On-line fluoride and pH measurement in outlet water for monitoring membrane degradation in polymer electrolyte membrane fuel cells

The chemical degradation of the polymer electrolyte membrane was monitored using on-line measurements of F − and pH in the outlet water during the operation of the fuel cell. The effectiveness of the technique was evaluated under nominal, non-degrading conditions, a chemical stability accelerated stress test (AST), and conditions to eliminate H 2 O 2 production from O 2 crossover. The on-line measurements showed significant decreases in pH from 6 to 4 when degradation occurred and fluoride release rates of 12.1-13.8 μ g cm −2 h −1 during the first 24 h of the AST. The estimation of the total F mass loss using the ion-selective electrode method showed good agreement with the estimation of loss using imaging of membrane thinning. To further evaluate the on-line measurement technique, the effect of using a total ionic strength adjustment buffer (TISAB) solution was investigated on the anode and cathode outlet water samples, the results showed that F − concentrations may be underestimated at pH < 3.5 when the solution was not used. However, the trends and values were largely comparable even without the solution added, which benefits on-line measurement. The on-line technique was also applied to an AST campaign to investigate the effects of varying membrane characteristics, e.g., thickness, side chain length, use of radical scavengers, and presence of F. The highest F − release was observed for the long side chain Nafion membranes, followed by the shorter side chain 3M membranes. Nafion membranes with radical scavengers showed negligible release during the 200 h AST, while the hydrocarbon membranes also showed negligible release as expected, even when using Nafion ionomer in the catalyst layer. • On-line measurement of PEMFC chemical membrane degradation. • Monitoring of F − and pH in PEMFC outlet water. • Comparison of F mass loss determined using on-line technique with F mass loss calculated using ex situ X-ray computed tomography of membrane thinning. • On-line technique used to study effects of membrane characteristics such as thickness, side chain length, presence of radical scavengers, and use of hydrocarbon ionomer. • Discussion on the effect of total ionic strength adjustment buffer (TISAB) in relation to on-line measurements.