| The oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)are the most critical processes in electrochemical energy conversion and storage technologies,such as electrocatalytic water splitting,fuel cells,and rechargeable metalair batteries.The design of low-cost,highly active electrocatalysts toward OER and ORR is the most crucial step for the large-scale application of these devices.This project aims to addressing these issues through a detailed study of electronic structure and OER/ORR activity of Mn-based transitional metal oxides with element doping and spin state engineering.High-resolution X-ray photoemission spectroscopy(XPS)and X-ray absorption spectroscopy(XAS)were used to study the electronic-activity relationship of the catalysts at molecular level for designing highly active electrocatalysts.We find that the appearance of hole states,which is of paramount importance to facilitate the OER by reducing the energy barrier for the electron transfer associated with the OER process.Furthermore,we have modulated the spin state of La1-xSrxMnO3 by coupling external magnetic field with electrochemistry to achieve highly active electrocatalyst for OER.The main contents are shown below:(1)We report a systematical study on the tuning of the electronic structure of La2-xSrxNiMnO6 with 0≤x≤1.0 to promote the bifunctional OER/ORR activity.The bifunctional index(ΔE)is substantially reduced with increasing of Sr contents and achieves an optimal value of 0.85 V for La1.4Sr0.6NiMnO6,exceeding that of the widely studied LaNiO3.Our study on electronic structures reveals that the enhancement of ORR and OER activities strongly correlates with the appearance of Ni3+oxidation states and the up-shift of O 2p band center promoted by Sr doping.In particular,an increase of hole states,derived from the Ni3+ states,reduces the energy barrier for electrons transfer from 0.44 eV to 0.12 eV,and hence improves the intrinsic OER activities.The tuning of electronic structure that leads to higher OER and ORR activities in La2-xSrxNiMnO6 can be extended to other materials for design of active bifunctional electrocatalysts.(2)We have modulated the spin state of LaMnO3 via Sr doping(La1-xSrxMnO3)and coupled external magnetic field with electrochemistry to achieve highly active electrocatalyst for OER.Comprehensive characterizations were carried out on the magnetic properties and OER activity for La1-xSrxMnOO.We found that the spin parallel arrangement is thermodynamically more favored to the gas-phase O2 molecule that leads to a high OER activity.The spin state of La1-xSrxMnO3 is decided by the competitive relationship between double exchange and super exchange interactions.Mn3+-O-Mn4+double exchange interaction dominates when the Sr level is 0.35,which results in the spin parallel behavior in La1-xSrxMnO3.The Mn3+-O-Mn4+double exchange induces the electrons delocalized,hence improves the intrinsic OER activities.Furthermore,once the magnetic field has been applied,the electrons in the two Mn ions are oriented parallel for Sr-dopped La1-xSrxMnO3,and the OER performance is enhanced obviously.The understanding of spin related electron transfer in oxygen electrocatalysis will be of considerable interest for those working on the theory and mechanism of spin-catalyzed oxygen electrocatalysis. |
