Chemical Stability of PFSA Membranes in Heavy-Duty Fuel Cells: Fluoride Emission Rate Model

Abstract

Laboratory data from in-cell tests at and near open circuit potentials (OCV) and ex-situ H2O2 vapor exposure tests are used to develop a fluoride emission rate (FER) model for a state-of-the-art 12-µm thin, low equivalent weight, long-chain perfluorosulfonic acid (PFSA) ionomer membrane that is mechanically reinforced with expanded PTFE and chemically stabilized with 2 mol% cerium as an anti-oxidant. The anode FER at OCV linearly correlates with O2 crossover from the cathode and the high yield of H2O2 at anode potentials, as observed in rotating ring disk electrode (RRDE) studies. The cathode FER may be linked to the energetic formation of reactive hydroxyl radicals (·OH) from the decomposition of H2O2 produced as an intermediate in the two-electron ORR pathway at high cathode potentials. Both anode and cathode FERs are significantly enhanced at low relative humidity and high temperatures. The modeled FER is strongly influenced by the gradients in water activity and cerium concentration that develops in operating fuel cells. Membrane stability maps are constructed to illustrate the relationship between the cell voltage, temperature, and relative humidity for FER thresholds that define H2 crossover failure by chemical degradation over a specified lifetime.

Key findings

• ▸Anode FER linearly correlates with O2 crossover and H2O2 yield at OCV.
• ▸Cathode FER linked to the formation of reactive hydroxyl radicals from H2O2 decomposition.
• ▸FER significantly enhanced at low relative humidity and high temperatures.

Keywords

Identifiers

DOI

10.3390/electrochem6030025

Journal

Electrochem

Year

2025