Structural Design Of Nanoarray And Electrocatalytic Research

As an efficient,green,sustainable energy resource,hydrogen energy has been widely viewed as promising alternatives for fossil fuels to address the issues of energy shortage and environmental pollutions.In the aspect of hydrogen production,electrolytic water method can directly convert electric energy into hydrogen which has the advantages of environmental friendly and high hydrogen purity.It is an ideal hydrogen production technology.However,the two half-reactions of overall water splitting,those are,the anodic oxygen evolution reaction（OER）and the cathodic hydrogen precipitation reaction（HER）,suffering from slow kinetics.Electrocatalysts are often required to accelerate the two half-reactions for improving the efficiency of overall water splitting.The precious metals Pt-based and Ru/Ir-based catalysts have excellent HER and OER activity respectively,the rare and high price of precious metals greatly limit their application in practical electrolytic water.Therefore,the development of low-cost and efficient non-precious metal-based electrocatalysts is the focus of current research.Recent studies have shown that the construction of nano-array materials on conductive substrates is an effective way to prepare high-performance catalysts for water splitting.Compared with ordinary powder materials,nano-array electrocatalysts have the following advantages:（1）directly growing on conductive substrate can reduce contact resistance and satisfy the requirement of fast charge transfer in electrocatalytic process;（2）the large gap between nanoarrays can not only provide sufficient channels for electrolyte mass transfer,but also increase the specific surface area and the number of active sites;（3）the nano-array material also has high stability,which can guarantee the long-term and sustainable electrocatalytic reaction.Therefore,in this thesis,the nano-array material is taken as the research object,and the chemical composition and structure of the nano-array materials are regulated to improve the electrolytic water performance of the nano-array material,and to explore the composition of the nano-array material.The relationship between the structure and the catalytic performance provides a certain method guidance and theoretical basis for the construction of a new type of efficient and low-cost electrolytic water catalyst.The main contents of the study are as follows:（1）the oxygen-rich titanium dioxide（TiO₂）shell was modified on the surface of porous bimetallic phosphates nanoarray by surface modification,and the effect of TiO₂shell on alkaline HER reaction activity was studied.The results show that the abundant oxygen defects on the surface of TiO₂ can greatly improve the ability of catalyst to adsorb and dissociate water,and thus accelerate the slow kinetic Volmer step in HER reaction to produce more active hydrogen.At the same time,porous（Co,Ni）₂P nanoparticles with large surface area can provide sufficient active sites for adsorption and recombination of active hydrogen to produce hydrogen.Due to the synergistic effect between the porous（Co,Ni）₂P nanoparticles and the defective TiO₂ shell,the electrocatalyst has higher HER activity and faster reaction kinetics:the overpotential with current density of 10 mA/cm² is only 92 mV,.Tafel slope is 49 mV/dec.（2）The alkaline HER activity of metal hydroxide nanowires array was enhanced by the regulation of electronic structure.By using the difference of precipitation constants,phosphate ions were modified on the surface of porous CoNi-hydroxide nanowires arrays by ion exchange method.It was found that the modification of phosphate radical could effectively regulate the electronic structure of Co²⁺and Ni²⁺.Therefore,the ability of catalyst to adsorb and dissociate water is improved.Moreover,the porous structure on nanowires can not only improve the mass transfer in HER reaction,but also provide more active sites for HER reaction.Due to the unique composition of the catalyst and the favorable nano-structure,the prepared P-CoNi（OH）_x exhibited excellent electrocatalytic properties under alkaline conditions:A current density of 10 mA/cm² overpotential is 39 mV,Tafel slope is 31.4 mV/dec,ultra-high stability（more than 20 h）.（3）The ultrathin metal sulfide（thickness:2.2 nm）composite MnO₂ nano-array was constructed by in-situ oxidation-reduction method,and its completely electrolytic water performance was studied.The process are as follows:Firstly,CoNi-hydroxide nanoarray was synthesized.Although the array has certain electrocatalytic performance,it has higher overpotential and Tafel slope than commercial Pt/C.In order to reduce the overpotential,further optimization has been carried out.Metal hydroxides are converted to metal sulfides by hydrothermal vulcanization,and then reduced by spontaneous redox reaction with potassium permanganate solution to reduce the thickness of the sulfides and to modify the surface of the sulphide nanoparticles with MnO₂ NPs.The prepared nano-array material is used as anode and cathode to set up the complete electrolysis water device.It is found that the prepared array material has excellent performance of complete electrolysis water,and the cell voltage with current density of 10 mA/cm² is only 1.51 V.Further studies show that the reduction of the thickness of sulfide nanoparticles can greatly increase the surface area of the catalyst and thus increase the number of surface active sites of the catalyst,so that the electrocatalytic activity can be enhanced.