Strategies to mitigate Pt dissolution in low Pt loading proton exchange membrane fuel cell: II. A gradient Pt loading design

In the second companion paper on low Pt-loaded gradient cathode, the degradation of Type II cathode with higher Pt loading (60 wt%) near the cathode/membrane interface was studied and compared to a control cathode with uniform Pt loading (40 wt%). All catalyst coated membranes were deposited through reactive spray deposition technology with average Pt particle size of 2 nm and Pt loadings of 0.05 mg cm-2 and 0.1 mg cm-2 on anode and cathode, respectively. The Type II cathode showed improved fuel cell performance at beginning-of-test (BOT) due to higher Pt utilization benefited from high Pt loading close to the membrane. Although the loss of electrochemical surface area for the Type II cathode was ∼80%, similar to that of the control cathode, the end-of-test performance (EOT) was improved by 50-100 mV, which can be attributed to the reduction of Pt loss in the Pt depletion zone (from 80% to 60%) as well as from 30% to 18% for the entire cathode. A comparison between Type I and Type II cathode was made in terms of Pt retention and particle size distribution. Type II cathode turned out to be more effective than Type I cathode as a result of being able to maintain a higher percentage of smaller particles (<5 nm), even though the Pt loss% was a slightly higher than the Type I cathode. Therefore, when evaluating the end-of-test performance, two crucial parameters need to be considered: (1) the retention the Pt particles in the depletion zone within 0-2 um from the cathode/membrane interface and (2) the percentage of small Pt particles in the cathode at EOT. The usual performance indicator, the electrochemical surface area, ceased to be an effective parameter when the Pt/C catalyst has been degraded.