Porous Silica Nanospheres Functionalized with Phosphonic Acid as Intermediate-Temperature Proton Conductors Yong Gang Jin, Shi Zhang Qiao,* Zhi Ping Xu, Joa˜o C. Diniz da Costa, and Gao Qing Lu* ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and Australian Institute of Bioengineering and Nanotechnology, The UniVersity of Queensland, QLD 4072, Australia ReceiVed: NoVember 18, 2008; ReVised Manuscript ReceiVed: December 23, 2008 This paper reports the first study of developing porous silica nanospheres functionalized with phosphonic acid as intermediate-temperature (above 100 °C) proton conductors. These materials were synthesized by the co-condensation of diethylphosphatoethyltriethoxysilane (DPTS) and tetraethoxysilane using surfactant cetyltrimethylammonium bromide as a template, followed by acidification of phosphonate to phosphonic acid. With more DPTS used in the synthesis, the pore structure of the sample changes from the ordered mesoporous MCM-41 structure to the microporous structure, and the particle size simultaneously increases from ca. 80 to ca. 200 nm. The prepared silica nanospheres show promising proton conductivity above 100 °C; for example, the conductivity is as high as 3.0 × 10 -4 to 0.015 S cm -1 at 130 °C when the relative humidity increases from 20 to 100%. The proton conductivity increases with an increase in the content of functionalized phosphonic acid, whereas the conductivity under low humidity conditions is significantly enhanced by the high surface area of the porous structure. Because of their nanosized monodisperse spherical morphology, these novel intermediate-temperature proton conductors are promising in applications as inorganic fillers for preparing composite proton exchange membranes. 1. Introduction The current state-of-the-art proton exchange membrane fuel cells (PEMFCs), based on perfluorosulfonic acid polymer membranes, such as Nafion, are only capable of operating at temperatures below 100 °C (typically at 80 °C) due to the inability of the membrane to function at intermediate temper- atures above 100 °C. 1,2 Increasing the operating temperature to over 100 °C promises important benefits with respect to the complexity, cost, and performance of the fuel cell system. 3 Recently, acid-functionalized proton conductors based on me- sostructured silica have been prepared by pore filling, 4 postgrafting, 5,6 and one-pot co-condensing functional silica precursors 7 or functional surfactants. 8 These materials exhibit promising proton conductivity, especially above 100 °C and under low humidity conditions, resulting from their superior water adsorption properties of the porous structure. Furthermore, some attempts have been made to use these proton conductors as inorganic fillers to modify polymer PEMs. 9-12 The incorpora- tion of these fillers improves water retention of the membrane to enable the intermediate-temperature operation and simulta- neously reduces the methanol crossover when the modified membrane is used in the direct methanol fuel cell. However, the functionalization of these proton conductors is mainly related to sulfonic acid as the protogenic group. Alternatively, phosphoric acid and phosphonic acid have recently attracted much attention for the development of intermediate-temperature PEMs due to their high charge carrier concentration, thermal stability, and oxidation resistance. More importantly, amphoteric phosphoric/phosphonic acids, which are advantageous over sulfonic acid, can facilitate proton conduction in the dry state by forming dynamic hydrogen bond networks. 13,14 So far, the functionalization with phosphoric acid has been widely studied, 15-17 but acid leaching by water is problematic for applications in fuel cells due to the hydrolytically unstable linkage of phosphoric acid with silicate or polymer backbones, such as Si-O-P and C-O-P bonds. In this regard, the phosphonic acid functionalization via hydrolytically stable Si-C and C-P bonds is more attractive. On the other hand, most of these previously reported proton conductors have a large particle size and an irregular morphology, and, thus, the agglomeration of fillers is generally found when they are incorporated as fillers in the composite membrane. 9,10 It has been reported that the higher surface-to-volume ratio of fillers as a result of a smaller agglomerate size is more favorable in the property enhancement of the composite membrane. 18,19 Here we report the preparation of phosphonic acid function- alized silica nanospheres with a porous structure, such as the MCM-41 mesostructure, followed by a systematic investigation of their proton conductivity at temperatures up to 140 °C and under a variety of humidity conditions. The proton conduction of the prepared materials has been elucidated in relation to the content of functionalized phosphonic acid and the pore structure. To the best of our knowledge, this represents the first study of developing porous silica nanospheres functionalized with phos- phonic acid as intermediate-temperature proton conductors. In addition, the nanosized monodisperse proton conductors pre- pared here have promise as inorganic fillers to facilitate the preparation of homogeneous organic-inorganic composite PEMs. This has been confirmed by our very recent work, 20 where the homogeneous Nafion composite membranes have been easily prepared using MCM-41 pure silica nanospheres as fillers. However, incorporating these unfunctionalized silica fillers decreases the proton conductivity because they are much less conductive than Nafion. Hence, our present functionalized materials more favor the conductivity enhancement of the composite membrane. * To whom correspondence should be addressed. E-mail: s.qiao@uq.edu.au (S.Z.Q.), maxlu@uq.edu.au (G.Q.L.). Phone: +61 7 33463815. Fax: +61 7 33656074. J. Phys. Chem. C 2009, 113, 3157–3163 3157 10.1021/jp810112c CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009