Research On Degradation Mechanism Of Proton Exchange Membrane

Proton exchange membrane(PEM)is one of the key materials in the PEMFC. Durability of the proton exchange membranes is not only one of the most important criterion of proton exchange membranes performance,but also technical obstacle to the commercial viability of PEM fuel cell.The practical improvement of the PEM opertation lifetime is dissatisfied due to the lack of understanding of degradation mechanism.This thesis evaluate the durability of PEMs from chemical durability and mechanical stability.We use ex situ accerlate tests to study membrane degradation,soaking a piece of membrane in Fenton's regent.Cu²⁺/H₂O₂ and Cr³⁺/ H₂O₂ were also used to compare degradation mechanism of various ions.Besides, mechanical durability is also very important to the PEM since mechanical fission of membranes in the form of pinholes and tears has been frequently observed in the decomposed PEMs.Desiccation-moisture experiment was used to compare the physical degradation behavor of Nafionl 11 membranes and PTFE/Nafion composite membranes.The results and conclusions are summarized as following:(1)In Fenton's accerlated system,the appearance of the deteriorated membranes aslo changed a lot.The SEM micrographs of the PEMs show that there are many small bubbles appeared in the PEMs.The physical peroperties of the membrane were affected because of the changed molecule structure of Nation and the lost groups.With the lose of-SO₃^-,the proton conductivity of PEMs changed. The mechnical strength of Nafionlll membranes decreased because of the scattered structure of deteriorated Nafionl11 membranes.The decreased proton conductivity and mechnical strength of PEMs just mean a weaker membrane.H₂ crossover rate which is caused by pinholes and bubbles is a key point to the failure of the PEMs after sudden jump.(2)The degradatioin rates of Nafionl11 membranes are different in various ions accerlated systems.It was found that the presense of Fe²⁺and Cu²⁺greatly enchances the decomposition rate of PEMs.This is due to the formation of strongly nucleophilic radicals such as hydroxy and hydroperoxy radicals upon decomposition of H₂O₂ with the catalyst ions.(3)The NMR measurements of deteriorated Nafion-Fe²⁺solution demonstrated that the fragments of the polymer were mostly existed as whole side chains of the Nation membranes.This result mentioned that the degradation was generated from the decomposition the polymer main chain.The results of FTIR and NMR measurements of deteriorated membranes revealed that both the main and side chains are decomposed at similar rates.So we concluded that the degradation of the PEMs starts from the defect ends of the main chain and result in the loss of the polymer repeat units.With the increase of repeat unit loss,little voids and pinholes would appear in the pronton exchange membrane.These little voids and pinholes gave the membrane hazards of gas crossover and led to a catastrophic failure of the proton exchange membrane.(4)The desiccation-moisture experiment was used to study the mechnical degradation of PEMs under the low stress in a long time.The Nation211 membrane is stable when stress is lower than 1.70 MPa,while the water-uptake generating stress can reach to 2.23 MPa soaked in surroundings with condition of the 25%RH and temperature at 25℃.The study also found that PTFE/Nafion composite PEMs are more durable than the Nation membrane mostly due to the lower water-uptake generating shinkage stress of 0.4 Mpa(25%RH 25℃).The thickness of Nation membranes became thinner and swelling rate decreased after hundreds of cycles, while there is no obverous change of thickness and swelling rate for PYFE/Nafion composite PEMs.