Activity Regulation Of Carbon-based Electrocatalysts

Since the fifties of the twentieth century,modern people’s life is closely related to energy.The increasing global energy consumption and the environmental pollution caused by fossil fuels and irrefragable make the acceleration of new energy development and utilization.The development of new energy is the key to realize low carbon development and has become the new power of a global energy growth momentum.Metal-air batteries is used light metals as negative active material,oxygen in the air as positive active material,and oxygen reaches gas-liquid-solid three-phase interface through gas diffusion electrode and reacts with metal anode to output electric energy.It has the advantages of high specific energy,stable discharge voltage,low cost,non-toxicity,low pollution,simple structure and so on.It is considered as a new energy with great development and application prospect in the future.Due to the high cost and scarce reserves of precious metals,it is urgent to develop non-precious metal catalysts.In this dissertation,based on the Schiff base reaction of 2,6-diaminopyridine and formaldehyde,a series of non-noble metals-based hybrid materials were designed and synthesized by the direct pyrolysis of the Schiff base microspheres obtained from the reaction of 2,6-diaminopyridine and formaldehyde and transition metal-coordinated Schiff based microspheres,which can be used as efficient electrocatalysts for oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）.The electrochemical properties of the catalysts were characterized by using electrochemical test techniques.Many meaningful results are obtained and summarized as follows.1.A Schiff base microsphere was synthesized by the condensation reaction of 2,6-diaminopyridine and formaldehyde,and subsequent pyrolysis of the Schiff base microsphere at inert atmosphere led to nitrogen-doped carbon microspheres,named NCSs-T.The optimized NCSs-800 electrocatalyst possessed a large specific surface area,high conductivity,and numerous active nitrogen species,and therefore showed excellent electrocatalytic activity for ORR in alkaline medium.The onset potential and half-wave potential and limiting current density of the catalyst toward ORR were estimated as 0.96 V vs.RHE,0.84 V vs.RHE,and 5.5 m A cm^-2,respectively.The assembled zinc-air battery（ZAB）with NCSs-800 as the air cathode presented high power density of 136.1 m W cm^-2 and good rate performance.3.2.To improve the catalytic activity of above-mentioned carbon microspheres,transition metal ions(Fe³⁺,Co²⁺,Ni²⁺)were introduced into the as-obtained Schiff base microspheres through the coordination of metal ions with pyridinyl groups in microspheres.The pyrolysis of transition metals-coordinated Schiff base microspheres resulted in novel Fe and N co-doped carbon microspheres,labelled as Fe-NCSs-T.The as-obtained Fe-NCSs-900 catalyst showed excellent electrocatalytic activity ORR,with a positive onset potential(E_onset potential)of 1.02 V vs.RHE,a positive half-wave potential(E_1/2)of 0.89 V vs.RHE and a limiting current density of 5.05 m A cm^-2.When used as an air cathode,Fe-NCSs-900 based ZAB showed high power density of 128.3m W cm^-2,good stability excellent rate performance and long-term discharge cycle stability at the discharge current density of 2 m A cm^-2,superior to Pt/C based battery.The remarkable performance of Fe-NCSs-900 was ascribed to its large BET surface area,large amount of defective carbon and high active iron nitride and Fe-N_x sites.3.To realize bifunctional electrocatalytic activity of the nitrogen-doped carbon microspheres,dual transition metal ions（Co and Ni）-coordinated Schiff base microspheres was fabricated.The sulfurization and subsequent pyrolysis of Co/Ni-coordinated Schiff base microspheres gave rise to transition metals-based nanoparticles loaded NCSs,named Co/Ni-NCSs-T（where T denotes the pyrolysis temperature）.The optimized Co/Ni-NCSs-900 catalyst displayed excellent bifunctional electrocatalytic performance,with a half-wave potential of 0.80 V vs.RHE and a limiting current density is 6.21 m A cm^-2 for ORR and a low overpotential of 200 m V at 10 m A cm^-2 for OER.The Co/Ni-NCSs-900 based ZAB exhibited high power density of 127.9 m W cm^-2and excellent long-term cycle stability.