GDE Stability in CO₂ Electroreduction to Formate: The Role of Ionomer Type and Loading

The electrochemical reduction of CO₂ (ERCO₂) to formate is a promising decarbonization strategy, yet the long-term stability of gas diffusion electrodes (GDEs) remains a major bottleneck for large-scale implementation and technoeconomic viability. This study systematically investigates the role of catalyst layer (CL) composition in enhancing GDE performance and durability, focusing on ionomer selection, catalyst-to-ionomer ratio optimization, and the use of additives (such as PTFE) to tune the CL hydrophobicity. As a catalyst, (BiO)₂CO₃ is used as an active material thanks to its selectivity toward formate. The impact of the ionomer type is evaluated by comparing <i>Nafion</i>, a proton-conducting ionomer, with <i>Sustainion</i>, an anion-conducting ionomer. While <i>Nafion</i>-based GDEs exhibit competitive selectivity toward formate at low ionomer content, with Faradaic efficiencies (FE) around 85%, increasing the ionomer concentration can promote hydrogen evolution reaction (HER), with FEs for H₂ even exceeding 60%, due to worsened catalyst distribution and the clogging of CO₂ pathways to the active catalyst sites. In contrast, <i>Sustainion</i>-based GDEs effectively suppress HER across all catalyst-to-ionomer ratios, achieving high FEs for formate, in the range of 60-90%. However, even with <i>Sustainion</i>, excessive ionomer loading leads to pore clogging, limited CO₂ accessibility, and decreased formate production. To further enhance stability, PTFE is introduced as an additive alongside <i>Sustainion</i>, tuning the hydrophobicity of the CL. By optimizing the amount of PTFE to add, we achieve continuous operation for 24 h, maintaining a high FE for formate (∼85%) and keeping HER below 10%, with formate rates of 8.92 mmol m^-2 s^-1 and single-pass conversion efficiencies of 5.81%. Stability studies reveal that <i>Nafion</i>- and <i>Sustainion</i>-only GDEs suffer from electrolyte flooding over time, which limits the CO₂ transport and accelerates HER. In contrast, flooding can be prevented on PTFE-modified GDEs, enabling permanent catalyst accessibility and preventing high HER rates. These findings underscore the critical role of CL composition in achieving prolonged GDE stability. By leveraging anion-conducting ionomers and optimizing hydrophobicity, this work provides a pathway toward the scalable deployment of ERCO₂ in formate technology.