The membrane is a key component in the fuel cell and acts as an electrolyte for transporting hydrogen protons from anode to cathode, while the electrons flow through an external circuit. Also the membrane has to separate the gas spaces of the anode and cathode and prevent hydrogen to cross to the cathode side. Meanwhile membranes are constructed getting thinner, because reduced thickness improves proton conductivity and water balance of the membrane. On the contrary the electron conductivity and the gas permeability are increased, in particular to hydrogen. This leads to a reduction in efficiency due to parasitic hydrogen consumption as well as by mixed potentials on the cathode electrode. In addition the mixing of hydrogen and oxygen possibly leads to formation of hydrogen peroxide. This causes degradation of the membrane and generation of pinholes, whereby the gas permeability is increased. Hence the hydrogen crossover represents a fundamental factor for the lifetime of a fuel cell.
The most frequently used techniques for the evaluation of hydrogen crossover are linear sweep voltammetry (LSV) or cyclic voltammetry (CV). The cathode is supplied with humidified nitrogen, while the anode is feed with humidified hydrogen and functions as counter and reference electrode. A potentiostat is used to apply a potential ramp with a small slope (e.g., 2mV/s) between to potentials. At the same time the resultant current is measured. Since the cathode is only fed with humidified nitrogen, the measured current results from electrochemical oxidation of hydrogen that has crossed through the membrane from anode to cathode, and from the electrical short of the fuel cell. The results of a CV are shown in “Comparison of CV and PSM”, blue curve. It can clearly be seen that the results are falsified, despite the low potential rate by adsorption and desorption of hydrogen, as well as loading and unloading of the capacitive double layer of the MEA. Furthermore, due to the long measurement time this method is very sensitive to variations of the operation parameters (e.g., pressure, temperature, humidity…).
With the new established Potential Step Method (PSM) the current is only measured at a few different potentials. Here, the potential is kept constant until adsorption, desorption and capacitive loading processes are completed. Thereby the current solely results from the electrical short and from the oxidation of hydrogen that crossed over from the anode to the cathode. In Figure “Potential Step Method (PSM)”, the chronological progress of such a measurement is illustrated. In this measurement, constant currents were usually achieved after less than one minute per point. Since, in theory, only two data points are required, the necessary measurement time for the evaluation of hydrogen crossover can significantly be reduced compared to CV or LSV. Figure “Comparison of CV and PSM” shows the results of the PSM measurement as red dots. If the data is fitted to a linear equation, the y-intercept equals the hydrogen crossover expressed in mA/cm² and the slope correspondes to the electrical short of the membrane.
This method is now used at ZBT for qualification of membranes additionally to direct permeation measurement and is available for service.
- Division fuel cells and systems
- Division electrochemistry and coating
- service portfolio: component qualifcation
