The electrochemical reduction of carbon dioxide (ECR) to chemical fuels presents a large-scale solution for storing renewable electricity. Methane (CH4), with its high energy density (55.5 MJ kg⁻¹), widespread use, and large market share (~23% of global energy use), is a particularly attractive target. Producing CH4 from CO2 and renewable electricity would allow direct integration into existing natural gas infrastructure, paving the way for decarbonization. However, practical ECR systems require operation at high current densities (>300 mA cm⁻²) with high selectivity and energy efficiency. High product concentrations and avoidance of CO2 loss to carbonate crossover are crucial for reducing separation costs. Gas-phase ECR systems, using flow cells or membrane electrode assemblies (MEAs), have shown promising results, achieving high Faradaic efficiencies (FEs) (60–70%) at relatively high current densities (200–300 mA cm⁻²). Yet, these systems often rely on alkaline electrolytes or anion exchange membranes, leading to significant carbonate formation or crossover, requiring additional energy for electrolyte or CO2 recycling. Bicarbonate-fed systems offer an integrated approach, generating CO2 in situ within the reactor through the reaction of protons (from a bipolar membrane) with bicarbonate ions. This approach offers effective carbon utilization, bypassing energy-intensive CO2 extraction. It also allows for higher gas product concentrations due to the reduced mixing with the CO2 feedstock. Although bicarbonate-fed systems have demonstrated high CO and formate selectivities at relatively high current densities, CH4 selectivity has remained relatively low compared to gas-phase systems. The authors hypothesized that this limited performance results from an uncontrolled reaction environment (local CO2 concentration and pH) and the lack of highly selective catalysts, particularly given that Cu reconstruction at high current density tends to favor the hydrogen evolution reaction. This research aims to address these limitations.

Previous research on electrochemical CO2 reduction (ECR) to produce methane has explored various approaches. Gas-phase systems, utilizing either flow cells or membrane electrode assemblies (MEAs), have demonstrated success in achieving high Faradaic efficiencies (FEs) at significant current densities. However, many of these systems employ alkaline electrolytes or anion exchange membranes, resulting in challenges related to carbonate formation or crossover, thus increasing energy consumption for electrolyte or CO2 recycling. More recently, bicarbonate-fed systems have emerged as an alternative, integrating CO2 capture and conversion steps. These systems generate CO2 in situ, enhancing carbon utilization and enabling higher product concentrations. However, existing bicarbonate-fed systems have exhibited comparatively lower CH4 selectivity than their gas-phase counterparts. This study addresses the need for improved CH4 selectivity and efficiency in bicarbonate-fed ECR systems. The researchers aimed to overcome the limitations of previous approaches by focusing on optimizing the reaction environment and employing an in-situ catalyst activation strategy.

The study employed a multi-faceted approach combining experimental and computational techniques. A one-dimensional multiphysics model was developed to investigate the role of CO2 availability in a bicarbonate-fed ECR system. The model considered key physical phenomena, including species and charge transport, electrocatalytic reactions, CO2 phase transfer, and buffer equilibrium reactions. This modeling allowed for a detailed CO2 accounting at different current densities to understand the dynamics within the system. The experimental setup utilized a two-electrode MEA flow cell with a copper mesh cathode and a nickel foam anode, separated by either a bipolar membrane (BPM) or an anion exchange membrane (AEM). A 0.3 M KHCO3 solution saturated with CO2 served as the electrolyte. Electrochemical CO2 reduction was performed under various conditions, including constant current, cyclic voltammetry (CV) for catalyst activation, and square-wave alternating current operation. The effects of oxidation and reduction current and time on CH4 selectivity were investigated systematically. Catalyst surface changes were characterized using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Product concentrations in the outlet stream were analyzed using gas chromatography (GC) and nuclear magnetic resonance spectroscopy (NMR). The Faradaic efficiency (FE) of various products was calculated, considering the reduction and oxidation cycles. Electrochemical double-layer capacitance measurements were used to estimate the surface roughness factors of the catalysts. The model was validated against experimental data obtained under different configurations, including using an AEM, varying sparging gases, and at various electrolyte concentrations.

The modeling revealed the importance of electrode pore size on local CO2 availability. Large Cu pores enhanced the transport of dissolved CO2 and favored bicarbonate conversion into CO2, leading to higher local CO2 concentrations. An open matrix design significantly improved performance at high current densities compared to a dense matrix. The in-situ catalyst activation strategy, employing alternating current operation, proved crucial for maintaining high CH4 selectivity. This strategy periodically oxidized and reconstructed the Cu catalyst surface, suppressing the hydrogen evolution reaction and promoting CH4 production. Optimization of oxidation and reduction conditions further enhanced CH4 selectivity. At an oxidation current density of 2.5 mA cm⁻² and a reduction time of 5 s, CH4 FEs exceeding 70% were achieved across a wide current density range (100–750 mA cm⁻²). A record-high CH4 partial current density of over 500 mA cm⁻² was achieved. SEM analysis showed that the alternating current operation resulted in a porous Cu surface morphology, contrasting with the Cu nanoparticle formation observed under constant current conditions. XPS analysis revealed similar ratios of copper oxide and metallic copper in both cases. Control experiments showed that both low surface roughness factors and alternating current operation were essential for achieving high CH4 FEs. A high surface area Cu electrode, with significantly increased roughness, primarily produced ethylene instead of methane. Experimental validation confirmed that both dissolved CO2 and in-situ generated CO2 were necessary for high CH4 selectivity at high current densities. The system demonstrated a record-high CH4 concentration of up to 23.5% in the gas outlet stream. Long-term stability tests showed CH4 FEs remained above 70% for at least 12 h at current densities of 250 and 500 mA cm².

The findings demonstrate a significant advancement in electrochemical CO2 reduction to methane. The combination of an open matrix Cu electrode and an in-situ catalyst activation strategy addresses key limitations of previous systems. The open matrix design effectively utilizes both dissolved CO2 and in-situ generated CO2, maximizing CO2 utilization. The alternating current activation maintains high selectivity towards CH4 even at high current densities, significantly improving energy efficiency and productivity compared to prior art. The achieved CH4 partial current density surpasses most previously reported values, and the CH4 product concentration is also the highest reported. The model accurately predicted experimental trends, confirming the critical role of CO2 transport and the effectiveness of the open matrix design. This work highlights a new strategy for designing and optimizing electrochemical CO2 reduction systems, moving closer towards practical and economically viable CO2 conversion.

This study demonstrates a highly efficient and selective electrochemical system for converting CO2 to methane. The use of an open matrix copper electrode coupled with an in-situ catalyst activation strategy significantly improved methane selectivity and production rate. The system achieved record-high methane partial current density, product concentration, and stability, surpassing previous results obtained with either gas-phase or bicarbonate-fed systems. Future research could focus on further optimizing the gas-liquid separator to reduce CO2 loss in the outlet stream and exploring different electrode materials and architectures to further enhance performance.

While this study achieved impressive results, some limitations exist. The model is a one-dimensional representation, which simplifies the complex transport phenomena within the porous electrode. The long-term stability tests were conducted for a maximum of 12 hours, and longer-term studies are needed to fully assess the system's durability. The impact of potential impurities leached from the anode requires further investigation, despite experiments using different anodes showing similar results. The study focused on a specific electrolyte concentration, and further investigation is required to explore the performance at broader ranges.