In-situ Growth Of Manganese Oxide Nanowires On Carbon Substrate And Its Application In Electrocatalysis

Because of global climate change and the prominent contradiction between the supply and demand of traditional fossil energy,the energy structure of major countries in the world is gradually transforming from a highly polluting fossil energy system to an environment-friendly renewable energy system.Therefore,electrochemical energy storage and conversion technologies,such as proton exchange membrane（PEM）water electrolysis technology,have received worldwide attention,because they can effectively solve the intermittent energy supply defects of renewable clean energy and has environmentally friendly characteristics.However,high electricity consumption is required for water electrolysis,and large energy loss happened at the anode,on which oxygen evolution reaction（OER）happens.The performance of PEM water electrolysis is still unable to meet the requirements of large-scale commercialization.As a representative noble metal OER catalytic material,IrO₂ has limited catalytic activity despite its high stability at present.Therefore,the development of acidic OER catalysts with both high stability and activity is the key to overcome the bottleneck of the commercialization of PEM electrolytic hydrogen production technology.In response to this challenge,we have systematically studied MnO₂ nanomaterials.Combined with in-situ growth and cation exchange strategies,we have successfully constructed high-performance acidic OER catalysts by modifying atomic Ru components on the surface of MnO₂ nanomaterials.The specific research contents are as follows:（1）KMnO₄ can react with carbon to generate the MnO₂ crystal nucleus.One-step hydrothermal method was employed for the in-situ growth of MnO₂ nanocrystals on the high electronic conductive carbon substrate using KMnO₄ as the manganese source.MnO₂ is a kind of metal oxide with diverse polymorphism.And its phase transition behavior between metastable phases during the reaction is rooted in the non-equilibrium nature of crystallization,which is sensitive to the surface free energy.During the hydrothermal reaction process,H⁺and K⁺in the solution can effectively regulate the surface free energy through surface adsorption or ion intercalation during the nucleation and growth processes,thus effectively controlling the crystalline phase structure of MnO₂.Therefore,the MnO₂ with different structural polymorphs has been successfully in-situ grown on the carbon paper substrate by changing the concentrations of KMnO₄ and H₂SO₄ during the reaction process.Theα-MnO₂ growth conditions was further optimized to produceα-MnO₂ crystal nanowires of high specific areas.The relationship between polymorphs of MnO₂ and OER activity has been investigated and theα-MnO₂ exhibited the best OER catalytic properties.This is mainly because of its high electrochemical specific surface area with unique Mncoordination environment,compared with other crystal forms of MnO₂ materials,which have more moderate adsorption energy intensity for OER reaction intermediates.（2）Highly dispersed Ruthenium（Ru）species were successfully deposited on the surface ofα-MnO₂nanocrystals by the cation exchange strategy.The structural characterization analysis results suggested the atomic dispersion of Ru species on the surface of MnO₂ nanostructures.The electrochemical analysis results indicate that the optimized 4.2 wt.%Ru/MnO₂ catalyst shows an overpotential of 128 m V at an OER current density of 10 m A cm^-2,which is much better than that of a commercial Ru O₂ catalyst.The long-term stability test for up to 110 hours confirms its excellent OER stability.XPS analysis results prove the presence of induced effect in Ru/MnO₂,which can enhance the intrinsic catalytic activity by adjusting the adsorption energy of reaction intermediates on Ru sites.The excellent catalytic stability can be attributed to the cation exchange reaction between MnO₂ support and ruthenium element during oxygen evolution reaction,which can effectively inhibit the leaching of Ru during OER.The electrochemical characterization confirmed that the OER catalytic path of 4.2 wt.%Ru/MnO₂ followed the lattice oxygen reaction mechanism（LOM）.