Characterization methodology for anode starvation in HT-PEM fuel cells

Degradation caused by fuel starvation may be an important reason for limited fuel cell lifetimes. In this work, we present an analytical characterization of the high temperature polymer exchange membrane fuel cell (HT-PEM FC) behavior under cycled anode starvation and subsequent regeneration conditions to investigate the impact of degradation due to H2 starvation. Two membrane electrode assemblies (MEAs) with an active area of 21 cm2 were operated of up to 550 min, which included up to 14 starvation/regeneration cycles. Overall cell voltage as well as current density distribution (S++ unit) were measured simultaneously each minute during FC operation. The cyclicity of experiments was used to check the long term durability of the HT-PEM FC. After FC operation, micro-computed tomography (µ-CT) was applied to evaluate the influence of starvation on anode and cathode catalyst layer thicknesses. During starvation, cell voltage and current density distribution over the active area of the MEA significantly differed from nominal conditions. A significant drop in cell voltage from 0.6 to 0.1 V occurred after approx. 20 min for the first starvation step, and after 10 min for all subsequent starvation steps. By contrast, the voltage response is immediately stable at 0.6 V during every regeneration step. During each starvation, the local current density reached up to 0.3 A point-1 at the area near the gas inlet (9 cm2) while near the outlet it drops to 0.01 A point-1. The deviation from a balanced current density distribution occurred after 10 min for the first starvation step, and after ca. 2 min for the subsequent starvation steps. Hence, compared to the voltage drop, the deviation from a balanced current density distribution always starts earlier. This indicates that the local current density distribution is more sensitive to local changes in the MEA than overal cell voltage drop. This finding may help to prevent undesirable influences of the starvation process. The µ-CT images showed that H2 starvation lead to thickness decrease of ca. 20-30% in both anode and cathode catalyst layers compared to a fresh MEA. Despite of the 14 starvation steps and the thinning of the catalyst layers the MEA presents stable cell voltage during regeneration

Exploring critical parameters of electrode fabrication in polymer electrolyte membrane fuel cells

Microstructure and electrochemical properties of the cathode catalyst layers (CCL) of a polymer electrolyte membrane fuel cells (PEMFC) have great impact on the performance and durability of the cell. The aim of this work is to establish a link between CCL structure and fuel cell performance. To obtain CCLs with unique structures six types of electrodes were prepared, each with a different coating technique but with the same Pt loading. The coating techniques are airbrush, screen printing, inkjet printing, dry spraying, doctor blade and drop casting. Moreover, intrinsic properties of PEMFC electrodes such as porosity, permeability, diffusivity as well as ionomer distribution are determined by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) and Atomic Force Microscopy (AFM). Overall, 12 parameters have been evaluated. Generally, CCLs with low fractions of uncovered Pt/C show higher performance at low current densities. In this case the more homogeneous ionomer distribution leads to a higher catalyst utilization. At high current densities transport properties of the CCL have to be considered in addition to the catalyst utilization to explain their performance. The CCL prepared by screen printing shows a low fraction of uncovered Pt/C in combination with good transport properties, leading to the best performance at high currents

Analysis of the regeneration behavior of high temperature polymer electrolyte membrane fuel cells after hydrogen starvation

Two high temperature polymer exchange membrane fuel cells (HT-PEM FCs) were operated under repeated starvation/regeneration steps for 550 min (DPS-1) and 12 days (DPS-2). Concerning the investigation of the irreversible degradation under fuel (H2) starvation the focus was put on the electrochemical characteristics during regeneration, since during this step the system operates under normal conditions. Electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis were used as electrochemical characterization methods. DRT analysis of selected impedance spectra during regeneration steps for the first seven days revealed an intensity increase of the charge transfer kinetics of the cathode (ca. 10 Hz) and the anode (ca. 100 Hz). From day 3 on, an additional peak at 300-800 Hz appeared likely pointing to the formation of surface oxides at the anode side. The EIS and DRT results were verified with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM), Microcomputed Tomography (µ-CT) and Transmission Electron Microscopy (TEM). While TEM indicated advanced Pt particle agglomeration pointing to carbon corrosion, µ-CT measurements showed an increase of void volume fraction and a decrease of the tortuosity value. The combined results show that the anode gas outlet region of the FC is more degraded than the inlet region