Engineering Novel Transition Metal Catalysts And Corresponding Applications For Solar-to-hydrogen Conversion

As a clean and high-enthalpy energy carrier,hydrogen has been widely used in traditional industrial production,such as petroleum hydrocracking and ammonia synthesis.Solar-driven electrochemical water splitting for hydrogen evolution is a promising technology for green hydrogen production.It mainly includes converting solar energy into electricity to drive electrochemical hydrogen evolution reaction(ECHER)and the direct excitation of semiconductors for photoelectrochemical hydrogen evolution reaction(PEC-HER).However,the HER process of water splitting is affected by active area,charge/mass transport,and ad-/desorption energy barriers of intermediates,resulting in a high overpotential and low energy conversion efficiency.Besides,the PEC-HER relies on photogenerated electrons in semiconductors,which is heavily influenced by electron-hole recombination rate,interfacial charge transfer kinetics,etc.The novel transition metal-based catalysts were developed to reduce the HER overpotential and improve the energy conversion efficiency of EC-HER and PECHER.The main research contents of this paper are as follows:(1)To overcome the problems of high overpotential,poor electrical conductivity,and low utilization of active sites under low current density EC-HER,a metal oxide Sr4Ru2O9 with Ru-Ru metallic bond characteristics was synthesized by changing the connection method of the[RuO6]octahedral motif.The electrochemical test results show that Sr4Ru2O9 only needs a small overpotential(η10=28 mV)to reach-10 mA cm-2 when compared with SrRuO3(η10=57 mV).The activatio1 energy and the intermediates on the active sites during the HER process were further explored using density functional theory(DFT).The calculations results indicate that the electronic structure of the Ru-Ru metal bond inherent in the face-sharing[RuO6]motif of Sr4Ru2O9 determines catalytic activity.The Ru-Ru metallic bond provides top sites and bridge sites for enhancing the electrical conductivity and active area,which also promotes intermediate H*adsorption and the dissociation of H2O.The reasonable utilization of active sites reduced the HER overpotential.(2)Aiming at the problems of high overpotential,poor mass/charge transfer,and insufficient active area of EC-HER under industrial current density(>200 mA cm-2),this work combined the morphology and architecture control at the microscopic level with the electronic interaction at the atomic scale.The Ce-doping and CoP/Ni3P hybridization synergistically activate the catalyst to design a nanosheet-like,threedimensional Ce0.20-CoP/Ni3P@NF electrode for EC-HER at high current density.The unique morphology and structure enlarge the contact area between the electrode and electrolyte,enhancing mass transfer and active sites exposure.The electrode requires overpotentials(77500)of only 185 mV,162 mV,and 406 mV to drive-500 mA cm-2 of EC-HER under alkaline,acidic,and neutral conditions,respectively.Moreover,the overall water splitting employing Ce0.2-CoP/Ni3P@NF as cathode can reach 500 mA cm-2 with only a cell voltage of 1.775 V under quasi-industrial conditions of 25 wt%KOH electrolyte and 50℃.XPS and DFT calculations show that the charge redistribution induced by Ce-doping and the electronic interaction of CoP/Ni3P hybridization synergistically activate the electronic structure of the catalyst,leading to enhanced conductivity and a large active area.Moreover,the downshift of the d-band center(εd=-2.0 eV)optimizes the energy barriers of intermediate H*/H2 desorption(ΔGH*=-0.11 eV)and H2O dissociation(ΔGTs2==1.09 eV),thereby reducing the HER overpotential at large current density.(3)To resolve the PEC-HER problems of high overpotential,severe electron-hole recombination,and slow charge transfer kinetics at the photoelectrode/electrolyte interface,a hierarchical Ru@MoS2 heterostructure HER catalyst was designed on the light-absorbing substrate of n+p-Si/Ti surface via a metal-organic vapor deposition method,to enlarge the photoelectrode/electrolyte contact area and enhance mass transfer.The electronic interaction between Ru@MoS2 and n+p-Si/Ti substrate enhances the band bending of the photoelectrode,promotes electron-hole separation,and accelerates the interfacial charge transfer.Therefore,the n+p-Si/Ru@MoS2 photocathode achieves photocurrent density of j0=-43 mA cm-2 at 0 VRHE,solar-tohydrogen conversion efficiency of HC-STH=7.28%,and stability for 20 h.The XPS and DFT calculations demonstrate that the charge transfer caused by the difference in work function of Ru/MoS2 enhances the electron transport properties and active area,which also downshifts the d-band center of Ru@MoS2(εd=-1.688 eV)for the optimization of H*/H2 desorption energy barrier,thereby reducing the HER overpotential and improving the j0 value and STH efficiency.(4)To further reduce the overpotential caused by the high H*binding intensity at the Ru site of the heterostructural interface,a more thermodynamically neutral and stable heterostructure HER catalysts of RuSe2@WO3 was designed by chemical vapor deposition using n+p-Si/Ti as the light-absorbing substrate.The charge redistribution between the uniformly dispersed RuSe2@WO3 catalyst and the n+p-Si/Ti substrate optimizes the bandgap and band structure of the photoelectrode,which promotes electron-hole separation.The n+p-Si/Ti/WO3@RuSe2 photocathode achieves j0=-36 mA cm-2,HC-STH=9.43%,and stability for 60 h.The XPS and DFT calculations demonstrate that the charge redistribution induced by the electronic interaction between RuSe2 and WO3 increased conductivity and active area,which also upshifts the d-band center(εd=-1.688 eV)of RuSe2@WO3 for the reduction of H*adsorption energy barrier and accelerates the HER kinetics at the photoelectrode/electrolyte interface.Thereby,WO3@RuSe2 reduced the HER overpotential and improved the STH efficiency and stability.In this work,efficient transition metal-based HER catalysts and electrodes are designed based on solar-driven water splitting for HER.To reduce the HER overpotential,the electronic structure of the catalyst and the energy barrier of the intermediates are systematically engineered to increase the active area and improve the electron/mass transport properties at the electrode/electrolyte interface.The methods further suppress the electron-hole recombination and improve STH efficiency under PEC conditions.The research results provide theoretical methods and ideas for designing HER catalysts and constructing EC-HER and PEC-HER systems.