A PEFC system consists of a cell stack and ancillary units, such as cooling, hydrogen and air feeds and humidification systems. A number of single cells are normally connected in series to produce high output power, and the abnormal operation of even a single cell can result in malfunction of the entire cell stack. Therefore, detecting the abnormal operational conditions is essential to enhance the durability and reliability of marine PEFCs and to ensure long-term safe and stable operation.
Electrochemical impedance spectroscopy (EIS) is a technique suitable for the real-time diagnosis of operating PEFCs. The present work investigated the effects of abnormal variations in the cell temperature and the flow rate and relative humidity of supplied gases (caused by malfunctions of ancillary units) on the cell resistance and capacitance values in an equivalent circuit model using EIS analysis. Both increasing the cell temperature and decreasing the supplied gas humidity degrade the proton conductivity, thereby raising the ohmic resistance. It is therefore essential to maintain the hydration of the membrane so as to obtain sufficient proton conductivity. However, overly high gas humidity will allow liquid water to accumulate in the electrode, the gas diffusion layer and the separator flow channels. This reduces the supply of oxygen to the electrode, thereby increasing the mass transport resistance. The gas diffusion layer coated with a microporous layer containing hydrophilic carbon nanotubes (CNTs) was effective at preventing both membrane dehydration and flooding regardless of the humidity conditions.
The effect of injecting a sodium chloride (NaCl) solution into the cathode inlet air on the PEFC performance was also evaluated. The cell voltage at a constant current density of 0.5 Acm-2 decreased from 0.71V to 0.62V after injecting a NaCl solution for a span of 2 h. The reduced cell voltage could be attributable to lowered proton conductivity due to the displacement of protons in the membrane by sodium ions. The presence of chloride ions would block active Pt sites and accelerate the dissolution of Pt. These effects reduced the total active area at the catalyst layer during operation. The presence of NaCl in the cathode air stream increased all the resistances, thus degrading the cell performance. However, the increase in the charge transfer resistance was the most significant.
The effect of injecting distilled water instead of a NaCl solution after detecting performance degradation due to NaCl contamination was evaluated. The extent of chlorine ions adsorption on Pt in the catalyst layer was relatively low during the initial stage of performance degradation. The cell voltage increased gradually for 6 h after initiating the injection of distilled water, and reached to a constant value of 0.69V, which represented a recovery value of 97% compared with that obtained under normal conditions without NaCl contamination. Injecting distilled water after the initial detection of abnormal operational conditions was evidently effective at recovering the cell performance at this early stage.