Mechanical fatigue modeling of reinforced membranes in fuel cells

A fuel cell system is an electrochemical device that converts chemical energy from a fuel, such as hydrogen, into electricity, water, and heat. In Polymer electrolyte fuel cells (PEFC), the membrane acts as an ion-conducting component between the anode and cathode, plays a predominant role in the fuel cell's performance and lifetime. Humidity and temperature cycling during fuel cell operation cause the membrane to expand and contract, leading to mechanical fatigue over time. In this study, by developing a constitutive model, the mechanical fatigue responses of a reinforced membrane in varied environmental conditions have been investigated in both ex-situ and in-situ. Both ex-situ fatigue tests by dynamic mechanical analysis (DMA) and in-situ fatigue test through pressure-differential accelerated mechanical stress tests (∆P-AMST) in a fuel cell setup, have been implemented and modeled for the fatigue evaluation of the reinforced membrane. Based on the critical accumulated plastic dissipation energy (CAPDE) criterion, the membrane's fatigue lifetime estimation model for a complete fuel cell condition has been built. Projecting of fatigue lifetime from ∆P-AMST to a full fuel cell model presented a novel approach: this evaluation could be completed in less than one week as opposed to the 2-3 months required for accelerated mechanical stress tests (AMST) based on U.S. Department of Energy (DOE) protocol. This method considered all significant factors that affect the fatigue lifetime of a reinforced membrane as a viscoelastic material, such as temperature, humidity, and strain rate due to different humidity cycles. The impacts of dry phase durations in every RH cycle, the level of dryness, the types of clamping pressure, the properties of the catalyst layer (CL) and gas diffusion layer (GDL), and the ratio of the membrane's width under the channel to under the land on the membrane's fatigue lifetime have been investigated. It was learned that having shorter dry phase durations and lower levels during relative humidity (RH) cycles, using pneumatic or hydraulic pressure for clamping, and minimizing the membrane's width under the channel to below the land can increase the membrane's fatigue lifetime. Having a stronger CL or GDL does not play an effective role in the mechanical failure of the membrane. Finally, chemical degradation effects on membrane fatigue lifetime were investigated and incorporated into the fatigue lifetime estimation algorithm, considering influences on membrane thinning and plastic dissipation energy.