Phosphoric Acid Transfers Protons Cooperatively

High proton conductivity in liquid phosphoric acid arises from concerted transfer across chains of H₃PO₄ molecules rather than through individual transfer events (Nat. Chem., DOI: 10.1038/nchem.1329). Neat liquid phosphoric acid has the highest known intrinsic proton conductivity, and understanding the mechanism may help improve proton-conducting fuel-cell electrolytes. In the new work, a team led by Mark E. Tuckerman of New York University and Klaus-Dieter Kreuer of Germany’s Max Planck Institute for Solid State Research used ab initio molecular dynamics simulations to study proton transfer in liquid phosphoric acid. They found that transfer occurs in a cooperative manner across chains of as many as five H₃PO₄ molecules. This mechanism contrasts with that of proton transfer in water, which happens in discrete steps from one H₂O molecule to another with accompanying solvent reorganization. The difference arises from the different hydrogen-bonding networks in the liquids, Tuckerman says. The hydrogen bonds in liquid phosphoric acid are shorter than those in water, making it easier for H₃PO₄ to auto­ionize. Also, hydrogen bond rearrangement to resolve charge buildup from proton transfer requires only rotation around a P–O bond rather than reorientation of an entire H₃PO₄ molecule.