The escalating atmospheric CO₂ levels demand innovative solutions beyond carbon capture and storage. Electrochemical CO₂ reduction (CO₂RR) offers an attractive alternative, converting CO₂ into valuable chemicals and fuels using renewable energy. To achieve economic viability, CO₂ electrolyzers must operate at low cell voltage, high current density, high product selectivity, and exhibit long-term durability. Aqueous electrolytes limit CO₂ availability due to low solubility, resulting in a thick diffusion layer and hindering reaction rates. Gas diffusion electrodes (GDEs), consisting of a catalyst layer on a GDL, address this by directly supplying gaseous CO₂. The GDL's role is crucial; it provides a high surface area for the catalyst, ensures electrical contact, facilitates CO₂ transport, and manages water removal. While carbon-based GDLs are frequently used due to their properties (abundance, customizable porosity, conductivity, stability, and cost-effectiveness), their behavior in CO₂RR remains insufficiently understood. Previous studies in other fields (e.g., fuel cells) cannot be directly translated to CO₂ electrolyzers due to different operational conditions (temperature, water presence, ion concentrations). Commercial GDLs vary widely in composition (MPL presence, PTFE content, thickness, fiber structure), potentially impacting CO₂RR significantly. This study aims to provide a comprehensive comparison of commercially available GDLs, identifying key structural parameters that influence CO₂RR performance and guiding the rational design of improved GDLs.

Existing literature highlights the importance of gas diffusion electrodes (GDEs) for efficient CO2 electrolysis, focusing on overcoming limitations posed by the low solubility of CO2 in aqueous electrolytes. Studies have explored various aspects of GDE design, including the impact of catalyst materials, the catalyst layer structure, and the overall cell architecture. The role of the gas diffusion layer (GDL) has also been investigated, with some research emphasizing the effects of hydrophobicity and PTFE content on selectivity and performance. However, a systematic, comprehensive comparison of commercially available GDLs under identical conditions, considering multiple parameters simultaneously, has been lacking. This gap in the literature underscores the need for this study.

The researchers employed a zero-gap electrolyzer cell (8 cm²) with a Sustainion™ X37-50 anion exchange membrane (AEM). Commercially available silver nanoparticles were spray-coated onto 20 different commercially available GDLs (from six manufacturers) to create GDEs with consistent catalyst loading (1.0 mg cm⁻²). The GDLs varied in MPL presence, PTFE content (in CFL and MPL), thickness, and carbon fiber structure (woven vs. non-woven, straight vs. spaghetti-like fibers). A PTFE gasket was used to precisely control the compression ratio of each GDL, which was individually optimized for best performance. The anode used an Ir-coated porous titanium frit. Chronoamperometric measurements (1-hour) were performed at various cell voltages (2.4-3.2 V) to assess CO and H₂ production. The electrolyte (0.1 M CsOH for short-term and 0.05 M CsOH for long-term) was recirculated through the anode, and humidified CO₂ (12.5 cm³ cm⁻² min⁻¹) was fed to the cathode at 60 °C. GDL characterization included SEM, micro-CT, and contact angle measurements. Long-term electrolysis (100 h) was conducted with selected GDLs to evaluate durability. Data analysis involved correlating electrochemical performance (partial current densities, Faradaic efficiency) with GDL parameters (MPL presence, PTFE content, thickness, fiber structure, hydrophobicity, surface cracking).

The study revealed several key findings: 1. **The absence of an MPL significantly reduced CO₂RR selectivity**, favoring HER regardless of other GDL parameters. GDLs with MPLs exhibited significantly higher CO₂RR selectivity (above 90% FE) compared to their MPL-free counterparts. 2. **GDL thickness showed a minimal effect on short-term performance**, but thicker GDLs performed more stably during long-term electrolysis, demonstrating improved durability and maintaining selectivity. 3. **The impact of PTFE content was complex**. While increased hydrophobicity (higher PTFE content) is generally beneficial for CO₂RR, excessively high PTFE levels can hinder gas transport and decrease conductivity. 4. **Cracks in the MPL exhibited a non-obvious effect**: GDLs with higher crack density showed superior CO partial current density. This is linked to better water management in the zero-gap cell design. However, this conclusion is context-dependent, as water management varies with electrolyzer configuration and electrolysis conditions. 5. **Carbon fiber structure had a less significant influence.** Woven versus non-woven or straight versus spaghetti-like fibers showed minor differences in CO₂RR rate. Spaghetti-like fibers, however, were observed to be more prone to wetting. 6. **High contact angle did not directly correlate with superior electrochemical performance.** The optimal balance between hydrophobicity and gas permeability is crucial. 7. **Six top-performing GDLs consistently exceeded 200 mA cm⁻² CO partial current density at 2.6 V.** At 3.0 V, they delivered 460-580 mA cm⁻², showing excellent performance. Long-term (100 h) electrolysis demonstrated stable operation for the best GDLs with minimal degradation. Specific data points are presented in Figures 1, 2, 3, 4, 5, and 6, including partial current densities for CO and H₂, Faradaic efficiency, contact angles, and micro-CT/SEM images showcasing the GDL structures and crack formation.

This study successfully identified crucial GDL parameters influencing CO₂RR performance. The significant role of the MPL in improving selectivity underscores its importance for efficient CO₂ electrolysis. The beneficial role of cracks in GDLs in the context of water management within a zero-gap cell highlights the complexity of this system. These findings directly address the need for improved GDL design in CO₂ electrolyzers, suggesting that achieving high performance requires balancing hydrophobicity for water management, gas permeability for CO₂ transport, and electronic conductivity for efficient current flow. Future work should focus on further optimizing GDL design to mitigate the observed trade-offs, with more research into the precise influence of crack formation and water management in different cell configurations.

This comprehensive study of 20 commercially available GDLs demonstrates that the presence of a microporous layer is essential for high CO₂RR selectivity and that the optimal balance of PTFE content, thickness and crack formation in the MPL significantly influences performance. The identified relationships between GDL structure and electrochemical performance provide crucial insights for the rational design of next-generation GDLs tailored for efficient and durable CO₂ electrolysis. Future research should explore GDLs with optimized architectures, focusing on finely tuned hydrophobicity, porosity, and optimized crack structures to further enhance performance and stability across various operational conditions.

The study focused on a specific zero-gap electrolyzer cell configuration and operational conditions. The generalizability of the findings to other cell designs and conditions requires further investigation. The long-term stability experiments, while providing valuable insights, were limited to 100 hours, and longer-term tests are needed to fully assess durability under various conditions. The characterization of GDLs was performed on unused samples, and changes during operation might influence the observed correlations.