Carbothermal synthesis of titanium oxycarbide as electrocatalyst support with high oxygen evolution reaction activity
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Yangchuan Xinga) Department of Chemical Engineering, University of Missouri, Columbia, Missouri 65211 (Received 7 May 2012; accepted 21 September 2012)
Carbothermal reduction of semiconducting TiO2 into highly conductive titanium oxycarbide (TiOxCy) was investigated. The thermally produced uniform carbon layer on TiO2 (Degussa P25) protects the TiO2 nanoparticles from sintering and, at the same time, supplies the carbon source for doping TiO2 with carbon. At low temperatures (e.g., 700 °C), carbon only substitutes part of the oxide and distorts the TiO2 lattice to form TiO2 xCx with only substitutional carbon. When the carbon-doped TiO2 is annealed at a higher temperature (1100 °C), x-ray diffraction and x-ray photoelectron spectroscopy results showed that TiOxCy, a solid solution of TiO and TiC, was formed, which displays different diffraction peaks and binding energies. It was shown that TiOxCy has much better oxygen revolution reaction activity than TiO2 or TiO2 xCx. Further studies showed that the TiOxCy obtained can be used as a support for metal electrocatalyst, leading to a bifunctional catalyst effective for both oxygen reduction and evolution reactions.
I. INTRODUCTION
Carbon black has for a long time served as electrocatalyst support for porous electrodes in fuel cells and metal–air batteries.1–7 These carbon particles contribute as reaction sites for the oxygen reduction reaction (ORR) or oxygen evolution reaction (OER), especially during discharge and recharge processes. However, the fact of severe corrosion of carbon materials at high potentials during electrochemical reactions8 makes such applications impractical under longterm cell operation in acidic media.9 Therefore, alternative catalyst supports have to be sought, and recent attention has been focused on TiO2 as a stable support candidate.10–12 Due to the semiconducting nature of TiO2, great efforts were taken to improve its electronic conductivity. One type of reduced TiO2, called Magneli phase (TixO2x 1), has been shown to have high stability and electron conductivity in corrosive electrolytes and at high potentials.13–16 Yet, the reduction condition for making the Magneli phase is generally rather harsh, requiring very high temperatures under hydrogen, leading to severe sintering and consequently significant loss of surface areas. On the other hand, carbon-modified anatase TiO2 nanoparticles and nanotube arrays under relatively mild conditions have been demonstrated to be effective in photocatalysis and applied in dye-sensitized solar cells.17–21 The TiO2 carbon modification methods include carbon a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.353 454
J. Mater. Res., Vol. 28, No. 3, Feb 14, 2013
http://journals.cambridge.org
Downloaded: 13 Jan 2015
monoxide reduction,20 acetylene reduction,22 flame made with TiC,19,23 or polymer TiO2 composites.17,24 X-ray photoelectron spectroscopy (XPS) results of these samples revealed the presence of carbon in all of them.21 However, the particle size dis
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