The electrochemical reduction of CO₂ (CO₂R) to valuable liquid fuels offers a sustainable solution for storing renewable energy and mitigating CO₂ emissions. Significant progress has been made in understanding CO₂R pathways and developing gas-diffusion-electrode-based electrolyzers capable of high current densities. However, improving full-cell energy efficiency and maximizing CO₂ utilization remains crucial for industrial viability. Acidic electrolytes offer advantages by minimizing carbonate formation, but the slower CO₂R kinetics and competing HER in these conditions pose challenges. Previous strategies using alkali metal cations to enhance CO₂R are limited by high-potential turbulence. This study explores an alternative approach: enhancing the surface coverage of CO₂R intermediates to promote CO₂R and suppress HER, particularly in acidic conditions and at high current densities. This strategy aims to achieve industrially relevant CO₂R performance with high energy and carbon efficiencies.

Previous research has demonstrated that modifying Cu-based alloy surfaces with elements like Zn, Al, or Pb can tune CO binding and OCHO binding energies, improving selectivity for specific products. For example, incorporating Zn or Al weakens CO adsorption, favoring C2+ production, while adding Pb enhances formic acid selectivity. Strategies to curb HER in strong acids include using alkali metal cations to hinder proton diffusion or increasing the coverage of CO₂R intermediates. However, the cation approach faces limitations at high current densities due to turbulent electroconvective flows. Using nanoparticle coatings has shown some success in mitigating this, but bicarbonate precipitation can still be problematic. Employing cation-free systems, such as solid-state electrolyte (SSE) based membrane electrode assemblies (MEAs), offers a promising alternative. This study builds upon these existing approaches by focusing on the inherent promotion of CO₂R over HER through catalyst design and optimizing surface intermediate coverage in a cation-free acidic environment.

This research employed a combined theoretical and experimental approach. Density Functional Theory (DFT) calculations using the Vienna Ab-initio Simulation Package (VASP) were conducted to investigate CuSnx catalysts, exploring different Cu/Sn ratios to identify optimal compositions for maximizing FA production and minimizing HER. The Gibbs free energy differences between OCHO and COOH intermediates, as well as H adsorption energies, were calculated to assess the thermodynamic feasibility of FA production and HER suppression. The differential charge density analysis was used to study the electronic properties of the surface active sites and their influence on selectivity. Experimentally, CuSnx catalysts were synthesized via thermal evaporation onto PTFE gas diffusion electrodes. Electrochemical CO₂R performance was evaluated in both alkaline (1 M KOH) and acidic (3 M KCl and 0.05 M H₂SO₄, pH 1) electrolytes using a three-electrode flow cell. Gas and liquid products were quantified using gas chromatography (GC), nuclear magnetic resonance spectroscopy (NMR), and ion chromatography (IC). In situ electrochemical attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was used to examine the surface coverage of CO₂R intermediates under various electrochemical conditions. Finally, a cation-free SSE-based MEA electrolyzer was employed for the continuous production of pure FA solutions. The electrolyzer utilized an anion exchange membrane (AEM) and a proton exchange membrane (PEM) along with a solid-state electrolyte layer. The anode was IrOx/Ti foam, and the cathode was the optimized CuSnx catalyst. Formic acid production rates, energy efficiency, and stability were examined. Characterization techniques included SEM, TEM, HRTEM, SAED, XRD, and XPS to analyze the catalyst's morphology, structure, and composition.

DFT calculations identified Cu₆Sn₅ as a promising catalyst due to its strong OCHO affinity and weak H binding, leading to high selectivity for FA production and suppressed HER. Experimentally, Cu₆Sn₅ exhibited exceptional performance in acidic electrolytes. A Faradaic efficiency (FE) exceeding 90% for FA production was achieved across a wide current density range (0.4–1.2 A cm⁻²), reaching a peak of 91% at 1.2 A cm⁻². At a current density of 0.5 A cm⁻², the single-pass carbon efficiency (SPCE) reached a remarkable 77.4%, a significant improvement over previous reports. This high performance was maintained for over 300 hours of continuous operation. In situ ATR-FTIR spectroscopy confirmed a significant (2.8x) enhancement in OCHO surface coverage on Cu₆Sn₅ compared to Sn under identical acidic conditions, supporting the mechanism of enhanced intermediate coverage promoting selectivity and efficiency. The cation-free SSE-based MEA electrolyzer demonstrated stable production of pure FA solution (0.36 M) with 88% FE and a full-cell energy efficiency of 37% over 130 hours of continuous operation. The Cu₆Sn₅ catalyst exhibited a uniform distribution of Cu and Sn, and the catalyst layer displayed hydrophobicity, promoting CO₂ diffusion. The formation of monoclinic Cu₆Sn₅ crystals was confirmed by TEM, HRTEM, and SAED analysis. The experimental data strongly supports the DFT predictions regarding the superior performance of Cu₆Sn₅ in this context.

The findings demonstrate the effectiveness of enhancing surface intermediate coverage as a strategy for high-performance acidic CO₂R. The Cu₆Sn₅ catalyst's superior performance is attributed to its ability to selectively bind OCHO while repelling H, promoting FA formation and suppressing the competing HER. The use of in situ ATR-FTIR spectroscopy provided direct experimental evidence of enhanced OCHO coverage, corroborating the theoretical predictions. The success of the cation-free SSE-based MEA electrolyzer showcases the viability of this approach for continuous, high-efficiency production of pure FA solutions. These results represent a significant advancement in CO₂R technology, paving the way for the development of more efficient and sustainable carbon capture and utilization processes.

This study successfully demonstrates the enhancement of acidic CO₂ electroreduction to formic acid through controlled intermediate coverage on Cu₆Sn₅ catalysts. The combined theoretical and experimental approach confirmed the superior performance of Cu₆Sn₅ in achieving high Faradaic efficiency, single-pass carbon efficiency, and long-term stability under acidic conditions. The successful implementation of a cation-free solid-state electrolyte MEA electrolyzer further highlights the potential for industrial-scale application of this technology. Future research could focus on further optimizing catalyst design to enhance activity and durability and exploring different MEA configurations for improved scalability and cost-effectiveness.

While this study presents significant advancements, certain limitations should be noted. The DFT calculations were performed under idealized conditions, and the actual electrochemical environment could differ. The long-term stability tests were conducted under specific conditions, and further investigation into the catalyst's behavior under wider operating parameters is warranted. Scaling up the SSE-based MEA electrolyzer for industrial applications will require further engineering and optimization efforts. Furthermore, the cost-effectiveness of the materials and manufacturing process needs to be assessed for large-scale implementation.