High‐Power Alkali‐Free Direct Formate Fuel Cell Enabled by Optimized Ionomer Loading With a Cation‐Exchange Membrane

ABSTRACT Direct formate fuel cells (DFFCs) provide a safe liquid‐fuel pathway for renewable energy storage, yet achieving high performance under alkali‐free conditions remains challenging due to limitations in ion transport and catalyst‐layer structure. Here, for the first time, a cation‐exchange membrane (CEM) was combined with cationic ionomers (CI) in both catalyst layers to establish a fully alkali‐free configuration, and the effects of ionomer loading were systematically examined. Optimizing the anode ionomer content to ionomer‐to‐carbon (I/C) ratio of 0.83 produced a well‐balanced liquid–catalyst–ionomer triple‐phase boundary and improved reaction kinetics. Fuel‐composition analysis revealed that Na + transport across the CEM accounted for only 20%–30% of the theoretical value, indicating that proton transport dominates charge compensation under alkali‐free operation. At the cathode, reducing CI content enhanced oxygen transport by thinning the ionomer film and increasing access to catalytic sites, achieving a peak power density of 92 mW·cm −2 —over twice that of previously reported alkali‐free Na‐ion‐conducting DFFCs. Although lower ionomer loading increased HCOO − crossover and accelerated voltage decay, these results demonstrate that appropriate CI tuning in both electrodes effectively balances oxygen transport, crossover and ion conduction, thereby enabling substantially improved performance in alkali‐free DFFCs without external alkali additives.