The development of redox-active conjugated polymers for energy storage and electrocatalysis
Department of Material Science and Engineering, Stanford University, CA, USA
Redox-active conjugated polymers are an interesting class of materials for electrochemical devices since their properties can be tuned by chemical design to enable redox activity in various electrolytes. In my talk, I will introduce design rules for the development of conjugated polymers with high electronic and ionic charge transport properties to enables fast charging of single-phase electrodes in aqueous electrolytes [1,2]. While the choice of the polymer backbone is important for achieving electrochemical stability of the charged polymer states, it is found that the tuning of the local environment of the polymers is equally important. The tuning of the local environment of the materials is achieved by attaching hydrophilic (polar) side chains to the polymer backbone to enable fast ion transport, swelling, and reversible phase changes. Electrodes fabricated with these polymers can utilize more than 70 % of the available redox-active state in the bulk of the polymer while maintaining high electrochemical stability during continuous cycling. In the second part of my talk, I will show how this concept can be utilized for developing a single-phase, metal-free, and solution processible electrocatalyst for the oxygen reduction reaction in aqueous electrolytes. I will explain the working principle of polymeric electrocatalyst, for which the polymeric organic semiconductor is first activated by an electrochemical doping reaction that increases the reactivity of the material towards molecular oxygen. By employing in-situ spectroelectrochemical measurements and rotating ring disk electrode (RRDE) measurements, the performance of electrocatalyst is evaluated across various pH values where the highest activity is found in alkaline electrolytes. In summary, I will show how the tuning of the polymer's energy levels and side chains is a successful strategy for the development of low-cost, metal-free, and solution-processable electrocatalysts for energy conversion technologies.
 A. Giovannitti, C. B. Nielsen, D.-T. Sbircea, S. Inal, M. Donahue, M. R. Niazi, D. A. Hanifi, A. Amassian, G. G. Malliaras, J. Rivnay, I. McCulloch, Nat. Commun.2016, 7, 13066.
 D. Moia, A. Giovannitti, A. A. Szumska, I. P. Maria, E. Rezasoltani, M. Sachs, M. Schnurr, P. R. F. Barnes, I. McCulloch, J. Nelson, Energy Environ. Sci.2019, 12, 1349.
We present the development of electron-transporting polymeric organic semiconductors as a new class of metal-free electrocatalysts for the oxygen reduction reaction in aqueous electrolytes. The polymeric organic semiconductors are based on conjugated polymers with large electron affinities (equivalent to materials with low lying lowest unoccupied molecular orbital (LUMO)), where polar side chains are attached to the backbone to process materials from solution and to improve the ionic charge transport properties. This design concept results in fast charging polymer electrodes where volumetric charging of thick electrodes (> 1 μm) is achieved due to the balanced ionic and electronic charge transport properties of the polymer. The outstanding mixed ionic/electronic transport properties also enable the utilization of single-phase electrodes where no additives or binders are needed for the electrode to function in aqueous electrolytes. We will further explain the working principle of polymeric electrocatalyst, for which the polymeric organic semiconductor is first activated by an electrochemical doping reaction (reduction, n-type doping) that increases the reactivity of the material towards molecular oxygen. By employing in-situ spectroelectrochemical measurements and rotating ring disk electrode (RRDE) measurements, we find that the polymer achieves its highest performance when charged to the polaronic, singly charged, state. The polymers predominantly yield hydrogen peroxide through the 2-electron reduction of oxygen. The selectivity towards peroxide and water (4-electron product) is influenced by electrolyte pH. We hypothesize that chemically tuning the polymer's energy levels and side chains will pave the way for the successful development of low-cost, metal-free, and solution-processable electrocatalysts for energy conversion technologies.