Facile Synthesis of Porous Metal Oxide Nanotubes and Modified Nafion Composite Membranes for Polymer Electrolyte Fuel Cells Operated under Low Relative Humidity Kriangsak Ketpang, Kibong Lee, and Sangaraju Shanmugam* Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST) 50-1, Sang-Ri, Hyeonpung-Myeon, Dalseong-Gun, Daegu, Republic of Korea, 711-873 * S Supporting Information ABSTRACT: We describe a facile route to fabricate mesoporous metal oxide (TiO 2 , CeO 2 and ZrO 1.95 ) nanotubes for efficient water retention and migration in a Nafion membrane operated in polymer electrolyte fuel cell under low relative humidity (RH). Porous TiO 2 nanotubes (TNT), CeO 2 nanotubes (CeNT), and ZrO 1.95 (ZrNT) were synthesized by calcining electrospun polyacrylonitrile nano- fibers embedded with metal precursors. The nanofibers were prepared using a conventional single spinneret electrospinning technique under an ambient atmosphere. Their porous tubular morphology was observed by SEM and TEM analyses. HR- TEM results revealed a porous metal oxide wall composed of small particles joined together. The mesoporous structure of the samples was analyzed using BET. The tubular morphology and outstanding water absorption ability of the TNT, CeNT, and ZrNT fillers resulted in the effective enhancement of proton conductivity of Nafion composite membranes under both fully humid and dry conditions. Compared to a commercial membrane (Nafion, NRE-212) operated under 100% RH at 80 °C, the Nafion-TNT composite membrane delivered approximately 1.29 times higher current density at 0.6 V. Compared to the Nafion-TiO 2 nanoparticles membrane, the Nafion-TNT membrane also generated higher current density at 0.6 V. Additionally, compared to a NRE-212 membrane operated under 50% RH at 80 °C, the Nafion-TNT composite membrane exhibited 3.48 times higher current density at 0.6 V. Under dry conditions (18% RH at 80 °C), the Nafion-TNT, Nafion-CeNT, and Nafion- ZrNT composite membranes exhibited 3.4, 2.4, and 2.9 times higher maximum power density, respectively, than the NRE-212 membrane. The remarkably high performance of the Nafion composite membrane was mainly attributed to the reduction of ohmic resistance by the mesoporous hygroscopic metal oxide nanotubes, which can retain water and effectively enhance water diffusion through the membrane. KEYWORDS: Nafion composite membrane, mesoporous TiO 2 nanotubes, metal oxide, PEFCs, electrospinning ■ INTRODUCTION Recent research in polymer electrolyte fuel cells (PEFCs) operated under low relative humidity (RH) has focused on simplifying thermal management and reducing the cost of the system. 1 However, the current PEFC technology, which utilizes perfluorosulfonic acid (PFSA) polymer membranes, for example, Nafion, as an electrolyte, exhibits poor performance under low RH. 1 The deterioration of PEFCs performance under this condition is mainly due to the loss of proton conductivity of the Nafion membrane, which results in a drastic increase of ohmic overpotential. 2 The proton conductivity of Nafion membrane is highly influenced by the amount of water present in the membrane, and the proton conductivity is maximal when the membrane is fully saturated with water. 3,4 Operating PEFCs under low RH leads to membrane dehydration, which remarkably reduces the proton conductiv- ity. 4 It is thus highly desirable to enhance the proton conductivity of the Nafion membrane under low RH so as to achieve higher PEFCs performance. An effective approach to improve the proton conductivity of Nafion membranes under low RH is to incorporate bifunctional ceramic/inorganic fillers in the membrane, such as SiO 2 , 5-12 TiO 2 , 11-25 ZrO 2 , 11-13,26,27 heteropolyacids, 28 and zeolites, 29 etc., which are both hygroscopic and proton conductors. Hygroscopic inorganic fillers contain water in the molecular framework which does not evaporate even at a temperature greater than 100 °C because of electrostatic attraction within the electrical double layer (EDL). 19 The insolubility of these compounds in aqueous media also prevents them leaching out from the membrane. 19 Nevertheless, incorporating hygroscopic nanoparticle fillers in Nafion membrane does not enhance fuel Received: June 16, 2014 Accepted: September 9, 2014 Published: September 9, 2014 Research Article www.acsami.org © 2014 American Chemical Society 16734 dx.doi.org/10.1021/am503789d | ACS Appl. Mater. Interfaces 2014, 6, 16734-16744