1: Proton diffusion pathway in water estimated using Quantum Hopping Molecular
Dynamics at 300 K. Yellow indicates events for which the transfer occurred
without a barrier (purely quantum event). Magenta depicts events for which
transition state theory applies.The vehicular diffusion of the H3O species,
diffusion alone, is depicted in cyan. Notice how short vehicular diffusion is compared
to proton transfer events, either classical transition state events or quantum tunneling events.
Proton transfer in non-aqueous solvents and anhydrous environments
Several types of "water free" proton conducting membranes that incorporate
quaternary nitrogen atoms are under investigation: a) polymeric materials with
heterocyclic pending groups, such as imidazole, pyrazole or benzimidazole; b)
new dendrimer-PTFE copolymers combining hydrophilic dendrimers with hydrophobic
linear polymers; and c) novel Bronsted acid based ionic liquids, made by
combining organic amines above their melting temperatures with bis
(trifluoromethanesulfonyl) amide (HTFSI,these are electroactive
for H2 oxidation and O2 reduction
at a Pt electrode under non-humidifying conditions at moderate temperatures
(ca. 130oC) (Figure 2),
d) most promising are simple protic ionic liquid mixtures such as a 4:6 mixture of methyl and
dimethyl ammonium nitrate that become superionic at 25oC. At this temperature conductivities are
similar to Nafion at 80oC, 150 versus 100 mS/cm. At 100oC,
the conductivity is 470 mS/cm, a factor of 4 larger than the conductivity of Nafion.
Figure 2: Examples of proton acceptor for a Bronsted acid HTFSI
for non-aqueous proton transfer membrane media.
To explain this superionic proton conductivity we have studied the quantum
mechanical barrier for proton transfer in the methyl-ethyl ammonium nitrate system
(Figure 3). For distance lower than 2.5 Angstroms proton transfer is barrier less that
may help explain the superionic nature of this ionic liquid.
Figure 3: Proton transfer barriers as a function of donor-acceptor (N...O)
distance (in Angstroms) in the methylethylammonium nitrate ionic liquid.
Other alternative amine systems under study are:
Organic bases (such as cyclic amines and heterocyclic
compounds with multiple substituents, including but not limited to F, O, S, N, P)
Polymeric versions of the most promising organic bases
with organic acids (polyphosphoric acid, sulfonic acid, etc.)
Inorganic systems containing high density of hydrogen
donor/acceptors but low-lying temperature/pressure phase transitions.
Personnel: Dr. Mario Blanco