Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes

Abstract

Conjugated polymers achieve redox activity in electrochemical devices by combining redox-active, electronically conducting backbones with ion-transporting side chains that can be tuned for different electrolytes. In aqueous electrolytes, redox activity can be accomplished by attaching hydrophilic side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes. While this approach has been beneficial for achieving fast electrochemical charging in aqueous solutions, little is known about the relationship between water uptake by the polymers during electrochemical charging and the stability and redox potentials of the electrodes, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes harms the reversibility of electrochemical processes and results in irreversible swelling of the polymer. We show that small changes of the side chain composition can significantly increase the reversibility of the redox behavior of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. Finally, we show that tuning the local environment of the redox-active polymer by attaching hydrophilic side chains can help to reach high fractions of the theoretical capacity for single-phase electrodes in aqueous electrolytes. Our work shows the importance of chemical design strategies for achieving high electrochemical stability for conjugated polymers in aqueous electrolytes.

The effect of side chain engineering on conjugated polymers in organic electrochemical transistors for bioelectronic applications

“…Despite this, as exemplified throughout, EG based side chains have undoubtably bolstered the field, improved OECT device performance by orders of magnitude, increased operational stability and have allowed for the swellability of materials to be controlled. 77,79,84,91…”

“…With increasing hydrophilicity in the system, uneven swelling and water retention upon cyclic electrochemical doping processes can indeed permanently disrupt the microstructure permanently and hence weakens material stability and hinders charge carrier mobility. 50,84 Consequently, one of the major challenges of material design rises from the trade-off between the electronic conductance and ionic conductance, which are opposingly correlated with the swelling behaviour. The introduction of mixed types of side chains into these materials could serve to optimise the charge mobility and the volumetric capacitance simultaneously.…”

“…The introduction of mixed types of side chains into these materials could serve to optimise the charge mobility and the volumetric capacitance simultaneously. In studies examining the potential application of D–A copolymers in OECTs, side chains of different natures could be separately engineered to the donor or acceptor monomers, achieving the goal of controlled swelling 84 and improved stability. 41,86…”

“…Moreover, the accumulation of the remaining water in each doping/dedoping cycle, due to the strong interaction between the glycol side chains and water molecules, leads to irreversible morphological changes that sets limits on the reversibility, stability and finally the performance of materials. 84,85…”

“…13(a) . 84 This study examines the impact of side chain nature in a similar way to the EG percentage study previously discussed, 37 but with a different focus on the role of alkyl side chains to control the swelling behaviour. Regardless of the slight difference in the donor units, the increasing amount of hydrophobic alkyl side chains results in a decrease in the degree of swelling in both types of polymers.…”