Study of Tantalum Carbide Supported Iridium-Based Catalyst and Sulfonated Phenylated Polyphenylene as Materials for Polymer Electrolyte Membrane Water Electrolysis

Abstract

The use of iridium-based electrocatalyst supported on tantalum carbide (IrOx/TaC) was investigated for polymer electrolyte membrane water electrolysis (PEMWE) applications. The IrOx/TaC electrocatalyst was used to prepare anodes for perfluorosulfonic acid (PFSA) based membrane electrode assemblies (MEAs) and sulfonated phenylated polyphenylene (sPPP) based MEAs. The Mayer bar method and the automated fluid dispenser method were compared as methods to prepare the electrodes. The Mayer bar method was selected due to the small amount of ink required to prepare electrodes as well as the consistency of the fabricated electrodes. A catalyst ink formulation that incorporated diethylene glycol butyl ether (DEGBE) was developed for use with the Mayer bar method. The effect of the electrocatalyst and ionomer content combination was investigated using Nafion 115 which is a PFSA membrane. Tantalum carbide supported iridium based (IrOx/TaC) electrocatalysts with IrOx/TaC ratios of 7.5 wt%, 15 wt% and 30 wt% were synthesized using the surfactant mediated method. The electrocatalysts were used to prepare anodes where the ionomer content was varied from 10 wt% to 15 wt% to 22 wt%. Electron microscopy was used to analyse the morphology of the catalysts and MEAs. The electrochemical performance of the MEAs was determined using steady state polarization and electrochemical impedance spectroscopy (EIS). It was found that the ionomer content and the iridium distribution played an important role in the MEA performance. MEAs that had 22 wt% anode ionomer content were found to have the best performance. The MEA made with 30 wt% IrOx/TaC had the best overall performance. At 1 A/cm2 it had an iR free potential of 1.64 V and 1.57 V at 60 °C and 80 °C, respectively. The IrOx/TaC MEAs had 40 - 45% higher specific hydrogen production rate and comparable cell efficiency and energy consumption compared to commercial MEAs. Conventional catalyst coated membrane (CCM) and porous transport electrode (PTE) methods were tested as methods to prepare sPPP based MEAs. The CCM-based MEAs were found to be unstable and failed during the conditioning step, whereas it was possible to generate a polarization curve using the PTE-based MEAs. The iR free potential was 1.9 V at 1 A/cm2.