Design And Synthesis Of Bi-Component Electrocatalysts For Electrochemical Water Splitting

Converting the electricity produced from renewable energy sources like solar energy,wind energy and hydroenergy into storable chemical fuel is beneficial for the integration of renewable energy into present energy system.Hydrogen energy is an ideal energy carrier due to its diverse sources,zero carbon emission in utilization and high energy density.Electrolysis of water is a promising technique for the hydrogen production because of its high compatibility with renewable energy electricity and potential of achieving decarbonation throughout the whole process.However,the large-scale commercialization of water electrolysis technique is hindered by its high cost,mainly resulting from the intensive energy consumption during operation,and the high cost of noble-metal-based catalysts.To this end,the development of non-precious hydrogen-evolving and oxygen-evolving catalysts with high activity and durability is of great significance,which can enhance the water electrolysis efficiency and lower the energy consumption and catalyst cost.Based on the above considerations,the works in this thesis focus on the development of novel catalysts,the controllable synthesis of catalysts with specific composition and structure,the study of the relationship between catalyst structure and catalytic activity,and the machanitic study of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.The five aspects of works are described in the following:（1）Based on the synergitic catalytic effect of bimetallic composition and the effect of regulating metal oxidation state on the catalytic activity,a novel bimetallic Co₂Mo₃O₈suboxide was developed as the HER catalyst.Co₂Mo₃O₈was synthesized via the reduction of Co Mo O₄·0.9H₂O precursor under high-temperature H₂ atomosphere.In-situ grown on Ni foam（NF）substrate,the fabricated Co₂Mo₃O₈/NF electrode exhibited high alkaline HER activity in electrocatalytic measurements,requiring an overpotential of 85m V to achieve a current density of 10 m A cm^-2.To improve the Co₂Mo₃O₈electrode conductivity and enable more catalytically active sites,Co₂Mo₃O₈/Co/NF with 3D hierarchical structure was designed and synthesized by the reduction of Co Mo O₄·0.9H₂O/Co₃O₄/NF precursor under high-temperature H₂ atomosphere.The internal Co nanowires can facilitate the charge transfer during catalytic process and serve as the nucleation sites for the growth of abundant Co₂Mo₃O₈active sites.Benefiting from the above aspects,Co₂Mo₃O₈/Co/NF exhibited better HER catalytic activity than Co₂Mo₃O₈/NF,with only an overpotential of about 50 m V to affort a current density of10 m A cm^-2.（2）In order to simultaneously lower the energy barrier of water dissociation and adsorption/desorption of hydrogen intermediate,an in-situ electrochemical surface modification method was developed to fabricate Co-Mo_（18%）/Co（OH）₂composite catalyst toward alkaline hydrogen evolution reaction（HER）.The Co-Mo alloys were prepared by the cathodic electrodeposition under large current density.The hydrogen gas generated in the electrodeposition process played a pore-forming role and resulted in the formation of porous nanostructure.Among the Co-Mo series alloys,Co-Mo_（18%）exhibited the highest HER activity,and this is originated from the modulation of electronic structure by the Co/Mo atomic ratio.In the study of relationship between catalyst structure/composition and catalytic activity,it was revealed that Co-Mo alloy during long-term electrolysis test evolved into an amorphous Co（OH）₂surface,which further enhanced the HER activity.Combined with kinetic studies,a bifunctional HER mechanism was proposed:the in-situ formed amorphous Co（OH）₂on the alloy surface has higher oxophicility,which is beneficial for the adsorption of water and its dissociation;Meanwhile,the adjacent Co-Mo alloy sites can stabilize the hydrogen intermediates and promote the hydrogen release.（3）The controllable synthesis of catalysts with specific structure and morphorlogy can be assisted by hard-template and soft-template methods.However,the template material has to be removed when using a hard-template method;And soft-template has relatively poorer control in the catalyst morphorlogy.In this work,a self-templing method was developed for the construction of Co M_xO_y/Co₃O₄/NF（M=Mo,W,V）nanoarrays with 3D hierarchical structure.In the synthesis process,the Co₃O₄ nanowires as the self-template partially dissolved under weakly acidic condition and the Co²⁺ions released reacted with Mo O₄^2-/WO₄^2-/VO^3-to form Co M_xO_y precipitates which redeposited on the surface of residual Co₃O₄ nanowires.Electrocatalytic measurements showed that Co M_xO_y/Co₃O₄/NF has excellent alkaline OER activity,even superior to commercial Ru O₂catalyst.Almost no activity degradation was observed during long-term stability tests,while the formation of oxyhydroxide species were observed in structure/composition characterizations.This oxyhydroxide as the actual active site participated in the OER catalysis.（4）The understanding in the local environments of Fe sites and how Fe structure affect the OER activity is still insufficient and under debate.In this work,an experimental technique involving the spiking of Fe³⁺ions into the KOH electrolyte and cyclic voltammetry（CV）measurement was developed.It is found that the introduced Fe sites in the initial voltammetry cycles can dramatically enhance the OER activity of Ni O_xH_yand Co O_xH_y films,with the electrochemical properties of host material almost unaffected,such as the position and integrated area of redox peaks.More Fe species were incorporated during the subsequent voltammetry cycles and the Ni redox peaks shifted anodic.This process is,however,accompanied by the moderate increase in the OER current.The above results give the experimental support fot the view that there are at least two types of Fe sites existing in the host materials,and the correlation between the local structure of Fe species and the OER activity was proposed:one type of Fe species specially adsorbed on the surface of host materials,such as the surface defect sites or high-index planes,showing superior OER activity and limited influence in the electronic structure of host materials,and anothod type of Fe species within the lattice of host materials,showing relatively lower activity but prominent influence in the electronic structure of host materials.（5）To further study the OER catalytic activity and reaction mechanism of surface Fe species on transition-metal hydroxides,a chronoamperometry（CA）based experimental technique was developed,and introduced Fe species can be successfully confined to the specific surface sites of Ni O_xH_yand Co O_xH_y.Based on this experimental technique,the intrinsic OER activity of surface Fe sites was determined by the calculation of turnover frequency(TOF_Fe).The results reveal that the intrinsic activity of Fe increases with its accumulation on the surface of Ni O_xH_yand Co O_xH_y.Besides,the Fe sites on the surface of Ni OOH has prominently higher OER activity compared to those located on Co OOH surface,suggesting the critical role of electronic interaction between surface Fe species and host materials in affecting the catalytic activity.The results of DFT-based theoretical calculations reveal the significant correlation between the cooperative oxidative and catalytic mechanism upon surface Fe dimers and the high OER activity.