Effect of Relative Humidity on Membrane Degradation Rate and Mechanism in PEM Fuel Cells

Abstract

The degradation rates and mechanisms of catalyst coated Nafion 212 membranes were investigated at a series of relative humidifies (RH). Degradation studies at open circuit voltage (OCV) showed that membrane degradation rates were faster at 60% RH than at 20% or 100% RH. Degradation studies performed under load (0.6V) showed that membrane degradation rates significantly increased after switching from high RH (100%) to low RH (25%) after 500 hours. This accelerated degradation was likely due to the dual effects of mechanical and chemical degradation. Gas chromatography performed on fuel cell exhaust gases has shown increased gas permeability at increased RH, but cannot fully explain the change of degradation rates with RH. A model is proposed to explain the effect of RH on membrane degradation rate. RH impacts the formation of oxygen radicals via both reactant concentration and reaction rate. Increased RH leads to higher gas permeability of hy drogen and oxygen thus higher reactant concentrations for oxygen radical formation. Conversely, the platinum surface tends to be more highly oxidized at high RH, lowering the reaction rates for oxygen radical formation. The tradeoffs between these phenomenon result in a higher membrane degradation rate at 60% RH than at either 20% or 100% RH for the conditions presented.