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Electrochemical transformation processes involving carbon, hydrogen, oxygen, nitrogen, and small‐molecule chemistries represent a promising means to store renewable energy sources in the form of chemical energy. However, their widespread deployment is hindered by a lack of efficient, selective, durable, and affordable electrocatalysts. Recently, grain boundary (GB) engineering as one category of defect engineering, has emerged as a viable and powerful pathway to achieve improved electrocatalytic performances. This review presents a timely and comprehensive overview of recent advances in GB engineering for efficient electrocatalysis. The beneficial effects of introducing GBs into electrocatalysts are discussed, followed by an overview of the synthesis and characterization of GB‐enriched electrocatalysts. Importantly, the latest developments in leveraging GB engineering for enhanced electrocatalysis are thoroughly examined, focusing on the electrochemical utilization cycles of carbon, hydrogen, oxygen, and nitrogen. Future research directions are proposed to further advance the understanding and application of GB engineering for improved electrocatalysis. [ABSTRACT FROM AUTHOR]

Development of cost‐effective water splitting technology that allows low‐overpotential operation at high current density with non‐precious catalysts is the key for large‐scale hydrogen production. Herein, it is demonstrated that the versatile perovskite‐based oxides, usually applied for operating at low current density and room temperature in alkaline solution, can be developed into low‐cost, highly active and durable electrocatalysts for operating at high current densities in a zero‐gap anion exchange membrane electrolyzer cell (AEMEC). The composite perovskite with mixed phases of Ruddlesden–Popper and single perovskite is applied as the anode in AEMEC and exhibits highly promising performance with an overall water‐splitting current density of 2.01 A cm−2 at a cell voltage of only 2.00 V at 60 °C with stable performance. The elevated temperature to promote anion diffusion in membrane boosts oxygen evolution kinetics by enhancing lattice‐oxygen participation. The bifunctionality of perovskites further promises the more cost‐effective symmetrical AEMEC configuration, and a primary cell with the composite perovskite as both electrodes delivers 3.00 A cm−2 at a cell voltage of only 2.42 V. This work greatly expands the use of perovskites as robust electrocatalysts for industrial water splitting at high current density with great practical application merit. [ABSTRACT FROM AUTHOR]

The study of atomically dispersed metal‐nitrogen electrocatalysts is still limited in terms of understanding their catalytic mechanism because of the inability to precisely regulate the coordination number and type of N in combination with the metal elements. Inspired by the high catalytic activity and selectivity of natural enzymes, herein, we have designed and fabricated ultrathin carbon nanosheet‐supported Mn single‐atom catalysts (SACs) with a precise pyrrole‐type Mn‐N4 (PT‐MnN4) configuration using a bio‐mimicking strategy. The PT‐MnN4 SACs display outstanding oxygen reduction reaction (ORR) activity, with a half‐wave potential (E1/2) of 0.88 V (vs. revisible hydrogen electrode [RHE]) and extremely high stability in alkaline media. Moreover, superior ORR activities are also obtained, E1/2 of 0.73 V and 0.63 V in acid and neutral electrolytes, respectively, indicating the efficient pH‐universal ORR performances. The assembled zinc–air battery using the PT‐MnN4 SACs as air cathodes exhibits a high peak power density (175 mW cm−2) and long‐term stability up to 150 h, implying its promising application in metal–air batteries. This study has paved the way toward the rational design and precise regulation of single‐atom electrocatalysts. [ABSTRACT FROM AUTHOR]

Secondary Zn‐air batteries (ZABs) are recognized as one of the most promising power sources for the future with lucrative features of low cost, high energy density, eco‐friendliness, and high safety. However, the widespread implementation of ZABs is still hampered by the sluggish oxygen redox reactions. Thus the deployment of cost‐effective and highly efficient air electrodes to substitute precious metals (Pt/Ir), is highly challenging, however, highly desired. Zeolitic imidazolate frameworks (ZIFs) are emerging functional materials, which demonstrate several outstanding characteristics, such as high specific surface area, high conductivity, self‐doped N, open pore structure, versatile compositions and favourable chemical stability. Through varying the metal/organic moiety or by employing different synthesis protocols, ZIFs with different properties could be obtained. Being adaptable, desired functionalities may be further incorporated into ZIFs through pre‐treatment, in situ treatment, and post treatment. Thus, ZIFs are the ideal precursors for the preparation of variety of bi‐functional air electrodes for ZABs by materials tuning, morphological control, or by materials hybridization. Here, the recent advances of ZIFs‐based materials are critically surveyed from the perspective of synthesis, morphology, structure and properties, and correlated with performance indicators of ZABs. Finally, the major challenges and future prospects of ZIFs associated with ZABs are discussed. [ABSTRACT FROM AUTHOR]

Oxygen reduction reaction (ORR) is a crucial reaction for various energy conversion and storage devices, but the sluggish kinetics and the usage of noble metals greatly restrict its practical device applications. In this work, a well-designed high-performance catalyst for ORR was synthesized via a facile one-step Co-MOF carbonization method, in which Co-MOF was prepared using the cobalt acetate tetrahydrate and 2,2′-bipyridyl-5,5′-dicarboxylic acid (H2bpydc) as the only raw materials and H2bpydc as the favorable carbon and nitrogen source for in situ nitrogen doping and metallic cobalt reduction. The resultant 3D mesoporous carbon foam catalyst with embedded Co nanoparticles (CoN-CF) is enriched with nitrogen, which exhibits high specific surface area and abundant N-doping active sites for catalytic ORR. In particular, the optimized CoN-CF-700 sample displays the best catalytic performances including onset potential of 0.94 V, half-wave potential of 0.85 V, long-term durability and superior resistance to methanol poisoning. The demonstrated synthetic strategy provides a new insight into easy-synthesis and high-economy routes for metal-N-C catalysts and a deeper understanding of the effects of microstructures on catalytic mechanisms. [ABSTRACT FROM AUTHOR]

Abstract: Water electrolysis is regarded as a green and highly efficient approach to producing high‐purity hydrogen, but commercialization of this technology still requires the development of high‐performance and affordable electrocatalysts for the hydrogen evolution reaction (HER). Currently, because of its excellent electrical conductivity and good corrosion resistance in acidic media, cobalt phosphide (CoP) has become a representative non‐noble‐metal HER catalyst despite its inadequate catalytic activity. Herein, a strategy of multiple catalyst‐structure engineering, which simultaneously includes doping, nanostructuring, and in situ nanocarbon coating, was employed to significantly improve the HER performance of CoP. CoP with optimized ruthenium doping and covered by ultrathin graphitic carbon shells shows remarkably high HER catalytic behaviour with a low overpotential of only 73 mV at a current density of 10 mA cm−2 and a small Tafel slope of 46 mV dec−1, close to that of the Pt/C benchmark, while maintaining excellent durability. Moreover, the ultrathin graphene shell has a significant positive effect on catalytic activity. This work demonstrates the necessity and validity of multifold structural control, which can be widely used to design various materials for different catalytic processes. [ABSTRACT FROM AUTHOR]

Abstract: Active and affordable electrocatalysts made from Earth‐abundant elements are central to water electrolysis involving the oxygen evolution reaction (OER). While nonprecious transition metal‐based (Co, Ni, Fe) materials are extensively studied as cost‐effective, high‐performance alternatives to noble metal‐based catalysts, iron‐based catalysts with Fe acting as the single active site are rarely investigated due to their intrinsically low reactivity. Here, by taking advantage of the versatility of perovskite structure, metalloid Si, the second most abundant element in Earth's crust, is doped into an Fe‐based perovskite oxide (SrFeO3−δ), to obtain SrFe0.9Si0.1O3−δ as a highly efficient OER catalyst. Remarkably, SrFe0.9Si0.1O3−δ shows an approximately threefold growth in OER activity relative to SrFeO3−δ with a small Tafel slope of 58 mV dec−1, comparing favorably to or even exceeding many Co‐/Ni‐based perovskite catalysts. This enhancement can be ascribed to a structural transition from tetragonal to cubic symmetry upon Si‐doping, which brings about an optimized iron oxidation state, rich oxygen vacancies, and fast charge transfer that are beneficial to the OER catalysis. This work highlights the importance of how incorporating Earth‐abundant, nonprecious elements (e.g., Si) into a perovskite oxide structure can be used to create novel materials for the electrochemical water oxidation. [ABSTRACT FROM AUTHOR]

The development of highly efficient and low-cost electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting associated with the storage of clean and renewable energy. Here, this study reports its findings in the development of a nanostructured perovskite oxide as OER/HER bifunctional electrocatalyst for overall water splitting. Prepared by a facile electrospinning method, SrNb0.1Co0.7Fe0.2O3-δ perovskite nanorods (SNCF-NRs) display excellent OER and HER activity and stability in an alkaline solution, benefiting from the catalytic nature of perovskites and unique structural features. More importantly, the SNCF-NR delivers a current density of 10 mA cm−2 at a cell voltage of merely ≈1.68 V while maintaining remarkable durability when used as both anodic and cathodic catalysts in an alkaline water electrolyzer. The performance of this bifunctional perovskite material is among the best ever reported for overall water splitting, offering a cost-effective alternative to noble metal based electrocatalysts. [ABSTRACT FROM AUTHOR]

Manganese oxides are attractive catalyst candidates for the oxygen reduction reaction (ORR) given their natural abundance and low toxicity. α-MnO2, in particular, exhibits high ORR activity in an alkaline medium. The hybrid concept provides a way to obtain enhanced ORR performance and long-term durability through an optimized metal oxide-support interaction. Herein, we synthesized a carbon nanotube (CNT)-graphene-α-MnO2 hybrid in a hydrothermal reaction in which the MnO2 nanosheets were deposited on the interior and exterior surfaces of the CNT channels. The resultant hybrid displayed very high ORR activity that is only marginally less than the performance of a commercial 20 wt % Pt/C catalyst and showed even better stability. The excellent ORR activity was attributed to two main factors, that is, the mesoporous architecture of the catalyst and the strong electron coupling between the encapsulated metal oxide and the support. We also showed that the preferential deposition of MnO2 nanosheets within the CNT channels provides enhanced ORR performance relative to deposition on the exterior surfaces of the channels only. This in turn demonstrates unequivocally the confinement effect that the CNT exerts on the encapsulated metal oxide component, which can be exploited as a route to enhanced ORR activity. [ABSTRACT FROM AUTHOR]

Developing cost-effective and efficient electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the storage of renewable energies. Perovskite oxides serve as attractive candidates given their structural and compositional flexibility in addition to high intrinsic catalytic activity. In a departure from the conventional doping approach utilizing metal elements only, here it is shown that non-metal element doping provides an another attractive avenue to optimize the structure stability and OER performance of perovskite oxides. This is exemplified by a novel tetragonal perovskite developed in this work, i.e., SrCo0.95P0.05O3- δ (SCP) which features higher electrical conductivity and larger amount of O2 2−/O− species relative to the non-doped parent SrCoO3- δ (SC), and thus shows improved OER activity. Also, the performance of SCP compares favorably to that of well-developed perovskite oxides reported. More importantly, an unusual activation process with enhanced activity during accelerated durability test (ADT) is observed for SCP, whereas SC delivers deactivation for the OER. Such an activation phenomenon for SCP may be primarily attributed to the in situ formation of active A-site-deficient structure on the surface and the increased electrochemical surface area during ADT. The concept presented here bolsters the prospect to develop a viable alternative to precious metal-based catalysts. [ABSTRACT FROM AUTHOR]

Highlights: A novel high-performance bi-functional electrocatalyst was fabricated by in situ growth of ZIF-67 polyhedrons onto Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) particles. Strong coupling between different phases endowed the robust structure to the composite, which translated into stable charging/discharging operation for 300 h. And the new interfacial phases significantly promoted the catalytic activity, stability and electrical conductivity. The development of bi-functional electrocatalyst with high catalytic activity and stable performance for both oxygen evolution/reduction reactions (OER/ORR) in aqueous alkaline solution is key to realize practical application of zinc–air batteries (ZABs). In this study, we reported a new porous nano-micro-composite as a bi-functional electrocatalyst for ZABs, devised by the in situ growth of metal–organic framework (MOF) nanocrystals onto the micrometer-sized Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) perovskite oxide. Upon carbonization, MOF was converted to porous nitrogen-doped carbon nanocages and ultrafine cobalt oxides and CoN4 nanoparticles dispersing inside the carbon nanocages, which further anchored on the surface of BSCF oxide. We homogeneously dispersed BSCF perovskite particles in the surfactant; subsequently, ZIF-67 nanocrystals were grown onto the BSCF particles. In this way, leaching of metallic or organic species in MOFs and the aggregation of BSCF were effectively suppressed, thus maximizing the number of active sites for improving OER. The BSCF in turn acted as catalyst to promote the graphitization of carbon during pyrolysis, as well as to optimize the transition metal-to-carbon ratio, thus enhancing the ORR catalytic activity. A ZAB fabricated from such air electrode showed outstanding performance with a potential gap of only 0.83 V at 5 mA cm−2 for OER/ORR. Notably, no obvious performance degradation was observed for the continuous charge–discharge operation for 1800 cycles over an extended period of 300 h. [ABSTRACT FROM AUTHOR]

The discovery of new, inexpensive, and efficient electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solutions holds key to the realization of clean hydrogen production through water electrolysis. While molybdenum (Mo)-based inorganic compounds have been extensively investigated as catalyst candidates toward the alkaline HER, Mo-based oxides, in particular, complex oxides, are rarely explored. Here we report a Mo-containing, metallic BaMoO 3 perovskite, which is derived from a scheelite-type, insulating BaMoO 4 oxide through a thermal reduction-induced phase transformation, as a potential electrocatalyst for driving the alkaline HER. The BaMoO 3 perovskite made of interconnected MoO 6 octahedra is found to be more HER-active than the BaMoO 4 scheelite consisting of isolated MoO 4 tetrahedra, showing a significant increase in intrinsic activity by about one order of magnitude, which is due to the stabilized Mo tetravalent state, the increased surface oxygen vacancy concentration, and the improved electrical conductivity of BaMoO 3. Importantly, BaMoO 3 exhibits a smaller overpotential to deliver a geometric current density of −10 mA cm geo − 2 when compared with many of the bulk-sized perovskite catalysts comprising other transition metals (e.g., Mn, Fe, Co, and Ni) and the intensively studied Mo-based catalysts (e.g., MoS 2), making it highly promising as an alternative electrocatalyst for the alkaline HER. Image 1 • A Mo-containing BaMoO 3 perovskite shows efficient hydrogen evolution reactivity. • Metallic BaMoO 3 perovskite is more active than insulating BaMoO 4 scheelite. • BaMoO 3 stabilizes tetravalent Mo, increases oxygen vacancy concentration, and improves charge transfer. [ABSTRACT FROM AUTHOR]

Self-standing nanowires are the focus of intense interest as oxygen evolution reaction catalysts due to their natural advantages. However, the role of the substrate used for self-standing nanowires in oxygen evolution reaction is not fully investigated. Herein, we report that the oxygen evolution reaction activity is closely associated with the substrate. The oxygen evolution reaction activity of cobalt oxide catalysts is enhanced with the increasing electronegativity of the substrates, while iron oxide catalysts break this rule due to the stronger synergistic interplay between iron and copper compared to that between iron and gold. Under the guidance of these findings, we construct a highly active and robust non-precious-metal CuO@CoFeO x self-standing nanowire electrocatalyst, which shows a constant current density of 10 mA cm -2  at an overpotential of 0.36 V for 300 h continuously without noticeable degradation in 0.1 M KOH solution. Moreover, the CuO@CoFeO x nanowire anode coupled with a Pt-Ni foam cathode shows an attractive overpotential of 0.39 V to provide a current density of 10 mA cm −2 for water electrolysis in 1.0 M KOH solution. [ABSTRACT FROM AUTHOR]

• NiFe-LDH nanosheets aligned on Co-embedded N-doped carbon triangle plates with interwoven CNTs are achieved. • The unique 3D hierarchical structure acts as multifunctional electrocatalysts towards ORR, OER and HER. • The DFT results confirm the synergetic effect between the NiFe-LDH and the Co@NC-CNTs. • The fabricated Zn-air battery shows a high power density of 194 mW cm−2 and a long cycle life of up to 760 h. A structural engineering strategy is proposed to fabricate trifunctional electrocatalysts supported by carbon cloth, composed of NiFe-LDH nanosheets aligned on Co-embedded N-doped carbon triangle plates with interwoven carbon nanotubes (Co@NC-CNTs@NiFe-LDH), which is employed as a freestanding electrode for both flexible Zn-air batteries and self-powered water splitting. This unique 3D hierarchical structure and the strong interfacial coupling between the NiFe-LDH and Co@NC-CNTs facilitate the transport of oxygen species as well as electrons and achieve synergy between the integrated nano-scaled structures with different functional components. As expected, a large peak power density of 194 mW cm−2 and outstanding durability for 760 h are obtained in the as-assembled rechargeable Zn-air battery. The resultant water splitting device acquires a cell voltage of 1.66 V at 10 mA cm−2 and the as-assembled Zn-air battery has been successfully employed in a self-power water splitting system, demonstrating a great potential prospect for practical renewable energy applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Surface oxygen vacancy assists the construction of the FeOOH/CdS heterostructures. • Interface oxygen vacancy enhances electron interactions between components. • Oxygen vacancy promotes adsorption, activation, and conversion of CO 2 to CO. • FeOOH/CdS heterostructure is an efficient bifunctional photocatalyst for CO 2 RR and OER. An efficient bifunctional photocatalyst for CO 2 reduction and water oxidation based on a heterostructure with FeOOH nanospindles (NSs) and CdS nanocrystals (NCs) is reported. The microstructure characterizations and theoretical calculations reveal that surface oxygen vacancies (V O) at FeOOH NSs provide preferential deposition sites not only for CdS NCs to grow high-quality interface and form the heterostructure, but also for inert molecule adsorption. Owing to the synergistic effects of the V O -assisted interfacial charge transfer and surface reactions, the optimized FeOOH/CdS heterostructures display a single-carbon compound production rate of 18.43 μ mol g−1 h−1 (5.88 μ mol g−1 h−1 for CH 4 , 12.55 μ mol g−1 h−1 for CO), about 6.2 and 8.5 times higher than their separated counterparts, and an O 2 evolution rate of 676.50 μ mol g−1 h−1 under visible-light irradiation. This work demonstrates for the first time the essential role of surface V O in the FeOOH/CdS heterostructures and their synergistic effects on photocatalysis performance. [ABSTRACT FROM AUTHOR]

• The formation of CoOOH species at interface of Co x Ni-LDHs in the first anodic process before OER process. • The surface reconstruction with CoOOH affects OER catalytic activity and electrode reaction kinetics. • The co-existence of CoIII and NiIII at LDH electrode interface significantly enhances OER performance. • Bimetallic Co 1.8 Ni-LDH nanosheets exhibit a low overpotential and excellent stability at a high current densityof 100mA cm−2 Geo. Bimetallic CoNi-LDHs as catalyst candidates for OER have attracted increasing interests due to their high catalytic activities, low cost and earth abundance. In this work, a series of Co x Ni-LDH nanosheets were synthesized by a chemical co-precipitation method. The impact of Co roles on OER catalytic activities, oxidation conversion of nickel metal ions and resistance of charge-transfer reactions (R ct) was investigated. The electrochemical dynamic behavior of bivalent CoII was characterized by CV and EIS in combination with XPS. The surface morphologies and microstructure were studied by SEM and TEM, respectively. The results show that CoII is irreversibly oxidized to CoIIIOOH at interface in the first anodic polarization process prior to the occurrence of OER. The surface reconstruction with the formation of CoIIIOOH species reduces the R ct and favors the oxidation conversion of NiII hydroxide into NiIII(oxy)hydroxide at a relatively lower applied potential. The co-existence of these high-valence-state metals (CoIII and NiIII) on the LDH surface further boosts the catalytic activity and enhances the OER performance. The OER activity of the Co x Ni-LDHs increases with increase of the Co: Ni ratio, and reaches the maximum as the Co:Ni ratio raises to about 1.8:1.0. The Co 1.8 Ni-LDH shows the most active OER catalyst with an overpotential of 290 mV at 10 mA cm−2 Geo and a Tafel slop of 66 mV dec−1, as well as has a higher i ECSA and TOF. The carbon-free pasted Co 1.8 Ni-LDH electrode on Ni foam also exhibits excellent durability at 100 mA cm−2 in 1M KOH at room temperature. [ABSTRACT FROM AUTHOR]

• Hierarchically porous carbon microspheres (CS) electrocatalysts were prepared. • Porous CS is highly efficient and stable for ORR over a wide range of pH values. • Excellent performance is mainly attributed to the rich surface defects. • The encapsulated cobalt residual further enhances the ORR activity of catalysts. • Mechanism for the enhanced performance of the porous CS is revealed. Carbon materials are promising alternatives to noble metal catalysts for oxygen reduction reactions (ORR) in energy conversion devices. Here we report hierarchically porous carbon microspheres with a trace amount of encapsulated cobalt as highly active and stable electrocatalysts for the ORR under wide pH values ranged from high alkaline to high acidic conditions. We use renewable natural date pulp as the carbon precursor and a facile hydrothermal carbonization method with in situ formed recyclable cobalt as a template for the synthesis. These catalysts yield competitive catalytic activity (a small Tafel slope of 53 mV dec-1) as well as superb durability and methanol tolerance compared to the benchmark Pt/C catalyst in alkaline electrolyte. Even under harsh acidic conditions, the catalysts deliver a satisfactory catalytic performance and good stability, indicating their extensive applicability. This performance is mainly attributed to the rich surface defects and the hierarchically porous structure that provide abundant active sites for the ORR. In situ formed cobalt nanoparticles are critical to the creation of the abundant mesopores, high specific surface area, and catalytically active defect sites over the carbon material. The encapsulated cobalt residual further enhances the ORR activity of the catalysts, while having a negligible effect on the cost due to its trace level (0.2 at.%). [ABSTRACT FROM AUTHOR]

• Co/CoS/Metal-N-C@S, N-codoped mesoporous carbon polyhedrons are synthesized. • The specific roles of Co, CoS and (Co, Fe)-N-C active species are well recognized. • An ORR onset potential of 0.971 V and a half-wave potential of 0.906 V are achieved. • The assembled rechargeable Zn-air battery exhibits a high-power density of 213 mW cm−2. It remains a huge challenge to design and realize trifunctional high-efficiency and low-cost electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, with the introduction of CdS and ferrocene, a metal–organic framework (MOF)-induced strategy is reported to synthesize a hierarchical trifunctional electrocatalyst based on Co/CoS nanoparticles and metal (Co, Fe)-N-C species in a hairy S, N-codoped mesoporous carbon polyhedron interwoven with carbon nanotubes (Co/CoS/Fe-HSNC). The specific roles of Co, CoS and (Co, Fe)-N-C active species in Co/CoS/Fe-HSNC catalyst for ORR, OER and HER are also well recognized. Besides, CdS nanoparticles is demonstrated to act as a pore former and S sources both for reaction and doping, and ferrocene is employed to initiate the in-situ growth of carbon nanotubes and Fe-N-C active sites which both contribute to superior multifunctional activities. Benefitting from the rationally designed nanostructures, the ORR half-potential of the Co/CoS/Fe-HSNC electrocatalyst reaches 0.906 V in 0.1 M KOH, about 65 mV larger than commercial Pt/C electrocatalyst, and a low overpotential of 250 mV is achieved for OER and 138 mV for HER at a current density of 10 mA cm−2. Constructed with the Co/CoS/Fe-HSNC catalyst, a Zn-air battery exhibits a high power density of 213 mW cm−2 and a small charge/discharge voltage gap of 0.79 V at 2.0 mA cm−2. The presented strategy provides new insight into the construction of MOF-derived hierarchical multifunctional catalysts for Zn-air batteries and water electrolysis. [ABSTRACT FROM AUTHOR]