Investigation of Phosphoric Acid Distribution in PBI Based HT-PEM Fuel Cells

High temperature Polymer Electrolyte Membrane (HT-PEM) fuel cells provide a good alternative to low temperature PEMFCs. Operation at higher temperatures shows better CO tolerances and a simpler water management.

Nevertheless, the phosphoric acid (PA) doped PBI HT-PEMFC as the most successful HT-PEM fuel cell type shows long-term degradation effects resulting in lower performance and limited durability. The phosphoric acid loss of the membrane-electrode-assembly (MEA) during operation, which provides high proton conductivity within the membrane, may be identified as being one of the major degradation aspects (1-3). Due to that fact, it is important to understand the degradation mechanism and to get a more detailed insight view into the process of phosphoric acid loss of the MEA during fuel cell operation.

In this work acid distribution within the fuel cell test system and balancing is investigated within different test procedures.

An accelerated stress test (AST) with load cycling between high current densities of 0.6 A/cm² (4 minutes) and 1.0 A/cm² (16 minutes) can provoke degradation processes in a shorter time period and increase the effect of acid loss due to higher water production in comparison to standard AST (0.3-0.6 A/cm²).

Additionally long term tests at constant load conditions (0.3 A/cm²) will be performed.

During the tests the MEAs will be characterized with electrochemical measurements like polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and linear sweep voltammetry (LSV).

For a defined balancing of acid distribution a precise detection of acid content in product water is necessary. Therefore a test system with cold water traps positioned directly after gas outlets is established.

In addition, investigation of acid uptake of bipolar plates will be done with ex-situ storage in hot phosphoric acid as well as pristine bipolar plates that are installed for each new test. For the overall balance of acid distribution the values of phosphoric acid are determined in MEA, bipolar plates and product water.

The amounts of phosphoric acid in MEA and bipolar plates are identified by two methods, titration with sodium hydroxide after acid leaching in water or a solution of water and acetone. Ion chromatography (IC) and inductive coupled plasma method (ICP-MS) allows determining the content of PA in product water. To give a better indication of acid distribution the tests will be optimized to higher operation time of more than 500 hours. Further explanation about the variation of acid content will be given.

Figure 1: Distribution of phosphoric acid after 330 and 500 hours of load cycling between 0.6 and 1 A/cm², H₂/Air, 160°C.

In addition, the surface structures of the bipolar plates will be analyzed by atomic force microscopy (AFM), confocal and scanning electron microscope (SEM) in conjunction with energy dispersive X-ray spectroscopy (EDX) to get an impression into the morphological and structural changes in surface area and roughness changes, as well as the distribution of elementary Phosphor.

The combination of determination of phosphoric acid in MEA, bipolar plates and product water together with the imaging methods can provide a better understanding of the effects of acid distribution during different test procedures and give an overall review on various degradation steps in HT-PEMFC technology.