Alex Welch1 Joseph DuChene1 Giulia Tagliabue1 Artur Davoyan1 Wen-Hui Cheng1 Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States

Here we report a large-area, catalytically-active and highly stable nanoporous gold (np-Au) electrode for (photo)electrocatalytic CO2 reduction in aqueous electrolytes. A one μm thick np-Au electrode has a 27x larger electrochemical activity than a comparable planar Au film, owing to increased surface area. The np-Au structure is fabricated via electron beam co-deposition of a gold/silver (Au/Ag) alloy of tunable elemental composition (10/90 to 30/70) and variable thickness (0.1 μm to 2 μm), followed by a chemical etch of silver with nitric acid to yield a monolithic, np-Au structure. Depending on the etching temperature, we can alter the Au feature size from 10 nm to 25 nm. The np-Au also possesses near-unity absorption throughout a broad portion of the visible spectrum from 400 nm to 600 nm. Gas chromatography indicates H2 and CO as the main CO2 reduction products from np-Au electrodes in a 50 mM K2CO3 buffer. Although produced in a similar CO:H2 product ratio vs applied potential as that obtained for planar Au electrodes, the increased surface area results in significantly higher partial current densities for CO than planar Au. Chronoamperometry measurements indicate stable np-Au electrode operation over periods exceeding 24 h at a potential of −0.5 V vs. RHE and a cathode current of −6 mA/cm2. The faradaic efficiency remains similar across different thickness and etch temperatures, although the current does increase as the surface area of the np-Au layer increases. These np-Au electrodes show a photocatalytic response marked by an increase in current density of ~1mA/cm2 upon irradiation with white light at an incident power of 1.5 W/cm2. Changes in the product selectivity observed from np-Au photocathodes as a function of electrolyte temperature, illumination wavelength, and incident light power will be discussed. In summary, np-Au electrodes exhibit both high electrocatalytic activity and sustained electrochemical stability, making them interesting candidates as cathodes for (photo)electrochemical CO2 reduction.