From batch to flow: the effect of pH, current, and the crystal facets of Cu2O on electrochemical CO2 reduction

Abstract

As humanity is confronted by climate change, electrochemical CO₂ reduction has become an important strategy for generating value-added chemicals whilst lowering carbon emissions. In this work, Cu₂O nanoparticles with different morphologies and predominant exposed crystal facets, including nanocubes (Cu₂O-NC) with (100) facets, nanoflowers (Cu₂O-NF) with (110) facets, and octahedral structures (Cu₂O-O) with (111) facets, are prepared, and compared as catalysts for the electrochemical CO₂ reduction to C₂₊ products in a flow cell electrolyzer, to overcome the mass transfer limitations of CO₂ in an H-cell and reach industrially relevant currents. To maximize the performance towards C₂₊ products, a parameter optimization (i.e. pH and current) was performed. Under the conditions of 150 mA cm⁻² and pH of 8.5, the Cu₂O-NC revealed a maximum faradaic efficiency of 58% for C₂₊ products. Similar studies in an H-cell system have shown lower total C₂₊ FE of around 35–40% for the nanocubes, indicating an improvement and showcasing the advantages of using a gas-fed flow electrolyzer. Finally, the long-term stability of these materials was also evaluated. The results revealed that C₂₊ activity remains constant for four hours at 50%. However, a sharp decline was observed after five hours when GDE flooding occurs, leading to a dominant HER. To confirm, the electrode was washed and dried before re-utilizing it. Since the Cu₂O largely recovers its initial C₂₊ activity (from 50% to 43%) albeit with a slightly different product composition, this confirms GDE flooding as the main cause of degradation. During the eCO₂RR, Cu₂O is reduced to metallic Cu, as proven by in situ Raman. As a result, the particle morphology is roughened, which is proven by ex situ SEM images. Subsequently, water penetrates the gas diffusion electrode more easily, inhibiting the diffusion of CO₂, and alongside the electrowetting effect results in GDE flooding. In conclusion, this work explores the utilization of different Cu₂O catalysts in a flow electrolyzer, revealing insights into higher currents, their stability issues, crystal facet dependency, and reaction environment, which was unexplored in previous literature where the focus was on H-cell testing, which evaluates the catalysts under less demanding and more controlled conditions that are less relevant for up-scaling and eventual commercialization. Based on these new insights, further improvements can be made to enhance the total C₂₊ FE and improve stability.