Shorting in Polymer Electrolyte Fuel Cells: Aspects of Material Testing and Operating Conditions

Abstract

Ohmic shorting through the membrane has been identified as one of the major failure modes in polymer electrolyte fuel cells (PEFC) [1]. Shorting occurs when electrons flow directly from the anode to the cathode instead of through the device being powered. Ohmic shorting not only can reduce the fuel cell performance, but also can lead to local heat generation in the vicinity of the short, causing membrane damage that can ultimately result in gas crossover failure. Two types of membrane shorts have been observed during the fuel cell testing: soft shorts and hard shorts. A soft short is a sub-critical short that results from the conductive carbon fibers or debris penetrating through the membrane. The soft shorts do not immediately lead to fuel cell failure; however, significant accumulation of soft shorts can reduce the overall cell resistance and compromise fuel cell durability through cell voltage degradation. A hard short is a critical short that is the result of significant heat release from an existing soft short. A hard short can directly lead to membrane crossover and cell failure. Hard shorts occur suddenly in an operating fuel cell stack where one cell develops a cell voltage reversal well below -1 V [2]. The present study is composed of two parts. The first is on the development of a test method to determine the density of soft shorts and their ohmic resistance that may lead to hard shorts when the adverse fuel cell operating conditions are met. The second part of the study is to investigate these adverse fuel cell operating conditions that lead to cell reversal and the potential hard shorts. The ultimate objective of the study is to prevent the fuel cell shorting failure through better material selection/designs and fuel cell operation controls. In the first part of this study, we use a current distribution circuit board [3] in a fixture that induces uniform compression over a 32 cm2 area of a sample that consists of a piece of proton exchange membrane or a membrane electrode assembly sandwiched between two gas diffusion layers. By incrementally increasing the compressive pressure and subjecting the sample to a voltage of 0.6 V, we can measure increasing shorting currents in some of the 0.5 cm2 current distribution segments. By de-convoluting the current density distribution measured in this experiment, the number and severity of each short can be identified and the robustness of various material sets against soft shorts can be compared. In the second part of the study, we stepwise dried out a single large-active-area fuel cell by decreasing the inlet gas dew points while under galvanostatic control. At a critical dew point, the cell potential dropped rapidly to reach a negative voltage that was limited to -1 V. Tests were conducted at various cell current densities and inlet temperatures, which showed that the corresponding cell resistance at the onset of rapid cell voltage drop decreases as the current density increases or as the temperature decreases. Good correlation is found between the cell resistance and the onset of cell reversal at various current densities. When the cell reversal limit of -1V was removed, we found that hard shorts can develop at a cell voltage around -3V, confirming the importance of operating the fuel cell within the operating window developed in this study. A.B. LaConti, M. Hamdan, R.C. McDonald, Mechanism of Membrane Degradation for PEMFCs, in Mechanisms of Membrane Degradation for PEMFCs, in: W. Vielstich, A. Lamn, H.A. Gasteiger (Eds), Handbook of Fuel Cells: Fundamentals, Technology and Applications, Vol. 3, John Wiley & Sons, New York, 2003, pp. 647–662.Gittleman, C. S., Coms, F. D., and Lai, Y. H., "Chapter 2: Membrane Durability: Physical and Chemical Degradation", Polymer Electrolyte Fuel Cell Degradation, Editors: Matthew M. Mench, E. Caglan Kumbur, T. Nejat Veziroglu, Elsevier (2012).J.J. Gagliardo, J.P. Owejan, T.A.Trabold, and T.W.Tighe, Nucl. Instrum. Meth. A, 605 (2009) 115-118.