Study On The Transition Metal Sulphides And Phosphides Composite Nanomaterials For Electrocatalytic Water Splitting

Hydrogen production from electrolytic water splitting has been considered as one of the most promising technologies for hydrogen production in current research,which can effectively convert intermittment electrical energy into chemical energy,and has the advantages of high product purity and no pollution.The electrolytic water splitting is divided into two half reactions:the oxygen evolution reaction(OER)occurring at the anode and the hydrogen evolution reaction(HER)occurring at the cathode.Developing efficient electrocatalytic materials to lower the half-reaction overpotential and reduce energy consumption is a crucial way to improve the efficiency of water splitting.To date,most efficient catalysts for HER and OER(Ru O₂,Ir O₂ Pt-based materials,etc.)are precious metals,but their scarcity and high price have limited their large-scale application.Therefore,it is necessary to develop low-cost,efficient,and stable non-noble elecrtocatalysts for OER and HER.Transition metal sulfides and phosphides are a type of potential non-noble electrocatalytic materials due to the advantages of tunable electronic structure,special morphology,and low price.In this paper,we use them as the main part and compounding with other functional components by various ways(introducing dianions simultaneously,substitutions,and heterojunctions,etc.)to prepare efficient composite nanomaterials for water splitting.The electronic structure and morphology of materials can be modulated using multicomponent interactions.The activity of the materials was further analyzed by density function theory(DFT).It mainly includes the following contents:(1)Design of the NiCoDH nanoparticles/NiCoS nanosheets and their performance for oxygen evolution reaction:We report a lamellar reverse micelle directed assembly strategy to integrate Ni Co layered hydroxides(NiCoDH)nanodots and Ni Co sulfide(NiCoS)nanosheets(NiCoDH/NiCoS)by one-step solvothermal method.The NiCoDH/NiCoS features with an interconnected 3D architecture composed of ultrathin amorphous NiCoS nanosheets(?1.5 nm),decorated by ultra-small NiCoDH nanodots(2?3 nm).The optimal NiCoDH/NiCoS exhibits a low overpotential of 303 mV at current density of 20 mA cm^-2,together with a small Tafel slope of 77.6 mV dec^-1,the high Faraday efficiency of about 95%as well as good stability.The excellent OER performance and enhanced stability are derived from the strong interface-coupling of NiCoDH nanodots/NiCoS nanosheets,highly synergistic effect of bimetal Co and Ni,amorphous-like structure to expose more active sites,and interconnected 3D architecture for effective mass transport and structural stability,(2)Design of the MoS₂/MoO₂/MF and their performance for hydrogen evolution reaction:Efficient water splitting requires not only good OER catalytic materials,but also optimization of HER performance.The powder catalyst is unstable and easy to fall off during high current testing.Besides,the binder is required,which can increase the resistance and cover the active site.Therefore,A facile oxidation-sulfidation strategy is proposed to fabricate the 3D amorphous MoS₂ nanosheets on MoO₂ films/Mo foil(MF)as free-standing electrode,which features as the integration of three merits(high conductivity,abundant exposures of active sites,and enhanced mass transfer)into one electrode for hydrogen evolution reaction(HER).Density functional theory(DFT)calculations reveal the strong interaction between MoS₂ and MoO₂,which can enhance the intrinsic conductivity with narrow bandgap,and decreases hydrogen adsorption free energy(?GH*=?0.06 eV)to facilitate the HER process.Benefiting from the unique 3D structure with amorphous MoS₂ nanosheets on conductive MoO₂ films/MF to facilitate the electron/mass transfer by eliminate contact resistance,controllable number of stacking layers and size of MoS₂ slabs to expose more edge sites,the optimal MoS₂/MoO₂/MF exhibits outstanding activity with overpotential of 154 mV at the current density of 10 mA cm^-2,Tafel slope of 52.1 mV dec^-1,and robust stability.(3)Design of the heterojunction CoP/MoO₂ composites and their performance for hydrogen evolution reaction under acidic and alkaline media:Cobalt phosphide has been widely reported as effective HER material,but limited to the weak ability of water adsorption/dissociation,the activity is still far from that of Pt-based materials.Heterostructured CoP nanoarrays/MoO₂ films on Mo foil(CoP-MoO₂/MF)were synthesized through low-temperature phosphorization process using pre-oxidized MF supported cobalt hydroxide-carbonate nanoneedles as precursor.Such CoP-MoO₂/MF heterostructure exhibits the superior HER performance with low overpotentials of 42 mV(alkaline)and 65 mV(acid)to achieve the current density of 10 mA cm^-2,robust stability,large exchange current density and high turnover frequency.The experimental characterizations and DFT calculations unveil that the presence of MoO₂ possess the inherent ability to adsorb water and cleave H-O bonds,and favoring hydrogen transfer to accelerateVolmer step.The heterointerfacial charge redistribution endows the optimal hydrogen adsorption ability on adjacent P sites to boost the Heyrovsky step,and as a result,the hydrogen adsorption free energy(?G_H*)can be reduced from 0.08 eV on CoP(011)and-0.78 eV on MoO₂(011)to 0.02 eV on the heterointerface of CoP-MoO₂,which significantly improves HER activity and accelerates catalytic kinetics.