The large-scale practical application of fuel cells will be
difficult to realize if the expensive platinum-based electrocatalysts
for oxygen reduction reactions (ORRs) cannot be replaced by
other efficient, low-cost, and stable electrodes. Here, we report
that vertically aligned nitrogen-containing carbon nanotubes
(VA-NCNTs) can act as a metal-free electrode with a much better
electrocatalytic activity, long-term operation stability, and
tolerance to crossover effect than platinum for oxygen reduction
in alkaline fuel cells. In air-saturated 0.1 molar potassium
hydroxide, we observed a steady-state output potential of –80
millivolts and a current density of 4.1 milliamps per square
centimeter at –0.22 volts, compared with –85 millivolts
and 1.1 milliamps per square centimeter at –0.20 volts
for a platinum-carbon electrode. The incorporation of electron-accepting
nitrogen atoms in the conjugated nanotube carbon plane appears
to impart a relatively high positive charge density on adjacent
carbon atoms. This effect, coupled with aligning the NCNTs,
provides a four-electron pathway for the ORR on VA-NCNTs with
a superb performance.
