The Isotope Effect on Proton Conduction and Glass Transition in Phosphoric Acid

Hydrogen fuel cells combine hydrogen and oxygen to create water and electricity. Polymer electrolyte membranes (PEM) make up barriers within the fuel cell allowing only protons to pass through, while keeping other components separate. Many PEM contain phosphoric acid (PA) as a building block due to its excellent proton conducting properties. Improved ionic conductivity in PEM can lead to the development of better, more efficient fuel cells.

While ionic conductivity in PA at high temperatures is extensively characterized, the low temperature dynamics are not so well explored. Below the glass transition, molecular motion is frozen and proton motion is forced to occur via local, intermolecular hopping. The Grotthuss mechanism of proton transport describes a relay-type proton motion, in which the jump of a proton to a neighboring molecule is followed by a succeeding proton jump of a different proton. Proton hopping is faster than molecular diffusion of a charged molecule, i.e. the Grotthuss mechanism allows for extraordinarily high proton conduction.

Isotopic substitution of hydrogen with deuterium alters the mass of the moving particle, while keeping electric charge unaffected. The isotope effect is defined as the ratio of reaction rates of different isotopes. Classically, the isotope effect on proton motion should be proportional to the ratio of the square root of mass. Experiments have shown that the isotope effect does not confine to the classical picture, but in reality is much larger. Isotopic exchange also affects glass transition. The temperature at which glass transition occurs is generally higher in materials containing the heavier isotope.

In the present work, effects of isotopic substitution on electrical and material characteristics of PA are investigated using broadband dielectric spectroscopy (BDS), differential scanning calorimetry (DSC) and Brillouin light scattering (BLS). Ionic conductivity was found to decrease in PA with the heavier isotope. Glass transition temperatures increased in the deuterated samples. In addition a strong change in ionic conductivity and glass transition temperature were observed between samples of different concentrations of acidity. Isotope effects do not follow the classical predictions, but exhibit much stronger changes.