H 3 PO 4 / oxide nanoparticles / polymer composites as proton conducting membranes

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H3PO4 / oxide nanoparticles / polymer composites as proton conducting membranes D. Carrière, P. Barboux, M. Moreau and J.-P. Boilot. Laboratoire de Physique de la Matière Condensée, CNRS UMR 7643C, Ecole Polytechnique, 91128 Palaiseau Cedex ABSTRACT Stable sols of 60 nm colloidal zirconia have been prepared by thermolysis of zirconium acetate. The surface complexing acetate groups have been replaced by phosphoric acid groups. Phosphate grafting has been characterized by dynamic light scattering, infrared spectroscopy, 31P nuclear magnetic resonance and impedance spectroscopy measurements. These systems give acid and proton conductive particles (4.10-5 S.cm-1 at 70 % relative humidity). H3PO4/ZrO2/PVDF-co-HFP composite membranes have been synthesized. Impedance spectroscopy measurements allow discrimination between proton conduction at the surface of the phosphated particles and within free H3PO4 in the polymer. For the highest H3PO4/ZrO2 ratios, the latter phenomenon prevails, giving a proton conductivity of 6.10-4 S.cm-1 at 70 % R.H. INTRODUCTION Proton-conductive membranes may find various applications in electrochromic devices, capacitors and fuel cells. This last application requires high proton conductivity, high chemical durability and a good stability against temperature and moisture modifications. Nafion®, a purely organic acid fluoropolymer used in such devices presents a swelling effect at high humidity and temperature but also a loss of proton conductivity at low water content [1]. Dispersions of H3PO4, H2SO4 or other acids in various polymers [2] may overcome this problem. However, stability of these systems towards hydrolysis can be poor. This leads to a loss of proton carriers during fuel cell running. An interesting alternative is dispersion of high specific surface acid oxides. Indeed, they show both good proton mobility and their acid character is stable towards hydrolysis [3]. We here present a study relative to phosphate grafting onto polycrystalline nanometric sols of ZrO2, and their use in H3PO4/ZrO2/PVDF-co-HFP composite membranes. EXPERIMENTAL The zirconium acetate solution (Zr ~ 15-16% wt) and triethyl phosphate were purchased from Aldrich Chemicals. Acetic acid was purchased from SDS. PVDF-co-HFP (Mw = 145000, HFP: 11% wt) has been friendly given by Atochem. Dynamic light scattering (DLS) measurements were obtained on a Malvern 4700 photocorrelator. 31P nuclear magnetic resonance was performed in a high-resolution liquid Bruker probe with a MSL 360 spectrometer. Impedance spectroscopy was performed with a HP4192A impedance analyzer. Silver electrodes were cast on the samples. Samples were equilibrated at 70 % relative humidity with a dilute sulfuric acid solution. X-ray powder diffraction was run with a Philips X Pert diffractometer at the CuKα wavelength. Infrared spectra were made with a Perkin Elmer 783 spectrometer on samples diluted in KBr powder. V12.5.1

Zirconia sol in water Stable aqueous sols of crystalline zirconia were prepared as described by Matchett and al. [4] as follows: 20