Evaluation of Degradation Processes in Alkaline and PEM Water Electrolyzers

Abstract

In the struggle against climate change, hydrogen has been considered as a key player, since it can be used as a fuel in electrochemical devices, but also as feed in the chemical and metallurgical industry. However, only if this hydrogen is produced by CO2-free process, the net zero-emission goals will be achieved. Green hydrogen can be obtained by water electrolysis using renewable electricity as power source. While alkali (AEL) and acid (PEMEL) electrolyzers are the most advanced technologies, further improvements especially in terms of catalyst stability are still needed to increase their lifetime and economical profitability for megawatt and gigawatt scaling-up.In this work, we report on electrochemical degradation tests of commercially available AEL and PEMEL single cells with 5 cm2 active, geometrical surface. The AEL anode and cathode consist of Raney-Ni electrodes with PTFE binder supported on a Ni grid, separated by a Zirfon diaphragm. 30% KOH at 80°C was used as electrolyte in both chambers. The commercial catalyst coated membrane (CCM) for PEMEL consists of 2 mgIr/cm2 anode and 1 mgPt/cm2 cathode coated on Nafion N117. Titanium felt and carbon paper were used as porous transport layers in the anode and cathode side, respectively. Ultrapure water at 60 °C was used as electrolyte in both sides. For long-term electrochemical tests, current density of 0.5 A/cm2 and 2 A/cm2 was applied to the AEL and PEMEL cells, respectively, for 1000 h. Since at nominal current density, low degradation rate was expected, accelerated degradation tests (ADT) were performed at two identical cells by using triangular polarization profile aiming to reflect dynamic electrolyzer operation in case of e.g., direct coupling with wind turbine. Thereby, the current density was varied between 0.05 - 1 A/cm2 for AEL and 0.05-3 A/cm2 for PEMEL for at least 1000 h. After each 100 h period during both constant and triangular polarization experiments, the cell was electrochemically characterized by impedance spectroscopy and current/voltage (UI) profiles and anolyte and catholyte were collected for ICP-OES measurements in order to evaluate dissolved species. After test, the cells were disassembled and the cell components were analyzed by SEM/EDX, XRD and XPS technique. By correlating the physicochemical with the electrochemical changes in the components of the cell, main degradation mechanisms will be exposed.