High frequency, high current impedance spectroscopy: Experimental protocols enabling measurement up to 1MHz at high current densities
By John Harper, Mike Rust, Brian Sayers and Andrew Savage
Keywords:
GSM, frequency response analyzer (FRA), batteries, fuel cells, supercapacitors, multi-channel potentiostat, AC impedance
Recent advances in the development of high power electrochemical devices, such as fuel cells, batteries and supercapacitors, have been made possible by an improved understanding of the fundamental electrochemical processes. Electrochemical Impedance Spectroscopy (EIS) stands out amongst all other electrochemical techniques since information regarding ohmic losses, electrochemical kinetics and mass transfer processes can be characterised in a single experiment; in marked contrast with traditional DC techniques.
Until recently the bulk of research using impedance techniques focused on the characterisation of cell membranes and the determination of the electrochemical kinetics. The mid frequency range was therefore of most interest. Scientists and engineers have now realised that the entire frequency response curve yields useful data for example on non-Faradaic mechanisms, such as water management, ohmic losses and the ionic conductivity of proton exchange membranes. Therefore impedance is fast becoming a standard analytical tool in high energy, storage device applications.
With the availability of wide bandwidth instrumentation, measurements up to 1MHz are possible. However, due to the bandwidth limitations experienced by potentiostats when they are operating at high DC current levels, valid data acquisition in the high frequency domain is generally limited to below 10kHz. Unfortunately, this has limited the application of EIS to cells with current ratings typically less than 1A. An example of this limitation is the characterisation of solid oxide fuel cells where it is necessary to measure the impedance response at high frequencies c.f. 70kHz. Therefore, engineers have traditionally relied on DC techniques, such as current interrupt, to determine the ohmic losses associated with the stack whilst using the mid to low frequency spectrum from EIS to understand the remaining cell characteristics.
This technical note describes some experimental techniques that overcome the bandwidth limitation at high DC current levels and provide accurate impedance measurements up to 1MHz in potentiostatic mode or 125kHz in galvanostatic mode, thus affording the study of mechanisms that were previously beyond the range of traditional impedance techniques. These techniques apply to the study of high power devices such as fuel cells (SOFC, DMFC and PEMs), supercapacitors and batteries.