(Digital Presentation) Concentration Admittance Spectroscopy for Oxygen Transport Diagnostics in Polymer Electrolyte Fuel Cells
Authors: Sun Ying , Kadyk Thomas , Kulikovsky Andrei , Eikerling Michael 
Published: November 2022

Polymer electrolyte fuel cells will be crucial as efficient and environmentally benign energy conversion devices in a sustainable hydrogen economy. The further development and deployment of PEFCs require powerful diagnostic tools to assess the interplay of transport and reaction processes in an operating cell, extract the relevant parameters, and perform causal analyses of deviations from healthy cell operation. The diagnostic capabilities of electrochemical impedance spectroscopy (EIS) are widely known and extensively exploited in disentangling the transport and reaction processes in electrochemical cells. The kinetics of the oxygen reduction reaction (ORR) and thus the current density produced by a PEFC at a given cell voltage are not only sensitive to modulations in the electrode potential (or cell voltage), as used in EIS, but they are also affected by modulations in oxygen concentration. The latter effect gives rise to another impedance-type response referred to as concentration or pressure impedance. An oxygen concentration impedance (ζ = δE/δc, where δE and δc are the small-amplitude harmonic perturbations of cell voltage and oxygen concentration) could provide useful complementary capabilities to scrutinize oxygen transport processes. Various experimental works have explored the possibility of probing the response of the PEFC cell voltage with small-amplitude periodic perturbations in oxygen concentration or gas pressure1-6 and numerical models have been developed to rationalize these response functions.4,7-9 The presented work builds on a recently developed analytical model for the oxygen concentration/pressure impedance.10,11 In that work, the limit of large air flow stoichiometry and large oxygen transport loss in the catalyst layer was considered. The present work relaxes these assumptions and it focuses on the case of the so-called concentration admittance spectroscopy, which is based on the hitherto unexplored idea of measuring the response in the oxygen concentration variation to a voltage perturbation. We will present a newly developed quasi-2D model for the cathode side concentration admittance of a PEFC that accounts for oxygen transport in the flow-field channel, in the gas diffusion layer, and in the cathode catalyst layer. An analytical expression for the concentration admittance will be presented and parametric dependencies of the static admittance will be discussed. We will demonstrate how information on the oxygen transport coefficients in the flow field channel, gas diffusion layer, and catalyst layer can be drawn from the admittance at the air channel outlet.
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