Paraskevi Flouda

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• Randall, K., Enderlin, M., & Flouda, P. (2024). Architectural engineering of nanocomposite electrodes for energy storage. MRS Communications, 14(5). doi:10.1557/s43579-024-00601-z
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  The design of electrode architecture plays a crucial role in advancing the development of next generation energy storage devices, such as lithium-ion batteries and supercapacitors. Nevertheless, existing literature lacks a comprehensive examination of the property tradeoffs stemming from different electrode architectures. This prospective seeks to bridge this gap by focusing on the diverse nanocomposite electrode architectures. Furthermore, the challenges related to designing well-defined electrode architectures for enhanced energy storage are discussed. Finally, this review addresses the interdisciplinary nature of this field by examining the integration of advanced characterization and fabrication techniques, and machine learning methodologies for electrode optimization. Graphical abstract: Designing electrodes with controlled architecture and leveraging emerging tools such as in situ characterization, additive manufacturing methods, and machine learning facilitates the advancement of energy storage systems. (Figure presented.)
• Flouda, P., Choi, J., Buxton, M. L., Nepal, D., Lin, Z., Bunning, T. J., & Tsukruk, V. V. (2023). Synthesis and assembly of two-dimensional heterostructured architectures. MRS Communications, 13(5), 674-684.
• Flouda, P., Inman, A., Gumenna, M., Bukharina, D., Shevchenko, V. V., Gogotsi, Y., & Tsukruk, V. V. (2023). Ultrathin Films of MXene Nanosheets Decorated by Ionic Branched Nanoparticles with Enhanced Energy Storage Stability. ACS Applied Materials & Interfaces, 15(46), 53776-53785.
• Kim, M., Han, M. J., Lee, H., Flouda, P., Bukharina, D., Pierce, K. J., Adstedt, K. M., Buxton, M. L., Yoon, Y. H., Heller, W. T., Singamaneni, S., & Tsukruk, V. V. (2023). Bio-Templated Chiral Zeolitic Imidazolate Framework for Enantioselective Chemoresistive Sensing. Angewandte Chemie International Edition, 62(30), e202305646.
• Flouda, P., Bukharina, D., Pierce, K., Stryutsky, A., Shevchenko, V., & Tsukruk, V. (2022). Flexible Sustained Ionogels with Ionic Hyperbranched Polymers for Enhanced Ion-Conduction and Energy Storage. ACS Applied Materials and Interfaces, 14(23). doi:10.1021/acsami.2c04502
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  Flexible and mechanically robust gel-like electrolytes offer enhanced energy storage capabilities, versatility, and safety in batteries and supercapacitors. However, the trade-off between ion conduction and mechanical robustness remains a challenge for these materials. Here, we suggest that the introduction of ionic hyperbranched polymers in structured sustained ionogels will lead to both enhanced ion conduction and mechanical performance because of the hyperbranched polymers' ionically conductive groups and the complementary interfacial interactions with ionic liquids. More specifically, we investigate the effect of hyperbranched polymers with carboxylate terminal groups and imidazolium counterions with various ionic group densities on the properties of ionogels composed of coassembled cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) as sustainable open pore frame for ionic liquid immersion. The addition of hyperbranched polymers leads to the formation of highly interconnected openly porous, lightweight, and shape-persistent materials by harnessing hydrogen bonding between the polymers and the CNFs/CNCs "frame". Notably, these materials possess a 2-fold improvement in ionic conductivity combined with many-fold increase in Young's modulus, tensile strength, and toughness, making them comparable to common reinforced nanocomposite materials. Furthermore, the corresponding thin-film gel supercapacitors possess enhanced electrochemical cycling stability upon repeated bending with an 85% capacitance retention after 10 000 cycles, promising new insight in the development of simultaneously conductive and flexible gel electrolytes with sustained performance.
• Flouda, P., Stryutsky, A., Buxton, M., Adstedt, K., Bukharina, D., Shevchenko, V., & Tsukruk, V. (2022). Reconfiguration of Langmuir Monolayers of Thermo-Responsive Branched Ionic Polymers with LCST Transition. Langmuir, 38(39). doi:10.1021/acs.langmuir.2c01940
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  Thermo-responsive ionic polymers have the ability to form adaptive and switchable morphologies, which may offer enhanced control in energy storage and catalytic applications. Current thermo-responsive polymers are composed of covalently attached thermo-responsive moieties, restricting their mobility and global dynamic response. Here, we report the synthesis and assembly at the water-air interface of symmetric and asymmetric amphiphilic thermo-responsive branched polymers with weakly ionically bound arms of amine-terminated poly(N-isopropylacrylamide) (PNIPAM) macro-cations. As we observed, symmetric branched polymers formed multimolecular nanosized micellar assemblies, whereas corresponding asymmetric polymers formed large, interconnected worm-like aggregates. Dramatic changes in localized and large-scale chemical composition confirmed the reversible adsorption and desorption of the mobile PNIPAM macro-cations below and above the low critical solution temperature (LCST) and their non-uniform redistribution within polymer monolayer. Increasing the temperature above LCST led to the formation of large interconnected micellar aggregates because of the micelle-centered aggregation of the hydrophobized PNIPAM macro-cationic terminal chains in the aqueous subphase. Overall, this work provides insights into the dynamic nature of the chemical composition of branched ionic polymers with weakly ionically bound thermo-responsive terminal chains and its effect on both morphology and local/surface chemistry of monolayers at LCST transition.
• Loufakis, D., Flouda, P., Oka, S., Pombo, J., Lagoudas, D., & Lutkenhaus, J. (2022). Mechanically Improved Zn-Ion Battery Cathodes Based on Branched Aramid Nanofibers. Journal of Physical Chemistry C, 126(48). doi:10.1021/acs.jpcc.2c06958
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  Zinc-ion batteries address common environmental and safety concerns by using nontoxic and abundant metals in conjunction with aqueous electrolytes. Nonetheless, for applications such as structural energy storage, where a multifunctional system aims to replace both the structural and energy storage subsystems of an electric vehicle, safe but mechanically strong components are desired. MnO2, a typical cathode material for Zn-ion batteries, however, exhibits poor mechanical performance. Therefore, a lack of knowledge remains for mechanically strong cathodes for safe, structural Zn-ion batteries. Here, we combine branched aramid nanofibers (BANFs) with MnO2 particles and reduced graphene oxide (rGO) to fabricate mechanically improved cathodes for Zn-ion batteries. The addition of BANFs allows for an increase in the MnO2 loading and significantly improved electrochemical properties (up to 190% increase of capacity). Additionally, hydrogen bonding between BANFs and rGO notably improved the mechanical properties (up to 51% increase in ultimate tensile strength and up to 593% increase in ultimate strain). Electrochemo-mechanical tests were also performed to assess coupling, showing that applied strain decreases the capacity, but no measurable internal stresses develop during electrochemical cycling. This work combines the multifunctionality of structural electrodes with the inherent safety of Zn-ion batteries.
• Aderyani, S., Flouda, P., Shah, S., Green, M., Lutkenhaus, J., & Ardebili, H. (2021). Simulation of cyclic voltammetry in structural supercapacitors with pseudocapacitance behavior. Electrochimica Acta, 390. doi:10.1016/j.electacta.2021.138822
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  Cyclic voltammetry is an important technique to characterize the electrochemical performance and reaction kinetics in electrical and electrochemical energy storage devices under various conditions. In this study, a physics-based model is developed to simulate cyclic voltammetry measurements of reduced graphene oxide with aramid nanofiber (rGO-ANF) composite structural supercapacitors through multiphysics computational modeling and compared against experimental results. The presence of asymmetric forward and reverse sweeps in the cyclic voltammetry curves suggests pseudo-capacitance behavior associated with the oxygen-functionalized group. A multistep kinetics modeling approach is used to evaluate various kinetic processes that can occur at different potential values employing both Butler-Volmer and Tafel equations. Parametric studies were also performed to investigate the effects of scan rate, equilibrium potential, exchange current density, and transfer coefficient on cyclic voltammetry curves. The results demonstrate that nonzero equilibrium potential is more accurate for rGO-ANF supercapacitors and a low exchange current density of 10−6 A m−2 shows better agreement with the experimental measurements. The multistep modeling of cyclic voltammetry accompanied with experimental cyclic voltammetry curves provides a more accurate and comprehensive analysis of the kinetics and thermodynamics of structural supercapacitors and enables better design and control of device performance and life cycle.
• Flouda, P., Oka, S., Loufakis, D., Lagoudas, D., & Lutkenhaus, J. (2021). Structural lithium-ion battery cathodes and anodes based on branched aramid nanofibers. ACS Applied Materials and Interfaces, 13(29). doi:10.1021/acsami.1c06413
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  Structural batteries and supercapacitors combine energy storage and structural functionalities in a single unit, leading to lighter and more efficient electric vehicles. However, conventional electrodes for batteries and supercapacitors are optimized for high energy storage and suffer from poor mechanical properties. More specifically, commercial lithium-ion battery anodes and cathodes demonstrate tensile strength values
• Ma, T., Easley, A., Wang, S., Flouda, P., & Lutkenhaus, J. (2021). Mixed electron-ion-water transfer in macromolecular radicals for metal-free aqueous batteries. Cell Reports Physical Science, 2(5). doi:10.1016/j.xcrp.2021.100414
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  Metal-free aqueous batteries offer environmentally friendly energy storage without depleting the global reserves of strategic elements. These batteries consist of solid, redox-active polymeric anodes and cathodes with a water-based electrolyte. However, the performance of available active polymers remains inferior, and design principles to guide future development are lacking. Here, we report on the coupled mass, electron, and water transfer behavior for a series of non-conjugated nitroxide radical polymers in a water-based electrolyte-containing organic salt for guiding future design. In situ monitoring of the charge-discharge process reveals the important role of polymer-water interactions. Specifically, improved polymer-water interactions manifest in accelerated kinetics, which promote capacity retention at higher discharge rates. The most hydrophilic polymer, poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl acrylamide) (PTAm), yielded a capacity of 115 mAh/g, or 97% of its theoretical value, at 0.05 mA/cm2. At 5 mA/cm2, the PTAm capacity remained relatively high at 50 mAh/g, whereas comparable polymer capacities were much lower.
• Sarang, K., Zhao, X., Holta, D., Cao, H., Arole, K., Flouda, P., Oh, E., Radovic, M., Green, M., & Lutkenhaus, J. (2021). Carbon Additive-Free Crumpled Ti3C2T XMXene-Encapsulated Silicon Nanoparticle Anodes for Lithium-Ion Batteries. ACS Applied Energy Materials, 4(10). doi:10.1021/acsaem.1c01736
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  Silicon anodes are promising for future lithium-ion battery applications, but the large volume expansion of silicon particles causes electrode disintegration and excessive solid electrolyte interphase buildup during charge-discharge. This results in diminished cycling capacities and Coulombic efficiencies. "Yolk-shell"structures, wherein silicon particles are encapsulated within a conductive coating, have been explored in the recent past to address this issue, but most use amorphous carbon shells that have poor interactions with silicon. Here, conductive Ti3C2TX MXene nanosheets are crumpled around silicon particles via a one-step spray-drying process to create carbon-free anodes. The hydroxyl (-OH) terminal groups on the MXene surface contribute to the formation of a robust electrode via hydrogen bonding interactions with neighboring MXenes and encapsulated silicon particles. The relative silicon and MXene contents are varied to obtain crumpled MX/Si capsules of varying compositions, leading to different particle morphologies and energy storage performance metrics. The best-performing anode contains crumpled MX/Si = 32/68 wt % without any carbon additives required, demonstrating the cycling capacities of ∼550 mAh/gtotal at a current density of ∼1.7 A/gtotal (0.5 C-rate). Compared to equivalent electrodes containing uncrumpled MX/Si or crumpled reduced graphene oxide/Si, the crumpled MX/Si was far superior.
• Yun, J., Echols, I., Flouda, P., Chen, Y., Wang, S., Zhao, X., Holta, D., Radovic, M., Green, M., Naraghi, M., & Lutkenhaus, J. (2021). Layer-by-Layer Assembly of Reduced Graphene Oxide and MXene Nanosheets for Wire-Shaped Flexible Supercapacitors. ACS Applied Materials and Interfaces, 13(12). doi:10.1021/acsami.0c19619
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  As the demand for wearable electronic devices increases, interest in small, light, and deformable energy storage devices follows suit. Among these devices, wire-shaped supercapacitors (WSCs) are considered key components of wearable technology due to their geometric similarity to woven fiber. One potential method for creating WSC devices is the layer-by-layer (LbL) assembly technique, which is a "bottom-up"method for electrode fabrication. WSCs require conformal and adhesive coatings of the functional material to the wire-shaped substrate, which is difficult to obtain with other processing techniques such as vacuum filtration or spray-coating. However, the LbL assembly technique produces conformal and robust coatings that can be deposited onto a variety of substrates and shapes, including wires. In this study, we report WSCs made using the LbL assembly of alternating layers of positively charged reduced graphene oxide functionalized with poly(diallyldimethylammonium chloride) and negatively charged Ti3C2Tx MXene nanosheets conformally deposited on activated carbon yarns. In this construct, the added LbL film enhances capacitance, energy density, and power density by 240, 227, and 109%, respectively, relative to the uncoated activated carbon yarn, yielding high specific and volumetric capacitances (237 F g-1, 2193 F cm-3). In addition, the WSC possesses good mechanical stability, retaining 90% of its initial capacity after 200 bending cycles. This study demonstrates that LbL coatings on carbon yarns are promising as linear energy storage devices for fibrous electronics.
• Easley, A., Vukin, L., Flouda, P., Howard, D., Pena, J., & Lutkenhaus, J. (2020). Nitroxide Radical Polymer-Solvent Interactions and Solubility Parameter Determination. Macromolecules, 53(18). doi:10.1021/acs.macromol.0c01739
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  Redox-active polymers such as macromolecular nitroxide radicals have been studied as electrode materials for organic batteries and electronics. Polymer-solvent interactions are essential to processing and device performance, but macromolecular nitroxide radical-solvent interactions are currently not well described. In this work, the Hildebrand and Hansen solubility parameters of poly(2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA), oxidized PTMA (PTMA+), and PTMA's precursor (PTMPM) are determined using both experimental and group contribution methods for the first time. This work indicates that the hydrogen-bonding Hansen solubility parameter (δh) and the solubility sphere radii (R) provide the best prediction of the macromolecular radical's interaction with solvents. From these solubility parameters, the group contribution values for the nitroxide and oxoammonium cation were then calculated. It is shown that this information allows for the prediction of polymer-solvent interactions for other nitroxide-based macromolecular radicals. Finally, the discovered solubility parameters are used to predict PTMA electrode formulations for batteries that outperform controls.
• Flouda, P., Quinn, A., Patel, A., Loufakis, D., Lagoudas, D., & Lutkenhaus, J. (2020). Branched aramid nanofiber-polyaniline electrodes for structural energy storage. Nanoscale, 12(32). doi:10.1039/d0nr04573j
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  Strong electrodes with good energy storage capabilities are necessary to accommodate the current needs for structural and flexible electronics. To this end, conjugated polymers such as polyaniline (PANI) have attracted much attention due to their exceptional energy storage performance. However, PANI is typically brittle and requires the use of substrates for structural support. Here, we report a strategy for developing free-standing structural supercapacitor and battery electrodes based on PANI. More specifically, aniline is polymerized in the presence of branched aramid nanofibers (BANFs) and single walled carbon nanotubes (SWCNTs). This results in a network morphology that allows for efficient load transfer and electron transport, leading to electrodes with capacity values up to 128 ± 5 mA h g-1 (vs. a theoretical capacity of 147 mA h g-1), Young's modulus of 4 ± 0.5 GPa, and tensile strength of 40 ± 4 MPa. Furthermore, the charge storage mechanism is investigated, in which both Faradaic and non-Faradaic contributions are observed. This work demonstrates an efficient strategy for designing structural electrodes based on conjugated polymers.
• Flouda, P., Yun, J., Loufakis, D., Shah, S., Green, M., Lagoudas, D., & Lutkenhaus, J. (2020). Structural reduced graphene oxide supercapacitors mechanically enhanced with tannic acid. Sustainable Energy and Fuels, 4(5). doi:10.1039/c9se01299k
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  Electrodes that combine energy storage with mechanical performance are desirable for structural and flexible applications. However, conventional electrodes fail to address mechanical performance. Here, noncovalent bonding of graphene/aramid nanofiber electrodes with tannic acid yields a significant improvement in tensile strength and Young's modulus while maintaining good energy storage.
• Lutkenhaus, J., & Flouda, P. (2020). Ceramic Electrolytes Get “Tough” on Lithium Metal Batteries. Matter, 3(1). doi:10.1016/j.matt.2020.06.009
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  Stiff ceramic electrolytes mitigate lithium dendrite formation for enhanced lithium metal battery safety, but these same electrolytes are undesirably brittle. In this issue of Matter, Athanasiou et al. arrest crack formation using reduced graphene oxide in a ceramic electrolyte, revealing the importance of electrolyte toughness. Stiff ceramic electrolytes mitigate lithium dendrite formation for enhanced lithium metal battery safety, but these same electrolytes are undesirably brittle. In this issue of Matter, Athanasiou et al. arrest crack formation using reduced graphene oxide in a ceramic electrolyte, revealing the importance of electrolyte toughness.
• Lutkenhaus, J., & Flouda, P. (2020). Structural batteries take a load off. Science Robotics, 5(45). doi:10.1126/SCIROBOTICS.ABD7026
• Patel, A., Loufakis, D., Flouda, P., George, I., Shelton, C., Harris, J., Oka, S., & Lutkenhaus, J. (2020). Carbon Nanotube/Reduced Graphene Oxide/Aramid Nanofiber Structural Supercapacitors. ACS Applied Energy Materials, 3(12). doi:10.1021/acsaem.0c01926
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  Reduced graphene oxide/aramid nanofiber (rGO/ANF) supercapacitor electrodes have a good combination of energy storage and mechanical properties, but ion transport remains an issue toward achieving higher energy densities at high current because of the tightly packed electrode structure. Herein, carbon nanotubes (CNTs) are introduced to prevent rGO flake stacking to improve the rate capability of the rGO/ANF structural supercapacitor. The effect of CNTs on the rGO/ANF composite electrode's mechanical and electrochemical properties is investigated by varying the composition. The addition of 20 wt % CNTs led to an increase in Young's modulus up to 10.3 ± 1.8 GPa, while a maximum in ultimate strain and strength of 1.3 ± 0.14% and 55 ± 6.8 MPa, respectively, was found at a loading of 2.5 wt % CNTs. At low specific currents, the electrodes performed similarly (160-170 F g-1), but at high specific currents (5 A g-1), the addition of 20 wt % CNTs led to a significantly higher capacitance (76 F g-1) as compared to that of rGO/ANF electrodes without CNTs (26 F g-1). In addition, the energy density also improved significantly at high power from 1.4 to 5.1 W h L-1 with the addition of CNTs. The improvement in mechanical properties is attributed to the introduction of additional hydrogen-bonding and π-πinteractions from the carboxylic acid-functionalized CNTs. The increase in capacitance at higher discharge rates is due to improved ion transport from the CNTs. Finally, in situ electrochemomechanical testing examines how capacitance varies with strain in these structural electrodes for the first time.
• Wang, S., Park, A., Flouda, P., Easley, A., Li, F., Ma, T., Fuchs, G., & Lutkenhaus, J. (2020). Solution-Processable Thermally Crosslinked Organic Radical Polymer Battery Cathodes. ChemSusChem, 13(9). doi:10.1002/cssc.201903554
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  Organic radical polymers are promising cathode materials for next-generation batteries because of their rapid charge transfer and high cycling stability. However, these organic polymer electrodes gradually dissolve in the electrolyte, resulting in capacity fade. Several crosslinking methods have been developed to improve the performance of these electrodes, but they are either not compatible with carbon additives or compromise the solution processability of the electrodes. A one-step post-synthetic, carbon-compatible crosslinking method was developed to effectively crosslink an organic polymer electrode and allow for easy solution processing. The highest electrode capacity of 104 mAh g−1 (vs. a theoretical capacity of 111 mAh g−1) is achieved by introducing 1 mol % of the crosslinker, whereas the highest capacity retention (99.6 %) is obtained with 3 mol % crosslinker. In addition, mass transfer was observed in situ by using electrochemical quartz crystal microbalance with dissipation monitoring. These results may guide future electrode design toward fast-charging and high-capacity organic electrodes.
• Aderyani, S., Flouda, P., Lutkenhaus, J., & Ardebili, H. (2019). The effect of nanoscale architecture on ionic diffusion in rGo/aramid nanofiber structural electrodes. Journal of Applied Physics, 125(18). doi:10.1063/1.5087280
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  Structural energy storage is a rapidly emerging area with tantalizing applications such as integrated devices in textiles, smart suits, and uniforms. Due to several outstanding properties, graphene oxide (rGO)/aramid nanofiber (ANF) composite material has emerged as a compelling choice as a structural electrode for supercapacitors and batteries. A key question of significant technological relevance pertains to what kind of nanoscale architecture motifs may lead to enhanced ionic diffusivity - the key characteristic dictating the overall performance of the electrode. In this study, we attempt to address precisely this question, through multiphysics simulations, in the context of several "experimentally realizable, layered" architectures. We investigate different arrangements (staggered and aligned) and various degrees of waviness of the rGO nanosheets inside the ANF polymer matrix. Our results indicate that decreasing waviness of the rGO sheets can enhance the ion diffusivity in the staggered and aligned arrangements of the electrode material, while this effect is stronger in the staggered arrangements than in the aligned arrangements. The insights obtained from this study can lead to a more effective design of electrode architectures.
• Flouda, P., Feng, X., Boyd, J., Thomas, E., Lagoudas, D., & Lutkenhaus, J. (2019). Interfacial Engineering of Reduced Graphene Oxide for Aramid Nanofiber-Enabled Structural Supercapacitors. Batteries and Supercaps, 2(5). doi:10.1002/batt.201800137
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  Structural energy storage systems can simultaneously store energy and bear mechanical loads and are emerging technologies for electric vehicles, aircrafts, and satellites since they offer significant mass and volume savings. However, many of the materials used in energy storage applications are not mechanically robust or, on the other hand, mechanically stiff materials are not necessarily electrochemically active. One solution is to combine reduced graphene oxide (rGO) sheets and aramid nanofibers (ANFs). Here we hypothesize that engineering the interfacial interactions between rGO sheets and the ANFs will lead to enhanced mechanical properties, as noncovalent interactions offer multiple sites for load transfer. rGO sheets are functionalized with carboxylic acid (−COOH) and amine (−NH2) groups, and the effect on the mechanical and electrochemical performance of nanocomposite supercapacitor electrodes is investigated. Notably, the −NH2 functionalization and the addition of ANFs leads to a dramatic 200 % improvement of the strength of the electrodes without significantly compromising the electrochemical performance. Furthermore, it is demonstrated that these electrodes’ multifunctionality is superior to other state-of-the-art structural electrodes and may pose possible replacements for steel or epoxy. These findings provide critical knowledge for the design of next-generation multifunctional electrodes in that we highlight the importance of interfacial engineering.
• Flouda, P., Shah, S., Lagoudas, D., Green, M., & Lutkenhaus, J. (2019). Highly Multifunctional Dopamine-Functionalized Reduced Graphene Oxide Supercapacitors. Matter, 1(6). doi:10.1016/j.matt.2019.09.017
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  Batteries and supercapacitors that possess the mechanical properties of structural composites are desirable for electric vehicles and aerospace applications, as energy may be stored within structural panels to realize significant mass and volume savings. However, current electrode materials suffer from poor mechanical performance, as described by a low multifunctional efficiency parameter (
• Yun, J., Echols, I., Flouda, P., Wang, S., Easley, A., Zhao, X., Tan, Z., Prehn, E., Zi, G., Radovic, M., Green, M., & Lutkenhaus, J. (2019). Layer-by-Layer Assembly of Polyaniline Nanofibers and MXene Thin-Film Electrodes for Electrochemical Energy Storage. ACS Applied Materials and Interfaces, 11(51). doi:10.1021/acsami.9b16692
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  The growing demand for compact energy storage devices may be met through the use of thin-film microbatteries, which generally rely on charge storage in thin or conformal layers. A promising technique for creating thin-film electrodes is layer-by-layer (LbL) assembly, based on the alternating adsorption of oppositely charged species to a surface to form a nanostructured electrode. Thin-film energy storage devices must have a high energy density within a limited space, so new electrode structures, materials, and assembly methods are important. To this end, both two-dimensional MXenes and polyaniline nanofibers (PNFs) have shown promising energy storage properties. Here, we report on the LbL assembly of positively charged PNFs and negatively charged Ti3C2Tx MXenes into hybrid electrodes for thin-film energy storage devices. The successful assembly is demonstrated in which MXenes and PNFs are deposited in films of 49 nm/layer pair thickness. The resulting composition was 77 wt % PNFs and 23 wt % MXenes. The charge storage process was deconvoluted into faradaic/non-faradaic contributions and separated into contributions from PNFs and MXenes. A sandwich cell showed a maximum areal capacity, energy, and power of 17.6 μA h cm-2, 22.1 μW h cm-2, and 1.5 mW cm-2, respectively, for PNF/MXene multilayers of about 2 μm thickness. This work suggests the possibility of using LbL PNF/MXene thin films as electrode materials for thin-film energy storage devices used in next-generation small electronics.
• De, S., Purcell, C., Murley, J., Flouda, P., Shah, S., Green, M., & Lutkenhaus, J. (2018). Spray-On Reduced Graphene Oxide-Poly(vinyl alcohol) Supercapacitors for Flexible Energy and Power. Advanced Materials Interfaces, 5(23). doi:10.1002/admi.201801237
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  Flexible and mechanically robust energy storage devices, such as capacitors, are future enablers of the next generation of structural energy and power, flexible electronics, and biometrics. These concepts require that the capacitor be seamlessly integrated into the device in a scalable process, while still maintaining mechanical and electrochemical properties. The challenge is that ease-of-processing, mechanical, and electrochemical properties are not mutually exclusive. Spraying allows for fast manufacture, but producing an ink amenable to spraying is challenging because of aggregation of the electroactive materials in the dispersion, resulting in poor performance. Here, sprayable, flexible supercapacitor electrodes are demonstrated containing reduced graphene oxide (rGO), poly(vinyl alcohol) (PVA), and carbon black achieved via high-throughput air-brushing process from aqueous media. The spray processing, mechanical properties, and electrochemical performance of various compositions of rGO, PVA, and carbon black are explored. rGO sheets provide capacitive energy storage, PVA facilitates air-brushing, and carbon black bridges rGO sheets to form electronic pathways. Due to the good combination of mechanical and electrochemical properties, a flexible supercapacitor shows steady capacitance retention after several bending cycles. This spray-on three-component composite electrode balances electrochemical and mechanical properties, which is otherwise difficult to realize for compact brick and mortar structures, especially from airbrushing.