Proton transfer driven by the fluctuation of water molecules in chitin film

Proton-transfer mechanisms and hydration states were investigated in chitin films possessing the functionality of fuel-cell electrolytes. The absolute hydration number per chitin molecule (N) as a function of relative humidity (RH) was determined from the OH stretching bands of H2O molecules, and the proton conductivity was found to enhance above N = 2 (80%RH). The FIR spectrum at 500-900 cm(-1) for 20%RH (N < 1) together with first-principles calculations clearly shows that the w(1) site has the same hydration strength as the w(2) site. The molecular dynamics simulations for N = 2 demonstrate that H2O molecules with tiny fluctuations are localized on w(1) and w(2), and the hydrogen-bond (HB) network is formed via the CH2OH group of chitin molecules. Shrinkage of the O-O distance (d(OO)), which synchronizes with the barrier height, is required for proton transfer from H3O+ to adjacent CH2OH groups or H2O molecules. Nevertheless, d(OO) is hardly modulated for N = 2 because H2O molecules are strongly constrained on w(1) and w(2), and therefore, the transfer probability becomes small. For N = 3, novel HBs emerged between the additional H2O molecules broadly distributed on the w(3) site and H2O molecules on w(1) and w(2). The transfer probability is enhanced because large fluctuations and diffusions in the whole H2O molecule yield large modulations of d(OO). Consequently, long-range proton hopping is driven by the Zundel-type protonated hydrates in the water network.