Precisely Modify The Interface Layer And Improve Water Dissociation Property Of Bipolar Membrane

Bipolar membrane is a special kind of ion-exchange membrane,which consists of anion exchange membrane,cation exchange membrane and interface layer between them.Due to such extraordinary structure,under reverse bias,the interface layer has a unique characteristic for dissociating water into protons(H+)and hydroxide ions(OH-).Owing to this unique property,bipolar membranes(BPMs)have been widely applied in waste water disposal,the generation of acids and bases,resource recovery and energy utilization.However,BPMs are still faced with problems such as high overpotential caused by large water dissociation resistance,weak or no interaction between the AEL and CEL which lead to ballooning and delamination.In order to meet the industrial application requirements,it is necessary to further optimize the water dissociation performance and improve the preparation process of BPMs.Thus,based on The Second Wien Effect and the protonation-deprotonation mechanism,this thesis is focus on the precise construction of the interface layer(IL)by the means of in-situ growth method and the ultrasonic spraying coating method so as to reduce the water dissociation resistance,improve the interface electric field strength,and reduce the water dissociation overpotential.Combined with a variety of characterization and electrochemical tests,we explore the the relationship between water dissociation rate and interface layer constructure.And at the same time,BPMs are introduced into the field of clean energy preparation to expand the scope of BPMs application.The specific work content is as follows:(1)In view of the problem that the catalyst particles have weak adhesion to the base membrane layer,thus resulting in catalyst shedding and increasing the overpotential across the BPMs,this work modifies the cation exchange layer(CEL)by in-situ growth nanosheet arrays,after that prepares the BPMs via ultrasonic spraying coating anion exchange layer(CEL).Potassium ferrate(K2FeO4)undergoes hydrolysis reaction to form Fe(OH)3 colloids and γ-FeOOH nanosheets,but when K2FeO4 is dissolved in alkaline solution such as sodium hydroxide solution,the stability of Fe(OH)3 colloids would be destroyed,thus deposited on the SPPO-CEL as nucleus,so as to realize the in-situ growth of γ-FeOOH nanosheets arrays on SPPO-CEL,thereby modifying the interface layer.The in-situ growth γ-FeOOH nanosheets arrays as IL catalyst effectively expands the reactive area and site for water dissociation reaction,as a result,the overpotential is 0.76 V at 50 mA cm-2.Even more improve the adhesion between the catalyst and the SPPO-CEL and also the stability of the BPMs.For after 20 h long stability test,there no leakage of the catalyst or delamination.(2)The above-mentioned single layer catalyst BPM is more suitable for the preparation of acids and alkalis from salt solution systems,but when in the field of water electrolysis cell,a certain pH gradient(ΔpH=14)needs to be maintained across the BPM.That’s to say,the AEL and CEL of the BPM are in the environment of strong alkali and acid solution respectively.What’s more,this also puts forward higher requirements for the water dissociation effect and stability of the catalyst.From the perspective of local pH difference,according to the existing point of zero charge(PZC)test results of metal oxides,we find two metal oxide catalysts with the best catalytic performance under strong acids as tin dioxide(SnO2)and strong alkalis as nickel oxide(NiO),and with the help of the ultrasonic spray-coating method to construct a doublelayer metal oxide catalyst in the IL.After precisely adjust the catalyst loading,it is realized that with the help of synergistic effect between the two catalysts to catalyze the water dissociation reaction in the optimal pH environment,thus reduce the overpotential required for the water dissociation reaction,and at the same time introduce the BPMs to the water electrolytic cell under strong acid and alkali conditions.In the electrolytic cell with strong acid and alkali,the introduction of NiO and SnO2 as IL catalyst can well reduce the water dissociation overpotential of the BPMs(at 50 mA cm-2,overpotential:0.80 V).And in the long term stability test,the overpotential across the BPMs can always be lower than the commercial Fumasep BPM.In this work,we construct and precisely modify the interface layer of bipolar membrane by in-situ growth and ultrasonic spray-coating water dissociation catalyst,and explore the water dissociation performance of bipolar membrane combined with a variety of electrochemical testing methods.By constructing and precisely modify the interface layer of the bipolar membrane,the water dissociation rate and the stability of the bipolar membrane are significantly improved.Further,the bipolar membrane will be introduced into acidic and alkaline water electrolyqtic devices in order to expand the application range.In the future,the bipolar membrane can be introduced into pure water system for electrolytic aquatic hydrogen and carbon dioxide reduction and other fields.