Transition-Metal Single Atoms in a Graphene Shell as Active Centers for Highly Efficient Artificial Photosynthesis

Utilizing solar energy to fix CO2 with water into chemical fuels and oxygen, a mimic process of photosynthesis in nature, is becoming increasingly important but still challenged by low selectivity and activity, especially in CO2 electrocatalytic reduction. Here, we report transition-metal atoms coordinated in a graphene shell as active centers for aqueous CO2 reduction to CO with high faradic efficiencies over 90% under significant currents up to ∼60 mA/mg. We employed three-dimensional atom probe tomography to directly identify the single Ni atomic sites in graphene vacancies. Theoretical simulations suggest that compared with metallic Ni, the Ni atomic sites present different electronic structures that facilitate CO2-to-CO conversion and suppress the competing hydrogen evolution reaction dramatically. Coupled with Li+-tuned Co3O4 oxygen evolution catalyst and powered by a triple-junction solar cell, our artificial photosynthesis system achieves a peak solar-to-CO efficiency of 12.7% by using earth-abundant transition-metal electrocatalysts in a pH-equal system. [Display omitted] •Single Ni atoms coordinated in graphene exhibit superb catalytic CO2RR performance•Atomic-resolution 3D APT provides direct identification of single Ni atoms•DFT simulations suggest that the tuning of electronic structure favors CO2RR over HER Using clean electricity to reduce CO2 to chemicals or fuels is becoming increasingly important to renewable energy applications and environmental protection. The challenge comes from the strong competition with the hydrogen evolution reaction in aqueous solutions, especially for those earth-abundant transition metals such as Ni, which dramatically lowers the CO2 reduction selectivity. Isolating the transition-metal single atoms into a graphene matrix can significantly tune their catalytic behaviors to favor the CO2-to-CO reduction pathway, reaching a high CO selectivity of more than 90%. This work creates an important platform in designing active and low-cost CO2 reduction catalysts with high selectivity toward fuels, opening up great opportunities for both technological applications in renewable energies and fundamental mechanism studies in catalysis. State-of-the-art three-dimensional atom probe tomography provides direct evidence of Ni single atoms coordinated in graphene vacancies for highly selective CO2 reduction to CO and suppressed hydrogen evolution in water.