Tae Kyoung Kim

Solution-Processed CoFe2O4 Nanoparticles on 3D Carbon Fiber Papers for Durable Oxygen Evolution Reaction

We report CoFe2O4 nanoparticles (NPs) synthesized using a facile hydrothermal growth and their attachment on 3D carbon fiber papers (CFPs) for efficient and durable oxygen evolution reaction (OER). The CFPs covered with CoFe2O4 NPs show orders of magnitude higher OER performance than bare CFP due to high activity of CoFe2O4 NPs, leading to a small overpotential of 378 mV to get a current density of 10 mA/cm(2). Significantly, the CoFe2O4 NPs-on-CFP electrodes exhibit remarkably long stability evaluated by continuous cycling (over 15 h) and operation with a high current density at a fixed potential (over 40 h) without any morphological change and with preservation of all materials within the electrode. Furthermore, the CoFe2O4 NPs-on-CFP electrodes also exhibit hydrogen evolution reaction (HER) performance, which is considerably higher than that of bare CFP, acting as a bifunctional electrocatalyst. The achieved results show promising potential for efficient, cost-effective, and durable hydrogen generation at large scales using earth-abundant materials and cheap fabrication processes.

Liquefied gas electrolytes for electrochemical energy storage devices

Separating charges is a gas Solid and liquid electrolytes allow for charges or ions to move while keeping anodes and cathodes separate. Separation prevents short circuits from occurring in energy storage devices. Rustomji et al. show that separation can also be achieved by using fluorinated hydrocarbons that are liquefied under pressure. The electrolytes show excellent stability in both batteries and capacitors, particularly at low temperatures. Science, this issue p. eaal4263 Electrolytes based on liquefied gas solvents show excellent conductivity at low temperatures. INTRODUCTION The vast majority of electrolyte research for electrochemical energy storage devices, such as lithium-ion batteries and electrochemical capacitors, has focused on liquid-based solvent systems because of their ease of use, relatively high electrolytic conductivities, and ability to improve device performance through useful atomic modifications on otherwise well-understood solvent molecules. However, with a delicate balance between electrochemical stability, ionic conductivity, temperature, and safety, there has understandably been little change over a number of decades in the electrolyte composition, which consists primarily of carbonate-type solvents and provides limited improvement in device performance. RATIONALE It is often assumed that molecules that are gaseous at room temperature are typically nonpolar and have low intermolecular attraction, which prevents them from condensing at room temperature. Although this may be true in general, there are a number of reasonably polar molecules that are gaseous at room temperature. It is hypothesized that these relatively polar gaseous molecules, when liquefied under pressure, could dissolve salts at room temperature to form liquefied gas electrolytes. Although a number of potential solvents were explored, the present study focuses on the use of hydrofluorocarbons, which are nontoxic and have relatively strong chemical bonds, allowing for a wide electrochemical window. Although these solvents generally have moderate dielectric constants, their exceptionally low viscosity allows for a superior solvent dielectric-fluidity factor that is higher than commonly used solvents, potentially allowing for relatively high electrolytic conductivities. Last, the low melting points of the solvents studied could allow for substantial improvements in device operation at low temperatures. Although they may require modified processes for manufacturability, the pressures of the proposed chemistry are moderate, and the solvents are compatible with commonly used separator and electrode materials. RESULTS All tests were conducted in high-pressure vessels in which the solvents were in a liquefied state under their own vapor pressure. We found that electrochemical capacitors that have a liquefied gas electrolyte based on difluoromethane (CH2F2) have an exceptionally wide operation temperature from –78° to +65°C, with similar resistance and capacitance to conventional devices. Further, we demonstrate an increase in operation voltage to 3.0 V—equivalent to a 23% increase in energy density—under accelerated life conditions. The use of a liquefied gas electrolyte based on fluoromethane (CH3F) show platting and stripping efficiencies on lithium metal of ~97% over hundreds of cycles under aggressive testing (1 mA cm−2, 1 C cm−2 each cycle), with no evidence of dendritic growth. This impressive behavior on lithium metal is thought to be due to the particular combination of an exceptionally low viscosity, high vapor pressure, and LiF chemical reduction products on the anode surface. The same fluoromethane-based liquefied gas electrolyte shows good cycling and rate performance on a LiCoO2 cathode. We demonstrate a high discharge capacity retention of 60.6% at –60°C, which is thought to be due to an ideal solid electrolyte interphase formed on the electrodes as observed through x-ray photoelectron spectroscopy analysis. Last, we show that there is a substantial drop in electrolytic conductivity at elevated temperatures because of salt precipitation out from solution as the supercritical point is approached (~+40° to 80°C) and recovery in the conductivity as the temperature is lowered. This reversible mechanism is demonstrated to effectively shutdown operation in an electrochemical capacitor device and may similarly enable reversible battery shutdown at increased temperatures, mitigating the potential of thermal runaway and improving safety. CONCLUSION A succinct background and demonstration of liquefied gas electrolytes for both electrochemical capacitors and lithium batteries are presented and show potential for substantial improvements in low-temperature operation, energy density, and safety. With their superior electrochemical and physical properties, further exploration and development of these liquefied gas solvents is warranted for their use in next-generation energy storage devices and beyond. Illustration of the electrolytic conductivity and pressure with temperature of the studied liquefied gas electrolytes. The electrolyte solvent is liquefied from a gaseous state under pressure. Exceptionally high electrolytic conductivities are observed at low temperatures. Further, a sharp drop in conductivity occurs as the salt precipitates near the supercritical temperature, which allows for a reversible shutdown mechanism to mitigate battery thermal runaway. Electrochemical capacitors and lithium-ion batteries have seen little change in their electrolyte chemistry since their commercialization, which has limited improvements in device performance. Combining superior physical and chemical properties and a high dielectric-fluidity factor, the use of electrolytes based on solvent systems that exclusively use components that are typically gaseous under standard conditions show a wide potential window of stability and excellent performance over an extended temperature range. Electrochemical capacitors using difluoromethane show outstanding performance from –78° to +65°C, with an increased operation voltage. The use of fluoromethane shows a high coulombic efficiency of ~97% for cycling lithium metal anodes, together with good cyclability of a 4-volt lithium cobalt oxide cathode and operation as low as –60°C, with excellent capacity retention.

Multi-wall carbon nanotube-embedded lithium cobalt phosphate composites with reduced resistance for high-voltage lithium-ion batteries

Lithium cobalt phosphate (LCP) is a high-voltage cathode material used in highenergy- density lithium-ion batteries. With a novel composite synthesis method, multi-wall carbon nanotube (MWCNT)-embedded LCP nanocomposites (LCPCNT composites) are synthesized to enhance the electrical conductance of LCP particles, reducing charge-transfer resistance. The LCP-CNT composites with enhanced electrical conductance approximately doubled cell capacity compared to a cell with a bare LCP cathode. The crystal structure of LCP-CNT composite particles is characterized by X-ray diffraction; the microstructures of the embedded MWCNTs inside LCP particles are confirmed by transmission and scanning electron microscopy with focused ion beam procedures. Electrochemical impedance spectroscopy shows the charge-transfer resistance of the cell with the LCP-CNT composite (1.0 wt. % CNT) cathode decreases to ~80 Ω, much smaller than the ~150 Ω charge-transfer resistance of the bare-LCP cathode cell. Based on battery test and impedance analysis, the main factors affecting the capacity increment are the reduced charge transfer resistance and the uniform distribution of MWCNTs, which is formed during the gelation step of the LCP synthesis procedure.

Vertical Si nanowire arrays fabricated by magnetically guided metal-assisted chemical etching.

In this work, vertically aligned Si nanowire arrays were fabricated by magnetically guided metal-assisted directional chemical etching. Using an anodized aluminum oxide template as a shadow mask, nanoscale Ni dot arrays were fabricated on an Si wafer to serve as a mask to protect the Si during the etching. For the magnetically guided chemical etching, we deposited a tri-layer metal catalyst (Au/Fe/Au) in a Swiss-cheese configuration and etched the sample under the magnetic field to improve the directionality of the Si nanowire etching and increase the etching rate along the vertical direction. After the etching, the nanowires were dried with minimal surface-tension-induced aggregation by utilizing a supercritical CO2 drying procedure. High-resolution transmission electron microscopy (HR-TEM) analysis confirmed the formation of single-crystal Si nanowires. The method developed here for producing vertically aligned Si nanowire arrays could find a wide range of applications in electrochemical and electronic devices.

Controlled metallic nanopillars for low impedance biomedical electrode

Radial metallic nanopillar/nanowire structures can be created by a controlled radiofrequency (RF) plasma processing technique on the surface of certain alloy wires, including important biomedical alloys such as MP35N (Co-Ni-Cr-Mo alloy), platinum-iridium and stainless steel. In electrode applications such as pacemakers or neural stimulators, the increase in surface area in elongated MP35N nanopillars allows for decreased surface impedance and greater current density. However, the nanopillar height on MP35N alloy tends to be self-limiting at ∼1-3μm. The objective of this study was to further elongate the radial nanopillars so as to reduce electrode impedance for biomedical electrode applications. Intelligent experimental design allowed for efficient investigation of processing parameters, including plasma material, process duration, power, pressure and repetition. It was found that multi-step repeated processing in the parameter-controlled RF environment could increase nanopillar height to ∼10μm, a 400% improvement, while the RF plasma processing with identical total duration but in a single step did not lead to desired nanopillar elongation. Measurement of electrode impedance in phosphate-buffered saline solution showed an associated decrease to one-fifth of the surface impedance of unprocessed wire for signals below 100Hz. For the purposes of this study, MP35N and Pt-Ir wires were characterized and demonstrated augmented surface impedance properties which, in combination with superior cell integration, enhanced biomedical electrode performance.