Edge Effects on Reference Electrode Measurements in PEM Fuel Cells

Abstract

A reference electrode specifically designed for proton exchange membrane ͑PEM͒ fuel cells ͑FCs͒ was developed. By using the Nafion membrane material as the electrolyte, this reference electrode eliminates the needs to modify the normal FC setup and can be easily integrated into FC systems. Experimental and modeling studies had been conducted to further understand the effects of misalignment in the main electrodes on the reference electrode signals. It was found that the reference sensing point is greatly affected by the relative geometry and alignment of the two electrodes on the main membrane and electrode assembly ͑MEA͒ and the nature of electrode kinetics. Even with a perfect alignment, the reference sensing point is no longer fixed at the middle of the membrane due to difference in the kinetic characteristics at the anode and the cathode. When the misalignment is greater than the membrane thickness, the potential that the reference electrode measures is overwhelmingly masked by the edge potential of the protruding electrode. Slow electrode kinetics can significantly magnify edge effects. Generally the reference sensing point varies significantly with the change of misalignment factor and cell voltage or current. The use of reference electrode as a qualitative tool to detect electrode flooding is also discussed.Half-cell reaction data on polymer electrolyte membrane ͑PEM͒ fuel cells are of great interest to researchers, both experimentalists and modelers. A reference electrode is needed when one tries to study the half-cell reaction of the cathode or the anode of an electrochemical system. The ultimate goal of using a reference electrode is to quantitatively measure the voltage losses due to intrinsic kinetics, mass transfer, and ohmic resistance of individual electrodes of an electrochemical system. Using this information, one can identify the problems in the cell and develop solutions. For modelers, this information will help to validate corresponding half-cell modules of the model and to generate more accurate model parameters.To quantitatively analyze the half-cell polarization data, one has to know the relationship between the electrolyte potential at the working electrode surface and the electrolyte potential measured by the reference electrode. In liquid electrolyte systems, this information can usually be acquired by placing a Luggin capillary potential probe in the main current flow path close to the working electrode. With known current and ohmic resistance between the electrode surface and the potential probe, the voltage loss due to electrolyte resistance can be compensated. However, for solid electrolyte systems with thin slab geometry, like PEMFCs and solid oxide fuel cells ͑SOFCs͒, the potential probe usually has to be placed outside of the main current flow path due to geometric restriction posed by the thin solid electrolyte. This is especially true for PEMFC systems, where the typical electrolyte thickness is about 50 m ͑for Nafion 112͒ or less. A schematic of a typical reference electrode co...

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