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.

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...

Owing to their zero pollution and high efficiency, fuel cells and metal–air batteries show great potential for broad application to sustainable energy technologies. However, the use of expensive and scarce Pt‐based materials as cathode catalysts to overcome the slow kinetics of oxygen‐reduction reaction (ORR) limits the scalability of such devices. Recently, considerable progress has been made in the development of nonprecious‐metal ORR catalysts. Although metal macrocyclic complexes (MMC) exhibiting a well‐defined M‐N4 (M = Fe, Co, Mn, Cu, etc.) structure (which can provide open sites to combine with oxygen and catalyze ORR) have attracted widespread attention, the MMC ORR performance is usually unsatisfactory because MCCs exhibit poor conductivity, symmetric electron distribution, inferior O2 adsorption, and low activation. However, MMC‐modified conductive‐carbon materials effectively solve such problems and simultaneously boost the ORR catalytic activity. In this review, the recent achievements in MMC‐functionalized carbon materials as ORR catalysts are summarized, and the current challenges and prospects of MMC‐functionalized carbon‐based ORR catalysts are discussed based on recent experimental and theoretical studies.

Single atom Fe–nitrogen–carbon (Fe–N–C) catalysts have high catalytic activity and selectivity for the oxygen reduction reaction (ORR), and are possible alternatives for Pt‐based materials. However, the reasonable design and selection of precursors to establish their relationship with Fe–N–C catalyst performance is still a formidable task. Herein, precursors with controllable structures are easily achieved through isomer engineering, with the purpose of regulating the active site density and microscopic morphology of the final electrocatalyst. As‐proof‐of‐concept, phenylenediamine isomers‐based polymers are used as precursors to fabricate Fe–N–C catalysts. The Fe–PpPD‐800 derived from p‐phenylenediamine shows that the best ORR activity with a half‐wave potential (E1/2) reaches 0.892 V vs reversible hydrogen electrode (RHE), which is better than the counterparts derived from o‐phenylenediamine (Fe–PoPD‐800) and m‐phenylenediamine (Fe–PmPD‐800), even surpassing commercial Pt/C (E1/2 = 0.881 V vs RHE). Furthermore, the self‐made zinc–air battery based on Fe–PpPD‐800 achieves high power density and specific capacity up to 242 mW cm−2 and 873 mA h gZn−1 respectively, a stable open circuit voltage of 1.45 V, and excellent cycling stability. This work not only proves the practicability of adjusting the catalytic activity of single‐atom catalysts through isomer engineering, but also provides an approach to understand the relationship between precursors and target catalysts performance.

Catalysts with metal‐Nx sites have long been considered as effective electrocatalysts for oxygen reduction reaction (ORR), yet the accurate structure‐property correlations of these active sites remain debatable. Report here is a proof‐of‐concept method to construct 1,4,8,11‐tetraaza[14]annulene (TAA)‐based polymer nanocomposites with well‐managed electronic microenvironment via electron‐donors/acceptors interaction of altering electron‐withdrawing β‐site substituents. DFT calculation proves the optimal −Cl substituted catalyst (CoTAA−Cl@GR) tailored the key OH* intermediate interaction with Co−N4 sites under the d‐orbital regulation, hence reaching the top of ORR performance with excellent turnover frequency (0.49 e s−1 site−1). The combination of in situ scanning electrochemical microscopy and variable‐frequency square wave voltammetry techniques contribute the great ORR kinetics of CoTAA−Cl@GR to the relatively high accessible site density (7.71×1019 site g−1) and fast electron outbound propagation mechanism. This work provides theoretical guidance for rational design of high‐performance catalysts for ORR and beyond.