Bio-enzymatic synthesis of nitrogen-doped porous carbon/SnO2/carbon microspheres as high-performance composite materials for lithium-ion and sodium-ion batteries

In this study, a nitrogen-doped 3D porous starch-derived carbon/SnO₂/carbon (PSC/SnO₂/C) composite is synthesized with porous starch as a carbon source by biological enzymatic hydrolysis. Compared with the traditional complex acid-base reagent method, the biological enzymatic method is more environmentally friendly and economical, and it can also naturally introduce nitrogen sources and dope the carbon layer. Many mesoporous nanostructures provide enough buffer space and promote the ions' and electrons’ transmission rate. The formation of the Sn–O–C bond between SnO₂ and carbon ensures the stability of the structure. As a result, the PSC/SnO₂/C composite exhibits a high initial discharge capacity (1802 mAhg⁻¹ at 0.2 A g⁻¹ for LIBs and 549 mAh g⁻¹ at 0.1 A g⁻¹ for SIBs) and good cycle stability (701 mAh g⁻¹ at 0.2 A g⁻¹ after 100 cycles for LIBs and 271 mAh g⁻¹ at 0.1 A g⁻¹ after 100 cycles for SIBs). This synthesis method can prepare other energy storage systems such as fuel cells, supercapacitors, and metal ion batteries.     

The host hollow carbon nanospheres as cathode material for nonaqueous room-temperature Al–S batteries

Aluminum-ion batteries have attracted great attention in virtue of their reliable safety performance and cost-effective raw materials. The sulfur element with a high specific capacity gives great development space for aluminum-sulfur (Al–S) battery. However, the dissolution of sulfur in electrolyte hinders the application of Al–S battery. Carbon materials with porous structure has larger specific surface area for adsorption of sulfur, and the porous carbon for sulfur cathode provides a certain barrier for the shuttle effect during the charge-discharge process. In this work, hollow carbon synthesized by template method is applied to Al–S batteries. It is found that the cave-like porous carbon material provides space for storing sulfur and polysulfides, alleviating the sulfur shuttle effect in Al–S batteries. The specific capacity of the hollow carbon materials for Al–S batteries is 1027 mAh g^?1 at the first cycle and the rechargeable specific capacity achieves 378 mAh g^?1 after 28 cycle.     

ZnSe/CoSe/NCDPH composites as anode materials for lithium ion batteries with high capacity and long cycle

In this paper, dopamine hydrochloride (DPH) is introduced to synthesize ZIF-8@ZIF-67@DPH in the preparation of ZIF-8@ZIF-67. ZnSe/CoSe/NC_DPH (N-doped carbon) composites are calcined in a high-temperature inert atmosphere with ZIF-8@ZIF-67@DPH as the precursor, selenium powder as the selenium source. ZnSe/CoSe/NC_DPH has high discharge specific capacity, good cycle stability and outstanding rate performance. The first discharge capacity of ZnSe/CoSe/NC_DPH is 1616.6 mAh g⁻¹ at the current density of 0.1 A g⁻¹, and the reversible capacity remains at 1214.2 mAh g⁻¹ after 100 cycles, the reversible capacity is 416.7 mAh g⁻¹ after 1000 cycles at 1 A g⁻¹. Therefore, ZnSe/CoSe/NC_DPH composites provide a new step for the research and synthesis of new stable, high-capacity, and safe high-performance lithium ion batteries. The bimetallic selenide composites not only have bimetallic active sites, but also can form synergistic effect between different metal phases, which can effectively reduce the capacity attenuation caused by volume expansion and reactive stress enrichment during lithium storage of metal oxide anode materials. Meanwhile, N-doped carbon can improve the conductivity and provide more active sites to store lithium, thus improving its lithium storage capacity.     

A freestanding MoO2‐decorated carbon nanofibers interlayer for rechargeable lithium sulfur battery

Lithium‐sulfur (Li‐S) battery based on sulfur cathodes is of great interest because of high capacity and abundant sulfur source. But the shuttling effect of polysulfides caused by charge‐discharge process results in low sulfur utilization and poor reversibility. Here, we demonstrate a good approach to improve the utility of sulfur and cycle life by synthesizing carbon nanofibers decorated with MoO₂ nanoparticles (MoO₂‐CNFs membrane), which plays a role of multiinterlayer inserting between the separator and the cathode for Li‐S battery. The S/MoO₂‐CNFs/Li battery showed a discharge capacity of 6.93 mAh cm^?2 (1366 mAh g^?1) in the first cycle at a current density of 0.42 mA cm^?2 and 1006 mAh g^?1 over 150 cycles. Moreover, even at the highest current density (8.4 mA cm^?2), the battery achieved 865 mAh g^?1. The stable electrochemical behaviors of the battery has achieved because of the mesoporous and interconnecting structure of MoO₂‐CNFs, proving high effect for ion transfer and electron conductive. Furthermore, this MoO₂‐CNFs interlayer could trap the polysulfides through strong polar surface interaction and increases the utilization of sulfur by confining the redox reaction to the cathode.     

Fabrication of ultrafine Gd2O3 nanoparticles/carbon aerogel composite as immobilization host for cathode for lithium‐sulfur batteries

Carbon aerogel (CA), possessing abundant pore structures and excellent electrical conductivity, have been utilized as conductive sulfur hosts for lithium‐sulfur (Li‐S) batteries. However, a serious shuttle effect resulted from polysulfide ions has not been effectively suppressed yet due to the weak absorption nature of CA, resulting in rapid decay of capacity as the cycle number increases. Herein, ultrafine (~3 nm) gadolinium oxide (Gd₂O₃) nanoparticles (with upper redox potential of ~ 1.58 V versus Li⁺/Li) are uniformly in‐situ integrated with CA through directly sol‐gel polymerization and high‐temperature carbonization. The Gd₂O₃ modified CA composites (named as Gdx‐CA, where x means molar ratio of Gd₂O₃ nanoparticles to carbon) are incorporated with S. Then, the products (S/Gdx‐CA) are acted as sulfur host materials for Li‐S batteries. The results demonstrate that adding ultrafine Gd₂O₃ nanoparticles can dramatically improve the electrochemical properties of the composite cathodes. The S/Gd2‐CA electrode (loading with 58.9 wt% of S) possesses the best electrochemical properties, including a high initial capacity of 1210 mAh g^?1 and a relatively high capacity of 555 mAh g^?1 after 50 cycles at 0.1 C. It is noteworthy that the performance of long‐term cycle (350 cycles) for the S/Gd2‐CA (317 mAh g^?1 after 100 cycles and 233 mAh g^?1 after 350 cycles at 1 C) is improved significantly than that of S/CA (150 mAh g^?1 after 150 cycles at 1 C). Overall, the enhancement of electrochemical performances can be due to the strong polar nature of the ultrafine Gd₂O₃ nanoparticles, which provide strong adsorption sites to immobilize S and polysulfide. Furthermore, the Gd₂O₃ nanoparticles present a catalytic effect. Our research suggests that adding Gd₂O₃ nanoparticles into S/CA composite cathode is an effective and novelty method for improving the electrochemical performances of Li‐S batteries.     

Graphene–sulfur–Ni(OH)2 sandwich foam composites as free-standing cathodes for high-performance Li–S batteries

Lithium–sulfur (Li–S) batteries are considered as a next-generation energy storage solution thanks to the dramatic increase in their energy density and the accompanying low fabrication cost. Nonetheless, their practical applications have been hampered by critical challenges such as cycling instability, low sulfur utilisation and efficiency. Here, the interfacial growth of Ni(OH)₂-encapsulated sulfur nanoparticles in a reduced graphene oxide free-standing matrix as Li–S cathodes has been highlighted, along with a detailed application investigation of the sandwich foam structure in Li–S batteries. Compared to sulfur composites based solely on a graphene host (S@rGO), this composite foam cathode could provide significant improvements in specific capacity and long-cycle stability with the effective confinement and promoted conversion of lithium polysulfides. Moreover, the sandwich foam composite cathode exhibits a high specific capacity of 1189 mA h g⁻¹ (0.1 C), better rating performance (691 mA h g⁻¹ at 2 C) and remarkable cycling stability, retaining 81% of the initial capacity after 200 cycles of charge–discharge at 0.2 C. Furthermore, Ni(OH)₂ wrapping at the cathode–electrolyte interface offers vastly improved polysulfide shuttle suppression, which provides a new cathode encapsulation method for further developments of advanced Li–S cathodes.     

Mesoporous MnO2 fibers as an efficient bifunctional absorber for high-performance lithium-sulfur batteries

A novel morphology of mesoporous MnO₂ fibers (MOF) are successfully prepared for the first time as host materials for lithium-sulfur (LiS) batteries. The as-prepared mesoporous MnO₂ fibers can restrain the polysulfides dissolution via chemical bonding and physical trapping at the same time. As a result, the mesoporous MnO₂ fibers sulfur (MOF/S) composites exhibit excellent cycle performance. The MOF/S composite electrodes deliver a high initial capacity of 1015 mAh g^?1 and maintain 815 mAh g^?1 after 200 cycles at 0.1 C.     

Facile synthesis of mesoporous TiNb2O7/C microspheres as long-life and high-power anodes for lithium-ion batteries

The unique ReO₃ crystallographic shear structure of TiNb₂O₇ has enabled its application as anode material for lithium-ion batteries, in which the lattice parameter and volume change of TiNb₂O₇ are often negligible during lithium-ion insertion/extraction. However, several intrinsic problems of TiNb₂O₇, including low electronic and ionic conductivity, can restrict its application significantly. In this study, carbon-coated mesoporous TiNb₂O₇ microspheres are fabricated through a simple solvothermal reaction. By combining the advantages of both the amorphous carbon and mesoporous structure, TiNb₂O₇/C composite exhibits superior lithium storage performance, with higher rate capability (200 mAh g⁻¹ at 30 C) and cyclability (191 mAh g⁻¹ at 10 C after 500 cycles). The improved performance is due mainly to the high pseudocapacitance and low charge transfer resistance obtained from the mesoporous structure and amorphous carbon layer. The study provides a new way of constructing TiNb₂O₇ for ultra-fast storage devices, demonstrating great potential for application in power batteries.     

Multiporous core-shell structured MnO@N-Doped carbon towards high-performance lithium-ion batteries

It is imminently to seek for high energy density in addition to a sensational lifetime of lithium-ion batteries (LIBs) to meet growing requisition in the energy storage application. Anode containing metal oxide composite is being thoroughly investigated for their higher capacity than that of the commercial graphite. A multiporous core-shell structured metal oxide composite anode possessing the excellent capacity and superb lifespan for LIBs is designed. In detail, metal oxide (i.e., MnO) is encapsulated in N-doped carbon shell (MnO@N–C) via coprecipitation-annealing technique. During annealing, abundant void space among MnO cores/between MnO cores and N–C shells is obtained. This space can efficaciously buffer volume changes of MnO upon cycles. Benefiting from the unique structure and heteroatom doping, the capacity of MnO@N–C microcube anode exhibits 576 mAh g⁻¹ at 5 A g⁻¹ with an ultra-long lifespan more than 3500 cycles. The connection between the electrode characteristics and structure is concurrently examined by adopting kinetic analysis. Finally, a full lithium-ion battery is presented, applying the MnO@N–C (anode) and Nick-rich layered oxide (cathode). It is believed that structural designing with heteroatom doping can be utilized in vaster fields for superior capabilities.     

Efficient MnO and Co nanoparticles coated with N-doped carbon as a bifunctional electrocatalyst for rechargeable Zn-air batteries

Developing the low-cost, durable, and efficient bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) plays an important role in the commercial implementation of the Zn-air batteries. Herein, we design and synthesize the MnO and Co nanoparticles coated with N-doped carbon (MC@NC) as an excellent bifunctional oxygen electrocatalyst. It is found that the optimal MC@NC-0.3 exhibits outstanding ORR performance with a positive half-wave potential of 0.82 V and excellent OER activity with a small overpotential of 360 mV at 10 mA cm⁻². When applied in the liquid Zn-air battery, MC@NC-0.3 displays a high maximum power density of 153 mW cm⁻², a large specific capacity of 776 mAh g⁻¹ and the excellent cycling stability with a negligible increase after 300 h. Furthermore, the fiber-shaped all-solid-state Zn-air battery also displays remarkable stability at high current density. This study offers a facile strategy to construct a high-efficient, low-cost, and durable transitional metal-based bifunctional electrode for renewable energy applications.     

Synergistic effect of titanium-oxide integrated with graphitic nitride hybrid for enhanced electrochemical performance in lithium-sulfur batteries

Lithium-sulfur (Li-S) secondary batteries have been limited by the poor cyclic stability, mainly caused by the dissolution polysulfide species into the electrolyte and subsequent irreversible shuttling effect. Recently, addition of the polysulfide adsorbents within sulfur cathode is effectively improving the electrochemical performance. Herein, TiO₂ integrated with g-C₃N₄ (TiO₂@g-C₃N₄: TOCN) hybrid was prepared by a facile heating treatment from the precursor of urea and TiO₂composites, which used as host material for elemental sulfur (TOCN@S) in Li-S batteries. The multifunctional TOCN not only reduced the electrochemical resistance but also provided strong adsorption sites to immobilize sulfur and polysulfide. As a result, the TOCN@S cathode with a sulfur content of 74.5 wt% and a sulfur loading of 3.1 mg cm⁻²exhibits a high initial capability of 804 mAh g⁻¹ at 0.5°C with capacity retention of 67.2% after 500 cycles. Additionally, the composite cathode also possesses excellent high-rate performance, retaining a remarkable specific capacity of 630 mAh g⁻¹ even at a rate of 2°C.     

Tunable Fe/N co-doped 3D porous graphene with high density Fe-Nx sites as the efficient bifunctional oxygen electrocatalyst for Zn-air batteries

Nitrogen-doped transition metal materials display promising potential as bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, Fe/N co-doped three-dimensional (3D) porous graphene (FeN-3D-PG) is prepared via a template method using sodium alginate as the carbon source and low polymerization degree melamine resin as the nitrogen source. The low polymerization degree melamine resin can form complexes with Fe³⁺ in the aqueous solution and further forms high density Fe-N_x active sites during pyrolysis. Meanwhile, the formed 3D porous structure efficiently promotes the uniform distribution of Fe-N_x active sites. The FeN-3D-PG catalyst exhibits pH-independent ORR activity. For OER, the catalyst possesses a low over potential (370 mV at 10 mA cm⁻²) in alkaline electrolyte. The Zn-air batteries (ZABs) using FeN-3D-PG as cathode exhibits a power density up to 212 mW cm⁻², a high specific capacity of 651 mAh g⁻¹, and the charge-discharge stability of 80 h. This work provides new sight to transition metal materials based ZABs with excellent performance.     

Nitrogen/sulfur co-doped disordered porous biocarbon as high performance anode materials of lithium/sodium ion batteries

Nitrogen/sulfur co-doped disordered porous biocarbon was facilely synthesized and applied as anode materials for lithium/sodium ion batteries. Benefiting from high nitrogen (3.38 wt%) and sulfur (9.75 wt%) doping, NS_1-1 as anode materials showed a high reversible capacity of 1010.4 mA h g⁻¹ at 0.1 A g⁻¹ in lithium ion batteries. In addition, it also exhibited excellent cycling stability, which can maintain at 412 mAh g^-1 after 1000 cycles at 5 A g⁻¹. As anode materials of sodium ion batteries, NS_1-1 can still reach 745.2 mA h g⁻¹ at 100 mAg^-1 after 100 cycles. At a high current density (5 A g^-1), the reversible capacity is 272.5 mA h g⁻¹ after 1000 cycles, which exhibits excellent electrochemical performance and cycle stability. The preeminent electrochemical performance can be attributed to three effects: (1) the high level of sulfur and nitrogen; (2) the synergic effect of dual-doping heteroatoms; (3) the large quantity of edge defects and abundant micropores and mesopores, providing extra Li/Na storage regions. This disordered porous biocarbon co-doped with nitrogen/sulfur exhibits unique features, which is very suitable for anode materials of lithium/sodium ion batteries.     

A thermo-stable poly(propylene carbonate)-based composite separator for lithium-sulfur batteries under elevated temperatures

Lithium-sulfur (Li-S) batteries have a great potential for the future development of energy industry. However, the high-temperature performance of Li-S batteries is still facing great challenge due to the high flammability of the electrolyte, sulfur cathode as well as the separator. The separator modification is an effective method to improve the thermal stability of separator and the electrochemical performance of Li-S batteries under elevated temperatures. However, the reported methods of separator coating are too complicated to be applied in the industrial production. Here, a novel thermo-stable composite separator (M-Celgard-p), in which a layer of silicon dioxide-poly (propylene carbonate) based electrolyte (nano-SiO₂@PPC) with a high ionic-conductivity of 1.03 × 10⁻⁴ S cm⁻¹ is coated on the commercial Celgard-p separator, is prepared by using a simple dipping method. Compared to the Li-S battery assembled with Celgard-p separator, the M-Celgard-p separator combined with a sulfur/polyacrylonitrile (S/PAN) cathode can improve the electrochemical performance of Li-S batteries, especially their high-temperature stability. As a result, the (S/PAN)/M-Celgard-p/Li cell delivers a high specific capacity of 724.7 mAh g⁻¹ at 1.0 A g⁻¹ after 200 cycles and presents a good rate capability of 1408 mAh g⁻¹ at 1.0 A g⁻¹ and 1216 mAh g⁻¹ at 2.0 A g⁻¹. More importantly, the (S/PAN)/M-Celgard-p/Li cell can exhibit a capacity retention ratio of 69.4% after 200 cycles at 60°C. The M-Celgard-p separator with high Li-ion conductivity can not only block the “shuttle-effect” of polysulfides during cycling but also enhance the thermal stability under elevated temperatures. This work presents a simple dipping method to prepare composite separator with excellent thermal stability, which enhance the rate performance and cyclic stability of Li-S batteries under elevated temperatures. We believe this work can provide a new way to develop more reliable Li-S batteries for practical applications.     

N-doped porous carbon-stabilized Pt in hollow nano-TiO2 with enhanced photocatalytic activity

In this study, Pt nanoclusters with small sizes (~1.8 nm) is designed and loaded onto the inner surface of hollow nano-TiO₂ through combining polymer anchoring and photochemical solid-phase reduction strategy. The N-doped mesoporous carbon derived from the polymer is beneficial for the exposure of Pt sites and stabilization of Pt nanoclusters (~1.8 nm), and supplying more active N-sites. More importantly, the pore structure of the N-doped mesoporous carbon layer prepared by air-etching method contributes to the diffusion of reactants/products and exposure of more active Pt sites to promote surface reactions. The as-prepared C/Pt@TiO₂-3% with 0.54 wt% of Pt shows the largest total pore volume (0.46 cm³ g⁻¹) with an average pore size of ~3 nm, and its catalytic activity is greatly improved with a H₂ evolution rate of 9086 μmol h⁻¹ g⁻¹, which is about 48 and 96 times that of bare hollow nano-TiO₂ and P25, respectively. In addition, the as-prepared C/Pt@TiO₂-3% shows good durability in a 40-h cyclic test.     

ZnMn2O4/milk-derived Carbon hybrids with enhanced Lithium storage capability

One of the effective ways to improve the conductivity and structural stability of binary metal oxide nanostructures is to tightly composite them with nano-carbon materials with excellent conductivity. However, the introduction of low density carbon materials also reduces the energy density of batteries. Therefore, we provides a new idea to enhance the lithium storage performance of carbon/binary transition metal oxide anode materials by multi-element co-doping carbon. ZnMn₂O₄ provides high lithium storage capacity; non-metallic heteroatoms in milk-derived carbon greatly improve the conductivity of carbon materials; metal heteroatoms in milk-derived carbon increase the density of carbon materials. Multicomponent co-doping carbon can build up the mass specific capacity, ratio performance, cyclic life and mechanical properties of binary metal oxides/porous carbon nanocomposites. As the anode materials of lithium-ion batteries, the ZnMn₂O₄/MC (milk-derived carbon) hybrids deliver a high reversible capacity of 1352 mAh g⁻¹ after 400 cycles at 0.1 A g⁻¹, and a remarkable long-term cyclability with 635 mAh g⁻¹ after 300 cycles at 1.0 A g⁻¹.     

A microporous carbon derived from metal-organic frameworks for long-life lithium sulfur batteries

A polyhedral microporous carbon derived from metal-organic frameworks (ZIF-8) could present good property for sulfur loading and trapping. A melting-evaporation route was adopted to synthesize two sulfur/microporous carbon (S/MC) composites, of which sulfur content is controllable, and ether-based or ester-based electrolytes were used to evaluate the synthesized composites for the lithium sulfur batteries. According to electrochemical results, the S/MC composite with 65.2 wt% S in the ether-based electrolyte exhibited optimized performance as compared with the composite with 65.2 wt% S in the ester-based electrolyte, as well as the composite with 58.6 wt% S in the two kinds of electrolytes. For the S/MC composite with 65.2 wt% S in ether-based electrolyte, the initial discharge capacity could reach up to 1505.9 mAh g⁻¹ and the reversible capacity could be 833.3 mAh g⁻¹ after 40 cycles at 0.1 C. Furthermore, while being respectively evaluated at 0.5, 1.0, and 2.0 C, the discharge capacities could still maintain at 544, 493 and 354 mAh g⁻¹ after 300, 500, and 800 cycles, demonstrating appreciable cyclic reversibility and rate capability.     

Engineering of the N-doped carbon support for improved performance of supported Pd catalysts in hydrogen production from gas-phase formic acid

Development of N-doped Pd/C catalysts for hydrogen production from gas-phase formic acid is a challenge. To elucidate the efficient routes of nitrogen insertion on the surface of a mesoporous carbon support, the latter was treated with melamine (Mel), dicyandiamide or NH₃ at 673 and 823 K. Pyrolysis of the melamine/carbon mixture taken in a 1:2 ratio provides an increase in the reaction rate by a factor of 5. The inserted N-sites strongly interact with Pd leading to the formation of highly dispersed Pd nanoparticles (∼1.6 nm) and active atomically dispersed Pd²⁺ species. With a further increase of the Mel/C ratio, the number of surface N-sites decreases due to occupation of carbon support pores with a g–C₃N₄–type residue. This provides a decrease in the Pd dispersion leading to lower reaction rates. Therefore, melamine is an efficient N precursor. The considered synthesis of N-doped catalysts could be scaled.     

New strategy to prepare nitrogen self-doped graphene nanosheets by magnesiothermic reduction and its application in lithium ion batteries

Nitrogen self-doped graphene (N/G) nanosheets were prepared through magnesiothermic reduction of melamine. The obtained N/G features porous structure consisting of multi-layer nanosheets. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectra and X-ray diffraction (XRD). As anode of lithium ion batteries (LIBs), it exhibits excellent reversible specific capacity of 1753 mAh g⁻¹ at 0.1 A g^-1 after 200 cycles. The reversible capacity can maintain at 1322 mAh g⁻¹ after 500 cycles at 2 A g⁻¹. At the same time, all results indicate remarkable cycle stability and rate performance as anode materials. Furthermore, this study demonstrates an economical, clean and facile strategy to synthesize N/G nanosheets from cheap chemicals with excellent electrochemical performance in LIBs.     

MOF-derived multi-metal embedded N-doped carbon sheets rich in CNTs as efficient bifunctional oxygen electrocatalysts for rechargeable ZABs

The rational design and preparation of bifunctional electrocatalysts with pleasant oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance is crucial for extensive commercial applications of rechargeable Zn–air batteries (ZABs). Herein, we report a simple method to obtain multi-metal (Fe, Ni, Zn) embedded in N-doped carbon sheets entangled with carbon nanotubes (CNTs) as superior oxygen electrocatalysts (FeNi-NCS-2). The resultant FeNi-NCS-2 exhibits an impressive electrochemical performance, providing a reversible oxygen overpotential as low as 0.758 V. The ZAB with FeNi-NCS-2 as the air cathode shows a promising capacity of 639.71 mAh g^?1 at 20 mA cm^?2, a power density of 109.8 mW cm^?2 and cycling stability of over 130 cycles at 10 mA cm^?2 with an energy efficiency of about 55%, superior to the ZAB based on Pt/C–IrO₂. The satisfactory electrocatalytic performance is mainly due to the Fe, Ni-based nanoparticles protected by graphitic carbon layers, hierarchical porous lamellar structures that promote the accessibility between the active centers and the electrolyte as well as self-growing tangled carbon nanotubes that provide fast transmission channels. This study presents a facile way for the synthesis of highly efficient ORR/OER bifunctional electrocatalysts for high-performance rechargeable ZABs.