Sensivity Analysis of a PEM Electrolyser Cathode with Respect to the Platinum and Nafion Loading

The energy excess of intermittent power sources can be stored as hydrogen. Polymer electrolyte membrane (PEM) electrolysis is an efficient way to produce electrolytic hydrogen.^[1] Due to the acidic environment only scarce and expensive noble metals like platinum and iridium are used as cathode and anode catalysts respectively.

To cut down the investment cost of a PEM electrolyzer, one of the main tasks is the reduction of the noble metal amount while retaining or increasing the performance of the electrolyser.^[2] Thereby the influence of the decrease of the noble metal loading and possible degradation of the active material on the durability of the HER catalyst are both essential.

In this study we investigated the degradation of catalyst coated membranes (CCM) under constant and intermittent PEM electrolysis conditions. For this purpose we conducted long term test (1000h) with 4 different commercial CCMs each having different cathode loadings. To date, the durability of CCMs is examined under moderate current densities (0.9 to 1.4 A cm^-2).^[3] However, in this study the long-term test were performed at a current density of 2 A cm^-2 to simulate real electrolysis conditions. As a next step we manufactured CCMs with a low platinum loading and different Nafion amounts at the cathode to examine the influence of the ionomer content on the long-term durability. TEM, XRD and EIS were applied to monitor physical and electrochemical changes during the durability test. The cathode and anode potentials were measured with the help of a reference electrode to determine the cause of degradation.^[4]

CCMs with a platinum loading of 0.8 mg_Pt cm^-2 showed a small degradation rate at a constant and dynamic operation mode. We observed that the reduction of the cathode loading to 0.05 mg_Pt cm^-2 lead to a 5 fold increase of the degradation rate at the constant and  the dynamic operation mode. The increase of the cathode potential during the durability test indicates a degradation of the cathode catalyst, which was verified by TEM and XRD analyses: Agglomeration of the platinum particles on the carbon support was observed.

Our study clearly identifies the mechanisms responsible for the degradation of the cathode. This information opens up new possibilities to implement counter measures against the deterioration of the Pt/C catalyst, and to produce cost effective CCMs with low loadings and high durability.

1)         M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hyd. Energ., 2013, 38(12),4901 - 34.

2)         K. Ayers, Annual Merit Review DOE Hydrogen and Fuel Cells and Vehicle Technologies Programs, Washington, DC, 2014. Http://www.hydrogen.energy.gov/pdfs/review14/pd098_ayers_2014_o.pdf

3)         M. Debe, J. Electrochem. Soc., 2012, 159(6), K165-K176.

4)         S.A. Grigoriev, P. Millet, V.N. Fateev, J. Power Sources, 2008, 177(2), 281 - 285.