Defects and morphology engineering for constructing V_(s)-Ni_(3)S_(2)@V_(s)-Cu_(2)S nanotube heterojunction arrays toward efficient bifunctional electrocatalyst for overall water splitting

The development of highly active,stable and inexpensive electrocatalysts for hydrogen production by defects and morphology engineering remains a great challenge.Herein,S vacancies-rich Ni_(3)S_(2)@Cu_(2)S nan-otube heterojunction arrays were in-situ grown on copper foam(V_(s)-Ni_(3)S_(2)@V_(s)-Cu_(2)S NHAs/CF)for efficient electrocatalytic overall water splitting.With the merits of nanotube arrays and efficient electronic mod-ulation drived by the OD vacancy defect and 2D heterojunction defect,the resultant V_(s)-Ni_(3)S_(2)@V_(s)-Cu_(2)S NHAs/CF electrocatalyst exhibits excellent electrocatalytic activity with a low overpotential of 47 mV for the hydrogen evolution reaction(HER)at 10 mA cm^(-2) current density,and 263 mV for the oxygen evolution reaction(OER)at 50 mA cm^(-2) current density,as well as a cell voltage of 1.48 V at 10 mA cm^(-2).Moreover,the nanotube heterojunction arrays endows V_(s)-Ni_(3)S_(2)@V_(s)-Cu_(2)S NHAs/CF with outstanding stability in long-term catalytic processes,as confirmed by the continuous chronopotentiom-etry tests at current densities of 10 mA cm^(-2) for 100 h.

Design Principles and Mechanistic Understandings of Non-Noble-Metal Bifunctional Electrocatalysts for Zinc-Air Batteries

Zinc-air batteries(ZABs)are promising energy storage systems because of high theoretical energy density,safety,low cost,and abundance of zinc.However,the slow multi-step reaction of oxygen and heavy reliance on noble-metal catalysts hinder the practical applications of ZABs.Therefore,feasible and advanced non-noble-metal elec-trocatalysts for air cathodes need to be identified to promote the oxygen catalytic reaction.In this review,we initially introduced the advancement of ZABs in the past two decades and provided an overview of key developments in this field.Then,we discussed the work-ing mechanism and the design of bifunctional electrocatalysts from the perspective of morphology design,crystal structure tuning,interface strategy,and atomic engineering.We also included theoretical studies,machine learning,and advanced characterization technologies to provide a comprehensive understanding of the structure-performance relationship of electrocatalysts and the reaction pathways of the oxygen redox reactions.Finally,we discussed the challenges and prospects related to designing advanced non-noble-metal bifunctional electrocatalysts for ZABs.

The component-activity interrelationship of cobalt-based bifunctional electrocatalysts for overall water splitting:Strategies and performance

Cobalt-based electrocatalysts take advantage of potentially harmonizable microstructure and flexible coupling effects compared to commercial noble metal-based catalytic materials.However,conventional water electrolysis systems based on cobalt-based monofunctional hydrogen evolution reaction(HER)or oxygen evolution reaction(OER)catalysts have certain shortcomings in terms of resource utilization and universality.In contrast,cobalt-based bifunctional catalysts(CBCs)have attracted much attention in recent years for overall water splitting systems because of their practicality and reduced preparation cost of electrolyzer.This review aims to address the latest development in CBCs for total hydrolysis.The main modification strategies of CBCs are systematically classified in water electrolysis to provide an overview of how to regulate their morphology and electronic configuration.Then,the catalytic performance of CBCs in total-hydrolysis is summarized according to the types of cobalt-based phosphides,sulfides and oxides,and the mechanism of strengthened electrocatalytic ability is emphasized through combining experiments and theoretical calculations.Future efforts are finally suggested to focus on exploring the dynamic conversion of reaction intermediates and building near-industrial CBCs,designing advanced CBC materials through micro-modulation,and addressing commercial applications.

Visible Light-Induced Photocatalysis:Self-Fenton Degradation of p-CIPhOH Over Graphitic Carbon Nitride by a Polyethylenimine Bifunctional Catalyst

Deep degradation of organic pollutants by sunlight-induced coupled photocatalytic and Fenton (photo-Fenton) reactions is of immense importance for water purification. In this work, we report a novel bifunctional catalyst (Fe-PEI-CN) by codoping graphitic carbon nitride (CN) with polyethyleneimine ethoxylated (PEI) and Fe species, which demonstrated high activity during p-chlorophenol (p-ClPhOH) degradation via H_(2)O_(2) from the photocatalytic process. The relationship between the catalytic efficiency and the structure was explored using diff erent characterization methods. The Fe modification of CN was achieved through Fe-N coordination, which ensured high dispersion of Fe species and strong stability against leaching during liquid- phase reactions. The Fe modification initiated the Fenton reaction by activating H_(2)O_(2) into ·OH radicals for deep degradation of p-ClPhOH. In addition, it eff ectively promoted light absorption and photoelectron-hole (e-h ^(+) ) separation, corresponding to improved photocatalytic activity. On the other hand, PEI could significantly improve the ability of CN to generate H_(2)O_(2) through visible light photocatalysis. The maximum H_(2)O_(2) yield reached up to 102.6 μmol/L, which was 22 times higher than that of primitive CN. The cooperation of photocatalysis and the self-Fenton reaction has led to high-activity mineralizing organic pollutants with strong durability, indicating good potential for practical application in wastewater treatment.

Enhanced bifunctional oxygen electrochemical catalytic performance using La-doped CoFe_(2)O_(4)spinel supported by 3D-G for Zn-air batteries

The preparation of bifunctional catalysts for oxygen reduction(ORR)and oxygen evolution(OER)is crucial for Zn-air batteries.Here,we report a La doped CoFe_(2)O_(4) spinel catalyst supported on threedimensional graphene(3D-G),where La can facilitate electron transfer from Co to Fe,leading to increased electron cloud density in Fe and improved catalytic performance.The redshift of the G peak in the Raman spectra indicates the interaction between theπbond of 3D-G and d orbitals in La_(0.2)CoFe_(1.8)O_(4).La_(0.2)CoFe_(1.8)/3D-G exhibits superior ORR performance(E_(1/2)=0.86 V vs.RHE)and OER performance(E_(j=10)=1.55 V vs.RHE)to CoFe_(2)O_(4)/3D-G(E_(1/2)=0.831 V vs.RHE,E_(j=10)=1.603 V vs.RHE).Furthermore,it demonstrates excellent bifunctional oxygen catalytic performance while maintaining high power density and stability in liquid zinc-air batteries(ZABs)and flexible ZABs(F-ZABs).This work presents a viable strategy for utilizing rare earth element doped spinels to enhance oxygen catalyst and ZABs performance.

Optimizing band structure of CoP nanoparticles via rich-defect carbon shell toward bifunctional electrocatalysts for overall water splitting

Transition-metal phosphides(TMPs)with high catalytic activity are widely used in the design of electrodes for water splitting.However,a major challenge is how to achieve the trade-off between activity and stability of TMPs.Herein,a novel method for synthesizing CoP nanoparticles encapsu-lated in a rich-defect carbon shell(CoP/DCS)is developed through the self-assembly of modified polycyclic aromatic molecules.The graft and removal of high-activity C-N bonds of aromatic molecules render the controllable design of crystallite defects of carbon shell.The density functional theory calculation indicates that the carbon defects with unpaired electrons could effectively tailor the band structure of CoP.Benefiting from the improved activity and corrosion resistance,the CoP/DCS delivers outstanding difunctional hydrogen evolution reaction(88 mV)and oxygen evolution reaction(251 mV)performances at 10 mA cm^(−2)current density.Furthermore,the coupled water electrolyzer with CoP/DCS as both the cathode and anode presents ultralow cell voltages of 1.49 V to achieve 10 mA cm^(−2)with long-time stability.This strategy to improve TMPs electrocatalyst with rich-DCS and heterogeneous structure will inspire the design of other transition metal compound electrocatalysts for water splitting.

Constructing P-CoMoO_(4)@NiCoP heterostructure nanoarrays on Ni foam as efficient bifunctional electrocatalysts for overall water splitting

Improving catalytic activity and durabilty through the structural and compositional development of bifunctional electrocatalysts with low cost,high activity and stability is a challenging issue in electrochemical water splitting.Herein,we report the fabrication of heterostructured P-CoMoO_(4)@NiCoP on a Ni foam substrate through interface engineering,by adjusting its composition and architecture.Benefitting from the tailored electronic structure and exposed active sites,the heterostructured P-CoMoO_(4)@NiCoP/NF arrays can be coordinated to boost the overall water splitting.In addition,the superhydrophilic and superaerophobic properties of P-CoMoO_(4)@NiCoP/NF make it conducive to water dissociation and bubble separation in the electrocatalytic process.The heterostructured PCoMoO_(4)@NiCoP/NF exhibits excellent bifunctional electrocatalysis activity with a low overpotential of 66 mV at 10 mA cm^(-2) for HER and 252 mV at 100 mA cm^(-2) for OER.Only 1.62 V potential is required to deliver 20 mA cm^(-2) in a two-electrode electrolysis system,providing a decent overall water splitting performance.The rational construction of the heterostructure makes it possible to regulate the electronic structures and active sites of the electrocatalysts to promote their catalytic activity.

Strengthening absorption ability of Co-N-C as efficient bifunctional oxygen catalyst by modulating the d band center using MoC

Co-N-C is a promising oxygen electrochemical catalyst due to its high stability and good durability.However,due to the limited adsorption ability improvement for oxygen-containing intermediates,it usually exhibits inadequate catalytic activity with 2-electron pathway and high selectivity of hydrogen peroxide.Herein,the adsorption of Co-N-C to these intermediates is modulated by constructing heterostructures using transition metals and their derivatives based on d-band theory.The heterostructured nanobelts with MoC core and pomegranate-like carbon shell consisting of Co nanoparticles and N dopant(MoC/Co-N-C)are engineered to successfully modulate the d band center of active Co-N-C sites,resulting in a remarkably enhanced electrocatalysis performance.The optimally performing MoC/Co-N-C exhibits outstanding bi-catalytic activity and stability for the oxygen electrochemistry,featuring a high wave-half potential of 0.865 V for the oxygen reduction reaction(ORR)and low overpotential of 370 mV for the oxygen evolution reaction(OER)at 10 mA cm^(-2).The zinc air batteries with the MoC/Co-N-C catalyst demonstrate a large power density of 180 mW cm^(-2)and a long cycling lifespan(2000 cycles).The density functional theory calculations with Hubbard correction(DFT+U)reveal the electron transferring from Co to Mo atoms that effectively modulate the d band center of the active Co sites and achieve optimum adsorption ability with"single site double adsorption"mode.

Bimetallic ZIFs-derived electrospun carbon nanofiber membrane as bifunctional oxygen electrocatalyst for rechargeable zinc-air battery

The recharged zinc-air battery(ZAB) has drawn significant attention owing to increasing requirement for energy conversion and storage devices.Fabricating the efficient bifunctional oxygen catalyst using a convenient strategy is vitally important for the rechargeable ZAB.In this study,the bimetallic ZIFs-containing electrospun(ES) carbon nanofibers membrane with hierarchically porous structure was prepared by coaxial electrospinning and carbonization process,which was expected to be a bifunctional electrocatalyst for ZABs.Owing to the formed dual single-atomic sites of Co-N_(4) and Zn-N_(4),the obtained ES-Co/ZnCNZIFexhibited the preferable performance toward oxygen reduction reaction(ORR) with E1/2of 0.857 V and JLof 5.52 mA cm^(-2),which were more than Pt/C.Meanwhile,it exhibited a marked oxygen evolution reaction(OER) property with overpotential of 462 mV due to the agglomerated metallic Co nanoparticles.Furthermore,the ZAB based on the ES-Co/Zn-CNZIFcarbon nanofibers membranes delivered peak power density of 215 mW cm^(-2),specific capacity of 802.6 mA h g^(-1),and exceptional cycling stability,far larger than Pt/C+RuO_(2)-based ZABs.A solid-state ZAB based on ES-Co/Zn-CNZIFshowed better flexibility and stability with different bending angles.

A benzenesulfonic acid-modified organic polymer monolithic column with reversed-phase/hydrophilic bifunctional selectivity for capillary electrochromatography

Here,a styrene-based polymer monolithic column poly(VBS-co-TAT-co-AHM)with reversed-phase/hydrophilic interaction liquid chromatography(RPLC/HILIC)bifunctional separation mode was success-fully prepared for capillary electrochromatography by the in situ polymerization of sodium p-styrene sulfonate(VBS)with cross-linkers 3-(acryloyloxy)-2-hydroxypropyl methacrylate(AHM)and 1,3,5-triacryloylhexahydro-1,3,5-triazine(TAT).The preparation conditions of the monolith were optimized.The morphology and formation of the poly(VBS-co-TAT-co-AHM)monolith were confirmed by scanning electron microscopy(SEM)and Fourier transform infrared spectroscopy(FT-IR).The separation perfor-mances of the monolith were evaluated systematically.It should be noted that the incorporation of VBS functional monomer can provideπ-πinteractions,hydrophilic interactions,and ion-exchange in-teractions.Hence,the prepared poly(VBS-co-TAT-co-AHM)monolith can achieve efficient separation of thiourea compounds,benzene series,phenol compounds,aniline compounds and sulfonamides in RPLC or HILIC separation mode.The largest theoretical plate number for N,N0-dimethylthiourea reached 1.7×10^(5)plates/m.In addition,the poly(VBS-co-TAT-co-AHM)monolithic column showed excellent reproducibility and stability.This novel monolithic column has great application value and potential in capillary electrochromatography(CEC).

Compositional engineering of HKUST-1/sulfidized NiMn-LDH on functionalized MWCNTs as remarkable bifunctional electrocatalysts for water splitting

Water-splitting reactions such as the hydrogen evolution reaction(HER)and the oxygen evolution reaction(OER)typically require expensive noble metal-based electrocatalysts.This has motivated researchers to develop novel,cost-effective electrocatalytic systems.In this study,a new multicomponent nanocomposite was assembled by combining functionalized multiwalled carbon nanotubes,a Cu-based metal–organic framework(MOF)(HKUST-1 or HK),and a sulfidized NiMn-layered double hydroxide(NiMn-S).The resulting nanocomposite,abbreviated as MW/HK/NiMn-S,features a unique architecture,high porosity,numerous electroactive Cu/Ni/Mn sites,fast charge transfer,excellent structural stability,and conductivity.At a current density of 10 mA cm-2,this dual-function electrocatalyst shows remarkable performance,with ultralow overpotential values of 163 mV(OER)or 73 mV(HER),as well as low Tafel slopes(57 and 75 mV dec-1,respectively).Additionally,its high turnover frequency values(4.43 s-1 for OER;3.96 s-1 for HER)are significantly superior to those of standard noble metal-based Pt/C and IrO2 systems.The synergistic effect of the nanocomposite's different components is responsible for its enhanced electrocatalytic performance.A density functional theory study revealed that the multi-interface and multicomponent heterostructure contribute to increased electrical conductivity and decreased energy barrier,resulting in superior electrocatalytic HER/OER activity.This study presents a novel vision for designing advanced electrocatalysts with superior performance in water splitting.Various composites have been utilized in water-splitting applications.This study investigates the use of the MW/HK/NiMn-S electrocatalyst for water splitting for the first time to indicate the synergistic effect between carbon-based materials along with layered double hydroxide compounds and porous compounds of MOF.The unique features of each component in this composite can be an interesting topic in the field of water splitting.

The Preparation of Nanosized Pd/ZSM-23 Bifunctional Catalysts for n-Hexadecane Hydroisomerization by Employing PHMB as the Growth Modifi er

The development of highly effective metal-zeolite bifunctional catalysts for the hydroisomerization of n-alkanes is a paramount strategy to produce second-generation biofuels with high quality.In this study,polyhexamethylene biguanide hydrochloride(PHMB)is precisely added to the initial gel to synthesize nanosized ZSM-23 zeolites(Z23-x PH).Due to orientation adsorption and steric hindrance effects of PHMB,each sample of Z23-x PH demonstrates enhanced mesoporosity in comparison with the conventional Z23-C zeolite.Furthermore,the Bronsted acid density of the Z23-x PH samples is also signifi cantly reduced due to a reduction in the distribution of framework Al at T2-T5 sites.The corresponding Pd/23-C and Pd/Z23-x PH bifunctional catalysts with 0.5 wt%Pd loading for n-hexadecane hydroisomerization are prepared by incorporating ZSM-23 zeolites as acid supports.According to the catalytic test results,the suitable addition of PHMB can effectively promote the iso-hexadecane yield.The Pd/Z23-2PH catalyst with an n_(PHMB)/n(_Si)molar ratio of 0.002 demonstrates the highest maximum iso-hexadecane yield of 74.1%at an n-hexadecane conversion of 88.3%.Therefore,the employment of PHMB has provided a simple route for the development of highly effective Pd/ZSM-23 catalysts for n-alkane hydroisomerization.

Manipulating oxygenate adsorption on N-doped carbon by coupling with CoSn alloy for bifunctional oxygen electrocatalyst

Highly active bifunctional oxygen electrocatalysts accelerate the development of high-performance Zn-air battery,but suffer from the mismatched activities of oxygen evolution reaction(OER)and oxygen reduced reaction(ORR).Herein,highly integrated bifunctional oxygen electrocatalysts,cobalt-tin alloys coated by nitrogen doped carbon(CoSn@NC)are prepared by MOFs-derived method.In this hybrid catalyst,the binary CoSn nanoalloys mainly contribute to highly active OER process while the Co(or Sn)-N-C serves as ORR active sites.Rational interaction between CoSn and NC donates more rapid reaction kinetics than Pt/C(ORR)and IrO_(2)(OER).Such CoSn@NC holds a promise as air-cathode electrocatalyst in Zn-air battery,superior to Pt/C+IrO_(2)catalyst.First-principles calculations predict that CoSn alloys can upgrade charge redistribution on NC and promote the transfer to reactants,thus optimizing the adsorption strength of oxygen-containing intermediates to boost the overall reactivity.The tuning of oxygenate adsorption by interactions between alloy and heteroatom-doped carbon can guide the design of bifunctional oxygen electrocatalysts.

Facile synthesis of V-doped CoP nanoparticles as bifunctional electrocatalyst for efficient water splitting

Adjusting the intrinsic activity and conductivity of electrocatalysts may be a crucial way for excellent performance for water splitting.Herein,the rational design of vanadium element doped cobalt phosphide(V-doped CoP)nanoparticles has been investigated through a facile gaseous phosphorization using cobalt vanadium oxide or hydroxide(Co-V hydr(oxy)oxide)as precursor.The physical characterization shows that the homogeneous dispersion of V element on V-doped CoP nanoparticles have obtained,which may imply the enhanced electrocatalytic activity for hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).The electrochemical measurements of the prepared V-doped CoP in alkaline electrolyte demonstrate the superior electrocatalytic activity for both HER(overpotential of 235 mV@10 mA cm^-2)and OER(overpotential of 340 mV@10 mA cm^-2).Further,V-doped CoP nanoparticles used as anode and cathode simultaneously in a cell require only 370 mV to achieve a current density of 10 mA cm^-2.The outstanding electrocatalytic activity may be ascribed to the improved conductivity and intrinsic activity owing to phosphating and the doping of V element.In addition,the long-term stability of V-doped Co P has been obtained.Therefore,metal doping into transition metal-based phosphides may be a promising strategy for the remarkable bifunctional electrocatalyst for water splitting.

Bifunctional Electrocatalysts Based on Mo-Doped NiCoP Nanosheet Arrays for Overall Water Splitting

Rational design of efficient bifunctional electrocatalysts is highly imperative but still a challenge for overall water splitting.Herein,we construct novel freestanding Mo-doped NiCoP nanosheet arrays by the hydrothermal and phosphation processes,serving as bifunctional electrocatalysts for overall water splitting.Notably,Mo doping could effectively modulate the electronic structure of NiCoP,leading to the increased electroactive site and improved intrinsic activity of each site.Furthermore,an electrochemical activation strategy is proposed to form Mo-doped(Ni,Co)OOH to fully boost the electrocatalytic activities for oxygen evolution reaction.Benefiting from the unique freestanding structure and Mo doping,Mo-doped NiCoP and(Ni,Co)OOH show the remarkable electrochemical performances,which are competitive among current researches.In addition,an overall water splitting device assembled by both electrodes only requires a cell voltage of 1.61 V to reach a current density of 10 mA cm?2.Therefore,this work opens up new avenues for designing nonprecious bifunctional electrocatalysts by Mo doping and in situ electrochemical activation.

In Situ Coupling Strategy for Anchoring Monodisperse Co_9S_8 Nanoparticles on S and N Dual?Doped Graphene as a Bifunctional Electrocatalyst for Rechargeable Zn–Air Battery

An in situ coupling strategy to prepare Co_9S_8/S and N dual?doped graphene composite(Co_9S_8/NSG) has been proposed. The key point of this strategy is the function?oriented design of organic compounds. Herein, cobalt porphyrin derivatives with sulfo groups are employed as not only the coupling agents to form and anchor Co_9S_8 on the graphene in situ, but also the heteroatom?doped agent to generate S and N dual?doped graphene. The tight coupling of multiple active sites endows the composite materials with fast electrochemical kinetics and excellent stability for both oxygen reduction reaction(ORR) and oxygen evolution reaction(OER). The obtained electrocatalyst exhibits better activity parameter(ΔE = 0.82 V) and smaller Tafel slope(47.7 mV dec^(-1) for ORR and 69.2 mV dec^(-1) for OER) than commercially available Pt/C and RuO_2. Most importantly, as electrocatalyst for rechargeable Zn–air battery, Co_9S_8/NSG displays low charge–discharge voltage gap and outstanding long?term cycle stability over 138 h compared to Pt/C–RuO_2. To further broaden its application scope, a homemade all?solid?state Zn–air battery is also prepared, which displays good charge–discharge performance and cycle performance. The function?oriented design of N_4?metallomacrocycle derivatives might open new avenues to strategic construction of high?performance and long?life multifunctional electrocatalysts for wider electro?chemical energy applications.

Hierarchical sulfur and nitrogen co-doped carbon nanocages as efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air battery

Exploring inexpensive and efficient bifunctional electrocatalysts for oxygen reduction reaction(ORR) and oxygen evolution reaction(OER) is critical for rechargeable metal-air batteries. Herein, we report a new 3D hierarchical sulfur and nitrogen co-doped carbon nanocages(hSNCNC) as a promising bifunctional oxygen electrocatalyst by an in-situ MgO template method with pyridine and thiophene as the mixed precursor. The as-prepared h SNCNC exhibits a positive half-wave potential of 0.792 V(vs. reversible hydrogen electrode, RHE) for ORR, and a low operating potential of 1.640 V at a 10 mA cm-2 current density for OER. The reversible oxygen electrode index is 0.847 V, far superior to commercial Pt/C and IrO2,which reaches the top level of the reported bifunctional catalysts. Consequently, the hSNCNC as air cathodes in an assembled Zn-air battery features low charge/discharge overpotential and long lifetime. The remarkable properties arises from the introduced multiple heteroatom dopants and stable 3D hierarchical structure with multi-scale pores, which provides the abundant uniform high-active S and N species and efficient charge transfer as well as mass transportation. These results demonstrate the potential strategy in developing suitable carbon-based bi-/multi-functional catalysts to enable the next generation of the rechargeable metal-air batteries.

Synthesis of 3D Hexagram-Like Cobalt–Manganese Sulfides Nanosheets Grown on Nickel Foam: A Bifunctional Electrocatalyst for Overall Water Splitting

The exploration of low-cost and efficient bifunctional electrocatalysts for oxygen evolution reaction and hydrogen evolution reaction through tuning the chemical composition is strongly required for sustainable resources. Herein, we developed a bimetallic cobalt–manganese sulfide supported on Ni foam(CMS/Ni) via a solvothermal method. It has discovered that after combining with the pure Co_9S_8 and Mn S, the morphologies of CMS/Ni have modulated. The obtained three-dimensionally hexagram-like CMS/Ni nanosheets have a significant increase in electrochemical active surface area and charge transport ability. More than that, the synergetic effect of Co and Mn has also presented in this composite. Benefiting from these, the CMS/Ni electrode shows great performance toward hydrogen evolution reaction and oxygen evolution reaction in basic medium, comparing favorably to that ofthe pure Co_9S_8/Ni and Mn S/Ni. More importantly, this versatile CMS/Ni can catalyze the water splitting in a twoelectrode system at a potential of 1.47 V, and this electrolyzer can be efficiently driven by a 1.50 V commercial dry battery.

In-situ emb e dding zeolitic imidazolate framework derived Co–N–C bifunctional catalysts in carbon nanotube networks for flexible Zn–air batteries

Recently, the development of high-performance bifunctional oxygen catalysts integrated with flexible conductive scaffolds f or rechargeable metal-air batteries has attracted considerable interest, driving by fastgrowing wearable electronics. Herein, we report a flexible bifunctional oxygen catalyst thin film consisting of Co–N–C bifunctional catalysts embedding in carbon nanotube(CNT) networks. The catalyst is readily prepared by pyrolysis of cobalt-based zeolitic imidazolate frameworks(ZIF-67) that are in-situ synthesized in CNT networks. Such catalyst film demonstrates very high catalytic activities for oxygen reduction(onset potential: 0.91 V, and half-wave potential: 0.87 V vs. RHE) and oxygen evolution(10 m Acm^-2 at 1.58 V) reactions, high methanol tolerance property, and long-term stability(97% current retention). Moreover, our integrated catalyst film shows very good structure flexibility and robustness. Based on the obtained film air electrodes, flexible Zn–air batteries demonstrate low charging and discharging overpotentials(0.82 V at 1 m A cm^-1) and excellent structure stability in the bending tests. These results indicate that presently reported catalyst films are potential air electrodes for flexible metal–air batteries.

Gold-iridium bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions

Carbon supported gold-iridium composite(Au Ir/C) was synthesized by a facile one-step process and was investigated as the bifunctional catalyst for oxygen reduction reaction(ORR) and oxygen evolution reaction(OER). The physical properties of the Au Ir/C composite were characterized by transmission electron microscopy(TEM), X-ray diffraction(XRD) and X-ray photoelectron spectroscopy(XPS). Although the Au and Ir in the Au Ir/C did not form alloy, it is clear that the introduction of Ir decreases the average Au particle size to 4.2 nm compared to that in the Au/C(10.1 nm). By systematical analysis on chemical state of metal surface via XPS and the electrochemical results, it was found that the Au surface for the Au/C can be activated by potential cycling from 0.12 V to 1.72 V, resulting in the increased surface roughness of Au,thus improving the ORR activity. By the same potential cycling, the Ir surface of the Ir/C was irreversibly oxidized, leading to degraded ORR activity but uninfluenced OER activity. For the Au Ir/C, Ir protects Au against being oxidized due to the lower electronegativity of Ir. Combining the advantages of Au and Ir in catalyzing ORR and OER, the Au Ir/C catalyst displays an enhanced catalytic activity to the ORR and a comparable OER activity. In the 50-cycle accelerated aging test for the ORR and OER, the Au Ir/C displayed a satisfied stability, suggesting that the Au Ir/C catalyst is a potential bifunctional catalyst for the oxygen electrode.