Abstract
Non-precious-metal or metal-free catalysts with stability are desirable but challenging for proton exchange membrane fuel cells. Here we partially unzip a multiwall carbon nanotube to synthesize zigzag-edged graphene nanoribbons with a carbon nanotube backbone for electrocatalysis of oxygen reduction in proton exchange membrane fuel cells. Zigzag carbon exhibits a peak areal power density of 0.161 W cm−2 and a peak mass power density of 520 W g−1, superior to most non-precious-metal electrocatalysts. Notably, the stability of zigzag carbon is improved in comparison with a representative iron-nitrogen-carbon catalyst in a fuel cell with hydrogen/oxygen gases at 0.5 V. Density functional theory calculation coupled with experimentation reveal that a zigzag carbon atom is the most active site for oxygen reduction among several types of carbon defects on graphene nanoribbons in acid electrolyte. This work demonstrates that zigzag carbon is a promising electrocatalyst for low-cost and durable proton exchange membrane fuel cells.
Highlights
Non-precious-metal or metal-free catalysts with stability are desirable but challenging for proton exchange membrane fuel cells
An N-doped graphene/carbon nanotube (CNT) composite has been reported to exhibit a stable Proton exchange membrane fuel cells (PEMFCs) performance, though at a relatively low activity, which attracted a great deal of interest in metal-free electrocatalysts for the PEMFCs16
The CNT backbones in the middle of graphene nanoribbons (GNRs), coupled with a carbon black spacer, effectively exposes the zigzag carbon for oxygen reduction reaction (ORR)
Summary
Non-precious-metal or metal-free catalysts with stability are desirable but challenging for proton exchange membrane fuel cells. We partially unzip a multiwall carbon nanotube to synthesize zigzag-edged graphene nanoribbons with a carbon nanotube backbone for electrocatalysis of oxygen reduction in proton exchange membrane fuel cells. Density functional theory calculation coupled with experimentation reveal that a zigzag carbon atom is the most active site for oxygen reduction among several types of carbon defects on graphene nanoribbons in acid electrolyte. The easy restacking of GNRs could lead to low activity since most of the GNR edges in the restacked materials are blocked from the reactants, leading to extremely low utilization of the active sites This blocking effect will be more serious in a PEMFC because the catalyst layer in a PEMFC is much thicker than that in a half cell. A density functional theory (DFT) calculation reveals zigzag carbon is the active site on the GNR
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