Water transport study in a high temperature proton exchange membrane fuel cell stack

A study of water transport in a high temperature phosphoric acid doped polybenzimidazole (PBI) membrane fuel cell stack is reported. Tests with different stoichiometries of dry cathode and different humidity levels of anode are performed. It is found that water transport across the membrane electrode assembly (MEA) is noteworthy and that water vapor partial pressure on the anode outlet is almost always higher than on the cathode outlet, even when using dry hydrogen. The water transport is a strong function of current density but it also depends on stoichiometry and humidity level. In a series of tests with dry nitrogen on one side and humid nitrogen on the other side, the membrane's water permeability coefficient is determined to be 2.4 × 10-13 mol s-1 cm-1 Pa-1 at 160 °C which is more than an order of magnitude higher than the values previously reported in the literature. Also, the results indicate that the permeability coefficient might be relative humidity dependent and could even be somewhat higher than the value reported here, but further investigation is needed. The experimental findings are reproduced and explained with a 2D steady state computational fluid dynamics (CFD) model. Internal water transport profiles across the membrane and along the gas flow channels are presented and discussed.This work is partially funded by the project of CICYTDPI2011-25649 MICINN. Finally, the authors highly appreciate the support of the Institut de Robòtica i Informàtica Industrial in enabling a research stay of Dario Bezmalinović at the Fuel Cell Laboratory in Barcelona.Peer Reviewe

D5.3: Condition monitoring and health assessment of PEM fuel cells

A novel equivalent circuit model is made by adding additional resonance loops comprising of a resistance, capacitance and inductance, for both anode and cathode, representing mass transport and resistive losses within the catalyst layer. Such a model is able to match and explain low frequency inductance observed in all Electrochemical Impedance Spectroscopy measurements taken during durability tests. The model was used to monitor the state of health of a fuel cell exposed to a controlled accelerated stress test. The results indicate that resistance, capacitance and inductance representing the cathode catalyst layer change dramatically during the accelerated stress test, and this shows good agreement with the findings of the periodic diagnostic tests

D5.1: Analysis of Degradation Mechanisms

PEM fuel cells degrade over time. The loss of cell potential is the most obvious symptom of degradation. Degradation may affect either one of the four major losses in fuel cells, namely activation polarization, ohmic losses, concentration polarization and hydrogen crossover losses. The report first addresses these symptoms, and then degradation mechanisms of different fuel cell components are discussed, namely catalyst and catalyst layer, membrane, and gas diffusion layer. For each of these some mitigation strategies are discussed. For each of degradation mechanism the key stressors are identified and standardized accelerated test protocols are presented. Experiments that were conducted at FESB with the goal of gaining practical experience and understanding of underlying degradation mechanisms. Two series of experiments were conducted with two of the most severe stressors, namely prolonged exposure to open circuit voltage, and potential cycling

D2.4: Characterisation and Measurement Techniques

The aim of this deliverable is to define tests protocol, characterization and measurement techniques for long duration term tests. According to the deliverable 2.3, a cycle is proposed. On one hand, at FCLAB, a 2000 hours duration test is a succession of daily cycles and then a succession of ten same cycles per day. On the other hand, accelerated test will start at FESB and ZSW. Furthermore, tests on the system will occur on Dantherm system. ZSW provides stacks and the conditioning of them. This deliverable presents the measurement techniques and their periodicity

Low-Frequency EIS for PEM Fuel-Cell Diagnostics

<p>In order to estimate fuel-cell degradation status on-line and inexpensively, a diagnostic technique based on relay feedback is developed. The technique can obtain critical parameters within seconds of start-up and is robust to measurement bias.</p> <p>Electrochemical impedance spectroscopy (EIS) is a popular laboratory technique to perform diagnostics on electrochemical systems such as fuel cells, but its application to real-life fuel-cell systems is difficult because of the size and cost of the apparatus. In this study, we present a more detailed equivalent-circuit model for a PEM fuel cell, able to explain the positive reactance shown at low frequencies.</p> <p>Some of these characteristics, measured at several stages during an Accelerated Stress Test (AST), progress gradually with catalyst degradation, providing an effective prognostic variable. In order to measure these characteristics, a relay-based feedback excitation algorithm is developed to estimate the low-frequency intercept in the Nyquist plane of the cell impedance without resorting to a full-fledged EIS.</p> <p>The simulations indicate that the algorithm converges to an estimate within about 5 seconds, and is robust to bias. The algorithm can be run within the standard control system that fuel cells are usually equipped with, with no additional hardware.</p