The Proton Conducting Mechanism of PEG-Polyoxometalates Nanocomposites Is Revealed
The developing of next generation proton conductors is quite critical because of the urgent need of high efficient fuel cells. The design of proton conductors with appropriate mechanical properties that can be operated at high temperature and low environment humidity is the key issue in this field. Polyoxometalates (POMs), a group of nano-scaled metal oxide clusters with well-defined, mono-dispersed structures, show promising proton conductivity and high stability against high temperature. However, the proton conductivity of pure POMs requires high environmental humidity. Moreover, the high crystalline energy of POMs materials makes them hard for further processing. Polymers own appreciable and tunable mechanical properties and some polymer also possess high ion conductivity. Therefore, the developing of novel POM-polymer hybrid materials is used to address all these drawbacks of POM proton conductors. The POM-polymer hybrid also owns tunable phase separation behavior in their bulk state, which can be used to further increase the proton conductivity of the thin films processed from these hybrid materials. Specifically, researchers have successfully prepared the poly(ethylene glycol) (PEG)-POM hybrid materials with PEG molecules inserted inside the nano-channels defined by POMs. The obtained anhydrous hybrid materials show high proton conductivity at high temperature and low humidity (<1% RH). However, the design of these PEG-POM hybrid materials still need optimization to achieve the quality of commercial proton conductors. The understanding of proton conductivity mechanism is the key to the rational design of the PEG-POM hybrid materials.
A joint team led by Prof. Panchao Yin from South China University of Technology and Prof. Zhe Wang of Tsinghua University with collaborators from the University of Tokyo and Oak Ridge National Laboratory applied advanced neutron scattering techniques to study the structures and dynamics of these hybrid nanocomposites and provided the proton conducting mechanism at molecular scale.
Due to their sensitivity to hydrogen and its isotopes, neutron scattering techniques have been under intense development, and their applications for research on structures and dynamics of polymers have been significantly expanded. Suggested from inelastic neutron scattering (INS) results, due to the strong space confinement and POM-PEG interaction, the PEG chain possesses a distorted helix structure with a number of ttt geometries in the polymer main chain.
Fig. 1 Structures of PEG-POM nanocomposites.
QENS was further used to explore the spatial-temporal features of the confined PEG motion and their relation to the proton conduction process of the material. Indicated from QENS results analysis, and , parameters that describe the confining geometry for the motion of C atoms, are found to be around 13 Å and 0.3 Å, respectively. It can be concluded that the C atoms indeed perform a highly longitudinal motion, as we have assumed in the modelis the characteristic time of the motion of an H atom relative to its associated C atom. Their analysis shows thatis ca. several picoseconds for all measured temperatures.
Fig. 2 Conformation of the PEG inside the 1D nano-channel defined by the framework of POMs. In panel (a), the PEG chain is seen to stay as a distorted helix. Panel (b) illustrates the meanings of fitting parameters. The cylinder with a length of Ｌ and a radius of Ｒ denotes the space that a C atom can explore.Ｄ is the diffusivity of a C atom, andis the characteristic time of the two-side jump of an H atom in a methylene group. The double-headed arrow denotes the direction of the 1D nano-channel.。
A nice Arrhenius behavior is observed on the temperature dependence of both proton conductivityand self-diffusion coefficientＤ of PEG chain. It is observed that the slopes ofＤ andare close to each other, suggesting that these two processes have similar activation energies. This results suggest that the proton conduction of POMs highly relies on the successful hopping of proton among POM clusters, which is strongly suppressed in anhydrous POM samples since protons are localized around single POMs and separated from each other by counterion barriers. For these POM-PEG hybrids, PEG chain segment can bridge the hopping of protons from one POM to their neighbor POMs. A proton can temporarily associate with an O atom on PEG and move together with the polymer segment. The motion of O atoms along the longitudinal direction makes it much easier for the proton to hop between different POMs. When an external electrical field is applied, the proton can perform a long-range transport through the PEG with the assistance of the localized longitudinal motion of O atoms.
Fig. 3 Proton conductivity (blue) and self-diffusion coefficientＤ(red) as a function of. The symbols are experimental results, the solid lines denote the fitting results with Arrhenius law.
Strategies regarding the design of high-efficiency proton conductors can be conjected from the above proton conduction mechanism. The activation energy of proton conducting process can be lowered by optimizing the structure of POM-PEG hybrid materials that favors the 1D diffusion of PEG. The POM units can be altered to those with less negative charges in order to weaken the supramolecular interaction between PEG and POMs and consequently lower the energy barrier for PEG motions. On the other hand, giving the fact that the dynamics of PEGs usually heavily rely on their molecular weight, PEGs with low molecular weight are preferred for the design of hybrid proton conducting materials.
This work was published in the Journal of Physical Chemistry Letters. This story was also selected as supplementary cover to highlight its contribution in nanoscience, physical chemistry, polymer science, and proton conducting.
Wu, H.; Li, L.; Tsuboi, M.; Cheng, Y.; Wang, W.; Mamontov, E.; Uchida, S.; Wang, Z.*; Yin, P.* Spatial-Temporal Characteristics of Confined Polymer Motion Determine Proton Conduction of Polyoxometalate–Poly(ethylene glycol) Hybrid Nanocomposites. The Journal of Physical Chemistry Letters2018, 5772-5777.
Link：https://pubs.acs.org/doi/10.1021/acs.jpclett.8b02113
Prof. Panchao Yin started his independent research career in 2015 and have independently published 16 papers as corresponding author, including 2 in J. Am. Chem. Soc. （Link1, Link2）、1 in Nano Lett. (Link)、1 in J. Phys. Chem. Lett.(Link)、1 in Mater. Chem. Front.(Link)、1 in Chem. Commun.(Link)、2 in Chem. Eur. J. （Link1, Link2）、1 in Inorg. Chem.(Link)和1 in Langmuir (Link)。
Source from South China Advanced Institute for Soft Matter Science and Technology