Localized Electrochemical Performance Degradation in Polymer Electrolyte Fuel Cells (PEFCs)

Abstract

Pt electrocatalyst durability in polymer electrolyte fuel cells (PEFCs) is generally evaluated through an accelerated stress test (AST); for example, one AST features repeated square-wave cycling with H2/N2 between 0.6 V to 0.95 V vs. reversible hydrogen electrode (RHE) [1]. A separate triangular-wave AST with a higher potential range (1 – 1.5 V vs. RHE) assesses the durability of carbon-based supports [2]. Recent studies [3]–[5] have revealed the heterogeneous nature of cathode catalyst layer degradation. In general, Pt particle size growth mimics the flow field geometry with greater particle size growth under lands compared to channels. Additionally, growth is typically shown to be greater near the air outlet than the inlet and is assumed to be correlated to local performance decay. The impact of such localized degradation on distributed cell performance is investigated in this work. In the present study, a segmented cell is used to quantify functional dependence of local current distributions in aged samples to heterogeneous catalyst layer degradation (Pt particle size growth and carbon support corrosion). The outcome of this enhanced understanding is to identify limiting factors in cell performance at end-of-life (EOL).Single cell studies with 25-cm2 active area are performed using catalyst-coated Nafion® XL membranes (Ion Power Inc.) and SGL-22 BB gas diffusion layers (GDLs) as membrane electrode assembly (MEA) materials and 7-channel serpentine flow field. An S++® current scan shunt (CSS) sensor plate (25-cm2) with 100 current and 25 temperature measurement segments is utilized for current and temperature mapping, respectively. The MEAs are subjected to DOE’s square-wave cycling (0.6 to 0.95 V vs. RHE), triangular-wave cycling (1 to 1.5 V vs. RHE), and a sequence of square-wave cycling followed by triangular-wave cycling. Complete in-situ electrochemical characterization and post-mortem ex-situ diagnostics such as micro-X-ray diffraction (micro-XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are used to obtain particle size distribution and spatial degradation profiles. Results indicate a strong dependence of current distributions on localized catalyst layer degradation. For example, MEA with relatively uniform current distributions at the beginning-of-life (BOL) in Figure 1 exhibits severe mass transport limitations (significantly higher current at the air inlet than the outlet) at EOL when subjected to triangular-wave carbon corrosion AST. Furthermore, an increase of ~1.5x in Tafel slope is observed for the aged sample at EOL, highlighting increased transport losses. These mass transport losses are believed to originate from loss of catalyst layer porosity and subsequent compaction due to carbon support corrosion [2].This work seeks to achieve a greater understanding of the functional dependence between catalyst growth and carbon corrosion and observed local performance.