In situ preparation of multi-wall carbon nanotubes/ Au composites for oxygen electroreduction† Na Li, ab Zhenghua Tang, * ac Likai Wang, a Qiannan Wang, a Wei Yan, a Hongyu Yang, a Shaowei Chen ad and Changhong Wang * b Multi-wall carbon nanotubes (CNTs)/Au nanocomposites have been prepared by the in situ reduction approach. The as-prepared hybrid materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The composition of the nanocomposites was fine tuned by varying the mass ratio of Au-to-CNTs. Among a series of samples tested, hybrid materials with a Au/carbon nanotubes (Au/CNTs) ratio ¼ 1 : 2 demonstrated the best activity towards oxygen reduction reaction (ORR), as most positive onset potential, largest kinetic current density, highest amount of electron transfer and lowest H 2 O 2 yields were obtained. Notably, the Au/ CNTs also exhibited remarkable long-term durability higher than commercial Pt/C. Introduction A proton exchange membrane fuel cell (PEMFC) has been considered one of the most promising technologies to tackle the global energy crisis and environmental problems. 1,2 Oxygen reduction reaction is a key process for energy conversion in fuel cells, unfortunately, the slow reaction kinetics of ORR as well as the high price of the Pt based catalysts signicantly hinder the widespread commercialization of fuel cells. 3,4 Moreover, the Pt based catalysts also suffer from the low abundance of Pt on earth as well as the poor stability of such catalysts. Therefore, a great deal of research efforts have been continuously devoted to developing new strategies to lower the amount of Pt or substituting Pt with other non-precious metals or even using metal-free materials. 5–15 Bulk gold has been considered to be catalytically inert previ- ously for a long time. However, when the dimension reaches nanometer scale, gold nanoparticles demonstrated excellent catalytic activity in multiple organic reactions 16 and electro- chemical reaction. Particularly, for those ultrasmall nanoclusters with core diameter less than 2 nm, excellent activity toward oxygen electroreduction was observed by Chen's group. 17 Inter- estingly, strong size effects were observed and the activity increased with the decreasing of the core size. Smaller sized particles possess higher fraction of low-coordinated surface atoms, 18,19 which can facilitate the oxygen adsorption on the surface and making them easily activated. However, when employing gold nanoparticles or clusters alone for catalyzing electrochemical process, the surface ligands may block some active sites hence detrimental to the mass transport behaviors and electron transfer kinetics, 20 meanwhile, gold nanoparticles tend to aggregate, dissolve, sinter or decom- pose during the electrochemical reaction. 21 To conquer these issues, a variety of substrates including porous carbon, 22 carbon nanosheets, 23 carbon nanotubes, 24 TiO 2 25 as well as other materials 26 have been employed as supports to stabilize or entrap the gold nanoparticles. For instances, Alexeyeva et al. found that glassy carbon electrode modied by gold nanoparticle (AuNP)/ multi-walled carbon nanotubes (MWCTs) possessed effective ORR activity in acidic media, and a two-electron reduction pathway was taken. 27 By employing layer-by-layer deposition technique, nanocomposite catalysts of AuNP/PDDA-MWCTs were synthesized and enhanced activity was acquired. 28 Inter- estingly, the hybrid materials of AuNP/MWCTs can also be prepared by sputter deposition of gold on MWCTs followed by heat-treatment. 29 To enhance the activity, these hybrid materials were gener- ally subjected to pyrolysis or elevated temperature calcination to remove the surface ligands. Note that, these processes normally require high energy input and sophisticated sample prepara- tion procedures. Surfactant-free Au nanoclusters in graphene a New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, P. R. China. E-mail: zhht@scut.edu.cn b School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, P. R. China. E-mail: wangchh@gdut.edu.cn c Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, Guangdong Provincial Engineering and Technology Research Center for Environmental Risk Prevention and Emergency Disposal, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou, 510006, China d Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95064, USA † Electronic supplementary information (ESI) available: TEM images of CNTs, HR-TEM images of the Au/CNTs ¼ 1 : 2 composite, additional linear scanning voltammetric (LSV) curves, and repeated LSV measurements of the Au/CNTs ¼ 1 : 2 composite. See DOI: 10.1039/c6ra16533h Cite this: RSC Adv. , 2016, 6, 91209 Received 27th June 2016 Accepted 16th September 2016 DOI: 10.1039/c6ra16533h www.rsc.org/advances This journal is © The Royal Society of Chemistry 2016 RSC Adv. , 2016, 6, 91209–91215 | 91209 RSC Advances PAPER Published on 19 September 2016. Downloaded by University of California - Santa Cruz on 21/11/2016 17:00:46. View Article Online View Journal | View Issue