Fast Proton Conduction Facilitated by Minimum Water in a Series of Divinylsilyl-11-silicotungstic Acid-co-Butyl Acrylate-co-Hexanediol Diacrylate Polymers James L. Horan, † Anitha Lingutla, § Hui Ren, § Mei-Chen Kuo, † Sonny Sachdeva, † Yuan Yang, ‡ Soenke Seifert, ⊥ Lauren F. Greenlee, # Michael A. Yandrasits, ∥ Steven J. Hamrock, ∥ Matthew H. Frey, § and Andrew M. Herring* ,† † Department of Chemical and Biological Engineering, and ‡ Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401, United States § 3M Corporate Research Materials Laboratory, and ∥ 3M Component Fuel Cell Program, 3M Center, St. Paul, Minnesota 55144, United States ⊥ X-ray Sciences Division, Argonne National Laboratory, Argonne, Illinois 60439, United States # Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, Colorado 80305, United States * S Supporting Information ABSTRACT: Studies of proton transport in novel materials are important to enable a large array of electrochemical devices. In this study, we show that heteropoly acids (HPAs) when immobilized in polymer matrixes have highly mobile protons. Divinyl-11-silicotungstic acid, an HPA, was copolymerized with butyl acrylate and hexanediol diacrylate at various weight percentage loadings from 25% to 85% using UV initiated polymerizations. The resultant films were tan colored flexible sheets of ca. 120 μm thickness. The morphology of these films varied with loading, showing phase separation into clustered HPA above a 50 wt % loading and lamella morphologies above an 80 wt % loading. Water uptake was strongly associated with the HPA clusters, which facilitated transport of protons. This was realized by proton conductivities as high as 0.4 S cm −1 at 95 °C and 95% RH and 0.1 S cm −1 at 85 °C and 50% RH. Pulse field gradient spin echo NMR measurements indicated that water self- diffusion was fast (1.4 × 10 −5 and 4.4 × 10 −5 cm 2 s −1 for 50% and 100% RH, respectively) at 80 °C. We show that the water in these systems is highly associated with the HPA clusters and that fast proton transport is facilitated by as few as 3 water molecules per proton. 1. INTRODUCTION The study of proton conduction in novel materials is of increasing importance. 1 The ability to operate devices at temperatures >100 °C under ambient humidity conditions would greatly improve efficiency with the potential to simplify system design and lower costs. 2 The dominant types of membrane materials used in proton conductors are perfluoro- sulfonic acid (PFSA) polymers that must be saturated with water to obtain practical levels of proton conduction. 3 The need for a hydrated membrane restricts practical operating temperatures to lower than 100 °C, resulting in parasitic loads such as heat rejection and water management being placed on the operating system of devices using PFSA membranes. 4 A large effort has been applied to this problem, and there have been some promising materials developed recently that have much higher proton conductivities under hotter and drier conditions than has been possible in the past. The approach we take in this work is to replace the traditional hydrated sulfonic acid group or recently developed proton-conducting groups such as sulfonimides, 3,5 phosphorus-based acids, 6 and function- alized zirconium phosphonates 7−10 in PEMs with heteropoly acids (HPAs) as the sole proton conductors. Supporting Information Table S1 compares the reported proton conductivity of these materials. The development of advanced sulfonated hydrocarbon polymers has resulted in proton conductivities on the order of 0.1 S cm −1 above 100 °C, which represents a dramatic improvement on the conductivity of PFSA polymers under similar conditions. 11,12 To date, the use of inorganic additives to proton-conducting systems has not yielded any dramatic improvement in conductivity at temper- atures between 100 and 120 °C (Supporting Information Table S1), unless the inorganic moiety is incorporated into the polymer backbone. 10 In this Article, we describe a novel proton conduction pathway using very few water molecules mediated by HPAs in a novel film. The HPAs are a subset of the well-known large class of metal oxides called polyoxometalates (POMs), 13 in which a central heteroatom is surrounded by typically 12 or more tungsten or Received: September 6, 2013 Revised: December 13, 2013 Published: December 13, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 135 dx.doi.org/10.1021/jp4089657 | J. Phys. Chem. C 2014, 118, 135−144