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Hierarchical 3D CoP micro-pompon assembled by 1D-nano-needle-array decorated 2D-micro-sheets provided facile charge transfer and lots of electrocatalytic active sites to deliver decent DSSC performance (8.80% at 1 sun; 17.27% at 7 klux).

Supercapacitors have gained significant attention owing to their exceptional performance in various applications, such as mobile devices, electric vehicles, and renewable energy storage systems.

The performance of supercapacitors strongly depends on the electrochemical characterizations of electrode materials. Herein, a composite material consisted of iron(III) oxide (Fe2O3) and multilayer graphene-wrapped copper nanoparticles (Fe2O3/MLG-Cu NPs) is fabricated on a flexible carbon cloth (CC) substrate via two-step synthesis process for supercapacitor application. Where, MLG-Cu NPs are prepared on CC by one-step chemical vapor deposition synthesis approach; thereafter, the Fe2O3 is further deposited on the MLG-Cu NPs/CC via successive ionic layer adsorption and reaction method. The related material characterizations of Fe2O3/MLG-Cu NPs are well investigated by scanning electron microscopic, high resolution transmission electron microscopy), Raman spectrometer and X-ray photoelectron spectroscopy; the electrochemical behaviors of the pertinent electrodes are studied by cyclic voltammogram, galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy measurements. The flexible electrode with Fe2O3/MLG-Cu NPs composites exhibits the best specific capacitance of 1092.6 mF cm−2 at 1 A g−1, which is much higher than those of electrodes with Fe2O3 (863.7 mF cm−2), MLG-Cu NPs (257.4 mF cm−2), multilayer graphene hollow balls (MLGHBs, 14.4 mF cm−2) and Fe2O3/MLGHBs (287.2 mF cm−2). Fe2O3/MLG-Cu NPs electrode also exhibits an excellent GCD durability, and its capacitance remains 88% of its original value after 5000 cycles of the GCD process. Finally, a supercapacitor system consisted of four Fe2O3/MLG-Cu NPs/CC electrodes can efficiently power various light-emitting diodes (i.e. red, yellow, green, and blue lights), demonstrating the practical application of Fe2O3/MLG-Cu NPs/CC electrode.

Materials with different nanostructures can have diverse physical properties, and they exhibit unusual properties as compared to their bulk counterparts. Therefore, the structural control of desired nanomaterials is intensely attractive to many scientific applications. In this brief review, we mainly focus on reviewing our recent reports based on the materials of graphene and the transition metal chalcogenide, which have various low-dimensional nanostructures, in relation to the use of electrocatalysts in electrochemical energy applications; moreover, related literatures were also partially selected for discussion. In addition, future aspects of the nanostructure design related to the further enhancement of the performance of pertinent electrochemical energy devices will also be mentioned.

Novel polymorphic MoxW1-xTe2-based counter electrodes possess high carrier mobility, phase-dependent lattice distortion, and surface charge density wave to boost the charge-transfer kinetics and electrocatalytic activity in dye-sensitized solar cells (DSSCs). Here, we report the syntheses of stoichiometry-controlled binary and ternary MoxW1-xTe2 nanowhiskers directly on carbon cloth (CC), denoted by MoxW1-xTe2/CC, with an atmospheric chemical vapor deposition technique. The synthesized MoxW1-xTe2/CC samples, including 1T'-MoTe2/CC, Td-WTe2/CC, Td-Mo0.26W0.73Te2.01/CC, and 1T'- & Td-Mo0.66W0.32Te2.02/CC, were then employed as different counter electrodes to study their electrochemical activities and efficiencies in DSSCs. The photovoltaic parameter analysis manifests that MoxW1-xTe2/CCs are more stable than a standard Pt/CC in the I-/I3- electrolyte examined by cyclic voltammetry over 100 cycles. A 1T'- & Td-Mo0.66W0.32Te2.02/CC-based DSSC can achieve a photocurrent density of 16.29 mA cm-2, a maximum incident photon-to-electron conversion efficiency of 90% at 550 nm excitation, and an efficiency of 9.40%, as compared with 8.93% of the Pt/CC counterpart. Moreover, the 1T'- & Td-Mo0.66W0.32Te2.02/CC shows lower charge-transfer resistance (0.62 Ω cm2) than a standard Pt/CC (1.19 Ω cm2) in electrocatalytic reactions. Notably, MoxW1-xTe2 nanowhiskers act as an electron expressway by shortening the path of carrier transportation in the axial direction from a counter electrode to electrolytic ions to enhance the reaction kinetics in DSSCs. This work demonstrates that the nanowhisker-structured 1T'- & Td-Mo0.66W0.32Te2.02/CC with high carrier mobility and robust surface states can serve as a highly efficient counter electrode in DSSCs to replace the conventional Pt counter electrode for electrocatalytic applications.

Boron nitride (BN) is newly introduced as a non-metal electro-catalyst for the counter electrode of a dye-sensitized solar cell (DSSC). By applying a conductive binder of sulfonated poly(thiophene-...

The performance of supercapacitors strongly depends on the electrochemical characterizations of electrode materials. Herein, a composite material consisted of polypyrrole (PPy) and multilayer graphene-wrapped copper nanoparticles (PPy/MLG-Cu NPs) is fabricated on a flexible carbon cloth (CC) substrate via two-step synthesis process for supercapacitor application. Where, MLG-Cu NPs are prepared on CC by one-step chemical vapor deposition synthesis approach; thereafter, the PPy is further deposited on the MLG-Cu NPs/CC via electropolymerization. The related material characterizations of PPy/MLG-Cu NPs are well investigated by scanning electron microscopic, high resolution transmission electron microscopy, Raman spectrometer and x-ray photoelectron spectroscopy; the electrochemical behaviors of the pertinent electrodes are studied by cyclic voltammogram, galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements. The flexible electrode with PPy/MLG-Cu NPs composites exhibits the best specific capacitance of 845.38 F g−1 at 1 A g−1, which is much higher than those of electrodes with PPy (214.30 F g−1), MLG-Cu NPs (6.34 F g−1), multilayer graphene hollow balls (MLGHBs; 52.72 F g−1), and PPy/MLGHBs (237.84 F g−1). Finally, a supercapacitor system consisted of four PPy/MLG-Cu NPs/CC electrodes can efficiently power various light-emitting diodes (i.e. red, yellow, green and blue lighs), demonstrating the practical application of PPy/MLG-Cu NPs/CC electrode.

Nowadays, the requirement of energy increases every year, however, the major energy resource is fossil fuel, a limiting source. Dye-sensitized solar cells (DSSCs) are a promising renewable energy source, which could be the major power supply for the future. Recently, the transition metal component has been demonstrated as potential material for counter electrode of platinum (Pt)-free DSSCs owing to their excellent electrocatalytic ability and their abundance on earth. Furthermore, the transition metal components exist different special nanostructures, which provide high surface area and various electron transport routs during electrocatalytic reaction. In this chapter, transition metal components with different nanostructures used for the application of electrocatalyst in DSSCs will be introduced; the performance of electrocatalyst between intrinsic heterogeneous rate constant and effective electrocatalytic surface area are also be clarified. Final, the advantages of the electrocatalyst with different dimensions (i.e., one to three dimension structures) used in DSSCs are also summarized in the conclusion.

A cobalt oxide (Co3O4)-decorated silicon carbide (SiC) nano-tree array (denoted as Co3O4/SiC NTA) electrode is synthesized, and it is investigated for use in micro-supercapacitor applications. Firstly, the well-standing SiC nanowires (NWs) are prepared by nickel (Ni)-catalyzed chemical vapor deposition (CVD) method, and then the thin layer of Co3O4 and the hierarchical Co3O4 nano-flower-clusters are, respectively, fabricated on the side-walls and the top side of the SiC NWs via electrodeposition. The deposition of Co3O4 on the SiC NWs benefits the charge transfer at the electrode/aqueous electrolyte interface due to its extremely hydrophilic surface characteristic after Co3O4 decoration. Furthermore, the Co3O4/SiC NTA electrode provides a directional charge transport route along the length of SiC nanowires owing to their well-standing architecture. By using the Co3O4/SiC NTA electrode for micro-supercapacitor application, the areal capacitance obtained from cyclic voltammetry measurement reaches 845 mF cm−2 at a 10 mV s−1 scan rate. Finally, the capacitance durability is also evaluated by the cycling test of cyclic voltammetry at a high scan rate of 150 mV s−1 for 2000 cycles, exhibiting excellent stability.

Since Prof. Grätzel and co-workers achieved breakthrough progress on dye-sensitized solar cells (DSSCs) in 1991, DSSCs have been extensively investigated and wildly developed as a potential renewable power source in the last two decades due to their low cost, low energy-intensive processing, and high roll-to-roll compatibility. During this period, the highest efficiency recorded for DSSC under ideal solar light (AM 1.5G, 100 mW cm−2) has increased from ~7% to ~14.3%. For the practical use of solar cells, the performance of photovoltaic devices in several conditions with weak light irradiation (e.g., indoor) or various light incident angles are also an important item. Accordingly, DSSCs exhibit high competitiveness in solar cell markets because their performances are less affected by the light intensity and are less sensitive to the light incident angle. However, the most used catalyst in the counter electrode (CE) of a typical DSSC is platinum (Pt), which is an expensive noble metal and is rare on earth. To further reduce the cost of the fabrication of DSSCs on the industrial scale, it is better to develop Pt-free electro-catalysts for the CEs of DSSCs, such as transition metallic compounds, conducting polymers, carbonaceous materials, and their composites. In this article, we will provide a short review on the Pt-free electro-catalyst CEs of DSSCs with superior cell compared to Pt CEs; additionally, those selected reports were published within the past 5 years.

A hierarchical urchin-like structure of CoSe2/CoSeO3 (denoted as CoSe2/CoSeO3-UL) is synthesized by a one-step hydrothermal method and investigated as the electro-catalyst for the counter electrode (CE) of dye-sensitized solar cells (DSSCs). The evolution of the CoSe2/CoSeO3 nanostructures as a function of the synthesis time has been identified by scanning electron microscopy. The growth mechanism is proposed and the desired CoSe2/CoSeO3 hierarchical UL structure is noticed at 4 h. CoSe2/CoSeO3-UL consists of nanoparticles and hexagonal prisms for providing a high surface area for catalytic reactions and a one dimensional charge transport route, respectively. The results show that DSSCs using the CoSe2/CoSeO3-UL CE exhibit a higher power conversion efficiency (η) of 9.29 ± 0.24% than that of the cell using a Pt CE (8.33 ± 0.07%). Under various dim light conditions (i.e., a T5 lamp), the DSSC with the CoSe2/CoSeO3-UL CE shows outstanding η's of 19.88 ± 0.10%, 18.24 ± 0.06%, and 16.00 ± 0.13% at 7000 lux (2.21 mW cm−2), 6000 lux (1.89 mW cm−2) and 4800 lux (1.55 mW cm−2), respectively. This study demonstrates that CoSe2/CoSeO3-UL has great potential to replace Pt in DSSCs, and the application of DSSCs can be extended from outdoor to indoor conditions.

The production of hydrogen via the water splitting process is one of the most promising technologies for future clean energy requirements, and one of the best related challenges is the choice of the most highly efficient and cost effective electrocatalyst. Conventional electrocatalysts based on precious metals are rare and very-expensive for large-scale production of hydrogen, demanding the exploration for low-cost earth abundant alternatives. In this context, extensive works from both theoretical and experimental investigations have shown that two-dimensional (2D) layered materials have gained considerable attention as highly effective electrocatalytic materials for electrical-driven hydrogen production because of their unique layered structure and exciting electrical properties. This review highlights recent advancements on 2D layered materials, including graphene, transitional metal dichalcogenides (TMDs), layered double hydroxides (LDHs), MXene, and graphitic carbon nitride (g-C3N4) as cost-effective and highly efficient electrocatalysts for hydrogen production. In addition, some fundamental aspects of the hydrogen evolution reaction (HER) process and a wide ranging overview on several strategies to design and synthesize 2D layered material as HER electrocatalysts for commercial applications are introduced. Finally, the conclusion and futuristic prospects and challenges of the advancement of 2D layered materials as non-precious HER electrocatalysts are briefly discussed.

Sustainable energy technology has received enormous attention in recent years. Specifically, electrochemical water splitting is considered to be the cleanest technique for the production of promising fuels, for example, hydrogen and oxygen, where transition metal (di)chalcogenides (TMCs) as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been a growing interest. In this chapter, the typical preparation methods of TMCs such as chemical vapor phase deposition (CVD) and solvothermal synthesis are introduced. Then, several TMC materials for catalyzing HER and OER are reviewed. Most importantly, this chapter also introduced some in situ approaches to realize the mechanism of electrocatalytic behavior toward HER and OER. Finally, the conclusion and futuristic prospects of TMCs in HER and OER are discussed.

The formation of thin and uniform capacitive layers for fully interacting with an electrolyte in a supercapacitor is a key challenge to achieve optimal capacitance. Here, we demonstrate a binder-free and flexible supercapacitor with the electrode made of cobalt oxide nanoparticle (CoO NP)-wrapped graphene hollow nanoballs (GHBs). The growth process of Co(OH)2 NPs, which could subsequently be thermally annealed to CoO NPs, was monitored by in situ electrochemical liquid transmission electron microscopy (TEM). In the dynamic growth of Co(OH)2 NPs on a film of GHBs, the lateral formation of fan-shaped clusters of Co(OH)2 NPs spread over the surface of GHBs was observed by in situ TEM. This CoO-GHBs/CC electrode exhibits high specific capacitance (2238 F g-1 at 1 A g-1) and good rate capability (1170 F g-1 at 15 A g-1). The outstanding capacitive performance and good rate capability of the CoO-GHBs/CC electrode were achieved by the synergistic combination of highly pseudocapacitive CoO and electrically conductive GHBs with large surface areas. A solid-state symmetric supercapacitor (SSC), with CoO-GHBs/CCs used for both positive and negative electrodes, exhibits high power density (6000 W kg-1 at 8.2 Wh kg-1), high energy density (16 Wh kg-1 at 800 W kg-1), cycling stability (∼100% capacitance retention after 5000 cycles), and excellent mechanical flexibility at various bending positions. Finally, a serial connection of four SSC devices can efficiently power a red light-emitting diode after being charged for 20 s, demonstrating the practical application of this CoO-GHBs/CC-based SSC device for efficient energy storage.

Novel polymorphic Mo

Dye-sensitized solar cell (DSSC) is one of the third generation solar cells converting solar light into electricity that attracted a remarkable research interest globally due to their simple device fabrication processes in ambient conditions and reasonable cell efficiency for commercialization; moreover, the advantages on colorfulness, possible plasticity, and high conversion efficiencies under indoor illumination also render DSSC to play a crucial role in energy-harvesting systems for green buildings. To eliminate the electrolyte leakage issues in traditional DSSCs (i.e., cells with organic solvent-based electrolytes), new class of ionic liquids, namely poly(ionic liquid)s, are recently investigated and used as the quasi/all–solid state electrolyte to improve the cell durability. Herein, we give a short review on the poly(ionic liquid)s to be used as the quasi/all–solid state electrolytes in DSSCs. Finally, the future perspectives for the development of poly(ionic liquid)s for DSSCs are also briefly discussed.

Nitrogen-doped graphene hollow nanoballs (N-GHBs) were synthesized in chemical vapor deposition (CVD) reaction using melamine as a chemical precursor via an in situ nitrogen-doping approach. In the CVD reaction, N-GHBs were deposited directly on carbon cloth (CC) to be used as an efficient metal-free electrocatalyst for dye-sensitized solar cell (DSSC) applications. The highly curved N-GHBs could avoid the self-assembly restacking of planar graphene sheets, which usually occurred during the film preparation. Keeping oxygen contaminations from N-GHBs, the characteristic electrical conductivity of graphene was preserved in the as-synthesized N-GHBs. By controlling the evaporation temperature of melamine, the nitrogen-doping content of 8.7–14.0% and different nitrogen-doped configurations in N-GHBs could be adjusted. The catalytic activities of different nitrogen-doped states in N-GHBs toward the triiodide (I 3− ) reduction in DSSCs were investigated, revealing that the pyridinic and quaternary nitrogens, rather than the total nitrogen doping level, in N-GHBs are mainly responsible for their catalytic activities in DSSCs. For solar cell applications, the high surface area and heteroatomic nitrogens of GHBs can remarkably improve the catalytic activity toward the triiodide reduction, lower the charge-transfer resistance, and enhance the corresponding photovoltaic performance (7.53%), which is comparable to that (7.70%) of a standard sputtered Pt counter electrode-based cell. These exceptional properties allow N-GHBs/CC to act as a promising electrocatalytic electrode for DSSC and other electrochemical energy applications.

Cobalt diselenide/cobalt selenite (CoSe2/CoSeO3 ) composite crystals with nanocube (NC), nanorod (NR), and nanoparticle (NP) structures were obtained through a microemulsion-assisted hydrothermal synthesis by incorporating the anionic surfactant docusate sodium salt (AOT), the cationic surfactant cetyltrimethyl ammonium bromide (CTAB), and the non-ionic surfactant Triton ® X-100, respectively. The as-prepared CoSe2/CoSeO3 crystals were utilized as the electrocatalysts in dye-sensitized solar cells (DSSCs). Among all the crystals, the one obtained through Triton ® X-100 enables its DSSC to achieve the best power conversion efficiency (η ) of 9.27%, which is even better than that of the DSSC with platinum (Pt) (7.91%). The catalytic abilities of the counter electrodes were analyzed by cyclic voltammetry (CV), Tafel polarization plots, and electrochemical impedance spectra (EIS). The photovoltaic behaviors are substantiated by incident photon-to-electron conversion efficiency (IPCE) spectra. The DSSC with CoSe 2/CoSeO3-NP exhibited an η of 9.27% at 100 mW cm−2, a better η of 9.31% at 50 mW cm−2, and the best η of 9.41% at 10 mW cm−2, thereby extending the scope of application of this DSSC from outdoor to indoor regions. The earth abundant, low-cost material of CoSe2/CoSeO3-NP can be a promising electrocatalytic material to replace the expensive Pt in a DSSC.

An ionic liquid (IL), 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-1-oxyl-4-yl)amino]ethyl}-1H-imidazol-3-ium tetrafluoroborate (TILBF4), containing a stable radical, 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO), was synthesized successfully. The TILBF4 shows two distinctive properties having the characteristics of ion-storage and supporting electrolyte materials. In this study, a hybrid electrochromic device (ECD) with a high coloration efficiency based on this TILBF4 and a poly(3,3-diethyl-3,4-dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) thin film was proposed. The maximum absorbance of the PProDOT-Et2/TILBF4 ECD was noticed at 590 nm upon applying the cell potential between −1.0 and 1.0 V. The device exhibits a transmittance change (ΔT) of 62.2% at 590 nm when being switched in the first transition, with short switching times of 4.0 s for bleaching and 3.6 s for coloring. A relatively high coloration efficiency (η) of 983.0 cm2/C was achieved at 590 nm. Furthermore, the ECD maintained 98.0% of its initial ΔT after consecutive operation for 1000 cycles between −1.0 and 1.0 V.

We have synthesized a metal-free composite ink that contains graphene dots (GDs) and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) that can be used on paper to serve as the counter electrode in a flexible dye-sensitized solar cell (DSSC). This paper-based GD/PEDOT:PSS electrode is low-cost, light-weight, flexible, environmentally friendly, and easy to cut and process for device fabrication. We determined the GD/PEDOT:PSS composite effectively fills the dense micro-pores in the paper substrate, which leads to improved carrier transport in the electrode and a 3-fold enhanced cell efficiency as compared to the paper electrode made with sputtered Pt. Moreover, the DSSC with the paper electrode featuring the GD/PEDOT:PSS composite did not fail in photovoltaic tests even after bending the electrode 150 times, whereas the device made with the Pt-based paper electrode decreased in efficiency by 45% after such manipulation. These exceptional properties make the metal-free GD/PEDOT:PSS composite ink a promising electrode material for a wide variety of flexible electronic applications.

A titanium oxide-selenide (TiO1.1Se0.9) composite material was successfully obtained on a flexible substrate of carbon cloth (CC); this TiO1.1Se0.9/CC electrode was used as the counter electrode (CE) in a dye-sensitized solar cell (DSSC). Each carbon fiber in the CC was wrapped with a composite layer of TiO1.1Se0.9, and the composite layer contained TiO1.1Se0.9 nanospheres and one-dimensional (1D) nanorods. Thus, each carbon fiber with its TiO1.1Se0.9 layer has a hierarchical core–shell structure, in which the carbon fiber acts as a one-dimensional conductive core for electron transfer and the TiO1.1Se0.9 layer around the fiber functions as an electro-catalytic shell. In iodide-based electrolyte, the DSSC with the TiO1.1Se0.9/CC exhibits a power conversion efficiency (η) of 9.47%, which is higher than cells with CEs of Pt/CC (7.75%) and pristine TiO2/CC (4.90%). Under dim light conditions, the best TiO1.1Se0.9/CC electrode rendered its cell an η of 10.00% at 0.5 sun and the best η of 10.39% at 0.1 sun. In cobalt-based electrolytes, a DSSC with the TiO1.1Se0.9/CC shows an attractive η of 10.32% at 1.0 sun, the best η of 10.47% at 0.5 sun, and an impressive η of 10.20% at 0.1 sun. The earth abundant and low-cost TiO1.1Se0.9 is a promising alternative to Pt for the CE of a DSSC in both iodide and cobalt electrolytes.

A new series of metal-free alkylated tetrathienoacene (TTAR)-based organic chromophores, TPA–TTAR–TA (R = branched-C8H17, 1, TTAR-b8; R = C15H31, 2, TTAR-15; R = C9H19, 3, TTAR-9), are synthesized for application in dye-sensitized solar cells (DSSCs). Due to the extensively conjugated TTAR π-bridge, all three dyes exhibit high extinction coefficients (1 × 105 M−1 cm−1). By systematically exploring the effects of the TTAR alkyl chain substituents, a significant influence of the dye coverage (orientation) on the TiO2 surfaces is observed. The branched-alkyl TTAR-b8 (1) promotes significant tilting and packing distortion on TiO2 in comparison to more ordered monolayers of linear long alkyls TTAR-15 (2) and TTAR-9 (3). Photophysical measurements on the dye-grafted TiO2 films reveal that the branched-alkylated TTA unit in 1 enhances the electron injection efficiency, in agreement with the high quantum efficiency. Notably, by utilizing a three-dimensional (3D) photonic crystal (PhC) layer to enhance the coherent scattering an increase the light absorption, TTAR-b8 exhibits higher short-circuit current densities and achieved a high PCE of 11.18%. TTAR-b8 is thus the best performing fused-thiophene-based organic DSSC dye reported to date.

Two-dimensional-layered transition metal dichalcogenides (2D-layered TMDs) are a chemically diverse class of compounds having variable band gaps and remarkable electrochemical properties, which make them potential materials for applications in the field of electrochemical energy. To date, 2D-layered TMDs have been wildly used in water-splitting systems, dye-sensitized solar cells, supercapacitors, and some catalysis systems, etc., and the pertinent devices exhibit good performances. However, several reports have also indicated that the active sites for catalytic reaction are mainly located on the edge sites of 2D-layered TMDs, and their basal plane shows poor activity toward catalysis reaction. Accordingly, many studies have reported various approaches, namely active-site engineering, to address this issue, including plasma treatment, edge site formation, heteroatom-doping, nano-sized TMD pieces, highly curved structures, and surface modification via nano-sized catalyst decoration, etc. In this article, we provide a short review for the active-site engineering on 2D-layered TMDs and their applications in electrochemical energy. Finally, the future perspectives for 2D-layered TMD catalysts will also be briefly discussed.

A novel organic sensitizer, coded CR147, is applied to sensitize a ZnO–based dye–sensitized solar cell (DSSC). The common problem of ZnO dissolution and Zn 2+ /dye agglomeration, caused by the use of ruthenium–based dyes, is solved by the application of this CR147 dye. The highest power conversion efficiency ( η ) of 4.77% is achieved for the DSSC using a photoanode with a film of commercial ZnO nanoparticles (C ZnO) and the CR147 dye, while the η is only 3.41% for the DSSC with the commercial N719 dye. The cell performance with the CR147 dye is further improved by using a photoanode with the double layer ZnO film (D–ZnO), composed of an underlayer with coral–like ZnO nanocrystals and an overlayer with hexagonal club–like ZnO submicrocrystals. The DSSC with the D–ZnO film exhibits an η of 6.89%, which is ca. 45% higher than that of the DSSC with the C ZnO film (4.77%). This higher efficiency is attributed to the superior charge transfer and light–scattering abilities provided by coral–like ZnO nanocrystals and hexagonal clubs–like ZnO submicrocrystals, respectively, with reference to these parameters of C ZnO.

A composite thin film of nitrogen-doped graphene and molybdenum disulfide (NGr/MoS2) was prepared via a simple drop-coating method, and used as an electrocatalytic film for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The NGr was intended for increasing the conductivity and serving as the skeleton of the composite film, while the MoS2 with remarkable electrocatalytic ability increased the effective electrocatalytic sites in the composite film. The NGr acts as the architecture framework for MoS2 sheet, which could expose the active sites on its edge, as demonstrated in scanning electron microscopy (SEM). The weight percent (wt %) of the NGr in the composite film was optimized for obtaining the best performance to the pertinent DSSC. The NGr/MoS2 composite film shows much higher electrocatalytic ability for the reduction of I−/I3−, compared to the films of bare NGr and bare MoS2. Electrocatalytic abilities of the films were estimated by cyclic voltammetry (CV), rotating disc electrode (RDE), Tafel polarization plot, and electrochemical impedance spectroscopy (EIS) analyses. The DSSC with the NGr/MoS2 CE exhibits a solar-to-electricity power conversion efficiency of 7.82%, close to 8.25% of the DSSC with a platinum CE film. The low-cost NGr/MoS2 composite film is a potential alternative to replace the expensive platinum film for use as the catalytic film on the counter electrode of a DSSC.

A composite film of molybdenum diselenide nanosheets and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (MoSe 2 NS/PEDOT:PSS) was prepared for the counter electrode (CE) of a dye-sensitized solar cell (DSSC), via a low-cost drop-coating method. The two-dimensional (2D) nanosheets of MoSe 2 (MoSe 2 NS) were aimed to provide the composite film not only a large amount of electrocatalytic active sites but also orientational pathways for electron transfer, from the conducting substrate of the CE to the interface at the CE electrolyte. The PEDOT:PSS acts as the conductive matrix for the composite film, and also enables multiple interfacial electron transfer pathways between the MoSe 2 NS and the substrate. Owing to the combination of the advantages of both MoSe 2 NS and PEDOT:PSS, the DSSC with the composite film of MoSe 2 NS/PEDOT:PSS on its CE exhibited a power conversion efficiency ( η ) of 7.58 ± 0.05%, which is comparable to that of the cell with a Pt CE (7.81 ± 0.03%). The counter electrode films were characterized by scanning electron microscopy. Various electrochemical analyses, including those with cyclic voltammetry, rotating disk electrode, Tafel polarization curves, and electrochemical impedance spectroscopy, were performed intending to quantify the electrocatalytic ability of the counter electrode films; these techniques were able to precisely distinguish the functions of MoSe 2 NS and PEDOT:PSS in the composite films; the former worked as an electrocatalyst and the latter as a conductive binder. Incident photon-to-current conversion efficiency (IPCE) was used to substantiate the photovoltaic parameters. When the MoSe 2 NS/PEDOT:PSS composite film was coated on a flexible titanium (Ti) foil, the pertinent DSSC showed even a higher η of 8.51 ± 0.05%, as compared to a lower η of 8.21 ± 0.02% using the Pt-coated Ti foil CE. The results indicate the promising potential of MoSe 2 NS/PEDOT:PSS composite film to replace the expensive Pt.

Zinc oxide (ZnO) crystals with structures of hexagonal club (HC), nanoparticle (NP), and hexagonal plate (HP) are synthesized via microemulsion technique by using the surfactants of cationic cetylmethylammonium bromide (CTAB), nonionic Triton ® X-100, and anionic docusate sodium (AOT), respectively, and they are used for the photoanodes of dye-sensitized solar cells (DSSCs). Phase diagrams are constructed in mapping the compositions of the microemulsions to guide the synthesis of ZnO crystals. All the synthesized ZnO crystals exhibited wurtzite crystal phase of ZnO in their X-ray diffraction (XRD) patterns, and their morphologies and structures were investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM), respectively. The sub-micro sized HC and HP ZnO crystals are meritorious in the present work as large particles are effective for light-scattering in the photoanode; the nano-sized NP ZnO crystal is meritorious in the present work as small particles are effective for dye adsorption in the photoanode of DSSCs. By using the photoanodes with bi-layer NPs/HPs and NPs/HCs ZnO films for the ZnO-based DSSCs with a novel organic dye (coded DJ149), we obtained good cell efficiencies of 7.40% and 6.95%, respectively. Incident photon-to-current conversion efficiency (IPCE) curves, and UV–vis reflectance spectra are also used to substantiate the results.

A polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN(-), which is aimed to form a SeCN(-)/(SeCN)3(-) redox couple with a more positive redox potential than that of traditional I(-)/I3(-), (c) chelates the potassium cations through the lone pair electrons of the oxygen atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the recombination of photoinjected electrons with (SeCN)3(-) ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC due to its functions of b-d and prolongs the stability of the cell due to its function of a. The QSS-DSSC with the gel electrolyte containing 30 wt % of the POEI-IS in liquid selenocyanate electrolyte exhibited a high VOC of 825.50 ± 3.51 mV and a high power conversion efficiency (η) of 8.18 ± 0.02%. The QSS-DSSC with 30 wt % POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h.

The development of earth-abundant and efficient electrocatalysts for the hydrogen evolution reaction (HER) is one of the keys to success for future green energy systems using hydrogen fuel. Nanostructuring of electrocatalysts is a promising way to enhance their electrocatalytic performance in the HER. In this study, pure pyrite-type beaded stream-like cobalt diselenide (CoSe2) nanoneedles are directly formed on flexible titanium foils through treating a cobalt oxide (Co3O4) nanoneedle array template with selenium vapor. The beaded stream-like CoSe2 nanoneedle electrode can drive the HER at a current density of 20 mA cm−2 with a small overpotential of 125 mV. Moreover, the beaded stream-like CoSe2 nanoneedle electrode remains stable in an acidic electrolyte for 3000 cycles and continuously splits water over a period of 18 hours. The enhanced electrochemical activity is facilitated by the unique three-dimensional hierarchical structure, the highly accessible surface active sites, the improved charge transfer kinetics and the highly attractive force between water and the surface of the nanoneedles that exceeds the surface tension of water.

A composite film based on carbon black nanoparticles and sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)ethoxy]-2,5-diyl) (CB-NPs/s-PT) is formed on a flexible titanium foil for the use as the electro-catalytic counter electrode (CE) of dye-sensitized solar cells (DSSCs). The CB-NPs provide the large amount of electro-catalytic active sites for the composite film, and the s-PT polymer serves as a conductive binder to enhance the inter-particle linkage among CB-NPs and to improve the adhesion between the composite film and the flexible substrate. The flexible CB-NPs/s-PT composite film is designed to possess good electro-catalytic ability for I−/ I 3 − redox couple by providing large active sites and rapid reduction kinetic rate constant of I 3 − . The cell with a CB-NPs/s-PT CE exhibits a good cell efficiency (η) of 9.02 ± 0.01% at 100 mW cm−2, while the cell with a platinum CE shows an η of only 8.36 ± 0.02% under the same conditions. At weak light illuminations (20–80 mW cm−2), a DSSC with CB-NPs/s-PT CE still exhibits η's of 7.20 ± 0.04–9.08 ± 0.02%. The low-cost CB-NPs/s-PT CE not only renders high cell efficiency to its DSSC but also shows a great potential to replace the expensive platinum; moreover it is suitable for large-scale production or for indoor applications.

A series of new organic dyes containing fluoreneamine-based donor and carbazole π-spacer are reported. These dyes displayed longer wavelength charge transfer absorption when compared to that of the corresponding phenylamine analogs. Fluoreneamine-based donors shifted the lowest unoccupied molecular orbital upwardly which is beneficial for the injection of electrons from the excited dye into the conduction band of TiO 2 and lowered the oxidation potential of the dyes. The electronic perturbations caused by the fluorene unit have been confirmed by the density functional theoretical calculations. A device fabricated using a dye featuring difluorenylaminocarbazole donor and thiophene in the π-spacer exhibited promising power conversion efficiency (6.4%). The interfacial electron transfer kinetics has been elucidated by electrochemical impedance spectroscopy which showed the beneficial role of difluorenylamine unit in suppressing the electron recombination.

We synthesize heteroatoms-doped graphene hollow nanoballs (GHBs) on flexible carbon cloth (CC) substrates via chemical vapor deposition (CVD) reaction to be used as an efficient non-noble electrocatalyst in dye-sensitized solar cells (DSSCs). The as-synthesized heteroatoms-doped GHBs/CC, including nitrogen-doped GHBs, sulfur-doped GHBs, and nitrogen and sulfur-codoped GHBs (denoted by N-GHBs/CC, S-GHBs/CC and N,S-GHBs/CC, respectively), are used as an efficient counter electrode (CE) in DSSCs. Unlike planar graphene sheets, the highly curved GHBs can avoid self-assembly restacking to provide high surface areas for electrocatalytic reactions. In addition, the heteroatomic incorporation in GHBs can reduce the charge-transfer resistance to enhance the electrocatalytic activity. Among these doped GHB samples, N,S-GHBs show the best catalytic performance due to the synergistic effect from both electronic and geometric changes, caused by the N- and S-dopings, respectively. The DSSC with a N,S-GHB CE exhibits the power conversion efficiency of 9.02%, comparable to that (8.90%) of a Pt-based counterpart.

A hierarchical cobalt oxide (Co3O4)-functionalized silicon carbide nanowire (SiC NW) array electrode is synthesized on 4H-SiC(0001) substrates and investigated for use in the oxygen evolution reaction (OER) of water splitting. Ni-catalyzed chemical vapor deposition is employed to obtain vertically-aligned SiC NWs, and electrodeposition is used to functionalize the nanowires with hierarchical Co3O4 nano-clusters and a thin Co3O4 layer on the top side and the side-walls of the NWs, respectively. By using the Co3O4-functionalized SiC NW electrode for OER, the overpotential required to drive an anodic current density (J(anodic)) of 10 mA cm(-2) is 0.22 V, a value much lower than those of Co3O4 nano-clusters (0.42 V), of bare SiC NWs (1.27 V) electrodes and of Pt foil (0.56 V). The overpotential value of 0.22 V is among the most active for the noble metal-free electro-catalytic electrode to produce J(anodic) of 10 mA cm(-2). The deposition of Co3O4 on the SiC NW substrate produces an extremely hydrophilic surface, which improves the charge transfer characteristics at the interface between the electrode and the aqueous electrolyte. Moreover, the vertically-aligned SiC NWs provide a directional charge transport route along the nanowire length, as seen from a lower charge transfer resistance (R-ct) in the Co3O4-functionalized SiC NW electrode compared to the Co3O4-functionalized SiC electrode under the same OER operating conditions. Finally, the Co3O4-functionalized SiC NW electrode maintains stable performance in an alkaline electrolyte (i.e., 1 M KOH aqueous solution) during continuous electrolysis testing over 16 h. The outstanding electro-catalytic performance and stability make the Co3O4-functionalized SiC NW electrode a promising noble metal-free electro-catalytic electrode for the OER and other renewable energy applications. (C) 2017 Elsevier Ltd. All rights reserved.

Graphene dots (GDs) are used for enhancing the performance of the poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)-based counter electrodes in Pt-free dye-sensitized solar cells (DSSCs). As compared to PEDOT:PSS CEs, GD–PEDOT:PSS films possess a rough surface morphology, high conductivity and electrocatalytic activity, and low charge-transfer resistance toward I − /I 3 − redox reaction, pushing cell efficiency to 7.36%, which is 43% higher than that of the cell with PEDOT:PSS CEs (5.14%). Without much impact on efficiency, the DSSCs with GD–PEDOT:PSS CEs work well under low-light conditions (light intensity −2 and angle of incidence >60°), such as indoor and low-level outdoor lighting and of the sun while the other traditional cells would fail to work. The concurrent advantage in low cost in Pt-free materials, simple fabrication processes, comparable efficiency with Pt CEs, and high performance under low-light conditions makes the DSSC with GD–PEDOT:PSS CEs suitable to harvest light for a diverse range of indoor and low-level outdoor lighting locations.

A novel ionic-liquid mediator, 1-butyl-3-{2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-yl)amino]ethyl}-1H-imidazol-3-ium selenocyanate (ITSeCN), has been successfully synthesized for dye-sensitized solar cells (DSSCs). ITSeCN possesses dual redox channels, imidazolium-functionalized 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and selenocyanate, which can serve as the cationic redox mediator and the anionic redox mediator, respectively. Therefore, ITSeCN has a favorable redox nature, which results in a more positive standard potential, larger diffusivity, and better kinetic heterogeneous rate constant than those of iodide. The DSSC with the ITSeCN electrolyte shows an efficiency of 8.38 % with a high open-current voltage (VOC ) of 854.3 mV, and this VOC value is about 150 mV higher than that for the iodide-based DSSC. Moreover, different electrocatalytic materials were employed to trigger the redox reaction of ITSeCN. The ITSeCN-based DSSC with the CoSe counter electrode achieved the best performance of 9.01 %, which suggested that transition-metal compound-type materials would be suitable for our newly synthesized ITSeCN mediator.

A honeycomb-like composite film of nitrogen-doped graphene and poly(3,4-ethylenedioxythiophene) (NGr/PEDOT) was prepared as a highly-efficient catalytic material for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). The NGr was intended for increasing the conductivity and electrocatalytic ability of the composite film, and the PEDOT was used for a strong adhesion of the composite film to the substrate and for large surface area. The DSSC with the NGr/PEDOT composite CE exhibited a power conversion efficiency ( η ) of 8.30±0.03% for front-side illumination and 6.10±0.02% for back-side illumination. The DSSC with a Pt CE showed an η of 8.17±0.01% for front-side illumination and 5.76±0.05% for back-side illumination. The composite catalytic film of NGr/PEDOT is a low-cost alternative for replacing the conventional and expensive Pt film. Another DSSC was fabricated, in which its counter electrode contained a flexible Ti foil as the substrate and the NGr/PEDOT film as the catalytic film. This DSSC with its flexible CE exhibited an η of 8.53±0.02%.

Polypyrrole (PPy) with various morphologies were synthesized by chemical oxidative polymerization and further used as the counter electrode (CE) in dye–sensitized solar cells (DSSCs). The conventional anionic surfactant, docusate sodium salt (AOT), cationic surfactant, cetylmethyl ammonium bromide (CTAB), and the newly developed polymeric dispersant, poly(oxyethylene)–imide (POEM), were employed in the PPy synthesis. Scanning electron microscopy images (SEM) revealed diversified morphologies of the synthesized PPy nanoparticles with irregular sheet (IS), hierarchical nanosphere (HNS), and nanosphere (NS), corresponded to the choice of the commercial surfactants AOT and CTAB, as well as the home–made POEM, respectively. Fourier transform spectroscopy (FT–IR) and X–ray diffraction (XRD) analysis were used to confirm the PPy structures and crystalline properties. When fabricated into films and used as CE in DSSCs, the PPy–HNS demonstrated the superior cell efficiency of 6.71 ± 0.16% to those of PPy–IS (5.46 ± 0.31) and PPy–NS (6.31 ± 0.24), respectively. The excellent electrocatalytic ability of PPy–HNS was essentially attributed to its high electrochemical surface area ( A e ), which was quantitatively calculated through a rotating disk electrode system by using the Koutecky–Levich equation. Brunauer–Emmett–Teller (BET) surface area measurement, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were also used to substantiate the explanation for the DSSC performances.