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In this study, we designed a unique method for increasing F-doping concentration in F-doped SnO2 (FTO) films, without the extra addition of NH4F as a doping source, using NaOH acting as a functional additive during ultrasonic spray pyrolysis. The NaOH triggers a chemical reaction with HF, resulting in the presence of dissociated F− acting as a doping source. To optimize the NaOH effect on the transparent conducting performance in the FTO films, we adjusted the volume percentages of the NaOH to 0, 1, 5, and 10 vol% during FTO deposition. Compared with other FTO films, the FTO film prepared with 5 vol% NaOH revealed enhanced carrier concentration (7.81 × 1020 cm−3) generated by the increased F-doping concentration (3.57 at%) and high Hall mobility (27.18 cm2/(V S)) through smooth surface morphology. Such behaviors through the NaOH effect resulted in FTO films with decreased sheet resistance (5.3 ± 0.16 Ω/□), leading to improved electrochromic (EC) energy storage performances of fast switching speed (6.6 s for coloration speed and 8.4 s for bleaching speed) due to faster electrochemical kinetics at the active electrodes, high coloration efficiency (58.1 cm2/C) and high specific capacitance (65.2 F/g at 2 A/g) via enhanced electrochemical activity in the active electrodes that widens the transmittance modulation. Therefore, our study suggests a novel method to improve the transparent conducting performances of FTO films for application in EC energy storage devices.

In this study, Fe co-doping into fluorine-doped tin oxide (FTO) films using horizontal ultrasonic spray pyrolysis deposition (HUSPD) is reported as a novel approach to improve their transparent conduction for high-performance electrochromic (EC) devices. To optimize their transparent conducting performance, we adjusted the Fe to F ratio to an atomic percentage of 0, 1, 2, and 3 at% during the film deposition. With the optimized Fe co-doping effect induced at Fe 2 at%, the resulting Fe-doped FTO films exhibited simultaneous improvements in the carrier concentration and Hall mobility which are, attributed to Vo induced by a Fe substitution, and the smooth surface morphology interfering with the growth of the FTO crystallites, respectively. As a result, the Fe-doped FTO films prepared at Fe 2 at% achieve a higher transparent conduction (4.4 ± 0.14 Ω/□ sheet resistance and 83.1% optical transmittance) than the other films with a figure of merit of 3.56 × 10−2 Ω−1. Hence, when these unique films are used in EC devices as a transparent conductive oxide, fast switching speeds (6.1 s coloration speed and 5.1 s bleaching speed) and a high coloration efficiency (CE) of 48.2 cm2/C occur as unique effects of the Fe-doped FTO films, accelerating the insertion of Li+ and electrons into the WO3 films, which act as an active EC material and enhancing the electrochemical activity. Therefore, our study provides promising insight into unique transparent conductive oxide films for high-performance EC devices.

Activated carbon has received enormous global attention as an electrode material for electrical double layer capacitors (EDLCs) due to its superior electrochemical performance. But while manufacturing high-quality activated carbon as an electrode material, overcoming limitations such as depletion of raw material resources, poor high-rate performance, and cycle stability is still a significant challenge. To address these limitations, we firstly suggest a novel mesoporous activated carbon derived from ecklonia cava (Meso-ACDE) using an acid-base reaction and KOH activation processes. The optimized Meso-ACDE electrode showed a superior specific capacitance of 182 F g−1 at the current density of 0.2 A g−1 and a high-rate capacitance of 154 F g−1 at the current density of 20.0 A g−1 with an excellent cycling stability up to 2000 cycles. The enhanced electrochemical performance of the Meso-ACDE electrode was mainly attributed to the well-developed mesoporous structure with a high concentration of oxygen related functional groups, which were acquired during the activation process. Thus, we believe that the Meso-ACDE could be a promising alternative to conventional activated carbon for high-performance EDLCs.

In recent years, advanced designs of non-precious electrocatalysts, such as those with transition metals and heteroatoms into iron‑nitrogen-doped mesoporous carbon, have been actively studied to replace precious-metal electrocatalysts for oxygen reduction reaction (ORR), which are used by future energy storage and conversion devices such as metal-air batteries and fuel cells. In the present study, we propose a noble non-precious electrocatalyst through the introduction of fluorine into iron‑nitrogen doped mesoporous carbon. To this end, we synthesized Tri(Fe/N/F)-doped mesoporous carbon nanofiber (MCNF) using electrospinning, the precursor coating method, and carbonization. Tri(Fe/N/F)-doped MCNFs exhibited an improved onset potential of ~0.9 V, the half-wave potential of ~0.82 V, and limiting-current density of −4.76 mA cm−1, with a four-electron pathway. In addition, Tri(Fe/N/F)-doped MCNFs showed remarkable long-term stability and endurance of methanol-crossover. Therefore, Tri(Fe/N/F)-doped MCNFs exhibited improved ORR performance, which could be explained by the increased specific surface area by mesoporous structures and improved oxygen adsorption by the synergy effects by Fe-Nx macrocycles and a high pyridinic- and pyrrolic-N species resulting from F doping.

With the increase in global challenges related to energy depletion, there is significant emphasis on studies involving next-generation optoelectronic applications such as smart windows and electronic displays. In particular, electrochromic devices (ECDs) have been identified as strategic innovations for energy-saving “smart windows” to address these challenges. Despite this increased level of attentions, ECDs have not yet attained broad commercial acceptance because of their limited electrochromic (EC) properties including coloration efficiency (CE, 10.0 s). To address these limitations, critical effort is required to enhance the EC properties by tuning the film structure and electronic structure of ECDs. In this study, we demonstrated the effect of nanocomposite structure of conductive metal oxides and WO3 EC films. Antimony-doped tin oxide nanoparticles (ATO NPs) were utilized because of their superior electrical conductivity and large band gap. To achieve the optimum addition amount of ATO NPs in EC films, we adjusted the amount as 0, 0.6, 1.2, 2.4 wt%. WO3 EC films with the optimum addition amount (1.2 wt%) of ATO NPs exhibited improved EC performance including both the switching speeds (5.4 s for the coloration speed and 2.4 s for the bleaching speed) and CE value (48.2 cm2/C). The enhancement of EC performance was attributed to the well-dispersed ATO NPs in the WO3 films that can effectively improve electrical conductivity via the formation of by forming preferred electron pathway. In addition, the large band gap of ATO NPs broadens the transmittance modulation of the EC layer which contributed to the increment of the CE value. Therefore, our results suggest a strategy to obtain the enhanced WO3 films with superior EC performances using conductive metal oxides nanocomposite structure.

As transparent conducting oxides (TCOs) have been widely used as a common component of many optoelectronic applications, ensuring high conductivity and transparency TCOs has become a pivotal concern. In the present study, we report developing the combustion-activated pyrolysis route of horizontal ultrasonic spray pyrolysis deposition (HUSPD) as a novel strategy to form highly transparent conducting fluorine-doped tin oxide (FTO) films. Compared to the basic route, the combustion-activated FTO films showed an attractive transparent conducting performance (figure of merit of 5.34 × 10−2 Ω−1) with a highly improved optical transparency (90.1%) due to the formation of a smooth and dense film structure to reduce light scattering on the surface, and a decrease of oxygen vacancies to broaden the optical bandgap, all of which yielded an excellent performance as compared to the previously reported studies on the FTO films. Moreover, when the combustion-activated FTO films were used as TCOs of electrochromic devices and dye-sensitized solar cells, they acquired multifunctional effects of (a) an efficient electron transfer by (200) preferred orientations of the FTO; (b) a relaxed light scattering on the interface due to smooth and dense surface morphology of the FTO films; and (c) a broad optical bandgap by decreased oxygen vacancies, resulting in an impressive improvement of both electrochromic and photovoltaic performances. Taken together, our results demonstrate that combustion-activated FTO films are an attractive technique for forming high-performance TCOs that can further be used in multifunctional optoelectronic devices.

In the present study, V2O5 nanorod (VR)/graphene oxide (GO) nanocomposite films that can be used as an electrochromic (EC) material have been developed using the sol-gel spin-coating method. In order to optimize the nanocomposite effect of GO consisting of the VR films on EC performances, we controlled the volume percentage of GO compared to vanadium precursor to 0, 1, 2, and 3 vol%. With the increase of GO in VR films to 2 vol%, the resultant films exhibited a densely percolated structure, which was due to the nanocomposite effect of GO to generate the VR growth by an interaction with the oxygen-containing functional groups of GO. Due to the effect of optimized GO, the VR/GO nanocomposite films fabricated with 2 vol% GO revealed fast switching speeds (2.5 s for the bleaching speed and 1.4 s for the colouration speed). This was the result of an improved electrical conductivity and ion diffusion coefficient and high cycling retention of transmittance modulation (94.90% after 500 cycles) stemming from the remarkable electrochemical stability of the films as compared to those of bare VR films.

Providing a crystallographic and electronic modification of carbon-based materials in electrical double-layer capacitors (EDLCs) is an essential technology needed to improve the charge storage kinetics and to enhance ultrafast cycling performances. However, despite numerous structural composite and morphological modification efforts focused on active materials, ultrafast charge storage kinetics still indicated a poor ultrafast capacitance and low cycling stability. To solve these problems, in the present study, we propose a novel heteroatom (N, P, and B)-doped activated carbon (AC) that has the synergistic effects of N-, P-, and B-doping using the one-pot doping calcination process. Compared to the bare-AC, N-doped AC, P-doped AC, and B-doped AC, the novel heteroatom-doped AC indicates an improved ultrafast charge storage kinetics, such as high specific capacitance (243.9 F g−1 at the scan rate of 10 mV s−1), good cycling stability (216.7 F g−1 at 100 mV s−1 after 500 cycles), and superb ultrafast cycling capacitance (199.7 F g−1 at 300 mV s−1). These superb electrochemical performances can be attributed by synergistic effects of increased active sites by N-doping related to a high charge storage area, improved functional groups by P-doping related to an excellent wettability between the electrode and the electrolyte, and enhanced electrical properties by B-doping related to a good electron acceptability.

Due to its high electrical conductivity and excellent chemical/physical durability, the nickel (Ni) foam is conventionally used as the current collector of supercapacitors that are characterised by high power density, rapid charge/discharge cycles, and long lifespan. However, the limitation of the current collector is its flat surface, which leads to a low rate performance and cycling stability. Therefore, a rational design of the current collector and the electrode material is an essential interfacial engineering technology to be developed to improve the electrochemical performance of the collector. In the present study, applying the surface functionalization of the Ni current collector with the nano-sized stairs of the terraced surface resulted in the electrochemical performance of a remarkable capacitance (210 F g−1 at the current density of 0.5 A g−1), excellent rate performance of 83%, and outstanding cycling stability of 89% after 10,000 cycles. The proposed design has obvious advantages in terms of the high contact area between the current collector and electrode material, on the one hand, and the uneven surface, one the other hand, leading to an excellent rate performance and an outstanding cycling stability. This remarkable capability demonstrates that the surface functionalization of the current collector is a promising technology for high-performance supercapacitors.

The ability to provide a reasonable design and a practical modification of the interfacial structure in lithium-ion batteries (LIBs) is a key technology to enhance electrochemical kinetics and to improve ultrafast cycling performances. However, in recent years, despite considerable structural engineering efforts focused on active materials, in practice, ultrafast LIB performances still are characterized by low ultrafast cycling capacity and poor stability at the current density above 6 C. To solve these problems, in the present study, we propose a unique interface modification of the Al foil having crater porous interface using the electrochemical etching process. The cathode fabricated with a crater porous interface of the Al foil exhibits a remarkably enhanced ultrafast storage performance, such as high ultrafast cycling capacity (91.0 mAh g−1 at 10 C) and superb ultrafast cycling stability (76.2 mAh g−1 with the capacity retention of 84.8% at 10 C after 250 cycles) as compared to those of the cathode fabricated with the bare Al foil and commercial-etched Al foil. These improved electrochemical performances can be explained by the combined effects of improved electrical conductivity related to fast charge transport and increased interface contact area between the active material and the current collector related to enhanced interface adhesion during ultrafast cycling performance.

Characterized by high power density and long cycle life, electric double layer capacitors have been considered to be most promising energy storage devices. However, the use of low electrolyte concentration due to the degradation of current collector in high electrolyte concentration involves their limitation for ionic diffusion at high current densities during cycling, leading to a low electrochemical behavior. Therefore, the sensible design of the interfacial structure between the current collector and the active material is a supreme technology for accomplishing desired requirements as the improved performance of current collector. In the present paper, by applying the graphite nanosheet as an interlayer on the nickel (Ni) current collector, the electrochemical performance is improved with high specific capacitance (236 F g−1 at the current density of 0.2 A g−1), outstanding high-rate ability (87%), and excellent cycling stability (93% after 10,000 cycles in the high-concentration electrolyte). Hence, the advantages of this unique approach include the improvement of the contact area between the current collector and the active material and prevention of the oxidation of the Ni current collector. This enhanced electrochemical performance suggests that this interface engineering is a powerful strategy for potential applications in electric double layer capacitors.

In the present study, Fe-doped V2O5 films showing impressive electrochromic (EC) performance were developed using the sol-gel spin-coating method. To confirm the optimized Fe-doping effect on the V2O5 films for the EC performance, we adjusted the Fe atomic percentages to 0.0, 0.5, 1.0, and 1.5 at%, respectively. With the effect of Fe doping on the V2O5 films, the obtained films resulted in the formation of the oxygen vacancies. As the result, when the optimum Fe atomic percentage was 1.0 at%, the enhanced switching speeds (3.7 s for the bleaching speed and 2.0 s for the coloration speed) and enhanced coloration efficiency value (47.3 cm2/C) compared to the other films were implemented. This can be attributed to the improved electrical conductivity and Li+ diffusion coefficient that led to efficient generation of the EC reaction activity and narrowing the optical bandgap at the coloration state to increase transmittance modulation. Therefore, this unique film can be a promising EC material to improve the performance for the EC devices.

Due to their excellent chemical stability, as well as low operating temperature, high energy density, and environment-friendliness, carbon supports are a prospective candidate for platinum (Pt) nanocatalysts in direct methanol fuel cells (DMFCs). However, numerous efforts to achieve the high efficiency for the energy conversion by carbon supports have faced considerable challenges owing to an inefficient utilization of the inside region, leading to the low electrochemical performance. Thus, in the present study, we propose an advanced surface technology for the mesoporous structure. The obtained Pt nanocatalyst-decorated mesoporous carbon nanofiber support offers a high anodic current density of 732 mA mgPt−1, and an excellent catalytic stability as compared to the commercial Pt/C (20 wt% Pt on Vulcan carbon, De Nora S.P.A.) and Pt/CNF. Due to these characteristics, this advanced carbon support provides several, benefits such as the well-dispersed Pt nanocatalysts on the surface, as well as achieves a superb catalytic stability. In sum, the advanced carbon support is a promising candidate to improve the electrochemical performance of DMFCs.

Carbon has received an intensive consideration in view of its application as an anode in lithium storage and is characterized by high electrical conductivity, excellent chemical and physical properties, and outstanding stability for insertion and deinsertion of Li ions. However, the due to the high-cost production requiring a high temperature process, a limited storage capacity, and a poor rate capability. In the present study, we suggest a novel protein as a raw material of carbon using simply carbonization. The nitrogen-doped carbon indicates the nitrogen (N)-doped sites with graphitic–N and pyridinic–N sites, as well as high crystallizability. The optimized electrode delivers an excellent cycling stability (284 mA h g−1 after 100 cycles at 100 mA g−1), an impressive rate performance (154 mA h g−1 at 2000 mA g−1), and a remarkable ultrafast cycling stability (112 mA h g−1 after 500 cycles at 2000 mA g−1). Therefore, this unique nitrogen-doped carbon offers attractive advantages in terms of the functional N-doped sites, a simple fabrication process, and a low-cost production.

In this paper, a battery-supercapacitor system is developed and its electrochemical performance is investigated. The battery-supercapacitor system is composed of a separated LiFePO4/activated carbon cathode and a separated Li4Ti5O12/activated carbon anode onto both sides of a piece of aluminum foil. We demonstrated the superior electrochemical performance of this battery-supercapacitor system, such as its energy density of 4.9–48.5 Wh/kg, power density of 167.7–5243.2 W/kg, rate capability of 73.9% at a current density of 20 A and cycle life (91.5% after 1800 cycles) which outperforms that of a hybrid supercapacitor. This can be explained by the synergistic effect of a Faradaic and non-Faradaic system in a single cell. The results clearly show that the battery-supercapacitor system, including a LiFePO4 cathode/Li4Ti5O12 anode and an activated carbon anode/activated carbon cathode, has great potential for use in advanced energy storage devices.

Due to their excellent mechanical durability and high electrical conductivity, carbon and silicon composites are potentially suitable anode materials for Li-ion batteries with high capacity and long lifespan. Nevertheless, the limitations of the composites include their poor ionic diffusion at high current densities during cycling, which leads to low ultrafast performance. In the present study, seeking to improve the ionic diffusion using hydrothermal method, electrospinning, and carbonization, we demonstrate the unique design of excavated carbon and silicon composites (EC/Si). The outstanding energy storage performance of EC/Si electrode provides a discharge specific capacity, impressive rate performance, and ultrafast cycling stability.

Carbon nanofibers (CNFs) are used as active materials for electrodes in various energy devices, such as lithium ion secondary batteries, supercapacitors, and fuel cells. Recent studies have shown that nanoscale coatings on carbon nanotubes increase the output and lifespan of these devices owing to the improvement of their mechanical and chemical properties. Among various coating methods, atomic layer deposition (ALD) can adjust the thickness of the coating layer conformally without any directional growth. Therefore, ALD can coat particles with high aspect ratios, such as CNFs, even at nanometer levels of thickness. In this work, we grew two different morphologies of a SnO2 layer on CNF. We used two types of ALD equipment: flow-type ALD (static ALD), and fluidized bed reactor-type ALD (dynamic ALD). Static ALD could form a discontinuous SnO2, while a uniform SnO2 layer was formed by pre-inserting a layer of Al2O3. On the other hand, dynamic ALD formed a uniform SnO2 layer without pre-insertion of an Al2O3 layer. X-ray photoelectron spectroscopy analysis revealed that both Sn4+ and Sn2+ were present in SnO2 on the CNF deposited by static ALD, probably due to the formation of an interfacial layer between the SnO2 and CNF. When the dynamic ALD method was used, only Sn4+ was present in the SnO2 on CNF. Cyclic voltammetry analysis was performed to characterize the electrochemical properties of the SnO2-coated CNF as an electrode on a direct methanol fuel cell. It was revealed that the discontinuous SnO2 on CNF deposited by static ALD showed the highest current efficiency as well as enhanced electrocatalytic stability.

Owing to their superb mechanical durability resulting from the dramatic volume changes of the Sn nanoparticles and high electrical conductivity, carbon and tin (Sn) nanocomposites have received an increasing attention in view of their application as anode materials for lithium ion batteries (LIBs). However, due to the poor ionic diffusion capability for Li ions during the cycling, the low ultrafast performance for energy storage remains rather limited. In the present study, aiming to improve the ionic diffusion capability for Li ions, we suggest a novel design of the debossed structure of carbon and Sn nanocomposites by electrospinning, carbonization, and the debossing process. The electrode based on the debossed structure exhibits a noticeable cycling stability and high discharge capacity (677 mA h g−1 after 100 cycles at 100 mA g−1), an excellent rate capability (482 mA h g−1 at 2000 mA g−1), and an outstanding ultrafast cycling stability (275 mA h g−1 after 500 cycles at 2000 mA g−1). Therefore, this novel design of the debossed structure based on carbon and Sn nanocomposites offers attractive effects, such as the effective accommodation of dramatic volume changes for the Sn nanoparticles, as well as an improved ionic diffusion performance of Li ions.

Carbon has acquired considerable attention in view of its application as supports for platinum (Pt) catalyst in direct methanol fuel cells (DMFCs) with promising renewable energy source due to their high surface area and excellent chemical stability. However, the progress of carbon supports still needs to move towards the practical utilization of high-performance DMFCs. In the present study, we propose a novel support of nitrogen (N)-doped carbon and nickel (Ni) composites produced from protein using an impregnation process and carbonization to increase the electrical conductivity. To this end, we fabricated the Pt nanoparticles on N-doped carbon and Ni composites (Pt@NC/Ni). To obtain the optimized electrochemical performance, the amount of Ni components into carbon supports was controlled by three types. Specifically, as compared to commercial Pt/C and other samples, the optimized Pt@NC/Ni with the high electrical conductivity of 0.75 S cm−1 shows the lowest onset potential of 0.03 V, the highest anodic current density of 744 mA mgPt−1, and an excellent catalytic stability with the highest retention rate of 86%. Accordingly, this novel support provides multiple advantages in terms of the well-dispersed Pt nanoparticles on the surface, N-doping effect of carbon supports, and an increased electrical conductivity by the introduction of Ni components. Therefore, Pt@NC/Ni is a promising novel catalyst to enhance electrochemical performance of methanol oxidation reaction.

This study describes the radiolytic fabrication of large Pt nanoparticles (NPs)-decorated reduced graphene oxide (L-Pt/rGO) and its use as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). A homogenous aqueous solution of GO in 10 wt% isopropyl alcohol (IPA) aqueous solution containing Pt precursor was irradiated with γ-ray at room temperature to prepare the L-Pt/rGO. The analytic results from TEM, EDX, XPS, and Raman revealed that the rGO decorated with 100 nm Pt NPs was successfully formed by the γ-ray irradiation-induced reduction of both the GO and Pt precursor in aqueous IPA solution. Based on the results of the DSSCs performance test, the energy conversion efficiency of the DSSCs with the L-Pt/rGO-based CE outperformed that with the rGO-based one due to the lower sheet resistance, and even was comparable to that of the Pt-based CE. This L-Pt/rGO fabricated by a simple, room temperature and scalable radiolytic method can be used as a promising CE material for low-cost and high-performance DSSCs.

In recent years, owing to the capability to reversibly adjust transparency, reflection, and color by the low electric field, electrochromic devices (ECDs) have received an extensive attention for their potential use in optoelectronic applications. However, considering that the performances of the ECDs, including coloration efficiency (CE, 10.0 s), are still low for an effective applied use, critical efforts are needed to push the development of a unique nanostructure film to improve electrochromic (EC) performances. Specifically, as the large-scale applications (e.g. refrigerators, vehicles, and airplanes) of the ECDs have been recently developed, the study for improving switching speed is urgently needed for commercialization of the devices. In this context, the present study reports a novel nanostructure film of Fe-doped WO3 films with highly developed mesopores and oxygen vacancy defects, fabricated using the Fe agent and the camphene-assisted sol-gel method. Fe-doped WO3 films with highly developed mesopores and oxygen vacancy defects show remarkable EC performances with both fast switching speed (2.8 s for the coloration speed and 0.3 s for the bleaching speed) and high CE (71.1 cm2/C). These two aspects contribute to the synergistic effects of optimized Fe doping and camphene on the films and have outstanding values as compared to previously reported results of WO3-based materials. Specifically, the fast switching speed is attributed to the shortened Li+ diffusion pathway of the highly developed mesopores; and the other is the improved electrical conductivity of the highly increased oxygen vacancy defects. In addition, the high CE value is due to an efficient charge transport as the result of a more effective electroactive contact of the morphology with highly developed mesopores, resulting in a large transmittance modulation with a small intercalated charge density.

Owing to the characteristics of high energy density, low operating temperature, and environmentally-friendly features, direct methanol fuel cells (DMFCs) are a promising renewable energy source. However, the electrocatalysts of the anode are vulnerable in terms of their electrochemical performance, as they can be easily toxified by CO and other hydrocarbons, which might lead to a break-up of the methanol oxidation reaction (MOR). For further advances in the DMFC industry with improved electrochemical performance, this issue should be urgently resolved. Thus, this study proposes a novel approach to synthesize protein-based carbon as platinum electrocatalyst supports (PCPs) from tofu using a carbonization for the improved methanol oxidation activities. Among commercial Pt/C and other samples, the composite loaded 10 wt% Pt electrocatalyst showed the highest anodic current density of 510 mA mgPt−1, the excellent electrocatalytic stability, and the highest retention of 86%. The improved electrochemical performances can be attributed to the good dispersion of Pt electrocatalysts and N-doping effect of protein-based carbon supports. These results suggest that PCPs derived from tofu will be one of promising candidates as platinum catalyst supports to improve methanol oxidation activities.

In the present study, we fabricated fluorine-doped tin oxide (FTO) films with different sheet resistances (~10 Ω/□, ~6 Ω/□, and ~3 Ω/□) prepared through the adjustment of deposition time during the horizontal ultrasonic spray pyrolysis deposition (HUSPD) and investigated the effect of electrochromic (EC) performances with different sheet resistances of the FTO films used as transparent conducting electrodes. The results demonstrated that, owing to the increased electrochemical activity, the decrease of sheet resistance accelerated switching speeds of the EC devices. However, for the coloration efficiency (CE), the FTO films with the optimum sheet resistance of ~6 Ω/□ exhibited the highest value as compared to the other samples. The improvement of the CE value can be mainly attributed to high transmittance modulation by the uniform surface morphology of the FTO films to reduce interfacial light-scattering between the WO 3 films and FTO films. Therefore, our results provide a valuable insight into the improvement of the performance of the EC devices using the optimum sheet resistance (~6 Ω/□) of the FTO films.

We prepare granule Li4Ti5O12/carbon nanotube composite (LTO/CNT) using intermediate Li2TiO3 and then used as an anode of hybrid supercapacitors. The hybrid supercapacitors fabricated with LTO/CNT anode with 3 wt% CNT and activated carbon (AC) cathode deliver superior electrochemical performances. It is mainly ascribed to the reduced charge-transfer resistance and improved ionic and electronic conductivity, resulting from smooth and rapid lithium ion kinetics and electron diffusion by suppression of particle growth and aggregation. Moreover, elastic properties of CNT can alleviate the volume change of LTO during charge-discharge process, resulting in excellent cycle performance. The performance improvement such as discharge capacitance, rate capability, and cyclability as well as energy and power density demonstrate that 3 wt% CNT addition in LTO anode appeal for high performance hybrid supercapacitors.

The design of a low-cost, stable, and highly efficient electrocatalyst using a non-precious material and carbon composites for oxygen reduction reaction (ORR) activity to replace platinum-based electrocatalyst is essential for future energy conversion devices, such as fuel cells and metal air batteries. However, previous efforts to acquire the high ORR activity by non-precious material and carbon composites faced substantial challenges due to a few active sites during electrochemical reactions. Herein, we synthesize an advanced composite of iron carbide nanoparticles on nitrogen and cobalt-doped carbon nanofiber (Fe3C/N@Co-doped CNF) by electrospinning, a precures coating process and carbonization. Fe3C/N@Co-doped CNF offers a high onset potential of 0.9 V, high half-wave potential (E1/2) potential of 0.8 V, and a nearly four-electron pathway (n = 3.9). Therefore, this unique composite provides multi-active sites using the doping system and metal carbide nanoparticles for the ORR activity, as well as an outstanding tolerance to methanol crossover.

In this study, barnacle-like manganese oxide (MnO 2 ) decorated porous carbon nanofibers (PCNF) were synthesized using electrospinning and the chemical precipitation method for high-performance asymmetric supercapacitors. The porous structure of PCNF was acquired using poly(styrene- co -acrylonitrile) in the electrospinning solution. In order to obtain the optimized barnacle-like MnO 2 on PCNF (MnO 2 -PCNF), the barnacle-like MnO 2 was synthesized using different synthetic times (namely, 1.5, 3.0, and 7.0 min) of the chemical precipitation. Among them, the optimized MnO 2 -PCNF for 3.0 min exhibited the well-dispersed MnO 2 on the PCNF with the nano-size of 190–218 nm. The optimized MnO 2 -PCNF showed the superior specific capacitance of 209.8 F g −1 at 10 mV s −1 and the excellent high-rate performance of 160.3 F g −1 at 200 mV s −1 with the capacitance retention of 98.7% at 100 mV s −1 for 300 cycles. In addition, electrochemical performances of asymmetric cell (constructed activated carbon and MnO 2 -PCNF) showed the high specific capacitance of 60.6 F g −1 at the current density of 0.5 A g −1 , high-rate capacitance of 30.0 F g −1 at the current density of 10 A g −1 , and the excellent energy density of 30.3–15.0 Wh kg −1 in the power density range from 270 to 9000 W kg −1 . The enhanced electrochemical performance can be explained by the synergistic effects of barnacle-like MnO 2 nanoparticles with a high active area related to high specific capacitance and well-dispersed MnO 2 with a short ion diffusion length related to the excellent high-rate performance.

Hybrid supercapacitors are being studied as a next-generation energy storage device that bridge supercapacitor and lithium-ion battery. However, an imbalance of two electrodes of hybrid supercapacitors, especially due to the slow lithium ion kinetics at anode is the one of the most important problems. To solve this problem, we have demonstrated optimized hybrid supercapacitor performance by applying the punched granule Li 4 Ti 5 O 12 (P-LTO) via mechanical punching as a high power anode. The hybrid supercapacitors based on P-LTO anode and activated carbon delivers not only high specific capacity of 57 F/g and excellent long-term cycling of 99% after 14,000 cycles but also remarkable rate capability of 90.6% even at a high rate of 4 A/g, mainly due to i) increase in the total effective contact area, ii) efficient electrolyte impregnation and iii) effective dispersion of hydrogen fluoride attack.

In the present study, in order to demonstrate the one-pot synthesis of aluminum oxide (Al 2 O 3 ) coating and aluminum doping, we synthesized aluminum oxide (Al 2 O 3 )-coated LiAl x Mn 2-x O 4 (LAMO) NPs using a sequential process of the as-spun nanofiber templates, chemical precipitation, and calcination as a cathode material in lithium ion batteries (LIBs). To find the optimum condition of Al 2 O 3 coating layer and Al doping, we performed the simple calcination methods at 300 °C using the Al(OH) 2 -coated LMO NPs. The resultant Al 2 O 3 -coated LAMO NPs exhibited the highest capacity of 111.1 mAh g −1 with the capacity retention of 94.4% after 90 cycles at 1 C, excellent rate performance, and the highest high-rate capacity of 81.4 mAh g −1 at 10 C as compared to bare LMO NPs without Al 2 O 3 coating and Al(OH) 2 -coated LMO NPs without calcination. The improved electrochemical performance can be defined by the co-effect of Al 2 O 3 coating and Al doping on bare LMO NPs. The former is related to cycle stability that increased due to the prevention of volume expansion and Mn dissolution as a physical buffer layer. The latter is related to high-rate performance improved due to the enhanced bonding energy of Al O bond. Therefore, it can be concluded that Al 2 O 3 -coated LAMO NPs are promising candidate cathode materials for high-performance LIBs.

The existing synthetic approaches for high-performance WO3-based electrodes require energy-intensive instrumentation and complex processing, which hinder the development of flexible electrochromic (EC) energy storage devices. Herein, new low-temperature-synthesized amorphous (a)-quantized WO3·H2O films for application in EC energy storage devices are proposed. The WO3·H2O films are fabricated by the spontaneous hydrolysis of a spin-coated WCl6 solution by the water molecules in the surrounding atmosphere followed by annealing at 80 °C. This is an original and unique concept in that the induced quantization of a-WO3 increases the number of electroactive sites, provides abundant oxygen vacancies, and widens the band gap, while the intercalated water molecules stabilize the structure, resulting in efficient charge transfer and stress alleviation during electrochemical reactions. Consequently, the transmittance (53.8% at 633 nm) and specific capacitance (78.5 F g−1 at 1 A g−1) of the flexible electrodes improves. Additionally, the carrier concentration and mobility increase due to a-WO3 quantization, thereby increasing the electrical conductivity, resulting in the rapid switchability (3.7 s for coloration and 2.9 s for bleaching) and high rate capability of the flexible electrodes. The flexible solid-state devices light a 1.5 V white-light-emitting diode and maintain their good EC energy storage performance (76.1% transmittance modulation and 73.1% specific capacitance) even after 300 bending cycles at a bending radius of 1.3 cm. Therefore, the proposed a-quantized WO3·H2O films are promising active electrodes for flexible EC energy storage devices.

We newly developed amorphous WO3·H2O (a-WO3·H2O) films with porosity and oxygen vacancy (VO) defects through a humidity adjustment causing the accelerated hydrolysis of WOCl4 with H2O during spin-coating and during low-temperature annealing for flexible electrochromic (EC) energy-storage devices. Optimizing the hydrolysis effect in all a-WO3·H2O films, we adjusted the humidity to 25, 35, and 45% in a humid chamber. Specifically, the a-WO3·H2O film fabricated at 35% exhibited a developed porous morphology and an increased number of VO defects, providing increased electrochemically active sites and enhanced electrical conductivity, respectively, due to the accelerated hydrolysis of WOCl4 and the increased intercalation of water molecules. Such behaviors of the a-WO3·H2O film bring about superior flexible EC energy-storage performances of widened transmittance modulation (60.0% at 633 nm), fast switching speeds (3.4 s for coloration speed and 4.2 s for bleaching speed), a high CE (62.7 cm2/C), good specific capacitance (94.2 F/g at 2 A/g), and rate capability (74.3%). Specifically, the increased transmittance modulation and specific capacitance stem from the increased electrochemical activity caused by the enriched electrochemically active sites. Moreover, the fast switching speeds and good rate capability are generated by electrochemical kinetics improved with the porous morphology and the increased VO.

Nitrogen-doped carbon quantum dots (N-CQDs) decorated on platinum (Pt) nanoparticles (Pt@N-CQDs) were synthesized for use as catalysts in the oxygen reduction reaction (ORR). Pt@N-CQDs were prepared by hydrothermal and reduction methods, and an optimum concentration of N-CQDs was demonstrated. The weight percentage of CQDs to Pt was tested at 10, 20, and 30 wt% to optimize the ORR performance. In particular, Pt@20N-CQDs with 20 wt% N-CQDs exhibit superior electrochemical performance, such as onset potential (Eonset) of ~0.925 V, half-wave potential (E1/2) of ~0.834 V, and limited-current density of −3.83 mA cm−2 at 0.5 V. In the durability test for ORR catalytic activity, Pt@20N-CQDs showed low potential degradation and superior long-term stability of 20 mV (90.6%) at E1/2 after 5000 cycles in 0.1 M KOH electrolyte. These performance improvements are owing to improved electrical properties by N doping in the CQDs and the increased number of active sites with oxygen-containing functional groups.

Interface engineering of LiMn2O4 (LMO) is a promising strategy to enhance the lithium storage capability and cycling stability of cathode materials in Li-ion batteries (LIBs). This is because the strategy prevents structural degradation; however, Li storage kinetics remains unsatisfactory, resulting in poor ultrafast cycling performance. Therefore, we fabricated an Nb-doped TiO2 (NTO) functional layer as a conductive passivation layer on the LMO surface by horizontal ultrasonic spray pyrolysis deposition. The NTO functional layer suppressed the volume expansion of LMO and exhibited high electrical and ionic conductivity, which resulted in improved structural stability of LMO (related to cycling stability) and increased electron/ion transfer rate (related to ultrafast cycling performance). In the TiO2 structure, Ti4+ ions were replaced by Nb5+ ions, which possess high electrical conductivity and a wide c-axis as a Li-ion diffusion route. As a result, the NTO-coated LMO cathode material showed an outstanding specific capacity of 112.7 mAh/g with a remarkable capacity retention of 96.2% after 100 cycles at a current density of 1 C and excellent ultrafast cycling capacity and stability of 70.0 mAh/g after 500 cycles at a current density of 10 C.

The interface engineering of anode electrodes in lithium-ion batteries (LIBs) is a key strategy in improving the Li-ion storage kinetics and interface stability to achieve outstanding ultrafast cycling capacities and cycling stabilities of LIBs. However, despite extensive research on the interfacial engineering of electrode materials, studies on the interface design between the electrode and current collector to improve the ultrafast cycling performance are rare. In this study, we designed a novel interface architecture for a fluorine-doped carbon quantum dot (F-CQD) interfacial layer on a stockade-like etched Cu foil via electrochemical modification and a spray coating process. The anode electrode assembled with the resultant Cu foil showed enhanced adhesion, high reaction kinetics, and excellent interface stability between the electrode and Cu foil due to the F-CQD interfacial layer on the stockade-like etched Cu foil, leading to an improved ultrafast cycling performance. Consequently, the novel architecture of a Cu foil having stockade-like etching patterns with an F-CQD interfacial layer showed an increased ultrafast cycling capacity of 82.9 mAh g−1 and excellent ultrafast cycling stability of 94.1% after 500 cycles under ultrafast cycling conditions. These improved ultrafast cycling performances are due to the high contact area between the electrode and Cu foil, excellent reaction sites, and superb corrosion resistance.

Owing to the increase in need for high-performance energy storage devices, activated carbon has attracted considerable attention as an electrode material for electrical double-layer capacitors (EDLCs). However, the conventional manufacturing process of activated carbon is seriously limited by the depletion of raw material sources. Biomass-based activated carbon, which is currently considered as a breakthrough, exhibits performance limitations such as low specific capacitance and poor high-rate performance. In this study, we propose the use of fluorine-doped mesoporous carbon (F-MAC), which is derived from Ecklonia cava using fluorine impregnation and KOH activation, to address the above challenges. F-MAC exhibits high specific capacitance (184 F/g), excellent high-rate performance (155 F/g at a current density of 20 A/g), and superior cycle stability (82.8% capacitance retention after 2,000 cycles). These performance improvements are attributed to the increased surface area, high mesopore volume fraction, fluorine-doping effect, and high concentration of oxygen functional groups.

Recently, transition metal dichalcogenides (TMDs) have attracted considerable attention as anode materials in ultrafast lithium ion batteries because of their high theoretical capacity and outstanding ion diffusion kinetics. Despite these remarkable properties, TMDs exhibit fast capacity fading and insufficient Li storage kinetics, owing to the excessive volume expansion and low electric/ionic transfer rate. The aim of this study is to reinforce the structural stability and Li storage kinetics of TMDs through the use of well-dispersed CoS2 and WS2 ultrasmall particles (USPs) embedded in hierarchical porous carbon nanofibers, including micro/mesoporous composite sturctures. As expected, this architecture offers a high specific capacity (718.0 mAh g−1) with the capacity retention of 93.4% after 100 cycles at 0.1 A g−1 owing to increased Li storage sites and prevention of volume expansion of CoS2 and WS2 USPs. In particular, a remarkable fast discharge capacity (444.5 mAh g−1) with the capacity retention of 90.2% after 1000 cycles are noted. These results are related to the high number of Li ion storage sites, effective prevention of volume expansion of well-dispersed CoS2 and WS2 USPs, short Li ion diffusion length, and favorable Li ion acceptability, which is caused by the hierarchical porous structure containing meso/micropores.

Carbon materials as intra-electrode frameworks for energy storage devices have received noticeable attention due to their high porosity, high electrical conductivity, and excellent chemical and physical stability. Nevertheless, the utilisation of these frameworks still faces significant challenges such as limited raw material resources, complicated synthetic process, high-temperature synthesis, and high production costs. Herein, we report a unique and facile approach to synthesise a spoilt tofu-derived carbon framework with embedded ultrasmall Sn nanocrystals (SCS) derived from food waste (such as spoilt tofu) using simple impregnation and carbonisation. The unique architecture of SCS was due to the porous structure of spoilt tofu and was utilised as an anode for Li-ion batteries. The optimised SCS architecture shows excellent electrochemical performance with outstanding cycling stability (621 mA h g −1 capacity retention up to 100 cycles) and excellent high-rate performance (250 mA h g −1 at 2000 mA g −1 ). Thus, this facile approach provides helpful synergistic effects in terms of structural stability, electrochemical active surface area, and shorter diffusion pathways for Li ions. Consequently, the recycling strategy of spoilt tofu food waste could provide a unique route to low-cost production of high-performance Li-ion batteries.

Activated porous carbon (APC), which is used as the electrode material in electrical double layer capacitors (EDLCs), is constantly being developed to enhance its electrochemical performance with low cost. Nevertheless, APC still encounters important challenges due to the limited amount of raw material resources. Herein, we propose a novel and simple approach to synthesize APC from tofu using a simple synthesis process involving carbonization with KOH activation. To obtain the optimized APC, three types of APCs are prepared using different weight ratios of 1:3, 1:4, and 1:5 (WTofu/WKOH) during the carbonization. The optimized APC electrode shows an outstanding specific capacitance of 207 F g−1 at a current density of 0.1 A g−1, a remarkable high-rate performance, an impressive energy density of 25.7–19.9 W h kg−1, and an excellent cycling stability for up to 2000 cycles. Therefore, this unique approach to obtain APC using tofu yield a material with some useful qualities in terms of an increased number of electroactive sites based on a high surface area, and an improved wettability based on the oxygen-containing functional groups on the surface of the APC. Accordingly, tofu-based APC is a promising alternative to conventional APC.

Silicon and carbon composites as an anode material in lithium-ion batteries are the most promising candidates to replace conventional graphite, owing to their synergetic effects of high capacity and excellent mechanical stability. Despite these appealing merits, the significant challenge is directly related to the poor dispersion of Si nanoparticles in the carbon nanofiber (CNF) matrix, leading to a structural collapse after cycling due to the large volume change (∼300%) of Si. Thus, we synthesized unique composites of high-dispersed Si and titanium oxide (TiO2) nanoparticles in the CNF matrix (Si/TiO2/CNF) using the complexation of TiO2 nanoparticles in order to the high-dispersed Si nanoparticles. The Si/TiO2/CNF electrode presents enhanced electrochemical properties including excellent cycling stability and high specific capacity (947 mA h g−1 at 100 mA g−1 after 100 cycles), remarkable high-rate performance (612 mA h g−1 at 2000 mA g−1), and outstanding ultrafast cycling stability (478 mA h g−1 at 2000 mA g−1 after 100 cycles). It is revealed that the high-dispersed Si nanoparticles can maintain the initial structure after cycling, which definitely demonstrates the superiority of our concept.

We fabricated transparent indium tin oxide (ITO)/antimony tin oxide (ATO) electrodes using a combined process of spin-coating of hybrid ITO nanoinks, electrospraying of ATO, and hydrogen (H2) activation carried out at a low annealing temperature of 200 °C. The produced ITO electrode exhibited an enhanced surface densification and phase conversion of In(OH)3 to ITO. As a result, the H2-activated ITO/ATO electrodes exhibited excellent transparent conducting performances with a superior sheet resistance of ~47.5 Ω/□ and a good transmittance of ~85.3% as compared to the ITO and ITO/ATO electrodes. Despite the use of the low annealing temperature, the achieved improvement in the conducting performance could be attributed to the synergistic effect of the enhanced carrier concentration and the Hall mobility related to the improved surface densification achieved with the electrosprayed ATO thin film and reduction of the residual In(OH)3 phase by H2 activation. Therefore, our method can be used as a novel strategy for obtaining high-performance solution-processed transparent conducting oxides at a low annealing temperature of 200 °C for use in various optoelectronic applications.

To improve the electrochemical performance of hybrid capacitors, hollow lithium manganese oxide (LiMn 2 O 4 , LMO) nanotubes (NTs) as cathode materials were synthesized by a solid-state reaction, using MnO 2 coated on a porous carbon nanofiber (PCNF) templates. To determine the optimum shell thickness of hollow LMO, the time of MnO 2 coating on PCNF was adjusted to 10, 30, and 60 min. Among these, hollow LMO NTs, which were synthesized by 30-min coating with MnO 2 on the PCNFs, have superior performance. They exhibited an excellent reversible capacity (∼72.8 mAh g −1 ) at 1 C, capacity retention of ∼98.4% after 100 cycles, and an excellent high-rate capability. This superior performance can be explained by the hollow structure giving a reduced diffusion distance for Li-ions, the networked structure of one-dimensional NTs allowing fast charge transfer, and the achievement of the optimal stoichiometric ratio of the LMO phase.

Great efforts have been devoted for the synthesis of Li 4 Ti 5 O 12 nanoparticles as anode material, because particle size less than 100 nm is the main factor for achieving a high rate capability. In this study, we fabricated nano-sized Li 4 Ti 5 O 12 by high-energy milling operated at 3500 rpm, that it was treated afterwards by post-calcination to improve its pulverized crystallinity. Post-calcination at moderate temperature is found to be effective for improving the electrochemical properties of Li 4 Ti 5 O 12 prepared by mechanical pulverization. Finally, a 100 nm-sized Li 4 Ti 5 O 12 was successfully obtained by optimizing the high-energy milling passing 10 times in chamber and setting post-calcination to 700 °C. The specific capacity significantly depended on the crystallinity of Li 4 Ti 5 O 12 after treated by the post-calcination. The 100 nm-sized Li 4 Ti 5 O 12 showed high-rate capability and good capacity retention of 99.9% after 1500 cycles. It can be explained by the synergistic effect of reduced particle size and improved crystallinity.