Origin of Instability in Fe-N-C Oxygen-Depolarized Cathodes in HCl Electrolysis

Oxygen depolarized cathode (ODC) catalysts, especially for HCl electrolysis, face challenging corrosive environments that narrow the choice of materials to the current state-of-theart RhxSy/C. Nitrogen-coordinated Fe on carbon defect surfaces, commonly referred to as Fe-N-C, offers a low-cost, highly active, and Cl-poisoning-resistant alternative. However, its long-term durability remains unexplored under the unique operating conditions of the HCl electrolysis. To address this, we investigate the stability of Fe-N-C under simulated uncontrolled shutdowns, mimicking the harsh transients of industrial operation. Post-mortem analysis of Fe-N-C using Xray absorption and 57 Fe Mössbauer spectroscopy reveals a transition from the Fe-N4 coordinating environment to Fe2O3. Despite these structural changes, Fe-N-C maintained a competitive low voltage of 1.33 V at 0.5 A cm⁻ 2 , consistent with a relatively low fraction of sites being affected. Accelerated stress tests simulating uncontrolled shutdown conditions, combined with Raman spectroscopy, showed that Fe-N-C and RhxSy/C share a common carbon degradation mechanism. This work demonstrates the viability of Fe-N-C as a robust ODC catalyst and provides guidance for improving its durability by mitigating demetallation and carbon oxidation.