Monolithic Photoelectrochemical CO 2 Reduction Producing Syngas at 10% Efficiency

Increasing anthropogenic carbon dioxide emissions have prompted the search for photoelectrochemical (PEC) methods of converting CO 2 to useful commodity products, including fuels. Ideally, such PEC approaches will be sustained using only sunlight, water, and CO 2 as energetic and reactant inputs. However, low peak conversion efficiencies (<5%) have made commercialization of fully‐integrated PEC devices prohibitive. Here, a 4 cm 2 monolithic PEC device exceeding 10% solar‐to‐fuel efficiency with principal fuel products of carbon monoxide and hydrogen is reported. The corresponding solar‐to‐CO and solar‐to‐H 2 efficiencies are 7% and 3.5%, respectively. Screening of a range of operating conditions reveals a tunable product mixture of H 2 and CO using a gold electrocatalyst. Accordingly, it is shown that device optimization yields a H 2 ‐to‐CO ratio of 1:2 commonly present in synthesis gas (syngas). Notably, the modularity and facile fabrication of this device permit the incorporation of a broad array of materials for various applications. For example, the electrocatalyst may easily be swapped to target a different set of products.