title: Electrochemical methods to characterize cell performance and in situ degradation at planar and tubular PEM water electrolyzers with low catalyst loading
Creators: name: Sánchez Batalla, Beatriz
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: PEM water electrolysis
subject: degradation
subject: electrochemical methods
subject: distribution of relaxation times

description: Climate change is one of the most relevant topics nowadays. In the last century, the global surface temperature has increased by 1.1 °C and the main contributor is the emission of greenhouse gases, which comes in its majority from the energy sector. Our detachment from fossil fuels is urgently needed but can only be achieved if the intermittent nature of renewable energies will be overcome. For that, storage devices such as batteries and electrolyzers can be used. Proton Exchange Membrane water Electrolyzers (PEMEL) are a promising technology amongst water electrolyzers, but costs have to be reduced and lifetime has to be improved. Within the joint project “Tubulyze” two approaches are suggested: reduction of electrolyzer costs by decreasing the catalyst loading and using Atomic Layer Deposition (ALD) and reduction of manufacturing costs by co-extrusion of tubular half-cells consisting of 3D printed anodes and membrane. However, the influence of these approaches on the lifetime of the cell must be studied. In this work, a variety of planar PEM electrolysis cells (PEMEC) with low catalyst loading are assembled and characterized. Different anodes coated by ALD with low iridium loading are used, starting with reference titanium felts and followed by 3D printed Ti6AlV. Degradation protocols are developed, and the cells are characterized in situ by means of polarization curves (PolCurvs) and electrochemical impedance spectroscopy (EIS). The different overpotentials that occur in the cell can be calculated by fitting the PolCurvs, while the resistances in the cell are identified using EIS. However, for the application of EIS a prior knowledge of the system is needed to quantify the contribution of its components to the overall resistance by using a suitable equivalent circuit model (ECM) but the overlap of different processes in the Nyquist and Bode plots hinders the identification of single components of the system. Thus, a successful identification of the different processes responsible for the resistances in the cell is needed. By applying the distribution of relaxation times (DRT) analysis to evaluate the EIS spectra, the contribution of each component to the full cell resistance can be identified by varying different operation parameters. In this work DRT analysis is used to allocate the different processes taking place in the cells and to optimize the cell configuration varying the cathode and anode catalyst loading as well as the cathode thickness. Furthermore, the components and processes that limit cell performance during the degradation tests are successfully identified and corroborated by postmortem physicochemical analysis of the cell components. The influence of the electrolyte used in the cell performance and cell lifetime is also investigated. Thus, cells operated in deionized water are compared to cells operated in 1 M sulfuric acid. Cell performance decreases while operating with deionized water, due to an increase in the ionic resistance of the cell and cell components. However, a degradation rate 7 times lower is found. Main degradation processes in the different electrolytes are successfully identified by DRT analysis of the EIS spectra. Electrocatalyst activity loss on the anode side is mainly responsible for the decrease in performance of a cell operated with sulfuric acid, while an increase of the ionic resistance and contamination of the membrane are the main processes degrading the cell operated in deionized water. Thereupon, the electrochemical methods used to characterize the degradation of the planar cells are used to characterize tubular cells. The co-extrusion of the membrane and the inner electrode reduces the costs of a PEMEC, but the membrane must be activated before its application in the cell. This activation includes corrosive solvents that damage the inner electrode. Thus, a new, milder method to activate the membrane is developed and optimized. Membranes pretreated with this method are characterized and compared to commercial membranes by means of conductivity, proton diffusion, ion exchange capacity, water uptake and dimensional swelling. Tubular PEMEC activated with this new method are successfully tested and the degradation processes are also identified.