Study on Carbon Corrosion Induced Crack Propagation on Cathode Catalyst Layer in Polymer Electrolyte Membrane Fuel Cell Under Startup-Shutdown Operating Condition

Abstract

Polymer electrolyte membrane fuel cell (PEMFC) has the advantages of high energy density, high efficiency and theoretically zero emission [1], and is expected to replace the traditional internal combustion engine [2] to solve the problems of severe environmental pollution and resource shortage. However, the wide-spread application of FC generators in vehicles faces two major bottlenecks, i.e. cost and durability. Complex vehicle operating conditions including speed change, startup and shutdown, idling and high power pose great challenges to PEMFC's durability. It is found that under the adverse operating conditions such as startup-shutdown, carbon corrosion induced crack propagation [3] in the cathode catalyst layer is an inevitable attenuation and aging behavior, which ultimately leads to fuel cell performance degradation and failure (Figure 1). This study intends to use the combination of experimental research, numerical calculation and theoretical analysis. The formation of CO2 is quantified to characterize carbon corrosion rate. The kinetics of carbon corrosion is slow in normal operation. Therefore, the accelerated stress test conditions used in this work were adopted from Ref. [4]. The cell voltage was cycled from a lower potential of 0.6 V for 30 s, to an upper potential of 1.4 V for 60 s, to simulate the high potential during startup-shutdown conditions. The crack propagation before and after degradation are observed and measured using an optical microscope. The existence and growth of cracks aggravate the carbon corrosion [5], destroy the continuity of catalyst layer, or cause local water flooding [6], which increase the mass transfer resistance and results in a significant performance degradation. A multiple physical model is set up, that couples carbon corrosion, mechanical properties change and crack growth during the startup-shutdown cycling. The carbon corrosion model and CL agglomerate model are combined to predict performance of CL during startup-shutdown cycles. In addition, the model of stress corrosion crack is established to simulate the crack growth with the carbon corrosion and mechanical properties change. A parametric study was presented to analyze the effect of operation condition (e.g., relative humidity, pressure, maximum and minimum voltage) and structural parameters (e.g., carbon loading, porosity, and ionomer fraction) on the performance degradation and crack growth at various numbers of cycles. With the wide application of fuel cell, the degradation of catalyst performance is inevitable. This study can fill the gap in the mechanism of crack growth caused by carbon corrosion, as well as provide critical guidance for PEMFC lifetime prediction. Figure 1