Synthesis Of Anionic Ionomer Based On Piperidine Groups And Study Of Its Catalytic Layers

Anion-exchange membrane fuel cells(AEMFCs)have potential cost advantages over proton-exchange membrane fuel cells(PEMFCs)due to the use of non-precious metal catalysts.The membrane electrode assembly(MEA),as the heart of the fuel cell,is mainly composed of a gas diffusion layer,a catalytic layer and an anion exchange membrane(AEM).The catalytic layer where chemical reactions take place plays a key role in the performance of membrane electrodes.The catalytic layer is composed of a catalyst and an ionomer,the properties of the ionomer will affect the stability of the catalytic layer,and the distribution of the ionomer will affect the microstructure inside the catalytic layer.In order to avoid the separation phenomenon between the catalytic layer and the exchange membrane caused by the incompatibility between the anion exchange membrane and the ionomer in the membrane electrode,the same polymer as the anion exchange membrane is often used as the ionomer in current research and applications.However,this kind of polymer generally has the problems of high swelling and poor solubility.In actual working conditions,repeated changes in temperature and humidity easily lead to changes in the physical structure of the catalytic layer and affect the stability of the fuel point cell.The development of low-swelling and highly soluble catalytic layer ionomers is imminent.However,existing researches mainly focus on the development of anion exchange membranes and non-precious metal catalysts,ignoring the research on ionomers.Based on this,this paper designed a low-swelling soluble ionomer by structural modification of anion exchange membrane materials,tested its structural properties and explored the regulation mechanism of molecular weight on the structure and properties of ionomers.In addition,the structure of the catalytic layer was prepared based on the modified ionomer proposed in this paper,and the effects of the molecular weight of the ionomer and the solvent of the ionomer on the microscopic morphology and performance of the catalytic layer were studied.Finally,the effect of low-swelling ionomer against cracks in the catalytic layer was tested by accelerated aging experiments.The main conclusions are as follows:1.Quaternary ammonium poly(N-methyl-piperidine-m-terphenyl)ionomers were prepared using m-terphenyl with kink structure and highly stable piperidine groups,and the structure and properties of the ionomers were characterized.From the molecular conformation map of the ionomer,it can be seen that the m-terphenyl backbone ionomer displays a kink structure that folds back,forming a microscopic phase-separated structure that facilitates ion conduction.The special structure of the m-terphenyl backbone leads to a reduced degree of piperidination on the backbone and thus a low ion exchange capacity.At the same time,due to the special structure of the main chain,the ionomer has the characteristics of low swelling,high solubility and high ionic conductivity.In addition,ionomers with three molecular weights were synthesized by controlling the stoichiometric ratio of the reactants based on the development of the molecular structure of the meta-terphenyl chain ionomer,and the effect of molecular weight on the structure and properties of the ionomer was explored.It was found that the molecular weight of the ionomer affects the degree of polymerization of the ionomer.With the increase of the molecular weight,the piperidine group on the main chain increases,the viscosity,swelling property and ionic conductivity of the ionomer all increase.2.Six groups of catalytic layers were prepared using three ionomer solvents and ionomers with three molecular weights,respectively.The influence mechanism of ionomer solvent and molecular weight on the structure of catalytic layers was explored through morphology characterization methods and performance tests at different scales.The results show that the solvent with the highest polarity has the strongest solvation ability,enabling the ionomer to be uniformly and thinly distributed on the catalyst surface.The solvent with the lowest boiling point and the highest viscosity has the highest deposition rate,and is rapidly deposited on the catalyst surface during the spraying process of the catalytic layer,forming evenly distributed secondary pores,and the internal structure of the catalytic layer is the most uniform.At the same time,the larger the molecular weight of the ionomer,the greater the intermolecular interaction force,and the stronger the viscosity of the ionomer.The viscosity of the ionomer plays an important role in the distribution of the ionomer and the formation of pores in the catalytic layer.In the primary pores in the agglomerates formed by the catalyst and ionomers,the ionomers with moderate viscosity are uniformly distributed on the outside of the agglomerates,retaining moderate gas transport mesoporous channels and providing sufficient ion transport channels.The internal structure of the catalyst layer prepared by medium molecular weight ionomer is the most uniform.Finally,the catalytic layer was prepared based on the developed low-swelling ionomer and the modified high-swelling ionomer,and the effect of swelling degree on the mechanical decay of the catalytic layer was tested by accelerated aging test.The results show that the catalyst layer prepared by the newly developed low-swelling ionomer has a 75% lower crack growth rate during the dehydration process than the high-swelling ionomer catalyst layer,and the mechanical attenuation of the catalyst layer is obviously delayed and the durability is improved.