An important challenge for PEMFC is stability and durability of the membrane separator. In this dissertation, we applied both experimental and modeling methods to investigate the chemical durability of PFSA membranes for fuel-cell applications.
Degradation data were collected and the membrane samples were analyzed by XPS after Fenton’s test; FTIR was also invoked to validate the XPS results. The effects of Fe2+ concentration and temperature on membrane degradation were discussed. Following fuel-cell durability tests, the degraded MEA was also analyzed and results were compared with those from a fresh MEA. This is the first application of XPS to correlate surface analysis and degradation studies of Nafion® membranes. The experimental results provide evidence of chemical attack of the CF2 backbone.
In-situ fuel-cell tests were conducted to study the effect of operating conditions such as relative humidity, oxygen partial pressure and temperature on membrane degradation by measuring FER from exhaust water. The impact of water transport was investigated by varying the humidity at each electrode of the fuel cell. An example of a long-term durability test result was also presented.
Since the level of H2O2 was found to be key to membrane degradation, we designed a novel spectrophotometric method to quantitatively determine H2O2 concentration in a fuel cell by using a multilayer MEA. The effects of relative humidity, oxygen partial pressure and membrane thickness on H2O2 concentration were studied. H2O2 emission rates were measured in anode- and cathode-only MEAs to separately study H2O2 formation at each electrode. In addition, a model for H2O2 formation, transport, and reaction in PEMFCs is established for the first time to validate experimental data and study formation mechanism.
Catalyst agglomerates were included in this H2O2 formation model, thereby allowing profiles of oxygen and H2O 2 concentration inside the fuel cell to be simulated. The average H 2O2 concentration in the membrane was predicted under different operating conditions. Membrane properties, including membrane thickness, levels of metal ion contaminants, oxygen diffusivity, were varied to evaluate their effects on H2O2 concentration in the membrane. Moreover, electrode properties such as thickness, catalyst activity, etc. were studied to minimize H2O2 formation in the fuel cell. Insights to reduce the formation of H2O2 and to extend membrane lifetime were suggested.
The humidity effect on membrane degradation was studied by collecting vent water during the tests. The membrane conductivities and mechanical properties were measured by ex-situ high-throughput instruments. The ion exchange capacity of membrane samples was determined by ICP Emission Spectrometer. FTIR was applied to study both the formation of new groups and the relative abundance of existing groups in the degraded membrane. The thermal stability of degraded membranes was determined by TGA. The cross section of a degraded MEA sample was imaged with SEM to investigate the mechanical structure change. Simulation results from a simple degradation model were compared with experimental data. The representative reaction pathway in each degradation scheme was also postulated.
The effect of temperature on membrane degradation was also investigated. FERs were determined, and the apparent activation energy was calculated from an Arrhenius Equation. XPS spectra were collected from both anode and cathode sides of fuel-cell membrane to compare the effect of temperature on each side. Atomic analysis was performed to study the impact of temperature on both backbone decomposition and side group degradation. A multilayer MEA was used to study the effects of location and thickness on membrane degradation. An improved kinetic model of membrane degradation was built to simulate the experimental data.
Finally, an attempt to mitigate membrane degradation by using peroxide decomposition reagent was performed. OCV curves were recorded during two fuel-cell durability tests with and without the addition of this additive. Both FER and TER were compared. Recommendations for the improvement of peroxide decomposition additive were suggested.