Supported single-atom catalysts (SACs) possess high catalytic activity, selectivity, and atom utilizations. However, the atom coordination environments of SACs are difficult to accurately regulate due to the high complexity of coordination site and local environment of support. Herein, we develop an in-situ electrochemical cation-exchange method to fill the cation vacancies in MnO₂ with Ru single atoms (SAs). This obtained catalyst exhibits high mass activity, which is ~ 44 times higher than commercial RuO₂ catalyst and excellent stability, superior to the most state-of-the-art oxygen evolution reaction (OER) catalysts. The experimental and theoretical results confirm that the doped Ru can induce charge density redistribution, resulting in the optimized binding of oxygen species, and the strong covalent interaction between Ru and MnO₂ for resisting oxidation and corrosion. This work will provide a new concept in the synthesis of well-defined local environments of supported SAs.

Hydrogen, especially the "green hydrogen" based on water electrolysis, is of great importance to build a sustainable society due to its high-energy-density, zero-carbon-emission features, and wide-range applications. Today's water electrolysis is usually carried out in either low-temperature (< 100 ℃), e.g., alkaline electrolyzer, or high-temperature (> 700 ℃) applications, e.g., solid oxide electrolyzer. However, the low-temperature devices usually suffer from high applied voltages (usually > 1.5 V @0.01 A cm⁻²) and high cost; meanwhile, the high-temperature ones have an unsatisfied lifetime partially due to the incompatibility among components. Reasonably, an intermediate-temperature device, namely, proton ceramic cell (PCC), has been recently proposed. The widely-used air electrode for PCC is based on double O^2-/e^- conductor or composited O^2-/e^-−H⁺ conductor, limiting the accessible reaction region. Herein, we designed a single-phase La_0.8Sr_0.2Co_1-xMn_xO_3-δ (LSCM) with triple H⁺/O^2-/e^- conductivity as the air electrode for PCCs. Specifically, the La_0.8Sr_0.2Co_0.8Mn_0.2O_3-δ (LSCM8282) incorporates 5.8% proton carriers in molar fraction at 400 ℃, indicating superior proton conducting ability. Impressively, a high current density of 1580 mA cm⁻² for hydrogen production (water electrolysis) is achieved at 1.3 V and 650 ℃, surpassing most low- and high-temperature devices reported so far. Meanwhile, such a PCC can also be operated under a reversible fuel cell mode, with a peak power density of 521 mW cm⁻² at 650 ℃. By correlating the electrochemical performances with the hydrated proton concentration of single-phase triple conducting air electrodes in this work and our previous work, a principle for rational design of high-performance PCCs is proposed.