PtCo-alloy catalysts have achieved mainstream application approximately 20 years after they were first reported. The current state of art PEM fuel cell cathode catalyst is represented by Pt-alloy nanoparticles dispersed on high surface area carbon (HSC) supports. [1] Mass activity > 0.6 A/mg_Pt has been reported consistently for such catalysts. Unfortunately, the high BOL activity of the Pt-alloy nanoparticle catalysts can suffer fairly rapid degradation in mass activity (MA) over time on exposure to fuel cell operating conditions, especially under voltage cycling (V-cycling). The prevalent mechanism for loss of specific activity is due to dissolution of the non-noble alloying element and thickening of Pt shell during voltage cycling. [2] Not withstanding the advances and numerous impactful studies, the degradation of Pt-alloy nanoparticles and its impact on electrode performance is poorly understood. For example, a recent article reviews the conflicting results on the impact of alloying on Pt nanoparticle stability, from positive, to neutral, to negative. [3] Additionally, the existing literature on durability of PtCo alloys are focused on the activity and nanoparticle characterization rather than on associated V-loss in the composite electrode. Given the criticality of this issue, a careful systematic study of this phenomenon with best-in-class materials in a state of art (SOA) membrane electrode assembly (MEA) is desired.

In this present work, the use of operating conditions as a key factor to mitigate electrode degradation is explored. This was accomplished by conducting voltage cycling tests in H₂/N₂ environment at different operating conditions up to 60,000 voltage cycles. To identify the factors affecting the degradation, fractional factorial design of experiments was implemented. The fractional design allows to identify the impact of individual factors and its interactions while reducing the total number of tests needed to be run. Factors in the test matrix include temperature, relative humidity, upper potential limit and upper potential hold time etc. In-situ electrochemical diagnostics such as electrochemical surface area (ECSA), mass activity (MA), specific activity (SA) was measured to understand the kinetics and degradation mechanism. Transport properties such as proton transport resistance and oxygen transport resistance was measured using H₂-N₂ impedance [4] and oxygen limiting current measurements [5]. The end of test (EOT) MEAs were characterized by ex-situ characterization measurements such as TEM for particle size distribution, electron micro probe analysis (EPMA) and EDS measurements to quantify the cobalt composition and loss up on voltage cycling at different operating conditions.

In addition to multi-factor design of experiments, few key factors like RH and temperature were studied independently. Figure 1 shows the impact of relative humidity (RH) on H₂-air performance and measured electrochemical surface area. Up to 100 mV loss at high current density (2.0 A/cm²) and ~30% ECSA loss observed for MEAs cycled up to 30,000 voltage cycles. Both performance and ECSA loss decreases with decreasing relative humidity. Almost zero ECSA loss after 30,000 voltage cycles at 25% RH observed. This example indicates that PtCo alloy catalyst degradation can be mitigated by carefully identifying the operating condition. The effect of various such operating condition and degradation mechanisms will be discussed.

References

1. S. Kumaraguru, 2018 Annual Progress Report, https://www.hydrogen.energy.gov/pdfs/progress18/fc_kumaraguru_2018.pdf
2. C.E. Carlton, S. Chen, P.J. Ferreira, L.F. Allard, Y. Shao-Horn, J. Phys. Chem.Lett. 3, 161 (2012)
3. J.A. Gilbert, A.J. Kropf, N.N. Kariuki, S. DeCrane, X. Wang, S. Rasouli, K. Yu, P.J. Ferreira, D. Morgan, D.J. Meyers, J. Electrochem. Soc., 162 (14), F1487 (2015)
4. R. Makharia, M. F. Mathias, D. R. Baker, J. Electrochem. Soc., 152 (5), A970 (2005)
5. T.A. Greszler, D.A. Caulk, P. Sinha, J. Electrochem. Soc., 159, F831 (2012)