Role of SnO ₂ Nanorods Loading in Governing Flooding Stability of Gas Diffusion Electrodes for CO ₂ Reduction to Formate

Electrochemical CO2 reduction to formate is a promising pathway for carbon utilization, yet long-term operation at industrially relevant current densities is often limited by gas diffusion electrode (GDE) flooding. Here, we investigate flooding mitigation through catalyst-layer design using SnO2 nanorods grown directly on carbon paper via a hydrothermal method. By systematically varying the growth-solution concentration, we tune catalyst coverage, loading, thickness, and porosity while preserving intrinsic catalytic activity and formate selectivity. Electrochemical testing in a GDE flow cell reveals a strong dependence of operational stability on catalyst loading, with an intermediate growth concentration delivering optimal performance. The optimized SnO2 nanorod electrode sustains stable formate production at −100 mA/cm2 for 25 h without catastrophic flooding. In situ Raman spectroscopy and postelectrolysis K+ penetration analysis indicate that excessive catalyst thickness accelerates microscale flooding through local alkalization, whereas insufficient catalyst coverage promotes premature flooding via exposed substrate regions. A one-dimensional transport–reaction model quantitatively links catalyst-layer macrostructure to CO2 mass-transfer limitations and local reaction distributions, revealing a transition from kinetic to transport-limited regimes with increasing thickness. These results establish catalyst loading and morphology as critical design parameters for stabilizing gas–liquid interfaces in GDE-based CO2 electrolyzers and provide practical guidelines for achieving high-rate, high-stability formate production.