Graphitic Carbon Quantum Dots Modified Nickel Cobalt Sulfide as Cathode Materials for Alkaline Aqueous Batteries

Carbon quantum dots(CQDs)as a new class of emerging materials have gradually drawn researchers’concern in recent years.In this work,the graphitic CQDs are prepared through a scalable approach,achieving a high yield with more than 50%.The obtained CQDs are further used as structure-directing and conductive agents to synthesize novel N,S-CQDs/NiCo2S4 composite cathode materials,manifesting the enhanced electrochemical properties resulted from the synergistic effect of highly conductive N,S-codoped CQDs offering fast electronic transport and unique micro-/nanostructured NiCo2S4 microspheres with Faradaic redox characteristic contributing large capacity.Moreover,the nitrogen-doped reduced graphene oxide(N-rGO)/Fe2O3 composite anode materials exhibit ultrahigh specific capacity as well as significantly improved rate property and cycle performance originating from the high-capacity prism-like Fe2O3 hexahedrons tightly wrapped by highly conductive N-rGO.A novel alkaline aqueous battery assembled by these materials displays a specific energy(50.2 Wh kg^−1),ultrahigh specific power(9.7 kW kg^−1)and excellent cycling performance with 91.5%of capacity retention at 3 A g^−1 for 5000 cycles.The present research offers a valuable guidance for the exploitation of advanced energy storage devices by the rational design and selection of battery/capacitive composite materials.

Nano-Au-decorated hierarchical porous cobalt sulfide derived from ZIF-67 toward optimized oxygen evolution catalysis:Important roles of microstructures and electronic modulation

Enhancing both the number of active sites available and the intrinsic activity of Co-based electrocatalysts simultaneously is a desirable goal.Herein,a ZIF-67-derived hierarchical porous cobalt sulfide decorated by Au nanoparticles(NPs)(denoted as HP-Au@CoxSy@ZIF-67)hybrid is synthesized by low-temperature sulfuration treatment.The well-defined macroporous-mesoporous-microporous structure is obtained based on the combination of polystyrene spheres,as-formed CoxSy nanosheets,and ZIF-67 frameworks.This novel three-dimensional hierarchical structure significantly enlarges the three-phase interfaces,accelerating the mass transfer and exposing the active centers for oxygen evolution reaction.The electronic structure of Co is modulated by Au through charge transfer,and a series of experiments,together with theoretical analysis,is performed to ascertain the electronic modulation of Co by Au.Meanwhile,HP-Au@CoxSy@ZIF-67 catalysts with different amounts of Au were synthesized,wherein Au and NaBH4 reductant result in an interesting“competition effect”to regulate the relative ratio of Co^(2+)/Co^(3+),and moderate Au assists the electrochemical performance to reach the highest value.Consequently,the optimized HP-Au@CoxSy@ZIF-67 exhibits a low overpotential of 340 mV at 10 mA cm^(-2)and a Tafel slope of 42 mV dec-1 for OER in 0.1 M aqueous KOH,enabling efficient water splitting and Zn-air battery performance.The work here highlights the pivotal roles of both microstructural and electronic modulation in enhancing electrocatalytic activity and presents a feasible strategy for designing and optimizing advanced electrocatalysts.

Hierarchical polypyrrole@cobalt sulfide-based flexible on-chip microsupercapacitors with ultrahigh energy density for selfcharging system

Herein,we prepare the unique hierarchical polypyrrole@cobalt sulfide(PPy-hs@CoS)hollow sphere-based nanofilms as interdigitated electrodes for flexible on-chip micro-supercapacitors(MSC).Benefiting from the excellent flexibility and high electrical conductivity of PPy-hs combined with the great electrochemical activity of CoS,such PPy-hs@CoS composite material can not only inhibit the volume expansion of PPy but also promote the diffusion of the electrolyte ions.The PPy-hs@CoS filmbased electrode delivers a greatly improved specific capacitance and small resistance.Density functional theory calculations infer that OH−prefers to bind to PPy on CoS@PPy and confirms the synergistic effect of each component for enhanced reaction kinetics.A quasi-solid-state on-chip flexible asymmetric MSC based on PPy-hs@CoS and activated carbon(AC)microelectrodes exhibits large areal-specific capacitance(131.9 mF/cm2 at 0.3 mA/cm2),ultrahigh energy density(0.041 mWh/cm2@0.224 mW/cm2 and 25.6 mWh/cm3@140.6 mW/cm3),and long cycle lifespan.We demonstrate the possibility to scale up the PPy-hs@CoS nanofilm microelectrode by arranging two of our asymmetric MSC in series and parallel connections,which respectively increase the output voltage and current.A self-charging system by connecting our asymmetric MSCs with a piece of commercial solar cells is developed as a potential possible mode for future highly durable and high-voltage integrated electronics.

Interfacial engineering of heterostructured carbon-supported molybdenum cobalt sulfides for efficient overall water splitting

Constructing hetero-structured catalyst is promising but still challenging to achieve overall water splitting for hydrogen production with high efficiency.Herein,we developed a sulfide-based MoS_(2)/Co_(l-x)S@C hetero-structure for highly efficient electrochemical hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).The carbon derived from the filter paper acts as a conducting carrier to ensure adequate exposure of the active sites guaranteed with improved catalytic stability.The unique hierarchical nano-sheets facilitate the charge and ion transfer by shortening the diffusion path during electro-catalysis.Meanwhile,the robust hetero-interfaces in MoS_(2)/Co_(1-x)S@C can expose rich electrochemical active sites and facilitate the charge transfer,which further cooperates synergistically toward electro-catalytic reactions.Consequently,the optimal MoS_(2)/Co_(1-x)S@C hetero-structures present small over-potentials toward HER(135 mV@10 mA·cm^(-2))and OER(230 mV@10 mA·cm^(-2)).The MoS_(2)/Co_(1-x)S@C electrolyzer requires an ultralow voltage of 1.6 V at the current density of 10 mA·cm^(-2)with excellent durability,outperforming the state-of-the-art electro-catalysts.This work sheds light on the design of the hetero-structured catalysts with interfacial engineering toward large-scale water splitting.

Integrating ceria with cobalt sulfide as high-performance electrocatalysts for overall water splitting

The development of bifunctional electrocatalysts for overall water splitting is highly desired for converting electricity into chemical energy.However,the synthesis of high-performance bifunctional electrocatalysts remains a pressing challenge.Here,we found that both the oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)performance of the Co_(3)S_(4) electrode can be significantly improved by integration with CeO_(2).Specifically,as-prepared 5%Ce-Co_(3)S_(4) and 1%Ce-Co_(3)S_(4) delivered low overpotentials of 290 and 257 mV to achieve 10 mA cm^(-2) for the OER and HER in 1.0 M KOH,respectively.The crucial role of CeO_(2) originated from its unique surface with abundant oxygen vacancies,which were beneficial for the stabilization of Co^(2+)sites with high OER activity and both the adsorption and dissociation of water molecules in the HER process.This work is expected to provide a general approach to prepare a wide range of high-performance electrode materials for energy-related applications.

Construction of nickel cobalt sulfide nanosheet arrays on carbon cloth for performance-enhanced supercapacitor

Materials featured with self-supported three-dimensional network,hierarchical pores and rich electrochemical active sites are considered as promising electrodes for pseudocapacitors.Herein,a novel strategy for the growth of nickel-cobalt bisulfide(Ni Co S)nanosheets arrays on carbon cloth(CC)as supercapacitor electrodes is reported,involving deposition of two-dimensional metal-organic framework(MOF)precursors on the CC skeletons,conversion of MOF into nickel-cobalt layered double-hydroxide by ion exchange process and formation of Ni Co S by a sulfidation treatment.The Ni Co S nanosheets with rough surface and porous structures are uniformly anchored on the CC skeletons.The unique architecture endows the composite(Ni Co S/CC)with abundant accessible active sites.Besides,robust electrical/mechanical joint between the nanosheets and the substrates is attained,leading to the improved electrochemical performance.Moreover,an asymmetric supercapacitor device is constructed by using Ni Co S/CC and activated carbon as a positive electrode and a negative electrode,respectively.The optimized device exhibits a high specific capacitance,large energy density and long cycle life.The Ni Co S/CC electrode with intriguing electrochemical properties and mechanical flexibility holds great prospect for next-generation wearable devices.

Polydopamine(PDA)-activated cobalt sulfide nanospheres responsive to tumor microenvironment(TME)for chemotherapeutic-enhanced photothermal therapy

Most recently,cobalt sulfide(CoS)nanospheres(NSs)have been demonstrated as an ideal high-efficient photothermal agent for tumor elimination.Howeve r,the surface of CoS NSs is lack of functio nal chemical groups or active radicals to incorporate therapeutic agents,which tremendously hinders their versatile utilization in medical field.Here,surface activation of CoS NSs was realized through the growth of polydopamine(PDA)in situ via alkaline-triggered polymerization.Upon the formation of CoS@PDA NSs,thiol-polyethylene glycol(SH-PEG)and chemotherapeutic agent of doxorubicin(DOX)were loaded onto the particle surface by means ofπ-πelectrostatic interaction and Michael addition reactions.Assynthesized CoS@PDA/PEG/DOX(CoPPD)NSs exhibited an admirable photothermal property and high loading capacity of DOX(44.6%).Furthermore,drug release can be accelerated under a more acidic pH condition mimicking tumor microenvironment(TME),ascribed to the protonation of amino group in DOX molecules.Finally,a strong chemotherapeutic-enhanced photothermal therapeutic effect was demonstrated toward solid tumor under near-infrared(NIR)light irradiation without causing significant systemic toxicity.In this regard,this paradigm may offer valuable guidance for the design of multifunctional CoS-based nanoagents for medical treatment.

Two-dimensional porous zinc cobalt sulfide nanosheet arrays with superior electrochemical performance for supercapatteries

Unique two-dimensional(2D)porous nanosheets with overwhelmingly rich channels and large specific surface area exhibit superior electrochemical capacitance performance,as compared to the conventional zero-and one-dimensional counterparts.As ternary transition metal sulfides(TMSs)are well recognized for their high electrochemical activity and capacity,and the replacement of oxygen with sulfur may result in high stability and flexible properties of the nanomaterials,as compared to transition metal oxides,herein we report the synthesis of 2D porous nanosheet arrays of Zn_(x)Co_(1-x)S(x=0,0.25,0.5,0.75,and 1)via a facile hydrothermal process.Due to the synergistic effect of the metal components and a unique 2D porous structure,the Zn_(0.5)Co_(0.5)S electrode was found to stand out as the best among the series,with a high specific capacity of 614 C g^(-1)at 1 A g^(-1)and excellent cycle retention rate of 90%over 10,000 cycles at 10 A g^(-1).Notably,a supercapattery based on a Zn_(0.5)Co_(0.5)S positive electrode and an activated carbon(AC)negative electrode(Zn_(0.5)Co_(0.5)S//AC)was found to display a 1.6 V voltage window,a 61 mA h g^(-1)specific capacity at 1 A g^(-1),a 49 Wh kg^(-1)energy density at 957 W kg^(-1)power density,and excellent cycling performance(88%over 10,000 cycles),suggesting tremendous potential of Zn_(0.5)Co_(0.5)S in the development of high-performance supercapattery devices.

Bimetallic nickel cobalt sulfides with hierarchical coralliform architecture for ultrafast and stable Na-ion storage

A series of bimetallic nickel cobalt sulfides with hierarchical micro/nano architectures were fabricated via a facile synthesis strategy of bimetallic micro/nano structure precursor construction-anion exchange via solvothermal method. Among the nickel cobalt sulfides with different Ni/Co contents, the coral-like Ni1.01Co1.99S4 (Ni/Co, 1/2) delivers ultrafast and stable Na-ion storage performance (350 mAh·g−1 after 1,000 cycles at 1 A·g−1 and 355 mAh·g−1 at 5 A·g−1). The remarkable electrochemical properties can be attributed to the enhanced conductivity by co-existence of bimetallic components, the unique coral-like micro/nanostructure, which could prevent structural collapse and self-aggregation of nanoparticles, and the easily accessibility of electrolyte, and fast Na+ diffusion upon cycling. Detailed kinetics studies by a galvanostatic intermittent titration technique (GITT) reveal the dynamic change of Na+ diffusion upon cycling, and quantitative kinetic analysis indicates the high contribution of pseudocapacitive behavior during charge-discharge processes. Moreover, the ex-situ characterization analysis results further verify the Na-ion storage mechanism based on conversion reaction. This study is expected to provide a feasible design strategy for the bimetallic sulfides materials toward high performance sodium-ion batteries.

Towards high-performance anodes:Design and construction of cobalt-based sulfide materials for sodium-ion batteries

Sodium-ion batteries are increasingly becoming important in the energy storage field owing to their low cost and high natural abundance of sodium.Cobalt-based sulfide materials have been extensively studied as anode materials owing to their remarkable Na storage capability.Nevertheless,the application of cobalt-based sulfides is hampered by their serious capacity degradation and unsatisfactory cycling stability due to severe structural changes during cycling.Therefore,it is important to comprehensively summarize advances in the understanding and modification of cobalt-based sulfides from various perspectives.In the present review,recent advances on various cobalt-based sulfides,such as CoS,CoS_(2),Co_(3)S_(4),Co_(9)S_(8),NiCo_(2)S_(4),CUCo_(2)S_(4),and SnCoS_(4),are outlined with particular attention paid to strategies that improve their sodium storage performance.First,the mechanisms of charge storage are introduced.Subsequently,the key barriers to their extensive application and corresponding strategies for designing high-performance cobalt-based sulfide anode materials are discussed.Finally,key developments are summarized and future research directions are proposed based on recent advancements,aiming to offer possible fascinating strategies for the future promotion of cobalt-based sulfides as anode materials applied in sodium-ion 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.

CoS Nanosheets Coated with Dopamine-Derived Carbon Standing on Carbon Fiber Cloth as Binder-Free Anode for Li-ion Batteries

Cobalt sulphides attract much attention as anode materials for Li-ion batteries(LIBs).However,its poor conductivity,low initial column efficiency and large volume changes during cycling have hindered its further development.Herein,novel interlaced CoS nanosheets were firstly prepared on Carbon Fiber Cloth(CFC)by two hydrothermal reactions followed with carbon coating via carbonizing dopamine(CoS NS@C/CFC).As a freestanding anode,the nanosheet structure of CoS not only accommodates the volume variation,but also provides a large interface area to proceed the charge transfer reaction.In addition,CFC works as both a three-dimensional skeleton and an active substance which can further improve the areal capacity of the resulting electrode.Furthermore,the coated carbon combined with the CFC work as a 3D conductive network to facilitate the electron conduction.The obtained CoS NS@C/CFC,and the contrast sample prepared with the same procedure but without carbon coating(CoS NS/CFC),are characterized with XRD,SEM,TEM,XPS and electrochemical measurements.The results show that the CoS NS@C/CFC possesses much improved electrochemical performance due to the synergistic effect of nanosheet CoS,the coated carbon and the CFC substrate,exhibiting high initial columbic efficiency(~87%),high areal capacity(2.5 at 0.15 mA cm−2),excellent rate performance(1.6 at 2.73 mA cm−2)and improved cycle stability(87.5%capacity retention after 300 cycles).This work may provide a new route to explore freestanding anodes with high areal specific capacity for LIBs.

Interface enhanced well-dispersed Co9S8 nanocrystals as an efficient polysulfide host in lithium–sulfur batteries

The high specific capacity and energy density of lithium-sulfur batteries have attracted strong considerations on their fundamental mechanism and energy applications.However,polysulfide shuttle is still the key issue that impedes the development of Li-S batteries.Exploring nanocrystal hosts for polysulfide immobilization and conversion is a promising way.In this contribution,we have investigated well-dispersed Co9S8 nanocrystals grown on graphene oxide(GO)nanosheets with different degrees of dispersion as cathode host materials for Li-S batteries.The Co9S8-GO composite with 1 wt%GO(GCS1)has an average crystal size of 76 nm and shows the strongest adsorption capability toward lithium polysulfides.When used as the host material for the cathode of Li-S batteries,the GCS1-sulfur composite exhibits an initial specific capacity of^-1000 mAh g^-1 at 0.5 C and shows an average decay rate of 0.11%for 500 cycles.This work on the dispersion control of Co9S8 nanocrystals may inspire more investigations on well-dispersed nanocrystal based hosts for Li-S batteries.

Ultrasmall CoS nanoparticles embedded in heteroatom-doped carbon for sodium-ion batteries and mechanism explorations via synchrotron X-ray techniques

Transition metal sulfides have been regarded as promising anode materials for sodium-ion batteries(SIB).However,they face the challenges of poor electronic conductivity and large volume change,which result in capacity fade and low rate capability.In this work,a composite containing ultrasmall CoS(~7 nm)nanoparticles embedded in heteroatom(N,S,and O)-doped carbon was synthesized by an efficient one-step sulfidation process using a Co(Salen)precursor.The ultrasmall CoS nanoparticles are beneficial for mechanical stability and shortening Na-ions diffusion pathways.Furthermore,the N,S,and O-doped defect-rich carbon provides a robust and highly conductive framework enriched with active sites for sodium storage as well as mitigates volume expansion and polysulfide shuttle.As anode for SIB,CoS@HDC exhibits a high initial capacity of 906 mA h g^(-1)at 100 mA g^(-1)and a stable long-term cycling life with over 1000 cycles at 500 mA g^(-1),showing a reversible capacity of 330 mA h g^(-1).Meanwhile,the CoS@HDC anode is proven to maintain its structural integrity and compositional reversibility during cycling.Furthermore,Na-ion full batteries based on the CoS@HDC anode and Na_(3)V_(2)(PO_(4))_(3)cathode demonstrate a stable cycling behavior with a reversible specific capacity of~200 m A h g^(-1)at least for 100 cycles.Moreover,advanced synchrotron operando X-ray diffraction,ex-situ X-ray absorption spectroscopy,and comprehensive electrochemical tests reveal the structural transformation and the Co coordination chemistry evolution of the CoS@HDC during cycling,providing fundamental insights into the sodium storage mechanism.

Sandwiched N-carbon@Co_(9)S_(8)@Graphene nanosheets as high capacity anode for both half and full lithium-ion batteries

Transition metal sulfides(TMSs) are promising candidates for replacing graphite anode in LIBs. However,the low conductivity and structural collapse caused by the large volume change during lithium insertion and extraction greatly limit its application. Herein, we report a unique design of a two-dimensional(2 D) sandwich structure of N-doped carbon@Co9 S8@graphene(N–C@Co9 S8@G) with multilayer structure. Electrochemical tests reveal that the N–C@Co9 S8@G nanosheets possess a high reversible capacity(1009 mAhg^(-1) at 0.1 Ag^(-1)), and excellent rate capability(422mAhg^(-1) at 10 Ag^(-1)) and long cycle life(853 m Ahg^(-1) at 1Ag^(-1) for 500 cycles). Experimental studies reveal that capacitive storage contributes to the high reversible capacity. The lithium storage kinetics are studied by Galvanostatic intermittent titration technique(GITT) and electrochemical impedance spectroscopy(EIS). Meanwhile, the potential of N–C@Co9 S8@G anode in a full cell using Li Co O2 as the cathode is also demonstrated, exhibiting a high reversible capacity of 300mAhg^(-1) cycles at 0.1Ag^(-1). The strategy proposed in this work paves the way to engineering high performance anodes in LIBs.

Interface Engineering of CoS/CoO@N‑Doped Graphene Nanocomposite for High‑Performance Rechargeable Zn–Air Batteries

Low cost and green fabrication of high-performance electrocatalysts with earth-abundant resources for oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)are crucial for the large-scale application of rechargeable Zn-air batteries(ZABs).In this work,our density functional theory calculations on the electrocatalyst suggest that the rational construction of interfacial structure can induce local charge redistribution,improve the electronic conductivity and enhance the catalyst stability.In order to realize such a structure,we spatially immobilize heterogeneous CoS/CoO nanocrystals onto N-doped graphene to synthesize a bifunctional electrocatalyst(CoS/CoO@NGNs).The optimization of the composition,interfacial structure and conductivity of the electrocatalyst is conducted to achieve bifunctional catalytic activity and deliver outstanding efficiency and stability for both ORR and OER.The aqueous ZAB with the as-prepared CoS/CoO@NGNs cathode displays a high maximum power density of 137.8 mW cm^−2,a specific capacity of 723.9 mAh g^−1 and excellent cycling stability(continuous operating for 100 h)with a high round-trip efficiency.In addition,the assembled quasi-solid-state ZAB also exhibits outstanding mechanical flexibility besides high battery performances,showing great potential for applications in flexible and wearable electronic devices.

Self-supporting and dual-active 3D Co-S nanosheets constructed by ligand replacement reaction from MOF for rechargeable Al battery

Metal sulfides with high theoretical capacities are expected as promising cathode materials of Al batteries(AIBs). However, powdery active materials are mainly synthesized and loaded on current collector by insulating binder without capacity. Meanwhile, S as inert element in metal sulfides can not usually provide capacity. So, powdery metal sulfides only exhibit limiting practical capacity and poor cycling stability due to weak conductivity and low mass utilization. Herein, the novel self-supporting and dual-active Co-S nanosheets on carbon cloth (i.e. Co-S/CC) with hierarchically porous structure are constructed as cathode of AIBs. Co-S nanosheets are derived from ZIF-67 nanosheets on CC by a facile ligand replacement reaction. As a result, the binder-free Co-S/CC cathode with good conductivity delivers excellent initial discharge capacity of 383.4 m Ah g^(-1)(0.211 m Ah cm^(-2)) at current density of 200 m A g^(-1)and maintain reversible capacity of 156.9 m Ah g^(-1)(0.086 m Ah cm^(-2)) with Coulombic efficiency of 95.8% after 500 cycles,which are much higher than those of the traditional slurry-coating cathodes. Both Co and S as active elements in Co-S/CC contribute to capacity, which leads to a high mass utilization. This work provides a significant strategy for the construction of self-supporting metallic cathode for advanced high-energy density Al battery.

Co9S8 Nanotubes as an Efficient Catalyst for Hydrogen Evolution Reaction in Alkaline Electrolyte

Cobalt sulfide (Co9S8) nanotubes were found to be an electrocatalyst for the hydrogen evolution reaction under alkaline condition. An electrode comprising of Co9S8 nanotubes on a glass carbon electrode (GCE) (mass loading: 0.855 mg·cm-2) produced a cathodic current density of 20 mA·cm-2 at an overpotential of 320 mV. The Co9S8/GCE electrode was stable over 20,000 s during potentiostatic electrolysis. Minor degradation of reduction current after 4000 cyclic voltammetric sweeps suggests the long-term viability under operating conditions. The faradaic efficiency of Co9S8 nanotubes is nearly 100% during the electrolysis of water.

Sulfur–nitrogen co-doped graphene supported cobalt–nickel sulfide rGO@SN-CoNi_(2)S_(4) as highly efficient bifunctional catalysts for hydrogen/oxygen evolution reactions

Designing highly active and stable electrocata-lysts for hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)is a challenge for energy con-version and storage technology.In this work,a S and N co-doped graphene supported cobalt–nickel sulfide composite catalyst(rGO@SN-CoNi_(2)S_(4))was synthesized simply via a one-step hydrothermal method.The as-synthesized CoNi_(2)S_(4)particles grew in a mosaic manner inside GO lamellae and were encapsulated with graphene.As a bifunctional catalyst,the r GO@SN-CoNi_(2)S_(4)exhibits excellent electrocatalytic performance under alkaline con-ditions,which only required the overpotential of 142.6 mV(vs.RHE)and 310 m V(vs.RHE)to deliver a current density of 10 mA·cm^(-2) for HER and OER,respectively.The good hydrophilicity of the r GO@SN,the pure phase of bimetallic structure,and the chemical coupling/interaction between the CoNi_(2)S_(4)and the rGO@SN are attributable to be the possible reasons responsible for the higher HER and OER catalytic activities.Additionally,the rGO@SN-CoNi_(2)S_(4)also shows a great potential for serving as an excellent cathode and anode electrolyzer during the water splitting process.

Phosphorus-doped Ni–Co sulfides connected by carbon nanotubes for flexible hybrid supercapacitor

As promising electrode materials for supercapacitors,nickel-cobalt bimetallic sulfides render the advantages of abundant redox reactions and inherently high conductivity.However,in general,unsatisfactory performance of low specific capacity,low rate capability,and fast capacity loss exist in Ni–Co sulfide electrodes.Herein,we rationally regulate phosphorus-doped nickel–cobalt sulfides(P-NCS)to enhance the electrochemical performance by gas–solid phosphorization.Moreover,carbon nanotubes(CNTs)as conductive additives are added to improve the cycle stability and conductivity and form the composite P-NCS/C/CNT.According to density functional theory,more electrons near the Fermi surface of P-NCS are demonstrated notionally than those of simple CoNi2S4.Electrochemical results manifest that P-NCS/C/CNT exhibits superior electrochemical performance,e.g.,high specific capacity(932.0 C·g^(-1)at 1 A·g^(-1)),remarkable rate capability(capacity retention ratio of 69.1%at 20 A·g^(-1)),and lower charge transfer resistance.More importantly,the flexible hybrid asymmetric supercapacitor is assembled using P-NCS/C/CNT and activated carbon,which renders an energy density of 34.875 W·h∙kg^(-1)at a power density of 375 W∙kg^(-1).These results show that as-prepared P-NCS/C/CNT demonstrates incredible possibility as a battery-type electrode for high-performance supercapacitors.