Study Of Aromatic Hydrocarbon Ionomers For Membrane Electrode Assembly Ofproton Exchange Membrane Fuel Cell

The membrane electrode assembly（MEA）is the core component of the proton exchange membrane fuel cell（PEMFC）,which determines the performance and durability of the PEMFC.The commonly used perfluorosulfonic acid type proton exchange membrane（PEM）in the MEA has high cost and their preparation is not environmentally friendly.Researchers have developed a series of sulfonated polyaromatic hydrocarbon-based polyelectrolyte materials as the proton exchange membrane and have achieved good results.However,the development and application of hydrocarbon ionomer materials in the catalyst layer have not kept pace with the development of hydrocarbon PEMs,and the poor interfacial compatibility between perfluorosulfonic acid-based ionomer catalyst layers and aromatic hydrocarbon-based PEMs has limited the improvement of hydrocarbon performance of PEMFC.Therefore,the study of hydrocarbon ionomers is of great significance for the development of PEMFCs.In this paper,several sulfonated poly（aryl ether）polymers（SPAEK-C,SPAEK-TM,SPEEK）,which are soluble in low boiling point proton solvents such as water/isopropyl alcohol,were synthesized and used as catalyst layer ionomers for PEM fuel cells in Chapter 2 to investigate the effect of ionomer structure on the structure and performance of membrane electrodes.From the experimental results in Chapter 2,SPAEK-TM ionomers with two symmetric dimethyl groups on the backbone of the main chain are more suitable for aromatic hydrocarbon PEMFC catalyst layers.The excellent catalyst layer structure and outstanding electrochemical performance provide SPAEK-TM-based MEA excellent single cell performance,with a maximum power density of 92 m W/cm² in direct methanol fuel cell（DMFC）and514 m W/cm² in H₂/air fuel cell,both of which are better than Nafion^?-MEA.The introduction of SPAEK-TM ionomer improves the interfacial compatibility between CL and PEM and avoids the delamination between Nafion^?-CL and SPAEK-based PEM after long-term operation,which effectively improves the lifetime of the PEMFC.In addition,the introduction of hydrocarbon ionomers can inhibit the penetration of methanol fuel.Based on the relevant findings in Chapter 2,a series of SPAEK-TM ionomers with different degree of sulfonation were prepared and applied to the PEMFC catalyst layer in Chapter 3.Combined with the experimental results in Chapter 2,we found that the degree of sulfonation of the ionomers played an important role in determining the performance of PEMFC.The ECSA of catalyst gradually increased as the degree of sulfonation increased from 0.6 to 1.4,and the ECSA of SPAEK-1.4 was as high as59.53 m²/g _Pt.However,the output power of single cell was the highest for SPAEK-1.2 and then slightly decreased for SPAEK-1.4,because the water management of the cathode catalyst layer became poor when the sulfonation degree was too high,thus decreasing part of the catalyst activity and cell power.After the same level of accelerated stress test（AST）,the performance of SPAEK-1.4 with higher sulfonation degraded more significantly.The above results indicate that increasing the sulfonation degree of the ionomer within a certain range can improve the proton conductivity of the catalyst layer,resulting in increased ECSA and higher cell output power,but further increasing the sulfonation degree can impair the output power and stability of the single cell.For SPAEK ionomers,the degree of sulfonation of 1.2 is a better choice.Based on the findings of the previous two chapters,SPAEK ionomers with a degree of sulfonation of 1.2 were selected and the effect of ionomer molecular weight on the structure of catalyst layer and MEA performance was investigated.The size of the prepared ionomer-catalyst agglomerates of the four SPAEK-TM ionomers with Pt/C decreased with the increasing molecular weight,and the thickness of the catalyst layer became thinner with the increasing ionomer molecular weight.Furthermore,the ionomer with high molecular weight improves the interfacial contact behavior between the ionomer and catalyst,which is beneficial to the proton transfer,and the ECSA and maximum power density of single cell increases.Among them,SPAEK-90K shows the best performance.However,the performance of ECSA and single cell decreases significantly when the molecular weight increases further to 140kg mol^-1,and the resistance to material transport increases.Comparing SPAEK-60K and SPAEK-90K,the degree of decrease in maximum power density after aging is close,indicating the effect of molecular weight on thestability of fuel cell is not obvious.As seen in the previous chapters,the introduction of hydrocarbon ionomers in cathode catalyst layer（CCL）can increase the output power of the cell,but this highly sulfonated poly（aryl ether）ionomer makes the catalyst layer more hydrophilic and the water flooding of CCL is more severe.To solve this problem,in Chapter 5,we added hydrophobic polytetrafluoroethylene（PTFE）to CCL to optimize the water management of CCL and explored the effect of PTFE content on the performance of MEA.According to the experiments in Chapter 5,the addition of a small amount of PTFE to the catalyst layer was able to increase its hydrophobicity without losing its ECSA,where the ECSA was still as high as 48.56 m²/g _Pt when 20 wt%PTFE was added,and the catalyst layer did not become thicker to affect the transport of substances.The polarization curves of each MEA in the H₂/air fuel cell showed that MEA-20PT had the maximum power density of 555 m W/cm²,which is higher than MEA without PTFE（514 m W/cm²）,while the maximum power density just decreased by only 4.32%after 3000 AST aging cycles,compared to 8.56%without hydrophobic treatment.There is a corresponding performance improvement in DMFC.However,when more content of PTFE was added,the catalyst layer became thicker,then the pore structure was blocked by PTFE agglomerates.The ECSA decreased significantly,and the output power of the single cell was also reduced,sothe overall performance was worse than that without hydrophobic treatment.