Single-Particle Evaluation of Nano and Micro-Sized Redox-Active Polymers for Energy Storage

Abstract

Redox active polymers (RAPs) and redox active colloids (RACs) are new energy storage materials for size-exclusion redox flow batteries (RFBs).1-3 RAPs show beneficial characteristics for RFBs including high capacity, high solubility, electrochemical reversibility and tailorable properties via synthetic methods.1,2,4 Their crosslinked counterparts, RACs, are well defined spheres that can be nano- or micro-sized and have many similar characteristics to RAPs while showing even higher capacities and greater crossover rejection.3 As with other macromolecular systems, much remains unknown about the applicability and practicality of energy storage systems involving organic macromolecules capable of storing 106-109 electrons per molecule. In this presentation, I want to discuss methodologies we have been using to analyze stability and evolution of the RAC charge/discharge process during cycling with a focus on measuring a single or a limited number of particles. As with other non-conjugated RAPs, the mechanism of charge transfer across a RAC involves electron hopping between redox active pendants and simultaneous counterion movement.3,5 We recently investigated the electron transfer process within viologen-based RACs through various analytical methods based on scanning electrochemical microscopy (SECM) and modeling through COMSOL.6 SECM is a scanning probe technique that can assess local redox reactivity at the micro or nanoscale in a liquid electrolyte amenable to real battery environments.7 With SECM, single RACs could be imaged, sized and directly reduced/oxidized through making contact with an SECM probe. We were also able to evaluate monolayer films of nanometer sized RACs with a spectroelectrochemical setup using a coupled Raman-SECM instrument. This study provided some useful fundamental information and analytical tools/methods to guide future development and refinement for long time-scale experiments on RACs. I will present on recent cycling experiments involving SECM and modified electrodes. Both chronoamperometric steps and cyclic voltammetry (CV) have been used to measure the cycling process and to manipulate capacity access during cycling. Our data indicate that RACs undergo some conditioning before higher charge recovery and the proportion of reduced to oxidized viologen, or state-of-charge (SoC), will affect the amount of charge that can be recollected from the RAC. I will also discuss measurements involving Raman coupled to electrochemistry for single and few RACs. Refining and developing effective analytical methods is the key to understanding electron transfer, storage and failure within RACs and all battery materials. Our methods are pushing the limits of electrochemistry on single macromolecules but hopefully will be extended to other energy storage and electron transfer systems. J. Am. Chem. Soc. 2014, 136 (46), 16309-16316. Nature 2015, 527 (7576), 78-81. J. Am. Chem. Soc. 2016, 138 (40), 13230-13237. Chem. Mater. 2016, 28 (20), 7362-7374. J. Phys. Chem. 1991, 95 (16), 6383-6389. Langmuir 2017, 33 (37), 9455-9463. Scanning electrochemical microscopy. CRC Press: 2012.