Electrochemical oxygen reduction could proceed via either 4e⁻-pathway toward maximum chemical-to-electric energy conversion or 2e⁻-pathway toward onsite H₂O₂ production. Bulk Pt catalysts are known as the best monometallic materials catalyzing O₂-to-H₂O conversion, however, controversies on the reduction product selectivity are noted for atomic dispersed Pt catalysts. Here, we prepare a series of carbon supported Pt single atom catalyst with varied neighboring dopants and Pt site densities to investigate the local coordination environment effect on branching oxygen reduction pathway. Manipulation of 2e⁻ or 4e⁻ reduction pathways is demonstrated through modification of the Pt coordination environment from Pt-C to Pt-N-C and Pt-S-C, giving rise to a controlled H₂O₂ selectivity from 23.3% to 81.4% and a turnover frequency ratio of H₂O₂/H₂O from 0.30 to 2.67 at 0.4 V versus reversible hydrogen electrode. Energetic analysis suggests both 2e⁻ and 4e⁻ pathways share a common intermediate of *OOH, Pt-C motif favors its dissociative reduction while Pt-S and Pt-N motifs prefer its direct protonation into H₂O₂. By taking the Pt-N-C catalyst as a stereotype, we further demonstrate that the maximum H₂O₂ selectivity can be manipulated from 70 to 20% with increasing Pt site density, providing hints for regulating the stepwise oxygen reduction in different application scenarios.

High-loaded oxygen reduction reaction (ORR) Pt intermetallic compounds with high performance expression under PEMFC operating conditions are prerequisite for practical application. Nevertheless, high metal-loading would lead to the severe agglomeration and nonuniformity of nanoparticles, imposing an enormous challenge on efficient synthesis. Herein we present a cobalt oxide aided structural evolution strategy to controllably synthesize high-loaded (44.7 wt%) sub-6 nm Pt1Co1 intermetallic compounds with a Pt-rich shell (Pt1Co1-IMC@Pt). Experiments and theory explicitly reveal that the ordered arrangement of Pt–Co atoms endows surface Pt with a lowered d band centre and enhanced oxidation-resistance of Pt/Co sites, thus simultaneously boosting the ORR activity and durability. The preeminent intrinsic ORR activity on an optimized catalyst reaches a mass activity as high as 0.53 A mg_(Pt) ⁻¹@0.90 V RHE⁻¹ (MA@0.9 V) in rotating disk electrode measurements. PEMFCs with such a catalyst deliver record-high power density (2.30/1.23 W cm⁻² under H₂–air/O₂ conditions at 80 °C) and extraordinary stability, ranking the highest fuel cell performance among the Pt-based electrocatalysts. Significantly, the MA@0.9 V calculated from fuel cell attains 0.46 A mg_(Pt) ⁻¹, exceeding the 2020 DOE target (0.44 A mg_(Pt) ⁻¹) and very close to the intrinsic value, definitely confirming the superiority of high-loaded Pt1Co1-IMC@Pt/C in activity expression under fuel cell conditions. This study paves a new way for the future practical application of low-Pt catalysts in PEMFCs.

With the conventional Haber–Bosch NH₃ synthesis in industry requiring harsh pressures and high temperatures, artificial N₂ fixation has been long sought after. The electrochemical nitrogen reduction reaction (NRR) could offer a solution by allowing NH₃ production under ambient conditions. In this review, important recent findings on theoretical calculations and experimental exploration on the NRR at room temperature are systematically reviewed. Firstly, we discuss the mechanism of electrochemical heterogeneous catalysis for the NRR. The NRR is a multi-proton coupled electron transfer (PCET) process which implies that in addition to catalyst surface size effects, ligand and strain effects will also significantly influence the binding energy of the adsorbed N atoms, reaction intermediates and product species. Electrocatalysts including metals, metal nitrides, metal oxides and carbon-based materials will also be discussed at length. A linear scaling relationship seems to limit the NRR activity on most metals and metal oxides. Metal nitrides, however, follow the Mars–van Krevelen (MvK) mechanism which usually shows a lower potential energy barrier compared to the associative mechanism. Carbon-based materials and some single atom catalysts exhibit improved activity and selectivity due to ligand effects. Thus, electrolytes containing a proton donor might play a crucial role in the NRR. The limiting concentration of proton donors and the rate of proton transport to the active sites might be effective factors in boosting the selectivity of the NRR. Specifically, ionic liquids with high N₂ solubility demonstrate much larger faradaic efficiency and would be promising candidates for use in NRR processes. Inspired by the characteristics of PCET, four strategies of electrode engineering were introduced including limiting protons, tuning the electron transport, modifying the electrode structure facilitating mass transport, and completely changing the NRR mechanism inspired by bio-nitrogenase and Li mediated N₂ fixation.