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[Display omitted] • A comprehensive overview of atomically dispersed M−N−C catalysts for ORR. • Focus on active site engineering to improve activity and stability in fuel cells and Zn-air batteries. • Future directions of developing advanced M−N−C catalysts for ORR are outlined. Atomically dispersed metal-nitrogen-carbon (M−N−C) oxygen reduction reaction (ORR) catalysts have been proven as one of the most promising non-precious metal catalysts. In this review, we summarize recent advances in the active site engineering toward M−N−C catalysts for ORR, which focuses on modulating coordination environment of active sites to enhance intrinsic activity, such as coordination structures, heteroatomic doping M−N−C, dual-atom sites, coupled catalytic sites, and defective M−N−C. Moreover, the strategies for increasing the density and utilization of active sites are discussed, including chelation, stepwise atom doping, spatial confinement, surface exposed M−N x sites, and porous structure optimization. Thereafter, the performance decline mechanisms are introduced and the strategies for improving stability are systematically analyzed by regulating the structure of the active center and carbon support. Finally, we also proposed the future perspectives of M−N−C catalysts. This review aims to provide some guidelines towards synthesis of M−N−C catalysts for fuel cells and Zn-air batteries. [ABSTRACT FROM AUTHOR]

Solvent environment engineering is developed to synthesize ZIF-derived hierarchically porous cubic Fe N C(C) catalysts with densely Fe-N x sites. [Display omitted] • The Fe-N x site density of ZIF-derived Fe N C is controlled by changing the solvent. • The Fe N C(C) synthesized in water has a high single Fe atom loading (0.6 at%). • The catalyst displays satisfactory ORR activity under alkaline/acidic environment. • The catalyst presents nice performance in liquid and solid-state Zn-air batteries. Zeolitic imidazole framework (ZIF)-derived iron–nitrogen-carbon (Fe N C) materials are expected to be high-efficiency catalysts for oxygen reduction reaction (ORR). However, increasing the density of active sites while avoiding metal accumulation still faces significant challenges. Herein, solvent environment engineering is used to synthesize the Fe N C containing dense Fe-N x moieties by adjusting the solvent during the ZIF precursor synthesis process. Compared with methanol and water/methanol, the aqueous media can provide a more moderate Fe content for the ZIF precursor, which facilitates the construction of high-density Fe-N x sites and prevent the appearance of iron-based nanoparticles during pyrolysis. Therefore, the Fe N C(C) nanocubes synthesized in an aqueous media have the highest single atom Fe loading (0.6 at%) among the prepared samples, which presents excellent oxygen reduction properties and durability under alkaline and acidic conditions. The advantage of Fe N C(C) is proven in Zn-air batteries, with outstanding performance and long-term stability. [ABSTRACT FROM AUTHOR]

Heteroatoms-modified Fe–N–C catalysts have garnered significant attention for enhancing the oxygen reduction reaction (ORR). However, revealing the correlation between the type of heteroatoms used for doping and catalytic performance still faces significant challenges. Herein, based on the density functional theory (DFT), a series of heteroatom-modified Fe–N–C models with the tailored Fe–N 4 -X (X = S, P or B) site are constructed to explore the regulatory mechanism of heteroatoms on the electronic structure and adsorption behavior of Fe centers. Theoretical calculations reveal that the doping of S atoms can optimize the electronic environment of Fe atoms, thus leading the favorable interaction between Fe–N 4 –S site and OH intermediates. As a result, the Fe–N 4 –S site possess a lower energy barrier for the desorption of OH* than that of Fe–N 4 –B and Fe–N 4 –P, indicating higher catalytic activity and kinetics of S-modified Fe–N–C catalysts. [Display omitted] • Heteroatom-modified Fe–N 4 models are analyzed by theoretical calculations. • S doping can optimize the electronic structure around Fe–N 4 sites. • S doping promotes the OH* desorption, thus leading to enhanced kinetics process. • Fe–N 4 –S model exhibits higher activity and stability than Fe–N 4 –B and Fe–N 4 –P. [ABSTRACT FROM AUTHOR]

The Zn-air battery is a promising next-generation battery because of the higher energy density and safety compared with the Li-ion battery. However, the Zn-air battery suffers from huge energy loss and poor cycling performance, which are due to the sluggish kinetics of oxygen reduction/evolution reactions (ORR/OER) and the instability of bifunctional catalysts. The state-of-the-art bifunctional catalyst is the composite of Pt and Ir/Ru oxides, which is hindered by the scarce resource and high cost. Therefore, developing low-cost and high-efficient bifunctional non-noble metal-based catalysts is essential to the deployment of Zn-air batteries. Herein, we report a dual-nitrogen-source mediated pyrolysis route to rationally design and synthesize the hybrid Co/Co–N–C bifunctional catalyst. The tailored catalyst consisting of carbon confined Co nanoparticles and atomically dispersed Co–N–C moiety is identified. Remarkably, the dual-nitrogen-source derived Co/Co–N–C catalyst demonstrates excellent electrochemical performances with a half-wave potential of 0.882 V for ORR and a potential of 1.640 V at 10 mA cm−2 for OER; reasonably, the bifunctionality of Co/Co–N–C, which is supported by the excellent self-breathing Zn-air battery performance (the maximum energy density of 897.1 Wh kg Zn −1; no obvious degradation within 167h cycling). The abundant Co-Nx sites and metallic Co nanoparticles synergistically boost the ORR and OER. Image 1 • A dual-nitrogen-source method is proposed to synthesize porous Co-based catalysts. • The dual nitrogen sources play different roles in morphology and structure control. • The hybrid carbon-confined Co nanoparticles and Co-N x sites are well tailored. • The good bifunctionality is due to the hybrid active site configuration. • Excellent performance and lifetime are obtained in a Zn-air battery. [ABSTRACT FROM AUTHOR]

Coordination environment engineering is developed to synthesize Fe SA /NSC catalysts with the tailored N, S co-coordinated Fe atomic site. [Display omitted] • The Fe-N 3 S active site with the optimized electronic structure is constructed. • S doping promotes the OH* desorption, thus leading to enhanced kinetics process. • The catalyst displays impressive ORR activity in both alkaline and acidic mediums. • The catalyst exhibits excellent performance in Zn-air batteries and fuel cells. Atomically dispersed iron–nitrogen-carbon (FesbndNsbndC) materials have been considered ideal catalysts for the oxygen reduction. Unfortunately, designing and adjusting the electronic structure of single-atom Fe sites to boost the kinetics and activity still faces grand challenges. In this work, the coordination environment engineering is developed to synthesize the Fe SA /NSC catalyst with the tailored N, S co-coordinated Fe atomic site (Fe-N 3 S site). The structural characterizations and theoretical calculations demonstrate that the incorporation of sulfur can optimize the charge distribution of Fe atoms to weaken the adsorption of OH* and facilitate the desorption of OH*, thus leading to enhanced kinetics process and intrinsic activity. As a result, the S-modified Fe SA /NSC exhibits outstanding catalytic activity with the half-wave potentials (E 1/2) of 0.915 V and 0.797 V, as well as good stability, in alkaline and acidic electrolytes, respectively. Impressively, the excellent performance of Fe SA /NSC is further confirmed in Zn-air batteries (ZABs) and fuel cells, with high peak power densities (146 mW cm−2 and 0.259 W cm−2). [ABSTRACT FROM AUTHOR]

[Display omitted] • HP-Fe-N-C/2 exhibits eminent ORR activity and stability in alkaline and acidic media. • The Zn-air battery shows high power density and superior cyclability for 1342 h. • The power density of HP-Fe-N-C/2-based PEM fuel cell is up to 0.66 W cm−2. Atomically dispersed Fe-N-C catalysts are a promising alternative to platinum group metal (PGM) catalysts for oxygen reduction reaction (ORR). However, attaining efficient ORR activity and superior stability in Fe-N-C catalysts is still crucial yet challenging. Herein, we report a rational SiO 2 -mediated two-step pyrolysis strategy for stabilizing densely exposed Fe-N 4 active sites on hierarchically porous carbon (HP-Fe-N-C/2). Benefiting from the high density of accessible Fe-N 4 sites and the high graphitization degree of carbon support, the obtained HP-Fe-N-C/2 catalyst demonstrates outstanding activity and stability for ORR in both alkaline and acidic media. When used as the cathode catalyst, the assembled Zn-air battery shows a high peak power density of 217 mW cm−2 and ultra-long cycling stability for 1342 h without noticeable degradation. As a PGM-free cathode in proton exchange membrane fuel cells, it delivers a maximum output power density of 0.66 W cm−2. [ABSTRACT FROM AUTHOR]

[Display omitted] • A Zn 5 (OH) 6 (CO 3) 2 -assisted pyrolysis strategy is proposed to prepare a coupled catalyst. • Fe nanoparticles can optimize electronic structures and d-band center of Fe center. • Fe-FeN-C exhibits eminent ORR activity with a E 1/2 of 0.921 V in alkaline media. • The Zn-air batteries deliver high power density and long-cycle life for 408 h. Fe single-atom catalysts still suffer from unsatisfactory intrinsic activity and durability for oxygen reduction reaction (ORR). Herein, the coexisting Fe single atoms and nanoparticles on hierarchically porous carbon (denoted as Fe-FeN-C) are prepared via a Zn 5 (OH) 6 (CO 3) 2 -assisted pyrolysis strategy. Theoretical calculation reveals that the Fe nanoparticles can optimize the electronic structures and d-band center of Fe active center, hence reducing the reaction energy barrier for enhancing intrinsic activity. The Zn 5 (OH) 6 (CO 3) 2 self-sacrificial template not only can promote the formation of Fe single atoms, but also contributes to the construction of microporous/mesoporous/macroporous structures. Therefore, the obtained Fe-FeN-C exhibits impressive ORR activity with a half-wave potential of 0.921 V, which far exceeds Pt/C. With Fe-FeN-C as the cathode catalyst, the assembled Zn-air batteries delivered a maximum power density of 206 mW cm−2 and a long-cycle life over 400 h. [ABSTRACT FROM AUTHOR]

The reasonable design of porous structures is important but usually overlooked for nonprecious metal ORR catalysts. In this study, a facile dual melt-salt-mediated templating method is developed to prepare a Fe-N-C catalyst with tailored porous framework. The ZnCl 2 and NaCl are employed to construct abundant micropores and promote the transformation of partial micropores to mesopores, respectively, reasonably forming a 3D hierarchically porous framework. The catalyst demonstrates a satisfactory surface area (1605 m2/g), promoting mass transport and exposure of FeN 4 sites. Interestingly, the dual melt-salt templates avoid rapid loss of nitrogen during pyrolysis, thus enhancing Fe-N 4 active center density. Therefore, the obtained Fe-N-C material presents outstanding ORR performance in both alkaline and acid media, as well as good stability. The advances of this catalyst are further proved in liquid and solid-state Zn-air battery, with nice discharge stability and high peak power densities. The dual melt-salt-mediated templating strategy is developed to prepare the hierarchically porous Fe-N-C catalyst incorporated with abundant Fe-N x site. [Display omitted] • 3D hierarchically porous structure is engineered using ZnCl 2 and NaCl as templates. • The protective molten salts can sharply increase the Fe-Nx active site density. • The catalyst presents outstanding ORR activity under alkaline/acidic electrolytes. • The catalyst exhibits superior performance in practical Zn-air batteries. [ABSTRACT FROM AUTHOR]

Rational design and fabrication of high-efficiency electrocatalysts is essential for improving the kinetics of both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), so as to constructing high-performance rechargeable zinc-air batteries (ZABs). Among the promising candidates, transition metal-based (oxy)hydroxides have attracted extensive attention for their considerable catalytic potential on oxygen-involved reaction, while unsatisfactory intermediate adsorption/desorption strength and low conductivity limit their practical feasibility. In this work, interface engineering modification strategy is proposed, and a metal–organic framework derived heterostructure catalyst of CoFeO x H y -CoP@CC is fabricated. Comprehensively analyses demonstrate that the interaction between phosphide-(oxy)hydroxide induces the interfacial electronic redistribution, while the formed Schottky heterostructure facilitates rapid electron transfer. All these kinetically favorable for both OER and ORR, and leads to remarkable bifunctional performance on CoFeO x H y -CoP@CC, with the OER overpotential of 240 mV to deliver the current density of 10 mA·cm−2, which is better than CoP@CC, CC and RuO 2. In practical applications, liquid zinc-air cells assembled by CoFeO x H y -CoP@CC deliver the peak power density up to 117.55 mW cm−2, which is close to that of 20% Pt/C catalyst (108.48 mW cm−2), and considerable operational performance has also been achieved in all solid-state flexible ZABs. • A metal–organic framework derived phosphide-hydroxide heterostructure of CoFeO x H y -CoP@CC was fabricated. • Heterogenous interaction tunes the electronic configuration and facilitates rapid electron transfer, so that enhances electrocatalytic OER and ORR performance. • CoFeO x H y -CoP@CC employed as cathode in both liquid and solid zinc-air cells show considerable operational performance. [ABSTRACT FROM AUTHOR]

The systematic summarization of the synthesis-structure-performance-mechanism correlations provides guidance for the rational design of S-doped M-N-C catalysts for ORR. [Display omitted] • Recent review on preparation of S-doped M-N-C catalysts for ORR. • Effects of doping S atoms on the ORR performance of M-N-C catalysts. • The advanced characterization methods and computational methods for S-doped M-N-C catalysts are introduced. • The future development direction of M-N-C catalysts is described. Due to the strong electronegativity of the N atom, the adsorption energy of the transition metal –nitrogen active site for the ORR intermediate is too high to facilitate the subsequent elementary reactions. It is worth noting that introducing sulfur atoms in transition metal–nitrogen–carbon catalysts can optimize the charge distribution of nitrogen atoms around metal sites to improve the electronic structure and structural defects of the catalysis, so as to boost the adsorption and transfer of oxygen-containing intermediates. Meanwhile, sulfur-doping can also inhibit metal nanoparticle formation and promote hierarchical porous carbon formation. This review will afford the S-doped M-N-C catalysts rational design principle and strategies and introduce the corresponding ORR electrochemical properties. We will combine advanced characterization techniques and DFT calculation to discuss the coordination environment of active sites and the ORR mechanism of S-doped M-N-C catalysts. Finally, a general perspective on the challenges and opportunities for S-doped M-N-C catalysts will be proposed to facilitate further development. [ABSTRACT FROM AUTHOR]

The oxygen reduction reaction (ORR) is a key process for the operation of fuel cells. To accelerate the sluggish kinetics of ORR, a wide range of catalysts have been proposed and tested. In this work, a nano-dispersed copper-impregnated platinum catalyst prepared by electrodeposition of platinum on a poly[Cu(Salen)] template followed by polymer destruction is described. In addition to the high activity of the thus prepared catalyst in the oxygen reduction reaction surpassing that of both polycrystalline platinum catalyst and the commercial carbon-platinum catalyst ("E-TEK"), it showed remarkable tolerance to the presence of methanol in solution. [ABSTRACT FROM AUTHOR]