Electrochemically active porous carbon nanospheres prepared by inhibition of pyrolytic condensation of polymers.

Porous carbon is a pivotal material for electrochemical applications. The manufacture of porous carbon has relied on chemical treatments (etching or template) that require processing in all areas of the carbon/carbon precursor. We present a unique approach to preparing porous carbon nanospheres by inhibiting the pyrolytic condensation of polymers. Specifically, the porous carbon nanospheres are obtained by coating a thin film of ZnO on polystyrene spheres. The porosity of the porous carbon nanospheres is controlled by the thickness of the ZnO shell, achieving a BET-specific area of 1,124 m2/g with a specific volume of 1.09 cm3/g. We confirm that under the support force by the ZnO shell, a hierarchical pore structure in which small mesopores are connected by large mesopores is formed and that the pore-associated sp3 defects are enriched. These features allow full utilization of the surface area of the carbon pores. The electrochemical capacitive performance of porous carbon nanospheres was evaluated, achieving a high capacitance of 389 F/g at 1 A/g, capacitance retention of 71% at a 20-fold increase in current density, and stability up to 30,000 cycles. In particular, we achieve a specific area-normalized capacitance of 34.6 µF/cm2, which overcomes the limitations of conventional carbon materials.

Complete encapsulation of sulfur through interfacial energy control of sulfur solutions for high-performance Li-S batteries.

Complete encapsulation of high-content sulfur in porous carbon is crucial for high performance Li-S batteries. To this end, unlike conventional approaches to control the pore of carbon hosts, we demonstrate controlling the interfacial energy of the solution in the process of penetrating the sulfur-dissolved solution. We unveil, experimentally and theoretically, that the interfacial energy with the carbon surface of the sulfur solution is the key to driving complete encapsulation of sulfur. In the infiltration of sulfur solutions with N-methyl-2-pyrrolidone, we achieve complete encapsulation of sulfur, even up to 85 wt %. The sulfur fully encapsulated cathode achieves markedly high volumetric capacity and stable cycle operation in its Li-S battery applications. We achieve a volumetric capacity of 855 mAh/cm3 at 0.2C and a capacity reduction of 0.071% per cycle up to 300 cycles at 1C.

Balancing Electrolyte Donicity and Cathode Adsorption Capacity for High-Performance LiS Batteries.

LiS batteries with high theoretical capacity are attracting attention as next-generation energy storage systems. Much effort has been devoted to the introduction of cathode materials with strong adsorption to sulfide species, but it is presented that this selection should be refined in the application of high donicity electrolytes. The oxides with different adsorption capacities are explored while controlling the electrolyte donicity, confirming the trade-off effect between the donicity and the adsorption capacity for sulfur conversion. Specifically, a cathode substrate containing oxide nanoparticles of MgO, NiO, Fe2 O3 , Co3 O4 , and V2 O5 is prepared with spectra in adsorption capacity as well as low and high donicity electrolytes by controlling the concentration of LiNO3 salt. Strong adsorbent oxides such as Co3 O4 and V2 O5 cause competitive adsorption of electrolyte salts in high donicity electrolytes, resulting in poor cell performance. High cell performance is achieved on weakly adsorbing oxides of MgO or NiO with high donicity electrolytes; the MgO-containing cathode cell delivers a high discharge capacity of 1394 mAh g-1 at 0.2 C. It is believed that understanding the interactions between electrolytes and adsorbent substrates will be the cornerstone of high-performance LiS batteries.

A Layer-by-Layer Assembly Route to Electroplated Fibril-Based 3D Porous Current Collectors for Energy Storage Devices.

Electrical conductivity, mechanical flexibility, and large electroactive surface areas are the most important factors in determining the performance of various flexible electrodes in energy storage devices. Herein, a layer-by-layer (LbL) assembly-induced metal electrodeposition approach is introduced to prepare a variety of highly porous 3D-current collectors with high flexibility, metallic conductivity, and large surface area. In this study, a few metal nanoparticle (NP) layers are LbL-assembled onto insulating paper for the preparation of conductive paper. Subsequent Ni electroplating of the metal NP-coated substrates reduces the sheet resistance from ≈103 to <0.1 Ω sq-1 while maintaining the porous structure of the pristine paper. Particularly, this approach is completely compatible with commercial electroplating processes, and thus can be directly extended to electroplating applications using a variety of other metals in addition to Ni. After depositing high-energy MnO NPs onto Ni-electroplated papers, the areal capacitance increases from 68 to 811 mF cm-2 as the mass loading of MnO NPs increases from 0.16 to 4.31 mg cm-2 . When metal NPs are periodically LbL-assembled with the MnO NPs, the areal capacitance increases to 1710 mF cm-2 .

Enhanced Photoelectrochemical Water Splitting through Bismuth Vanadate with a Photon Upconversion Luminescent Reflector.

As the performance of photoanodes for solar water splitting steadily improves, the extension of the absorption wavelength in the photoanodes is highly necessary to substantially improve the water splitting. We use a luminescent back reflector (LBR) capable of photon upconversion (UC) to improve the light harvesting capabilities of Mo:BiVO4 photoelectrodes. The LBR is prepared by dispersing the organic dye pair meso-tetraphenyltetrabenzoporphine palladium and perylene capable of triplet-triplet annhilation-based UC in a polymer film. The LBR converts the wavelengths of 600-650 nm corresponding to the sub-band gap of Mo:BiVO4 and the wavelengths of 350-450 nm that are not sufficiently absorbed in Mo:BiVO4 to a wavelength that can be absorbed by a Mo:BiVO4 photoelectrode. The LBR improves the water splitting reaction of Mo:BiVO4 photoelectrodes by 17 %, and consequently, the Mo:BiVO4 /LBR exhibits a photocurrent density of 5.25 mA cm-2 at 1.23 V versus the reversible hydrogen electrode. The Mo:BiVO4 /LBR exhibits hydrogen/oxygen evolution corresponding to the increased photocurrent density and long-term operational stability for the water splitting reaction.

Three-Dimensional Polymeric Mechanical Metamaterials Fabricated by Multibeam Interference Lithography with the Assistance of Plasma Etching.

The pentamode structure is a type of mechanical metamaterial that displays dramatically different bulk and shear modulus responses. In this study, a face-centered cubic (FCC) polymeric microstructure was fabricated by using SU8 negative-type photoresists and multibeam interference exposure. Isotropic plasma etching is used to control the solid-volume fraction; for the first time, we obtained a structure with the minimum solid-volume fraction as low as 15% that still exhibited high structural integrity. Using this method, we reduced the width of atom-to-atom connections by up to 40 nm. We characterize the effect of the connection area on the anisotropy of the mechanical properties using simulations. Nanoindentation measurements were also conducted to evaluate the energy dissipation by varying the connection area. The Young's/shear modulus ratio is 5 times higher for the etched microstructure than that of the bulk SU8 materials. The use of interference lithography may enable the properties of microscale materials to be engineered for various applications, such as MEMS.

In Situ Gelation of Poly(vinylidene fluoride) Nanospheres for Dye-Sensitized Solar Cells: The Analysis on the Efficiency Enhancement upon Gelation.

The in situ gelation that utilizes the dissolution of polymers inside the cell is allowed high concentration polymer gel without concerns regarding high viscous electrolyte incorporation into the cell as in the conventional approach. We demonstrate the in situ gelation of polymer composite electrolytes using poly(vinylidene fluoride) nanospheres (PVdF NSs). The PVdF NSs were synthesized by high pressure emulsion polymerization using gaseous vinylidene fluoride monomers. Compared to the liquid electrolyte (LE) DSCs without PVdF gelation, the PVdF polymer gel electrolyte (PGE) DSCs displayed higher η than the LE DSCs; specifically, the 10 wt % PVdF PGE DSCs display 8.1% of the η, while the LE DSCs only display 6.5%. We characterized the effect of PVdF PGE on the photovoltaic parameters in detail. We also compared the long-term stability of DSCs containing LE and PVdF PGE. The DSCs with PVdF PGE exhibited high stability compared to the LE DSCs, similar to a conventional PGE system. We believe that this facile in situ gelation approach could be utilized for not only the practical application of polymer gel electrolytes DSCs but also for various energy-storage devices.

Particulate inverse opal carbon electrodes for lithium-ion batteries.

Inverse opal carbon materials were used as anodes for lithium ion batteries. We applied particulate inverse opal structures and their dispersion in the formation of anode electrodes via solution casting. We prepared aminophenyl-grafted inverse opal carbons (a-IOC), inverse opal carbons with mesopores (mIOC), and bare inverse opal carbons (IOC) and investigated the electrochemical behavior of these samples as anode materials. Surface modification by aminophenyl groups was confirmed by XPS measurements. TEM images showed mesopores, and the specific area of mIOC was compared with that of IOC using BET analysis. A half-cell test was performed to compare a-IOC with IOC and mIOC with IOC. In the case of the a-IOC structure, the cell test revealed no improvement in the reversible specific capacity or the cycle performance. The mIOC cell showed a reversible specific capacity of 432 mAh/g, and the capacity was maintained at 88%-approximately 380 mAh/g-over 20 cycles.

Characterization of charge transport properties of a 3D electrode for dye-sensitized solar cells.

Electron transport and recombination in three-dimensionally-ordered (3D-ordered) structure electrodes were investigated using intensity-modulated photocurrent and photovoltage spectroscopy. The surface-modified TiO2 inverse opal structure was applied as a 3D electrode. The morphology, crystalline structure and surface states of the 3D-ordered structure were characterized by SEM, TEM and XPS and compared to those of the conventional nanoparticulate TiO2 structure. The performance of the 3D electrode was also evaluated by comparing the transport time and recombination lifetime to those of the conventional electrodes. Remarkably, the recombination lifetime in inverse opal was found to be greater than in nanocrystalline TiO2 by 4.3-6.2 times, thus improving the electron collection efficiency by 10%. Comparing the photovoltaic performance, although the dye adsorption of the 3D-ordered porous electrode is lower, the electrode achieves a photocurrent density comparable to that of a nanoparticulate TiO2 electrode due to the higher light scattering as well as the higher collection efficiency.

Constructing inverse opal structured hematite photoanodes via electrochemical process and their application to photoelectrochemical water splitting.

In this study, we constructed an inverse opal structured hematite (IOS α-Fe2O3) as the photoanode of a photoelectrochemical (PEC) cell for efficient solar water splitting via a simple electrochemical process. At the same time, a series of affecting factors (template particle size, electrodeposition time and annealing temperature) to construct the IOS α-Fe2O3 photoanode on the photoelectrochemical water splitting were considered. Optimized PEC efficiency was observed for the IOS α-Fe2O3 photoanode annealed at 400 °C using the 250 nm sized-polystyrene (PS) colloid template and 9 minutes of electrodeposition time for the given specific Fe precursor solution. This resulted in the highest photocurrent density compared to other crossed conditions, which significantly achieved 3.1 mA cm(-2) at 0.5 V vs. Ag/AgCl reference electrode. The synthesis of the IOS α-Fe2O3 via an easy-to-control electrochemical process is described for first time that opens a possibility for constructing other oxide semiconductor photoanodes (not only well-known Si, Ti and Zr) with inverse opal structure.

The Polysulfide-Cathode Binding Energy Landscape for Lithium Sulfide Growth in Lithium-Sulfur Batteries.

A cathode substrate with strong adsorption of lithium polysulfides (LiPSs) has been preferred for lithium-sulfur (Li-S) batteries. However, the recent finding that controlled growth of lithium sulfides (Li2 S) during discharge is crucial for S utilization stimulates improvement of this preference. Here, the Li2 S growth and cell capacity in the LiPS binding energy landscape of cathode substrates are investigated. Specifically, Co-based ternary oxides are employed to obtain binding energies in the range of 4.0-7.4 eV. Of these substrates, only the MnCo2 O4 substrate with moderate LiPS affinity exhibits 3D Li2 S growth. The MnCo2 O4 cells achieve high sulfur utilization up to 84% at 0.2 C and excellent performance even under high sulfur loading/lean electrolyte conditions. In contrast, weak affinity substrates such as ZnCo2 O4 and strong affinity substrates such as NiCo2 O4 and CuCo2 O4 exhibit low discharge capacity with 2D Li2 S growth. For optimal LiPS affinity driving 3D growth, a balance between promoting LiPS adsorption and diffusion limitation in the LiPS adsorption layer is suggested.

Hierarchical twin-scale inverse opal TiO2 electrodes for dye-sensitized solar cells.

We describe the preparation of three-dimensional hierarchical twin-scale inverse opal (ts-IO) electrodes for dye-sensitized solar cells (DSSCs). The ts-IO TiO(2) structure was obtained from a template fabricated via the assembly of mesoscale colloidal particles (40-80 nm in diameter) in the confined geometry of a macroporous IO structure. The photovoltaic properties of ts-IO electrodes were optimized by varying the layer thickness or the size of mesopores in the mesoscale colloidal assembly. Electron transport was investigated using impedance spectroscopy. The result showed that due to the competing effects of recombination and dye adsorption, the maximum efficiency was observed at an electrode thickness of 12 µm. The electrodes of smaller mesopores diameters yielded the higher photocurrent density due to the decrease in the electron transport resistance at the TiO(2)/dye interface. A maximum efficiency of 6.90% was obtained using an electrode 12 µm thick and a mesopore diameter of 35 nm.

Inverse opal carbons for counter electrode of dye-sensitized solar cells.

We investigated the fabrication of inverse opal carbon counter electrodes using a colloidal templating method for DSSCs. Specifically, bare inverse opal carbon, mesopore-incoporated inverse opal carbon, and graphitized inverse opal carbon were synthesized and stably dispersed in ethanol solution for spray coating on a FTO substrate. The thickness of the electrode was controlled by the number of coatings, and the average relative thickness was evaluated by measuring the transmittance spectrum. The effect of the counter electrode thickness on the photovoltaic performance of the DSSCs was investigated and analyzed by interfacial charge transfer resistance (R(CT)) under EIS measurement. The effect of the surface area and conductivity of the inverse opal was also investigated by considering the increase in surface area due to the mesopore in the inverse opal carbon and conductivity by graphitization of the carbon matrix. The results showed that the FF and thereby the efficiency of DSSCs were increased as the electrode thickness increased. Consequently, the larger FF and thereby the greater efficiency of the DSSCs were achieved for mIOC and gIOC compared to IOC, which was attributed to the lower R(CT). Finally, compared to a conventional Pt counter electrode, the inverse opal-based carbon showed a comparable efficiency upon application to DSSCs.

Synthesis of porous carbon balls from spherical colloidal crystal templates.

Spherical inverse opal (IO) porous carbon was produced utilizing silica colloidal crystal spheres as templates. The spherical colloidal crystals were obtained through the self-assembly of monodisperse particles inside an emulsion droplet with confined geometry. The templates were inverted using a carbon precursor, phenol-formaldehyde (PF) resol. We demonstrated a two-step synthesis involving the subsequent infiltration of the PF resol precursor into the spherical colloidal crystal template and a one-step synthesis using a silica colloidal solution containing dissolved PF resol. In the former case, the sizes of the IO carbon balls were controlled by the size of the colloidal crystal templates, and diameters of a few micrometers up to 50 µm were obtained. The average diameter of the macropores created by the silica particles was 230 nm. Moreover, meso-/macroporous IO carbon balls were created using block-copolymer templates in the PF resol. In the one-step synthesis, the concentration of PF resol in the colloidal solution controlled the diameter of the IO carbon balls. IO balls smaller than 3 µm were obtained from the direct addition of 5% PF resol. The one-step synthesis produced rather irregular porous structures reflecting the less ordered crystallization processes inside the spherical colloidal crystals. Nitrogen adsorption and cyclic voltammetry measurements were conducted to measure the specific area and electroactive surface area of the IO carbon balls. The specific area of the mesopores-incorporated IO carbon balls was 1.3 times higher than that of bare IO carbon balls. Accordingly, the meso-/macroporous porous carbon balls exhibited higher electrocatalytic properties than the macroporous carbon balls.

Discovery of Dual-Functional Amorphous Titanium Suboxide to Promote Polysulfide Adsorption and Regulate Sulfide Growth in Li-S Batteries.

Lithium-sulfur (Li-S) batteries are promising as next-generation energy storage systems. Adsorbents for sulfide species are favorably applied to the cathode, but this substrate often results in a surface-passivating lithium sulfide(Li2 S) film with a strong adsorption of Li2 S. Here, an amorphous titanium suboxide (a-TiOx) is presented that strongly adsorbs lithium polysulfides (Li2 Sx , x < 6) but relatively weakly adsorbs to Li2 S. With these characteristics, the a-TiOx achieves high conversion of Li2 Sx and high sulfur utilization accompanying the growth of particulate Li2 S. The DFT calculations present a mechanism for particulate growth driven by the promoted diffusion and favorable clustering of Li2 S. The a-TiOx -coated carbon nanotube-assembled film (CNTF) cathode substrate cell achieves a high discharge capacity equivalent to 90% sulfur utilization at 0.2 C. The cell also delivers a high capacity of 850 mAh g-1 even at the ultra-high-speed of 10 C and also exhibits high stability of capacity loss of 0.0226% per cycle up to 500 cycles. The a-TiOx /CNTF is stacked to achieve a high loading of 7.5 mg S cm-2 , achieving a practical areal capacity of 10.1 mAh cm-2 .

Bilayer inverse opal TiO2 electrodes for dye-sensitized solar cells via post-treatment.

We investigated the formation of bilayer inverse opal TiO(2) (io-TiO(2)) structures via post-treatment with a TiO(2) precursor solution and characterized the photovoltaic performances of the resulting electrodes for use in dye-sensitized solar cells. The post-treatment of TiO(2) inverse opals in a precursor solution grew rutile TiO(2) nanoparticles on anatase crystalline phase io-TiO(2) surfaces, resulting in anatase/rutile bilayer structures. We achieved a maximum photovoltaic conversion efficiency of 4.6% using a 25 µm thick electrode formed with the post-treated io-TiO(2) under simulated AM 1.5 light. This efficiency represents a 183% improvement over the non-post-treated io-TiO(2) electrodes. The shell thickness was controlled by the post-treatment time. The effects of shell thickness on photovoltaic performance were investigated by measuring the morphologies and electrochemical impedance of the post-treated io-TiO(2). We found that post-treatment up to a certain period of time increased the surface area and electron lifetime, but further treatment resulted in decreased area and saturated lifetimes. The optimal post-treatment time was identified, and the optimal io-TiO(2) electrodes were characterized.

Facile synthesis of TiO2 inverse opal electrodes for dye-sensitized solar cells.

Engineering of TiO(2) electrode layers is critical to guaranteeing the photoconversion efficiency of dye-sensitized solar cells (DSSCs). Recently, a novel approach has been introduced for producing TiO(2) electrodes using the inverted structures of colloidal crystals. This paper describes a facile route to producing ordered macroporous electrodes from colloidal crystal templates for DSSCs. Using concentrated colloids dispersed in a volatile medium, the colloidal crystal templates were obtained within a few minutes, and the thickness of the template was easily controlled by changing the quantity of colloidal solution deposited. Here, the effects of the structural properties of the inverse opal TiO(2) electrodes on the photovoltaic parameters of DSSCs were investigated. The photovoltaic parameters were measured as a function of pore ordering and electrode film thickness. Moreover, DSSC applications that used either liquid or viscous polymer electrolyte solutions were investigated to reveal the effects of pore size on performance of an inverse opal TiO(2) electrode.

Enhanced photocurrent density of tungsten oxide hollow particle arrays produced by colloidal template synthesis.

Tungsten oxide hollow particle arrays were fabricated for use as the photoanode in photoelectrochemical cells. These arrays were constructed through an electrochemical infiltration method in a three-dimensional colloidal array template. The enhanced contact area between the tungsten oxide and the electrolyte resulted in an increase in photocurrent when compared to a cell prepared with no template. The porous structures were greatly influenced by the temperature used during the thermal annealing step. This simple and cost-effective approach resulted in photocatalytic activity for hydrogen production that was three times higher than that of tungsten oxide prepared without the template.

Unveiling the Effects of Nanostructures and Core Materials on Charge-Transport Dynamics in Heterojunction Electrodes for Photoelectrochemical Water Splitting.

Understanding the photogenerated charge-transport dynamics of metal oxide electrodes is the key to providing a strategy for practical improvement in the photoelectrochemical reaction activity. Here, we analyze the electron transport of a 3D bicontinuous SnO2/BiVO4 nanostructured photoelectrode by intensity-modulated photocurrent spectroscopy. We compare this electrode with 3D WO3/BiVO4 and planar-type bilayer SnO2/BiVO4 electrodes. In the results, we observe an order of magnitude faster electron transport in the 3D electrodes relative to the bilayer electrode. Moreover, we observe trap-limited transport on widely applied WO3/BiVO4 electrodes but confirm rapid trap-free transport on 3D SnO2/BiVO4. We also characterize the effect of electron transport on the water-splitting reaction. The electron-transport rate is directly related to the charge-separation efficiency in the water-splitting reaction. The fast transport time of the 3D SnO2/BiVO4 leads to the achievement of a significantly higher charge separation efficiency of 94%.

Photon upconversion-assisted dual-band luminescence solar concentrators coupled with perovskite solar cells for highly efficient semi-transparent photovoltaic systems.

A luminescent solar concentrator-based photovoltaic system (LSC-PVs) is highly transparent because it harvests solar light via the LSC, a transparent panel containing only fluorophores, and is, therefore, promising as a PV window. However, for the practical use of LSC-PV, achieving high efficiency remains a challenge. Here, we demonstrate an LSC-PV, which is based on the combination of an upconversion (UC)-assisted dual band harvesting LSC and perovskite solar cells (PSCs). We prepare a dual LSC panel consisting of a downshift (DS) LSC that absorbs violet light and an LSC that upconverts the red light. We apply a highly efficient mixed halide PSC with an efficiency of 17.22%. We control the thickness of the LSC panel as well as the dye concentration to maximize the emission from dual LSCs. The dual LSCs coupled with a PSC exhibit a high average-visible-transmittance of 82% and achieve a maximum efficiency of 7.53% at 1 sun (AM 1.5G) illumination. The dual LSC-PSC exhibits a constant efficiency even under oblique solar light illumination and a stable operation with an efficiency retention of 80%.