Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: A progress report By courtesy of: H. Steininger, M. Schuster, K. D. Kreuer,* A. Kaltbeitzel, B. Bingol, W. H. Meyer, S. Schauff, G. Brunklaus, J. Maiera and H. W. Spiess Phys. Chem. Chem. Phys., 2007, 9, 1764–1773Â