Effect of Hydrophobicity Gradients within MPL and MEA on PEM fuel cell performance

Abstract

The water inventory of PEMFC components strongly influences the performance under two-phase operating conditions. Within this work, the influence of hydrophobicity variation within electrode and microporous layer (MPL) on cell performance will be presented. Different weight percentages of PTFE is used in the catalyst and MPL inks as an additive in order to increase the hydrophobicity. The fabrication method for the membrane electrode assembly (MEA) is based on spraying the catalyst ink onto the membrane and coating MPL ink onto the GDL substrate using a doctor blade method. Fuel cell tests are carried out with a 25 cm² active area single cell at different relative humidities (RH) and oxygen flow rates to investigate the effect of PTFE on the ability of MEA and MPL on water management. Electrochemical impedance spectroscopy is performed as a diagnostic tool to investigate the reaction kinetics and mass transport. In order to assess the wettability of MEAs contact angle measurements were carried out. The materials are characterized by mercury porosimetry and scanning electron microscopy. Figure 1 shows the performance curve for cell 1 (conventional MEA +Conventional MPL), cell 2( MEA with 5 wt.% PTFE +Conventional MPL) and cell 3 (Conventional MEA+ MPL with 40 wt.% PTFE ) at different relative humidities. At dry conditions (RH=70%) cell 2 has a higher limiting current density compared to the other cells, which could be the indication that the hydrophobic electrode keeps the membrane material in the catalyst layer better hydrated. At over humidified conditions (RH=120%) the limiting current density value has decreased for cell 1 and 2 compared to the standard conditions (RH=100%) as a result of mass transport limitations. On contrary, Cell 3 shows an improved performance in high current density region at humid conditions, illustrating the ability of hydrophobic MPL to prevent the accumulation of liquid water at the interphase of MPL and catalyst layer at cathode side. In the presentation of the ongoing work, additional results, including HFR data will be reported. Figure 1

Key findings

• ▸Hydrophobicity variation within electrode and MPL influences cell performance under two-phase operating conditions.
• ▸PTFE additives in catalyst and MPL inks improve water management and cell performance at different relative humidities and oxygen flow rates.
• ▸Electrochemical impedance spectroscopy and contact angle measurements reveal improved reaction kinetics and wettability with PTFE additives.

Keywords

Identifiers

DOI

10.1149/ma2018-02/44/1514

Journal

ECS Meeting Abstracts

Year

2018