Single‐atom catalysts (SACs) with highly active sites atomically dispersed on substrates exhibit unique advantages regarding maximum atomic efficiency, abundant chemical structures, and extraordinary ...catalytic performances for multiple important reactions. In particular, M–N–C SACs (M=transition metal atom) demonstrate optimal electrocatalytic activity for the oxygen reduction reaction (ORR) and have attracted extensive attention recently. Despite substantial efforts in fabricating various M–N–C SACs, the principles for regulating the intrinsic electrocatalytic activity of their active sites have not been sufficiently studied. In this Review, we summarize the regulation strategies for promoting the intrinsic electrocatalytic ORR activity of M–N–C SACs by modulation of the center metal atoms, the coordinated atoms, the environmental atoms, and the guest groups. Theoretical calculations and experimental investigations are both included to afford a comprehensive understanding of the structure–performance relationship. Finally, future directions of developing advanced M–N–C SACs for electrocatalytic ORR and other analogous reactions are proposed.
Regulation strategies for enhancing the intrinsic electrocatalytic oxygen reduction reaction activity of M–N–C single‐atom catalysts are summarized in this review. Four components are considered in the optimization of the catalyst: the center metal atoms, the coordinated atoms, the environmental atoms, and the guest groups.
Hydrogen peroxide (H2O2) is a green oxidizer widely involved in a vast number of chemical reactions. Electrochemical reduction of oxygen to H2O2 constitutes an environmentally friendly synthetic ...route. However, the oxygen reduction reaction (ORR) is kinetically sluggish and undesired water serves as the main product on most electrocatalysts. Therefore, electrocatalysts with high reactivity and selectivity are highly required for H2O2 electrosynthesis. In this work, a synergistic strategy is proposed for the preparation of H2O2 electrocatalysts with high ORR reactivity and high H2O2 selectivity. A Co−Nx−C site and oxygen functional group comodified carbon‐based electrocatalyst (named as Co–POC–O) is synthesized. The Co–POC–O electrocatalyst exhibits excellent catalytic performance for H2O2 electrosynthesis in O2‐saturated 0.10 m KOH with a high selectivity over 80% as well as very high reactivity with an ORR potential at 1 mA cm−2 of 0.79 V versus the reversible hydrogen electrode (RHE). Further mechanism study identifies that the Co−Nx−C sites and oxygen functional groups contribute to the reactivity and selectivity for H2O2 electrogeneration, respectively. This work affords not only an emerging strategy to design H2O2 electrosynthesis catalysts with remarkable performance, but also the principles of rational combination of multiple active sites for green and sustainable synthesis of chemicals through electrochemical processes.
A synergistic strategy of rational combination of multiple active sites is proposed for high‐performance H2O2 electrosynthesis. Comodification of atomic Co–Nx–C sites and oxygen functional groups on noble‐metal‐free nanocarbon electrocatalysts synergistically renders high reactivity for oxygen reduction and high selectivity for the two‐electron pathway. Consequently, high H2O2 productivity is achieved through a green and sustainable electrochemical approach.
Rechargeable zinc–air batteries constitute promising next‐generation energy storage devices due to their intrinsic safety, low cost, and feasibility to realize high cycling current density and long ...cycling lifespan. Nevertheless, their cathodic reactions involving oxygen reduction and oxygen evolution are highly sluggish in kinetics, requiring high‐performance noble‐metal‐free bifunctional electrocatalysts that exceed the current noble‐metal‐based benchmarks. Herein, a noble‐metal‐free bifunctional electrocatalyst is fabricated, which demonstrates ultrahigh bifunctional activity and renders excellent performance in rechargeable zinc–air batteries. Concretely, atomic Co–N–C and NiFe layered double hydroxides (LDHs) are respectively selected as oxygen reduction and evolution active sites and are further rationally integrated to afford the resultant CoNC@LDH composite electrocatalyst. The CoNC@LDH electrocatalyst exhibits remarkable bifunctional activity delivering an indicator ΔE of 0.63 V, far exceeding the noble‐metal‐based Pt/C+Ir/C benchmark (ΔE = 0.77 V) and most reported electrocatalysts. Correspondingly, ultralong lifespan (over 3600 cycles at 10 mA cm−2) and excellent rate performances (cycling current density at 100 mA cm−2) are achieved in rechargeable zinc–air batteries. This work highlights the current advances of bifunctional oxygen electrocatalysis and endows high‐rate and long‐cycling rechargeable zinc–air batteries for efficient sustainable energy storage.
Ultrahigh bifunctional electrocatalytic activity for oxygen reduction and evolution is achieved with the indicator ΔE = 0.63 V, far exceeding the noble‐metal‐based benchmark and most reported electrocatalysts. Corresponding rechargeable zinc–air batteries afford ultralong lifespan over 3600 cycles at 10 mA cm−2 and ultrahigh cycling current density of 100 mA cm−2.
High‐performance bifunctional oxygen electrocatalysis constitutes the key technique for the widespread application of clean and sustainable energy through electrochemical devices such as rechargeable ...Zn–air batteries. Single‐atom electrocatalysts with maximum atom efficiency are highly considered as an alternative of the present noble‐metal‐based electrocatalysts. However, the fabrication of transition metal single‐atoms is very challenging, requiring extensive attempts of precursors with novel design principles. Herein, an all‐covalently constructed cobalt‐coordinated framework porphyrin with graphene hybridization is innovatively designed and prepared as the pyrolysis precursor to fabricate single‐atom Co–Nx–C electrocatalysts. Excellent electrochemical performances are realized for both bifunctional oxygen electrocatalysis and rechargeable Zn–air batteries with regard to reduced overpotentials, improved kinetics, and prolonged cycling stability comparable with noble‐metal‐based electrocatalysts. Design principles from multiple scales are proposed and rationalized with detailed mechanism investigation. This work not only provides a novel precursor for the fabrication of high‐performance single‐atom electrocatalysts, but also inspires further attempts to develop advanced materials and emerging applications.
Cobalt‐coordinated framework porphyrin hybridized with graphene is employed as the pyrolysis precursor to fabricate single‐atom Co–Nx–C electrocatalysts. Excellent ORR/OER bifunctional electrocatalytic performances are achieved with a small overpotential gap of 0.87 V, and corresponding Zn–air batteries afford higher power density, improved rate performance, and cycling stability for over 200 cycles beyond the noble‐metal‐based electrocatalysts.
The rechargeable zinc–air battery (ZAB) is a promising energy storage technology owing to its high energy density and safe aqueous electrolyte, but there is a significant performance bottleneck. ...Generally, cathode reactions only occur at multiphase interfaces, where the electrocatalytic active sites can participate in redox reactions effectively. In the conventional air cathode, the 2D multiphase interface on the surface of the gas diffusion layer (GDL) inevitably results in an insufficient amount of active sites and poor interfacial contact, leading to sluggish reaction kinetics. To address this problem, a 3D multiphase interface strategy is proposed to extend the reactive interface into the interior of the GDL. Based on this concept, an asymmetric air cathode is designed to increase the accessible active sites, accelerate mass transfer, and generate a dynamically stabilized reactive interface. With a NiFe layered‐double‐hydroxide electrocatalyst, ZABs based on the asymmetric cathode deliver a small charge/discharge voltage gap (0.81 V at 5.0 mA cm−2), a high power density, and a stable cyclability (over 2000 cycles). This 3D reactive interface strategy provides a feasible method for enhancing the air cathode kinetics and further enlightens electrode designs for energy devices involving multiphase electrochemical reactions.
A 3D multiphase reactive interface strategy is proposed to enhance the reaction kinetics in the air cathode of a rechargeable zinc–air battery. As a proof of concept, an asymmetric air cathode is designed, which exhibits an increased amount of accessible active sites, accelerated mass transfer, and a dynamically stabilized reactive interface.
Lithium–sulfur (Li–S) batteries are deemed as future energy storage devices due to ultrahigh theoretical energy density. Cathodic polysulfide electrocatalysts have been widely investigated to promote ...sluggish sulfur redox kinetics. Probing the surface structure of electrocatalysts is vital to understanding the mechanism of polysulfide electrocatalysis. In this work, we for the first time identify surface gelation on disulfide electrocatalysts. Concretely, the Lewis acid sites on disulfides trigger the ring‐opening polymerization of the dioxolane solvent to generate a surface gel layer, covering disulfides and reducing the electrocatalytic activity. Accordingly, a Lewis base triethylamine (TEA) is introduced as a competitive inhibitor. Consequently, Li–S batteries with disulfide electrocatalysts and TEA afford high specific capacity and improved rate responses. This work affords new insights on the actual surface structure of electrocatalysts in Li–S batteries.
Surface gelation on disulfide electrocatalysts in Li–S batteries is identified for the first time. The gel layer, formed through the solvent polymerization triggered by the Lewis acid sites, covers the active electrocatalytic sites and renders reduced redox kinetics. Herein, a Lewis base triethylamine is introduced to suppress the surface gelation and promote the electrocatalytic activity of disulfide electrocatalysts.
Rechargeable zinc–air batteries afford great potential toward next‐generation sustainable energy storage. Nevertheless, the oxygen redox reactions at the air cathode are highly sluggish in kinetics ...to induce poor energy efficiency and limited cycling lifespan. Air cathodes with asymmetric configurations significantly promote the electrocatalytic efficiency of the loaded electrocatalysts, whereas rational synthetic methodology to effectively fabricate asymmetric air cathodes remains insufficient. Herein, a strategy of asymmetric interface preconstruction is proposed to fabricate asymmetric air cathodes for high‐performance rechargeable zinc–air batteries. Concretely, the asymmetric interface is preconstructed by introducing immiscible organic–water diphases within the air cathode, at which the electrocatalysts are in situ formed to achieve an asymmetric configuration. The as‐fabricated asymmetric air cathodes realize high working rates of 50 mA cm−2, long cycling stability of 3400 cycles at 10 mA cm−2, and over 100 cycles under harsh conditions of 25 mA cm−2 and 25 mAh cm−2. Moreover, the asymmetric interface preconstruction strategy is universal to many electrocatalytic systems and can be easily scaled up. This work provides an effective strategy toward advanced asymmetric air cathodes with high electrocatalytic efficiency and significantly promotes the performance of rechargeable zinc–air batteries.
An asymmetric interface preconstruction strategy to fabricate asymmetric air cathodes for rechargeable zinc–air batteries is proposed. Precise loading of the electrocatalysts at the asymmetric interface is achieved to obtain zinc–air batteries cycling at 25 mAh cm−2 and 25 mA cm−2. The asymmetric interface preconstruction strategy is universal to various electrocatalytic systems and can be easily scaled up.
Efficient energy storage at low temperatures starves for competent battery techniques. Herein, inherent advantages of zinc–air batteries on low‐temperature electrochemical energy storage are ...discovered. The electrode reactions are resistive against low temperatures to render feasible working zinc–air batteries under sub‐zero temperatures. The relatively reduced ionic conductivity of electrolyte is identified as the main limiting factor, which can be addressed by employing a CsOH‐based electrolyte through regulating the solvation structures. Accordingly, 500 cycles with a stable voltage gap of 0.8 V at 5.0 mA cm−2 is achieved at −10 °C. This work reveals the promising potential of zinc–air batteries for low‐temperature electrochemical energy storage and inspires advanced battery systems under extreme working conditions.
The low‐temperature performances of zinc–air batteries are systematically investigated from both theoretical and experimental aspects in terms of feasibility verification, bottleneck analysis, and promotion strategies.
Lithium–sulfur (Li–S) batteries are highly regarded as the next‐generation energy‐storage devices because of their ultrahigh theoretical energy density of 2600 Wh kg−1. Sulfurized polyacrylonitrile ...(SPAN) is considered a promising sulfur cathode to substitute carbon/sulfur (C/S) composites to afford higher Coulombic efficiency, improved cycling stability, and potential high‐energy‐density Li–SPAN batteries. However, the instability of the Li‐metal anode threatens the performances of Li–SPAN batteries bringing limited lifespan and safety hazards. Li‐metal can react with most kinds of electrolyte to generate a protective solid electrolyte interphase (SEI), electrolyte regulation is a widely accepted strategy to protect Li‐metal anodes in rechargeable batteries. Herein, the basic principles and current challenges of Li–SPAN batteries are addressed. Recent advances on electrolyte regulation towards stable Li‐metal anodes in Li–SPAN batteries are summarized to suggest design strategies of solvents, lithium salts, additives, and gel electrolyte. Finally, prospects for future electrolyte design and Li anode protection in Li–SPAN batteries are discussed.
Increased attention SPAN: Recent advances in electrolyte regulation towards stable lithium‐metal anodes for Li‐sulfurized polyacrylonitrile (SPAN) batteries are summarized to afford design strategies of solvents, lithium salts, additives, and gel electrolyte.
Zinc–air batteries deliver great potential as emerging energy storage systems but suffer from sluggish kinetics of the cathode oxygen redox reactions that render unsatisfactory cycling lifespan. The ...exploration on bifunctional electrocatalysts for oxygen reduction and evolution constitutes a key solution, where rational design strategies to integrate various active sites into a high‐performance air cathode remain insufficient. Herein, a multiscale construction strategy is proposed to rationally direct the fabrication of bifunctional oxygen electrocatalysts for long‐lifespan rechargeable zinc–air batteries. NiFe layered double hydroxides and cobalt coordinated framework porphyrin are selected as the active sites considering their high intrinsic activity at the molecular level, and the active sites are successively integrated on three‐dimensional conductive scaffolds at mesoscale to strengthen ion transportation. Consequently, the multiscale constructed electrocatalyst exhibits excellent bifunctional performance (ΔE = 0.68 V), which is even better than that of the noble metal based benchmarks. The corresponding air cathodes endow zinc–air batteries with a reduced voltage gap of 0.74 V, a high power density of 185.0 mW cm−2, and an ultralong lifespan of more than 2400 cycles at 5.0 mA cm−2. This work demonstrates a feasible strategy to rationally integrate various active sites to construct multifunctional electrocatalysts for energy‐related processes.
A multiscale construction strategy is proposed to rationally integrate multiple active sites into composite electrocatalysts. NiFe‐layered double hydroxides and cobalt coordinated framework porphyrin are selected as the active sites and successively integrated on 3D conductive scaffolds. The as‐obtained electrocatalyst exhibits remarkable bifunctional oxygen reduction/evolution reaction performances, outperforming noble metal‐based benchmarks and realizes ultralong lifespan of 2400 cycles in rechargeable zinc–air batteries.
