Synthesis Of Ultrathin Two-Dimensional Nanomaterials And Their Applications In Electrochemical Energy Conversion

Two-dimensional ultrathin nanomaterials,due to their unique structural characteristics and exposure of many active sites,have potential applications in the field of energy,catalysis and so on.How to prepare nanomaterials with ultra-thin two-dimensional structures is still a major challenge,especially in terms of some metal-based layered nanomaterials.In recent years,a series of non-graphene-based ultrathin two-dimensional nanomaterials have been successfully synthesized and at the same time,these ultrathin two-dimensional materials in the relevant catalytic reactions have shown excellent catalytic activities.Therefore,the main research direction of this thesis is to prepare some novel thin two-dimensional materials,study their formation mechanism,and apply them to the relevant energy electrocatalytic reaction.The full dissertation consists of 8 chapters,each chapter reads as follows:Chapter 1:The first chapter introduces preparation methods of various ultra-thin nanomaterials and their related applications,and expounds the basis topic and research contents of this doctoral thesis.Chapter 2:PtCu alloy nanosheets were prepared by a CO-assisted method.The thickness of the nanosheets was within 2 nm.It was found that the adsorption of CO on PtCu(111)and the introduction of KI changed the reaction rate of the whole system during the formation of nanosheets,which were the key factors in the formation of the sheet structure.It has also been found that two-dimensional PtCu alloy nanosheets have good electrochemical hydrogen evolution and hydrogenation activity compared to nanoparticles due to the exposure of abundant active sites.Chapter 3:In this paper,we used the strong coordination of between ammonia molecules and Co(Ⅲ)to synthesize ultrathin Co3O4 nanosheets with(111)exposed crystal planes.It was found that the surface of the nanosheets was with high oxidation state Co(Ⅲ),while the Co(Ⅲ)atoms have been currently widely considered to be the active sites of some oxidation reactions.Therefore,the prepared Co3O4 was used as a catalyst for water oxidation to exhibit superior oxygen evolution performance,which is much higher than that of other Co-based catalysts previously reported.However,the oxygen produced in this process was worthless.In view of this problem,we proposed a new idea of "partial reforming of ethanol electrochemical".In the process of electrolysis hydrogen production,the aqueous solution containing ethanol-water mixed solution was used instead of electrolytic water.While reducing the electrolysis over-potential at the lower potential of ethanol can be highly selective oxidation of ethyl acetate,alcohol to achieve high value-added chemicals preparation.Based on this finding,we further constructed the electrochemical reactor with ethanol reforming by using NiMo composite nanoparticles as cathodic hydrogen evolution catalyst and ultrathin CO3O4 nanosheets as the anodic oxidation catalyst,and successfully realized the ethanol at room temperature by a 1.5 V battery.The small-scale practical application of high selectivity reforming resulted in a high value-added anode product ethyl acetate while obtaining the hydrogen.Chapter 4:Preparation of ultrathin two-dimensional metal materials,is a worldwide problem.As an important member of transition metals,Cu nanomaterials are widely used in various fields,especially in the field of energy catalysis.But at present,ultrathin Cu nanosheets have not been reported so far,the biggest challenge is that Cu nanomaterials are easily oxidized and can not be stably present in the air.In this system,we use a one-pot method to obtain an ultrathin hybird Cu/Ni(OH)2.The prepared CufNi(OH)2 nanosheets can be stably present in the air for 3 months.we also found that their stability is closely related to the carboxylate salts we have used.The oxidation potential of the carboxylate salt is lower than that of Cu.Therefore,the presence of carboxylate can retard the oxidation of Cu so that we can obtain stable and thin hybird Cu/Ni(OH)2.Cu is currently the most suitable catalyst for CO2 reduction,but their reduction product is not uniform,the main reason is that Cu is easily oxidized to its oxidation state,Cu valence changes lead to different products of CO2 reduction.And highly selectivity of CO2 to CO at lower potentials was achieved on the Cu/Ni(OH)2,with a Faraday efficiency of CO up to 92%.At the same time,the catalyst also showed excellent electrocatalytic stability.After more than 20 hours of electrolysis,the activity had no significantly decrease.Chapter 5:In the system of Chapter 4,we also captured a Cu2O/Ni(OH)2 nanosheets.The nanosheets were able to reduce CO2 to CH3OH.The highest Faraday efficiency of CH3OH was 63%at-0.35 V vs.RHE.It is important to note that this composite nanosheet also exhibits an ideal catalytic activity in the water oxidation reaction under neutral conditions.In view of their performances,We have established a solar cell driven electrochemical reforming system that operates at a voltage of about 2 V to allow CO2-H2O to be reformed to methanol and oxygen at room temperature.Chapter 6:the ultrathin Ni(OH)2 is modified by atom-dispersed Pt material to make it have excellent electrochemical properties.In alkaline,the Pt/Ni(OH)2 exhibits an excellentcatalytic property over commercial Pt/C in HER,which only needs 32 mV overpotential to acquire a current density of 10 mA/cm2.This composite of Pt/Ni(OH)2 also exhibits excellent properties in urea oxidation reaction.At the potential of 0.35 V,the current density of urea oxidation of Pt/Ni(OH)2 nanosheets(110 mA/cm2)is 4.4 times of Ni(OH)2 nanosheets(24 mA/cm2).It is presumed that the introduction of Pt promotes the process of Ni(OH)2 → NiOOH during electrochemical oxidation,and the Ni(III)content on Ni(OH)2 surface is increased,and the increase of active site directly enhances their catalytic performance.Chapter 7:Ultrafine One-dimensional Precious Metal Alloy Nanowires are also considered to be excellent electrocatalyst candidates for a variety of energy conversion and electrochemical sensing applications.In this work,we found that the CO-assisted synthesis method can also be used to prepare ultra-fine PtCu3 alloy wavy nanowires.For the overall formation of ultra-thin wave nanowires,the use of CO proved to be critical.Combined with its ultrafine characteristics,higher electrochemical surface area,the richness of twin defects and the synergistic effect of the two metal components,the prepared PtCu3 wavy wires are shown excellent electrocatalytic activity in the methanol oxidation reaction and in the electrochemical sensing of glucose.This wave shows the largest linear interval(0.1-42 mmol)at the time of detection of glucose.At the same time this method can also be used as other Pt-based alloy wavy line synthesis preparation.Chapter 8:With the use of CO and I ions,ultrathin Pd nanosheets will be assembled into an one-dimensional nanowires,and their length can reach micron grade.At the same time,a series of results show that the assembling sites were on the face of each Pd nanosheets.