Study On Anion/Cation-doped Non-noble Metal-based Catalyst For Water Electrolysis

Energy crisis and environmental pollution have increasingly restricted the development of global economy,and the exploration new energy has been imminent.Among many new energy sources,the natural-born merits of high energy density,strong renewable ability,carbon-free emission make hydrogen as the reliable choice to replace traditional fossil energy.At present,as a new method of hydrogen production with high benefit,environmental protection and great promise,the progress for acquiring hydrogen by water electrolysis has received extensive attention.In theory,water electrolysis can be achieved by applying a voltage of 1.23 V,and hydrogen is harvested at the cathode by hydrogen evolution reaction（HER）,and oxygen is generated at the anode by oxygen evolution reaction（OER）.However,because of the slow climb of four-electron and the difficult formation of O-O bond and other internal factors,as well as concentration polarization,contact resistance and other external factors,the actual potential of water electrolysis is much higher than that the theoretical potential.It is of great significance to change anodic oxidation type and synthesize high-efficiency,stable and cheap electrocatalyst to improve anodic oxidation kinetics,thus increasing economic benefit and production efficiency of water electrolysis.Urea electrolysis can achieve hydrogen production at the cathode with a smaller reaction potential（0.37 V）and obtain faster anode reaction kinetics,which is more energy saving and environmental protection,while urea oxidation reaction（UOR）still limited by the complex six-electron transfer of the difficult gas release process.Precious metals and their derivatives（Pt,Pd,RuO₂,IrO₂,etc.）have excellent electrocatalytic activity,but their large-scale application is limited by high price and scarce storage.In recent years,anion/cationic doped transition metal compounds have been widely reported as new electrocatalytic materials.Heterogeneous atom doping can bring about certain lattice defects,promoting the generation of more active sites and the enhancement of conductivity,and significantly improve the catalytic activity of materials.Based on the existing research progress,in this paper,the catalysts were designed and synthesized from the perspectives of performance and stability,respectively.On the one hand,nickel-based sulfide with good catalytic performance was doped with metal cation,and the stability of the material was optimized on the basis of activity through composite strategy;on the other hand,cobalt-based oxide with good stability was doped with nonmetal anion,and the activity of the material was optimized on the basis of stability by means of control variable strategy.The catalysts obtained by these two experimental approaches have excellent performance in HER,OER and UOR processes.The main research results of this paper are as follows:（1）Cu-Ni₃S₂/NF-1/4 cluster nanoflowers structure for catalyzing HER process.Cu-Ni₃S₂/NF nanostructures were directly synthesized by a simple two-step hydrothermal method.The experimental results show that heterogeneous element doping has a significant effect on the morphology of the material.After doping with Cu²⁺,the planar Ni₃S₂ cross nanosheets were transformed into 3D cluster nanoflower structures,effectively increasing the specific surface area and active site of the material.These properties endue Cu-Ni₃S₂/NF-1/4with excellent HER catalytic performance(92 mV@10 mA cm^-2)and stability（>12 h）,which shows the outstanding performance in HER catalysts of the same period.Nevertheless,the OER performance and stability of the catalyst are not ideal,so it can only catalyze HER process alone,but cannot as a bifunctional catalyst to catalyze overall water splitting.（2）Cu-Ni₃S₂ nanoflowers structure decorated with ultrathin NiFeLDH（Cu-Ni₃S₂@NiFeLDH）for catalyzing water electrolysis and urea electrolysis process.On the basis of the previous experiment,we combined NiFeLDH with excellent OER catalytic performance and Cu-Ni₃S₂ with excellent HER performance to achieve complementary advantages.Benefiting from the large specific surface area and high electrical conductivity of Cu-Ni₃S₂ nanoflowers,the strong corrosion resistance from NiFeLDH ultrathin nanosheets and the heterojunction effect between the two,Cu-Ni₃S₂@NiFeLDH shows satisfied catalytic activity and stability for both water and urea electrolysis.Among which,Cu-Ni₃S₂@NiFeLDH-100 served as HER catalyst which only needs a overpotential of 95 mV to drive the current density of 10 mA cm^-2;Cu-Ni₃S₂@NiFeLDH-200 served as both OER and UOR catalyst which only requires the potentials of 1.459 V and 1.299 V to drive the current density of 20 mA cm^-2;the resulted two-electrode system of Cu-Ni₃S₂@NiFeLDH-200^（+）//Cu-Ni₃S₂@NiFeLDH-100^（-）can stably catalyze water electrolysis and urea electrolysis for more than 20 h with cell voltages of only1.502 V and 1.413 V,respectively.（3）CeO₂ nanoparticle-dotted Se-doped Co₃O₄ nanoneedle arrays（Se-Co₃O₄@CeO₂/NF, abbreviated as SCCN）for catalyzing overall water splitting process.We firstly synthesized the SCCN nanostructure on the substrate of nickel foam（NF）through three continuous operations of mild hydrothermal,air calcination and selective selenation.In SCCN,Se was selectively doped into Co₃O₄ to form the main body of stable catalysis,and the introduced CeO₂ not only can stimulates Co₃O₄ to produce more oxygen defects,but also combines with Co₃O₄ to generate bimetallic synergistic effect.Density functional theory（DFT）calculation reveals that the real catalytic active center in SCCN is Se-Co₃O₄ rather than CeO₂.Performance test results show that SCCN-1 equips with excellent catalytic activity and stability of HER(48 mV@10 mA cm^-2)and OER(188 mV@10 mA cm^-2).The resulted SCCN-1^（+/-）electrode couple can stably deliver a current density of 10 mA cm^-2 at a cell voltage of 1.49 V for catalyzing overall water splitting,which far more than the similar catalysts recently reported...