Preparation Of Perfluorinated Ion Exchange Membrane For High-performance Chlor-alkali And Study On Its Interface Properties

Chlor-alkali industry is the basis for Chinese industry and national economy.The production capacity of chlorine and caustic soda is close to100 million tons,contributing almost 6 trillion to the Gross Domestic Product（GDP）.Perfluorinated ion exchange membrane（PIEM）,as the core component of the chlor-alkali electrolysis cell,is critical in realizing high-efficiency chlor-alkali electrolysis.Its main performance parameters include cell voltage,current efficiency,chlorine purity,and the salt content in alkali etc.The cell voltage is the most important indicator for evaluating the efficiency of chlor-alkali electrolysis,which is directly related to the production energy consumption.A lower cell voltage means a lower power consumption.In a electrolysis cell with fixed anode and cathode,the cell voltage is mainly influenced by the structure and interface properties of the chlor-alkali ion exchange membrane.The membrane interface modification is the core element for a high-performance chlor-alkali ion exchange membrane,and it is pivotal toward highly efficient chlor-alkali electrolysis.However,there are very few studies reporting on the relationship of surface composition,morphology,and membrane performance,which is lack of systematic research and conclusions.In this thesis,we investigated the interface properties of the PIEM and the design of the surface morphology.A variety of surface bubble-repellent coatings were designed and constructed.Surface morphology optimization technology was developed to control the morphology.The bubble wettability and ion transport properties of the surface could be tuned.Some research results were applied to industrial products,and a new-generation high-performance chlor-alkali PIEM was successfully developed,creating a theoretical and experimental basis for the preparation of other high-performance chlor-alkali perfluorinated ion exchange membranes.The main content can be summarized in following sections:1.Mechanistic study of bubble adhesion on PIEM surfaces:It was found that changing the chemical composition of the PIEM in different solutions was the main cause of the varying bubble adhesion behavior.The sodium form-perfluorosulfonic acid（PFSA-Na）and sodium form-perfluorocarboxylic acid（PFCA-Na）membranes in pure water were relatively weak in bubble adhesion.However,when the membrane was immersed in electrolyte（a Na Cl solution）,the bubbles adhesion on membrane surface was significantly enhanced.The maximum bubble adhesion volume was increased from 4～6μL to 14～16μL,and the bubble adhesion force was increased from 39.9～76.6μN to 176.1～178.8μN.By studying the bubble adhesion behavior on the surface of the membrane immersed in pure water and electrolyte,and comparing the surface morphology,it is confirmed that the PFSA-Na/PFCA-Na membrane shows different properties in different solutions.The difference in morphology and chemical composition could originate from the molecular rearrangement at the polymer/solution interface to minimize the surface energy of the system.This study revealed that a PIEM with excellent anti-bubble properties was of great significance for achieving high energy efficiency during chlor-alkali electrolysis.2.Preparation of aerophobic chlor-alkali perfluorinated ion exchange membranes and study of the influence of bubble adhesion on the membrane surface:Based on previous studies on the interfacial properties of the PIEM,a superaerophobic ZrO₂ coating layer was constructed on the surface of the PIEM to induce aerophobic properties of the membrane surface.The effect of membrane surface roughness on bubble wettability in brine,and the impact of bubble adhesion on Na Cl transport and electrochemical performance were investigated.The ZrO₂coating（SPIEM-6）obtained under the optimum conditions endowed the membrane with a low bubble adhesive force of 72.81μN and a high bubble contact angle of 151.3oin brine solution.The superaerophobic properties of the membrane are favorable for reducing bubble adhesion,which maximized the effective working area of the memebrane,and promoted ion transport.By employing SPIEM-6 as the ion-exchange membrane,the electrolysis cell of the Na Cl aqueous solution exhibited a low power consumption of 1923 k Wh t^-1,saving about 100 k Wh and 90k Wh per ton of Na OH compared to PIEM(2030 k Wh t^-1)and Nafion?N2030(2010 k Wh t^-1)at 4.5 k A m^-2,respectively.3.Preparation and performance evaluation of aerophobic chlor-alkali perfluorinated ion exchange membranes with efficient transmembrane transport properties:To reduce membrane resistance and improve the ion transport properties,a PIEM with"sacrificial material"nano-particles distributed on the surface was prepared.After etching,a meso-porous interface was formed on the surface of the PIEM,effectively preventing gas adsorption during the electrolysis process and greatly reducing the membrane resistance.The porous gas-repellent coatings can effectively improve the low transmission efficiency of traditional ZrO₂ coatings.The impact of the terephthalic acid/ZrO₂ ratio in the spray emulsion on the surface morphology,bubble wettability,Na Cl transport,and electrochemical performance was investigated.It was verified that the significantly improved ion transport efficiency of the obtained surfaces could be attributed to the introduced terephthalic acid.When the terephthalic acid/ZrO₂ ratio in the spray emulsion was 17:100（PIEM-D-40%）,the membrane surface exhibited superaerophobic properties with a bubble contact angle of 151.3°.Remarkably,PIEM-D-40%showed the lowest voltage and power consumption in chlor-alkali electrolysis.This excellent performance of PIEM-D-40%was attributed to superaerophobicity that reduces bubble adhesion while maintaining efficient Na Cl transport.4.Construction and performance evaluation of micro-nano-structures on the surface of chlor-alkali perfluorinated ion exchange membranes:Based on the experiences from a previous study and the theoretical exploration of ZrO₂ coating,the surface morphology of the PIEM is one of the key factors affecting the gas adhesion on the membrane surface.In this chapter,we used ZrO₂ with different particle size and distributions as a coating material and perfluorosulfonic acid polymer as a binding material to construct three heterogeneous ZrO₂coatings.By adjusting the particle size distribution of ZrO₂,the roughness,pore size and surface coverage of the coating could be adjusted.The results showed that ZrO₂ with a particle size distribution of 0.4-75μm could be used to prepare a"popcorn-like"micro/nano hierarchical ZrO₂coating（M-II）,which exhibited superaerophobicity and efficient ion transport.M-II was used for chlor-alkali electrolysis,and the power consumption was as low as 2037.8 k Wh t^-1 at 6 k A m^-2.The power required to produce 1 ton of 100%Na OH was reduced by 166.2 k Wh and113.2 k Wh compared with pristine PIEM(2204 k Wh t^-1)and commercial Nafion ^TM N2030(2151 k Wh t^-1)at 6 k A m^-2,respectively.The stability test revealed excellent stability of M-II.Negligible cell voltage fluctuations were seen during continuous 360-hour chlor-alkali electrolysis.5.A new-generation domestic chlor-alkali ion exchange membrane（DF2807）was developed based on the research in the previous chapters.From an engineering perspective,the IEC and the microstructure of the domestic chlor-alkali ion exchange membrane were regulated.The bond strength,loading,and uniformity of the functional surface coating were optimized,and a new generation of domestic chlor-alkali ion exchange membrane（DF2807）was produced.The DF2807 and the representative imported chlor-alkali ion exchange membrane were compared,the cell voltage,Cl₂ purity,and cell voltage escalation rate of DF2807 were comparable to those of Nafion ^TM N2050,confirming the superior performance of DF2807 membrane.