Structure Design And Performance Regulation Of Catalysts For Electrochemical Water Splitting

Accompanied by the rapid development of the global economy,the energy shortage and environmental pollution problems are being increasingly prominent.At present,the rapidly diminishing reserves of fossil fuels and resulting global climate change and environmental pollution problems by their combustion make the current energy structure face a significant challenge.The exploitation of renewable and clean energy is crucial for the current energy system to get out of a tight corner.But their intermittent and uncertainty characteristics trap the integration to public power systems.Efficient energy storage/conversion systems have been regarded as one of the most efficient approaches to regulate the output of electricity.Among the various energy storage systems,hydrogen is generally considered as an important energy carrier in future energy systems because of its high energy density,clean and zero-pollution.Compared with traditional hydrogen source from industrial by-products,electrochemical water splitting driven by renewable energy has many advantages,such as high hydrogen purity and environmental friendliness.However,due to the higher cost of hydrogen production from electrochemical water splitting,the proportion of the produced hydrogen in the total global hydrogen production does not exceed 5%.An efficient electrocatalyst for water splitting is essential to cut down unnecessary energy consumption and increase the efficiency of conversion of electrical energy to hydrogen chemical energy,and then obtain cheap hydrogen gas.To date,great efforts have been devoted to the development of highly efficient electrocatalysts for water splitting.However,most of the reported electrocatalysts still face high overpotential and poor stability,which cannot meet the needs of industrial production.Therefore,it is urgent to develop new catalyst synthesis methods and regulation strategies to optimize the electronic structure of catalyst active sites,accelerate the adsorption/desorption process of reaction intermediates on the surface of catalysts,and improve the catalytic performance of catalysts.In view of this,the electronic structure of the active sites of catalysts is modulated to enhance their electrochemical water splitting performance through synergistic effect of multiple sites,element doping,heterostructure engineering,and strain engineering in this dissertation.Meanwhile,advanced characterization techniques combined with theoretical calculations are employed to deeply study the reaction mechanism of catalysts and reveal the structure-activity relationship between structural features of catalysts and catalytic activity,which provides reference and inspiration for the development of highly efficient water splitting catalysts.This dissertation content specifically as follows:（1）Based on synergistic effect of multiple sites,we constructed catalysts comprising ruthenium single-atom（Ru1）and nanoparticle（Run）loaded on nitrogendoped carbon substrate（denoted as Ru1-Run/CN）.Benefitting from the synergistic effect between single-atom and nanoparticle,Ru1-Run/CN exhibits excellent catalytic performance for neutral hydrogen evolution reaction（HER）,superior to most previously reported catalysts.Specifically,Ru1-Run/CN catalyst exhibits a very low overpotential down to 32 mV at a current density of 10 mA cm^-2,while maintaining excellent stability up to 700 h at a current density of 20 mA cm^-2 during the long-term test.To reveal the details of the synergistic effect,we constructed atomic structure models of single atoms and nanoparticles and calculated the free energy profiles of neutral HER.Computational calculations reveal that,nanoparticles and single atom coexisting systems exhibit a lower free energy barrier for H2O splitting than isolated single-atom and nanoparticle,in which the existence of nanoparticle optimize the interactions between single-atom site and reactants.（2）Based on the cation doping strategy,we designed and fabricate Ce doped amorphous NiFe hydroxide catalysts（denoted as Ce-NiFe）.The catalyst enables high activity and outstanding stability toward alkaline oxygen evolution reaction（OER）,significantly better than that of undoped NiFe hydroxide（denoted as NiFe）.The overpotential of Ce-NiFe is 195 mV at 10 mA cm^-2.Meanwhile,the durability of the Ce-NiFe is maintained 300 h at 100 mA cm^-2.Using a variety of structural characterization techniques,we compared the structural features of Ce-NiFe and NiFe to reveal the effect of Ce doping on the NiFe.On the one hand,Ce doping breaks the long-range ordered structure of NiFe catalyst,so that Ce-NiFe exhibits amorphous characteristic,which is beneficial to expose more highly active sites.On the other hand,Ce-doping increases the oxidation states of active sites（Ni and Fe）,which is favorable for the OER process.（3）Base on the single atom doping strategy,we incorporated atomically dispersed Ir atoms into the nanosheet spinel Co₃O₄ lattice（denoted as Ir1-Co₃O₄-NS）.Ir1-Co₃O₄NS,as an efficient acidic OER catalyst,significantly lowers the overpotential down to 226 mV at 10 mA cm^-2 with an ultrahigh turnover frequency value of 3.15 s-1（η=300 mV）,three orders of magnitude higher than that of commercial IrO2.More importantly,Ir1-Co₃O₄-NS reaches a lifespan of up to 500 h at 10 mA cm^-2 for acidic OER,superior to most previously reported low-Ir catalysts.First-principles calculations reveal that the key*OOH intermediate can be stabilized by the lattice oxygen coordinated to Ir active site via hydrogen bond formation,which substantially regulates the rate-limiting step and lowers the activation free energy of OER process.Therefore,Ir1-Co₃O₄-NS exhibits excellent OER catalytic activity and low overpotential.（4）Based on heterostructure engineering and strain engineering,we constructed RuCo/RuCoO_x core/shell structured catalysts with strained Schottky heterojunction（denoted as sS-RuCoO_x/RuCo）.For acidic OER,sS-RuCoO_x/RuCo required an ultralow overpotential of 170 mV to achieve a current density of 10 mA cm^-2.Remarkably,sS-RuCoO_x/RuCo exhibits an unprecedented durability of 2500 h at 10 mA cm^-2 in 0.5 M H2SO4.Structure analyses combined with theoretical calculations reveal that strain engineering and local charge transfer caused by Schottky heterojunction modulate the electronic structure of active site,endowing an energetically favorable Ru-Co dual-sites reaction pathway and improving the OER catalytic activity of sSRuCoO_x/RuCo.Meanwhile,strained Schottky heterojunction decreases the oxidation state of the Ru active sites and Ru-O covalency,which suppresses the over-oxidation of Ru and participation of lattice oxygen,respectively.Therefore,sS-RuCoO_x/RuCo exhibits excellent durability for acidic OER.