Evaluation of temperature and electrolyte concentration dependent Oxygen solubility and diffusivity in phosphoric acid

Abstract

Reactant diffusion and solubility is of utmost important for providing sufficient reactant concentrations at a catalytic active surface. In high temperature proton exchange membrane fuel cells (HT-PEMFCs) the electrolyte consists of concentrated phosphoric acid embedded into a polybenzimidazole (PBI) membrane. Despite the elevated operation temperature of 150°C, the observed oxygen reduction reaction (ORR) rates in HT-PEMFCs are significantly lower than in low temperature PEMFCs. Reduced reactant transport in combination with site blocking is often mentioned as possible inhibition factors. As a step towards studying the ORR under realistic conditions in half-cell setups, in the present study we investigate the oxygen solubility and diffusivity in diluted phosphoric acid electrolyte in a temperature range of 5 to 80 °C. The presented 2 work demonstrates and discusses under which conditions reliable data for these constants can be obtained. In particular it is shown that the product of oxygen solubility and diffusivity, which determines the oxygen mass transport, first increases as function of temperature, but then decreases again above 60 °C. It seems that the decrease in oxygen solubility at higher temperatures is correlated to an increase in the apparent ORR activation energy.

Zigzag carbon as efficient and stable oxygen reduction electrocatalyst for proton exchange membrane fuel cells

“…All these catalysts displayed increased ORR activities with increasing temperature from 5 to 35 °C. Since the practical PEMFCs are usually operated at tens of degrees Centigrade, this temperature dependence implied a promising future for metal-free electrocatalysts to achieve high PEMFC performance 38 .…”

Probing the Effects of Acid Electrolyte Anions on Electrocatalyst Activity and Selectivity for the Oxygen Reduction Reaction

“…The total disk current density seen in H 3 PO 4 is lower owing to the decreased O 2 solubility with increased electrolyte concentration (1.55 M H 3 PO 4 is required to achieve pH 1 compared to 0.1 M for all the other acids investigated here and for which solubility is expected to be around the same order of magnitude). [52][53][54] The CVs demonstrate similarly shaped profiles in the kinetic region, with the ORR onset potential (at À 0.1 mA cm À 2 geo ) decreasing from 0.31 V to À 0.09 V vs. RHE with the following acid electrolyte trend HClO 4 > HNO 3 > H 2 SO 4 > H 3 PO 4 > HCl @ HBr (Figure 1a,c). Moreover, the Ag Tafel slopes (Figure 1f) are between 90 to 100 mV dec À 1 in all tested acid solutions, indicating that the overall reaction mechanism is not being significantly affected by the different anion species.…”

Testing fuel cell catalysts under more realistic reaction conditions: accelerated stress tests in a gas diffusion electrode setup

“…Only few efforts exist that combine a simple setup with a more realistic environment [22][23][24][25][26][27][28][29]. Here, we demonstrate how our recently introduced gas diffusion electrode (GDE) setup [30][31][32] can be used for benchmarking the stability of high surface PEMFC catalysts under realistic reaction conditions.…”