Water condensation behavior on the surface of a network of superhydrophobic carbon fibers with high-aspect-ratio nanostructures Tae-Jun Ko a,b , Eun Kyu Her a,b , Bongsu Shin b,c , Ho-Young Kim c , Kwang-Ryeol Lee b , Bo Ki Hong d , Sae Hoon Kim d , Kyu Hwan Oh a , Myoung-Woon Moon b, * a Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea b Future Conversion Technology Research Division, Korea Institute of Science and Technology, Seoul 130-650, Republic of Korea c School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea d Fuel Cell Vehicle Team 1, Eco-Technology Center, Hyundai-Kia Motors, Yongin-Si, Gyeonggi-Do 446-912, Republic of Korea ARTICLE INFO Article history: Received 3 May 2012 Accepted 25 June 2012 Available online 2 July 2012 ABSTRACT We have explored the condensation behavior of water on a superhydrophobic carbon fiber (CF) network with high-aspect-ratio hair-like nanostructures. Nanostructures ranging from nanopillars to hairy shapes were grown on CFs by preferential oxygen plasma etching. Superhydrophobic CF surfaces were achieved by application of a hydrophobic siloxane- based hydrocarbon coating, which increased the water contact angle from 147° to 163° and decreased the contact angle hysteresis from 71° to below 5°, sufficient to cause droplet roll-off from the surface. Water droplet nucleation and growth on the superhydrophobic CF were significantly retarded due to the high-aspect-ratio nanostructures under super- saturated vapor conditions. CFs are observed to wet with condensation between fibers of the pristine surface under super-saturated vapor conditions, which eventually leads to flooding. However, dropwise condensation became dominant in the superhydrophobic CF network, allowing for easy removal of the condensed droplets, which largely allowed the interstitial spaces of the fiber network to remain dry. It is implied that superhydrophobic CF can provide a passage for vapor or gas flow in wet environments such as a gas diffusion layer requiring the effective water removal in the operation of proton exchange membrane fuel cell. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Numerous studies have been conducted on the unique char- acteristics of carbon fiber (CF) such as high strength, light weight, and high porosity, which are applicable in various fields ranging from conventional areas such as aircraft, auto- mobile, and sports [1,2] to more recent energy storage and transfer technologies such as electrodes for super-capacitor [3,4] and lithium ion batteries [5,6] and gas diffusion layer (GDL) for proton exchange membrane fuel cell (PEMFC) [7]. Especially, GDL is composed of microscale CF networks such as CF-felt, paper, or cloth, and that provides the microscale porosity which allows for effective flow of reactant gases in PEMFC. However, during the PEMFC operation, water is gener- ated from the oxygen reduction reaction, and sometime excessive water is condensed on CF network, which would block the transition sites of CF networks of GDL, resulting in water flooding. This phenomenon inhibits the transport of 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.06.048 * Corresponding author: Fax: +82 2 958 5487. E-mail address: mwmoon@kist.re.kr (M.-W. Moon). CARBON 50 (2012) 5085 – 5092 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon