Controlled Synthesis And Performance Investigation Of Transition Metal-based Electrocatalysts For Water Splitting

Hydrogen,as a clean energy carrier with high energy density,is recognized as one of the promising alternatives for fossile fuels,has aroused wide research interest.Currently,the hydrogen production technology mainly depends on methane reforming and coal gasification,the product purity of these processes is very low.In addition,these procedures have caused environmental pollution and green-house effect inevitably.Electrochemical water splitting technology is the hot topic and research frontier in the field of hydrogen energy due to its wide availability of reactant,high efficiency,clean and sustainable features.The exploration of high-performance catalysts is the bottleneck to achieve the industrial application of hydrogen production from electrochemical water splitting process.At present,Pt-based catalysts exhibit high performance for hydrogen evolution reaction（HER）,and Ir/Ru-based oxides exhibit high performance toward oxygen evolution reaction（OER）.However,the high cost and low earth abundance features of Pt,Ir,Ru metals restrict their large-scale industrial application.In non-noble metal-based catalysts series,transition metal-based catalysts are the promising candidates due to their high charge conductivity and unique d electronic structures to dissociate H₂O molecules effectively.This essay proposes a heteroatom doping strategy to efficiently promote the alkaline OER performance of transition metal oxides,and a heterostructure interface construction strategy to efficiently improve the alkaline HER performance of transition metal-based catalysts.The main contents are as follows:1.OER is restricted by its 4 charges transfer property and slow reaction kinetics.Further application of 3d transition metal oxides as OER catalysts is prohibited by their low electronic conductivity and weak adsorption strength for OH^-ions.High-efficiency and stable OER process is promising by tuning the electronic structure of active sites in 3d transition metal oxides.On the basis of these considerations,the OER performance of S-NiCoVO_xnanosheets is enhanced via S doping strategy.S-NiCoVO_xnanosheets catalyst achieves the current densities of 10,100 mA cm^-2at overpotentials of 248,289 mV with a Tafel slope of 46.2 mV/dec in 1.0 M KOH electrolyte.In addition,negligible overpotential decation is observed in the 30 h stability test process at the current density of 50 mA cm^-2.Theoretical calculation results reveal that the electronic structure of active metal Co in S-NiCoVO_xnanosheets is tuned via S doping,which causes up shift of Co 3d band centre,strengthens the OH^-ions adsorption energetics on active Co sites,thus optimizes the adsorption energetics of transient intermediate OOH^*on active Co centre and decreases the energy barrier of rate-determining step in OER.2.Electrochemical water splitting process in alkaline media is an ideal system for achieving the industrial application for hydrogen production due to its wide availability of reactants,high gas product purity and strong corrison resistance of non-noble metal-based catalysts.However,HER,as another important half-reaction in water splitting process,performs low activity under alkaline conditions due to the obstacle to achive H₂O molecue cleavage（Volmer step）and favorable H^*adsorption（Heyrovsky/Tafel step）simultaneously on single active site.On the basis of these considerations,the Volmer and Heyrovsky/Tafel steps can be optimized simultaneously on dual active centres through synergetic effect via construction of heterostructure interfaces.Via constructing Co/CoMoN/NF heterostructure,the alkaline HER performance is enhanced due to the interfacial synergistic effect.For alkaline HER process,Co/CoMoN/NF heterostructure only needs overpotential of 173 mV to achieve the current density of 100 mA cm^-2associated with the Tafel slope of 68.9 mV/dec.Theoretical calculation results indicate that the active Co site on Co/CoMoN interface is favorable for water dissociation,the produced H^*adsorbs on CoMoN slab and CoMoN slab is responsible for H^*desorption and H₂release.Co/CoMoN/NF heterostructure exhibits high performance for alkaline HER through synergistic effect via construction of Co/CoMoN interfaces.Morever,Co/CoMoN/NF heterostructure exhibits impressive OER and overall water splitting performance.3.Since construction of heterostructure interfaces can largely improve alkaline HER performance through synergistic effect,it is rational to think of further promoting alkaline HER performance via increasing interfacial active sites.On the basis of these considerations,we constructed Co₃Mo nanoparticles/porous CoMoO₃nanosheets heterostructures.Co₃Mo nanoparticles/porous CoMoO₃nanosheets heterostructures exhibit excellent electrochemical water splitting performance due to its high specific surface areas,abundant interfacial active sits,porous structure favorable for aqueous electrolyte penetration and gas product transfer.For alkaline HER,Co₃Mo nanoparticles/porous CoMoO₃nanosheets heterostructures only need the overpotential of 103 mV to achieve the current density of 100 mA cm^-2associated with the Tafel slope of 46.4 mV/dec,which performance further outperforms the Co/CoMoN/NF heterostructure catalyst.Benefiting from the synergistic effect on Co₃Mo/CoMoO₃interfaces and increased interfacial active sites,Co₃Mo nanoparticles/porous CoMoO₃nanosheets heterostructures exhibit enhanced alkaline HER performance.Morever,Co₃Mo nanoparticles/porous CoMoO₃nanosheets heterostructures exhibit enhanced alkaline OER and overall water splitting performance.