Preparation And Electrocatalytic Properties Of Polyoxometalate Derived And Composite Materials

Since the beginning of 21st century,with the rapid population growth and the booming economy,the global energy consumption has grown dramatically.With severe environmental pollution and energy shortages,we are always striving to explore low-emission and clean energy.H2 is considered as a perspective renewable resource to substitute fossil fuels due to its high energy density as well as zero poisonous and green house gases emission.Hydrogen production from electrolyzed water has two important reaction processes:hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).Under the action of the HER electrocatalyst,a hydrogen evolution reaction occurs on the anode(HER,2H++2e→H2),and H+is reduced to form H2.At the same time,under the action of a suitable OER electrocatalyst,the cathode undergoes an oxygen evolution reaction(OER,2H2O+4e→4H++2O2),O2 is formed at the cathode.Electrocatalyst play an important role in increasing the efficiency of these reaction processes.Finding the right high-efficiency catalyst has become a major problem in the research of electrolyzed water hydrogen and metal-air batteries.This thesis consists of three parts:designing and preparing four kinds of Mo-based and Co-based electrocatalysts based on Polyoxometalate(POM),characterizing their structures.Their HERs,OERs and ORRs were also studied.In the first part:We selected Zn(OAc)2·2H2O,MoO3 and H3PO4 to synthesized polyoxometallate.{Zn0.5[Mo6O12(OH)3(HPO4)3(PO4)][Zn5(PO4)2(dpphen)3(H2O)4]}·3H2O(ZnP4Mo6,dpphen=4,7-diphenyl-1,10-phenanthroline),a highly reduced POM compound was synthesized.With ZnP4Mo6 as precursor,a MoP based electrocatalyst(MoP/PNC)was synthesized successfully.In MoP/PNC,MoP particles with the size about 30 to 50 nm disperse homogenously in P,N co-doped mesoporous carbon carrier.MoP/PNC exhibits promising HER activity with low overpotential.Its stability is also attractive,which tolerates long time and multi-cycle experiments.With MoP/PNC as cathode active material and Sn slice as anode,Sn-H+ battery was assembled.This battery can deliver open circuit voltage achieves 0.83 V with peak power density 53.5 mW·cm-2.Besides electricity production,this battery can also produce H2 on cathode with Faradic efficiency 95.7%and H2 generation rate 179 μmol·h-1.Sn-H+ battery can generate both hydrogen and electricity.In the second part,we synthesized the POM based compound of P4Mo6 type under hydrothermal reaction.According to different nitrogrnous ligend,BPP and BPE,two CoP4Mo6 based compounds were synthesized.CoMoP type bimetallic electrocatalysts were synthesized by high temperature calcination in a tube furnace,all of which were doped with nitrogen and carbon.For CoMoP@NC.For HER,in 1 M KOH,CoMoP@NC requires only an overpotential of 80 mV to obtain a current density of 10 mA·cm-2.For OER,in 1 M KOH,CoMoP@NC requires only 81 mV to obtain a current density of 10 mA·cm-2.CoMoP@NC also has excellent stability under the current.It remains stable after 1000 cycles and 12 hours of long-term testing.We have established an electrolytic cell with CoMoP@NC(A)as the cathode and anode.Unde the current density of 10 mA·cm-2,only 1.85 V is required to obtain the current.CoMoP@NC(A)can be used to decompose water with a 1.75 V battery.We hope that CoMoP@NC(A)electrocatalyst can be used as a new material to achieve overall water splitting.In the third part,a flaky shaped Co-Nx based electrocatalyst,Co-Nx/Co@NMC had been synthesized through the calcination of 2D MOF directly.In Co-Nx/Co@NMC,Co-Nx and metallic Co particles with the size about 15-20 nm reside homogenously in N-doped mesoporous carbon matrix.Co-Nx/Co@NMC possesses excellent ORR and OER activities in acidic electrolyte.In ORR,it exhibits a four-electron route with the electron transfer number in the range of 3.92-3.98.In OER,to obtain 10 mA·cm-2,the overpotential is merely 328 mV.With Co-Nx/Co@NMC as air cathode material,an efficient asymmetric Zn-air battery is constructed.The open circuit voltage of this asymmetric Zn-air battery is 2.16 V,with peak power density 300 mW·cm-2.Furthermore,it exhibits excellent charge-discharge stability,which keeps stable for 15 h with current density is 20 mA·cm-2.We can believe 2D MOF derived Co-Nx based electrocatalyst is an ideal air cathode to assemble asymmetric Zn-air battery with high voltage and power density.