Denis Bona, Francesco Pedrazzo, Ilaria Rosso, Dennis E. Curtin, Elena M. Tresso
International Workshop on characterization and quantification of MEA degradation processes, Grenoble, France // 26 – 27 September 2012
Carbon black based electrodes are generally recognized as state of art for PEM fuel cell technology due to the high performance achieved with relatively low Pt content. However, the catalyst carbon support is prone to carbon oxidation. This leads to loss of catalyst area and overall performance, as well as higher mass transport loss due to an increased flooding tendency.
This phenomenon is particularly severe when the fuel cell experiences repetitive start-stop cycles. Therefore, specific countermeasures against catalyst layer carbon oxidation are required, especially for automotive and backup power applications, where the startup/shutdown rate is considerably high. The authors evaluated a basic design that uses a stack shunt. A properly modified control protocol, which includes the stack shunt, is able to avoid high cathode potential peaks, which are known to accelerate catalyst carbon support corrosion and its negative effects. During two separate durability tests, one adopting the shunt design and another using non-protected shutdown, a 24-cell stack was subjected to continuous starts and stops for several months and its performance constantly monitored. Results show that when the shunt is used, there is a 37 percent reduction in the voltage degradation rate for each startup/shutdown cycle, and a two-fold increase in the number of startup/shutdown cycles before an individual cell reached the specified “End of Life” voltage criteria. Furthermore, ex-situ FE-SEM analysis revealed cathode catalyst layer thinning, an indication that the emerging degradation mechanism is the catalyst support carbon corrosion, as expected.
This provides further support that the constant voltage degradation rate typically experienced in PEMFCs can be attributed primarily to the catalyst support carbon corrosion rate. The proposed shunt protocol is very cost-effective and does not require any substantial changes in the system. For this reasons, its adoption is recommended as a viable method to decrease the catalyst support carbon corrosion rate and extend the operating life of the PEMFC stack.