Sang-Eun Chun

Electrochemically active nitrogen-enriched nanocarbons with well-defined morphology synthesized by pyrolysis of self-assembled block copolymer.

Novel nanoporous nitrogen-enriched carbon materials were prepared through a simple carbonization procedure of well-defined block copolymer precursors containing the source of carbon, i.e., polyacrylonitrile (PAN), and a sacrificial block, i.e., poly(n-butyl acrylate) (PBA). The preparation of nitrogen-enriched nanocarbons with hierarchical pore structure was enabled by the high fidelity preservation of the initial phase-separated nanostructure between two polymer blocks upon carbonization. Supercapacitors fabricated from the prepared carbons exhibited unusually high capacitance per unit surface area (>30 μF/cm(2)) which was attributed to the pseudocapacitance resulting from the high nitrogen content originating from the PAN precursor. Electrochemical availability of the nitrogen species was also evident from the results of oxygen reduction experiments. The hierarchical pore structure and the high nitrogen content in such materials make them particularly promising for use in supercapacitor and electrocatalyst applications.

Biologically derived melanin electrodes in aqueous sodium-ion energy storage devices

Significance Here we present important findings related to biologically derived pigments for potential use as battery electrodes. Namely, we report the synthesis, fabrication, and characterization of melanins as materials for use in aqueous sodium-ion batteries. We demonstrate the use of naturally occurring melanins as active electrode materials in charge storage devices. Furthermore, the performance of melanin anodes is comparable to many commonly available synthetic organic electrode materials. The structure–property relationships that govern the storage capacity in melanin materials were also elucidated. These findings suggest that the unique chemistry and nanostructure in natural melanins increase the charge storage capacity compared with synthetic melanin analogues. Biodegradable electronics represents an attractive and emerging paradigm in medical devices by harnessing simultaneous advantages afforded by electronically active systems and obviating issues with chronic implants. Integrating practical energy sources that are compatible with the envisioned operation of transient devices is an unmet challenge for biodegradable electronics. Although high-performance energy storage systems offer a feasible solution, toxic materials and electrolytes present regulatory hurdles for use in temporary medical devices. Aqueous sodium-ion charge storage devices combined with biocompatible electrodes are ideal components to power next-generation biodegradable electronics. Here, we report the use of biologically derived organic electrodes composed of melanin pigments for use in energy storage devices. Melanins of natural (derived from Sepia officinalis) and synthetic origin are evaluated as anode materials in aqueous sodium-ion storage devices. Na+-loaded melanin anodes exhibit specific capacities of 30.4 ± 1.6 mAhg−1. Full cells composed of natural melanin anodes and λ-MnO2 cathodes exhibit an initial potential of 1.03 ± 0.06 V with a maximum specific capacity of 16.1 ± 0.8 mAhg−1. Natural melanin anodes exhibit higher specific capacities compared with synthetic melanins due to a combination of beneficial chemical, electrical, and physical properties exhibited by the former. Taken together, these results suggest that melanin pigments may serve as a naturally occurring biologically derived charge storage material to power certain types of medical devices.

Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge

Electrochemical double-layer capacitors exhibit high power and long cycle life but have low specific energy compared with batteries, limiting applications. Redox-enhanced capacitors increase specific energy by using redox-active electrolytes that are oxidized at the positive electrode and reduced at the negative electrode during charging. Here we report characteristics of several redox electrolytes to illustrate operational/self-discharge mechanisms and the design rules for high performance. We discover a methyl viologen (MV)/bromide electrolyte that delivers a high specific energy of ∼14 Wh kg−1 based on the mass of electrodes and electrolyte, without the use of an ion-selective membrane separator. Substituting heptyl viologen for MV increases stability, with no degradation over 20,000 cycles. Self-discharge is low, due to adsorption of the redox couples in the charged state to the activated carbon, and comparable to cells with inert electrolyte. An electrochemical model reproduces experiments and predicts that 30–50 Wh kg−1 is possible with optimization.

Catechol-Mediated Reversible Binding of Multivalent Cations in Eumelanin Half-Cells

Electrochemical storage systems that utilize divalent cations such as Mg2+ can improve the volumetric charge storage capacities compared to those that use monovalent ions. Here, a cathode based on naturally derived melanin pigments is used in secondary Mg2+ batteries. Redox active catechol groups in melanins permit efficient and reversible exchange of divalent Mg2+ cations to preserve charge storage capacity in biopolymer cathodes for more than 500 cycles.

Self-deployable current sources fabricated from edible materials

Flexible biodegradable electronics have the potential to serve as the centerpiece for temporary electronically active medical implants. Biodegradable electronics may exhibit many advantages over traditional chronic implants. Two important long-term goals for biodegradable electronics are (1) supplying sufficient power and (2) reducing the invasiveness of device deployment. Edible electronic devices are capable of addressing both challenges. Here, we introduce electrochemical electronic power sources that are compatible with non-invasive deployment strategies and are composed entirely of edible materials and naturally occurring precursors that are consumed in common diets. The current sources developed herein are powered by onboard sodium ion electrochemical cells. Potentials up to 0.6 V and currents in the range of 5-20 μA can be generated routinely. These devices could serve as an enabling platform technology for edible electronics used in non-invasive sensing and stimulation of tissues within the human body.

High Energy Density Aqueous Electrochemical Capacitors with a KI-KOH Electrolyte

We report a new electrochemical capacitor with an aqueous KI-KOH electrolyte that exhibits a higher specific energy and power than the state-of-the-art nonaqueous electrochemical capacitors. In addition to electrical double layer capacitance, redox reactions in this device contribute to charge storage at both positive and negative electrodes via a catholyte of IOx-/I- couple and a redox couple of H2O/Had, respectively. Here, we, for the first time, report utilizing IOx-/I- redox couple for the positive electrode, which pins the positive electrode potential to be 0.4-0.5 V vs Ag/AgCl. With the positive electrode potential pinned, we can polarize the cell to 1.6 V without breaking down the aqueous electrolyte so that the negative electrode potential could reach -1.1 V vs Ag/AgCl in the basic electrolyte, greatly enhancing energy storage. Both mass spectroscopy and Raman spectrometry confirm the formation of IO3- ions (+5) from I- (-1) after charging. Based on the total mass of electrodes and electrolyte in a practically relevant cell configuration, the device exhibits a maximum specific energy of 7.1 Wh/kg, operates between -20 and 50 °C, provides a maximum specific power of 6222 W/kg, and has a stable cycling life with 93% retention of the peak specific energy after 14,000 cycles.

Evidence of Porphyrin‐Like Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting

Eumelanins are extended heterogeneous biopolymers composed of molecular subunits with ambiguous macromolecular topology. Here, an electrochemical fingerprinting technique is described, which suggests that natural eumelanin pigments contain indole-based tetramers that are arranged into porphyrin-like domains. Spectroscopy and density functional theory calculations suggest that sodium ions undergo occupancy-dependent stepwise insertion into the core of porphyrin-like tetramers in natural eumelanins at discrete potentials.

Porous Carbon Networks with Nanosphere-Interconnected Structure via 3-Aminophenol-Formaldehyde Polymerization

Although mesoporous carbon materials with hierarchical nanostructures have been produced by the synthesis of hybrid nanoparticles with a silicon dioxide (SiO2) core and a shell of resorcinol formaldehyde resin, it still remains a challenge to effectively tune the pore size distribution. Among a series of phenol derivatives, 3-aminophenol was found to exhibit not only excellent tunability of the size and low roughness of the sphere surface but also high pyrolysis yields in the synthesis of carbon nano/microspheres. Here, we report that mesoporous carbon networks with a bimodal pore size distribution in their hierarchical nanostructure were prepared by 3-aminophenol and formaldehyde polymerization on the SiO2 cores. In particular, the systematic control of the ratio of carbon precursors and silica nanoparticles provides a better control of the microstructure in hybrid nanoparticles with a shell of variable thickness composed of well-defined 3-aminophenol and formaldehyde resins, resulting in the tunability of their pore size distribution.

Characterization of Electric Double-Layer Capacitor with 0.75M NaI and 0.5 M VOSO 4 Electrolyte

We describe a redox-enhanced electric double-layer capacitor (EDLC) that turns the electrolyte in a conventional EDLC into an integral, active component for charge storage—charge is stored both through faradaic reactions with soluble redoxactive molecules in the electrolyte, and through the double-layer capacitance in a porous carbon electrode. The mixed-redox electrolyte, composed of vanadium and iodides, was employed to achieve high power density. The electrochemical reaction in a supercapacitor with vanadium and iodide was studied to estimate the charge capacity and energy density of the redox supercapacitor. A redox supercapacitor with a mixed electrolyte composed of 0.75 M NaI and 0.5 M VOSO4 was fabricated and studied. When charged to a potential of 1 V, faradaic charging processes were observed, in addition to the capacitive processes that increased the energy storage capabilities of the supercapacitor. The redox supercapacitor achieved a specific capacity of 13.44 mAh/g and an energy density of 3.81 Wh/kg in a simple Swagelok cell. A control EDLC with 1 M H2SO4 yielded 7.43 mAh/g and 2.85 Wh/kg. However, the relatively fast self-discharge in the redox-EDLC may be due to the shuttling of the redox couple between the polarized carbon electrodes.

Tailoring the porous texture of activated carbons by CO2 reactivation to produce electrodes for organic electrolyte-based EDLCs

Highly microporous carbon obtained by KOH etching of carbohydrates exhibited enhanced specific capacitance due to the increased adsorption of electrolyte ions on its large surface, which renders it a promising electrode material. However, the KOH-activated carbon electrode did not achieve its optimum charge capacity in organic electrolytes due to the limited accessibility of the electrolyte ions to the micropores, which hindered the adsorption of ions. The electrode performance was enhanced by enlarging the micropores of KOH-activated carbon to mesopores via reactivation in a stream of CO2, which allowed the mesopore/micropore ratio to be increased without compromising the originally high specific surface area. The extended amount of mesopores increased the charge capacity of the electrode by enabling the large organic electrolyte ions to access the porous surface, as compared to untreated KOH-activated carbon.