Characterization methodology for anode starvation in HT-PEM fuel cells

Abstract

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 H 2 starvation. Two membrane electrode assemblies (MEAs) with an active area of 21 cm 2 were operated of up to 550 minutes, 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 occured after approx. 20 minutes for the first starvation step, and after 10 minutes 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 cm 2 ) while near the outlet it drops to 0.01 A•point -1 . The deviation from a balanced current density distribution occurred after 10 minutes for the first starvation step, and after ca. 2 minutes 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 H 2 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.

Analysis of HT-PEM MEAs’ Long-Term Stabilities

“…Striking is the strong decrease of the CL layer on the anode and the adhesion of this layer to the membrane. These facts support the abovementioned assumption that the process of anode starvation was responsible for a main part of the degradation as observed by Yezerska et al [23]. Figure 5 shows the H 3 PO 4 contents of the tested MEA before (solid pillar) and after (striped pillar) the two long-term tests at a constant load of 0.3 A/cm 2 .…”

“…The next recorded voltage reduction at approximately 1250 h was caused by a faulty supply of hydrogen. This fuel starvation also caused damage [23], causing the voltage to drop to 620 mV. From this incident, there was a slight regeneration [23] over the period up to approximately 1600 h to 625 mV but subsiding from here with a degradation rate of 23.3 µV/h to 604 mV after approximately 2450 h of operation.…”

“…This fuel starvation also caused damage [23], causing the voltage to drop to 620 mV. From this incident, there was a slight regeneration [23] over the period up to approximately 1600 h to 625 mV but subsiding from here with a degradation rate of 23.3 µV/h to 604 mV after approximately 2450 h of operation. The subsequent increase to 612 mV can be attributed to an increase in the cathode stoichiometry from 2 to 3.5.…”

Factors influencing the performance of PEM fuel cells: A review on performance parameters, water management, and cooling techniques

“…Another responsible parameter for degrading PEM fuel cell performance and durability is starvation 117 . Starvation refers to an inadequate supply of fuel or oxidant under rapid load‐changing conditions 118 . Local starvation and overall starvation are two major types of reactant starvation 119 .…”

“…117 Starvation refers to an inadequate supply of fuel or oxidant under rapid load-changing conditions. 118 Local starvation and overall starvation are two major types of reactant starvation. 119 Both local starvation and overall starvation are the inadequate supply of reactant gas in a partial area of the CL and stack level, respectively.…”