Electrochemical Synthesis And Electrocatalytic Properties Of Non-Noble Transition Metal-Based Nanomaterials

Energy crisis and environmental pollution have posed challenges to human survival and development.Based on the strategic needs of sustainable development,human demand for energy is trying to transit from the traditional fossil energy to the new energy source.As a result,vigorous development of clean new energy technology is the inevitable choice of the world.As two green electrochemical energy conversion and production technology,fuel cell and hydrogen evolution based on electrochemical water splitting,are considered to be the effective ways to solve above problem.A lot of work has been done and thus a series of progress has been made.At present,the main problem that restrict the commercial application of direct methanol fuel cell and electrochemical water splitting is the dependence of precious metal Pt and RuO₂/IrO₂electrocatalytic materials with high cost.Therefore,the exploration,design and development of non-noble metal electrocatalytic materials with rapid methanol oxidation and water splitting kinetics are the main targets in this field.Combined with the new trends of the current electrocatalytic materials research,the object of the dissertation is to develop highly efficient and long-time stable non-noble metal electrocatalytic materials.The electrocatalytic performance of several non-noble metal based nanostructured materials was systematically studied,to explore their potential application value in the field of electrochemical energy conversion and production.Non-expensive transition metal based materials such as Ni-Co alloy,Ni-Cu alloy and CoS are selected as the research object in the dissertation.Through the way by adjusting the experimental parameters in the electrochemical deposition process,the composition,morphology,crystal structure,electronic structure and electrocatalytic performance of the obtained nanostructured materials were optimized.In general,the main content of the dissertation includes two parts:?1?In the context of anode reaction in direct methanol fuel cell,we chosen non-expensive transition metal Ni-based electrocatalytic materials for methanol oxidation.Ti supported bimetallic Ni-Co and Ni-Cu alloy nanomaterials were obtained by electrochemical deposition method.We systematically studied the methanol electrocatalytic oxidation properties of these obtained Ni-based alloy electrocatalysts and analyzed the mechanism of alloying for the electrocatalytic oxidation performance enhancement;?2?In the context of electrochemical water splitting,non-expensive transition metal compound Co S was chosen as the research object,Ni foam supported metal Ni doped Ni0.1Co0.9S nanosheets and metal Ni and nonmetal P co-doped Ni0.1Co0.9S|P nanosheets with higher electrocatalytic activity than intrinsic material CoS were obtained by electrochemical deposition and high temperature calcination method,respectively.After systematically characterized and tested of these two doped bifunctional electrocatalysts for overall water splitting,we explored the principle and practical effect of these two kinds of doping approach.The main research contents and conclusions are as follows:?1?Based on the principle that diffusion process controlled electrochemical deposition induced by large deposition current density favor the growth and crystallization of porous nanostructures,we prepared a series of bimetallic Ni_mCo_n?m+n=4?alloy micro/nano-materials with different Ni/Co atomic ratio.Due to the introduction of the hexagonal closest packed Co,the surface morphology and crystal structure of the Ni_mCo_n alloy micro/nano-materials are obviously changed with the increase of Co atom percentage.When the Ni/Co atomic ratio is 1:1,the Ni₂Co₂ alloy micro/nano-material is composed of many construction units with a‘holothurian'-like surface and micro/nano array cross-sectional morphology which are randomly distributed.The Ni₂Co₂ micro/nano-material has a large electrochemical active surface and surface redox Ni?II?/Ni?III?coverage due to the porous structure.Theoretical study based on density functional theory reveals that the Ni₂Co₂ alloy cluster with a Ni/Co atomic ratio of 1:1 has lowest adsorption energy of?CO?_ads,thus results in reduced poisoning of the Ni₂Co₂ electrocatalyst caused by?CO?_ads.Ni₂Co₂ alloy electrocatalyst shows the best methanol electrocatalytic oxidation activity and stability resulted from the interaction among electrochemical active surface,surface redox Ni?II?/Ni?III?coverage and adsorption energy of?CO?_ads.?2?Based on diffusion process controlled electrochemical deposition,a series of hierarchically structured Ni_xCu_y?x+y=1?alloy networks with different Ni/Cu atomic ratio were crafted by a viable hydrogen evolution-assisted electrodeposition route.The hydrogen evolution-assisted electrochemical deposition controlled by the diffusion process,resulting in the formation of the highly branched Ni_xCu_y nanostructures.The surface morphology and crystal structure of the obtained Ni_xCu_y alloys are very similar due to both Ni and Cu with the face centered cubic structure in the alloys.In addition,the crystal structure of Ni_xCu_y alloy catalyst shows?111?orientation growth.The Ni_0.75Cu_0.25 alloy electrocatalyst with the Ni/Cu atomic ratio of 3:1 shows the fastest electrochemical response.As anode electrocatalyst for methanol oxidation reaction in alkaline solution,Ni_0.75Cu_0.25 alloy electrocatalyst shows the highest oxidation peak current density(140 mA cm^-2)and a good long-time stability.?3?Through the electrochemical deposition route,we obtained the Ni doped bifunctional Ni0.1Co0.9S nanosheets electrocatalyst and Ni0.1Co0.9S@Ni Foam self-supported electrode for electrochemical overall water splitting.After metallic Ni doping,Ni0.1Co0.9S electrocatalyst shows both improved HER and OER activity.In an alkaline solution,Ni0.1Co0.9S electrocatalyst give only 108 mV and 287 mV for HER and OER,respectively,when current density reaching 10 mA cm^-2,significantly lower than CoS electrocatalyst which with 133 mV and 320 mV.Then,the self-supported Ni0.1Co0.9S@Ni Foam electrodes were used for electrochemical overall water splitting.Under a cell voltage of only 1.61 V,the current density of electrochemical overall water splitting can reach 10 m A cm^-2.?4?Based on the combination of electrochemical deposition with high temperature phosphating,we obtained the Ni and P co-doped bifunctional Ni0.1Co0.9S|P nanosheets electrocatalyst and Ni0.1Co0.9S|P@Ni Foam self-supported electrode for electrochemical overall water splitting.After metallic Ni and non-metallic P co-doping,Ni0.1Co0.9S|P electrocatalyst shows both improved HER and OER activity compared with Ni0.1Co0.9S electrocatalyst.In an alkaline solution,Ni0.1Co0.9S|P electrocatalyst give only 83 mV and 260 mV for HER and OER,respectively,when current density reaching 10 mA cm^-2.Then,the self-supported Ni0.1Co0.9S|P@Ni Foam electrodes were used for electrochemical overall water splitting.Under a cell voltage of only 1.57 V,the current density of electrochemical overall water splitting can reach 10 mA cm^-2.The study in the dissertation could provide some references to develop efficient and new electrocatalytic materials,and inspire relative research fields to explore new theory and direction.