Influence of thermal post-curing on the degradation of a cross-linked polybenzimidazole-based membrane for high temperature polymer electrolyte membrane fuel cells

• Thermal post-curing of cross-linked polybenzimidazole-based HTPEM under air.
• Post-cured membranes showed higher mechanical, thermal and chemical ex-situ stability.
• Durability in fuel cell cycled operation was improved by longer post-curing times.
• The occurring degradation mechanisms were determined.
• Longer post-curing period were shown to enhance the resistance against short circuits.

The lifetime stability of membranes is one of the main requirements regarding reliability of high temperature polymer electrolyte membrane fuel cells. The present work has improved durability under cycled operation by thermal post-curing of cross-linked polybenzimidazole (PBI)-based membranes. The membranes were dried over 1, 2 and 3 h at 250 °C under air. Ex-situ experiments proved an increase in stability by post-curing. The liquid uptake and swelling in phosphoric acid increased with longer curing periods. The effect of thermal treatments on cycle stability, lifetime and begin-of-life performance of the membrane electrode assemblies (MEAs) was investigated. Longer post-curing periods of the membranes had no influence on the MEAs' begin-of-life performance and constant current behavior over 2300 h. However, the 3 h post-cured MEAs showed enhanced cycle stability. Post-mortem analysis was carried out to identify the occurring degradation mechanisms. While a significant loss of phosphoric acid and a reduction of electrochemical surface activity on the cathode were observed for both post-cured MEAs, the 3 h dried membrane sample had a significantly higher resistance against pinhole formation during the long term test. Altogether, this work presents thermal post-curing as a promising method for the reduction of degradation determining effects in fuel cell membranes.