Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction

TLDR

Fuel cell deployment is limited by the cost and stability of platinum‑based oxygen‑reduction catalysts. The study aims to show that vertically aligned nitrogen‑doped carbon nanotubes (VA‑NCNTs) can serve as a superior, metal‑free ORR catalyst for alkaline fuel cells. Nitrogen doping generates high positive charge density on adjacent carbons, and the vertical alignment promotes a four‑electron ORR pathway. VA‑NCNTs delivered a steady‑state potential of –80 mV and 4.1 mA cm⁻² at –0.22 V, outperforming a Pt‑C electrode (–85 mV, 1.1 mA cm⁻² at –0.20 V) and demonstrating superior activity, stability, and crossover tolerance.

Abstract

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.