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A “vacuum vapor migration strategy” is employed to successfully prepare a novel CoNi–N–C catalyst containing uniformly dispersed CoNi alloy nanoparticles as a conceptually solid–ligand coupling with atomic Co–Nx–C active sites.

Atomically dispersed Fe–N–C catalysts have been extensively deemed as appealing substitutes for Pt-series catalysts towards oxygen reduction reactions (ORRs). Nevertheless, most reported Fe–N–C materials suffer from inefficient Fe-based nanoparticles and low-density Fe–Nx sites. Herein, a Zn/g-C3N4-mediated dual-template strategy was employed to synthesize densely populated atomic Fe–Nx center-embedded N-doped carbon nanosheets (SAs-Fe/N-CNSs) with adjustable porous structures by the simple pyrolysis of D-glucosamine/FeZn/g-C3N4 complexes. g-C3N4 works as a structure-guiding 2D template and offers abundant coordination-N trapping sites for anchoring Fe atoms, simultaneously. ZnCl2 serves as a self-sacrificial template creating a hierarchical porous structure by its volatilization as well as hinders the agglomeration of Fe atoms by spatial segregation during pyrolysis. Due to the high-density atomic Fe–Nx moieties, unique 2D structure, hierarchical porosity, and large surface area, the optimal SAs-Fe/N-CNS catalyst exhibits satisfying ORR performance including excellent activity (E1/2 = 0.91 V) and desirable durability, surpassing the Pt/C catalyst. Additionally, the superb performance of SAs-Fe/N-CNS-based Zn–air batteries with a maximum power density of 157.03 mW cm−2 verifies their promising application in practical electrochemical systems.

Development of high-efficiency non-noble metal materials to substitute Pt-based catalysts for oxygen reduction reactions is crucial in the commercial viability of Zn-air batteries technology. Nitrogen doped carbons (NDCs) are highly appealing as promising candidates. This study reports a facile molten salt (MS) method to synthesize aerogel-like nitrogen doped carbon (NDC-MS). The eutectic mixture acting as combined solvent and porogen leads to the obtained porous materials with extremely large surface area (1548.6 m2 g−1) and relatively high pore volume. The unique aerogel-like structure with hierarchical structure, increased catalytic active sits, extended surface area and large pore volume is beneficial for enhancing oxygen reduction reaction (ORR) performance. The resultant NDC-MS displays a superb ORR catalytic activity with high half-wave potential of 0.88 V, which is one of the most effective figures in previous literature of metal-free catalysts. The superb ORR performance can also be evaluated by Zn–air batteries with satisfactory power density and long-term operation stability. Therefore, such an efficient and green synthetic strategy can open up a new avenue for a wide range of commercial application of heteroatom doped carbon materials in advanced energy technologies.

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

Non-noble metal materials are regarded as the most promising catalysts for the oxygen reduction reaction (ORR) to overcome the inherent defects of Pt-based catalysts, like high cost, limited availability and insufficient stability. Here, we fabricate sandwich-like Co encapsulated nitrogen doped carbon polyhedron/graphene (s-Co@NCP/rGO) via a facile and scalable strategy by loading Co-based zeolitic imidazolate framework (ZIF-67) and graphene oxide (GO) layers individually on a polyurethane (PU) sponge template. The 3D sandwich structure is maintained with the assistance of the sponge template, which promotes the uniform dispersion of ZIF-67-derived Co embedded nitrogen doped carbon polyhedra (Co@NCP) and prevents the graphene plates from agglomerating during the annealing process. The final product demonstrates considerable catalytic performance for the ORR with a half-wave potential of 0.85 V, preferable stability and increased poisoning tolerance by comparison to 20 wt% Pt/C, which stems from the 3D sandwich-like structure, N/Co-doping effect, large accessible surface area and hierarchical porous structures. The excellent ORR performance of the catalysts means that they can be utilised in a Zn-air battery as cathode catalysts. During such a demonstration, s-Co@NCP/rGO shows a high open-circuit voltage of 1.466 V, remarkable long-term durability and an outstanding peak power density of 186 mV cm-2, which shows its high potential as a prospective alternative for widespread practical application in the field of non-noble metal ORR catalysts.

Nowadays, Zn-air batteries have been widely applied in miniature electronic devices and power stations. However, it is imperative but challenging to synthesize efficient non-precious metal electrocatalysts for popularizing application of Zn-air batteries. Herein, the three-dimensional porous Co@Co-NPC is prepared by doping P into flower-like Co/Zn-MOF-derived Co-N-C during the carbonization process. The obtained 3D-Co@Co-NPC possesses an excellent catalytic activity, showing a half-wave potential of 0.872 V for ORR and potential of 1.692 V at 10 mA cm−2 for OER. When employed as air-cathode catalyst in Zn-air battery, 3D-Co@Co-NPC shows a high power density of 182.5 mW cm−2 and a high capacity of 764 mAh g−1. Besides, it displays an excellent charging-discharging cycle stability (over 90 h). The outstanding ORR/OER performance of 3D-Co@Co-NPC are mainly attributed to the doping of P element, which can tune the electronic structure of Co-N-C matrix. The pickling experiment proves that Co nanoparticle can make contribution to ORR/OER activity.

Mn and N codoped carbon materials are proposed as one of the most promising catalysts for the oxygen reduction reaction (ORR) but still confront a lot of challenges to replace Pt. Herein, a novel gas-phase migration strategy is developed for the scale synthesis of atomically dispersed Mn and N codoped carbon materials (g-SA-Mn) as highly effective ORR catalysts. Porous zeolitic imidazolate frameworks serve as the appropriate support for the trapping and anchoring of Mn-containing gaseous species and the synchronous high-temperature pyrolysis process results in the generation of atomically dispersed Mn-N

Rational design and construction of highly efficient and durable non-noble-metal bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial to promote the widespread implementation of rechargeable Zn-air batteries. Herein, a bifunctional catalyst comprising Co nanoparticles uniformly embedded in hollow nitrogen doped carbon tubes (Co@hNCTs) is fabricated by a facile tube-directed templating strategy. In this strategy, surfactant-treated polypyrrole (PPy) nanotubes serve as the structure-guiding templates for efficient capture of Co2+, realizing the in-situ growth of zeolitic imidazolate frameworks-67 (ZIF-67) nanocrystals on PPy nanotubes. Sodium laurylsulfonate acts as anionic surfactant to endow PPy nanotubes with functional electronegative surface and strong anchoring effect toward ZIF-67, playing the pivotal role in binding of ZIF-67 nanocrystals with PPy nanotubes potently. Consequently, the developed catalyst presents a superior ORR activity with the half-wave potential of 0.87 V excellent durability with only a 7 mV loss of half-wave potential after 5000 cycles. The catalyst also exhibits superior catalytic performance for OER. When serving as an air electrode in Zn-air batteries, a large power density of 149 mW cm−2 and long-term cyclability for over 500 h are realized in ambient air, implying the great potential in practical application.