Transition Metals Oxides And Phosphides Supported On Nitrogen Doped Carbon For Oxygen Electrocatalysis

Electrocatalysis is critical to numerous renewable energy technologies such as fuel cells,metal air batteries and electrolyzers for the purpose of energy conversion and storage.These devices require electrocatalysts for the reactions such as oxygen reduction reaction,oxygen evolution reaction and hydrogen evolution reaction.These are multi-step and multi-electron electrochemical reactions that generally have high overpotentials and slow kinetics.While effective catalysts for these reactions are known and are commercially available,most of them are based on precious metals and hence are not economically viable for widespread application.Additionally,many of these catalysts still suffer from reaction poisoning and poor electrocatalytic selectivity and stability,further decreasing their overall utility.An ideal catalyst would be easily prepared,stable,and cost effective,and preferably exhibit bifunctional behavior,as such catalysts could simplify design protocols for rechargeable metal-air batteries and regenerative fuel cells/electrolyzers and lead to the realization of efficient and practical devices.With this in mind,in the present thesis we have reported two cobalt-based catalysts with superior ORR and bifunctional ORR/OER performance.Firstly Co particles coated by nitrogen-doped carbon（Co@NC）is synthesized by pyrolysis Co（OH）₂ with nitrogen-rich carbon precursor（HNO₃-treated melamine）to afford strong chemical bonding between the metallic Co and N.Due to the abundance Co-N bonding Co@NC shows superior ORR activity that outperforms state of the art noble metal 20%Pt/C from all aspect of on-set potential（1.05V）,overpotential at-1mA/cm²（0.97V）,half-wave potential（0.95）,stability and methanol crossover effect.Then to improve the OER activity some surface Co₂P is produced but keep the Co-N and N-C bonding almost unchanged（Co₂P/Co@NC）by adding additional phosphorus in the synthesis.This structure raises OER activity（with over-potential of290mV at 10mA/cm² on glassy electrode）better than Ru₂O and IrO₂,and retains ORR activity comparable with 20%Pt/C.Subsequently Co₂P/Co@NC shows reversible oxygen electrocatalysis comparable with the combination of Pt/C//RuO₂ and Pt/C//IrO₂,with the potential difference of OER and ORR as low as 0.63V.Therefore Co@NC and Co₂P/Co@NC can work as superior ORR and ORR/OER bifunctional catalyst to replace noble metals.And I also report an efficient electrocatalyst,with NiO/Co₃O₄ nanoparticles decorated on nitrogen-doped carbon（NiO/Co₃O₄@NC）.Abundant high-valence Ni³⁺and Co³⁺species were observed on the surface of hybrid due to the strong NC-metal oxide and NiO-Co₃O₄interactions.This unique structure leads to excellent OER performance,delivering a very low overpotential of 240 mV@10 mA·cm^-2 on glassy carbon and 200 mV@10 mA·cm^-2 on Ni foam in KOH and performing 6 and 16 times higher turnover frequency（@350 mV overpotential）than IrO₂ and RuO₂,respectively.