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Due to the increasing attention focused on global warming, many studies on reducing CO2 emissions and developing sustainable energy strategies have recently been performed. One of the approaches is CO2 methanation, transforming CO2 into methane. Such transformation (CO2 + 4H2 → CH4 + 2H2O) provides advantages of carbon liquification, storage, etc. In this study, we investigated CO2 methanation on nickel–magnesium–alumina catalysts both experimentally and computationally. We synthesized the catalysts using a precipitation method, and performed X-ray diffraction, temperature-programmed reduction, and N2 adsorption–desorption tests to characterize their physical and chemical properties. NiAl2O4 and MgAl2O4 phases were clearly observed in the catalysts. In addition, we conducted CO2 hydrogenation experiments by varying with temperatures to understand the reaction. Our results showed that CO2 conversion increases with Ni concentration and that MgAl2O4 exhibits high selectivity for CO. Density functional theory calculations explained the origin of this selectivity. Simulations predicted that adsorbed CO on MgAl2O4(100) weakly binds to the surface and prefers to desorb from the surface than undergoing further hydrogenation. Electronic structure analysis showed that the absence of a d orbital in MgAl2O4(100) is responsible for the weak binding of CO to MgAl2O4. We believe that this finding regarding the origin of the CO selectivity of MgAl2O4 provides fundamental insight for the design methanation catalysts.

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CuS is an encouraging photoelectrode candidate that meets the essential requirements for efficient solar‐to‐hydrogen production, but it has not been thoroughly studied. A CuS light absorber layer is grown by the self‐assembly of copper and sulfur precursors on a carbon paper (CP) electrode. Simultaneously, rGO is introduced as a buffer layer to control the optical and electrical properties of the absorber. The well‐ordered microstructural arrangement suppresses the recombination loss of electrons and holes owing to enhanced charge‐carrier generation, separation, and transport. The potential reaching 10 mA cm−2 in 1.0 m KOH solution is significantly lowered to 0.87 V, and the photocurrent density at 1.23 V is 94.7 mA cm−2. The computational result reveals that the potential‐determining step is sensitive to O* stability; the lower stability of O* in the thin layer of CuS/rGO decreases the free‐energy gap between the initial and final states of the potential‐determining step, resulting in a lowering of the onset potential. The faradaic efficiency for the photoelectrochemical oxygen evolution reaction in the optimized 2CuS/1rGO/CP photoanode is 98.60%, and the applied bias photon‐to‐current and the solar‐to‐hydrogen efficiencies are 11.2% and 15.7%, respectively, and its ultra‐high performance is maintained for 250 h. These record‐breaking achievement indices may be a trigger for establishing a green hydrogen economy.