEAs) in solid alkaline fuel cells (SAFCs) operating at high temperatures and in highly alkaline conditions is a critical issue. In this study, we developed a membrane–electrode assembly (MEA) using highly durable ether-linkage-free aromatic polyelectrolytes with a three-dimensionally twisted spirobifluorene (SBF) backbone for direct formate SAFCs (DF-SAFCs). Here, the SBF-based polyelectrolytes were employed as an ionomer for catalyst layers and as a protective thin layer (less than 1 μm in thickness) for the surface coating on a conventional anion-conducting pore-filling membrane. The simple method of the coating of durable polyelectrolyte layers on the membrane surfaces provided high chemical stability of the membrane against both alkaline and OH radicals, as well as a reduced permeability of formate through the membrane, while maintaining a low water uptake and high ionic conductivity. The MEA using these materials achieved a high performance with a maximum power density of 210 mW cm−2 in DF-SAFCs. More importantly, this study, for the first time, demonstrated the high stability of the MEA during long-term DF-SAFC operation (approximately 50 h in one week) in highly alkaline conditions and at a high temperature of 80 °C.