Biomethane production requires the upgrading of biogas streams, in which CO₂ must be removed or valorized to increase methane content and energy density. In this context, electrochemical CO₂ reduction to CH₄ offers a promising alternative to conventional upgrading processes by directly converting CO₂ into a valuable fuel using renewable electricity [1].

In this work, the electrochemical conversion of CO₂ to methane is evaluated in a membrane electrode assembly (MEA) reactor using humidified CO₂. Different Cu-based MOF materials are employed as catalysts [2]. The catalytic layer is spray‑coated onto a carbon substrate (Sigracet 39 BB) with a loading of 1.5 mg cm⁻², and electrodes are prepared using either Nafion (cation‑conducting) or Sustainion (anion‑conducting) ionomers.

The results clearly demonstrate that the electrode formulation strongly influences reaction selectivity. Nafion‑based electrodes promote hydrogen evolution, resulting in low methane Faradaic efficiencies (FEs) (5–10%). In contrast, the use of Sustainion significantly enhances CH₄ production, achieving FEs up to 25% at 50 mA cm⁻². This improvement is attributed to enhanced anion transport within the catalyst layer, which suppresses the competing hydrogen evolution reaction and favors CO₂ reduction pathways toward methane.

Once the electrode composition is optimized, the study focuses on the evaluation of advanced Cu‑based MOF catalysts, which show a strong potential to improve methane selectivity and activity. Among the materials investigated, Cu‑UiO66, Cu‑MOF‑5, and Cu‑ZIF‑8 derivatives exhibit markedly different performances due to their structural characteristics and metal distribution [3].

Cu‑UiO66, used as the reference catalyst, provides moderate methane selectivity, serving as a stable and well‑defined platform for electrode optimization. However, its performance is surpassed by other MOF‑based structures with higher surface areas and more favorable active site configurations. In particular, Cu‑MOF‑5 prepared via metalation displays the best catalytic performance, reaching methane FEs of up to 44.3% at 50 mA cm⁻². This enhanced activity is attributed to its high porosity, large surface area, and the presence of well‑dispersed Cu active sites, which facilitate CO₂ adsorption and the subsequent multi‑electron transfer steps required for CH₄ formation.

Similarly, Cu‑ZIF‑8 materials show promising results, especially for intermediate Cu/Zn ratios. The Cu‑ZIF‑8 (1:1 Cu/Zn) catalyst achieves methane FEs of up to 27.8%, highlighting the beneficial role of Zn in modulating reaction pathways. The presence of Zn is believed to promote the formation of CO intermediates, which are subsequently hydrogenated on Cu active sites to yield methane. This synergistic effect between Cu and Zn enhances overall selectivity toward CH₄ compared to single‑metal systems.

Overall, the study demonstrates that both electrode design and catalyst selection are critical parameters in maximizing methane production. While the use of Sustainion ionomer significantly improves selectivity at the electrode level, the choice of catalyst plays a decisive role in achieving high methane FEs. Among the materials tested, Cu‑MOF‑5 and Cu‑ZIF‑8 stand out as the most promising candidates, outperforming the benchmark Cu‑UiO66 and paving the way for further optimization and scale‑up of electrochemical biogas upgrading technologies.