(417a) Determination of the Cathode Degradation Mechanism in Hydroxide Exchange Membrane Fuel Cells

As the climate crisis worsens, decarbonizing the global economy has become a matter of increasing urgency. Today, the transportation sector accounts for roughly a quarter of all global greenhouse gas emissions. Promisingly, Proton Exchange Membrane Fuel Cells (PEMFCs) are emerging as a commercially viable power source for low-carbon intensity transportation applications. However, PEMFC costs today far exceed the DOE target, limiting their adoption. Hydroxide Exchange Membrane Fuel Cells (HEMFCs) hold the promise to be a cost-effective alternative to PEMFCs due to the potential for lower stack capital costs. Specifically, in alkaline, several inexpensive non-Platinum Group Metals (PGM) are stable and active catalysts, and stack bipolar plates can be manufactured using less expensive materials. Moreover, the Hydroxide Exchange Membranes (HEM) display lower gas crossover compared to PEMs. Currently, low chemical and mechanical durability is a key barrier to the commercialization of HEMFCs. However, the mechanisms of degradation in HEMFCs have not been thoroughly investigated and are not well understood thus far.

We examine the degradation of the cathode electrode in HEMFCs due to three possible contributions: alkaline instability of the ionomer, carbon corrosion of the catalyst support, and cleavage of the ionomer backbone due to oxygen radicals. A HEMFC constant current held at 0.5 A cm^-2 under pure H₂ and O₂ had a nonlinear voltage drop of 0.37 V over 180 h. The results from the fuel cell durability experiment were compared to data from durability tests designed to probe each mechanism. Using identical membrane electrode assemblies, we performed a HEM H₂ Pump Hold to evaluate ionomer alkaline stability, a High Voltage Hold to evaluate carbon corrosion of the support, and an Oxygen Limited Hold to evaluate radical oxidation of the ionomer. In the alkaline environment, the carbon support is active for 2 e- ORR, producing peroxide which is converted into hydroxyl and peroxyl radicals. At a constant current density of 0.5 A cm^-2, we show that degradation is primarily driven by radical degradation of the ionomer, accelerated by the carbon support. Radical-induced ionomer chemical degradation decreases the ionomer mechanical strength, leading to catalyst layer delamination. By eliminating the carbon support and adding ruthenium, a potential radical scavenger, the HEMFCs achieved a constant current hold at 0.5 A cm^-2 for 190 h with a degradation rate of 9 µV h^-1.