Correlating Electrolyte Inventory and Lifetime of HT-PEFC by Accelerated Stress Testing

Abstract

Phosphoric acid electrolyte evaporation in a polybenzimidazole based high temperature polymer electrolyte fuel cell is analyzed as a function of reactant gas stoichiometry and temperature. Based on these results a phosphoric acid vapor pressure curve is derived to predict the fuel cell liftetime with respect to electrolyte inventory. The predicted fuel cell life was validated by means of an accelerated stress test. Additionally, the correlation between electrolyte inventory and fuel cell performance was investigated by recording H 2 /air and H 2 /O 2 polarization curves during the course of the stress test to gain insight into the relation between acid inventory and the different degradation modes. © The Author High-temperature polymer electrolyte fuel cells (HT-PEFC) have the potential to become an important technology for small scale heat and power (CHP) applications. However, today, fuel cell based CHP applications are dominated by low-temperature PEFC (LT-PEFC), 1 even though the possibility to sustain high CO levels of up to 3%, 2 thermal integration of the fuel processing unit and no need of additional gas clean-up render HT-PEFCs especially suitable for operation on hydrocarbon-based fuels, i.e. natural gas. The high operating temperature of 160-200• C, reduced system complexity, due to the absence of additional gas humidification, and high system efficiencies are ideal properties of HT-PEFC for stationary CHP applications.Fuel cell durability, efficiency and cost are essential factors for commercialization. Durability is mainly determined by membrane electrode assembly (MEA) degradation. Amongst other degradation modes that HT-PEFCs share with low temperature PEFC, 3 electrolyte loss by evaporation and migration is exclusive to HT-technology and a limiting factor for CHP applications. We have recently demonstrated that PBI based membrane systems exhibit extensive electrolyte migration from cathode to anode under high current operation. 4 This was attributed to the high mobility of free hydrogen phosphate anions which carry part of the ionic current. While this work focuses on phosphoric acid loss by evaporation and its implication on lifetime and fuel cell performance, it cannot be excluded that the high PA mobility has an effect on electrolyte evaporation as it can influence the PA resupply and saturation of the electrodes.With respect to electrolyte evaporation, the phosphoric acid vapor pressure below temperatures of 300• C is extremely low, nevertheless it is expected to be significant considering the targeted lifetime of 50,000 h for CHP systems set out by the US Department of Energy (DOE) for 2015.5 Up to now, no literature data is available for the vapor pressure of phosphoric acid for temperatures below 200• C. 6-9Determining a phosphoric acid vapor pressure curve at the temperatures of interest for fuel cell operation (160-190• C) is a tedious task, due to the low phosphoric acid concentration in the gas phase and the accompanied analytical measurement complexity. Furthermore, phosphoric acid, b...

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

“…In addition, it is evident that the acid-loss rate depends also on the current density, even for samples that have been operated with approximately equal gas-flow rates. This observation is highly interesting because it directly contradicts the results by Eberhardt et al 23 who concluded that the acid-loss rate, when normalized in…”

“…Instead, we might ascribe the observation to decreasing ECSA due to loss of three-phase-boundary regions as a consequential secondary effect of severe acid loss whereby parts of the electrodes are rendered depleted of ion conducting phase. 23,37,38 While acid loss from the catalyst layer will also result in increased resistance, there should be no linear dependence on current in this case, so the effect should contribute neither to the ir-curves of Figure 2c nor to the ASR dependence shown in Figure 2d. However, the separation of ohmic and non-ohmic contributions by Equation 1 is a quite crude technique, and influence from some apparently linear contributions from the catalyst layer cannot be dismissed entirely.…”

“…Of the references, it is only in the work by Eberhardt et al 23 that re-condensation of acid in the exhaust tubing is explicitly mentioned as having been accounted for. This may explain why their values are somewhat higher than that of the other references.…”

Exceptional durability enhancement of PA/PBI based polymer electrolyte membrane fuel cells for high temperature operation at 200 °C

“…19 For example, the PA leaching rate at the cathode at 190 °C has been found to be about an order of magnitude higher than at 160 °C. 6,20 As a result, the durability data reported in the literature at 190-200 °C do not extend to much more than a few hundred hours of operation. 3,6,9 Increasing the operating temperature from 160 to 200 o C is, however, strongly beneficial due to further improved catalyst kinetics and fuel impurity tolerance in combination with better possibilities for thermal integration with fuel processing units.…”

In Situ Formed Phosphoric Acid/Phosphosilicate Nanoclusters in the Exceptional Enhancement of Durability of Polybenzimidazole Membrane Fuel Cells at Elevated High Temperatures

“…reported a 90 % loss of phosphoric acid in a fuel cell operated at 190 o C under a high gas flow rate [19].…”