Polyrotaxane helps proton transfer

Published in Chemistry
Polyrotaxane helps proton transfer

The paper “Thermally triggered polyrotaxane translational motion helps proton transfer” in Nature Communication can be found here: https://www.nature.com/articles/s41467-018-04733-4

Proton exchange membranes (PEMs) are negatively charged polymers in a film shape that conduct cations. PEMs have witnessed widespread applications in so many fields, including fuel cells (typical processes that convert chemical energy into electricity) and flow batteries (that can store megawatts of electricity in chemicals). For a traditional PEM, cations, for example, protons are exchanged between negatively charged carriers (for instance, sulfonates) and these carriers are covalently bonded to the polymer backbone, which has limited mobility due to large molecular weight. The limited mobility then results in low proton transfer rate, i.e. low proton conductivity and it remains the bottleneck to further enhance the efficiency of numerous energy conversion/storage processes, in which proton transfer is involved. A straightforward method to increase the proton transfer rate of PEMs is to incorporate more negatively charged carriers. However, mechanical properties of the PEMs are then compromised due to severe water swelling.

We report our recent strategy to deliver faster proton transfer while maintain the mechanical properties of the membranes. This strategy is inspired by the concept of molecular machines. A molecular machine, defined as an assembly of a distinct number of molecular components that are designed to perform machine-like movements as a result of an appropriate external stimulation, has been demonstrated and built, based on topological entanglement (mechanical bonds) or isomerisable unsaturated bonds. We proved in this work that by introducing the thermally triggered translational motion (although not in a controlled manner) of mechanically bonded rotaxane in a polymeric entity, exceptionally fast proton transfer could be attained at an external thermal supply. We implemented rotaxanes (based on a ring threaded over an axle with stoppers at both ends) in a polymer and found that as a result of a thermal input (an increase in temperature), the axle moves translationally. The axle has sulfonates on both ends (thereby negatively charged) and acts as proton transfer carrier. As a consequence, a proton conductivity (indicating proton transfer rate) of 260.2 mS cm-1, which is much higher than that of the state-of the-art Nafion, was achieved at a relatively low ion-exchange capacity of 0.73 mmol g-1(representing the amount of proton transfer carriers).

As opposed to the circumstance in a conventional proton transfer polymer to whose backbone the proton transfer carriers are covalently bonded, we included rotaxanes, which have proton transfer carriers that move when a thermal stimulus is applied, to prompt proton transfer rate. We proved that the thermally triggered movement of proton carriers help proton transfer at a much faster rate than that in traditional proton exchange membranes. This feature would benefit numerous applications of energy conversion and energy storage.

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