Investigation of Pt/C Catalysts Using Identical Location Transmission Electron Microscopy Under PEM Electrolysis Conditions

Abstract

Polymer exchange membrane (PEM) water electrolysers are based on scarce and expensive platinum nanoparticles materials, seriously affecting the manufacturing cost of commercial electrolyzers. In addition to the cost aspect, studies regarding the long term stability of PEM electrolysers have shown a coarsening of the carbon supported platinum particles leading to an increase of the cell voltage over time (degradation). Unfortunately, the mechanisms responsible for the cathode degradation were not identified in these studies.[1,2] The literature also indicates that the degradation of Pt/C starts at a potential > 0.4 vs. RHE (reversible hydrogen electrode).[3,4,5] However, the working potential of a PEM electrolyser cathode is lower than 0 V vs.RHE, and a degradation of the platinum catalyst on the cathode still occurs. Thus, conversely to fuel cells, it appears that different mechanisms are responsible for the degradation of the platinum active material under cathodic PEM electrolysis conditions. Here, we employed the Identical Location TEM method to unveil the mechanisms like particle coarsening, detachment, and migration of platinum under cathodic PEM electrolysis conditions. For this purpose we simulated the steady operation mode of an electrolyser in a three electrode setup with a catalyst-coated TEM-Grid as a working electrode under different overpotentials. Identical locations on the TEM-Grid were examined before and after the electrochemical experiments. In addition, the same experiments were conducted with catalyst coated gas diffusion electrodes in order to determine the change of the oxidation states of the catalytic material, and the amount of oxygen containing functional groups via X-ray photoelectron spectroscopy (XPS). In the fuel cell literature it is assumed that this functional groups work as an anchor holding the platinum particles in place. A decrease of these functional groups should induce a migration of platinum particles.[6] Our study reveals a dependence between the migration of platinum particles and the applied over potential. A more negative potential induces an increase of platinum particle migration leading to the formation of platinum aggregates. The XPS analysis shows no correlation between the applied potential and the amount of functional groups. Thus, the reduction of the functional groups is not responsible for the particle migration. Finally, we postulate that due to a more negative overpotential at the cathode, the hydrogen coverage rate of the platinum particles is increased leading to a bigger gap between the platinum particles and the carbon support. Consequently, the attractive Van-der-Waals forces are weakened, increasing the mobility of the platinum particles. It is clear from the literature that a reduction of the cathode platinum loading leads to a more negative cathode overpotential.[7] Our study adds a new contribution to this theme, suggesting that the same Pt loading reduction increases the degradation of the cathode. 1) P. Millet, R. Ngameni, S. A. Grigoriev, N. Mbemba, F. Brisset, A. Ranjbari, C. Etievant, Int. J. Hydrogen Energ., 2010, 35(10),5043 - 5042. 2) G. Wei, Yuxin Wang, C. Huang, Q. Gao, Z. Wang, L. Xu, Int. J. Hydrogen Energ., 2010, 35(9), 3951 - 3957. 3) Q. Xu, E. Kreidler, D. O. Wipf, and T. He, J. Electrochem Soc., 2008, 155(3), B228 – B231. 4) Q. Xu, D. O. Wipf, and T. He, Langmuir, 2007, 23 (17), 9098 – 9103. 5) J. Willsau, J. Heitbaum, J. electroanal. chem. interfacial electrochem., 1984, 161(1), 93 – 101. 6) L. Dubau, L. Castanheira, G. Berthome, F. Maillard, Electrochim. Acta, 2013, 110, 273 - 281. 7) H. Gasteiger, J. Power Sources, 2004, 127, 162 – 171.