Insight into Proton Transfer in Phosphotungstic Acid Functionalized Mesoporous Silica-Based Proton Exchange Membrane Fuel Cells

Abstract

We have developed for fuel cells a novel proton exchange membrane (PEM) using inorganic phosphotungstic acid (HPW) as proton carrier and mesoporous silica as matrix (HPW-meso-silica) . The proton conductivity measured by electrochemical impedance spectroscopy is 0.11 S cm–1 at 90 °C and 100% relative humidity (RH) with a low activation energy of ∼14 kJ mol–1. In order to determine the energetics associated with proton migration within the HPW-meso-silica PEM and to determine the mechanism of proton hopping, we report density functional theory (DFT) calculations using the generalized gradient approximation (GGA). These DFT calculations revealed that the proton transfer process involves both intramolecular and intermolecular proton transfer pathways. When the adjacent HPWs are close (less than 17.0 Å apart), the calculated activation energy for intramolecular proton transfer within a HPW molecule is higher (29.1–18.8 kJ/mol) than the barrier for intermolecular proton transfer along the hydrogen bond. We find that the overall barrier for proton movement within the HPW-meso-silica membranes is determined by the intramolecular proton transfer pathway, which explains why the proton conductivity remains unchanged when the weight percentage of HPW on meso-silica is above 67 wt %. In contrast, the activation energy of proton transfer on a clean SiO2 (111) surface is computed to be as high as ∼40 kJ mol–1, confirming the very low proton conductivity on clean silica surfaces observed experimentally.

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

“…1D) element mapping it can be seen that the mesoporous channels of mesoSiO2 are filled with PWA, anchored to the mesoporous silica network through ionic interactions. 34 The parallel white lines in the STEM image of PWA-meso-SiO2 (Fig. 1E) compliments Fig.1B by further confirming that the pores of meso-SiO2 are filled with PWA along the [001] direction in large scales.…”

“…Both experimental and theoretical modelling demonstrate that PWA-mesoSiO2 has the capability of high proton conductivity and high water retention due to the stability of PWA within the confined and ordered mesoporous silica structure. 34,38 Fuel cell durability studies were conducted at 200 °C and 200 mA cm -2 , using the isotropic composite membranes with PWA-meso-SiO2 loadings of 0, 5, and 15 wt%. The performance was evaluated from their steady state characteristics and from polarization curves, which were measured periodically during the test.…”

Mesoporous materials for energy conversion and storage devices

“…213). Moreover, proton-conducting mesoporous materials, such as phosphotungstic-acid-functionalized mesoporous silica, have shown great potential as membranes for fuel cells that operate at elevated temperatures because of their outstanding stability and efficient proton conductivity at temperatures higher than 100 °C and under low humidity 214,215 .…”

Proton conductivity and morphology of new composite membranes based on zirconium phosphates, phosphotungstic acid, and silicic acid for direct hydrocarbon fuel cells applications

“…Pandey et al [24] synthesized an ion exchange membrane by impregnating a porous poly vinyldene fluoride (PVDA) film with silica and PWA particles. Composite membranes based on Phosphotungstic acid (PWA) and silica were studied and reported to have a conductivity of 0.001-0.11 S cm -1 [23,25].…”