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

Abstract

. ¶ These authors contributed equally.P latinum (Pt) nanoparticles have long been regarded as the best catalyst for the oxygen reduction reaction (ORR) in fuel cells, though the Pt-based electrode still suffers from its susceptibility to timedependent drift and CO deactivation. 1Ϫ3 Furthermore, the high cost of the Pt catalysts, together with the limited reserves of Pt in nature, has been shown to be the major "showstopper" to mass market fuel cells for commercial applications. That is why the large-scale practical application of fuel cells has not been realized, though alkaline fuel cells with platinum as an ORR electrocatalyst were developed for the Apollo lunar mission in the 1960s. 4 Along with recent intensive research efforts in reducing or replacing Ptbased electrodes in fuel cells, 3,5Ϫ11 we have found that vertically aligned nitrogencontaining carbon nanotubes (VA-NCNTs) produced by pyrolysis of iron(II) phthalocyanine (a metal heterocyclic molecule containing nitrogen), 12 in either the presence or absence of additional NH 3 vapor, 3 could act as effective metal-free ORR electrocatalysts. The metal-free VA-NCNTs were shown to catalyze a four-electron ORR process free from CO "poisoning" with a 3-time higher electrocatalytic activity, smaller crossover effect, and better long-term operation stability than that of commercially available Pt-based electrodes (C2Ϫ20, 20% platinumonVulcanXC-72R; E-TEK) in alkaline electrolytes. 3 On the basis of the experimental observations and quantum mechanics calculations, we attributed the improved catalytic performance to the electronaccepting ability of the nitrogen atoms, which creates a net positive charge on adjacent carbon atoms in the nanotube carbon plane of VA-NCNTs to readily attract electrons from the anode for facilitating the ORR. 3 Uncovering this new ORR mechanism in nitrogen-doped carbon nanotube electrodes is significant as the same principle could be applied to the development of various other metal-free efficient ORR catalysts for fuel cell applications.The recent discovery of graphene has opened up a new era of 2-dimensional (2D) fundamental science and potential technology. 13Ϫ15 As mother of all graphitic forms, graphene is a building block for carbon materials of all other dimensionalities, such as 0D buckyballs, 1D nanotubes, and 3D graphite. Having many similarities to carbon nanotubes (CNTs) in structure and property, including its high aspect ratio (the ratio of lateral size to thickness), large surface, rich electronic states, and good mechanical properties, graphene is an attractive candidate for potential uses in many areas where the CNTs have been exploited. Superior to CNTs, the one atomic-thick graphene sheets with a 2D planar geometry will further facilitate electron transport, 16 and hence the more effective electrode materials.

Structure of graphene and its disorders: a review

“…Substitutional non-carbon atoms embedded in the graphitic lattice, such as nitrogen and boron dopants, possess more or less valence electrons than that of carbon atoms, and thus increase the surface reactivity [285]. Moreover, nitrogen-doped graphene is an efficient electrocatalyst for reduction processes [286,287]. Another efficient way for making graphene sheet less inert is by reacting it with halogen atoms such as chlorine [220,221] and fluorine [218,219].…”

“…Plasma treatment in ammonia gas are commonly used in efforts to dope graphene by nitrogen [287,346]. The nitrogen-containing radicals can readily form covalent bonds with the carbon lattice and retain stable in the post-annealing process.…”

Nitrogen-doped reduced graphene oxide electrodes for electrochemical supercapacitors

“…Chemical modification allows for tuning of graphene properties such as surface chemistry and electronic properties; 11 which renders them more suited to certain applications than pristine graphene. Nitrogen-doped (N-doped) graphene has been demonstrated to be of use as an electrocatalytic material for oxygen reduction in hydrogen fuel cells, 12 improves biocompatibility of carbon devices in biosensing 13 and enhances the performance of graphene-based supercapacitors. 14 Graphene may be synthesised via mechanical cleavage of graphite flakes, 15 Chemical Vapour Deposition (CVD) 16 or the decomposition of silicon carbide (SiC).…”

Practical Challenges Associated with Catalyst Development for the Commercialization of Li-air Batteries

“…Meanwhile, sputtering and heat treatment methods [22][23][24][25][26] have been newly explored in transition metal based catalysts (e.g., Fe, Co, Mo) and metal-free carbon catalysts. 27,28) In this section two main synthesis methods are briefly summarized.…”