Study of Mass Transport in the Anode of a Proton Exchange Membrane Fuel Cell with a New Hydrogen Flow-Rate Modulation Technique

TL;DR

This study introduces a novel spectroscopy technique, CH2S, to analyze hydrogen transport in PEMFC anodes, revealing key diffusion processes and effects of operational conditions on diffusivity.

Contribution

The paper presents the development of CH2S, a new analytical method for studying hydrogen transport and diffusivity in PEMFC anodes, with experimental validation under various conditions.

Findings

Identified two limiting transport processes: diffusion through GDL and MPL.

Hydrogen diffusivity decreases with current density in GDL and increases in MPL.

Superhydrophobic catalyst layer enhances hydrogen diffusivity.

Abstract

Hydrogen transport in the anode of a proton-exchange membrane fuel cell (PEMFC) has been studied with a modulation technique relating the hydrogen flow-rate $(\tilde{Q}_{H2})$ and the faradaic current $(\tilde{I})$, called $\textit{Current-modulated Hydrogen flow-rate Spectroscopy}$ (CH2S). A simple analytical expression for the transfer function, $H(j{\omega})=n \, F \, \tilde{Q}_{H2} \mathbin{/} \tilde{I}$, is provided, showing a skewed semicircle in Nyquist representation ($-H^{''}$ vs. $H^{'}$), extending from $H^{'}=0\:$ to $H^{'}=1$, and with the maximum frequency at ${\omega}_{max}=2.33\,(D_{H2}\mathbin{/}{L_{i}}^{2})$, where $D_{H2}$ is the effective hydrogen diffusivity and $L_i$ the thickness of the anode gas diffusion layer (GDL). The expression for CH2S is also calculated with an existing reversible chemical reaction in the GDL. Experimental results under different operation conditions show two transport processes limiting the anode reaction, one attributed to molecular diffusion through the partially saturated GDL, and the other to the microporous layer (MPL), or its interfaces with GDL or with the catalyst layer (CL). CH2S provides the hydrogen diffusivities $(D_{H2,i})$ associated to each process under the different conditions. Current density decreases slightly the diffusivity of the GDL, while it becomes activated in the MPL; using two GDLs in the anode improves both GDL and MPL diffusivities; humidification decreases the diffusivity in both, GDL and MPL; finally, a superhydrophobic anodic CL prepared by electrospray improves hydrogen diffusivity in GDL and MPL.