The prototype RAMMS, shown in Figure 1(a), was designed and developed with the following functions and features in mind: (1) rapid and simple sample loading and unloading of bare membrane, (2) safe, long-term, unattended operation, (3) stable control of the cell environment (temperature, RH), and (4) reproducible electrode clamping pressure, (5) user-selectable operating parameters (electrode pressure, temperature, RH, conditioning time, etc.) to facilitate evaluation of membranes of various properties (thickness, conditioning kinetics) and/or with different resistance specification conditions.
After developing a detailed specification for the RAMMS in collaboration with an industrial stakeholder, we designed, fabricated and tested a prototype unit. The system includes a novel humidification and environmental control system; integrated sample chamber, electrode and actuator system; impedance measurement for through-thickness membrane resistance; and application software.
Studies with the prototype revealed that electrode durability, in particular adhesion of platinum deposit on a porous metallic substrate, was the most significant technical challenge and performance limitation of the device. It was concluded that acceptable durability could not be obtained with any of the deposition methods and post-deposition treatments designed to improve coating adhesion. Custom-fabricated porous platinum electrodes exhibited acceptable durability.
In addition to conditioning time, the electrode squeeze pressure was identified as a primary test variable. Quantification of the effect of electrode squeeze pressure on the area-normalized HFR is shown in Figure 1(b). As hypothesized, lower HFR is strongly correlated with greater squeeze pressure. This is rationalized as being primarily driven by decreasing interfacial resistance with increasing electrode contact pressure, and secondarily, some compression and thinning of the membrane. In addition to the decrease in mean HFR, the range and standard deviation also decrease with increasing squeeze pressure. At the highest pressure investigated (2.36 MPa), the relative standard deviation (= standard deviation / mean) was 2.6% (n = 9).
The dependence of the HFR on the electrode contact pressure demonstrated by the data in Figure 1(b) is not inconsistent with the statements that variation in the pressure is not the primary source of variability in HFR. We observed that the effect of contact pressure on the HFR was -0.044 mΩ/kPa (-0.3 mΩ/PSI). That is, the resistance decreased approximately 0.044 mΩ per kPa (0.3 mΩ per PSI) increase in squeeze pressure. Other results indicated that the test-to-test variation in squeeze pressure was < 6.9 kPa (< 1 PSI). Therefore, < 0.3 mΩ of variation could be attributed to test-to-test variability in the electrode contact pressure, which is smaller than the observed variability. Thus, the bulk of the test-to-test variation in observed resistance was not due to variation in the electrode squeeze pressure.
Acknowledgement: This work was supported by a U.S. Department of Energy Small Business Innovation Research grant (DE-FG02-06ER84574). The support of Will Johnson (W.L. Gore & Associates) and the Gore Fuel Cell Group for helpful discussions and technical assistance is gratefully acknowledged.