Collective proton transport in polyphosphoric acid

One current goal of modern science is to rationalize and utterly exploit the factors governing proton conductivity for the next generation of energy storage technologies, including fuel cells. The so-called Grotthuss mechanism is generally considered to promote most efficient proton conductivity, implying cooperative, chain-like transfer of protons along the hydrogen-bonded networks of host molecules. However, such constructive type of collective dynamics has not been demonstrated experimentally so far. Employing dielectric spectroscopy, quasielastic neutron scattering, and ab initio molecular dynamic simulations we recently reported a direct experimental observation of proton transfer between the molecules of phosphoric acid. Intriguingly, although our analysis confirmed the existence of correlations between the proton jumps in this material, these lead to effective loops-like charge backflows which are reducing its conductivity, in contrast to the expectation for a Grotthuss mechanism. The present work will investigate how proton transport and its relationship with the structural rearrangements are influenced by the covalent bonding of phosphoric acid molecules. In particular, using polyphosphoric acid as a model system, we will discuss the possibility of tailoring a desired Grotthuss-like type of cooperativity for charge transport via a chain packing of the structural constituents in proton conducting materials.