The bifunctional electrocatalysis of oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) is critical for the development of rechargeable Zn–air batteries. Both ORRs and OERs ...suffer from sluggish kinetics and high overpotentials, and the bifunctional reactivity is limited by the scaling relationship. Therefore, smart designs on electrocatalysts are requested to achieve high bifunctional ORR/OER activity and excellent performance in Zn–air batteries. Herein, the requirements for bifunctional oxygen electrocatalysts are first introduced. Then the recent advances of precious‐metal‐free active materials and regulation methods of their intrinsic activity are introduced. The structural design principles to improve the accessibility of the active sites are further presented. Finally, a brief overview of the applications of bifunctional oxygen electrocatalysts in Zn–air batteries, including routine liquid batteries and flexible solid‐state batteries, is presented. This review affords rational design principles and strategies for nonprecious bifunctional ORR/OER electrocatalysts, which are expected to guide the targeted optimization of electrocatalysts and further exploration of emerging candidates.
The design strategies of nonprecious metal bifunctional oxygen electrocatalysts have been comprehensively summarized, considering the intrinsic activity and structure features. Recent progress on metal‐free and transition‐metal‐based bifunctional electrocatalysts for oxygen reduction and evolution reactions is reviewed. A brief overview of their applications in liquid and solid‐state Zn−air batteries is also presented.
Rechargeable flexible solid Zn‐air battery, with a high theoretical energy density of 1086 Wh kg−1, is among the most attractive energy technologies for future flexible and wearable electronics; ...nevertheless, the practical application is greatly hindered by the sluggish oxygen reduction reaction/oxygen evolution reaction (ORR/OER) kinetics on the air electrode. Precious metal‐free functionalized carbon materials are widely demonstrated as the most promising candidates, while it still lacks effective synthetic methodology to controllably synthesize carbocatalysts with targeted active sites. This work demonstrates the direct utilization of the intrinsic structural defects in nanocarbon to generate atomically dispersed Co–Nx–C active sites via defect engineering. As‐fabricated Co/N/O tri‐doped graphene catalysts with highly active sites and hierarchical porous scaffolds exhibit superior ORR/OER bifunctional activities and impressive applications in rechargeable Zn‐air batteries. Specifically, when integrated into a rechargeable and flexible solid Zn‐air battery, a high open‐circuit voltage of 1.44 V, a stable discharge voltage of 1.19 V, and a high energy efficiency of 63% at 1.0 mA cm−2 are achieved even under bending. The defect engineering strategy provides a new concept and effective methodology for the full utilization of nanocarbon materials with various structural features and further development of advanced energy materials.
A defect‐engineering strategy in nanocarbon is developed to generate atomically dispersed Co–Nx–C active sites for effective electrocatalysts. As‐obtained Co/N/O tri‐doped graphene exhibits excellent oxygen reduction reaction/oxygen evolution reaction bifunctional activities and impressive performances in rechargeable and flexible Zn‐air batteries. This work provides a novel concept to fully utilize the structural defects in nanocarbon for advanced energy storage and conversion.
It has always been critical to develop high‐performance polymeric materials with exceptional mechanical strength and toughness, thermal stability, and even healable properties for meeting performance ...requirements in industry. Conventional chemical cross‐linking leads to enhanced mechanical strength and thermostability at the expense of extensibility due to mutually exclusive mechanisms. Such major challenges have recently been addressed by using noncovalent cross‐linking of reversible multiple hydrogen‐bonds (H‐bonds) that widely exist in biological materials, such as silk and muscle. Recent decades have witnessed the development of many tailor‐made high‐performance H‐bond cross‐linked polymeric materials. Here, recent advances in H‐bond cross‐linking strategies are reviewed for creating high‐performance polymeric materials. H‐bond cross‐linking of polymers can be realized via i) self‐association of interchain multiple H‐bonding interactions or specific H‐bond cross‐linking motifs, such as 2‐ureido‐4‐pyrimidone units with self‐complementary quadruple H‐bonds and ii) addition of external cross‐linkers, including small molecules, nanoparticles, and polymer aggregates. The resultant cross‐linked polymers normally exhibit tunable high strength, large extensibility, improved thermostability, and healable capability. Such performance portfolios enable these advanced polymers to find many significant cutting‐edge applications. Major challenges facing existing H‐bond cross‐linking strategies are discussed, and some promising approaches for designing H‐bond cross‐linked polymeric materials in the future are also proposed.
Hydrogen‐bond cross‐linking has recently emerged as a promising strategy for creating high‐performance polymeric materials via self‐association of multiple hydrogen bonds or the addition of external cross‐linkers. These polymers exhibit a unique combination of high strength, large extensibility, thermostability, and even healable capability. Such a performance portfolio enables these polymeric materials to find many potential applications in the electronics and gas‐separation fields.
The emergence of van der Waals (vdW) heterostructures of 2D materials has opened new avenues for fundamental scientific research and technological applications. However, the current concepts and ...strategies of material engineering lack feasibilities to comprehensively regulate the as‐obtained extrinsic physicochemical characters together with intrinsic properties and activities for optimal performances. A 3D mesoporous vdW heterostructure of graphene and nitrogen‐doped MoS2 via a two‐step sequential chemical vapor deposition method is constructed. Such strategy is demonstrated to offer an all‐round engineering of 2D materials including the morphology, edge, defect, interface, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities. The hydrogen evolution is substantially accelerated on MoS2, while the oxygen reduction and evolution are significantly improved on graphene. This work provides a powerful overall engineering strategy of 2D materials for electrocatalysis, which is also enlightening for other nanomaterials and energy‐related applications.
A 3D mesoporous van der Waals heterostructure of graphene and nitrogen‐doped MoS2 is fabricated through a two‐step sequential chemical vapor deposition method. This affords an all‐round engineering of 2D materials including the morphology, edges, defects, interfaces, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities.
Bimetallic metal-organic frameworks (MOFs) have two different metal ions in the inorganic nodes. According to the metal distribution, the architecture of bimetallic MOFs can be classified into two ...main categories namely solid solution and core-shell structures. Various strategies have been developed to prepare bimetallic MOFs with controlled compositions and structures. Bimetallic MOFs show a synergistic effect and enhanced properties compared to their monometallic counterparts and have found many applications in the fields of gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. Moreover, bimetallic MOFs can serve as excellent precursors/templates for the synthesis of functional nanomaterials with controlled sizes, compositions, and structures. Bimetallic MOF derivatives show exposed active sites, good stability and conductivity, enabling them to extend their applications to the catalysis of more challenging reactions and electrochemical energy storage and conversion. This review provides an overview of the significant advances in the development of bimetallic MOFs and their derivatives with special emphases on their preparation and applications.
This review summarizes the design and synthesis of bimetallic MOFs and their derivatives, with superior performance to their monometallic counterparts in many applications.
Bifunctional electrocatalysis for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) constitutes the bottleneck of various sustainable energy devices and systems like rechargeable ...metal–air batteries. Emerging catalyst materials are strongly requested toward superior electrocatalytic activities and practical applications. In this study, transition metal hydroxysulfides are presented as bifunctional OER/ORR electrocatalysts for Zn–air batteries. By simply immersing Co‐based hydroxide precursor into solution with high‐concentration S2−, transition metal hydroxides convert to hydroxysulfides with excellent morphology preservation at room temperature. The as‐obtained Co‐based metal hydroxysulfides are with high intrinsic reactivity and electrical conductivity. The electron structure of the active sites is adjusted by anion modulation. The potential for 10 mA cm−2 OER current density is 1.588 V versus reversible hydrogen electrode (RHE), and the ORR half‐wave potential is 0.721 V versus RHE, with a potential gap of 0.867 V for bifunctional oxygen electrocatalysis. The Co3FeS1.5(OH)6 hydroxysulfides are employed in the air electrode for a rechargeable Zn–air battery with a small overpotential of 0.86 V at 20.0 mA cm−2, a high specific capacity of 898 mAh g−1, and a long cycling life, which is much better than Pt and Ir‐based electrocatalyst in Zn–air batteries.
Transition metal hydroxysulfides are proposed as bifunctional electrocatalysts in working Zn–air batteries with high oxygen evolution reaction/oxygen reduction reaction reactivities, high power densities, large capacities, and extraordinary stabilities. These transition metal hydroxysulfides are fabricated through a novel room‐temperature sulfurization strategy, which opens new doors to materials innovation of transition metal (hydro/oxy)sulfides and their practical applications in hetero/electrocatalysis, energy storage, and healthcare applications.
A cost‐effective and highly efficient oxygen evolution reaction (OER) electrocatalyst will be significant for the future energy scenario. The emergence of the core–shell heterostructure has invoked ...new feasibilities to inspire the full potential of non‐precious‐metal candidates. The shells always have a large thickness, affording robust mechanical properties under harsh reaction conditions, which limits the full exposure of active sites with highly intrinsic reactivity and extrinsic physicochemical characters for optimal performance. Herein, a nanosized CoNi hydroxide@hydroxysulfide core–shell heterostructure is fabricated via an ethanol‐modified surface sulfurization method. Such a synthetic strategy is demonstrated to be effective in controllably fabricating a core–shell heterostructure with an ultrathin shell (4 nm) and favorable exposure of active sites, resulting in a moderately regulated electronic structure, remarkably facilitated charge transfer, fully exposed active sites, and a strongly coupled heterointerface for energy electrocatalysis. Consequently, the as‐obtained hydroxide@hydroxysulfide core–shell is revealed as a superior OER catalyst, with a small overpotential of 274.0 mV required for 10.0 mA cm−2, a low Tafel slope of 45.0 mV dec−1, and a favorable long‐term stability in 0.10 M KOH. This work affords fresh concepts and strategies for the design and fabrication of advanced core–shell heterostructures, and thus opens up new avenues for the targeted development of high‐performance energy materials.
A CoNi hydroxide@hydroxysulfide core–shell heterostructure is fabricated via an ethanol‐modified surface sulfurization method with favorable electronic and interface engineering, resulting in excellent intrinsic activity, favorable extrinsic physicochemical characters, and thereby superior water oxidation performances.
Carbon micro/nanocages have received great attention, especially in electrochemical energy‐storage systems. Herein, as a proof‐of‐concept, a solid‐state gas‐steamed metal–organic‐framework approach ...is designed to fabricate carbon cages with controlled openings on walls, and N, P dopants. Taking advantage of the fabricated carbon cages with large openings on their walls for enhanced kinetics of mass transport and N, P dopants within the carbon matrix for favoring chemical adsorption of Zn ions, when used as carbon cathodes for advanced aqueous Zn‐ion hybrid supercapacitors (ZHSCs), such open carbon cages (OCCs) display a wide operation voltage of 2.0 V and an enhanced capacity of 225 mAh g−1 at 0.1 A g−1. Also, they exhibit an ultralong cycling lifespan of up to 300 000 cycles with 96.5% capacity retention. Particularly, such OCCs as electrode materials lead to a soft‐pack ZHSC device, delivering a high energy density of 97 Wh kg−1 and a superb power density of 6.5 kW kg−1. Further, the device can operate in a wide temperature range from −25 to + 40 °C, covering the temperatures for practical applications in daily life.
Just like steaming Chinese buns, a gas‐steamed metal–organic framework (MOF) approach is employed to prepare open‐wall carbon cages while simultaneously offering N, P dopants functionalized into a carbon matrix within a one‐step process. Taking the advantages of the fabricated carbon cages with controlled openings and N, P dopants, extraordinary Zn‐ion energy‐storage performance is displayed.
Abstract
Metal–organic frameworks (MOFs) have been increasingly applied in oxygen evolution reaction (OER), and the surface of MOFs usually undergoes structural transformation to form metal ...oxyhydroxides to serve as catalytically active sites. However, the controllable regulation of the reconstruction process of MOFs remains as a great challenge. Here we report a defect engineering strategy to facilitate the structural transformation of MOFs to metal oxyhydroxides during OER with enhanced activity. Defective MOFs (denoted as NiFc′
x
Fc
1‐x
) with abundant unsaturated metal sites are constructed by mixing ligands of 1,1′‐ferrocene dicarboxylic acid (Fc′) and defective ferrocene carboxylic acid (Fc). NiFc′
x
Fc
1‐x
series are more prone to be transformed to metal oxyhydroxides compared with the non‐defective MOFs (NiFc′). Moreover, the as‐formed metal oxyhydroxides derived from defective MOFs contain more oxygen vacancies. NiFc′Fc grown on nickel foam exhibits excellent OER catalytic activity with an overpotential of 213 mV at the current density of 100 mA cm
−2
, superior to that of undefective NiFc′. Experimental results and theoretical calculations suggest that the abundant oxygen vacancies in the derived metal oxyhydroxides facilitate the adsorption of oxygen‐containing intermediates on active centers, thus significantly improving the OER activity.
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