Preparation And Evaluation Of Electrochemical Catalysts For Non-noble Metal Compounds

While enjoying the achievements of technological progress and industrial development,we are facing an increasingly serious energy crisis and environmental degradation caused by the consumption of traditional fossil energy.It concerns the fate of humankind that developing the noval renewable and clean energy technologies,optimizing the energy structure,and maintaining sustainablem development.Among them,electrochemical energy conversion and storage devices,with their advantages of high theoretical energy density,high conversion efficiency and environmental friendliness,have attracted many attention and yield substantial results.Fuel cells and metal-air cells,in particular,are considered to be the most promising next-generation electrochemical energy conversion and storage technologies.However,as the key reactions of such electrochemical devices,oxygen reduction reaction(ORR)and oxygen precipitation reaction(OER)occurring on the oxygen electrode with a multi-electron transfer process,complex reaction mechanism and sluggish kinetic,which seriously limits the working efficiency of fuel cells and metal-air cells.Many researches show that catalysts can reduce the energy barrier and accelerating the reaction.And the precious metal materials based on Pt,Ir and Ru are commonly used for ORR/OER catalysts.Although these catalysts exhibite excellent activity,they cost expensive and yield poor stability,which extremely affect the industrial layout of fuel cells and metal-air cells.Therefore,it is of great practical significance to develop non-noble metal-based catalysts with satisfying activity and stability that will eliminate dependence on precious metal and promote the reform of energy structure.The catalytic activity of non-noble metals is far less than that of Pt,Ir and Ru materials,but their compounds show enhanced catalytic activity due to the optimization of electronic structure,synergistic effect with other elements.Moreover,the non-noble metal compounds generally show good stability in electrolyte environment.Among these non-noble metal compounds,their nitrides and oxides(e.g.spinel and perovskite structures)which have been widely used in other fields,and exhibite excellent ORR and OER catalytic activity.In addition,for the non-noble metal nitrides and oxides,they are facile for preparation with pollution-free,and the raw materials are abundant with a low cost,resulting in worldwide research attention as the most promising candidate for substituting the catalysts based on precious metals.It is of great research value that further improve their catalytic activity with the facile strategies,such as designing structure,controlling morphology,adjusting electronic structure.Therefore,the dissertation devotes to improve the ORR/OER catalytic property of several non-noble metal nitrides and oxides with spinel/perosikite structure through compositing with carbon materials,doping active elements and morphology and structure design.The research in this thesis was carried out as follows:(1)Graphite carbon enclosed non-noble metal nitrides nanoparticles.In this part,the novel C-TiN and C-AlN nanocomposites of TiN or AlN nanoparticles enclosed by graphite carbon are successfully synthesized via high temperature calcination method employing their corresponding oxcides(TiO2 and Al2O3)and melamine as raw materials under N2 atmosphere.Their catalytic activity,stability and methanol tolerance ability towards ORR are further studied.The results demonstrate that the oxides are completely nitrified to form corresponding nitride compounds,while the melamine is converted into graphitized carbon,coating on the surface of nitrides nanoparticles.The graphite carbon coating not only improves the ORR catalytic activity of metal nitrides,but also greatly improves the stability and methanol tolerance ability due to the coating of graphite carbon that isolates metal nitrides and complex electrolyte environment.The onset potential of C-AlN is 0.82 V,which is more positive than that of AlN(0.76 V).And the current density of C-AlN still remains 91.4%of its initial current during 10,000 s chronoamperometry test,while AlN only remains 70.4%.(2)Carbon nanotubes supported spinel oxides nanoparticles.In this part,carbon nanotubes(CNTs)are selected as the substrate to improve the catalyst's conductivity and dispersion,which is directly related to the charge transfer and active site exposure.Generally,metals and metal oxides are difficult to adhere to the surface of CNTs,because of its certain chemical inertia.In this reaserch,employing hydroxylated carbon nanotubes(CNTs-OH)as the carrier,Co-Mn spinel nanoparticles are grown in situ on the surface of CNTs.It is attributed to the surface of CNTs-OH is rich in hydroxyl functional groups,which provide enough active sites for the nucleation of Co-Mn complexus.And end up with,cubic MnCo2O4 and tetragonal CoMn2O4 nanoparticles with particle size of about 4-5 nm anchored on CNTs dispersedly.The ORR catalytic activity of the composite was significantly improved.Cubic MnCo2O4-CNTs showed the best performance,that its onset potential is 1.03 V,which is consistent with that of commercial Pt/C.And the current density of MnCo2O4-CNTs still remains 95.76%of its initial current during 10,000 s chronoamperometry test.Moreover,MnCo2O4-CNTs yield the optimal OER catalytic activity,exhibiting the potential for bi-functional catalyst.(3)Hierarchically porous flower-like spinel oxide assembled with ultrathin nanosheets.In this part,hierarchically porous flower-like MgMn2O4,which assembled with ultrathin nanosheets,have been synthesized though solvothermal process followed by calcination.Taking advantage of this unique morphology,the specific surface area of the catalyst increases,which exposing more active sites,promoting the contact between the catalyst and active substances in the electrolyte,and increasing the electron transport.Furthermore,the introduction of Mg2+ions causes the transformation of cubic Mn2O3 to tetragonal spinel MgMn2O4,which adjusts the electronic structure of Mn3+ions.It is confimed that,the hierarchically porous MgMn2O4 exhibit more positive onset potential and enhanced current density than the MgMn2O4 blocks and irregular sheets.In addition,their ORR catalytic performance is better than the pure manganese oxide(Mn2O3)which is prepared through the same method.The onset potential of h-MgMn2O4 is about 1.02 V,which is far more positive than that of b-MgMn2O4(0.78 V)and i-MgMn2O4(0.85 V).And the current density of h-MgMn2O4 still remains 91.7%of its initial current during 10,000 s chronoamperometry test.(4)B-site doped perovskite oxide porous cubic.Perovskite oxides LaNiO3 and Co doped LaNi1-xCoxO3(x=0.1,0.2,0.3,0.4)porous cubic were prepared by calcining the precursor.Benefitting from the glycine,which working as complexing agent and pore making agent,the LaNi1-xCoxO3 porous cube are obtained without lattice deformation.In the perovskite oxide,Co3+ ions drop into the B-site,partially replacing Ni3+ ions.And the perovskite LaNi1-xCoxO3 has rhombohedral structure.The results indicate that,the ORR and OER catalytic activity of LaNi1-xCoxO3 porous cube enhances at the early stage and then reduces with the increase of Co content.The sample of LaNi0.8Co0.2O3(x=0.2)obtained the best catalytic activity,and its ORR proceed via a quasi 4-electron pathway,with the onset potential of 0.94 V vs.RHE.Inaddition,the OER overpotential of LaNi0.8Co0.2O3 is 274 mV(at 10 mA cm-2).