Vasculature-on-a-Chip with a Porous Membrane Electrode for In Situ Electrochemical Detection of Nitric Oxide Released from Endothelial Cells.

Vasculature-on-a-chip is a microfluidic cell culture device used for modeling vascular functions by culturing endothelial cells. Porous membranes are widely used to create cell culture environments. However, in situ real-time measurements of cellular metabolites in microchannels are challenging. In this study, a novel microfluidic device with a porous membrane electrode was developed for the in situ monitoring of nitric oxide (NO) released by endothelial cells in real time. In this system, a porous Au membrane electrode was placed directly beneath the cells for in situ and real-time measurements of NO, a biomarker of endothelial cells. First, the device was electrochemically characterized to construct a calibration plot for NO. Next, NO released by human umbilical vein endothelial cells under l-arginine stimulation was successfully quantified. Furthermore, the changes in NO release with culture time (in days) using the same sample were successfully recorded by exploiting minimally invasive measurements. This is the first report on the combination of a microfluidic device and porous membrane electrode for the electrochemical analysis of endothelial cells. This device will contribute to the development of organ-on-a-chip technology for real-time in situ cell analyses.

Quinoid-Based Three-Dimensional Metal-Organic Framework Fe2(dhbq)3: Porosity, Electrical Conductivity, and Solid-State Redox Properties.

We report the synthesis, characterization, and electronic properties of the quinoid-based three-dimensional metal-organic framework [Fe2(dhbq)3]. The MOF was synthesized without using cations as a template, unlike other reported X2dhbq3-based coordination polymers, and the crystal structure was determined by using single-crystal X-ray diffraction. The crystal structure was entirely different from the other reported [Fe2(X2dhbq3)]2-; three independent 3D polymers were interpenetrated to give the overall structure. The absence of cations led to a microporous structure, investigated by N2 adsorption isotherms. Temperature dependence of electrical conductivity data revealed that it exhibited a relatively high electrical conductivity of 1.2 × 10-2 S cm-1 (Ea = 212 meV) due to extended d-π conjugation in a three-dimensional network. Thermoelectromotive force measurement revealed that it is an n-type semiconductor with electrons as the majority of charge carriers. Structural characterization and spectroscopic analyses, including SXRD, Mössbauer, UV-vis-NIR, IR, and XANES measurements, evidenced the occurrence of no mixed valency based on the metal and the ligand. [Fe2(dhbq)3] upon incorporating as a cathode material for lithium-ion batteries engendered an initial discharge capacity of 322 mAh/g.

Ultraporous, Ultrasmall MgMn2O4 Spinel Cathode for a Room-Temperature Magnesium Rechargeable Battery.

Magnesium rechargeable batteries (MRBs) promise to be the next post lithium-ion batteries that can help meet the increasing demand for high-energy, cost-effective, high-safety energy storage devices. Early prototype MRBs that use molybdenum-sulfide cathodes have low terminal voltages, requiring the development of oxide-based cathodes capable of overcoming the sulfide's low Mg2+ conductivity. Here, we fabricate an ultraporous (>500 m2 g-1) and ultrasmall (<2.5 nm) cubic spinel MgMn2O4 (MMO) by a freeze-dry assisted room-temperature alcohol reduction process. While the as-fabricated MMO exhibits a discharge capacity of 160 mAh g-1, the removal of its surface hydroxy groups by heat-treatment activates it without structural change, improving its discharge capacity to 270 mAh g-1─the theoretical capacity at room temperature. These results are made possible by the ultraporous, ultrasmall particles that stabilize the metastable cubic spinel phase, promoting both the Mg2+ insertion/deintercalation in the MMO and the reversible transformation between the cubic spinel and cubic rock-salt phases.

Electrochemical microwell sensor with Fe-N co-doped carbon catalyst to monitor nitric oxide release from endothelial cell spheroids.

Endothelial cells have been widely used for vascular biology studies; recent progress in tissue engineering have offered three-dimensional (3D) culture systems for vascular endothelial cells which can be considered as physiologically relevant models. To facilitate the studies, we developed an electrochemical device to detect nitric oxide (NO), a key molecule in the vasculature, for the evaluation of 3D cultured endothelial cells. Using an NO-sensitive catalyst composed of Fe-N co-doped reduced graphene oxide, the real-time monitoring of NO release from the endothelial cell spheroids was demonstrated.

A 3D-Printed, Freestanding Carbon Lattice for Sodium Ion Batteries.

Increasing mass loadings of battery electrodes critically enhances the energy density of an overall battery by eliminating much of the inactive components, while compacting the battery size and lowering the costs of the ingredients. A hard carbon microlattice, digitally designed and fabricated by stereolithography 3D-printing and pyrolysis, offers enormous potential for high-mass-loading electrodes. In this work, sodium-ion batteries using hard carbon microlattices produced by an inexpensive 3D printer are demonstrated. Controlled periodic carbon microlattices are created with enhanced ion transport through microchannels. Carbon microlattices with a beam width of 32.8 µm reach a record-high areal capacity of 21.3 mAh cm-2 at a loading of 98 mg cm-2 without degrading performance, which is much higher than the conventional monolithic electrodes (≈5.2 mAh cm-2 at 92 mg cm-2 ). Furthermore, binder-free, pure-carbon elements of microlattices enable the tracking of structural changes in hard carbon that support the hypothesized intercalation of ions at plateau regions by temporal ex situ X-ray diffraction measurements. These results will advance the development of high-performance and low-cost anodes for sodium-ion batteries as well as help with understanding the mechanisms of ion intercalations in hard carbon, expanding the utilities of 3D-printed carbon architectures in both applications and fundamental studies.

Series module of quinone-based organic supercapacitor (> 6 V) with practical cell structure.

Inexpensive, high-performing, and environmentally friendly energy storage devices are required for smart grids that efficiently utilize renewable energy. Energy storage devices consisting of organic active materials are promising because organic materials, especially quinones, are ubiquitous and usually do not require harsh conditions for synthesis, releasing less CO2 during mass production. Although fundamental research-scale aqueous quinone-based organic supercapacitors have shown excellent energy storage performance, no practical research has been conducted. In this study, we aimed to develop a practical-scale aqueous-quinone-based organic supercapacitor. By connecting 12 cells of size 10 cm × 10 cm × 0.5 cm each in series, we fabricated a high-voltage (> 6 V) aqueous organic supercapacitor that can charge a smartphone at a 1 C rate. This is the first step in commercializing aqueous organic supercapacitors that could solve environmental problems, such as high CO2 emissions, air pollution by toxic metals, and limited electricity generation by renewable resources.

Activity switching of Sn and In species in Heusler alloys for electrochemical CO2 reduction.

The electrochemical CO2 reduction reaction (CO2RR) activity of Ni2MnIn and Ni2MnSn Heusler alloys was investigated. Although pure In, Sn and Ni2MnIn generated formate as the major product, Ni2MnSn generated H2 as the major product. The CO2RR selectivity could be controlled by selecting the constituent elements of the intermetallic catalysts.

Are Redox-Active Organic Small Molecules Applicable for High-Voltage (>4 V) Lithium-Ion Battery Cathodes?

While organic batteries have attracted great attention due to their high theoretical capacities, high-voltage organic active materials (> 4 V vs Li/Li+ ) remain unexplored. Here, density functional theory calculations are combined with cyclic voltammetry measurements to investigate the electrochemistry of croconic acid (CA) for use as a lithium-ion battery cathode material in both dimethyl sulfoxide and γ-butyrolactone (GBL) electrolytes. DFT calculations demonstrate that CA dilitium salt (CA-Li2 ) has two enolate groups that undergo redox reactions above 4.0 V and a material-level theoretical energy density of 1949 Wh kg-1 for storing four lithium ions in GBL-exceeding the value of both conventional inorganic and known organic cathode materials. Cyclic-voltammetry measurements reveal a highly reversible redox reaction by the enolate group at ≈4 V in both electrolytes. Battery-performance tests of CA as lithium-ion battery cathode in GBL show two discharge voltage plateaus at 3.9 and 3.1 V, and a discharge capacity of 102.2 mAh g-1 with no capacity loss after five cycles. With the higher discharge voltages compared to the known, state-of-the-art organic small molecules, CA promises to be a prime cathode-material candidate for future high-energy-density lithium-ion organic batteries.

Copper Aluminum Layered Double Hydroxides with Different Compositions and Morphologies as Electrocatalysts for the Carbon Dioxide Reduction Reaction.

Electrochemical CO2 reduction (CO2 RR) is a key technology to convert greenhouse gas CO2 to value-added products, such as CO and formic acid (HCOOH). In the present study, two-dimensional Cu- and Al-based layered double hydroxides (Cu-Al/LDHs) were applied as CO2 RR catalysts. The catalysts were synthesized using a simple co-precipitation method employing sodium carbonate solutions with different pH and synthesis temperatures. The elemental ratio of Cu and Al, and sheet size were controlled. The most active Cu-Al/LDH showed a faradaic efficiency for CO generation of 42 % and one for formate generation of 22 % at the current density of 50 mA using a gas diffusion electrode system under galvanostatic conditions. Our result indicates that the sheet size of the LDH sheet is a critical parameter for determining CO2 RR activity.

A photo-curable gel electrolyte ink for 3D-printable quasi-solid-state lithium-ion batteries.

3D printing technologies have been adapted to enable the fabrication of lithium-ion batteries (LIBs), allowing flexible designs such as micro-scale 3D shapes. Here, we demonstrate 3D-printable gel electrolytes, printed at room temperature. The electrolyte gel solidified by UV irradiation maintains its structural integrity and high lithium-ion conductivity, enabling fully 3D-printed quasi-solid-state LIBs.

A cobalt-manganese layered oxide/graphene composite as an outstanding oxygen evolution reaction electrocatalyst.

To enhance the oxygen evolution reaction mass activity of cobalt-manganese layered oxide (CMO), we develop a one-pot synthetic process to anchor CMO onto graphene sheets (CMO/G). Its mass activity is 66-fold higher than that of physically mixed bare CMO with graphene and even better than those of previously reported graphene-supported first-row transition metal oxide-based electrocatalysts. The remarkable mass activity is attributed to the excellent intrinsic activity of CMO, small and well-dispersed CMO nanosheets on graphene sheets and hydrophilized graphene due to the synthetic process. Furthermore, CMO/G exhibits excellent stability.

Electron-Conductive Metal-Organic Framework, Fe(dhbq)(dhbq = 2,5-Dihydroxy-1,4-benzoquinone): Coexistence of Microporosity and Solid-State Redox Activity.

Redox-active metal-organic frameworks (MOFs) have great potential for use as cathode materials in lithium-ion batteries (LIBs) with large capacities because the organic ligands can undergo multiple-electron redox processes. However, most MOFs are electrical insulators, limiting their application as electrode materials. Here, we report an electron-conductive MOF with a 2,5-dihydroxy-1,4-benzoquinone (dhbq) ligand, Fe(dhbq). This compound had an electrical conductivity of 5 × 10-6 S cm-1 at room temperature due to d-π interactions between the Fe ion and the ligand and the permanent microporosity. Fe(dhbq) had an initial discharge capacity of 264 mA h g-1, corresponding to the two-electron redox process of dhbq.

Effect of Cobalt Speciation and the Graphitization of the Carbon Matrix on the CO2 Electroreduction Activity of Co/N-Doped Carbon Materials.

The electroreduction of carbon dioxide is considered a key reaction for the valorization of CO2 emitted in industrial processes or even present in the environment. Cobalt-nitrogen co-doped carbon materials featuring atomically dispersed Co-N sites have been shown to display superior activities and selectivities for the reduction of carbon dioxide to CO, which, in combination with H2 (i.e., as syngas), is regarded as an added-value CO2-reduction product. Such catalysts can be synthesized using heat treatment steps that imply the carbonization of Co-N-containing precursors, but the detailed effects of the synthesis conditions and corresponding materials' composition on their catalytic activities have not been rigorously studied. To this end, in the present work, we synthesized cobalt-nitrogen co-doped carbon materials with different heat treatment temperatures and studied the relation among their surface- and Co-speciation and their CO2-to-CO electroreduction activity. Our results reveal that atomically dispersed cobalt-nitrogen sites are responsible for CO generation while suggesting that this CO-selectivity improves when these atomic Co-N centers are hosted in the carbon layers that cover the Co nanoparticles featured in the catalysts synthesized at higher heat treatment temperatures.

Effect of Metal Coordination Fashion on Oxygen Electrocatalysis of Cobalt-Manganese Oxides.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the most critical reactions that limit the efficiency of fuel cells, water electrolyzers, and metal-air batteries. Therefore, a need exists to develop cost-effective and highly active alternative electrocatalysts for ORR and OER. This study investigates the influence of metal coordination fashion on electrocatalytic ORR and OER activities among three types of Co-Mn bimetallic oxides (CMOs): tunnel-type (CMO_T), layer-type (CMO_L), and spinel-type (CMO_S) structures. An electrochemical evaluation for CMOs verifies that CMO_L has the highest ORR and OER specific activities, which is relatively better than the previously reported bifunctional metal oxides. Additionally, atomic configuration analysis for the oxides suggests that the excellent ORR and OER activities of CMO_L result from the difference in Co and Mn coordination states. This paper not only presents an excellent electrocatalyst for alkaline fuel cells and water electrolyzers but also provides an important guideline for the design of oxygen electrocatalysts.

Rational Route for Increasing Intercalation Capacity of Hard Carbons as Sodium-Ion Battery Anodes.

Hard carbon (HC) is the most promising candidate for sodium-ion battery anode materials. Several material properties such as intensity ratio of the Raman spectrum, lateral size of HC crystallite (La ), and interlayer distance (d002 ) have been discussed as factors affecting anode performance. However, these factors do not reflect the bulk property of the Na+ intercalation reaction directly, since Raman analysis has high surface sensitivity and La and d002 provide only one-dimensional crystalline information. Herein, it was proposed that the crystallite interlayer area (Ai ) defined using La , d002 , and stacking height (Lc ) governs Na+ intercalation behavior of various HCs. It was revealed that various wood-derived HCs exhibited the similar total capacity of approximately 250 mAh g-1 , whereas the Na+ intercalation capacity (Ci ) was proportional to Ai with the correlation coefficient of R2 =0.94. The evaluation factor of Ai was also adaptable to previous reports and strongly correlated with their Ci , indicating that Ai is more widely adaptable than the conventional evaluation methods.

Cation-Disorder-Assisted Reversible Topotactic Phase Transition between Antifluorite and Rocksalt Toward High-Capacity Lithium-Ion Batteries.

Multielectron reaction electrode materials using partial oxygen redox can be potentially used as cathodes in lithium-ion batteries, as they offer numerous advantages, including high reversible capacity and energy density and low cost. Here, a reversible three-electron reaction is demonstrated utilizing topotactic phase transition between antifluorite and rocksalt in a cation-disordered antifluorite-type cubic Li6CoO4 cathode. This cubic phase is synthesized by a simple mechanochemical treatment of conventionally prepared tetragonal Li6CoO4. It displays a reversible capacity of 487 mAh g-1, a high value because of a reversible three-electron reaction using Co2+/Co3+, Co3+/Co4+, and O2-/O22- redox, occurring without O2 gas evolution. The mechanochemical treatment is assumed to reduce its lattice distortion by cation-disordering and facilitate a reversible topotactic phase transition between antifluorite and rocksalt structures via a dynamic cation pushing mechanism.

Supercritical hydrothermal synthesis of MoS2 nanosheets with controllable layer number and phase structure.

Molybdenum disulfide (MoS2), an attractive material for energy conversion devices, is known to exhibit varying properties depending on the number of layers and the phase structure. In this study, we developed a supercritical hydrothermal process that allows the controllable synthesis of MoS2 nanosheets with various structural and morphological characteristics. Detailed characterization of the synthesized materials confirmed that the number of layers in the MoS2 nanosheets could be controlled by varying the organic reducing agent under supercritical hydrothermal conditions. In addition, the MoS2 phase could be controlled kinetically by varying the reaction time with ascorbic acid as the reducing agent. Because water and elemental sulfur were respectively used as the solvent and sulfur source and as the reaction time was minimal, the developed hydrothermal process represents a facile processing method for different types of MoS2 nanosheets.

Electrical Conductivity-Relay between Organic Charge-Transfer and Radical Salts toward Conductive Additive-Free Rechargeable Battery.

In recent years, organic electrode materials have been strongly considered for use in sustainable batteries. However, most organic electrode materials have low electrical conductivity and require a lot of conductive additives, which decrease the effective capacity based on the entire electrode weight/volume. In this study, we propose a novel electrical conductivity-relay system that imparts electrical conductivity to organic small molecular electrodes without any conductive additive throughout the charge/discharge cycles. It consists of the combination of the charge-transfer phenomenon in a pristine state and the formation of organic radical salts in redox states. Herein, we demonstrate this electrical conductivity-relay system using a simply mixed molecular crystal couple of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) as a cathode without any conductive additive and aqueous sodium bromide as an electrolyte. During charge/discharge, the electrical conductivity of the cathode is supported by charge-transfer at the TTF/TCNQ interface and (TTF)Brn (0.7 ≤ n ≤ 0.8) and NaTCNQ radical salts, and the cathode exhibits a specific capacity of 112 mAh g-1 and a retention rate of 80.7% at the 30th cycle. Furthermore, the molecular crystal couple electrode of TTF and TCNQ shows better charge/discharge performance than the pure charge-transfer complex electrode, indicating that this system expands candidates for organic electrode materials to various pairs and mixing ratios of small molecules that do not form charge-transfer complexes.