Search

We report a novel detection method for single aqueous droplets in organic solvents by the collisional contact of the droplet, inducing the partial deformation of the ultramicroelectrode (UME) surface. For various chemical reactions in organic solvents, water impurities affect the catalytic activity, leading to a loss of productivity and selectivity. Therefore, it is necessary to monitor the water content of organic solvents in real time between many chemical production processes, from the laboratory to the industrial scale. Our method enables the detection of water contamination by real-time monitoring of the electrochemical signals or observing morphological changes in the microelectrode. When an aqueous droplet collides with the UME, the contact area of the electrode is electrolyzed, forming pits on the surface where the droplet falls. Current transient analysis shows a unique current spike corresponding to the reaction inside the adsorbed single aqueous droplet, which differs from those detected by the faradaic/nonfaradaic reaction of collision of other particles. Moreover, this analytical method can record the history of collision events from pits on the UME surface, implying that inspecting the UME surface could be a quick screening method for solvent contamination. Based on a comparison of the electrochemical signals and morphological changes of the electrode after each event, the sizes of the pits and droplets are related. A COMSOL simulation is performed to explain the shape of the peak current and pit formation during collision events. This experimental concept elucidates the dynamic behavior of aqueous droplets on a positively biased metal electrode.

Li–O2batteries have attracted considerable attention for several decades due to their high theoretical energy density (>3400 Wh/kg). However, it has not been clearly demonstrated that their actual volumetric and gravimetric energy densities are higher than those of Li-ion batteries. In previous studies, a considerable quantity of electrolyte was usually employed in preparing Li–O2cells. In general, the electrolyte was considerably heavier than the carbon materials in the cathode, rendering the practical energy density of the Li–O2battery lower than that of the Li-ion battery. Therefore, air cathodes with significantly smaller electrolyte quantities need to be developed to achieve a high specific energy density in Li–O2batteries. In this study, we propose a core–shell-structured cathode material with a gel-polymer electrolyte layer covering the carbon nanotubes (CNTs). The CNTs are synthesized using the floating catalyst chemical vapor deposition method. The polymeric layer corresponding to the shell is prepared by the layer-by-layer (LbL) coating method, utilizing Li-Nafion along with PDDA-Cl [poly(diallyldimethylammonium chloride)]. Several bilayers of Li-Nafion and PDDA, on the CNT surface, are successfully prepared and characterized via X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The porous structure of the CNTs is retained after the LbL process, as confirmed by the nitrogen adsorption–desorption profile and BJH pore-size distribution analysis. This porous structure can function as an oxygen channel for facilitating the transport of oxygen molecules for reacting with the Li ions on the cathode surface. These polymeric bilayers can provide an Li-ion pathway, after absorbing a small quantity of an ionic liquid electrolyte, 0.5 M LiTFSI EMI-TFSI [1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide]. Compared to a typical cathode, where only liquid electrolytes are employed, the total quantity of electrolyte in the cathode can be significantly reduced; thereby, the overall cell energy density can be increased. A Li–O2battery with this core–shell-structured cathode exhibited a high energy density of approximately 390 Wh/kg, which was assessed by directly weighing all of the cell components together, including the gas diffusion layer, the interlayer [a separator containing a mixture of LiTFSI, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR-14), and PDDA-TFSI], the lithium anode, and the LbL-CNT cathode. The cycle life of the LbL-CNT-based cathode was found to be 31 cycles at a limited capacity of 500 mAh/gcarbon. Although this is not an excellent performance, it is almost 2 times better than that of a CNT cathode without a polymer coating.

As potential successors to intercalation-based battery systems, Li-air batteries have attracted enormous research attention because of their higher theoretical specific energy compared with conventional batteries, in terms of unit of energy per active material mass. However, this unit complicates the estimation of realistically attainable cell-scale energy densities because all other indispensable cell components are excluded. Here, we report Li-O2batteries with an ultra-thin, customized gas-diffusion layer, highly conductive, stable polymeric ionic-liquid separator, and folded cell structure that reduce the volume and mass of multiple cell components. A complete cell-scale specific energy and energy density of 1,214 Wh kgcell−1and 896 Wh Lcell−1, respectively, were experimentally achieved. Cell-scale electrochemical phenomena were simulated using a multiphysics model that included the oxygen flow caused by the cathode reaction, dynamic porosity changes, and resistance increase during Li2O2formation. Both the cell geometry and cathode composition dramatically affected the cell's capacity.

Photoactivities of lead chromates with various combinations of Pb and Cr are rapidly screened using scanning electrochemical microscopy (SECM). In the rapid screening investigation, the metal oxide spot electrode with a Pb/Cr ratio of 2:1 exhibits the highest photoactivity among the semiconductor prepared with different compositions. The photoactivity and electrochemical properties of thin-film electrodes of PbCrO4, Pb2CrO5, and Pb5CrO8are further studied following the combinatorial screening. In the bulk electrode measurements, the Pb2CrO5bulk electrode displays the highest photocurrent of 0.23 mA/cm2for SO32–oxidation at 0.4 V vs Ag/AgCl under 100 mW/cm2UV–vis light irradiation. Pb2CrO5presents visible light activity with an absorption wavelength up to 550 nm and an incident photon to current conversion efficiency (IPCE) of 10% at the wavelength of 340 nm. The onset wavelength observed in the UV–vis absorption spectrum increases with increasing Pb contents in lead chromates. Optically obtained direct band gaps decreased from 2.38 to 2.25 to 2.07 eV for PbCrO4, Pb2CrO5, and Pb5CrO8, respectively. However, the onset wavelength that appeared in IPCE is 2.26 ± 0.02 eV for all three lead chromates where the photocurrent under longer wavelength light irradiation is insignificant. The results imply that more Pb 6s orbitals form interband states, increasing optical transitions in lead chromates. The band structures of PbCrO4, Pb2CrO5, and Pb5CrO8are also determined by electrochemical analyses and ultraviolet photoelectron spectroscopy (UPS).

A stable electrolyte is required for use in the open-packing environment of a Li-O2battery system. Herein, a gelled quasi-solid-state electrolyte containing SiO2nanoparticles was designed, in order to obtain a solidified electrolyte with a high discharge capacity and long cyclability. We successfully fabricated an organic–inorganic hybrid matrix with a gelled structure, which exhibited high ionic conductivity, thereby enhancing the discharge capacity of the Li-O2battery. In particular, the improved electrochemical stability of the gelled cathode led to long-term cyclability. The organic–inorganic hybrid matrix with the gelled structure played a beneficial role in improving the ionic conductivity and long-term cyclability and diminished electrolyte evaporation. The experimental and theoretical findings both suggest that the preferential binding between amorphous SiO2and polyethylene glycol dimethyl ether (PEGDME) solvent led to the formation of the solidified gelled electrolyte and improved electrochemical stability during cycling, while enhancing the stability of the quasi-solid state Li-O2battery.

We report that oxide composite electrodeposition can be used for the facile preparation of metal-doped BiVO4photoelectrodes for photoelectrochemical water oxidation. The photoactivity of electrodeposition film was improved by the addition of a small amount of tungstic acid particles during the electrodeposition. These particles are incorporated in the deposit and finally generate tungsten-doped bismuth vanadate. The suspended particles in the plating solution were electrostatically attracted to the cathode and accordingly incorporated into the deposit (electrostatic deposition). WO3nanoparticles (NPs) can be used instead of tungstic acid, to yield a BiVO4with different properties. Enhanced photoelectrochemical (PEC) water oxidation was confirmed via scanning electrochemical microscopy (SECM) by detecting increased oxygen evolution with using optical fiber incorporating a ring electrode.

A rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectrochemical (PEC) properties. We now extend this rapid screening to the examination of photocatalyst (semiconductor/semiconductor) composites: by examining a variety of ZnWO4/WO3composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO3with both UV and visible irradiation on a spot array electrode. With bulk films of various thickness formed by a drop-casting technique of mixed precursors and a one-step annealing process, the 9 atomic % ZnWO4/WO3resulted in a 2.5-fold increase in the photocurrent compared to pristine WO3for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulk-film electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO3and ZnWO4/WO3electrodes. X-ray diffraction showed the composite nature of the WO3and ZnWO4mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO3has a smaller band gap than ZnWO4, while Mott–Schottky analysis determined that ZnWO4has a more negative flat-band potential than WO3. A composite band diagram was created, showing the possibility of greater electron/hole separation in the composite material. Investigations on layered structures showed that the higher photocurrent was only observed when the ZnWO4/WO3composite was formed in a single annealing step.

Bi2WO6microelectrode arrays on FTO glass substrates were fabricated by a picoliter solution dispensing technique using Bi(NO3)3as the Bi source and (NH4)6H2W12O40as the W source in ethylene glycol. The scanning electrochemical microscope modified by using an optical fiber in place of an ultramicroelectrode was employed for rapid screening of the Bi2WO6arrays and to investigate the effect of 12 different dopants on the photocatalytic oxidation of SO32–. Among the different dopant compositions, addition of 12% Zn showed a photocurrent enhancement of up to 80% compared with that of the pure Bi2WO6. This result was further confirmed with bulk electrode studies for SO32–and water oxidation. UV–vis absorption, electrochemical impedance spectroscopy, scanning electron microscopy, and X-ray diffraction studies were carried out with the photocatalysts to elucidate the role of Zn in the bulk semiconductors. Absorbed photon-to-current efficiency and incident photon-to-current efficiency determinations further confirm the enhancement of photoelectrochemical behavior upon addition of Zn to Bi2WO6photocatalysts.

We report that conductive single nanoparticle (NP) collisions can involve a significant component of the mass transport to the electrode of the charged NPs by migration. Previously, collision events of catalytic NPs were described as purely diffusional using random walk theory. However, the charged NP can also be attracted to the electrode by the electric field in solution (i.e., migration) thereby causing an enhancement in the collision frequency. The migration of charged NPs is affected by the supporting electrolyte concentration and the faradaic current flow. A simplified model based on the NP transference number is introduced to explain the migrational flux of the NPs. Experimental collision frequencies and the transference number model also agreed with more rigorous simulation results based on the Poisson and Nernst–Planck equations.

Rechargeable batteries have emerged as the most promising energy storage devices in response to continually growing modern demands and are still being researched to attain higher energy densities, structural stability, and longer cycling and calendar life. Owing to the fact that battery electrodes are developed from various types of powders, incorporation of functional nanocoating of suitable materials on powder materials and/or nanosynthesis of active powder constituents have shown promising results regarding the aforementioned challenges associated with modern battery technology. Atomic layer deposition (ALD) has been demonstrated to be highly effective in fabricating inorganic films even at the subnanoscale, not only on flat surfaces but also on individual particles with high conformity, uniformity, and self-limiting growth, thus providing exceptional control over film thickness. Unlike conventional wet-chemical processes, powder ALD offers a unique opportunity to develop nano- and subnanoscale films of various compositions over a variety of substrate particles regardless of their size, morphology, and composition. Proper modifications made by powder ALD process are known to induce improvements in structural stability, electronic and ionic conductivity at the interface, and consequent charge–discharge properties of the batteries. This review comprehensively covers the main strategies and materials used over time to improve the performance of various types of batteries utilizing the powder ALD process.