Synthesis and Characterization of Redox-Responsive Disulfide Cross-Linked Polymer Particles for Energy Storage Applications

Cross-linking poly(glycidyl methacrylate) microparticles with redox-responsive bis(5-amino-l,3,4-thiadiazol-2-yl) disulfide moieties yield redox-active particles (RAPs) capable of electrochemical energy storage via a reversible 2-electron reduction of the disulfide bond. The resulting RAPs show improved electrochemical reversibility compared to a small-molecule disulfide analogue in solution, attributed to spatial confinement of the polymer-grafted disulfides in the particle. Galvanostatic cycling was used to investigate the impact of electrolyte selection on stability and specific capacity. A dimethyl sulfoxide/magnesium triflate electrolyte was ultimately selected for its favorable electrochemical reversibility and specific capacity. Additionally, the specific capacity showed a strong dependence on particle size where smaller particles yielded higher specific capacity. Overall, these experiments offer a promising direction in designing synthetically facile and electrochemically stable materials for organosulfur-based multielectron energy storage coupled with beyond Li ion systems such as Mg

Morphology controls the thermoelectric power factor of a doped semiconducting polymer.

The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with F n TCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (σ) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (μ). The Seebeck coefficient (α) is largely independent of OCL. This reveals that, unlike σ, leveraging strategies to improve μ have a smaller impact on α. Our best-performing sample with the largest OCL, vapor-doped PBTTT:F4TCNQ thin film, has a σ of 670 S/cm and an α of 42 μV/K, which translates to a large PF of 120 μW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics

Entropic Penalty Switches Li⁺ Solvation Site Formation and Transport Mechanisms in Mixed Polarity Copolymer Electrolytes

Emerging solid polymer electrolyte (SPE) designs for efficient Li-ion (Li+) conduction have relied on polarity and mobility contrast to improve conductivity. To further develop this concept, we employ simulations to examine Li+ solvation and transport in poly(oligo ethylene methacrylate) (POEM) and its copolymers with poly(glycerol carbonate methacrylate) (PGCMA). We find that Li+ is solvated by ether oxygens instead of the highly polar PGCMA, due to lower entropic penalties. The presence of PGCMA promotes single-chain solvation, thereby suppressing interchain Li+ hopping. The conductivity difference between random copolymer PGCMA-r-POEM and block copolymer PGCMA-b-POEM is explained in terms of a hybrid solvation site mechanism. With diffuse microscopic interfaces between domains, PGCMA near the POEM contributes to Li+transport by forming hybrid solvation sites. The formation of such sites is hindered when PGCMA is locally concentrated. These findings help explain how thermodynamic driving forces govern Li+ solvation and transport in mixed SPEs

Critical Percolation Threshold for Solvation-Site Connectivity in Polymer Electrolyte Mixtures

Poly(ethylene oxide) (PEO)-based polymer electrolytes are often mixed with rigid, nonconductive polymers to improve mechanical strength. The suppressed conductivity of the mixture typically arises from a reduced segmental mobility and a diminished connectivity between conductive PEO sites. To decouple these two mechanisms, we compare transport in symmetric miscible blends and disordered block copolymers (BCP) of PEO and poly(methyl methacrylate) (PMMA). Because the two systems have identical physicochemical properties, differences in their conductivity directly reflect the underlying PEO network connectivity. We find that, at short distances (+ solvation networks are identical for the two systems; however, a distinct variation in the network connectivity arises at length scales between 5 and 10 Å. Specifically, the BCP exhibits a lower connectivity, and therefore a lower conductivity than the blend. A quantitative model is proposed that associates long-range Li+ transport with local miscibility; the concept of network connectivity discussed here could be useful for designing polymer electrolyte systems

Morphology controls the thermoelectric power factor of a doped semiconducting polymer

The electrical performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution versus vapor) on the thermoelectric power factor (PF) of PBTTT molecularly p-doped with FnTCNQ (n = 2 or 4). The vapor-doped films have more than two orders of magnitude higher electronic conductivity (s) relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) (that is, length scale of aligned backbones) that correlates to a high apparent charge carrier mobility (m). The Seebeck coefficient (a) is largely independent of OCL. This reveals that, unlike s, leveraging strategies to improve m have a smaller impact on a. Our best-performing sample with the largest OCL, vapor-doped PBTTT: F4TCNQ thin film, has a s of 670 S/cm and an a of 42 μV/K, which translates to a large PF of 120 mW m-1 K-2. In addition, despite the unfavorable offset for charge transfer, doping by F2TCNQ also leads to a large PF of 70 μW m-1 K-2, which reveals the potential utility of weak molecular dopants. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics

Organo-disulfide-based particles enable controlled stimulus-triggered cleaning of electrode surfaces

Electrode fouling resulting in reduced performance is an ongoing challenge in electrochemical flow cells based on redox active polymers (RAPs). An avenue that holds substantial promise yet remains relatively unexplored involves the strategic design of RAPs capable of undergoing electrochemical stimulation to facilitate in situ electrode cleaning within a flow cell. Herein, a new electrode cleaning strategy is demonstrated through the application of redox-active poly(glycidyl methacrylate) particles crosslinked with 2-amino-1,3,4-thiadiazole disulfide (PGMA–ATDDS). The resulting particles can de-crosslink through cleavage of the disulfide bond using stimuli, such as electrochemical reduction or UV photoexcitation. Using a custom flow cell, applying such a stimulus to an ITO electrode artificially fouled with PGMA–ATDDS in the presence of a fluid flow leads to a significant particle removal (80%) that is over six times more efficient relative to the case when no stimulus is applied. Confocal fluorescence imaging of the electrochemically stimulated electrode highlighted localized disulfide reduction of particles near the electrode surface. It is posited that this selective de-crosslinking and concomitant electrolyte swelling at the particle/electrode interface facilitate particle removal in the presence of a fluid flow. In addition, the regeneration of electrode performance upon cleaning was demonstrated through charging of a redox-active particle suspension of poly(vinylbenzyl chloride) functionalized with dimethylaminoferrocene (PVBC–Fc). Upon electrochemical cleaning of the fouled ITO electrode, the accessible charge of PVBC–Fc was statistically equivalent to the accessible charge measured using a pristine ITO electrode. Overall, this study introduces a new approach for leveraging stimulus-responsive chemistries for RAPs to impart inherent functionality to facilitate in-line electrode cleaning in electrochemical flow cells

Tethered tertiary amines as solid-state n-type dopants for solution-processable organic semiconductors

A scarcity of stable n-type doping strategies compatible with facile processing has been a major impediment to the advancement of organic electronic devices. Localizing dopants near the cores of conductive molecules can lead to improved efficacy of doping. We and others recently showed the effectiveness of tethering dopants covalently to an electron-deficient aromatic molecule using trimethylammonium functionalization with hydroxide counterions linked to a perylene diimide core by alkyl spacers. In this work, we demonstrate that, contrary to previous hypotheses, the main driver responsible for the highly effective doping observed in thin films is the formation of tethered tertiary amine moieties during thin film processing. Furthermore, we demonstrate that tethered tertiary amine groups are powerful and general n-doping motifs for the successful generation of free electron carriers in the solid-state, not only when coupled to the perylene diimide molecular core, but also when linked with other small molecule systems including naphthalene diimide, diketopyrrolopyrrole, and fullerene derivatives. Our findings help expand a promising molecular design strategy for future enhancements of n-type organic electronic materials

Power Factor Enhancement in Solution‐Processed Organic n‐Type Thermoelectrics Through Molecular Design

A new class of high‐performance n‐type organic thermoelectric materials, self‐doping perylene diimide derivatives with modified side chains, is reported. These materials achieve the highest n‐type thermoelectric performance of solution‐processed organic materials reported to date, with power factors as high as 1.4 μW/mK^2. These results demonstrate that molecular design is a promising strategy for enhancing organic thermoelectric performance

Increasing the Thermoelectric Power Factor of a Semiconducting Polymer by Doping from the Vapor Phase

We demonstrate how processing methods affect the thermoelectric properties of thin films of a high mobility semiconducting polymer, PBTTT. Two doping methods were compared: vapor deposition of (tridecafluoro-1,1,2,2-tetrahydrooctyl)­trichlorosilane (FTS) or immersion in a solvent containing 4-ethylbenzenesulfonic acid (EBSA). Thermally annealed, thin films doped by FTS deposited from vapor yield a high Seebeck coefficient (α) at high electronic conductivity (σ) and, in turn, a large power factor (PF = α²σ) of ∼100 μW m^–1 K^–2. The FTS-doped films yield α values that are a factor of 2 higher than the EBSA-doped films at comparable high value of σ. A detailed analysis of X-ray scattering experiments indicates that perturbations in the local structure from either dopant are not significant enough to account for the difference in α. Therefore, we postulate that an increase in α arises from the entropic vibrational component of α or changes in scattering of carriers in disordered regions in the film