A scalable method for preparing Cu electrocatalysts that convert CO₂ into C₂⁺ products

The electrochemical CO₂ reduction reaction (CO₂RR) is a promising strategy to mitigate global warming caused by CO₂ accumulation from fossil fuel combustion. Efficient and selective electrocatalysts are crucial for the economic viability of CO₂RR. Copper (Cu) is unique among metals for its ability to convert CO₂ into hydrocarbons and alcohols. However, improving the efficiency and selectivity of Cu-based catalysts for CO₂RR, particularly towards multi-carbon (C₂⁺) products, remains a significant challenge. The main obstacles are competing reactions: the hydrogen evolution reaction (HER) and the formation of C₁ products (e.g., CH₄, HCOOH). These reactions reduce the faradaic efficiency (FE) of C₂⁺ products by consuming electrons and protons. C₁ production also reduces the availability of adsorbed carbon intermediates for C-C coupling reactions, which are essential for C₂⁺ formation. To minimize HER, the first step of CO₂RR (formation of *COOH) needs to be enhanced, and this is affected by the [CO₂]/[H⁺] ratio near the electrode surface. High current densities at rough surfaces cause rapid pH increases, reducing efficiency. Minimizing C₁ product formation requires a catalyst design that promotes C-C coupling. Simulations suggest that Cu (100) facets are favorable for C-C coupling, and a high density of surface defects (grain boundaries, step sites, vacancies) promotes the adsorption of reactive C₁ intermediates and C-C coupling. Previous studies using electrochemical cycling methods to create cubic Cu microstructures showed improved ethylene selectivity but limited FE (15-45%). This study aims to develop a novel electrochemical method to optimize Cu catalysts for selective C₂⁺ product formation by independently controlling parameters influencing catalyst performance, such as chemical species, potential, pH, and roughness.

The literature extensively explores Cu-based electrocatalysts for CO₂RR, with various approaches used to enhance performance, including porous Cu foams, Cu nanoparticle ensembles, oxide-derived Cu, plasma-treated Cu, Cu nanocubes, Cu nanowires, and single-crystal Cu. While progress has been made, achieving high selectivity for C₂⁺ products at scale remains a challenge. Research highlights the importance of minimizing competing HER and C₁ product formation. Simulations provide insights into the energy landscape of competing reactions on Cu surfaces, suggesting that specific facets (e.g., Cu (100)) and high *CO coverage are favorable for C-C coupling. The role of surface defects in promoting C-C coupling is also emphasized. Previous studies involving electrochemical cycling to create cubic Cu microstructures showed enhanced ethylene selectivity but with relatively low faradaic efficiency (15–45%). These findings motivated the current research to explore a more controlled approach to optimize catalyst characteristics.

The study introduces a three-step electrochemical method for preparing Cu electrocatalysts: (i) anodic halogenation of electropolished Cu foils in 0.1 M KX (X = Cl, Br, I) solutions; (ii) subsequent oxide formation in 0.1 M KHCO₃ electrolyte; and (iii) electroreduction in CO₂-saturated 0.1 M KHCO₃ by linear sweep voltammetry (LSV). The anodic halogenation step forms a layer of CuX (CuCl, CuBr, or CuI) on the Cu foil surface, confirmed by grazing-incidence X-ray diffraction (GI-XRD). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to analyze morphological and chemical changes during each step. The formation of Cu₂O during immersion in KHCO₃ is explained through the reaction of CuX with hydroxide ions. The subsequent LSV reduction aims to extract halide ions while retaining a high density of surface defects. The effects of halogenation time, applied potential, and electrolyte pH on the morphology, chemical composition, and catalytic performance were investigated. The catalytic performance was evaluated by bulk electrolysis of CO₂ at constant potentials in CO₂-saturated 0.1 M KHCO₃, analyzing gaseous and liquid products using gas chromatography (GC) and nuclear magnetic resonance (NMR) to determine faradaic efficiencies (FE). The roughness factor of the catalysts was determined using the double-layer capacitance method via cyclic voltammetry (CV).

GI-XRD confirmed the formation of CuCl, CuBr, or CuI on the Cu foil surface after anodic halogenation. SEM and EDS analysis revealed significant morphological changes during halogenation, oxide formation, and reduction. The Cu(I)-halide-derived catalysts exhibited high selectivity for C₂H₄, with its highest FE (45.1% on Cu_KCl, 49.5% on Cu_KBr, and 44.5% on Cu_KI) observed at approximately -1.15 V vs. RHE. In contrast, electropolished Cu primarily produced CH₄. The catalysts also produced significant amounts of CO, an important intermediate for C₂⁺ product formation. The optimal halogenation times were found to be 60-100 s for Cu_KCl, 60-90 s for Cu_KBr, and 10 s for Cu_KI, balancing a high density of defect sites with low roughness. At these optimized times, faradaic efficiencies for C₂⁺ products reached 70.7% (Cu_KCl), 71.5% (Cu_KBr), and 72.6% (Cu_KI). Increasing halogenation time beyond the optimum led to increased HER due to higher surface roughness. The study demonstrates a strong correlation between catalyst roughness, HER, and the [CO₂]/[H⁺] ratio near the electrode surface. Higher roughness leads to higher local pH, decreasing the [CO₂]/[H⁺] ratio and thus favoring HER. The findings strongly suggest that a balance between a high density of defect sites (promoting C-C coupling) and low roughness (minimizing HER) is crucial for efficient and selective C₂⁺ product formation in CO₂RR.

The results demonstrate that the developed method produces highly efficient Cu electrocatalysts for CO₂RR to C₂⁺ products, exceeding the performance of previously reported methods. The superior performance is attributed to the successful optimization of surface characteristics, namely, the high density of defect sites while maintaining low roughness. The defect sites facilitate C-C coupling by promoting the adsorption of reactive C₁ intermediates and stabilizing Cu⁺ species and subsurface oxygen, known to enhance C₂⁺ production. The low roughness, in turn, minimizes the competing HER by reducing the local pH increase during catalysis. The method's simplicity, consistency, and scalability make it a promising approach for industrial-scale CO₂ conversion technologies. The significant improvement in FE for C₂⁺ products, reaching up to 72.6%, positions this method as a key advancement in the field.

This study presents a novel and scalable method for preparing Cu electrocatalysts with significantly improved efficiency and selectivity for converting CO₂ to C₂⁺ products. The method utilizes anodic halogenation followed by surface reconstruction to create a catalyst with a high density of defect sites and low roughness, optimally balancing C-C coupling and HER suppression. The achieved faradaic efficiencies for C₂⁺ products up to 72.6% represent a significant advancement in CO₂ reduction catalysis. Future research could explore further optimization of the method, investigating different halide ions or surface treatments to enhance catalyst performance and durability.

The study focuses primarily on ethylene production and selectivity. While other C₂⁺ products were detected, a more detailed analysis of the complete product distribution and reaction pathways for other multi-carbon products may provide further insights. The long-term stability of the catalysts under continuous operation still needs comprehensive evaluation, as surface changes and degradation could impact efficiency over time. The scope of the study is limited to specific halide ions and electrolytes; exploring alternative chemistries could potentially identify even better performing catalysts.