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 new motif for highly catalytically active, large-area, stable nanoporous metal electrodes of Au and Cu with a nanofoam morphology, synthesized by dealloying of metal alloy thin films. This process yields 500 nm thick nanoporous Au electrodes with 5-15x larger electrochemical activity that than for comparable planar Au films, owing to increased surface area. The nanoporous Au (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 1 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 10-25nm. The np-Au also possesses near-unity absorption throughout a broad portion of the visible spectrum from (400nm to 600 nm). Gas chromatography indicates H2 and CO as reduction products of CO2 for np-Au electrodes in a 50 mM K2CO3 buffer at pH 6.8 with similar CO:H2 product ratio vs applied potential as that obtained for flat Au electrodes. Chronoamperometry measurements indicate stable np-Au electrode operation over periods > 6 hours at a potential of -1.34 V vs RHE and current of -10mA/cm2. Nanoporous Cu (np-Cu) electrodes are synthesized in a similar way, but using a Cu/Al alloy and HCl as the etchant for selective Al removal. Both np-Cu and np-Au electrodes show a photocatalytic response marked by a small increase in current density of ~5μA/cm2 upon irradiation with white light at a power of 300mW/cm2. Experiments to understand the product selectivity for np-Au and np-Cu photocatalytic electrodes as a function of temperature, illumination wavelength, and incident light power will be discussed. In summary, nanoporous metallic electrodes exhibit high activity and stability, and are interesting candidates as cathodes for electrochemical and photoelectrochemical CO2 reduction.