Long-Term Durability of PBI-Based HT-PEM Fuel Cells: Effect of Operating Parameters

Abstract

This work studies the long-term durability of high-temperature polymer electrolyte membrane fuel cells based on acid-doped polybenzimidazole membranes. The primary focus is on acid loss via the evaporation mechanism, which is a major cause of degradation in applications that involve long-term operation. Durability is assessed for 16 identically fabricated membrane electrode assemblies (MEAs), and evaluations are carried out using operating parameters as stressors with gas stoichiometries ranging from 2 to 25, current densities from 200 to 800 mA cm −2 , and temperatures of 160 or 180 • C. Cell diagnostics are composed of time resolved polarization curves, post mortem analysis, and in situ temperature measurements. A major part of the cell degradation during these steady-state tests can be ascribed to increasing area-specific series resistance. By means of post mortem acid-loss measurements, the degradation is correlated to the temperature and to the accumulated gas-flow volume. Such relations are indicative of acid loss via evaporation. Current density also plays a critical role for the acid loss and, thus, for the overall cell degradation. The effect of current is likely tied to mechanisms that involve water generation, migration of electrolyte ions, and locally elevated temperature inside the Since the polybenzimidazole (PBI) polymer acts as a Brønsted base in relation to phosphoric acid, membranes of PBI can be doped with phosphoric acid through acid-base complexation. Some of the H 3 PO 4 molecules, 2 per polymer repeat unit, are then bound to the basic functional sites of the polymer. However, excess acid is essential in order to attain a conductivity that is high enough for use as electrolyte in a polymer electrolyte membrane fuel cell (PEMFC). Fuel cells based on this type of electrolyte are known as high-temperature (HT-)PEMFCs, typically operating at temperatures of 140-180• C. Phosphoric acid-doped PBI membranes can be prepared either by direct (or so-called sol-gel) casting from a solution of PBI dissolved in polyphosphoric acid (PPA) or by solution casting of a PBI film via solvent evaporation, for which acid-doping follows subsequently. 1 For HT-PEMFCs, the cell performance usually increases to a stable state within the first few hundred hours of operation. This activation of the cell might be a consequence of acid redistribution in the membrane electrode assembly (MEA).2-4 Such redistribution of acid also implies susceptibility to its loss, however, because it is a testament to the mobility of the doping-acid. [5][6][7] As reviewed by Jakobsen et al., a number of mechanisms have been proposed to explain the loss of doping-acid from PBI membranes, including evaporation into the gas phase and a so-called steam distillation mechanism, the latter of which has been proposed governed by temperature as well as humidity. 9 Migration of electrolyte ions might also be an issue with respect to acid loss, particularly if the proton transference number deviates significantly from the maximum value for phosphori...

Acid Distribution and Durability of HT‐PEM Fuel Cells with Different Electrode Supports

“…It is not easy to find comparable durability data in the literature, because most durability testing is performed at 160 °C and not at 180 °C, a temperature that was deliberately chosen to accelerate the test. However, we do have other durability measurements at 180 °C from a parallel study in our group . At 200 mA cm −2 and 180 °C and with C2 on both sides and comparable membrane thickness, the decay rate was ca.…”

“…Yu et al operated cells at typical operating temperatures of 160 °C and 190 °C with a voltage degradation rate of 4.9 µV h −1 and 60 µV h −1 and an acid loss rate of 7.6 ng cm −2 h −1 and 110.4 ng cm −2 h −1 , respectively. Søndergård et al reported an even stronger temperature effect at higher stoichiometries of hydrogen and air. In this work, the fuel cell operational temperature of 180 °C was selected as a stressor to the long term durability test of HT‐PEMFCs with varied gas diffusion layer materials.…”

Fuel Cell Electrolytes of Polybenzimidazole Membranes Cross‐linked with Bis(chloromethyl) Arenes

“…The equilibrium acid doping level of the PBI‐CLI and PBI‐CLII was significantly lower than that of the membrane based on linear PBI, and nearly 200 h were required to reach equilibrium at room temperature. Increasing area specific resistance is mainly correlated to acid loss from the membrane , and the higher degradation rates of fuel cells based on PBI‐CLI and PBI‐CLII is likely a consequence of the rigid cross‐linked structure promoting acid leaching. The N ‐functionalization of the benzimidazole is also likely weakening the polymer‐phosphoric acid interaction, which could further promote the acid leaching.…”

“…However, the phosphoric acid tend to redistribute , or leach out during operation, which represents a predominating degradation mode of the fuel cell performance. This is particularly severe at high current loads, at high gas flow rates and at temperature in the higher end of the operating temperature window . Introduction of an inorganic phase, such as titanyl sulfate , titania , or mesoporous silica , , in the membrane matrix may stabilize the performance and extend the lifetime.…”

Enhancing the performance of polymer electrolyte membrane fuel cell by optimizing the operating parameter

“…In this way, the reactant gases transfer more easily and uniformly to the reaction zones; therefore, the intensity of the electrochemical reactions is amplified. Cleemann et al [26] investigated the performance of high-temperature polymer electrolyte membrane fuel cells based on acid-doped polybenzimidazole membranes. They found that high temperature enhances performance during drive cycles, and high gas flow rate for cooling increases acid loss.…”