Highly Efficient Metal-Free Growth of Nitrogen-Doped Single-Walled Carbon Nanotubes on Plasma-Etched Substrates for Oxygen Reduction

Abstract

Abstract:We have for the first time developed a simple plasmaetching technology to effectively generate metal-free particle catalysts for efficient metal-free growth of undoped and/or nitrogen-doped single-walled carbon nanotubes (CNTs). Compared with undoped CNTs, the newly produced metal-free nitrogen-containing CNTs were demonstrated to show relatively good electrocatalytic activity and long-term stability toward oxygen reduction reaction (ORR) in an acidic medium. Owing to the highly generic nature of the plasma etching technique, the methodology developed in this study can be applied to many other substrates for efficient growth of metal-free CNTs for various applications, ranging from energy related to electronic and to biomedical systems.The metal residuals from metal nanoparticles used as catalysts for the conventional CNT growth often cause detrimental effects undesirable for various applications, including electronic and biological systems. 1 The recent discovery of metal-free growth of carbon nanotubes (CNTs) has offered an alternative approach to carbon nanomaterials with novel properties attractive for many existing and new applications. 2,3 Along with the recent intensive research efforts in reducing or replacing the Pt-based electrode in fuel cells, 4 Gong et al. 5 have recently found that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) could act as extremely effective metal-free electrocatalysts for the oxygen reduction reaction (ORR), a key factor to limiting the performance of a fuel cell. The metal-free VA-NCNTs were shown to catalyze an efficient four-electron ORR process free from CO "poisoning" with a much higher electrocatalytic activity and better long-term operation stability than those of commercially available Pt-based electrodes in alkaline electrolytes. 5 This finding has a large impact on the fuel cell field and the catalyst community, and its repercussions are continuing. 6 However, the CNTs used in the previous report 5 were produced by pyrolysis of iron(II) phthalocyanine (a metal heterocyclic molecule containing nitrogen) 7 with the residual Fe catalyst particles being removed by electrochemical oxidation. Although great care was taken during the nanotube electropurification and similar ORR electrocatalytic performance has also been reported for nitrogen-doped ordered mesoporous graphitic arrays prepared by a metal-free nanocasting technology, 6h possible effects of metal contaminates in the NCNTs on the observed superb ORR performance could still be a matter of controversy, 8,9 unless NCNTs with good ORR electrocatalytic activities can be produced by a metal-free preparation procedure. In this regard, it will be a significant advancement if we can develop a metal-free growth process to produce NCNTs for the ORR and other applications. As far as we are aware, however, no NCNT has been produced by the metal-free growth and possible use of nitrogen-free CNTs produced by the metal-free growth has also been largely precluded by its low growth efficiency. 2,3 ...

Carbon nanotubes prepared by anodic aluminum oxide template method

“…Precursors containing carbon and hetero atoms are introduced simultaneously and then deposited onto the surface of the nanochannels, thus yielding outer surface doped CNTs. N and B are the most widely used hetero atoms for the doping of CNTs [102][103][104][105].…”

Highly efficient cathode catalyst layer based on nitrogen-doped carbon nanotubes for the alkaline direct methanol fuel cell

“…Generally, they have been synthesized with the help of metal catalysts (e.g Co or Fe), which had raised the concern that the ORR activity was attributable to the metal-nitrogen complexes as in the well-known case of metal-porphyrins [9]. However, synthesis methods that do not use any metal precursors or catalysts have been recently developed and ORR activity of the catalyst has been preserved [15,[18][19][20][21][22].…”

“…graphene [10,[23][24][25][26][27][28][29][30][31][32][33], carbon nanotubes [11,20,21,[34][35][36][37][38][39][40][41][42][43], carbon nanofibers [12,[44][45][46][47], mesoporous carbon [15,22], graphitic carbon [48,49], carbon spheres [19,[50][51][52][53], carbon nanocages 4 [54], flower-like carbon [55], carbon aerogel [56,57], vesicular carbon [58], nanodiamonds [59]) relatively few have been tested in actual fuel cell conditions [58,[60][61][62]…”

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

“…Whereas the majority of recent studies on carbon nanostructured metal-free electrocatalysts focus on ORR reactions in alkaline electrolytes, fuel cells that operate with acidic electrolytes, particularly polymer electrolyte membrane fuel cells (PEMFC) [14,15], can have a more significant economic impact. Therefore, the development of effective metal-free carbon-based catalysts for acidic electrolytes is important, though still challenging [16,[145][146][147][148][149][150].…”

“…N,S-co-doped CNTs (NS-CNTs) were also demonstrated to show enhanced ORR activity compared with CNTs, NCNTs, and SCNTs [150] with a much better durability than that of commercial 20 wt% Pt/C catalysts. Other examples of metalfree electrocatalysts for ORR in acidic electrolytes include N-doped SWCNTs [145], vapor-phase-polymerized PEDOT [152], pyridinic and pyrrolic N-doped graphene prepared by confined polymerization/carbonization within layered montmorillonite [90], graphene/CNT layer-by-layer composites [153], and N,F-co-doped graphite nanofibers [154]. Using pyrolysis of sulfur-doped rGO in the presence of Nafion and dimethyl formamide at 600 °C, Pham et al [17] reported the synthesis and characterization of F,N,S-tri-doped reduced graphene oxide (F,N,S-rGO) in both acidic and alkaline PEFCs (Fig.…”