Carbon Nanotubes as a Solid Acid Fuel Cell Cathode Material: Insights into In Operando Functional Stability

Abstract

AbstractCarbon nanomaterials, such as carbon nanotubes and graphene doped with heteroatoms, have drawn much attention, owing to their activity as a fuel cell electrocatalyst. This discovery is very important, as the wide application of low‐ and intermediate‐temperature fuel cells is partly hindered by the need for precious‐metals, such as platinum, as the electrocatalyst. Here, we report multi‐walled carbon nanotubes (MWCNTs) as the active electrode component for the oxygen reduction reaction (ORR) in solid acid fuel cells (SAFCs) and their in operando changes of surface chemistry and structure. We show, for the first time with fuel cell measurements, the effective (time‐dependent) electrocatalytic behavior of MWCNTs in SAFC cathodes. For all measurements, the MWCNTs were platinum free and were not doped with heteroatoms during or after the synthesis. AC impedance spectroscopy of the electrochemical cells, MWCNT|CsH2PO4|MWCNT, with and without a DC bias, were performed and the long‐term stability was evaluated over a 20 day period. Scanning electron microscopy, Raman, and XPS spectroscopy, before and after the electrochemical measurements, show significant changes in the surface chemistry and structure of the MWCNTs. We observe an increase in the surface oxygen concentration and an increased electrochemical activity. However, this active state is transient in nature, as other chemical degradation processes become more dominant. We conclude that carbon nanomaterials are promising as electrochemically active SAFC materials; however, challenges with stability need to be overcome for a realistic application.