Wetting properties of porous high temperature polymer electrolyte fuel cells materials with phosphoric acid.

Wetting properties of phosphoric acid in porous materials of high temperature fuel cells (HT-PEFC), operating at around 160 °C, are important for cell performance and durability, but the underlying wetting parameters have been unknown so far. Therefore, the influence of phosphoric acid temperature and concentration on the wetting behavior of porous HT-PEFC materials is investigated. The acid filling of gas diffusion and catalyst layers as function of capillary pressure is monitored with X-ray tomographic microscopy under the well defined conditions of an ex situ set-up at temperatures up to 160 °C. For the wetting of gas diffusion layers, with pore sizes in the order of few 10 µm, two opposing trends are shown. With increasing phosphoric acid concentration, less capillary pressure is required, while with increasing temperatures, higher capillary pressures are needed for filling up to a given saturation. The same trends are also found for the contact angle of phosphoric acid on PTFE. A higher contact angle is observed with increasing temperature while increasing the phosphoric acid concentration decreases the contact angle. As both trends are of a similar order of magnitude, the wetting behavior of concentrated (113 wt%) phosphoric acid at 160 °C is astonishingly similar to the wetting behavior of water at room temperature. Another important property for HT-PEFC operation is the filling of cracks in the catalyst layer, which have widths up to 100 µm. For large cracks (>60 µm), a capillary pressure of only 15 mbar was deduced from the measurement, increasing to 30 mbar for cracks between 20 and 60 µm. This, for the first time, allows for assessing the membrane phosphoric acid pressure during fuel cell operation. This can guide the development of improved porous materials for HT-PEFC.

Quantifying phosphoric acid in high-temperature polymer electrolyte fuel cell components by X-ray tomographic microscopy.

Synchrotron-based X-ray tomographic microscopy is investigated for imaging the local distribution and concentration of phosphoric acid in high-temperature polymer electrolyte fuel cells. Phosphoric acid fills the pores of the macro- and microporous fuel cell components. Its concentration in the fuel cell varies over a wide range (40-100 wt% H3PO4). This renders the quantification and concentration determination challenging. The problem is solved by using propagation-based phase contrast imaging and a referencing method. Fuel cell components with known acid concentrations were used to correlate greyscale values and acid concentrations. Thus calibration curves were established for the gas diffusion layer, catalyst layer and membrane in a non-operating fuel cell. The non-destructive imaging methodology was verified by comparing image-based values for acid content and concentration in the gas diffusion layer with those from chemical analysis.

Electrocatalytic reduction of hydrogen peroxide at a stationary pyrolytic graphite electrode surface in the presence of cytochrome c peroxidase: a description based on a microelectrode array model for adsorbed enzyme molecules.

Electrochemical reduction of H2O2 at pyrolytic graphite disc electrodes of radius 2.5 mm occurs at readily accessible potentials (600 mV versus the standard hydrogen electrode) in the presence of yeast cytochrome c peroxidase. Introduction of the enzyme into the electrolyte solution initiates large changes in the ellipsometric angles measured for the electrode-solution interface, consistent with time-dependent enzyme adsorption. This process may be correlated with changes in electrochemical activity. Over the same time course, linear-sweep voltammograms are characterized by a transition from a sigmoidal to a peak-type waveform. It is proposed that the time-dependent behaviour may be rationalized by use of a microscopic model for substrate mass transport, in which the two-electron reduction of peroxide occurs at electrocatalytic sites consisting of adsorbed enzyme molecules. A voltammetric theory based on treating the adsorbed redox enzymes as an expanding array of microelectrodes is in excellent agreement with experiment.