Design And Synthesis Of Self-Supported Transition-Metal-Based Electrocatalysts For Water Electrolysis

Developing renewable and eco-friendly energy sources like solar and wind energy is essential to our society due to the sharp deterioration of the environment caused by the excessive use of fossil fuels.While the intermittent nature of these energy sources impedes their continuous electric energy supply.The conversion of chemical energy stored by hydrogen fuel into electricity is an effective approach to solve this thorny issue.Meanwhile,the ever-decreasing electricity prices(average 6.41 cents/kwh for industrial electricity in the United States on Apr 2020,U.S.Energy Information Administration)have stimulated scientific researchers to explore electrocatalysts for the production of hydrogen from water electrolysis.However,the two half reactions of water splitting,namely hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),are thermodynamically unfavorable process that restricted by the high intrinsic activation barriers,thus the water electrolyzers usually operate at a much larger voltage of 1.8-2.0 V relative to the theoretical limit of 1.23 V.The use of efficient electrocatalysts for HER and OER can effectively improve the sluggish kinetics and therefore decrease the overpotential.Up to now,extensive efforts have been devoted to search the suitable self-supported transition-metal-based electrocatalysts such as oxides,hydroxides/oxyhydroxides,sulfides,alloys,etc.However,the activity and durability of most transition-metal-based electrocatalysts remain far from satisfactory.In view of this,we designed and synthesized a series of self-supported transition-metal-based electrocatalysts for efficient,stable and economical water electrolysis from the perspectives of optimizing element composition,adjusting microscopic morphology and improving bonding methods.This dissertation content specifically as follows:1.We synthesized a novel electrocatalyst of NiO@Ni decorated WS2 nanosheet array on carbon cloth(NiO@Ni/WS2/CC)for overall water splitting.WS2/CC was achieved through the sulfuration of the hydrothermally obtained WO3/CC,and then NiO@Ni was coated on WS2/CC via electrodeposition of Ni and subsequently thermal oxidation in the oven.This composite serves as a unique three-dimensional(3D)synergistic electrocatalyst that not only combines the intrinsic properties of individual NiO@Ni and WS2,but also exhibits significantly improved HER and OER activities.This electrocatalyst possesses Pt-like activity for HER and better OER performance than that of RuO2,as well as demonstrating superior long-term durability in alkaline media.Furthermore,it enables an alkaline electrolyzer with 10 mA cm-2 current density at 1.42 V cell voltage.The excellent performance is mainly attributed to the unique 3D configuration and multicomponent synergies among NiO,Ni and WS2.2.We reported the construction of MoNi nanoparticle decorated CoMoO3 cuboid arrays of vertical growth using Ni foam(NF)as the scaffold(MoNi/CoMoO3/NF).Owing to the high catalytic activity of MoNi nanoparticles,the interaction between different phases,the ability to increase the active sites of CoMoO3 cuboids and the synergistic effect on the whole,the MoNi/CoMoO3/NF exhibits high stability and activity with zero onset overpotential and a low Tafel slope of 35mV dec-1,and only 18 mV is needed to afford 10 mA cm-2 in 1 M KOH,which is even better than that of the state-of-the-art Pt/C catalyst.Density functional theory(DFT)calculations demonstrate that the CoMoO3 and MoNi in MoNi/CoMoO3 synergistically enhanced water splitting activity.3.We designed and developed the first type of single-atom Ru based catalyst applied for the HER under pH universal conditions.This binder-free catalyst consists of single Ru atoms anchored to the surface of MoS2 nanosheets array supported by a CC(Ru-MoS2/CC,Ru in Ru-MoS2:0.37 wt%).The abundant dispersed single Ru atoms are highly catalytically active,and the MoS2 array supported 3D porous structure offers more opportunities for active sites and serves as co-catalysts for synergetic catalysis,with accompanying enhanced the electrical conductivity of this array-catalyst.Remarkably,the Ru-MoS2/CC exhibits extremely enhanced catalytic activity compared to that of MoS2/CC and possesses good long-term stability.DFT calculations revealed that the improved catalytic activities stem from the synergistic effect between MoS2 and single-atom Ru.4.We reported here that a straightforward one-pot hydrothermal method involving the site-selective incorporation of Ru or Ir into NiV-LDH leads to the excellent bifunctional catalysts NiVRu-LDH and NiVIr-LDH on NF and discussed the results of comparative studies,and the origin of high HER and OER activities are analyzed at atomic level.X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectroscopy studies reveal synergistic electronic interactions among Ni,V,and Ru(Ir)cations.Raman spectra and Fourier and wavelet transform analyses of the extended X-ray absorption fine structure indicate modulated local coordination environments around the Ni and V cations,and the existence of V vacancies.The Debye-Waller factor suggests a severely distorted octahedral V environment caused by the incorporation of Ru and Ir.DFT calculations further confirm that Ru or Ir doping could optimize the adsorption energy of intermediates in the Volmer and Heyrovsky steps for HER and accelerate the whole kinetic process for OER.5.We combined the Ru-doping of NiCo2O4 spinel(NCO)and the surface modification with Ru nanoclusters through rational design and controllable fabrication(NCRO)as a dual modification method to markedly enhance the overall water splitting.Benefiting from the structure advantages,the synergistic electronic effects and optimal binding strength of the reaction intermediates,the NCRO exhibited excellent performance for both HER and OER in alkaline media.Specifically,it requires only 18 and 138 mV HER overpotential to achieve the current density of 10 and 1000 mA cm-2,respectively,with a small Tafel slope of 22 mV dec-1.Impressively,it can afford 1500 mA cm-2 with an overpotential of 420 mV for OER.As a result,an ultra low cell voltage of 1.45 V is needed at 10 mA cm-2 for overall water splitting.The DFT calculations suggest that the introduction of Ru could enhanced water dissociation ability,optimized the adsorption energy of reaction intermediates and altered the energy level of the d band center.