Kenneth Hernández-Burgos

Rapid and Efficient Redox Processes within 2D Covalent Organic Framework Thin Films

Two-dimensional covalent organic frameworks (2D COFs) are ideally suited for organizing redox-active subunits into periodic, permanently porous polymer networks of interest for pseudocapacitive energy storage. Here we describe a method for synthesizing crystalline, oriented thin films of a redox-active 2D COF on Au working electrodes. The thickness of the COF film was controlled by varying the initial monomer concentration. A large percentage (80-99%) of the anthraquinone groups are electrochemically accessible in films thinner than 200 nm, an order of magnitude improvement over the same COF prepared as a randomly oriented microcrystalline powder. As a result, electrodes functionalized with oriented COF films exhibit a 400% increase in capacitance scaled to electrode area as compared to those functionalized with the randomly oriented COF powder. These results demonstrate the promise of redox-active COFs for electrical energy storage and highlight the importance of controlling morphology for optimal performance.

Increasing the Gravimetric Energy Density of Organic Based Secondary Battery Cathodes Using Small Radius Cations (Li+ and Mg2+)

One of the major challenges in electrochemical energy storage (EES) is increasing the gravimetric capacity and energy density of the cathode material. Here we demonstrate how to increase the gravimetric energy density of electrical energy storage devices based on the use of organic materials through exploitation of the strong ionic coupling between a reduced carbonyl functionality and small cations such as lithium (Li(+)) and magnesium (Mg(2+)). Binding of the cation to the reduced carbonyl results in a positive shift of the formal reduction potential of the carbonyl couple. This has the effect of increasing the cell voltage which, in turn, results in an increase in the energy density. We show how this interaction can be used to dramatically increase, by up to a factor of 2, the energy density for a selected case study using 1,2-di(thiophen-2-yl)ethane-1,2-dione (DTED). We have carried out electrochemical and computational studies in order to understand the thermodynamic (positive shift of 250 mV and 1 V in the formal potential for the first and second reductions, respectively, of the carbonyl groups of DTED) and kinetic effects between small radii cations (Li(+) and Mg(2+)) and the reduced carbonyl functionality of carbonyl-based organic molecules (C-bOMs).

Cation-Dependent Stabilization of Electrogenerated Naphthalene Diimide Dianions in Porous Polymer Thin Films and Their Application to Electrical Energy Storage

Porous polymer networks (PPNs) are attractive materials for capacitive energy storage because they offer high surface areas for increased double-layer capacitance, open structures for rapid ion transport, and redox-active moieties that enable faradaic (pseudocapacitive) energy storage. Here we demonstrate a new attractive feature of PPNs--the ability of their reduced forms (radical anions and dianions) to interact with small radii cations through synergistic interactions arising from densely packed redox-active groups, only when prepared as thin films. When naphthalene diimides (NDIs) are incorporated into PPN films, the carbonyl groups of adjacent, electrochemically generated, NDI radical anions and dianions bind strongly to K(+), Li(+), and Mg(2+), shifting the formal potentials of NDIs second reduction by 120 and 460u2005mV for K(+) and Li(+)-based electrolytes, respectively. In the case of Mg(2+), NDIs two redox waves coalesce into a single two-electron process with shifts of 240 and 710u2005mV, for the first and second reductions, respectively, increasing the energy density by over 20u2009% without changing the polymer backbone. In contrast, the formal reduction potentials of NDI derivatives in solution are identical for each electrolyte, and this effect has not been reported for NDI previously. This study illustrates the profound influence of the solid-state structure of a polymer on its electrochemical response, which does not simply reflect the solution-phase redox behavior of its monomers.

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

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.

Modulation of the Electrochemical Reactivity of Solubilized Redox Active Polymers via Polyelectrolyte Dynamics

Redox active polymers (RAPs) are electrochemically versatile materials that find key applications in energy storage, sensing, and surface modification. In spite of the ubiquity of RAP-modified electrodes, a critical knowledge gap exists in the understanding of the electrochemistry of soluble RAPs and their relation to polyelectrolyte dynamics. Here, we explore for the first time the intersection between polyelectrolyte behavior and the electrochemical response that highly soluble and highly substituted RAPs with viologen, ferrocene, and nitrostyrene moieties elicit at electrodes. This comprehensive study of RAP electrolytes over several orders of magnitude in concentration and ionic strength reveals distinct signatures consistent with surface confined, colloidal, and bulk-like electrochemical behavior. These differences emerge across polyelectrolyte packing regimes and are strongly modulated by changes in RAP coil size and electrostatic interactions with the electrode. We found that, unlike monomer motifs, simple changes in the ionic strength caused variations over 1 order of magnitude in the current measured at the electrode. In addition, the thermodynamics of adsorbed RAP films were also affected, giving rise to standard reduction potential shifts leading to redox kinetic effects as a result of the mediating nature of the RAP film in equilibrium with the solution. Full electrochemical characterization via transient and steady-state techniques, including the use of ultramicroelectrodes and the rotating disk electrode, were correlated to dynamic light scattering, ellipsometry, and viscometric analysis. These methods helped elucidate the relationship between electrochemical behavior and RAP coil size, film thickness, and polyelectrolyte packing regime. This study underscores the role of electrostatics in modulating the reactivity of redox polyelectrolytes.