Electrochemical Impedance Spectra Measurements and Analysis of Polymer Electrolyte Membrane Electrolysis on Short Stack
Authors: Liebert Michael , Joerissen Ludwig , Scholta Joachim 
Published: December 2024

Energy storage is becoming increasingly urgent as the share of fluctuating renewable energies increases in order to utilize the full potential of power plants and stabilize the grid in times of lower yields. Water electrolysis is therefore a promising solution for converting energy into hydrogen, which offers the opportunity of long term, even seasonal energy storage, albeit with lower round-trip efficiency compared to established battery technology.

In a comparative technology assessment among AEM, SOEC, PEM, and AEL Polymer Electrolyte Membrane electrolysis excels in terms of power density >2 A/cm² and partial load stability, although it has disadvantages in terms on precious metal usage in catalysts and highly conductive corrosion resistant coatings for porous transport layers.

In this work the method of electrochemical impedance spectroscopy will be investigated to gain insight into kinetics and transport processes at different operation conditions. Furthermore, we will investigate degradation losses and establish a state of health monitoring at a short stack level. Since stack designs for atmospheric PEM-Electrolysis enables easy single cell voltage monitoring the Impedance response to a superimposed applied AC-current signal can be picked up from each cell individually. Yet, the large footprint of “technical cells” leads to very low overall impedance thus making signal pickup and analysis more challenging compared to single cell or RDE measurements on small active area. Applying a precise high power signal, precise measurement of small AC contributions at a high DC background and the reduction of interferences with other components such as interferences from the power line or ripples in the DC part as well as temporal and spatial homogeneity of the sample under test are posing constraints for the recording and interpretation of electrochemical impedance spectra.

We studied different setups of power electronics with add-on impedance spectrometers, both in serial and parallel configuration. The test object, shown in figure1b), was a 282 cm² active area PEM-electrolysis short stack of 3 cells derived from a an open source “generic PEM fuel cell stack” design which was operated at atmospheric pressure in a Greenlight G200 test rig (figure1a)) modified for electrolysis operation. A zahner IM6 impedance spectrometer in combination an EL1000 was used for the modulation of the alternating current signal. For comparison AC-modulation using a tunable AC frequency generator already implemented in the DC Power supply was studied. The current and voltage signals in this setup were recorded and analyzed using a high-speed transient recorder. The resulting impedance spectra of both devices are shown in Figure 1c) at different current densities. The spectra recorded were analyzed by standard methods via assignment of equivalent circuits. We studied the reproducibility and assignability of different time constants to different processes and equivalent circuit parameters depending on the operating conditions and the actuation amplitudes.

From the results of these studies, we derived optimization potential for the electrolysis stack design. Furthermore, we studied degradation processes as a consequence of accelerated stress tests providing a data basis for electrolyzer stack operation and predictive maintenance.

Figure 1