CVD grown CNTs within iron modified and graphitized carbon aerogel as durable oxygen reduction catalysts in acidic medium

Abstract

CNTs were grown on iron-modified mesoporous graphitized carbon aerogel (GCA) at 700°C, 800°C and 900°C using catalytic CVD method. Resultant CNT/GCA materials composition, morphology and structure were studied to understand their electrochemical stability and performance for oxygen reduction reaction (ORR) in acidic medium. CNT growth was increased from 700°C to 800°C, dominated by MWCNTs formation. In the temperature range from 800°C to 900°C, the growth was reduced by forming nanofiber/nanoribbon structures accompanied by MWCNTs. Mesoporosity of CNT/GCA composites declined at 700°C and 800°C due to MWCNT formation. However, CNT/GCA growth at 900°C improved mesoporosity with substantial increase in pore volume (∼3 times of GCA) due to formation of nanofibers and nanoribbons. The structure of CNT/GCA materials revealed nitrogen doping and dispersion of FeNx phase. A synergistic contribution of CNT/GCA material structure and morphology to ORR activity was noticed. Among CNT/GCA materials, CNT-800°C/GCA material showed ORR activity at lowest onset potential of 0.5V. However, CNT-900°C/GCA exhibits the highest ORR mass activity, with a half-wave onset potential difference of 120mV with Pt (40wt.%)/C. Moreover, CNT-900°C/GCA demonstrates high selectivity (>3.97) to 4 electron ORR path, excellent methanol tolerance and electrochemical durability which makes it a potential DMFC cathode candidate.