Surface/interfacial Chemical Regulation Of The Electrocatalytic Activity For Low-dimensional Cobalt-based Materials

With the over-use of traditional fossil energy,resulting in the problem of increasing environmental pollution and energy crisis,and researchers have been stimulated to explore and develop new and high-efficient alternative energy systems.Electrocatalytic water-splitting is one of the most ideal and effective ways to achieve hydrogen energy.However,the reaction involved in multi-step of electron transfer process limiting the kinetics of electrocatalytic water-splitting,lead to the requirement of additional potential to accelerate the electron transfer and overcome the energy barrier of reaction kinetics,thereby improving the conversion rate of intermediates to product.Electrocatalysts with high catalytic activity can effectively reduce the overpotential of electrocatalytic water-splitting.Even though precious metals electrocatalysts usually have better catalytic activity,but out of earth’s reserves and cost considerations,it has more application prospects to look for non-noble metal electrocatalysts with high catalytic activity for the sustainable development of hydrogen energy.This dissertation selected the low-dimensional cobalt-based materials with promising application in electrocatalytic water-splitting as the research object.Based on three key factors of low-dimensional cobalt-based materials,including electrical conductivity,catalytic active sites and reaction energy barrier and so on,explores various new surface and interfacial chemical regulation strategies to optimize the catalytic activities of low-dimensional cobalt-based materials.In guarantee the fully exposed active sites of low-dimensional cobalt-based materials,realizing the optimization of electrical behavior and reaction energy barrier.In this dissertation,we aim to employ the following strategies,including the surface reconstruction,phase transition,interface modulation and anion doping to effectively regulate the catalytic activities of low-dimensional cobalt-based materials,and obtain a series of high-active electrode materials.Moreover,we focus on the catalytic reaction mechanism of low-dimensional cobalt-based materials,providing a new way for understanding how to optimize the catalytic activities by surface and interfacial chemical regulation strategies.Detailed content includes the following several points:1.Concerning the problem that the electrocatalytic activity of most low-dimensional cobalt-based oxides materials has been limited because of their poor intrinsic conductivity and low active site exposure yield,the author optimize the OER catalytic activity of metallic Co4N material by surface chemical reconstruction strategy.Firstly,we prepared a series of metallic OER electrocatalyst system(Co2N,Co3N and Co4N)by electron delocalization modulation.The electrical behavior test confirms that the conductivity of this material system is increasing with the decreasing of N content.Benefitting from the high conductivity and optimized N content,metallic Co4N catalyst shows very promising OER catalytic activity.Moreover,the author further optimized the morphology of Co4N material and synthesized the Co4N porous nanowire arrays(Co4N NW/CC)with high OER catalytic activity.A series of ex-situ characterizations confirmed that the Co4N material happened surface construction during OER process,and the real active material is metallic Co4N core with a thin cobalt oxide/hydroxide shell for OER.This work verify the OER activity of electrocatalyst with high conductivity can be further optimized by surface chemical reconstruction strategy.2.Gain in-depth understanding of the relationship between crystal structure and intrinsic catalytic activity of electrocatalyst plays a role in designing novel and high-active electrocatalysts.The surface structure of material is closely to the adsorption free energy of reaction intermediates.Thus,optimizing the surface structure of material by chemical strategy is one of the most important ways to regulate their catalytic activity.In this work,the author selected the cubic and orthorhombic metallic CoSe2 material system as model,regulating the surface structure of CoSe2 material for optimal intrinsic electrical behavior and catalytic reaction energy barrier by a simple phase transformation strategy.Combining the XAFS and DFT theoretical calculation analysis,we revealed the relationship of the different Co-Se bond lengths with Hadsand water adsorption energy,and gain in-depth understanding of the close relationship between the crystal structure and the intrinsic HER electrocatalytic activity.Benefiting from phase-transformation engineering,which endows higher electrical conductivity,ideal water adsorption energy,and faster transformation efficiency of Had,into hydrogen,the metallic c-CoSe2 catalyst exhibits largely enhanced HER catalytic activity in alkaline medium.Based on phase-transformation strategy,this c-CoSe2 catalyst possesses high intrinsic conductivity and electrocatalytic activity,which is expected to been applied to the practical electrocatalytic water-splitting devices.3.Surface heterometal atom doping has been regard as a promising way to optimize the electrical behavior,active sites and reaction energy barrier of electrocatalysts.However,the drawback is that the subsequent complication of metal-active sites prejudice the in-depth study of intrinsic catalytic activity and the inset of metal ion with large atomic size is unfavorable for maintaining the stable crystal structure.In this work,the author highlighted a non-metal modulation,that is surface N-anion decoration strategy plays a role in improving the electrocatalytic activity of HER.Taking the metallic CoS2 material system as an example,we successfully prepared the N-anion decorated CoS2 porous nanowire arrays using the thiourea as both S and N sources by a simple precursor morphology-directed method.Based on theoretical and experimental result,we confirmed the introduction of N atoms enables synergistic advantages of improved electronic structure,modified the morphology of nanowires for more active sites exposure and optimized free energy of hydrogen adsorption,and thereby greatly improved HER catalytic performance of CoS2 material system.4.By heterogeneous hybridization between electrocatalyst and high conductive material can effectively improve the electrical behavior of low-dimensional cobalt-based materials.However,how to increase the interaction between electrocatalyst and supporter by interfacial modulation is the key to promote the catalytic activity of hybrids.In this regard,the author successfully prepared an intrinsically high-active cobalt borate nanosheet by a simple room temperature synthesis approach,and in-situ grown on high-conductive graphene nanosheet in the form of 2D/2D.This hybrid form increases the contact surface between cobalt borate nanosheet and graphene substrate,which is favor of the electron transfer during electrocatalytic process,and thereby realizing the optimization of reaction kinetics.Finally,bene fitting from the high surface active sites exposure yield,enhanced electron transfer capacity and strong synergetic coupled effect,this Co-Bi NS/G hybrid shows high OER catalytic activity and stability under alkaline and neutral conditions.Our work not only offered a new interfacial regulation strategy,but also experimentally confirmed this new Co-Bi nanosheet has a promising potential application in water oxidation.