Employing a low-cost and highly efficient electrocatalyst to replace Pt-based catalysts for hydrogen evolution reaction (HER) has attracted increasing interest in renewable energy research. Earth-abundant transition metals such as Fe, Co and Ni have been investigated as promising alternatives in alkaline electrolytes. However, these non-precious-metal catalysts are not stable in acids, excluding their application in the acidic solid polymer electrolyte (SPE). Herein, we report a strategy to encapsulate 3d transition metals Fe, Co and the FeCo alloy into nitrogen-doped carbon nanotubes (CNTs) and investigated their HER activity in acidic electrolytes. The optimized catalysts exhibited long-term durability and high activity with only an ∼70 mV onset overpotential vs. RHE which is quite close to that of the commercial 40% Pt/C catalyst, demonstrating the potential for the replacement of Pt-based catalysts. Density functional theory (DFT) calculations indicated that the introduction of metal and nitrogen dopants can synergistically optimize the electronic structure of the CNTs and the adsorption free energy of H atoms on CNTs, and therefore promote the HER with a Volmer–Heyrovsky mechanism.

Coordinatively unsaturated (CUS) iron sites are highly active in catalytic oxidation reactions; however, maintaining the CUS structure of iron during heterogeneous catalytic reactions is a great challenge. Here, we report a strategy to stabilize single-atom CUS iron sites by embedding highly dispersed FeN₄ centers in the graphene matrix. The atomic structure of FeN₄ centers in graphene was revealed for the first time by combining high-resolution transmission electron microscopy/high-angle annular dark-field scanning transmission electron microscopy with low-temperature scanning tunneling microscopy. These confined single-atom iron sites exhibit high performance in the direct catalytic oxidation of benzene to phenol at room temperature, with a conversion of 23.4% and a yield of 18.7%, and can even proceed efficiently at 0°C with a phenol yield of 8.3% after 24 hours. Both experimental measurements and density functional theory calculations indicate that the formation of the Fe═O intermediate structure is a key step to promoting the conversion of benzene to phenol. These findings could pave the way toward highly efficient nonprecious catalysts for low-temperature oxidation reactions in heterogeneous catalysis and electrocatalysis.

Coordinatively unsaturated Ni–N active sites facilitate CO₂electroreduction and inhibit the competitive hydrogen evolution reaction, demonstrating selective and high-rate CO₂electroreduction.