Transparent Metal Selenide Alloy Counter Electrodes for High‐Efficiency Bifacial Dye‐Sensitized Solar Cells

The exploration of cost‐effective and transparent counter electrodes (CEs) is a persistent objective in the development of bifacial dye‐sensitized solar cells (DSSCs). Transparent counter electrodes based on binary‐alloy metal selenides (M‐Se; M=Co, Ni, Cu, Fe, Ru) are now obtained by a mild, solution‐based method and employed in efficient bifacial DSSCs. Owing to superior charge‐transfer ability for the I^?/I₃^? redox couple, electrocatalytic activity toward I₃^? reduction, and optical transparency, the bifacial DSSCs with CEs consisting of a metal selenide alloy yield front and rear efficiencies of 8.30 % and 4.63 % for Co_0.85Se, 7.85 % and 4.37 % for Ni_0.85Se, 6.43 % and 4.24 % for Cu_0.50Se, 7.64 % and 5.05 % for FeSe, and 9.22 % and 5.90 % for Ru_0.33Se in comparison with 6.18 % and 3.56 % for a cell with an electrode based on pristine platinum, respectively. Moreover, fast activity onset, high multiple start/stop capability, and relatively good stability demonstrate that these new electrodes should find applications in solar panels.     

Nickel‐substituted cobalt ferrite nanoparticles supported on arginine‐modified graphene oxide nanosheets: Synthesis and catalytic activity

The amino acid arginine was used to modify the surface of graphene oxide nanosheets and then nickel‐substituted cobalt ferrite nanoparticles were supported on those arginine‐grafted graphene oxide nanosheets (Ni_0.5Co_0.5Fe₂O₄@Arg–GO). The prepared Ni_0.5Co_0.5Fe₂O₄@Arg–GO was characterized using flame atomic absorption spectroscopy, inductively coupled plasma optical emission spectrometry, energy‐dispersive spectroscopy, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, Raman spectroscopy, X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy. The application of Ni_0.5Co_0.5Fe₂O₄@Arg–GO as a catalyst was examined in a one‐pot tandem oxidative cyclization of primary alcohols with o ‐phenylenediamine to benzimidazoles under aerobic oxidation conditions. The results showed that 2‐phenylbenzimidazole derivatives were successfully achieved using Ni_0.5Co_0.5Fe₂O₄@Arg–GO nanocomposite catalyst via the one‐pot tandem oxidative cyclization strategy.     

Controlled Synthesis of Cr‐Co0.85Se Nanoarrays for Water Splitting at an Ultralow Cell Voltage of 1.43 V

Water splitting has attracted more and more attention as a promising strategy for the production of clean hydrogen fuel. In this work, a new synthesis strategy was proposed, and Co_0.85Se was synthesized on nickel foam as the main matrix. The doping of appropriate Cr amount into the target of Co_0.85Se and the Cr‐Co_0.85Se resulted in an excellent electrochemical performance. The doping of Cr introduces Cr³⁺ ions which substitute Co²⁺ and Co³⁺ ions in Co_0.85Se, so that the lattice parameters of the main matrix were changed. It is worth noting that the Cr0.15‐Co_0.85Se/NF material exhibits an excellent performance in the oxygen evolution reaction (OER) test. When the current density reaches 50 mA cm^?2 for OER, the overpotential is only 240 mV. For the hydrogen evolution reaction (HER) tests, the overpotential is only 117 mV to drive 10 mA cm^?2 of current density. Moreover, when the Cr0.15‐Co_0.85Se/NF material is used as a two‐electrode device for whole water splitting, the required cell voltage is only 1.43 V to reach a current density of 10 mA cm^?2, which is among the lowest values of the published catalysts up to now. In addition, the Cr0.15‐Co_0.85Se/NF catalyst also exhibits excellent stability during a long period of water splitting. The experimental result demonstrates that the change of the lattice structure has an obvious influence on the electrocatalytic activity of the material. When an external electric field is applied, it facilitates the rapid electron transfer rate and enhances the electrocatalytic performance and stability of the material.     

Low‐cost Nickel Complex Dye‐sensitized Titania Nanoparticle/nanotube Composites for Solar Cells

In this work, high‐performance dye‐sensitized solar cells (DSSCs) based on new low‐cost visible nickel complex dye (VisDye), TiO₂ nanoparticle/nanotube composites electrodes, carbon nanoparticles counter electrodes, and ionic liquids electrolytes have been fabricated. The electronic structure, optical spectroscopy, and electrochemical properties of the VisDye were studied. Experimental results indicate that it is beneficial to improve the electron transport and power conversion efficiency using the nickel complex VisDye and TiO₂ nanoparticle/nanotube composites. Under optimized conditions, the solar energy conversion efficiencies were measured. The short‐circuit current density (J_SC), the open‐circuit voltage (V_OC), the fill factor (FF), and the overall efficiency (η) of the DSSCs are 10.01 mA/cm², 516 mV, 0.68, and 3.52%, respectively. This study demonstrates that the combination of new VisDye with TiO₂ nanoparticle/nanotube composites electrodes and carbon nanoparticles counter electrodes provide a way to fabricate highly efficient dye‐sensitized solar cells in low‐cost production.     

Synthesis and photovoltaic properties of a low‐band‐gap polymer consisting of alternating thiophene and benzothiadiazole derivatives for bulk‐heterojunction and dye‐sensitized solar cells

We synthesized a novel low‐band‐gap, conjugated polymer, poly[4,7‐bis(3′,3′‐diheptyl‐3,4‐propylenedioxythienyl)‐2,1,3‐benzothiadiazole] [poly(heptyl₄‐PTBT)], consisting of alternating electron‐rich, diheptyl‐substituted propylene dioxythiophene and electron‐deficient 2,1,3‐benzothiadiazole units, and its photovoltaic properties were investigated. A thin film of poly(heptyl₄‐PTBT) exhibited an optical band gap of 1.55 eV. A bulk‐heterojunction solar cell with indium tin oxide/poly(3,4‐ethylenedioxythiophene)/poly(heptyl₄‐PTBT): methanofullerene [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) (1:4)/LiF/Al was fabricated with poly(heptyl₄‐PTBT) as an electron donor and PCBM as an electron acceptor and showed an open‐circuit voltage, short‐circuit current density, and power conversion efficiency of 0.37 V, 3.15 mA/cm², and 0.35% under air mass 1.5 (AM1.5G) illumination (100 mW/cm²), respectively. A solid‐state, dye‐sensitized solar cell with a SnO₂:F/TiO₂/N3 dye/poly(heptyl₄‐PTBT)/Pt device was fabricated with poly(heptyl₄‐PTBT) as a hole‐transport material. This device exhibited a high power conversion efficiency of 3.1%, which is the highest power conversion efficiency value with hole‐transport materials in dye‐sensitized solar cells to date. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1394–1402, 2007     

Size‐Selective Encapsulation of Hydrophobic Guests by Self‐Assembled M4L6 Cobalt and Nickel Cages

Subtle differences in metal–ligand bond lengths between a series of [M₄L₆]^4? tetrahedral cages, where M=Fe^II, Co^II, or Ni^II, were observed to result in substantial differences in affinity for hydrophobic guests in water. Changing the metal ion from iron(II) to cobalt(II) or nickel(II) increases the size of the interior cavity of the cage and allows encapsulation of larger guest molecules. NMR spectroscopy was used to study the recognition properties of the iron(II) and cobalt(II) cages towards small hydrophobic guests in water, and single‐crystal X‐ray diffraction was used to study the solid‐state complexes of the iron(II) and nickel(II) cages.     

A Hydrogen‐Deficient Nickel–Cobalt Double Hydroxide for Photocatalytic Overall Water Splitting

Developing highly efficient and low‐cost photocatalysts for overall water splitting has long been a pursuit for converting solar power into clean hydrogen energy. Herein, we demonstrate that a nonstoichiometric nickel–cobalt double hydroxide can achieve overall water splitting by itself upon solar light irradiation, avoiding the consumption of noble‐metal co‐catalysts. We employed an intensive laser to ablate a NiCo alloy target immersed in alkaline solution, and produced so‐called L‐NiCo nanosheets with a nonstoichiometric composition and O^2?/Co³⁺ ions exposed on the surface. The nonstoichiometric composition broadens the band gap, while O^2? and Co³⁺ ions boost hydrogen and oxygen evolution, respectively. As such, the photocatalyst achieves a H₂ evolution rate of 1.7 μmol h^?1 under AM 1.5G sunlight irradiation and an apparent quantum yield (AQE) of 1.38 % at 380 nm.     

Novel Thiazolo[5,4‐d]thiazole‐Based Organic Dyes for Quasi‐Solid‐State Dye‐Sensitized Solar Cells

A series of novel metal‐free organic dyes containing the thiazolo[5,4‐d]thiazole moiety were designed and synthesized for quasi‐solid‐state dye‐sensitized solar cells (DSSCs). Different alkoxy chains were introduced into the electron donor part of the dye molecules for comparison. The optical, electrochemical, and photovoltaic properties for all sensitizers were systematically investigated. It was found that the sensitizers with the different alkoxy groups have similar photophysical and electrochemical properties, such as absorbance and energy levels, owing to their close chemical structures. However, the quasi‐solid‐state DSSCs based on the resulting sensitizers exhibit different performance parameters. The quasi‐solid‐state DSSC based on sensitizer FNE74 with two octyloxy chains possessed the highest solar energy conversion efficiency of 5.10 % under standard AM 1.5G sunlight illumination without the use of coadsorbant agents.     

Benzothiadiazole Containing D‐π‐A Conjugated Compounds for Dye‐Sensitized Solar Cells: Synthesis,Properties, and Photovoltaic Performances

Two D ‐π‐A conjugated molecules, BzTCA and BzTMCA , were developed through facile synthetic approaches for dye‐sensitized solar cells. The investigation of the photophysical properties of BzTCA and BzTMCA both in dilute solutions and in thin films indicates that their absorption exhibits a wide coverage of the solar spectrum. The absorption features for BzTCA and BzTMCA commence at about 710 nm in solution, and at about 800 nm in the solid state. The absorption maxima (λ_max) for both BzTCA and BzTMCA on TiO₂ film are almost the same as those in dilute solution. Their HOMOs and LUMOs were found to partly overlap at the center of these dyes, which guarantees appreciable interactions between the donors and acceptors. The investigation of the performance of dye‐sensitized solar cells fabricated from BzTCA and BzTMCA indicated that the power‐conversion efficiencies are 6.04 % and 4.68 %, respectively, which could be comparable with the normal sensitizer N3. BzTMCA showed lower incident photon‐to‐electron conversion efficiency (IPCE) and J_sc values relative to BzTCA , which is probably because of the weaker driving force of dye regeneration and electron injection process of BzTMCA . The IPCE responsive area reached nearly 800 nm, which provides great potential for further improvement of the photocurrent density and power‐conversion efficiency. Our investigations demonstrate that both dyes BzTCA and BzTMCA could be promising candidates for dye‐sensitized solar cells.     

Nanosized Metal Phosphides Embedded in Nitrogen‐Doped Porous Carbon Nanofibers for Enhanced Hydrogen Evolution at All pH Values

Transition‐metal phosphides (TMPs) have emerged as promising catalyst candidates for the hydrogen evolution reaction (HER). Although numerous methods have been investigated to obtain TMPs, most rely on traditional synthetic methods that produce materials that are inherently deficient with respect to electrical conductivity. An electrospinning‐based reduction approach is presented, which generates nickel phosphide nanoparticles in N‐doped porous carbon nanofibers (Ni₂P@NPCNFs) in situ. Ni₂P nanoparticles are protected from irreversible fusion and aggregation in subsequent high‐temperature pyrolysis. The resistivity of Ni₂P@NPCNFs (5.34 Ω cm) is greatly decreased by 10⁴ times compared to Ni₂P (>10⁴ Ω cm) because N‐doped carbon NFs are incorporated. As an electrocatalyst for HER, Ni₂P@NPCNFs reveal remarkable performance compared to other previously reported catalysts in acidic media. Additionally, it offers excellent catalytic ability and durability in both neutral and basic media. Encouraged by the excellent electrocatalytic performance of Ni₂P@NPCNFs, a series of pea‐like M_xP@NPCNFs, including Fe₂P@NPCNFs, Co₂P@NPCNFs, and Cu₃P@NPCNFs, were synthesized by the same method. Detailed characterization suggests that the newly developed method could render combinations of ultrafine metal phosphides with porous carbon accessible; thereby, extending opportunities in electrocatalytic applications.     

Synthesis and Characterization of Ba0.5Sr0.5NixCo0.8‐xFe0.2O3‐δ (x=0 and 0.2) Perovskites as Electro‐catalysts for Methanol Oxidation in Alkaline Media

In this study, the perovskite‐type oxides Ba_0.5Sr_0.5Ni_0.2Co_0.6Fe_0.2O_3‐δ (BSNCF) with different B‐site transition metal elements were investigated as a new catalyst for methanol oxidation. They were prepared by the partial substitution of cobalt by nickel in Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ (BSCF) sample. Both materials prepared using a combined citrate/EDTA complexing method and their catalytic activitiesfor methanol oxidation were comparativelystudied using cyclic‐voltammetry (CV) and chronoamperometry (CA) in alkaline media at room temperature. Structural, textural, morphological and redox properties of the synthesized solids were investigated by several physicochemical methods such as XRD, FTIR, LRS, SEM‐EDX, BET and H₂‐TPR. The as‐prepared materials were obtained with high purity and high crystallinity. The partial substitution of cobalt by nickel in BSCF perovskite changes the structure of the solid which pass from cubic structure to a mixture of cubic and hexagonal forms. This change is accompanied by an increase in BET surface area and a decrease in crystalline size. For the electro‐catalysis measurements, results demonstrated that partially substituting of Co by Ni in BSCF structure enhance the electro‐catalytical activity towards the oxidation of methanol under alkaline conditions, making it an electro‐catalyst candidate for practical utilization in DMFCs.     

Fe/Co‐based Bimetallic MOF‐derived Co3Fe7@NCNTFs Bifunctional Electrocatalyst for High‐Efficiency Overall Water Splitting

Electrocatalytic water splitting to produce hydrogen and oxygen is regarded as one of the most promising methods to generate clean and sustainable energy for replacing fossil fuels. However, the design and development of an efficient bifunctional catalyst for simultaneous generation of hydrogen and oxygen remains extremely challenging yet is critical for the practical implementation of water electrolysis. Here, we report a facile method to fabricate novel N‐doped carbon nanotube frameworks (NCNTFs) by the pyrolysis of a bimetallic metal organic framework (MIL‐88‐Fe/Co). The resultant electrocatalyst, Co₃Fe₇@NCNTFs, exhibits excellent oxygen evolution reaction (OER) activity, achieving 10 mA/cm² at a low overpotential of just 264 mV in 1 M KOH solution, and 197 mV for the hydrogen evolution reaction. The high electrocatalytic activity arises from the synergistic effect between the chemistry of the Co₃Fe₇ and the NCNTs coupled to the novel framework structure. The remarkable electrocatalytic performance of our bifunctional electrocatalyst provides a promising pathway to high‐performance overall water splitting and electrochemical energy devices.     

One‐Step Synthesis of Self‐Supported Nickel Phosphide Nanosheet Array Cathodes for Efficient Electrocatalytic Hydrogen Generation

Nickel phosphide is an emerging low‐cost, earth‐abundant catalyst that can efficiently reduce water to generate hydrogen. However, the synthesis of nickel phosphide catalysts usually involves multiple steps and is laborious. Herein, a convenient and straightforward approach to the synthesis of a three‐dimensional (3D) self‐supported biphasic Ni₅P₄‐Ni₂P nanosheet (NS) array cathode is presented, which is obtained by direct phosphorization of commercially available nickel foam using phosphorus vapor. The synthesized 3D Ni₅P₄‐Ni₂P‐NS array cathode exhibits outstanding electrocatalytic activity and long‐term durability toward the hydrogen evolution reaction (HER) in acidic medium. The fabrication procedure reported here is scalable, showing substantial promise for use in water electrolysis. More importantly, the approach can be readily extended to synthesize other self‐supported transition metal phosphide HER cathodes.     

Enhanced Electrochemical Capacitive Properties of Nickel‐Cobalt Oxide Nano‐flakes Materials

Nickel‐cobalt oxide nano‐flakes materials are successfully synthesized by a facile chemical co‐precipitation method followed by a simple calcination process. The studies show that the as‐prepared nickel‐cobalt oxides with different Ni/Co ratio are composed of NiO and Co₃O₄ compounds. The Co_0.56Ni_0.44 oxide material, which exhibits a mesoporous structure with a narrow distribution of pore size from 2 to 7 nm, possesses markedly enhanced charge‐discharge properties at high current density compared with the pure NiO and pure Co₃O₄. The Co_0.56Ni_0.44 oxide electrode shows a specific capacitance value of 1227 F/g at 5 mA/cm², which is nearly three times greater than that of the pure NiO electrode at the same current density.     

A Computational Study of the Mechanism of Hydrogen Evolution by Cobalt(Diimine‐Dioxime) Catalysts

Cobalt(diimine‐dioxime) complexes catalyze hydrogen evolution with low overpotentials and remarkable stability. In this study, DFT calculations were used to investigate their catalytic mechanism, to demonstrate that the initial active state was a Co^I complex and that H₂ was evolved in a heterolytic manner through the protonation of a Co^II? hydride intermediate. In addition, these catalysts were shown to adjust their electrocatalytic potential for hydrogen evolution to the pH value of the solution and such a property was assigned to the presence of a H⁺‐exchange site on the oxime bridge. It was possible to establish that protonation of the bridge was directly involved in the H₂‐evolution mechanism through proton‐coupled electron‐transfer steps. A consistent mechanistic scheme is proposed that fits the experimentally determined electrocatalytic and electrochemical potentials of cobalt(diimine‐dioxime) complexes and reproduces the observed positive shift of the electrocatalytic potential with increasing acidity of the proton source.     

Dye‐Sensitized TiO2 Nanotube Solar Cells: Rational Structural and Surface Engineering on TiO2 Nanotubes

Owing to well‐defined structural parameters and enhanced electronic properties, highly ordered TiO₂ nanotube arrays have been employed to substitute TiO₂ nanoparticles for use in dye‐sensitized solar cells. To further improve the performance of dye‐sensitized TiO₂ nanotube solar cells, efforts have been directed toward the optimization of TiO₂ photoanodes, dyes, electrolytes, and counter electrodes. Herein, we highlight recent progress in rational structural and surface engineering on anodic TiO₂ nanotube arrays and their effects on improving the power conversion efficiency of dye‐sensitized TiO₂ nanotube solar cells.     

Enhanced Performance of Quasi‐Solid‐State Dye‐Sensitized Solar Cells by Branching the Linear Substituent in Sensitizers Based on Thieno[3,4‐c]pyrrole‐4,6‐dione

Thieno[3,4‐c]pyrrole‐4,6‐dione‐based organic sensitizers with triphenylamine ( FNE38 and FNE40 ) or julolidine ( FNE39 and FNE41 ) as electron‐donating unit have been designed and synthesized. A linear hexyl group or a branched alkyl chain, the 2‐ethylhexyl group, is incorporated into molecular skeleton of the dyes to minimize intermolecular interactions. The absorption, electrochemical, and photovoltaic properties for these sensitizers were then systematically investigated. It is found that the sensitizers have similar photophysical and electrochemical properties, such as absorption spectra and energy levels, owing to their close chemical structures. However, the quasi‐solid‐state dye‐sensitized solar cells (DSSCs) based on the two types of sensitizers exhibit very different performance parameters. Upon the incorporation of the short ethyl group on the hexyl moiety, enhancements in both open‐circuit voltage (V_oc) and short‐circuit current (J_sc) are achieved for the quasi‐solid‐state DSSCs. The V_oc gains originating from the suppression of charge recombination were quantitatively investigated and are in good agreement with the experimentally observed V_oc enhancements. Therefore, an enhanced solar energy conversion efficiency (η) of 6.16 %, constituting an increase by 23 %, is achieved under standard AM 1.5 sunlight without the use of coadsorbant agents for the quasi‐solid‐state DSSC based on sensitizer FNE40 , which bears the branched alkyl group, in comparison with that based on FNE38 carrying the linear alkyl group. This work presents a design concept for considering the crucial importance of the branched alkyl substituent in novel metal‐free organic sensitizers.     

Fe‐Doped Ni3C Nanodots in N‐Doped Carbon Nanosheets for Efficient Hydrogen‐Evolution and Oxygen‐Evolution Electrocatalysis

Uniform Ni₃C nanodots dispersed in ultrathin N‐doped carbon nanosheets were successfully prepared by carburization of the two dimensional (2D) nickel cyanide coordination polymer precursors. The Ni₃C based nanosheets have lateral length of about 200 nm and thickness of 10 nm. When doped with Fe, the Ni₃C based nanosheets exhibited outstanding electrocatalytic properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). For example, 2 at % Fe (atomic percent) doped Ni₃C nanosheets depict a low overpotential (292 mV) and a small Tafel slope (41.3 mV dec⁻¹) for HER in KOH solution. An outstanding OER catalytic property is also achieved with a low overpotential of 275 mV and a small Tafel slope of 62 mV dec⁻¹ in KOH solution. Such nanodot‐incorporated 2D hybrid structures can serve as an efficient bifunctional electrocatalyst for overall water splitting.     

NixCo3‐xO4 Nanoneedle Arrays Grown on Ni Foam as an Efficient Bifunctional Electrocatalyst for Full Water Splitting

Developing highly active, stable and robust electrocatalysts based on earth‐abundant elements for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is important for many renewable energy conversion processes. Herein, Ni_xCo_3‐xO₄ nanoneedle arrays grown on 3D porous nickel foam (NF) was synthesized as a bifunctional electrocatalyst with OER and HER activity for full water splitting. Benefiting from the advantageous structure, the composite exhibits superior OER activity with an overpotential of 320 mV achieving the current density of 10 mA cm^?2. An exceptional HER activity is also acquired with an overpotential of 170 mV at the current density of 10 mA cm^?2. Furthermore, the catalyst also shows the superior activity and stability for 20 h when used in the overall water splitting cell. Thus, the hierarchical 3D structure composed of the 1D nanoneedle structure in Ni_xCo_3‐xO₄/NF represents an avenue to design and develop highly active and bifunctional electrocatalysts for promising energy conversion.     

Ionic Liquid Based Electrolyte with Mesoporous Silica SBA-15 as Framework for Quasi-solid-state Dye-sensitized Solar Cells

Quasi-solid-state electrolytes were fabricated with mesoporous silica SBA-15 as a framework material. Ionic conductivity measurements revealed that SBA-15 can enhance the conductivity of the quasi-solid-state electrolyte. The diffusion coefficients of polyiodide ions such as Ⅰ3ˉ and Ⅰ5ˉ which were confirmed by Raman spectroscopic measurement, were about twice larger than that of I-. The optimized photoenergy conversion efficiency of dye-sensitized solar cells （DSSC） with the quasi-solid-state electrolyte was 4.3% under AM 1.5 irradiation at 75 mW·cm^-2 light intensity.