Electrochemical pressure impedance spectroscopy for transport characterization in electrochemical cells – EPISTEL

A single cell test bench has been developed for controlled EPIS operation, with the necessary metrology of physical and electrical variables. The control-and-data-acquisition card and PC made it possible to apply fluctuations of the outlet pressure at the cathode (air) compartment at given amplitude and frequency, below 1Hz. From the cell voltage response, the Matlab software developed leads to the voltage-pressure impedance (in V/bar) of the cell. P amplitude had to respect the linearity condition of impedance, with low distortion of the sine voltage. Operating conditions have been extensively varied to investigate very different hydration conditions.
A PEM fuel cell model for interpretation of transport data was developed and implemented. The pre-project in-house computational environment using various programming codes (Comsol, Matlab) and solvers, was improved by addition of modules for the polymer electrolyte membrane, for detailed modelling of gas and water transports in the electrode and gas diffusion layer (GDL), and water transport in the membrane. An EPIS model for simulation of pressure impedance spectra has been developed in parallel. Interpretation of experimental EPIS data could then be done for higher understanding of transport in the cell.

• Development of a fully equipped fuel cell test bench with measurements of electrochemical impedance (EIS), electrochemical pressure impedance (EPIS), and Inlet-outlet pressure impedance, for possible discussion and validation of physical models.
• Development of modelling and interpretation tools for characterization of transport phenomena, prediction of electrochemical impedance and electrochemical pressure impedance spectra.
• EPIS is very sensitive to gas diffusion as well as gas transport hindrance by liquid water.
• Based on systematic sensitivity analysis, the EPIS signal response from subdomains of the cell could be identified. EPIS allows separate observation of these subdomains, thus confirming a major assumption of the project proposal.

• Two major phenomena observed by EPIS are presently further investigated with other funds: huge EPIS modulus at moderate frequency upon hindered gas transport, and high EPIS phase shift at low frequency under flooding conditions.
• Although efficient, accurate EPIS acquisition requires more than four hours. A faster procedure with a restricted number of frequencies, is to be developed for sufficiently accurate diagnosis.
• Use of cheaper inducers of pressure fluctuation e.g. loudspeakers will be tested.

Four papers have been published on the principle of EPIS in experimental observation (2 papers) and describing the modelling (2 papers): these “ground papers” provide the potential of the method with preliminary results on the effect of operating conditions e.g. cell hydration, gas excess or oxygen fraction in the air fed. Two more manuscripts focused either on gas transport in dry conditions, or in operation under increasing amounts of liquid water, were recently submitted. Various communications either separate or joined, were presented in symposia and conferences. One PhD was defended in Sept 2021, the second one being planned in 2022.

Electrochemical pressure impedance spectroscopy (EPIS) shows a high sensitivity towards transport processes in cells with gaseous reactants (Grübl, Bessler et al. 2016). This novel technique is based on analyzing the dynamic current/voltage/pressure behavior by either current excitation/pressure detection or pressure excitation/voltage detection with frequencies in the range of 100 Hz to 1 mHz. For polymer electrolyte membrane fuel cells (PEMFC), EPIS is expected to allow the observation of flow and mass transport phenomena with high accuracy. These phenomena, covering gas transport in the gas diffusion layers and the catalyst support as well as evacuation of the produced water, govern the cell performance at high-current operation. The diversity of transport phenomena, their coupling with electrochemistry and temperature, and dependence on structural properties such as pore sizes and size distributions makes understanding and modeling difficult. With conventional techniques, in particular electrochemical impedance spectroscopy (EIS), transport phenomena are difficult to be distinguished from each other and their signal may be (partially) masked by charge-transfer processes.

The key hypothesis governing the present proposal is that EPIS can significantly increase measurement sensitivity and accuracy of transport-related phenomena in PEMFCs. We therefore propose a combined experimental and modeling study of EPIS for PEMFCs. A single-cell setup with pressure excitation and detection will be developed and operated by CNRS – Université de Lorraine (CNRS, France). A dynamic multi-physics model will be developed and used for data analysis by Offenburg University of Applied Sciences (HSO, Germany).
The primary goal of the study is the development and evaluation of EPIS for PEMFC diagnosis. In addition, the investigation is expected to bring a significant contribution on understanding, characterization and quantification of the different transport phenomena involving gases and liquid water in the cell. Finally, the use of low-cost pressure excitation and/or detection equipment such as a loudspeaker and/or pressure sensors will be tested, for the sake of significant reduction in the cost of the overall diagnosis technique. Our vision is that EPIS can become a standard diagnosis tool in everyday lab practice complementing EIS with limited add-on effort (financially and technically) but strongly enhanced output.

The three-year project will consist in continuous interaction between the two partners, CNRS being in charge of the experimental part and generation of data, HSO working on model development and data interpretation. The work packages include a preparative phase for experiments and models; the development and the use of the EPIS tool covering the measurement device and the simulation tool; interpretation of the results; and evaluation and assessment of the EPIS technique.