Proton-exchange membrane fuel cell and water electrolyzer (PEMFC and PEMWE) with high conversion efficiency and zero-carbon emission stand out as an attractive strategy for efficient conversion between hydrogen energy and renewable electricity. As a key component, efficient oxygen electrocatalyst for promoting sluggish reaction kinetics of oxygen reduction and evolution reaction (ORR and OER) under harsh operation conditions severely limited progress of these devices. Among various candidates, Pt-group (Pt, Ir, and Ru)-based electrocatalysts are still the most active ORR/OER catalysts. However, the scarcity, high cost, and questionable stability restrict the widespread applications and the commercialization of PEMWE/PEMFC. Progresses in synthesizing atomically dispersed single/multiple-atom catalysts (SACs/MACs) offer new opportunities to Pt-group ORR/OER catalysts owing to nearly 100% metal utilization and high catalytic activities. Extensive efforts have been continuously devoted to optimizing the local structure of Pt-group OER/ORR catalysts at atom-level for further enhancing stability and activity. In this review, universal synthesis methods to prepare Pt-group SACs are discussed first, highlighting crucial factors which affect the structure and catalytic performance. Afterward, advanced characterization techniques for directly confirming atomic dispersed metal atoms were introduced, including aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy. Importantly, considerations for rational catalyst design and typical Pt-group SACs/MACs are summarized regarding the regulation strategy of atomically dispersed metal sites and various supports, and effects of metal–support interaction on the catalytic performance. Finally, key challenges and proposed perspectives for future development of atomically dispersed Pt-group oxygen electrocatalysts for fuel cell and electrolyzer are briefly discussed.

Electrocatalytic reduction of nitrate (NO₃⁻) and nitride (NO₂⁻) to ammonia (NH₃) is of wide interest as a promising alternative to the energy-intensive Haber-Bosch route for mitigating the vast energy consumption and the accompanied carbon dioxide emission, as well as benefiting for the relevant sewage treatment. However, exploring an efficient and low-cost catalyst with high atomic utilization that can effectively facilitate the slow multi-electron transfer process remains a grand challenge. Herein, we present an efficient hydrogenation of NO₃⁻/NO₂⁻ species to NH₃ in both alkaline and neutral environments over the Fe₂(MoO₄)₃ derived hybrid electrocatalyst with the metallic Fe site on FeMoO₄ (Fe/FeMoO₄). The Mo ingredient can play a synergistically positive role in further promoting the NH₃ production on Fe. As a result, Fe/FeMoO₄ behaves well in the electrochemical NH₃ generation from NO₂⁻ with a maximum NH₃ Faradaic efficiency (FE) of 96.53% and 87.68% in alkaline and neutral electrolyte, corresponding to the NH₃ yield rate of 640.68 and 302.56 mg·h⁻¹·mg_cat. ⁻¹, respectively, which outperforms the Fe and Mo counterpart and other similar catalyst, showing the robust catalytic capacity of each active site.