Interrogating Charge Storage on Redox Active Colloids via Combined Raman Spectroscopy and Scanning Electrochemical Microscopy

Abstract

Redox active colloids (RACs) are dispersible, cross-linked polymeric materials that incorporate a high concentration of redox-active motifs, making them attractive for next-generation size-exclusion redox flow batteries. In order to tap into their full potential for energy storage, it is essential to understand their internal charge mobility, capacity, and cyclability. Here we focus on using a combined suite of Raman spectroscopy and scanning electrochemical microscopy (SECM) tools for evaluating three important parameters that govern charge storage in viologen-RACs: their intraparticle redox active concentration, their reduction/oxidation mechanism, and their charge transfer rate. We addressed RACs using SECM imaging and single-particle experiments, from which the intraparticle diffusion and concentration parameters were elucidated. By using Raman spectroscopy coupled to surface interrogation SECM, we further evaluated their reversible redox properties within monolayer films of 80- and 135-nm-sized RACs. Most notably we have confirmed that the concentration and redox mechanisms are essentially unchanged when varying the RAC size. As expected, we see that larger particles inherently require longer times for electrolysis independent of the methodology used for their study. Our simulations further verify the internal concentration of RACs and suggest that their porosity enables solution redox active mediators to penetrate and titrate charge in their interior. The combined methodology presented here sets an important analytical precedent in decoupling the charge storage properties of new bulk materials for polymer batteries starting from probing low-dimensional assemblies and single particles using nano- and spectroelectrochemical approaches.

Impact of Charge Transport Dynamics and Conditioning on Cycling Efficiency within Single Redox Active Colloids

“…Substrate Preparation: (1) RACs were dispersed in MeCN and dropcasted onto a glass substrate, (2) allowed to dry, (3) and then heated and cooled under ambient conditions. CV on multiple particles allowed us to estimate the diffusion coefficient for charge transfer, D CT , [18,41,42] at 8 10 À11 AE 3 10 À11 cm 2 /s (n = 7) in agreement with previous measurements on RAC monolayers. It was positioned 2.1 mm away from the surface to accommodate the RAC swollen height (~1.3 mm).…”

“…As depicted in Scheme 1, we approached a sharpened SECM probe with an exposed Pt microdisk of 300 nm in radius toward a glass surface containing Scheme 1. [18] We stepped the probe toward the particle in 100 nm increments while using cyclic voltammetry (CV). Substrate Preparation: (1) RACs were dispersed in MeCN and dropcasted onto a glass substrate, (2) allowed to dry, (3) and then heated and cooled under ambient conditions.…”

“…[12,17,18] Charge hopping on redox-active polymers has been investigated in solution, [19][20][21] as films on modified electrodes, [21,22] and with isolated molecules. Electron transfer at the electrode-particle interface and subsequent charge hopping enables particle electrolysis via the movement of charge between the oxidized (V 2 + ), and the reduced state (V + ).…”

“…Electron transfer at the electrode-particle interface and subsequent charge hopping enables particle electrolysis via the movement of charge between the oxidized (V 2 + ), and the reduced state (V + ). [18] We now turn to expanding these single-particle methodologies to evaluate performance-governing characteristics during cycling. [23] Recently, our group introduced spectroelectrochemical and nanoelectrochemical methodologies for evaluating electron transfer on single RAC particles and RAC monolayer assemblies.…”

“…battery nanomaterials. [18,[32][33][34][35][36][37] Through SECM feedback imaging, single RACs were located to make contact for direct charge/discharge measurements. To date the majority of these single entity studies has focused on micron-sized particles for ion intercalation.…”