From GDE half-cell screening to single-cell validation: impact of ionomer chemistry and I/C ratio on PEMFC catalyst-layer structure and performance

Understanding how ionomer chemistry governs catalyst-layer architecture is critical for the rational design of proton exchange membrane fuel cells (PEMFCs). In this work, we establish a multi-scale experimental framework that integrates gas diffusion electrode (GDE) half-cell screening, advanced structural characterization, and single- cell validation to directly correlate ionomer chemistry, catalyst-layer microstructure, and electrochemical performance. Using Nafion® and two short-side-chain Aquivion® ionomers (D79 and D98) as model systems, GDE screening identifies an optimal ionomer-to-carbon ratio (I/C = 0.6) and captures intrinsic activity trends that are subsequently validated under single-cell operation. Lower equivalent weight ionomers enhance proton transport by forming more connected ionomer networks, but also promote denser catalyst layers that reduce pore accessibility and lead to mass-transport losses at high current density. In contrast, more open architectures improve oxygen transport and water management, enabling superior full-cell performance despite lower intrinsic proton conductivity. These results establish a general design principle for PEMFC electrodes: optimal performance arises from balancing ionomer connectivity and pore accessibility, rather than maximizing ionomer conductivity alone. More broadly, this work demonstrates that GDE half-cell testing is a predictive and resource-efficient platform for catalyst-layer optimization, capable of decoupling proton transport from structural effects and linking electrochemical descriptors to microstructural features, while also highlighting its limitations under realistic PEMFC operating conditions.