Mitigating mass transport limitations: hierarchical nanoporous gold flow-through electrodes for electrochemical CO2 reduction

The reaction rates for electrochemical CO2 reduction in aqueous electrolytes can be limited by the low concentration and diffusion rate of the reactant CO2. To overcome this limitation, we fabricated and tested hierarchical nanoporous gold (hnpAu) flow-through electrodes that enable pumping CO2 saturated potassium bicarbonate electrolyte directly through the macropores of hnpAu. The hnpAu flow-through electrode was fabricated by ink casting followed by annealing and dealloying to generate a bimodal pore distribution where macropores (5-10 mu m) act as flow channels and nanopores (similar to 40 nm) provide high surface area. To minimize local CO2 depletion at the electrolyte-electrode interface we selected flow rates that warranted diffusion lengths exceeding the macropore diameter dimensions. We observed that the CO2 reduction rate increases with increasing flow rate through the electrode, but even for the highest overpotentials and flow rates, less than 20% of the dissolved CO2 was utilized despite the absence of diffusion limitations. We find that the competing hydrogen evolution reaction becomes increasingly suppressed with increasing flow rate, specifically in the low overpotential regime. These observations suggest that, beyond diffusion limitations, the CO2 reduction rate in aqueous electrolytes may become limited by the availability of free reaction sites, and that flowing the electrolyte through the electrode increases the CO2 reduction rate by facilitating the removal of the CO product.