Modelling of the mechanical durability of constrained Nafion membrane under humidity cycling

Abstract

Several degradation processes limit the lifetime of polymer electrolyte membranes in fuel cells. One of these processes is humidity changes during the operation of the cell, which causes swelling and shrinking of the polymer electrolyte membrane. Changes in membrane size lead to periodic mechanical stresses, which damage the polymer. This damage leads to the formation of cracks in the membrane and, thus, results in early failure of the fuel cell.This study presents a theoretical model that predicts the membrane lifetime depending on the thickness of the membrane and operating conditions of the periodic humidity cycle such as: amplitude of the humidity variation in the cycle, cycle duration, and temperature. The model is based on the assumption that mechanical destruction of the polymer electrolyte membrane, constrained in the fuel cell, occurs when the deformation energy applied to the membrane reaches a maximum value. The mechanical stress and deformation energy are estimated using the modified Eyring equation [Burlatsky et al., 2012]. The proposed model takes into account the influence of the temperature and water concentration on the membrane mechanical properties. The water concentration in the membrane is simulated by considering the water sorption/desorption kinetics and water sorption isotherm of the membrane.The model was used to predict the lifetime of the non-reinforced membrane Nafion. The calculation results show that membrane mechanical durability correlates positively with increasing membrane thickness and temperature. The increase in the amplitude of the humidity cycle decreases membrane mechanical durability. At very short humidity cycles (≤50−100 s), the membrane lifetime declines with increasing period of the humidity cycle. At higher humidity cycle period (>50−100 s), the increase of the cycle duration leads to continuous growth of the membrane lifetime.