Synthesis And Study Of Cobalt-based Oxides Electrocatalysts For Water Splitting

Electrocatalytic water splitting,involving hydrogen evolution reaction（HER）at cathode and oxygen evolution reaction（OER）at anode,is one of the most effective ways to achieve the hydrogen society.Currently,precious metals show excellent electrocatalytic performance for HER and OER.Unfortunately,their large-scale application is limited by the high costs and scarcity.Therefore,it is of importance to develop economic and efficient electrocatalysts for popularization of water splitting.Benefiting from the low price and abundant reserves,transition metal oxides have been widely concerned.However,such catalysts have poor stability and low conductivity.Based on this,cheap and easily prepared cobalt-based oxides are maily studied in this paper.Their electrocatalytic water splitting activity and stability are successfully optimized through crystal phase engineering、interface engineering and electrochemical in-situ reconstruction,etc.Firstly,a unique CeO_x@Co₃O₄ core-shell heterostructure is constructed by integrating the advantages of crystal phase engineering and interface engineering.There are abundant grain boundaries and heterogeneous interfaces in this structure,which can provide abundant electrocatalytic active sites,thus significantly improving the electrocatalytic water splitting activity of Co₃O₄.Furthermore,in view of the more excellent synergistic advantages of polymetallic oxides,Ni-Co-V trimetal oxide is used as a pre-catalyst in this work.And its in-situ surface reconstruction into highly efficient polymetallic hydroxyl oxides is successfully deepened during the anodic oxidation via FeOOH surface modification,which is conducive to the improvement of OER electrocatalytic performance.These studies not only achieve the expected goal of optimizing the structure and improving the performance of Co-based oxides,but also provide guidance for the large-scale application of transition metal oxides based electrocatalysts.The main research results abtained from this paper are as follows:（1）Cerium oxide ultrafine particles with low crystallinity is successfully introduced on the surface of Co₃O₄ grown on nickel foam（NF）substrate via rapid electrodeposition and the CeO_x@Co₃O₄/NF heterostructure with core-shell structure is constructed.Compared with Co₃O₄/NF,CeO_x@Co₃O₄/NF exhibits significantly improved OER and HER catalytic activity with overpotentials of 252 m V and 106 m V in 1 M KOH at 10m A·cm^-2 for OER and HER,respectively.And the two-electrode alkaline electrolyzer assembled by CeO_x@Co₃O₄/NF as both anode and cathode requires only 1.57 V to reach a water-splitting current density of 10 m A cm^-2.The good electrocatalytic activity can be mainly attributed to the following points:（1）Ultrafine cerium oxide particles with low crystallinity can not only provide abundant grain boundaries,but also protect the inner Co₃O₄ nanoneedles from damage;（2）The strong electron interaction between CeO_x and Co₃O₄ leads to the increase of Co³⁺content,which promotes the conversion of Co₃O₄ to highly active Co OOH in the OER process.Meanwhile,CeO_x can effectively promote the dissociation of water,thus improving the HER activity of Co₃O₄;（3）The abundant heterointerfaces in CeO_x@Co₃O₄/NF further enrich the active sites and facilitate the charge transfer kinetics and intermediate adsorption kinetics.（2）Ni_3-xCo_xV₂O₈/NF with flower-like hierarchical structure is synthesized on NF substrate by hydrothermal method(denoted as Ni_3-xCo_xV₂O₈/NF).And then,the trace FeOOH is introduced on the surface by one-step immersion method(denoted as Ni_3-xCo_xV₂O₈-Fe/NF).On the one hand,FeOOH,as a highly active OER catalytic species,can significantly improve the OER catalytic activity of Ni_3-XCo_xV₂O₈/NF.More importantly,it can lead to a faster and more complete dissolution of V species in Ni_3-xCo_xV₂O₈/NF during anodic oxidation,resulting in a faster and deeper surface reconstruction of Ni_3-xCo_xV₂O₈/NF into Ni-Co-Fehydroxyl oxides with ultra-high OER activity.The anodized Ni_3-xCo_xV₂O₈-Fe/NF is denoted as Ni_3-xCo_xV₂O₈-Fe/NF-14 h.Benefiting from the synergistic advantage between polymetallic ion,the Ni_3-XCo_xV₂O₈-Fe/NF-14 h only needs247 m V overpotential to drive OER current density of 10 m A cm^-2.Moreover,it can keep stable for more than 1600 h at 200 m A cm^-2 current density in 1 M KOH.