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 powerful 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 interpolation of the results to real 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 extend of the different degradation mechanisms such as Ostwald ripening, nanoparticle coalescence and platinum dissolution are strongly dependent on the environment and operating conditions, 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 Pt/Vulcan catalyst (20 %, Tanaka) based on the loss of electrochemically active surface area (ECSA) throughout a typical load cycle AST with a square wave profile and potential limits of 0.6-1.0 V/RHE. We systematically screened different parameters in the RDE such as temperature, pH, rotation, electrolyte and ionomer content to find an agreement with the degradation rate we observe in complementary 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 accelerates catalyst degradation in the liquid half-cell ASTs. [6] But even at equal temperature (80°C), the loss in active surface area is still much higher in the MEA with only 10 % remaining ECSA compared to 40 % in the RDE after 20k cycles, when the RDE catalyst films are prepared without ionomer. The severity is only slightly increased, when the RDE is being rotated at 1600 rpm. When we apply 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 see a strong increase in ECSA loss measured in 0.1 M HClO4 electrolyte. 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. 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 and another commercial catalyst (not shown in figure) that the RDE AST predicts catalyst degradation accurately, when relevant operating conditions (i.e. temperature and 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