The development of rechargeable metal–air batteries and water electrolyzers are highly constrained by electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, the construction of efficient trifunctional electrocatalysts for ORR/OER/HER are highly desirable yet challenging. Herein, hollow carbon nanotubes integrated single cobalt atoms with Co9S8 nanoparticles (CoSA + Co9S8/HCNT) are fabricated by a straightforward in situ self‐sacrificing strategy. The structure of the CoSA + Co9S8/HCNT are verified by X‐ray absorption spectroscopy and aberration‐corrected scanning transmission electron microscopy. Theoretical calculations and experimental results embrace the synergistic effects between Co9S8 nanoparticles and single cobalt atoms through optimizing the electronic configuration of the CoN4 active sites to lower the reaction barrier and facilitating the ORR, OER, and HER simultaneously. Consequently, rechargeable liquid and all‐solid‐state flexible Zn–air batteries based on CoSA + Co9S8/HCNT exhibit remarkable stability and excellent power density of 177.33 and 51.85 mW cm−2, respectively, better than Pt/C + RuO2 counterparts. Moreover, the as‐fabricated Zn–air batteries can drive an overall water splitting device assembled with CoSA + Co9S8/HCNT and achieve a current density of 10 mA cm−2 at a low voltage of 1.59 V, also superior to Pt/C + RuO2. Therefore, this work presents a promising approach to an efficient trifunctional electrocatalyst toward practical applications.

Zinc ion capacitors (ZICs) hold great promise in large-scale energy storage by inheriting the superiorities of zinc ion batteries and supercapacitors. However, the mismatch of kinetics and capacity between a Zn anode and a capacitive-type cathode is still the Achilles' heel of this technology. Herein, porous carbons are fabricated by using tetra-alkali metal pyromellitic acid salts as precursors through a carbonization/ self-activation procedure for enhancing zinc ion storage. The optimized rubidium-activated porous carbon (RbPC) is verified to hold immense surface area, suitable porosity structure, massive lattice defects, and luxuriant oxygen functional groups. These structural and compositional merits endow RbPC with the promoted zinc ion storage capability and more matchable kinetics and capacity with a Zn anode. Consequently, RbPC-based ZIC delivers a high specific energy of 178.2 W h kg −1 and a peak power density of 72.3 kW kg −1 . A systematic ex situ characterization analysis coupled with in situ electrochemical quartz crystal microbalance tests reveal that the preeminent zinc ion storage properties are ascribed to the synergistic effect of the dual-ion adsorption and reversible chemical adsorption of RbPC. This work provides an efficient strategy to the rational design and construction of high-performance electrodes for ZICs and furthers the fundamental understanding of their charge storage mechanisms or extends the understanding toward other electrochemical energy storage devices.

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.

Single‐atom catalysts (SACs) are attractive candidates for oxygen reduction reaction (ORR). The catalytic performances of SACs are mainly determined by the surrounding microenvironment of single metal sites. Microenvironment engineering of SACs and understanding of the structure–activity relationship is critical, which remains challenging. Herein, a self‐sacrificing strategy is developed to synthesize asymmetric N,S‐coordinated single‐atom Fe with axial fifth hydroxy (OH) coordination (Fe−N3S1OH) embedded in N,S codoped porous carbon nanospheres (FeN/SC). Such unique penta‐coordination microenvironment is determined by cutting‐edge techonologies aiding of systematic simulations. The as‐obtained FeN/SC exhibits superior catalytic ORR activity, and showcases a half‐wave potential of 0.882 V surpassing the benchmark Pt/C. Moreover, theoretical calculations confirmed the axial OH in FeN3S1OH can optimize 3d orbitals of Fe center to strengthen O2 adsorption and enhance O2 activation on Fe site, thus reducing the ORR barrier and accelerating ORR dynamics. Furthermore, FeN/SC containing H2O2 fuel cell performs a high peak power density of 512 mW cm−2, and FeN/SC based Znair batteries show the peak power density of 203 and 49 mW cm−2 in liquid and flexible all‐solid‐state configurations, respectively. This study offers a new platform for fundamentally understand the axial fifth coordination in asymmetrical planar single‐atom metal sites for electrocatalysis.

2D transition metal carbides/nitrides (MXenes) have excellent physicalchemical properties, which makes them promising for electrochemical energy storage devices. However, because of their inherent self-stacking and narrow interlayer spacing, it is rarely used in multivalent ion energy storage systems. In this study, fatty diamines and aromatic diamines with different molecular sizes are inserted between MXene interlayers as pillars through a one-step amination process to inhibit the self-stacking and obtain different expanded interlayer spacings with improved antioxidant stability. X-ray diffraction results show that interlayer spacing of MXene increases from 1.23 to 1.40 nm. The p-phenylenediamine-intercalated MXene (PDA-MXene) exhibits better matching interlayer spacing (1.38 nm) and pore structure for improved electrolyte-accessible surface area, enhanced charge-transport properties, and promoted Zn 2+ ions storage. Therefore, zinc-ion hybrid supercapacitor (ZHSC) using PDA-MXene as cathode exhibits higher specific capacitance (124.4 F g −1 at 0.2 A g −1 ) in 2 m ZnSO 4 electrolyte together with outstanding cycling stability (85% capacity retention after 10 000 cycles at 1 A g −1 ). This study provides a route for precise control of MXene interlayer spacing by small organic molecules, which can be used to observe efficient charge storage in MXene-based electrochemical energy storage devices by optimizing interlayer spacing.