Study On Alkaline Electrocatalytic Performance Of Metal Oxide Composite Catalyst For Water Splitting

Developing renewable and clean energy is an effective method to deal with the two major problems of fossil energy depletion and ecological environment destruction faced by human society.Among the many alternative energy sources,hydrogen is considered as the most promising clean energy source and an alternative to fossil fuels because of its abundance,high energy density and no carbon emissions.Among all kinds of hydrogen production technologies,electrochemical water splitting owns high working efficiency,simple operation,high purity,which is the most efficient method to produce hydrogen.The slow kinetics of the cathode hydrogen evolution(HER)and anode oxygen evolution(OER)reactions involved in the electrochemical water splitting and the use of precious metal catalyst limits the industrial production of water splitting.Therefore,developing of low-cost,highly efficient and stable catalysts and the in-depth understanding of catalytic reaction mechanism have become one of the core scientific issues of the electrochemical water splitting.In this study,two nanocomposites(Pt/MgO and NiFe-BMO/BCNT)were designed and synthesized to investigate the reaction mechanism of electrochemical water splitting at the electronic and atomic levels,respectively.Combined with in-situ synchrotron radiation X-ray absorption fine structure(XAFS)spectroscopy,in-situ synchrotron radiation infrared spectroscopy,the dynamic structural evolution of the catalystic site was monitored in real time.The research in this dissertation will provide significant guidance for the efficient and accurate design of electrocatalyst and regulating reaction mechanism in the future.The research contents of this dissertation are as follows:(1)Engineering a local acid-like environment in alkaline medium for efficient hydrogen evolution reaction.It is well known that the catalytic performance of HER in acidic medium is much higher than that in alkaline medium.Based on the understanding of the reaction process on the electrode surface,we believe that the reaction environment around the catalytic site is crucial for the catalytic activity,and it is an effective method to improve the performance of electrocatalyst by adjusting the local reaction environment around the catalyst.Therefore,we chose Pt nanoparticles(Pt/MgO)loaded on MgO nanosheets as the research system,and created a local acid-like environment in an alkaline medium successfully.The catalyst achieved comparable hydrogen evolution reaction(HER)performance to that of Pt/C in an acidic medium.In situ synchrotron infrared spectroscopy and X-ray absorption spectroscopy were used to investigate the presence of a key H3O+intermediate around the Pt site during HER process,which provides strong evidence for the formation of local acid-like environment around Pt nanoparticles in alkaline media.Combined with theoretical calculation and analysis,it is confirmed that the key factors for the formation of local acid-like environment include the following three points:(1)the oxygen vacancy(Vo)-rich MgO facilitates H2O dissociation to generate H3O+species.(2)Vo-rich MgO transfers part of its abundant electrons to Pt,leading to the formation of electron-enriched Ptδ-species.(3)H3O+migrates to Ptδ-and accumulates around Ptδ-due to the electrostatic interaction,which creates a local acidic environment in the alkaline medium.Our research provides a new perspective for designing high-performance electrocatalysts by adjusting the local reaction environment accurately.(2)Study on the performance of oxygen evolution reaction of NiFe-BMO/BCNT.Among the highly active OER materials,the conductivity and stability of transition metal-based catalysts are generally poor,while the electrochemical performance of carbon-based materials is unsatisfactory despite the high conductivity.Optimizing the interface structure of transition metal-carbon composites is an effective method to synthesize high performance and high stability electrocatalysts.We have prepared strong coupling NiFe-BMO clusters loaded B doped carbon nanotubes(NiFeBMO/BCNT)composites via simple annealing method successfully.The catalyst has a very prominent OER activity in alkaline medium.The overpotential is only 192 mV at 10mA cm-2,much lower than that of RuO2(269 mV).It also has good stability,with 20 hours of continuous reaction.Through XAFS characterization,we confirmed that the hybridization effect between C and metal Ni lead to the strong coupling between the carbon nanotubes and the metal oxide clusters on the surface.As an intermediate bridge,B successfully binds metal oxide clusters to carbon nanotubes closely.Due to the strong interaction and the increased interfacial area,NiFe-BMO/BCNT catalyst exhibits an efficient charge transfer process,which significantly enhances the catalytic activity of OER.This result will provide some theoretical support for us to design OER catalyst and understand its regulatory mechanism in the future.