Proton diffusion in the catalytic layer for high temperature polymer electrolyte fuel cells.

Marina Appel,Marie-Sousai Appavou,Dieter Richter,Reiner Zorn,Galin Borisov,Werner Lehnert,Olaf Holderer

doi:10.1039/c9ra06431a

Abstract

The present study focuses on quasielastic neutron scattering (QENS) of the proton dynamics in phosphoric acid (PA) inside the catalytic layer of high-temperature polymer electrolyte fuel cells (HT-PEFCs). The nanosecond proton dynamics is investigated on the local length scale around operating temperatures (300 K–430 K) using neutron backscattering spectroscopy. We have investigated the catalyst doped with different amounts of PA in order to understand the distribution of PA inside the layer. Three approaches are considered for the description of proton dynamics: the random jump diffusion model, distribution of diffusion constants and, finally, the trap model. Due to adsorption of the PA on the Pt particles the diffusion of protons in the catalytic layer is different in comparison to the bulk acid. The proton dynamics in the catalytic layer can be described by the random jump diffusion with traps. This diffusion is significantly slower than the diffusion of free PA; this also results in a lower conductivity, which is estimated from the obtained diffusion constant.

Highlights

Phosphoric acid-based polymer electrolyte fuel cells operating at elevated temperatures (160 C–180 C) attract increasing attention for different applications in the kW power range.[1,2,3,4,5] The advantages of this type of proton exchange fuel cells include low sensitivity to CO contamination,[6] no need for complicated water management and potentially high proton conductivity.[7,8] The conductivity of such cells is directly proportional to the amount of phosphoric acid (PA), which has the highest intrinsic proton conductivity of any known substance.[9]
We have compared the dynamics in the catalytic layers with high and low doping levels
Three models based on random jump diffusion have been used to describe the proton diffusion: random jump diffusion, jump diffusion with distribution of the diffusion constant and a trap model

Summary

Introduction

Phosphoric acid-based polymer electrolyte fuel cells operating at elevated temperatures (160 C–180 C) attract increasing attention for different applications in the kW power range.[1,2,3,4,5] The advantages of this type of proton exchange fuel cells include low sensitivity to CO contamination,[6] no need for complicated water management and potentially high proton conductivity.[7,8] The conductivity of such cells is directly proportional to the amount of phosphoric acid (PA), which has the highest intrinsic proton conductivity of any known substance.[9]. The experimental study of the proton dynamics of anhydrous PA on the local scale has been lately reported and

Methods

Results

Conclusion