Construction And Regulation Of Transition Metal Sulfides Nanoarrays Architectures As Electrocatalysts For Improved Water Splitting Performance

Cost-effective electrocatalytic water splitting holds great promise for the sustainable production of hydrogen fuel and powerfully advances the implementation of "Hydrogen Economy".At present,the recognized highly active noble metal(Pt,Ir,Ru)based materials can efficiently catalyze the process of hydrogen production through electrocatalytic water splitting,but their low storage and high cost restrict the large-scale commercial applications.Therefore,the development of cheap and efficient non-noble metal based electrocatalysts is the core of improving water splitting technology.Transition metal sulfides,with the advantages of abundant metal centers about adjustable valence state,high conductivity and regulatable structure,can be used as a kind of potential non-noble metal based electrocatalysts for hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).However,their catalytic activities are still limited to the insufficient exposure of catalytic active sites on the surface of catalysts,sluggish water dissociation kinetics and weak intermediates adsorption ability.In order to solve the above problems,in this thesis,Cu2S and Ni3S2 are used as the research objects.The catalytic activity and structural stability of transition metal sulfides are simultaneously improved through constructing the nano array structure and optimizing the chemical coordination environment of metal cations and anions.Meanwhile,the electrocatalytic water splitting performance and electrocatalytic reaction mechanism of transition metal sulfides nano arrays are further explored.The main achievements are as the follows.(1)The vanadium-doped Cu2S nanowall arrays assembled by nanowires(V-Cu2S-Nanowires NW)supported by copper foam are synthesized by a facile one-step solvothermal method,the nanoparticles furtherly constitute the smallest unit of nanowires.The three-dimensional(3D)hierarchical structure can more easily generate due to the addition of vanadium source,and the hydrogen adsorption free energy can be optimized due to vanadium doped effect.The 3D structure can offer the plentiful electronics/ions transport pathways and fully expose catalytic active sites.Thus V-Cu2S-Nanowires NW demonstrates enhanced electrocatalytic activity towards HER,with a low overpotential of 188 mV to drive 10 mA/cm2 and good durability for at least 20 h.(2)The V-doped NiS/Ni3S2 heterostructure nanorod arrays grown on nickel foam(VNS/NF-WM)are synthesized via a facile methanol-assisted hydrothermal method.We demonstrate that the morphology,phase composition and crystallinity of VNS/NF are well-modulated by tuning the ratios of water/methanol solvent.The optimized VNS/NF-WM heterostructured nanorod(the volume ratio of water/methanol is 3:1)exhibits superior HER electrocatalytic activity with the low overpotential of 85 and 218 mV to yield the current density values of 10 and 100 mA/cm2,respectively,and meanwhile sustains an excellent stability with almost unchanged current density of 10 mA/cm2 for 60 h.The excellent catalytic activity is attributed to the synergistic effects of morphology,heterogeneous structure and vanadium doping effect.(3)A novel hierarchical Ni3S2/VS4 nanohorn array supported on nickel foam(NS-horn/NF)is prepared by a self-driven synthesis strategy.The synergetic effects of geometric optimization and surface/interfacial modulation of electrocatalysts are realized.The results show that in-situ generation of VS4 in the NS-horn/NF not only triggers the formation of such unique hierarchical architecture,but favors the graft of enriched active bridging S22-on the strong coupling interface between Ni3S2 and VS4,and thus caused enhanced HER kinetics.More importantly,the abundant active nickel oxides for the OER availably form on the interface benefiting from the surface reconstruction of NS-horn/NF due to partial leaching of vanadium(?)of VS4 during the lasting electrolysis in alkaline medium,which promotes the adsorption of OH-and leads to the fast OER rate-determining step in alkaline media.The obtained NS-horn/NF exhibits superior HER and OER performance.When assembled to an alkaline electrolyzer,NS-horn/NF electrode only needs a small voltage of 1.57 V to yield 10 mA/cm2 and retains stability for a long time at least 70 h.(4)The Ni3S2 nanorod array decorated by(020)-oriented VS4 nanocrystalline grown on nickel foam(Shig-NS-rod/NF)is prepared via a facile solvothermal method.The NS-rod/NF materials with the controllable density of bridge S22-sites distributed on the coupling interface between Ni3S2 and VS4 are successfully prepared and their catalytic performances for HER were studied.Results show that Shig-NS-rod/NF catalyst with high density bridge S22-site could effectively graft more and uniformly distributed highly active bridge S22-sites on its coupling interface,which improved the microstructure,electronic structure of the materials with promoted OH-adsorption ability,thus accelerated HER Volmer step in alkaline medium.The resultant Shig-NS-rod/NF exhibits impressive catalytic performance toward HER,with a much low overpotential of 137 mV at 10 mA/cm2 and a long-term durability for at least 22 h,which is superior to Ni3S2 nanorod arrays with low-density bridging S22-(Slow-NS-rod/NF)and NS-film/NF counterparts(without VS4),even outperforming the NF-supported 20%Pt/C at large current density of over 120 mA/cm2.(5)The Ni3S2/VS4 heterostructure nanobelt arrays(Ni3S2/VS4/NF NBs)in situ grown on the surface of nickel foam are synthesized by one-step solvothermal method.The catalytic mechanism of highly active bridge S22-in Ni3S2 was revealed by the density functional theory(DFT).The influence of the reversible conversion of bridge S22-site catalytic mechanism and the enhanced hydrogen evolution kinetics process are proposed.The strong charge transfer process in the catalytic process and the enhanced adsorption capacity of active hydrogen protons were confirmed.The obtained Ni3S2/VS4/NF NBs exhibited excellent electrocatalytic activity for HER,with a low overpotential of 268 mV at 100 mA/cm2 and a long-term catalytic stability of at least 65 h.In addition,Ni3S2/VS4/NF NBs also had significantly enhanced OER catalytic activity and high stability.