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.

http://science.sciencemag.org/content/sci/323/5915/760.full.pdf

Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction.

Nitrogen-doped graphene (N-graphene) was synthesized by chemical vapor deposition of methane in the presence of ammonia. The resultant N-graphene was demonstrated to 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 via a four-electron pathway in alkaline fuel cells. To the best of our knowledge, this is the first report on the use of graphene and its derivatives as metal-free catalysts for oxygen reduction. The important role of N-doping to oxygen reduction reaction (ORR) can be applied to various carbon materials for the development of other metal-free efficient ORR catalysts for fuel cell applications, even new catalytic materials for applications beyond fuel cells.

/pdf/nitrogen-doped-graphene-as-efficient-metal-free-2c5sl934bm.pdf

Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells

Graphene and its derivatives are attractive for electrocatalytical application in fuel cells because of their unique structures and electronic properties. The electrocatalytical mechanism of nitrogen doped graphene in acidic environment was studied by using density functional theory (DFT). The simulations demonstrate that the oxygen reduction reaction (ORR) on N-doped graphene is a direct four-electron pathway, which is consistent with the experimental observations. The energy calculated for each ORR step shows that the ORR can spontaneously occur on the N-graphene. The active catalytical sites on single nitrogen doped graphene are identified, which have either high positive spin density or high positive atomic charge density. The nitrogen doping introduces asymmetry spin density and atomic charge density, making it possible for N-graphene to show high electroncatalytic activities for the ORR.

Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells

3.1.1. Covalent Interactions 1355 3.1.2. Noncovalent Interactions 1356 3.1.3. π-π Stacking 1356 3.1.4. Electrostatic Interactions 1357 3.2. In Situ Synthesis Directly on the CNT Surface 1358 3.2.1. Electrochemical Techniques 1358 3.2.2. Sol-Gel Process 1361 3.2.3. Hydrothermal and Aerosol Techniques 1363 3.2.4. Gas-Phase Deposition 1364 3.3. Comparison of Synthesis Techniques 1368 4. Properties and Potential Applications of CNT-Inorganic Hybrids 1368

Carbon Nanotube−Inorganic Hybrids

This paper reports a systematical study on the electrochemical properties of MnO2-based nanostructures as low-cost catalysts for oxygen reduction reaction (ORR) in alkaline media The results show 

MnO2-Based Nanostructures as Catalysts for Electrochemical Oxygen Reduction in Alkaline Media†

This study describes a facile and effective polymer-assisted route to synthesis of structurally uniform and electrochemically active manganese dioxide/multiwalled carbon nanotube (MnO2/MWNT) nanocomposite and investigates the electrocatalytic activity of the synthetic MnO2/MWNT nanocomposite toward the reduction of oxygen in alkaline media. Poly(sodium 4-styrene sulfonate) (PSS) used here as the polymer to assist the synthesis of the nanocomposite serves as a bifunctional molecule both for solubilizing MWNTs into an aqueous solution and for tethering Mn2+ precursor onto MWNT surfaces to facilitate the follow-up chemical deposition of MnO2 to eventually on-spot grow MnO2 nanoparticles onto MWNTs. The synthetic MnO2/MWNT nanocomposite has a uniform surface distribution and large coverage of MnO2 nanoparticles onto MWNTs, which was characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). The synthetic MnO2/MWNT nanoco...

Polymer-Assisted Synthesis of Manganese Dioxide/Carbon Nanotube Nanocomposite with Excellent Electrocatalytic Activity toward Reduction of Oxygen

Nanocomposite electrodes made of sol−gel-derived ceramic−carbon nanotube are fabricated by doping mutliwalled carbon nanotubes (MWNTs) into a silicate gel matrix. The electrochemical behavior and potential electrochemical applications of the ceramic−carbon nanotube nanocomposite electrodes (CCNNEs) are also studied. The as-prepared CCNNEs exhibit a tunable dimension ranging from conventional electrode to nanoelectrode ensemble (NEE), depending on the amount of the MWNT dispersed in the silica sol and finally doped within the gel matrix. A high content of the MWNT (i.e., higher than 1.5 mg/mL in the sol) leads to the formation of the CCNNE characteristic of an electrode of conventional dimension, while a low content (typically lower than 0.10 mg/mL) essentially yields the CCNNE like a nanoelectrode ensemble. The NEE is demonstrated to possess good electrocatalytic activity toward the oxidation of ascorbic acid (AA), and the CCNNE of conventional dimension is found to possess remarkable electrocatalytic act...

and Shaoxiang Xiong

Papers

Polymer-Assisted Synthesis of Manganese Dioxide/Carbon Nanotube Nanocomposite with Excellent Electrocatalytic Activity toward Reduction of Oxygen

Sol-gel-derived ceramic-carbon nanotube nanocomposite electrodes: tunable electrode dimension and potential electrochemical applications.