Engineering Bilayer Tandem Catalysts on Si-Based Photocathodes for High-performance CO₂ Reduction to Produce Methane

Solar-powered CO₂ reduction through photoelectrochemical (PEC) approaches to produce hydrocarbon fuels, such as methane (CH₄), is one of the most promising paths for supplying sustainable fuels. However, the limited light absorption capability and sluggish kinetics restrict the photocatalytic rate and selectivity for hydrocarbon production. Here, we introduce tandem catalysts on photocathodes designed to enhance controlled sequential reactions involving intermediates and thus the selectivity of CO₂ reduction. Specifically, when mounted on Cu/Ag-Cu bilayer catalysts, the p-type Si photocathode with a pyramid-structured surface dramatically improves CO₂-to-CH₄ conversion, achieving a selectivity of 60.2 ± 3.4% and a working current density of -32.9 ± 1.9 mA cm^-2 at -1.1 V vs. RHE. As identified by operando Raman and synchrotron-radiation Fourier transform infrared spectroscopy and Density Functional Theory, the bottom layer of the Cu/Ag-Cu catalysts comprises Ag and Cu nanoparticles, which catalyse the initial reduction of CO₂ to form *CO and the creation of *H species dissociated from H₂O, respectively. The top Cu layer subsequently enables the protonation of *CO to *CHO, ultimately yielding CH₄. This design of tandem catalysts, coupled with a thorough investigation of the reaction mechanisms, offers a powerful approach toward high-performance and selective pathways for solar-powered CO₂ reduction to targeted products.