Amorphous Bimetallic Oxide-Graphene Hybrids as Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

Recent Advances on Self-Supported Arrayed Bifunctional Oxygen Electrocatalysts for Flexible Solid-State Zn-Air Batteries

Flexible solid-state Zn-air batteries have been rapidly developed benefiting from the uprising demand for wearable electronic devices, wherein the air electrode integrated with efficient bifunctional oxygen electrocatalysts plays an important role to achieve high performance. Binder-free self-supported bifunctional catalysts can provide large active surface area, fast electron transport path, easy ion diffusion, and excellent structural stability and flexibility, thus acting as promising flexible air cathodes. In this review, recent advances on the application of nanoarrayed electrocatalysts as air cathodes in flexible Zn-air batteries are reviewed. Especially, various types of bifunctional oxygen electrocatalysts, including carbonaceous material arrays, transition metal compound arrays, transition metal/carbon arrays, transition metal compound/carbon arrays, and other hybrid arrays, are discussed. The applications of flexible Zn-air batteries with two configurations (i.e., planar stacks and cable fibers) are also introduced. Finally, perspectives on the optimization of arrayed air cathodes for future development to achieve high-performance flexible Zn-air batteries are shared.

Cu Nanoclusters/FeN4 Amorphous Composites with Dual Active Sites in N-Doped Graphene for High-Performance Zn-Air Batteries

Exploring inexpensive and earth-abundant transition metal-nitrogen-based carbon (MNC) catalysts to substitute the scarce and costly Pt-based electrocatalysts for the oxygen reduction reaction (ORR) is quite anticipated in metal-air batteries (MABs). Here, we demonstrate a facile vacuum-annealing method to synthesize Cu nanoclusters/FeN₄ amorphous composites embedded in N-doped graphene (Cu/Fe-NG). This approach avoids the long-term pyrolysis procedure and the use of an inert atmosphere in the conventional procedure for fabricating MNC catalysts. Interestingly, we discovered that the amorphous structure of Cu/FeN₄ composites can provide high-activity bimetallic M-N_x sites (M = Cu, Fe), because of which the Cu/FeN₄ composites exhibit boosted electrocatalytic activity with a positive half-wave potential of 0.88 V (vs RHE), long-term durability, and low hydrogen peroxide for the ORR. The origin of this enhancement was assigned to the concomitance of Fe-N₄ and Cu-N_x moieties in Cu/Fe-NG, favoring adsorption and activation of the O₂ molecule as suggested by X-ray absorption fine structure (XAFS) analyses and density functional theory (DFT) calculations. Moreover, examinations of Cu/Fe-NG in both liquid and quasi-solid-state Zn-air batteries (ZABs) can exhibit remarkable performances. This work may offer facile fabrication of enhanced performance MNC catalysts as well as a profound insight into the use of amorphous materials in the ORR and ZABs.

FeNi3-modified Fe2O3/NiO/MoO2 heterogeneous nanoparticles immobilized on N, P co-doped CNT as an efficient and stable electrocatalyst for water oxidation

As a rate-determining step, electrocatalytic water oxidation acts a pivotal role in the water splitting process. As a consequence, it is of great significance to explore low-cost, efficient and durable electrocatalysts for the oxygen evolution reaction (OER) to promote electrocatalytic splitting water. Herein, for the first time, FeNi₃-modified Fe₂O₃/NiO/MoO₂ heterogeneous nanoparticles immobilized on N, P co-doped CNT matrix materials (FNM/NPCNT) are synthesized via a facile solid-phase grinding of the precursor, composed of nickel hexacyanoferrate/phosphomolybdic acid/CNT, and subsequently pyrolyzing under nitrogen atmosphere without any further post-processing. Due to its significant enhancement of the charge transfer efficiency and prevention of the metallic-based catalysts from being corroded, the as-prepared FNM/NPCNT hybrid electrocatalyst shows a high OER activity with a low overpotential of 282 mV vs. RHE at 10 mA cm^-2 and a small Tafel slope of 46.2 mV dec^-1 in an alkaline electrolyte. Moreover, the as-prepared FNM/NPCNT hybrid delivers a large mass activity of 327.6 A g^-1 at the potential of 1.7 V and excellent stability (more than 20 h). This study opens up a new approach to design and synthesize non-precious transition metal-based composites immobilized N, P co-doped CNT materials as OER catalysts with high efficiency and long-term stability for promoting water splitting.

Study of “Ni-doping” and “open-pore microstructure” as physico-electrochemical stimuli towards the electrocatalytic efficiency of Ni/NiO for the oxygen evolution reaction

Added “Ni-doping” and “open-pore microstructure” act as physico-electrochemical stimuli towards enhanced electrocatalytic efficiency and electromechanical stability of Ni/NiO for the low-overpotential oxygen evolution reaction in alkaline medium.

Modulation of Inverse Spinel Fe3 O4 by Phosphorus Doping as an Industrially Promising Electrocatalyst for Hydrogen Evolution

Fe-based oxides have been seldom reported as electrocatalysts for the hydrogen evolution reaction (HER), limited by their weak intrinsic activity and conductivity. Herein, phosphorus doping modulation is used to construct inverse spinel P-Fe₃ O₄ with dual active sites supported on iron foam (P-Fe₃ O₄ /IF) for alkaline HER with an extremely low overpotential of 138 mV at 100 mA cm^-2 . The obtained inverse spinel Fe-O-P derived from controllable phosphorization can provide an octahedral Fe site and O atom, which bring about the unusual dissociation mechanisms of two water molecules to greatly accelerate the proton supply in alkaline media. Meanwhile, the ΔG_H of the P atom in Fe-O-P as an active site is theoretically calculated to be 0.01 eV. Notably, the NiFe LDH/IF⁽⁺⁾ ||P-Fe₃ O₄ /IF^(-) couple achieves an onset potential of 1.47 V (vs RHE) for overall water splitting, with excellent stability for more than 1000 h at a current density of 1000 mA cm^-2 , and even for 25 000 s at 10 000 mA cm^-2 in 6.0 m KOH at 60 °C. The excellent catalyst stability and low-cost merits of P-Fe₃ O₄ /IF may hold promise for industrial hydrogen production. This work may reveal a new design strategy of earth-abundant materials for large-scale water splitting.

In situ encapsulation of Co-based nanoparticles into nitrogen-doped carbon nanotubes-modified reduced graphene oxide as an air cathode for high-performance Zn-air batteries

Exploring highly efficient catalysts for the oxygen reduction/evolution reaction (ORR/OER) is very important in rechargeable Zn-air batteries. N-doped carbon coupled with transition metal-based species are among the most promising cathode catalysts for Zn-air batteries. However, the aggregation of metal-based sites during the synthetic/cycling process is a serious drawback of these catalysts. Herein, in situ encapsulation of ultra-small Co/Co4N nanoparticles into N-doping carbon nanotubes (N-CNTs) anchored on reduced GO (Co/Co4N@N-CNTs/rGO) has been achieved through pyrolyzing a core-shell-structured ZIF-8@ZIF-67-modified GO (ZIF-8@ZIF-67/GO) precursor; the nanoparticles have been further applied as a bifunctional catalyst in Zn-air batteries. Benefitting from its uniform dispersion of Co-based particles, close contact of Co/Co4N species and N-CNTs, and high N content, Co/Co4N@N-CNTs/rGO shows outstanding catalytic activity/stability towards ORR and OER. Moreover, Zn volatilization and rGO introduction in Co/Co4N@N-CNTs/rGO can effectively promote the reactions of Zn-air cells. Hence, the Co/Co4N@N-CNTs/rGO-based conventional Zn-air battery exhibits a fantastic specific capacity of 783 mA h gZn-1, a continuous discharge platform over 6 days, a high-power density of ∼200 mW cm-2 and an ultra-long cycling life of 440 h with a small overpotential of ∼0.8 V. Moreover, a flexible Co/Co4N@N-CNTs/rGO-based Zn-air cell was also designed and revealed outstanding mechanical flexibility and good electrochemical performance, which suggests its potential application prospects in wearable electronic devices.

Effective Oxygen Reduction Reaction Performance of FeCo Alloys In Situ Anchored on Nitrogen-Doped Carbon by the Microwave-Assistant Carbon Bath Method and Subsequent Plasma Etching

Electrocatalysts with strong stability and high electrocatalytic activity have received increasing interest for oxygen reduction reactions (ORRs) in the cathodes of energy storage and conversion devices, such as fuel cells and metal-air batteries. However, there are still several bottleneck problems concerning stability, efficiency, and cost, which prevent the development of ORR catalysts. Herein, we prepared bimetal FeCo alloy nanoparticles wrapped in Nitrogen (N)-doped graphitic carbon, using Co-Fe Prussian blue analogs (Co₃[Fe(CN)₆]₂, Co-Fe PBA) by the microwave-assisted carbon bath method (MW-CBM) as a precursor, followed by dielectric barrier discharge (DBD) plasma treatment. This novel preparation strategy not only possessed a fast synthesis rate by MW-CBM, but also caused an increase in defect sites by DBD plasma treatment. It is believed that the co-existence of Fe/Co-N sites, rich active sites, core-shell structure, and FeCo alloys could jointly enhance the catalytic activity of ORRs. The obtained catalyst exhibited a positive half-wave potential of 0.88 V vs. reversible hydrogen electrode (RHE) and an onset potential of 0.95 V vs. RHE for ORRs. The catalyst showed a higher selectivity and long-term stability than Pt/C towards ORR in alkaline media.

CuCo2S4 Nanosheets@N-Doped Carbon Nanofibers by Sulfurization at Room Temperature as Bifunctional Electrocatalysts in Flexible Quasi-Solid-State Zn-Air Batteries

The performance of quasi-solid-state flexible zinc-air batteries (ZABs) is critically dependent on the advancement of air electrodes with outstanding bifunctional electrocatalysis for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), together with the desired mechanical flexibility and robustness. The currently available synthesis processes for high-efficiency bifunctional bimetallic sulfide electrodes typically require high-temperature hydrothermal or chemical vapor deposition, which is undesirable in terms of the complexity in experimental procedure and the damage of flexibility in the resultant electrode. Herein, a scalable fabrication process is reported by combining electrospinning with in situ sulfurization at room temperature to successfully obtain CuCo2S4 nanosheets@N-doped carbon nanofiber (CuCo2S4 NSs@N-CNFs) films, which show remarkable bifunctional catalytic performance (Ej = 10 (OER) - E 1/2 (ORR) = 0.751 V) with excellent mechanical flexibility. Furthermore, the CuCo2S4 NSs@N-CNFs cathode delivers a high open-circuit potential of 1.46 V, an outstanding specific capacity of 896 mA h g-1, when assembled into a quasi-solid-state flexible ZAB together with Zn NSs@carbon nanotubes (CNTs) film (electrodeposited Zn nanosheets on CNTs film) as the anode. The ZAB also shows a good flexibility and capacity stability with 93.62% capacity retention (bending 1000 cycles from 0° to 180°), making it an excellent power source for portable and wearable electronic devices.

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.

Nature of Bimetallic Oxide Sb2MoO6/rGO Anode for High-Performance Potassium-Ion Batteries

Potassium-ion batteries (KIBs) are one of the most appealing alternatives to lithium-ion batteries, particularly attractive in large-scale energy storage devices considering the more sufficient and lower cost supply of potassium resources in comparison with lithium. To achieve more competitive KIBs, it is necessary to search for anode materials with a high performance. Herein, the bimetallic oxide Sb2MoO6, with the presence of reduced graphene oxide, is reported as a high-performance anode material for KIBs in this study, achieving discharge capacities as high as 402 mAh g-1 at 100 mA g-1 and 381 mAh g-1 at 200 mA g-1, and reserving a capacity of 247 mAh g-1 after 100 cycles at a current density of 500 mA g-1. Meanwhile, the potassiation/depotassiation mechanism of this material is probed in-depth through the electrochemical characterization, operando X-ray diffraction, transmission electron microscope, and density functional theory calculation, successfully unraveling the nature of the high-performance anode and the functions of Sb and Mo in Sb2MoO6. More importantly, the phase development and bond breaking sequence of Sb2MoO6 are successfully identified, which is meaningful for the fundamental study of metal-oxide based electrode materials for KIBs.

Ultra-High Mass-Loading Cathode for Aqueous Zinc-Ion Battery Based on Graphene-Wrapped Aluminum Vanadate Nanobelts

Rechargeable aqueous zinc-ion batteries (AZIBs) have their unique advantages of cost efficiency, high safety, and environmental friendliness. However, challenges facing the cathode materials include whether they can remain chemically stable in aqueous electrolyte and provide a robust structure for the storage of Zn2+. Here, we report on H11Al2V6O23.2@graphene (HAVO@G) with exceptionally large layer spacing of (001) plane (13.36 Å). The graphene-wrapped structure can keep the structure stable during discharge/charge process, thereby promoting the inhibition of the dissolution of elements in the aqueous electrolyte. While used as cathode for AZIBs, HAVO@G electrode delivers ideal rate performance (reversible capacity of 305.4, 276.6, 230.0, 201.7, 180.6 mAh g-1 at current densities between 1 and 10 A g-1). Remarkably, the electrode exhibits excellent and stable cycling stability even at a high loading mass of ~ 15.7 mg cm-2, with an ideal reversible capacity of 131.7 mAh g-1 after 400 cycles at 2 A g-1.

Recent Progress in Electrically Rechargeable Zinc-Air Batteries

Over the past decade, the surging interest for higher-energy-density, cheaper, and safer battery technology has spurred tremendous research efforts in the development of improved rechargeable zinc-air batteries. Current zinc-air batteries suffer from poor energy efficiency and cycle life, owing mainly to the poor rechargeability of zinc and air electrodes. To achieve high utilization and cyclability in the zinc anode, construction of conductive porous framework through elegant optimization strategies and adaptation of alternate active material are employed. Equally, there is a need to design new and improved bifunctional oxygen catalysts with high activity and stability to increase battery energy efficiency and lifetime. Efforts to engineer catalyst materials to increase the reactivity and/or number of bifunctional active sites are effective for improving air electrode performance. Here, recent key advances in material development for rechargeable zinc-air batteries are described. By improving fundamental understanding of materials properties relevant to the rechargeable zinc and air electrodes, zinc-air batteries will be able to make a significant impact on the future energy storage for electric vehicle application. To conclude, a brief discussion on noteworthy concepts of advanced electrode and electrolyte systems that are beyond the current state-of-the-art zinc-air battery chemistry, is presented.

Polyethyleneimine-Mediated Fabrication of Two-Dimensional Cobalt Sulfide/Graphene Hybrid Nanosheets for High-Performance Supercapacitors

Two-dimensional (2D) hybrid nanosheets made of electrochemically active materials and graphene, featuring fast reaction kinetics and excellent structural stability, are appealing as electrodes for supercapacitors. Herein, we report a general polyethyleneimine (PEI)-mediated fabrication of the 2D cobalt sulfide/graphene hybrid nanosheets via a facile hydrothermal strategy. Detailed analysis reveals that the uniform cobalt sulfide (CoS) nanoparticles are well-anchored on the 2D graphene surface. Thereinto, the nitrogen species originating from the PEI molecules as bridging sites are helpful in enhancing the coupling effect between the graphene and CoS species, which endows the hybrid nanosheets with high electroactivity and excellent structural stability. The as-obtained hybrid nanosheets with typical pseudocapacitive features demonstrate a high specific capacitance (320 F g^-1@1 A g^-1) and a superior electrochemical stability with the initial capacitance of 86.5% after 20 000 cycles as electrodes for supercapacitors. The assembled asymmetric supercapacitors by combining activated carbon electrodes further show a high charge storage of 28.8 Wh kg^-1 with an output power of 130 W kg^-1. More importantly, the similar strategy can be easily extended to fabricate other 2D hybrid nanosheets with extraordinary electrochemical performance with the maximum charge storage of 815 F g^-1 and energy density of 44. 6 Wh kg^-1. The present work, which achieves the general fabrication of 2D hybrid nanosheets with tuned compositions, may provide a new opportunity for the development of supercapacitors.

Metal and Nonmetal Codoped 3D Nanoporous Graphene for Efficient Bifunctional Electrocatalysis and Rechargeable Zn-Air Batteries

Developing bifunctional electrocatalysts with high activities and long durability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial toward the practical implementation of rechargeable metal-air batteries. Here, a 3D nanoporous graphene (np-graphene) doped with both N and Ni single atoms/clusters is reported. The predoping of N by chemical vapor deposition (CVD) dramatically increases the Ni doping amount and stability. The resulting N and Ni codoped np-graphene has excellent electrocatalytic activities for both the ORR and the OER in alkaline aqueous solutions. The synergetic effects of N and Ni dopants are revealed by density functional theory calculations. The free-standing Ni,N codoped 3D np-graphene shows great potential as an economical catalyst/electrode for metal-air batteries.

Identifying the Activation of Bimetallic Sites in NiCo2 S4 @g-C3 N4 -CNT Hybrid Electrocatalysts for Synergistic Oxygen Reduction and Evolution

Hybrid materials composed of transition-metal compounds and nitrogen-doped carbonaceous supports are promising electrocatalysts for various electrochemical energy conversion devices, whose activity enhancements can be attributed to the synergistic effect between metallic sites and N dopants. While the functionality of single-metal catalysts is relatively well-understood, the mechanism and synergy of bimetallic systems are less explored. Herein, the design and fabrication of an integrated flexible electrode based on NiCo₂ S₄ /graphitic carbon nitride/carbon nanotube (NiCo₂ S₄ @g-C₃ N₄ -CNT) are reported. Comparative studies evidence the electronic transfer from bimetallic Ni/Co active sites to abundant pyridinic-N in underlying g-C₃ N₄ and the synergistic effect with coupled conductive CNTs for promoting reversible oxygen electrocatalysis. Theoretical calculations demonstrate the unique coactivation of bimetallic Ni/Co atoms by pyridinic-N species (a Ni, Co-N₂ moiety), which simultaneously downshifts their d-band center positions and benefits the adsorption/desorption features of oxygen intermediates, accelerating the reaction kinetics. The optimized NiCo₂ S₄ @g-C₃ N₄ -CNT hybrid manifests outstanding bifunctional performance for catalyzing oxygen reduction/evolution reactions, highly efficient for realistic zinc-air batteries featuring low overpotential, high efficiency, and long durability, superior to those of physical mixed counterparts and state-of-the-art noble metal catalysts. The identified bimetallic coactivation mechanism will shed light on the rational design and interfacial engineering of hybrid nanomaterials for diverse applications.

Morphology control of zinc electrodeposition by surfactant addition for alkaline-based rechargeable batteries

The development of Zn-air batteries with a high energy density of 1350 W h kg-1 is one of the breakthroughs required to achieve a low carbon society. However, morphology control of the Zn negative electrode during charge/discharge (Zn-deposition/stripping) is essential for practical application. Considering the manufacturing process, a simple strategy is preferable. Herein, we employed surfactants as an inhibitor of the formation of mossy and dendrite Zn structures, and studied electrochemical Zn growth from the perspective of the electric charge of the surfactant. Even by using an additive free electrolyte of 0.25 M ZnO + 4 M KOH and with 1 mM sodium dodecyl sulfate (SDS: anionic surfactant) or polyacrylic acid (PAA: non-ionic surfactant), mossy and dendrite formations were unavoidable irrespective of the current density. On the other hand, a cationic surfactant, trimethyloctadecylammonium chloride (STAC), suppressed the shape change and resulted in a smooth and dense morphology. Zeta potential measurements, kinetic current densities observed from Tafel plots, and constant potential electrolysis indicate that quaternary ammonium cations (STAC) with bulky size adsorb onto protrusions which are the cause of shape change and suppress Zn deposition in the region to promote lateral growth. Although the adsorption of STAC increased the average overvoltage for Zn-deposition/stripping in a symmetric Zn|Zn cell under a current density of 10 mA cm-2, significantly stable behavior continued for 200 h. In contrast, the overvoltage of the additive free system suddenly increased after 156 h, associated with the accumulation of insulating ZnO and Zn(OH)2 formed on the Zn surface. In charge-discharge tests using an asymmetric Cu|Zn cell, the coulombic efficiency in the additive free electrolyte was less than 95%, whereas the addition of STAC at 1 mM achieved superior cycling performance without any capacity loss originating from the generation of dead Zn (electrical isolation). These results demonstrate that the addition of STAC is a promising method of controlling the Zn morphology.

CoP-Doped MOF-Based Electrocatalyst for pH-Universal Hydrogen Evolution Reaction

Although electrocatalysts based on transition metal phosphides (TMPs) with cationic/anionic doping have been widely studied for hydrogen evolution reaction (HER), the origin of performance enhancement still remains elusive mainly due to the random dispersion of dopants. Herein, we report a controllable partial phosphorization strategy to generate CoP species within the Co-based metal-organic framework (Co-MOF). Density functional theory calculations and experimental results reveal that the electron transfer from CoP to Co-MOF through N-P/N-Co bonds could lead to the optimized adsorption energy of H₂ O (ΔG H 2 O * ) and hydrogen (ΔG_H* ), which, together with the unique porous structure of Co-MOF, contributes to the remarkable HER performance with an overpotential of 49 mV at a current density of 10 mA cm^-2 in 1 m phosphate buffer solution (PBS, pH 7.0). The excellent catalytic performance exceeds almost all the documented TMP-based and non-noble-metal-based electrocatalysts. In addition, the CoP/Co-MOF hybrid also displays Pt-like performance in 0.5 m H₂ SO₄ and 1 m KOH, with the overpotentials of 27 and 34 mV, respectively, at a current density of 10 mA cm^-2 .

Advanced Carbon for Flexible and Wearable Electronics

Flexible and wearable electronics are attracting wide attention due to their potential applications in wearable human health monitoring and care systems. Carbon materials have combined superiorities such as good electrical conductivity, intrinsic and structural flexibility, light weight, high chemical and thermal stability, ease of chemical functionalization, as well as potential mass production, enabling them to be promising candidate materials for flexible and wearable electronics. Consequently, great efforts are devoted to the controlled fabrication of carbon materials with rationally designed structures for applications in next-generation electronics. Herein, the latest advances in the rational design and controlled fabrication of carbon materials toward applications in flexible and wearable electronics are reviewed. Various carbon materials (carbon nanotubes, graphene, natural-biomaterial-derived carbon, etc.) with controlled micro/nanostructures and designed macroscopic morphologies for high-performance flexible electronics are introduced. The fabrication strategies, working mechanism, performance, and applications of carbon-based flexible devices are reviewed and discussed, including strain/pressure sensors, temperature/humidity sensors, electrochemical sensors, flexible conductive electrodes/wires, and flexible power devices. Furthermore, the integration of multiple devices toward multifunctional wearable systems is briefly reviewed. Finally, the existing challenges and future opportunities in this field are summarized.

Research Advances of Amorphous Metal Oxides in Electrochemical Energy Storage and Conversion

Amorphous metal oxides (AMOs) have aroused great enthusiasm across multiple energy areas over recent years due to their unique properties, such as the intrinsic isotropy, versatility in compositions, absence of grain boundaries, defect distribution, flexible nature, etc. Here, the materials engineering of AMOs is systematically reviewed in different electrochemical applications and recent advances in understanding and developing AMO-based high-performance electrodes are highlighted. Attention is focused on the important roles that AMOs play in various energy storage and conversion technologies, such as active materials in metal-ion batteries and supercapacitors as well as active catalysts in water splitting, metal-air batteries, and fuel cells. The improvements of electrochemical performance in metal-ion batteries and supercapacitors are reviewed regarding the enhancement in active sites, mechanical strength, and defect distribution of amorphous structures. Furthermore, the high electrochemical activities boosted by AMOs in various fundamental reactions are elaborated on and they are related to the electrocatalytic behaviors in water splitting, metal-air batteries, and fuel cells. The applications in electrochromism and high-conducting sensors are also briefly discussed. Finally, perspectives on the existing challenges of AMOs for electrochemical applications are proposed, together with several promising future research directions.

Double-Holey-Heterostructure Frameworks Enable Fast, Stable, and Simultaneous Ultrahigh Gravimetric, Areal, and Volumetric Lithium Storage

Deliberate design of advantageous nanostructures holds great promise for developing high-performance electrode materials for electrochemical energy storage. However, it remains a tremendous challenge to simultaneously gain high gravimetric, areal, and volumetric capacities as well as high rate performance and cyclability to meet practical requirements mainly due to the intractable insufficient ion diffusion and limited active sites for dense electrodes with high areal mass loadings. Herein we report a double-holey-heterostructure framework, in which holey Fe₂O₃ nanosheets (H-Fe₂O₃) are tightly and conformably grown on the holey reduced graphene oxide (H-RGO). This hierarchical nanostructure allows for rapid ion and electron transport and sufficient utilization of active sites throughout a highly compact and thick electrode. Therefore, the free-standing flexible H-Fe₂O₃/H-RGO heterostructure anode can simultaneously deliver ultrahigh gravimetric, areal, and volumetric capacities of 1524 mAh g^-1, 4.72 mAh cm^-2, and 2621 mAh cm^-3, respectively, at 0.2 A g^-1 after 120 cycles, and extraordinary rate performance with a capacity of 487 mAh g^-1 (1.51 mAh cm^-2) at a high current density of 30 A g^-1 (93 mA cm^-2) as well as excellent cycling stability with a capacity retention of 96.3% after 1600 cycles, which has rarely been achieved before.

Boosting reversible oxygen electrocatalysis with enhanced interfacial pyridinic-N-Co bonding in cobalt oxide/mesoporous N-doped graphene hybrids

A hybrid electrocatalyst CoO/r-NLG with a high density of pyridinic-N-Co bonds was synthesized via pyrolysis treatments of a mixture of cobalt iron precursors and laser-induced mesoporous N-doped graphene, which achieved superb reversible ORR/OER activities, offering a low efficiency loss (ΔE) of 0.635 V in a 1.0 M KOH electrolyte.

All-in-One Bifunctional Oxygen Electrode Films for Flexible Zn-Air Batteries

As a promising energy-storage device, rechargeable Zn-air batteries have attracted considerable interests. Herein, a bifunctional oxygen electrode film prepared by adhering NiCo₂ O₄ nanosheets to a nitrogen and oxygen dual-doped carbon nanotubes film in a large scale is reported. The resulting self-supporting film electrode is multifunctional, which integrates a porous conducting structure for air diffusion and charge transfer, high-performance catalysts for oxygen reduction and evolution, and novel structural flexibility. The composite film demonstrates excellent oxygen reduction/evolution reaction catalytic activities with low Tafel slopes (50 mV dec^-1 for oxygen reduction reaction; 92 mV dec^-1 for oxygen evolution reaction). Without any additional current collector, gas diffusion layer, or binder, the obtained bifunctional film performs as an "all-in-one" air electrode in a Zn-air battery. A 50-cm-long cable-shaped Zn-air battery based on such a film air electrode exhibits high operating potentials (≈1.2 V at 0.25 mA cm^-2 ), low charging-discharging overpotentials (≈0.7 V), and stable cycling performance. Moreover, the flexible cable Zn-air batteries show excellent stability under different deformation conditions. The proposed concept of constructing scalable, all-in-one, freestanding, and flexible air electrodes would pave the way to develop next-generation wearable and portable energy-storage devices.

Recent Advances in Materials and Design of Electrochemically Rechargeable Zinc-Air Batteries

The century-old zinc-air (Zn-air) battery concept has been revived in the last decade due to its high theoretical energy density, environmental-friendliness, affordability, and safety. Particularly, electrically rechargeable Zn-air battery technologies are of great importance for bulk applications like electric vehicles, grid management, and portable electronic devices. Nevertheless, Zn-air batteries are still not competitive enough to realize widespread practical adoption because of issues in efficiency, durability, and cycle life. Here, following an introduction to the fundamentals and performance testing techniques, the latest research progress related to electrically rechargeable Zn-air batteries is compiled, particularly new key findings in the last five years (2013-2018). The strategies concerning the development of Zn and air electrodes are in focus. The design of other battery components, namely electrolytes and separators are also discussed. Poor performance of O₂ electrocatalysts and the lack of the long-term stability of Zn electrodes and electrolytes remain major challenges. Finally, recommendations regarding the testing routines and materials design are provided. It is hoped that this up-to-date account will help to shape the future research activities toward the development of practical electrically rechargeable Zn-air batteries with extended lifetime and superior performance.

Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide/Heteroatom (N,S)-Doped Double-Layered Carbon Tubes as an Efficient Bifunctional Oxygen Electrocatalyst

It is a critical challenge to construct efficient precious-metal-free bifunctional oxygen electrocatalysts for fuel cell and metal-air batteries via structural and component engineering. Herein, a one-dimensional mesoporous double-layered tubular structure, where Co₉S₈ nanocrystals are incorporated into nitrogen, sulfur codoped carbon, is successfully synthesized via the coordinated-assisted polymerization and sacrificial template methods. The double-layered tubular structure provides for a large electrochemically active surface area and promotes fast mass transfer. Cobalt oxides/oxyhydroxides, which are evolved from the sulfides during the catalytic processes, as the main active sites efficiently catalyze the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), in cooperation with the Co-N-C and heteroatom-induced active sites. Hence, it demonstrates excellent bifunctional electrocatalytic activity with the overvoltage between the OER potential at 10 mA cm^-2 ( E₁₀) and ORR half-wave potential ( E_1/2) of 0.707 V, which is superior to most of precious-metal-free bifunctional oxygen electrocatalysts reported recently, as well as the state-of-art Pt/C and RuO₂ catalysts.

Recent Advances toward the Rational Design of Efficient Bifunctional Air Electrodes for Rechargeable Zn-Air Batteries

Large-scale application of renewable energy and rapid development of electric vehicles have brought unprecedented demand for advanced energy-storage/conversion technologies and equipment. Rechargeable zinc (Zn)-air batteries represent one of the most promising candidates because of their high energy density, safety, environmental friendliness, and low cost. The air electrode plays a key role in managing the many complex physical and chemical processes occurring on it to achieve high performance of Zn-air batteries. Herein, recent advances of air electrodes from bifunctional catalysts to architectures are summarized, and their advantages and disadvantages are discussed to underline the importance of progress in the evolution of bifunctional air electrodes. Finally, some challenges and the direction of future research are provided for the optimized design of bifunctional air electrodes to achieve high performance of rechargeable Zn-air batteries.

Stable and Efficient Nitrogen-Containing Carbon-Based Electrocatalysts for Reactions in Energy-Conversion Systems

High activity and stability are crucial for the practical use of electrocatalysts in fuel cells, metal-air batteries, and water electrolysis, including the oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, and oxidation reactions of formic acid and alcohols. Electrocatalysts based on nitrogen-containing carbon (N-C) materials show promise in catalyzing these reactions; however, there is no systematic review of strategies for the engineering of active and stable N-C-based electrocatalysts. Herein, a comprehensive comparison of recently reported N-C-based electrocatalysts regarding both electrocatalytic activity and long-term stability is presented. In the first part of this review, the relationships between the electrocatalytic reactions and selection of the element to modify the N-C-based materials are discussed. Afterwards, synthesis methods for N-C-based electrocatalysts are summarized, and strategies for the synthesis of highly stable N-C-based electrocatalysts are presented. Multiple tables containing data on crucial parameters for both electrocatalytic activity and stability are displayed in this review. Finally, constructing M-N_x moieties is proposed as the most promising engineering strategy for stable N-C-based electrocatalysts.

Mn3O4 Quantum Dots Supported on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as Oxygen Reduction Electrocatalysts for High-Performance Zn-Air Batteries

Highly efficient and low-cost nonprecious metal electrocatalysts that favor a four-electron pathway for the oxygen reduction reaction (ORR) are essential for high-performance metal-air batteries. Herein, we show an ultrasonication-assisted synthesis method to prepare Mn₃O₄ quantum dots (QDs, ca. 2 nm) anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Mn₃O₄ QDs/N-p-MCNTs) as a high-performance ORR catalyst. The Mn₃O₄ QDs/N-p-MCNTs facilitated the four-electron pathway for the ORR and exhibited sufficient catalytic activity with an onset potential of 0.850 V (vs reversible hydrogen electrode), which is only 38 mV less positive than that of Pt/C (0.888 V). In addition, the Mn₃O₄ QDs/N-p-MCNTs demonstrated superior stability than Pt/C in alkaline solutions. Furthermore, a Zn-air battery using the Mn₃O₄ QDs/N-p-MCNTs cathode catalyst successfully generated a specific capacity of 745 mA h g^-1 at 10 mA cm^-2 without the loss of voltage after continuous discharging for 105 h. The superior ORR activity of Mn₃O₄ QDs/N-p-MCNTs can be ascribed to the homogeneous Mn₃O₄ QDs loaded onto the N-doped carbon skeleton and the synergistic effects of Mn₃O₄ QDs, nitrogen, and carbon nanotubes. The interface binding energy of -3.35 eV calculated by the first-principles density functional theory method illustrated the high stability of the QD-anchored catalyst. The most stable adsorption structure of O₂, at the interface between Mn₃O₄ QDs and the graphene layer, had the binding energy of -1.17 eV, greatly enhancing the ORR activity. In addition to the high ORR activity and stability, the cost of production of Mn₃O₄ QDs/N-p-MCNTs is low, which will broadly facilitate the real application of metal-air batteries.

B, N Codoped and Defect-Rich Nanocarbon Material as a Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions

The development of highly active, inexpensive, and stable bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts to replace noble metal Pt and RuO2 catalysts remains a considerable challenge for highly demanded reversible fuel cells and metal-air batteries. Here, a simple approach for the facile construction of a defective nanocarbon material is reported with B and N dopants (B,N-carbon) as a superior bifunctional metal-free catalyst for both ORR and OER. The catalyst is prepared by pyrolyzing the composites of ethyl cellulose and high-boiling point 4-(1-naphthyl)benzeneboronic acid in NH3 atmosphere with an inexpensive Zn-based template. The obtained porous B,N-carbon with rich carbon defects exhibits excellent ORR and OER performances, including high activity and stability. In alkaline medium, B,N-carbon material shows high ORR activity with an onset potential (Eonset) reaching 0.98 V versus reversible hydrogen electrode (RHE), very close to that of Pt/C, a high electron transfer number and excellent stability. This catalyst also presents the admirable ORR activity in acidic medium with a high Eonset of 0.81 V versus RHE and a four-electron process. The OER activity of B,N-carbon is superior to that of the precious metal RuO2 and Pt/C catalysts. A Zn-air battery using B,N-carbon as the air cathode exhibits a low voltage gap between charge and discharge and long-term stability. The excellent electrocatalytic performance of this porous nanocarbon material is attributed to the combined positive effects of the abundant carbon defects and the heteroatom codopants.

Tuning the Bandgap of Photo-Sensitive Polydopamine/Ag3PO4/Graphene Oxide Coating for Rapid, Noninvasive Disinfection of Implants

Bacterial infection and associated complications are threats to human health especially when biofilms form on biomedical devices and artificial implants. Herein, a hybrid polydopamine (PDA)/Ag3PO4/graphene oxide (GO) coating is designed and constructed to achieve rapid bacteria killing and eliminate biofilms in situ. By varying the amount of GO in the hybrid coating, the bandgap can be tuned from 2.52 to 2.0 eV so that irradiation with 660 nm visible light produces bacteria-killing effects synergistically in concert with reactive oxygen species (ROS). GO regulates the release rate of Ag+ to minimize the cytotoxicity while maintaining high antimicrobial activity, and a smaller particle size enhances the yield of ROS. After irradiation with 660 nm visible light for 15 min, the antimicrobial rates of the PDA/Ag3PO4/GO hybrid coating against Escherichia coli and Staphylococcus aureus are 99.53% and 99.66%, respectively. In addition, this hybrid coating can maintain a repeatable and sustained antibacterial efficacy. The released Ag+ and photocatalytic Ag3PO4 produce synergistic antimicrobial effects in which the ROS increases the permeability of the bacterial membranes to increase the probability of Ag+ to enter the cells to kill them together with ROS synergistically.

Identifying the Key Role of Pyridinic-N-Co Bonding in Synergistic Electrocatalysis for Reversible ORR/OER

For many regenerative electrochemical energy-conversion systems, hybrid electrocatalysts comprising transition metal (TM) oxides and heteroatom-doped (e.g., nitrogen-doped) carbonaceous materials are promising bifunctional oxygen reduction reaction/oxygen evolution reaction electrocatalysts, whose enhanced electrocatalytic activities are attributed to the synergistic effect originated from the TM-N-C active sites. However, it is still ambiguous which configuration of nitrogen dopants, either pyridinic or pyrrolic N, when bonded to the TM in oxides, predominately contributes to the synergistic effect. Herein, an innovative strategy based on laser irradiation is described to controllably tune the relative concentrations of pyridinic and pyrrolic nitrogen dopants in the hybrid catalyst, i.e., NiCo₂ O₄ NPs/N-doped mesoporous graphene. Comparative studies reveal the dominant role of pyridinic-NCo bonding, instead of pyrrolic-N bonding, in synergistically promoting reversible oxygen electrocatalysis. Moreover, density functional theory calculations provide deep insights into the corresponding synergistic mechanism. The optimized hybrid, NiCo/NLG-270, manifests outstanding reversible oxygen electrocatalytic activities, leading to an overpotential different ΔE among the lowest value for highly efficient bifunctional catalysts. In a practical reversible Zn-air battery, NiCo/NLG-270 exhibits superior charge/discharge performance and long-term durability compared to the noble metal electrocatalysts.

Reduced Graphene Oxide-Wrapped Co9-x Fex S8 /Co,Fe-N-C Composite as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution

Searching for highly efficient bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) using nonnoble metal-based catalysts is essential for the development of many energy conversion systems, including rechargeable fuel cells and metal-air batteries. Here, Co_9-x Fe_x S₈ /Co,Fe-N-C hybrids wrapped by reduced graphene oxide (rGO) (abbreviated as S-Co_9-x Fe_x S₈ @rGO) are synthesized through a semivulcanization and calcination method using graphene oxide (GO) wrapped bimetallic zeolite imidazolate framework (ZIF) Co,Fe-ZIF (CoFe-ZIF@GO) as precursors. Benefiting from the synergistic effect of OER active CoFeS and ORR active Co,Fe-N-C in a single component, as well as high dispersity and enhanced conductivity derived from rGO coating and Fe-doping, the obtained S-Co_9-x Fe_x S₈ @rGO-10 catalyst shows an ultrasmall overpotential of ≈0.29 V at 10 mA cm^-2 in OER and a half-wave potential of 0.84 V in ORR, combining a superior oxygen electrode activity of ≈0.68 V in 0.1 m KOH.

High Efficient Reduction of Graphene Oxide via Nascent Hydrogen at Room Temperature

To develop a green and efficient method to synthesize graphene in relative milder conditions is prerequisite for graphene applications. A chemical reducing method has been developed to high efficiently reduce graphene oxide (GO) using Fe₂O₃ and NH₃BH₃ as catalyst and reductants, respectively. During the process, environmental and strong reductive nascent hydrogen were generated surrounding the surface of GO sheets by catalyst hydrolysis reaction of NH₃BH₃ and were used for reduction of GO. The reduction process was studied by ultraviolet absorption spectroscopy, Raman spectroscopy, and Fourier transform infrared spectrum. The structure and morphology of the reduced GO were characterized with scanning electron microscopy and transmission electron microscopy. Compared to metal (Mg/Fe/Zn/Al) particles and acid system which also use nascent hydrogen to reduce GO, this method exhibited higher reduction efficiency (43.6%). Also the reduction was carried out at room temperature condition, which is environmentally friendly. As a supercapacitor electrode, the reversible capacity of reduced graphene oxide was 113.8 F g⁻¹ at 1 A g⁻¹ and the capacitance retention still remained at 90% after 200 cycles. This approach provides a new method to reduce GO with high reduction efficiency by green reductant.

A review of anion-regulated multi-anion transition metal compounds for oxygen evolution electrocatalysis

Recent advances in the anion regulation on multi-anion transition metal compounds as electrocatalysts for oxygen evolution reaction are reviewed.