Engineering Carbon Nanotubes As Oxygen Reduction Catalysts for Solid Acid Fuel Cells

We report on the effect of the chemistry of multi-walled carbon nanotubes (CNTs) on the oxygen reduction reaction (ORR) in solid acid fuel cells (SAFCs) based on the proton conducting electrolyte cesium di-hydrogen phosphate (CDP). CDP undergoes a super-protonic phase transition at ~230⁰C (10^-6 S/cm²-10^-2 S/cm²), under humidified gas atmosphere, making it a suitable fuel cell electrolyte. Due to the high cost of using platinum as the state of the art oxygen reduction reaction (ORR) catalyst, significant effort has been devoted to developing non-precious catalysts.

In this work we explore the suitability of functionalized carbon nanotubes as ORR catalysts. Composite electrodes of fine micron-sized CDP particles and CNTs were prepared by mechanical milling and applied to either side of a dense membrane of CDP, in turn, placed between carbon paper current collectors. The CNTs were acquiring commercially and included both hollow (standard) and bamboo CNTs. In both cases, in addition to non-functionalized forms of the CNTs, both amine-functionalized and acid-functionalized analogs were evaluated. Electrochemical activity was measured by A.C. impedance spectroscopy under humidified oxygen (pH₂0 of 0.4 atm) at 240⁰C. 

Of the non-functionalized CNTs, the bamboo structure was found to have a higher activity than the hollow structure, with an area specific electrode resistance of 1700 ohm cm² vs 3500 ohms cm². Functionalization of the CNTs led to orders of magnitude increase in the ORR activity. In particular, the electrode resistance values dropped to ~350 ohms cm² for the amine functionalized CNTs and ~40 ohms cm² for the carboxylic acid functionalized hollow CNTs. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) of the electrodes showed a relationship between the chemistry of the CNTs and the degree of mixing in the CNT/CDP composite, which may contribute to the differences in electrochemical behavior.