Engineering Bilayer Tandem Catalysts on Si‐Based Photocathodes for High‐performance CO 2 Reduction to Produce Methane

ABSTRACT Solar‐powered CO 2 reduction through photoelectrochemical (PEC) approaches to produce hydrocarbon fuels, such as methane (CH 4 ), 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 CO 2 reduction. Specifically, when mounted on Cu/Ag‐Cu bilayer catalysts, the p‐type Si photocathode with a pyramid‐structured surface dramatically improves CO 2 ‐to‐CH 4 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 CO 2 to form *CO and the creation of *H species dissociated from H 2 O, respectively. The top Cu layer subsequently enables the protonation of *CO to *CHO, ultimately yielding CH 4 . 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 CO 2 reduction to targeted products.