Operando Analysis of Cation Migration in PEMFC Electrodes Using Nanoscale X-Ray Computed Tomography

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) are beginning to be accepted as the electrification technology of choice for long-range, high-power vehicles due to their clean operation, high efficiency, and the ability to independently scale their range and power. However, PEMFC vehicles are still not cost-competitive with internal combustion engines, largely because of the high cost of the platinum (Pt) used as the electrocatalyst in the cathodes of PEMFCs (1). Recent efforts have succeeded in reducing the Pt content of PEMFC cathodes by using high-performance Pt alloys like PtCo or PtNi, which have improved catalytic activities over pure Pt (2, 3). While these alloys achieve higher power densities with smaller amounts of Pt in beginning-of-life performance, they experience significant degradation over time, including loss of Pt to particle coarsening and a Pt band and ionization and dissolution of the less-stable alloying element (i.e. Co or Ni). These contaminant cations are absorbed by the ionomer, replacing protons on the sulfonate groups and inhibiting proton conduction and affecting oxygen transport, leading to reduced performance at high current densities (4). While the overall loadings of these cations with respect to the total number of sulfonate groups in the membrane is low, these cations are suspected to migrate during operation, causing high local loadings of contaminant cations in the cathode catalyst layers.The focus of this work was the operando analysis of contaminant cation migration and accumulation using the nanoscale X-ray computed tomography (nano-CT) imaging technique. We designed a miniature nano-CT conducive fuel cell hardware with a Kapton-sealed X-ray viewing port, allowing us to monitor the temporal concentrations of contaminant cations (i.e. Co2+ and Cs+) at the catalyst layer/membrane interface using 2D absorption contrast imaging during cell polarization. The preliminary results from our method have demonstrated back-and-forth cation migration and accumulation under hydrogen pumping conditions, confirming the mobility of these cations under constant current application and demonstrating promise in the operando evaluation of their impact on proton and oxygen transport.This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.