Andreas Wohlfarth

Single Alkaline‐Ion (Li+, Na+) Conductors by Ion Exchange of Proton‐Conducting Ionomers and Polyelectrolytes

The vast majority of alkaline ion conductors used in electrochemical cells, especially batteries, are solutions of salts (such as LiClO4, LiCF3SO3 or LiPF6) in either aprotic polar solvents (such as ethylene carbonate) or polar polymers containing ether bridges (such as poly-ethyleneoxides, PEO). The solvating properties of such environments are poor compared to what is known for aqueous systems: there is still significant residual ionic interaction leading to correlations of the motion of cations and anions 4] and even the formation of contact ion pairs and triple ions. 6] Such effects are most pronounced for polymeric solvents. Here, the salt dissociation is driven by the coordination of alkaline ions with ether oxygens while the anions are chemically more free to move. As a consequence, such materials are more efficient anions than alkaline ion conductors. 4] Of course, such differences are smeared out in liquid electrolytes, but even for these there is strong indication for the formation of contact ion pairs and effective alkaline ion transference numbers smaller than the ones of anions (it is worth noting that alkaline ions can also be transferred by the counter diffusion of, for example, contact ion pairs and anions). The combination of relatively low total conductivity and low alkaline ion transference number is expected to lead to significant concentration polarization effects (formation of salt concentration gradients) even at moderate alkaline ion currents. Especially for high drain applications, such as high power batteries, electrolytes with a high single ion (Li, Na) conductivity may therefore help to reduce polarization effects within the electrolyte. This has been stimulating many attempts towards true single alkaline ion conductors, but in most cases either the transference number remained clearly below unity 11] or the conductivity was reduced to a non acceptable level. Most approaches aim at a retardation or complete immobilization of the anions. This may be achieved by introducing receptor functionalities for the anions (e.g. on polymers or particles) or by covalently binding the anion to a stationary phase (e.g. polymer, particle). Recently, Armand et al. have shown that by using anions with highly delocalized negative charge, even covalent immobilization may still allow for reasonable single ion conductivity. For a polystyrene functionalized with -[SO2-N-SO2-CF3] Li and solvated with PEO, they measured a conductivity of 3 10 5 S cm 1 at T = 60 8C. So far, the highest conductivities have been obtained for ionomers in their Li-form swollen with aprotic polar solvents. While for solvated polyphosphazenes with sulfonimide functional groups in their Li-form only moderate room temperature conductivities of up to 2.5 10 6 S cm 1 are reported, quite high conductivities reaching values of 7 10 4 S cm 1 have been measured for Li exchanged Nafion solvated with NMF. 21] These conductivities could be even further increased to values slightly above 1 mS cm 1 by using solvent mixtures (e.g. NMP/DMF) or by the additional use of crown-ethers as Licomplexing agent. To which extent the observed conductivities are carried by Li has not been examined yet. This Communication reports on polyelectrolytes solvated with aprotic polar solvents with conductivities reaching values so far known only for liquid aprotic electrolytes (i.e. larger 1 mS cm ), but with alkaline ion transference numbers close to unity, as evidenced by the comparison of conductivity and tracer diffusion coefficient data. Also, the relation between solvent and ion diffusion is investigated for the first time. The approach is based on our recent experience in the development of ionomers and polyelectrolytes for fuel cell applications showing high proton conductivity even at high temperature and low relative humidity. Under these conditions, also water becomes a poor solvent, that is, dissociation is no longer complete which has severe implications for the local structure and dynamics of the system. The clue for obtaining high proton conductivity turned out to be a very high local density of superacidic anions. These were sulfonic acid groups covalently bound to each phenyl ring of a poly phenylene-sulfone. Owing to the strong M effect of the sulfone links, the phenyl rings are low in electron density which is making the sulfonic acid functional group attached to these rings very acidic and the polymer backbone less sensitive towards electrophilic attack. 23] The very high density of the ionic groups allows for the formation of a continuous aqueous domain even at low hydration numbers l = [H2O]/[-SO3H] at which the very high acidity still leads to a substantial concentration of protonic charge carriers. These are actually hydronium ions and the conduction mechanism is of the vehicle type, that is, the diffusion of hydronium ions is a simple hydrodynamic process without significant proton exchange between hydronium ions and the solvent (water). In other words, the protonic charge carrier behaves like any other monovalent cation in this environment, and this suggests that also high alkaline ion conductivity may be observed in such systems. To test this hypothesis, we have ion exchanged highly sulfonated poly phenylene-sulfones in the acid form with Li and [a] Dr. K.-D. Kreuer, A. Wohlfarth, Dr. C. C. de Araujo, A. Fuchs, Prof. Dr. J. Maier Max-Planck-Institut f r Festkçrperforschung Heisenbergstraße 1, D-70569 Stuttgart (Germany) Fax: (+ 49) 711-689-1722 E-mail : kreuer@fkf.mpg.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201100506.

The solvation and ion condensation properties for sulfonated polyelectrolytes in different solvents : a computational study

In contrast to the broad knowledge about aqueous polyelectrolyte solutions, less is known about the properties in aprotic and apolar solvents. We therefore investigate the behavior of sulfonated polyelectrolytes in sodium form in the presence of different solvents via all-atom molecular dynamics simulations. The results clearly reveal strong variations in ion condensation constants and polyelectrolyte conformations for different solvents like water, dimethyl sulfoxide (DMSO) and chloroform. The binding free energies of the solvent contacts with the polyelectrolyte groups validate the influence of different solvent qualities. With regard to the ion condensation behavior, the numerical findings show that the explicit values for the condensation constants depend on the preferential binding coefficient as derived by the evaluation of Kirkwood–Buff integrals. Surprisingly, the smallest ion condensation constant is observed for DMSO compared to water, whereas in the presence of chloroform, virtually no free ions are present, which is in good agreement to the donor number concept. In contrast to the results for the low condensation constants, the sodium conductivity in DMSO is smaller compared to water. We are able to relate this result to the observed smaller diffusion coefficient for the sodium ions in DMSO.

Hypersulfonated polyelectrolytes: preparation, stability and conductivity

Specially tailored polyelectrolytes are becoming important as energy-related materials. Here we explore a synthetic strategy to prepare fully aromatic polymers containing single phenylene rings in the backbone functionalized with four sulfonic acid groups. Thioether bridges of semifluorinated poly(arylene thioether)s were oxidized to sulfone bridges, followed by substitution of all fluorines by NaSH and quantitative oxidation of the resulting thiol groups. This gave poly(arylene sulfone)s containing octasulfonated biphenyl units, reaching ion exchange capacities up to 8 meq g−1 and unprecedented high local sulfonic acid concentrations. These polyelectrolytes are stable up to 300 °C under air and achieve proton conductivities of up to 90 mS cm−1 at 120 °C and 50% relative humidity. Despite the excellent performance of this unique new class of hypersulfonated polymers, our data suggests that incomplete proton dissociation may ultimately limit the conductivity of highly sulfonated polymers.

Crystal Structures of New Alkali Metal-rich Oxometallates: Rubidium Aluminate Tetrahydroxide, Rb9(AlO4)(OH)4, Rubidium Orthogallate, Rb5GaO4, Cesiumbis-Chromate(IV) Oxide, Cs10(CrO4)2O, and Cesium Diindate, Cs8In2O7

Several new alkali metal oxometallates with anions built up from tetrahedral [MO4] units were obtained in reactions aimed at the formation of alkali metal suboxometallates or by thermally decomposing the latter. Rubidium orthoaluminate tetrahydroxide Rb9(AlO4)(OH)4 crystallizes with a new structure type (space group P21/c, a = 13.116(1), b = 6.9266(5), c = 18.934(2) A , β = 92.05(1)°, V = 1719.0(3) Å3, Z = 4, R1 = 0.0352) and contains orthoaluminate anions [AlO4]5− and isolated hydroxide anions. Rubidium orthogallate Rb5GaO4 crystallizes with the Na5GaO4 structure type (space group Pbca, a = 6.9318(5), b = 21.309(2), c = 11.740(1) Å, V = 1734.2(3) Å3, Z = 8, R1 = 0.0423) with isolated orthogallate anions [GaO4]5−. Cesium chromate oxide Cs10(CrO4)2O adopts the Cs10(GeO4)2O structure type (space group P21/c, a = 12.903(1), b = 11.4523(8), c = 19.074(3) Å , β = 127.903(8)°,V = 2223.9(4) Å3, Z = 4, R1 = 0.0326) with orthochromate(IV) anions [CrO4]4− and isolated oxide anions. In all orthometallates the anions [MO4]n− deviate only slightly from ideal tetrahedral symmetry. Cesium diindate Cs8In2O7 crystallizes with the Cs8Fe2O7 structure type (space group P21/c, a = 7.4307(6), b = 18.6181(14), c = 7.2639(6) Å , β = 119.225(8)°, V = 877.0(1) Å3, Z = 2, R1 = 0.0349). A single-crystal structure investigation at r. t. has shown linear diindate units, but the temperature dependence of the libration angles from TLS studies for the bridging oxygen atom suggests a slightly bent and dynamically disordered diindate anion. Graphical Abstract Crystal Structures of New Alkali Metal-rich Oxometallates: Rubidium Aluminate Tetrahydroxide, Rb9(AlO4)(OH)4, Rubidium Orthogallate, Rb5GaO4, Cesiumbis-Chromate(IV) Oxide, Cs10(CrO4)2O, and Cesium Diindate, Cs8In2O7