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1. Enhanced doping and structure relaxation of unsubstituted polythiophene through oxidative chemical vapor deposition and mild plasma treatment

   https://doi.org/10.1088/2515-7639/ad1c02  

   Zhang, Yuxuan; Liu, Mingyuan; Yeom, Hyo-Young; Jun, Byung-Hyuk; Baek, Jinwook; No, Kwangsoo; Song, Han-Wook; Lee, Sunghwan (January 2024, Journal of Physics: Materials)

   Abstract We report on the enhancement of electrical properties of unsubstituted polythiophene (PT) through oxidative chemical vapor deposition (oCVD) and mild plasma treatment. The work function of p-type oCVD PT increases after the treatment, indicating the Fermi level shift toward the valence band edge and an increase in carrier density. In addition, regardless of initial values, nearly the same work function is obtained for all the plasma-treated oCVD PT films as high as ∼5.25 eV, suggesting the pseudo-equilibrium state is reached in the oCVD PT from the plasma treatment. This increase in carrier density after plasma treatment is attributed to the activation of initially not-activated dopant species (i.e. neutrally charged Br), which is analogous to the release of trapped charge carriers to the valence band of the oCVD PT. The enhancement of electrical properties of oCVD PT is directly related to the improvement of the thin film transistor performance such as drain current on/off ratio, ∼10³and field effect mobility, 2.25 × 10⁻²cm²Vs⁻¹, compared to untreated counterparts of 10²and 0.09 × 10⁻²cm Vs⁻¹, respectively. 

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2. Effects of film thickness on electrochemical properties of nanoscale polyethylenedioxythiophene (PEDOT) thin films grown by oxidative molecular layer deposition (oMLD)

   https://doi.org/10.1039/D3NR00708A  

   Brathwaite, Katrina G.; Wyatt, Quinton K.; Atassi, Amalie; Gregory, Shawn A.; Throm, Eric; Stalla, David; Yee, Shannon K.; Losego, Mark D.; Young, Matthias J. (March 2023, Nanoscale)

   Poly(3,4-ethylene dioxythiophene) (PEDOT) has a high theoretical charge storage capacity, making it of interest for electrochemical applications including energy storage and water desalination. Nanoscale thin films of PEDOT are particularly attractive for these applications to enable faster charging. Recent work has demonstrated that nanoscale thin films of PEDOT can be formed using sequential gas-phase exposures via oxidative molecular layer deposition, or oMLD, which provides advantages in conformality and uniformity on high aspect ratio substrates over other deposition techniques. But to date, the electrochemical properties of these oMLD PEDOT thin films have not been well-characterized. In this work, we examine the electrochemical properties of 5–100 nm thick PEDOT films formed using 20–175 oMLD deposition cycles. We find that film thickness of oMLD PEDOT films affects the orientation of ordered domains leading to a substantial change in charge storage capacity. Interestingly, we observe a minimum in charge storage capacity for an oMLD PEDOT film thickness of ∼30 nm (60 oMLD cycles at 150 °C), coinciding with the highest degree of face-on oriented PEDOT domains as measured using grazing incidence wide angle X-ray scattering (GIWAXS). Thinner and thicker oMLD PEDOT films exhibit higher fractions of oblique (off-angle) orientations and corresponding increases in charge capacity of up to 120 mA h g −1 . Electrochemical measurements suggest that higher charge capacity in films with mixed domain orientation arise from the facile transport of ions from the liquid electrolyte into the PEDOT layer. Greater exposure of the electrolyte to PEDOT domain edges is posited to facilitate faster ion transport in these mixed domain films. These insights will inform future design of PEDOT coated high-aspect ratio structures for electrochemical energy storage and water treatment. 

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3. Enabling High-Rate Long-lifespan Lithium-Sulfur Batteries via Stereolithography Technique and Oxidative Chemical Vapor Deposition

   Zhang, Yuxuan; Lee, Sunghwan (June 2022, 64th Electronic Materials Conference)

   Enhancing battery energy storage capability and reducing the cost per average energy capacity is urgent to satisfy the increasing energy demand in modern society. The lithium-sulfur (Li-S) battery is especially attractive because of its high theoretical specific energy (around 2600 W h kg-1), low cost, and low toxicity.1 Despite these advantages, the practical utilization of lithium-sulfur (Li-S) batteries to date has been hindered by a series of obstacles, including low active material loading, shuttle effects, and sluggish sulfur conversion kinetics.2 The traditional 2D planer thick electrode is considered as a general approach to enhance the mass loading of the Li-S battery.3 However, the longer diffusion length of lithium ions, which resulted in high tortuosity in the compact stacking thick electrode, decreases the penetration ability of the electrolyte into the entire cathode.4 Although an effort to induce catalysts in the cathode was made to promote sulfur conversion kinetic conditions, catalysts based on transition metals suffered from the low electronic conductivity, and some elements (i.e.: Co, Mn) may even absorb and restrict polysulfides for further reaction. 5 To mitigate the issues listed above, herein we propose a novel sulfur cathode design strategy enabled by additive manufacturing and oxidative chemical vapor deposition (oCVD). 6,7 Specifically, the cathode is designed to have a hierarchal hollow structure via a stereolithography technique to increase sulfur usage. Microchannels are constructed on the tailored sulfur cathode to further fortify the wettability of the electrolyte. The as-printed cathode is then sintered at 700 °C in an N2 atmosphere in order to generate a carbon skeleton (i.e.: carbonization of resin) with intrinsic carbon defects. The intrinsic carbon defects are expected to create favorable sulfur conversion conditions with sufficient electronic conductivity. In this study, the oCVD technique is leveraged to produce a conformal coating layer to eliminate shuttle effects. Identified by scanning electron microscopy and energy-dispersive X-ray spectroscopy mapping characterizations, the oCVD PEDOT is not only covered on the surface of the cathode but also on the inner surface of the microchannels. High-resolution x-ray photoelectron spectroscopy analyses (C 1s and S 2p orbitals) between pristine and modified samples demonstrate that a high concentration of the defects has been produced on the sulfur matrix after sintering and posttreatment. In-operando XRD diffractograms show that the Li2S is generated in the oCVD PEDOT-coated sample during the charge and discharge process even with a high current density, confirming an eminent sulfur conversion kinetic condition. In addition, ICP-OES results of lithium metal anode at different states of charge (SoC) verify that the shuttle effects are excellently restricted by oCVD PEDOT. Overall, the high mass loading (> 5 mg cm-2) with an elevated sulfur utilization ratio, accelerated reaction kinetics and stabilized electrochemical process have been achieved on the sulfur cathode by implementing this innovative cathode design strategy. The results of this study demonstrate significant promises of employing pure sulfur powder with high electrochemical performance and suggest a pathway to the higher energy and power density battery. References: 1 Chen, Y. Adv Mater 33, e2003666. 2 Bhargav, A. Joule 4, 285-291. 3 Liu, S. Nano Energy 63, 103894. 4 Chu, T. Carbon Energy 3. 5 Li, Y. Matter 4, 1142-1188. 6 John P. Lock. Macromolecules 39, 4 (2006). 7 Zekoll, S. Energy & Environmental Science 11, 185-201. 

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4. POSS-ProDOT crosslinking of PEDOT

   https://doi.org/10.1039/c7tb00598a  

   Wei, Bin; Liu, Jinglin; Ouyang, Liangqi; Martin, David C. (January 2017, Journal of Materials Chemistry B)

   Alkoxy-functionalized polythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-propylenedioxythiophene) (PProDOT) have become promising materials for a variety of applications including bioelectronic devices due to their high conductivity, relatively soft mechanical response, good chemical stability and excellent biocompatibility. However the long-term applications of PEDOT and PProDOT coatings are still limited by their relatively poor electrochemical stability on various inorganic substrates. Here, we report the synthesis of an octa-ProDOT-functionalized polyhedral oligomeric silsesquioxane (POSS) derivative (POSS-ProDOT) and its copolymerization with EDOT to improve the stability of PEDOT coatings. The POSS-ProDOT crosslinker was synthesized via thiol–ene “click” chemistry, and its structure was confirmed by both Nuclear Magnetic Resonance and Fourier Transform Infrared spectroscopies. PEDOT copolymer films were then electrochemically deposited with various concentrations of the crosslinker. The resulting PEDOT- co -POSS-ProDOT copolymer films were characterized by cyclic voltammetry, Electrochemical Impedance Spectroscopy, Ultraviolet-Visible spectroscopy and Scanning Electron Microscopy. The optical, morphological and electrochemical properties of the copolymer films could be systematically tuned with the incorporation of POSS-ProDOT. Significantly enhanced electrochemical stability of the copolymers was observed at intermediate levels of POSS-ProDOT content (3.1 wt%). It is expected that these highly stable PEDOT- co -POSS-ProDOT materials will be excellent candidates for use in bioelectronics devices such as neural electrodes. 

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5. Sulfur Cathode Design Strategies Enabled by Stereolithography Technique and Oxidative Chemical Vapor Deposition

   Zhang, Yuxuan; Song, Han Wook; Lee, Sunghwan (May 2022, Materials Research Society)

   It is urgent to enhance battery energy storage capability to satisfy the increasing energy demand in modern society and reduce the average energy capacity cost. Among the candidates for next-generation high energy storage systems, the lithium-sulfur battery is especially attractive because of its high theoretical specific energy (around 2600 W h kg-1) and cost savings potential.1 In addition to the high theoretical capacity of sulfur cathode as high as 1,673 mA h g-1, sulfur is further appealing due to its abundance in nature, low cost, and low toxicity. Despite these advantages, the application of sulfur cathodes to date has been hindered by a number of obstacles, including low active material loading, low electronic conductivity, shuttle effects, and sluggish sulfur conversion kinetics.2 The traditional 2D planer thick electrode is considered as a general approach to enhance the mass loading of the lithium-sulfur (Li-S) battery.3 However, the longer diffusion length of lithium ions required in the thick electrode decrease the wettability of the electrolyte (into the entire cathode) and utilization ratio of active materials.4 Encapsulating active sulfur in carbon hosts is another common method to improve the performance of sulfur cathodes by enhancing the electronic conductivity and restricting shuttle effects. Nevertheless, it is also reported that the encapsulation approach causes unfavorable carbon agglomeration with low dimensional carbons and a low energy density of the battery with high dimensional carbons. Although an effort to induce defects in the cathode was made to promote sulfur conversion kinetic conditions, only one type of defect has demonstrated limited performance due to the strong adsorption of the uncatalyzed clusters to the defects (i.e.: catalyst poisoning). 5 To mitigate the issues listed above, herein we propose a novel sulfur electrode design strategy enabled by additive manufacturing and oxidative chemical vapor deposition (oCVD).6,7 Specifically, the electrode is designed to have a hierarchal hollow structure via a stereolithography technique to increase sulfur usage. Microchannels are constructed on the tailored sulfur cathode to further fortify the wettability of the electrolyte. The as-printed cathode is then sintered at 700 °C in a reducing atmosphere (e.g.: H2) in order to generate a carbon skeleton (i.e.: carbonization of resin) with intrinsic carbon defects. A cathode treatment with benzene sulfonic acid further induces additional defects (non-intrinsic) to enhance the sulfur conversion kinetic. Furthermore, intrinsic defects engineering is expected to synergistically create favorable sulfur conversion conditions and mitigate the catalyst poisoning issue. In this study, the oCVD technique is leveraged to produce a conformal coating layer to eliminate shuttle effects, unfavored in the Li-S battery performance. Identified by SEM and TEM characterizations, the oCVD PEDOT is not only covered on the surface of the cathode but also the inner surface of the microchannels. High resolution x-ray photoelectron spectroscopy analyses (C 1s and S 2p orbitals) between pristine and modified sample demonstrate that the high concentration of the defects have been produced on the sulfur matrix after sintering and posttreatment. In-operando XRD diffractograms show that the Li2S is generated in the oCVD PEDOT-coated sample during the charge and discharge process even with a high current density, confirming an eminent sulfur conversion kinetic condition. In addition, ICP-OES results of lithium metal anode at different states of charge (SoC) verify that the shuttle effects are excellently restricted by oCVD PEDOT. Overall, the high mass loading (> 5 mg cm-2) with elevated sulfur utilization ratio, accelerated reaction kinetics, and stabilized electrochemical process have been achieved on the sulfur cathode by implementing this innovative cathode design strategy. The results of this study demonstrate significant promises of employing pure sulfur powder with high electrochemical performance and suggest a pathway to the higher energy and power density battery. 

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