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 Nafion, a proton-conducting ionomer, with Sustainion, an anion-conducting ionomer. While Nafion-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, Sustainion-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 Sustainion, 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 Sustainion, 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 Nafion- and Sustainion-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.