Multiwalled Carbon Nanotubes As Non-Precious Metal Catalyst for the Oxygen Reduction Reaction

Abstract

Fuel cells are regarded as environmentally friendly energy converters. This, in combination with national and global intentions of energy transition from fossil based resources to renewable ones, makes fuel cells a possible key technology regarding the future energy economy. Because of their advanced state of development, proton exchange membrane fuel cells (PEMFCs) are considered to be the most promising type of fuel cells in terms of commercial use in automotive propulsion. Nevertheless, the high requirements of precious platinum metal for the cathodic oxygen reduction reaction (ORR) is one major reason that still prevents a wide-spread use of this technology to this day. A promising alternative to platinum based catalysts are the so-called non-precious metal based materials. To this day, these catalysts already achieve high current density during PEMFC operation but still need major improvements regarding their durability. In this work, the preparation and characteristics of nanotube based non-precious metal catalysts for the ORR are presented. For the commonly applied platinum based ORR catalysts the positive impact on durability and activity of using carbon nanotubes (CNTs) as carbon support has already widely been proven [1-3]. Here, three different catalyst preparations are presented. First, utilization of commercially available CNTs together with Fe-phenathroline leads to catalyst where FeNxCy moieties are present in-between the CNTs. Secondly, a preceding surface modification step of the commercially available CNTs leads to a material with the active sides directly attached onto the CNTs surface. The third preparation route gives a material where the FeNxCy moieties are directly incorporated into the nanotubes wall. TEM pictures are presented in order to show the structural morphology of the resulting catalysts and the nature of the iron nitrogen active sides is studied via Mößbauer spectroscopy. ORR activity and durability of the resulting nanotube based materials in acidic electrolyte is investigated. PEMFC performance tests clearly show that the connection between active side and nanotube play a crucial role for the activity of the catalyst. Hence, a significantly enhanced performance is found for the catalyst with the active sides incorporated into the nanotube walls.