Electrochemical Frequency Modulation (EFM) is a well-known technique which is mostly used for corrosion studies. It is generally performed in potentiostatic mode and allows calculating the corrosion current as well as Tafel parameters by rearranging the Butler‑Volmer equation. It was recently also used for testing fuel cells. However, for electrochemical power systems, galvanostatic control is much more beneficial. Even small potential changes can lead to high currents if a system has a very low internal resistance which can lead to irreversible damages at the cell and safety risks. Nonetheless, no practical and theoretical approach has been published so far for EFM measurement in galvanostatic mode.

This study presents first galvanostatic EFM measurements with fuel cells. The results are compared to other techniques which are typically used for analyzing fuel cells such as polarization experiments, electrochemical impedance spectroscopy (EIS), and current interrupt (CI). Measurements within this study are performed with a small Proton Exchange Membrane (PEM) fuel cell. During galvanostatic EFM experiments, a DC current signal is applied which is superimposed by a perturbation signal consisting of two sine waves with different frequencies (f₁, f₂). The measured voltage answer is more complex due to the non‑linear behavior of fuel cells. The voltage signal can be subdivided into its DC response (zero frequency), harmonics (integral multiples of both base frequencies; f = n×f_1/2 where n is an integer > 0), as well as interharmonics (sum and difference of multiple harmonics; f = n×f₁ ± m×f₂ where n, m are integers > 0). In order to determine these terms, a fast Fourier transform (FFT) algorithm is employed on the measured signal. The voltage response can be plotted versus frequency in a so‑called intermodulation spectrum. Harmonic and interharmonic amplitudes can be then used to calculate various parameters. As basis for all calculations serves the general equation for the cell voltage V_cellof a fuel cell:

V_cell = E_th - η_act - η_ohm - η_conc

The equation includes three major polarization terms that are responsible for voltage loss. These terms are the activation overpotential η_act, the ohmic overpotential η_ohm, and the concentration overpotential η_conc. E_th is the theoretical thermodynamic potential. The equation can be rearranged by expanding and substituting various terms. Similar to potentiostatic EFM, two causality factors can be extracted which can be used to proof validity of the measured results. Applicability of this method and its calculations are tested under various conditions such as different loads and oxygen concentrations. Finally, the results are compared with other techniques.