Modeling the Effect of Pt Precipitation on PEM Degradation

Abstract

Polymer electrolyte membranes (PEM) are critical components of the fuel cell that enables conduction of protons while preventing the flow of electrons through it. Additionally, the membrane separates the fuel and the oxidant streams that react electrochemically at the anode and cathode. The degradation and failure of proton exchange membranes (PEMs) in fuel cells remain a challenge for long-life applications. For many applications, a high durability of the PEM is required, often 5–20 khr. Accurate lifetime predictions are critical for the commercial viability of these applications because such long times are not usually available to assure the lifetime of the cell-stack design before fielding significant numbers of commercial units. The membrane in fuel cell conditions is subjects to chemical and mechanical degradation. The chemical degradation is catalyzed by Pt particles precipitated in the membrane during operation. Pt is a commonly used catalyst in the fuel cell electrodes. Instability of Pt at high potentials is greater and is aggravated by potential cycling. The Pt ions dissolved at the cathode, diffuse through the membrane towards the anode and precipitates in the membrane forming narrow band of Pt particles. The location of the Pt band in the membrane is governed by potential of Pt particles in the membrane, which is determined by competition of oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) at the particles [1]. The potential of Pt particle is high near the cathode and abruptly drops in some point X0 in the membrane bulk. Dissolved in the membrane Pt ions are reduced to Pt atoms in vicinity of X0 and then are agglomerated into Pt particles, which form Pt band [2]. Cross-overs of oxygen and hydrogen in the membrane react with each other at the Pt in the membrane. At high potential, catalytic oxygen reduction reaction (ORR) proceeds with H2O formation, while ORR involves H2O2 formation as by product at low potential. Direct formation of OH radicals by ORR is possible at intermediate potentials [3], which holds in the Pt band in the membrane. Therefore, oxygen cross-over is reduced in the membrane with generation of free OH radicals. The radicals attack the polymer molecules of the membrane causing chemical degradation which is accompanied by generation of hydrogen fluoride (HF). The rate of chemical degradation of the membrane is measured experimentally by fluoride emission rate (FER). Intensive chemical degradation leads to pinholes formation in vicinity of Pt band in the membrane [2,4]. A three-layer 1D model, including the anode, cathode, and membrane, was developed to describe the chemical degradation of the membrane. The model predicts dependence of FER on key factors that impact membrane degradation: amount of platinum, membrane thickness, the hydration level of the membrane, the concentration of the crossover reactants, and the temperature. Unexpected dependences of FER on Pt particles size and membrane thickness are predicted by the model. For low Pt loading in the membrane (small spacing between particles) dependences of FER on the Pt particle size and the membrane thickness have a maximum. That is due to twofold role of Pt in the membrane. On the one hand, the Pt particles are sources for free OH radicals in the membrane. On the other hand, the Pt particles are sinks for OH radicals due to diffusion-controlled quenching of the radicals at Pt surface. Competition of these two processes results in non-monotonic dependence of membrane degradation rate on Pt loading. The low loading of small Pt particles in membrane causes fast degradation of the membrane. In contrast, the high loading of large Pt particles in the membrane mitigates membrane from chemical degradation.