Spatially Resolved Electrochemical Impedance Measurement in a PEM Fuel Cell Using an Array of Reference Electrodes

Electrochemical impedance spectroscopy (EIS) is a widely used technique for fuel cell characterisation that can be applied in situ with minimal modification to commercial fuel cell hardware [1]. The technique involves applying a small ac stimulus to the system and measuring the response across a wide frequency range, typically 0.1 Hz - 10 kHz . The cell voltage is measured and various techniques are used to deconvolute the contributions from anode, cathode and membrane to the overall impedance. For PEMFCs, the anode component is often assumed to be negligible due to the more rapid kinetics of the hydrogen oxidation reaction compared to the oxygen reduction reaction. However, conventional EIS gives an average impedance across the entire area of the cell and provides no information on local variations that exist due to inhomogeneities in reactant concentration, water distribution and catalyst utilisation. The use of segmented cells to obtain a functional map of fuel cell performance is well established [2,3]. However, deconvolution of anode and cathode components in the impedance response is only possible if the electrode of interest is measured against a stable reference electrode, which is particularly important where air bleeding or fuels other than pure hydrogen are used in the anode. A reference electrode is also necessary for measurements to validate physical-based models used to fit impedance spectra.

Recently, an innovative reference electrode was developed at NPL that makes contact with the electrode of interest via a Nafion tube salt bridge inserted through the end plates of the cell [4]. An ionic pathway through the gas diffusion layer is achieved by impregnating with Nafion at the point of contact. This gives a localised measurement of electrode potential and avoids the problems of ohmic drop and edge effects experienced by other reference electrode configurations. A significant advantage of the design is that commercial MEAs can be tested directly, with minimal modification to cell hardware. Using an array of these reference electrodes, a spatial map of electrode potential can be generated, as previously demonstrated during start-up/shut-down [5]. Here we present the application of this reference electrode design to localised EIS measurement without the need for a segmented cell or complicated multiple frequency response analysis (FRA) equipment. The spatial variation in impedance in a commercial prototype single cell was mapped under a range of operating conditions using an array of reference electrodes and a single FRA with auxiliary voltage inputs. The advantages and disadvantages of this technique and the implications for modelling and optimisation of fuel cell design are discussed.

References

[1] X Yuan, H Wang, J Colinsun, J Zhang "AC impedance technique in PEM fuel cell diagnosis—A review" International Journal of Hydrogen Energy(2007) 32 (17) pp 4365-4380

[2] LC Pérez, L Brandão, JM Sousa, A Mendes “Segmented polymer electrolyte membrane fuel cells—A review” Renewable and Sustainable Energy Reviews (2011) vol. 15 (1) p. 169-185

[3] N. Zamel, A. Bhattarai, D. Gerteisen “Measurement of Spatially Resolved Impedance Spectroscopy with Local Perturbation” Fuel Cells(2013) 13 (5) pp 910–916

[4] G Hinds, E Brightman “In situ mapping of electrode potential in a PEM fuel cell” Electrochemistry Communications(2012) 17 pp 26-29

[5] E Brightman, G Hinds “In situ mapping of potential transients during start-up and shut-down of a polymer electrolyte membrane fuel cell” J. Power Sources (2014) 267 pp 160-170