Oxygen reduction reaction (ORR) is an essential process for sustainable energy supply and sufficient chemical production in modern society. Singleatom catalysts (SACs) exhibit great potential on maximum atomic efficiency, high ORR activity, and stability, making them attractive candidates for pursuing next-generation catalysts. Despite substantial efforts being made on building diversiform single-atom active sites (SAASs), the performance of the obtained catalysts is still unsatisfactory. Fortunately, microenvironment regulation of SACs provides opportunities to improve activity and selectivity for ORR. In this review, first, ORR mechanism pathways on N-coordinated SAAS, electrochemical evaluation, and characterization of SAAS are displayed. In addition, recent developments in tuning microenvironment of SACs are systematically summarized, especially, strategies for microenvironment modulation are introduced in detail for boosting the intrinsic 4e − /2e − ORR activity and selectivity. Theoretical calculations and cutting-edge characterization techniques are united and discussed for fundamental understanding of the synthesis-construction-performance correlations. Furthermore, the techniques for building SAAS and tuning their microenvironment are comprehensively overviewed to acquire outstanding SACs. Lastly, by proposing perspectives for the remaining challenges of SACs and infant microenvironment engineering, the future directions of ORR SACs and other analogous procedures are pointed out.

Atomically nitrogen‐coordinated iron atoms on carbon (FeNC) catalysts are emerging as attractive materials to substitute precious‐metal‐based catalysts for the oxygen reduction reaction (ORR). However, FeNC usually suffers from unsatisfactory performance due to the symmetrical charge distribution around the iron site. Elaborately regulating the microenvironment of the central Fe atom can substantially improve the catalytic activity of FeNC, which remains challenging. Herein, N/S co‐doped porous carbons are rationally prepared and are verified with rich Fe‐active sites, including atomically dispersed FeN4 and Fe nanoclusters (FeSA‐FeNC@NSC), according to systematically synchrotron X‐ray absorption spectroscopy analysis. Theoretical calculation verifies that the contiguous S atoms and Fe nanoclusters can break the symmetric electronic structure of FeN4 and synergistically optimize 3d orbitals of Fe centers, thus accelerating OO bond cleavage in OOH* for improving ORR activity. The FeSA‐FeNC@NSC delivers an impressive ORR activity with half‐wave‐potential of 0.90 V, which exceeds that of state‐of‐the‐art Pt/C (0.87 V). Furthermore, FeSA‐FeNC@NSC‐based Zn‐air batteries deliver excellent power densities of 259.88 and 55.86 mW cm–2 in liquid and all‐solid‐state flexible configurations, respectively. This work presents an effective strategy to modulate the microenvironment of single atomic centers and boost the catalytic activity of single‐atom catalysts by tandem effect.

High performance electrocatalysts play a vital role in various sustainable energy conversion and storage technologies. Though enormous carbon-based materials have been developed and exhibited efficient electrocatalytic ability comparable to precious...

As key parameters of electrocatalysts, the density and utilization of active sites determine the electrocatalytic performance toward oxygen reduction reaction. Unfortunately, prevalent oxygen electrocatalysts fail to maximize the utilization of active sites due to inappropriate nanostructural design. Herein, a nano‐emulsion induced polymerization self‐assembly strategy is employed to prepare hierarchical meso‐/microporous N/S co‐doped carbon nanocage with atomically dispersed FeN4 (denoted as Meso/Micro‐FeNSC). In situ scanning electrochemical microscopy technology reveals the density of available active sites for Meso/Micro‐FeNSC reach to 3.57 × 1014 sites cm−2, representing more than threefold improvement compared to micropore‐dominant Micro‐FeNSC counterpart (1.07 × 1014 sites cm−2). Additionally, the turnover frequency of Meso/Micro‐FeNSC is also improved to 0.69 from 0.50 e− site−1 s−1 for Micro‐FeNSC. These properties motivate Meso/Micro‐FeNSC as efficient oxygen electroreduction electrocatalyst, in terms of outstanding half‐wave potential (0.91 V), remarkable kinetic mass specific activity (68.65 A g−1), and excellent robustness. The assembled Zn–air batteries with Meso/Micro‐FeNSC deliver high peak power density (264.34 mW cm−2), large specific capacity (814.09 mA h g−1), and long cycle life (>200 h). This work sheds lights on quantifying active site density and the significance of maximum utilization of active sites for rational design of advanced catalysts.

Developing precious-metal-free bifunctional oxygen reduction and evolution reaction (ORR/OER) electrocatalysts is of great significance for several key energy conversion and storage systems, yet remains as a formidable challenge. Herein, a...