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

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 eCO(2)RR, 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.