Preparation And Investigation Of High Performance&Self-humidifying Membrane Electrode Assembly For Proton Exchange Membrane Fuel Cell Application

Proton exchange membrane fuel cell?PEMFC?is considered as a promising green energy conversion technology due to its advantages,such as high energy conversion efficiency,quick startup at low temperature,and zero or low exhaust,etc.Membrane electrode assembly?MEA?is the key component of PEMFC that consist of proton exchange membrane?Nafion membrane?,cathode/anode catalyst layer?CL?and cathode/anode gas diffusion layer?GDL?,and it plays a key role for the improvement of PEMFC performance and reducing the cost of PEMFC.Generally,Nafion membrane and Nafion ionomer used in the catalyst layer should be kept in wet in order to achieve optimal proton conductivity.The proton conductivity of Nafion membrane will decrease as the relative humidity decreases.Thus,in terms of the practical operating conditions of a PEMFC,an external humidifying device has to be used to maintain enough hydration of the cell system.However,the additional humidifying device complicates the cell's system,increases the cell's cost and reduces the cell's energy consumption.Indeed,developing a self-humidifying or non-humidifying MEA at low or non humidity condition has recently become an attractive topic in the field of fuel cell research.Realizing self-humidification or non-humidification of MEA could improve the cell's power density,decrease the cell's cost and energy consumption,as well as solve the problem of water management in PEMFC.Summarily,it is extremely significant for the commercialization of PEMFC.In this thesis,the preparation and investigation of self-humidifying MEAs were studied.Firstly,a novel self-humidifying membrane electrode assembly?MEA?was prepared by adding hydrophilic polymer agarose?ME?into anode CL for proton exchange membrane fuel cell?PEMFC?.The MEA with addition of ME gained good self-humidifying performance.The influence of hydrophilic organic polymer content,back pressure and relative humidity?RH?of anode and cathode were studied.It was found that the MEA prepared by using ME as hygroscopic agent gains good self-humidifying performance.The MEA with 4 wt% ME achieves the best low humidity performance.The performance of the self-humidifying MEA was hardly changed even if the RHs of both the anode and cathode decreased,but it was decreased as the back pressure decreased.Under a cell temperature of 60 oC,30 psi back pressure and 20% RH,the current density of MEA with 4 wt.% ME in CL is 500 mA cm-2 at 0.7 V and 960 mA cm-2 at 0.6 V,respectively,and stables at 820 mA cm-2 after 40 hours of long-term testing.Secondly,a self-humidifying MEA was prepared by using microcrystalline cellulose?MCC?as hydrophilic additives.The effect of MCC content and relative humidity of both anode and cathode were investigated.The results indicated that the MEA with MCC in anode gained better performance compared to that MEA without MCC;Under a cell temperature of 60 oC,20 psi back pressure and 20% RH,the current density of MEA with 4 wt% MCC in CL is 760 mA cm-2 at 0.6 V,and stables at 680 mA cm-2 after 22 hours of long-term testing.Thirdly,to further improve the performance and stability of PEMFC under high temperature with low humidity,a series of membrane electrode assemblies were prepared by adding hydrophilic organic polymer MCC at different locations?i.e.between membrane and catalyst layer?CL?,in the CL,and between the GDL and CL?,and the effect of addition location on the self-humidification performance of MEAs has been investigated.It was found that the MEA with MCC thin layer between the CL and GDL gained the best performance and high output: the current density of MEA with 0.5 mg cm-2 MCC between the CL and GDL?MEA-MCC5?reaches 1100 mA cm-2 at 0.6 V and the maximum power density is 751 mW cm-2,operating at 70 oC,30 psi and 30% RH,after 24 h operation,the MEA's current density at 0.6 V decreases by only 9.1%,while the performance of the MEA without MCC addition decreases 60% after 3 h operation under the same conditions,demonstrating the former's good self-humidification stability.The MEA with MCC could also exhibit good low humidity performance within the temperature range of 60-70 oC.Furthermore,a novel self-humidification Pt/SnO₂-SiO₂/C anode composite catalyst was successfully developed by using a binary oxide-modified carbon support on which Pt nanoparticles?NPs?were deposited.This work was based on previous study about developing a novel self-humidifying composite catalyst that modified by oxide.The MEA with Pt/SnO₂-SiO₂/C catalyst as anode exhibited good low-humidity performance and stability.It was found that the MEA with Pt/SnO₂-SiO₂/C?6.0 wt% SiO₂ and 4.0 wt.% SnO₂?in anode gained the best self-humidifying performance and stability: under a cell temperature of 60 oC,30 psi and 10% RH,its current density is 600 mA cm-2 at 0.7 V and 1050 mA cm-2 at 0.6 V,respectively,and the current density at 0.6 V decreases by only 16%?to ⁸80 mA cm-2?after 96 h of continuous operation,whereas the current density of the MEA with JM Pt/C as the anode decreases by 60% within 10 h operated under the same conditions.Finally,to further improve the self-humidifying performance of PEMFC,a new type of MEAs was prepared by introducing hydrophilic carbon nanotubes?HCNTs?in CL and GDL.The effect of HCNTs location,content,and relative humidity of both anode and cathode were investigated.It was found that the MEA with 20 wt% HCNTs in both the CL and the GDL?MEA-CLH20-GDLH20?achieves the best self-humidifying performance and stability: the current density of MEA-CLH20-GDLH20 is 800 mA cm-2 at 0.7 V and 1550 mA cm-2 at 0.6 V,respectively,with maximum power density of 991 mW cm-2,operating at 70 oC,30 psi and 30% RH.After 44 h continuous operation,its current density at 0.6 V remains at 1450 mA cm-2,with decreasing of only 6%,while the MEA without HCNTs addition decreases by 45% within 6 h test.