Ab initio calculations of the condensation of phosphonic acid and methylphosphonic acid:: Chemical properties of potential electrolyte materials for fuel cell applications

Organic and polymeric phosphonic acid derivatives are promising candidates as proton-conducting electrolytes in high-temperature (> 100 degrees C polymer electrolyte membrane fuel cells (PEM-FC). The proton conduction is adversely affected by the tendency of the amphoteric phosphonic acid groups to condensate (Steininger, H.; Schuster, M.; Kreuer, K. D.; Kaltbeitzel, A.; Bingol, B.; Meyer, W. H.; Schauff, S.; Brunklaus, G.; Maier, S.; Spiess, H. W. Phys. Chem. Chem. Phys. 2007, 9, 1764) to dimers, trimers, or higher polycondensates. Ab initio calculations have been performed to obtain the geometry of phosphonic acid (phosphorus acid), rnethylphosphonic acid, and their condensation products. The reaction energies for the formation of the dimer, the trimer, and the cyclic trimer have been calculated, including various corrections due to zero-point vibrations, entropy, and solvation. The results show that the formation of dimers and trimers is energetically possible both for unsubstituted phosphonic acid and for organic phosphonic acids, whereas the formation of the cyclic trimer is unfavorable because of the ring strain. Similarly, a direct mechanism for the dimerization of two molecules of phosphonic acid or methylphosphonic acid can be ruled out because of the high transition-state energies. In the condensed liquid, the condensation step proceeds most likely with the help of other proton-donating or proton-accepting phosphonic acid moieties.