Towards a Realistic Prediction of Catalyst Durability from Liquid Half-Cell Tests

Abstract

Half-cell measurements with rotating disc electrodes (RDE) in liquid electrolyte are a convenient and often used method to determine relevant electrochemical parameters of catalyst materials in early stage development, i.e. when small quantities and/or high number of samples need to be tested. The applicability of the obtained results to real-world systems, e.g. in form of membrane electrode assemblies (MEA) is, however, often challenging. [1] In the presented study, we focus on the reproducibility of voltage cycling-based accelerated stress testing (AST) of PEMFC cathode catalysts (Pt/C). The degradation, i.e. the extent of the different degradation mechanisms such as Ostwald ripening, nanoparticle coalescence, and loss of dissolved platinum into the electrolyte phase are strongly dependent on the environment and operating conditions, both of which differ significantly in RDE and MEA setups. [2-5] Nevertheless, the ability to make realistic predictions of catalyst durability from ASTs in early stage liquid half-cell measurements is highly desirable.To this end, we studied the degradation of a commercial 20%wt Pt/Vulcan catalyst (Tanaka) based on the loss of electrochemically active surface area (ECSA) throughout a typical voltage cycling AST with a square wave profile and potential limits of 0.6-1.0 VRHE. We systematically screened different parameters in the RDE, such as temperature, pH, rotation, electrolyte, and ionomer content, trying to determine the conditions that would yield a similar catalyst degradation rate in the RDE configuration compared to that in actual MEA measurements. Additionally, particle size distributions of the degraded catalysts were determined from TEM images to gain deeper insights into the changes in degradation mechanisms on the nanoscale. The excerpt of our results depicted in the attached figure show that the ECSA loss in the MEA could by far not be reached in typical RDE measurements carried out at room temperature (22°C). In accordance with Urchaga et al., we see that elevated temperature (80 °C) accelerates catalyst degradation in the liquid half-cell ASTs. [6] When we prepared catalyst films with Nafion® with an I/C ratio of 0.7, which is comparable to what we use in the MEA but at the same time beyond what is typically applied for RDE catalyst films, we observe an additional increase in ECSA loss in comparison to ionomer-free catalyst films cycled in 0.1 M HClO4. The high density of sulfonate groups in the ionomer lowers the local pH and they are known to adsorb on the Pt surface, which might be both factors for increased platinum dissolution-related degradation. ASTs in 1 M HClO4 and 0.05 M H2SO4 reveal that both lower pH and the presence of adsorbing anions accelerate the ECSA loss and influence the particle growth. We conclude, that high exposure of platinum particles to the sulfonate groups of the ionomer leads to faster degradation during potential cycling. We could show for Pt/Vulcan (as well as for other catalysts not shown in the attached figure) that the RDE AST can predict catalyst durability as accurately as am MEA, when relevant operating conditions (temperature) and electrode compositions (I/C ratio) are applied.[1] T. Lazaridis, B. M. Stühmeier, H. A. Gasteiger and H. A. El-sayed, Nat. Catal., 2022, 5, 363–373.[2] R. Sharma, S. M. Andersen, ACS Catal. 2018, 8, 3424–3434.[3] K. Ehelebe, D. Escalera-López, S. Cherevko, Curr. Opin. Electrochem. 2021, 100832.[4] H. Yu, M. J. Zachman, C. Li, L. Hu, N. N. Kariuki, R. Mukundan, J. Xie, K. C. Neyerlin, D. J. Myers, D. A. Cullen, ACS Appl. Mater. Interfaces 2022, DOI 10.1021/acsami.1c23281.[5] J. A. Gilbert, N. N. Kariuki, X. Wang, A. J. Kropf, K. Yu, D. J. Groom, P. J. Ferreira, D. Morgan, D. J. Myers, Electrochim. Acta 2015, 173, 223–234.[6] P. Urchaga, T. Kadyk, S. G. Rinaldo, A. O. Pistono, J. Hu, W. Lee, C. Richards, M. H. Eikerling, C. A. Rice, Electrochim. Acta 2015, 176, 1500–1510. Figure 1