Preparation And Properties Of Transition Metal Oxide-based Electrocatalysts For Oxygen Reduction And Evolution Reactions

Fuel cells and water electrolysis devices are ideal for energy storage and conversion.The development and utilization of these is one of the effective ways to alleviate the energy crisis and accelerate sustainable development.Oxygen electrode reactions include oxygen reduction reaction(ORR)and oxygen evolution reaction(OER),both of which are limited by their own characteristics such as slow kinetics,high overpotential and harsh working environment(highly oxidative),demanding the use of a large amount of highly efficient and stable electrocatalysts to accelerate electrochemical reactions.Excellent ORR and OER electrocatalysts are the cornerstone for the full deployment of renewable energy devices such as fuel cells,metal-air cells and water electrolyzers.Traditional ORR and OER electrocatalysts are dominated by Pt-based noble metal,Ir-based and Ru-based oxides(IrO₂ and Ru O₂),which are restricted by scarcity and high cost,greatly limiting the commercialization of related electrochemical devices.Therefore,it is particularly urgent to develop precious and non-precious metals catalysts with low cost,high activity and high stability.To solve the above problems,this study designed and constructed highly active and highly stable Pt based electrocatalysts and low cost efficient non-noble metal perovskite catalysts,and studied the structure-activity relationship between the composition of catalysts and ORR/OER catalytic activity and stability.The Pt particles were deposited and anchored at the interface of NbO₂ and graphene by traditional microwave assisted ethylene glycol method,and the Pt/NbO₂/graphene catalyst with three-phase boundary structure was prepared,and its ORR and OER activity and stability enhancement mechanism were analyzed in detail.It was found that the electron interaction between Pt and NbO₂ weakened the adsorption of OH_ads on Pt and promoted the dissociation of OH_ads,which was beneficial to the acceleration of ORR kinetics.OH_ads/O_2ads at the Pt-site and OH_ads species on the surface of NbO₂ are laterally repulsive,which weaken the Pt-OH bond and release the Pt active site at the same time,promoting ORR activity.The results shows that NbO₂ can improve the stability of Pt/NbO₂/graphene catalyst by anchoring Pt nanoparticles on the Pt and graphene interface,which effectively inhibits the dissolution,migration,detachment and redeposition of Pt particles due to Oswald ripening during long-term potential cycle,thus inhibiting the ORR activity degradation.In the OER process,and the added of NbO₂ significantly improves the OER activity of the Pt/graphene catalyst,and the constructed three-phase boundary structure promotes the improvement of stability,indicating that Pt/NbO₂/graphene is a catalyst with high ORR and OER activity and excellent electrochemical stability.A perovskite oxide(ABO₃)with a regulated electronic structure was further selected to analyze the structure-activity relationship between the electronic structure of the oxide and the catalytic activity.A series of LaMn_xCo_1-xO₃ perovskite catalysts with high specific surface area and high porosity were prepared by solvent thermal method.The electronic structure and energy band structure of the catalysts were optimized by Mn doping at the B position of the perovskite catalyst.It was found out that they exhibited excellent ORR and OER activity.The study shows that LaMn_0.3Co_0.7O₃ catalyst shows the best ORR activity(half wave potential of 0.72 V)because when the doping content of Mn is 0.3,the number of e_g orbital electron filling in LaMn_0.3Co_0.7O₃ catalyst decreases significantly and is close to the theoretical value 1.Theoretical analysis and discussion of the structure-activity relationship were carried out according to the band structure of the catalysts obtained by ultraviolet photoelectron.It was determined that the best LaMn_0.3Co_0.7O₃OER activity is mainly due to the the shift of O 2p to the Fermi level caused by Mn doping,enhanced Co-O covalent bond,and accelerated OER dynamics.In order to further improve the intrinsic activity of LaMn_xCo_1-xO₃ catalyst,a series of LaMn Ni Co O₃ catalysts were prepared by doping Ni element and changing its doping ratio based on LaMn_0.3Co_0.7O₃ catalyst.XPS,FT-IR and UPS techniques were used to systematically study the number of electrons filling in e_g orbitals of the catalyst,the covalency of B-O bonds and the energy gap between O 2p bands and Fermi levels.The results revealed the effect of Ni doping and different doping ratio on physical properties and electrochemical activity It is concluded that there is a structure-activity relationship between the electronic structure/energy band structure of perovskite catalyst and ORR/OER catalytic activity.The results show that the number of filling electrons in e_gorbitals is close to 1 and the enhanced B-O bond covalency are the main reasons for the enhancement of ORR intrinsic activity;the energy gap between O 2p bands and Fermi levels is found to be a description of OER activity.The above conclusion can provide theoretical guidance for the design and application of perovskite catalysts(ORR/OER).