Theoretical study on sulfonated and phosphonated poly[(aryloxy)phosphazenes] as proton-conducting membranes for fuel cell applications

Polyphosphazenes are considered to be more useful as proton-conducting membranes than Nafion due to their low methanol permeability, low water swelling ratios, satisfactory mechanical properties, and conductivities comparable to those of Nafion. In this work, compounds 1-6, six polyphosphazenes with different side groups, were designed and calculated. Structural parameters, proton affinities and water adsorptions were obtained on the basis of the optimized geometrical structures. Our calculations were in agreement with experimental results. It was found that the proton conductivities of the sulfonated poly[(aryloxy)phosphazenes] (R¹SO₃H) are higher than those of the phosphonated ones (R¹PO₃H₂), while the phosphonated poly[(aryloxy)phosphazenes] will retain water better at higher temperature than the sulfonated ones. The electron-withdrawing substituent of R² is beneficial to proton conductivities and water adsorptions of both sulfonated and phosphonated poly[(aryloxy)phosphazenes].