Preparation Of Biomass-drived Carbon-based Electrocatalysts For Oxygen Reduction Reaction

As an efficient energy conversion device,the development and application of fuel cells have received extensive attention.However,platinum-based precious metal catalysts for fuel cell cathodes are costly with poor stability,which limits their wide application.Therefore,the development of low-cost and high-activity non-precious metal carbon-based oxygen reduction reaction(ORR)electrocatalysts are of great significance.In this paper,sodium alginate and hemp as the precursors of cheap and resource-rich biomass materials were selected.Two transition metal-nitrogen-carbon(MNC)electrocatalysts were prepared by adding nitrogen source and metal source combined with carbonization process.Its morphology,structure,composition and electrocatalytic activity were characterized.The main contents and conclusions are as follows:(1)Co-SA/NaHCO3 aerogel microspheres were prepared by electrostatic spraying,ion exchange gelation and freeze-drying.ZIF-67 was loaded on aerogel microspheres by in-situ solvothermal growth to form ZIF-67@SA/NaHCO3 composite microspheres.A sodium alginate-derived Co/N co-doped porous carbon electrocatalyst(Co/NHPCMS)was prepared by high temperature carbonization.It has a three-dimensional cobalt/nitrogen doped hierarchically porous carbon network,which facilitates active site exposure and electron/substance transfer.Under the optimal conditions(the ratio of SA to NaHCO3 is 1:1,the carbonization temperature is 600?),Co/NHPCMS-600 has excellent ORR catalytic activity in alkaline medium,with the half-wave potential(E1/2)of 0.827 V,which is close to 20%Pt/C catalyst.Its long-term stability and methanol tolerance are better than 20%Pt/C catalyst.The zinc-air battery assembled with Co/NHPCMS-600 has an open circuit voltage of 1.318 V,a discharge specific capacity of 590 mAh g-1,and a maximum power density of 83 mA cm-2.(2)Nitrogen-doped porous carbon was prepared by using hemp as the precursor and NaHCO3 as the activator and melamine as the external nitrogen source.Then,Co/N co-doped hemp-derived porous carbon electrocatalyst(Co/NHPC)was prepared by adsorbing cobalt ions and secondary carbonization.The nitrogen-doped porous carbon(NHPC)prepared when hemp,NaHCO3 and melamine with a mass ratio of 1:3:1 exhibit the best ORR activity.Co/NHPC-90 prepared with a mass ratio of cobalt nitrate to NHPC of 0.9 has a high specific surface area(1251 m2 g-1)and excellent ORR catalytic performance.In an alkaline electrolyte,its half-wave potential is as high as 0.826 V,and it has excellent stability and methanol tolerance.The zinc-air battery assembled with Co/NHPC-90 has an open circuit voltage of 1.363 V and a specific capacity of 569 mAh g-1,and the maximum power density is about 76.7 mW cm-2.