Efficient Dye‐Sensitized Solar Cells with Potential‐Tunable Organic Sulfide Mediators and Graphene‐Modified Carbon Counter Electrodes

A new class of organic sulfide mediators with programmable redox properties is designed via density functional theory calculations and synthesized for efficient dye‐sensitized solar cells (DSCs). Photophysical and electrochemical properties of these mediators derived from systematical functionalization of the framework with electron donating and withdrawing groups (MeO, Me, H, Cl, CF₃, and NO₂) are investigated. With this new class of organic mediators, the redox potential can be fine‐tuned over a 170 mV range, overlapping the conventional I^?/I₃^?couple. Due to the suitable interplay of physical properties and electrochemical characteristics of the mediator involving electron‐donating MeO group, the DSCs based on this mediator behave excellently in various kinetic processes such as dye regeneration, electron recombination, and mass transport. Thus, the MeO derivative of the mediator is identified as having the best performance of this series of redox shuttles. As inferred from electrochemical impedance spectroscopy and cyclic voltammetry measurements, the addition of graphene into the normal carbon counter electrode material dramatically improves the apparent catalytic activity of the counter electrode towards the MeO derivative of mediator, resulting in N719 based DSCs showing a promising conversion efficiency of 6.53% under 100 mW·cm^?2 simulated sunlight illumination.     

Boosting Kinetics of Ce3+/Ce4+ Redox Reaction by Constructing TiC/TiO2 Heterojunction for Cerium-Based Flow Batteries

Cerium, a unique rare earth element, possesses a relatively high abundance, low cost, and high redox voltage, making it an attractive candidate for redox flow batteries. However, the sluggish kinetics and corrosion nature of the Ce³⁺/Ce⁴⁺ electrolyte result in overpotential and degradation of carbon felt (CF) electrodes, which hinders the development of cerium-based flow batteries. Therefore, it is essential to develop an electrode with high catalytic activity and corrosion resistance to the Ce³⁺/Ce⁴⁺ electrolyte. Herein, a TiC/TiO₂ coated carbon felt (TiC/TiO₂-CF) electrode is proposed. Remarkably, the TiC/TiO₂ coating effectively minimizes the exposure of the CF to the highly corrosive cerium electrolyte, consequently enhancing the electrode's corrosion resistance. Additionally, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy characterizations reveal the formation of a heterojunction between TiC and TiO₂, which significantly enhances the redox reaction kinetics of the Ce³⁺/Ce⁴⁺ redox couple. Eventually, the practical application of TiC/TiO₂-CF catalytic electrode in a Ce–Fe flow battery is demonstrated. This study sheds light on the synthesis conditions of the TiC/TiO₂-CF electrode, elucidates its heterojunction structure, and presents a novel Ce–Fe flow battery system.     

Detonation Nanodiamond and Onion‐Like‐Carbon‐Embedded Polyaniline for Supercapacitors

The detonation nanodiamond is a versatile low‐cost nanomaterial with tunable properties and surface chemistry. In this work, it is shown how the application of nanodiamond (ND) can greatly increase the performance of electrochemically active polymers, such as polyaniline (PANI). Symmetric supercapacitors containing PANI‐ND nanocomposite electrodes with 3–28 wt% ND show dramatically improved cycle stability and higher capacitance retention at fast sweep rate than pure PANI electrodes. Contrary to other PANI‐carbon nanocomposites, specific capacitance of the selected PANI electrodes with embedded ND increases after 10 000 galvanostatic cycles and reaches 640 F g^?1, when measured in a symmetric two‐electrode configuration with 1 M H₂SO₄ electrolyte. The demonstrated specific capacitance is 3–4 times higher than that of the activated carbons and more than 15 times higher than that of ND and onion‐like carbon (OLC).     

Switchable Supercapacitors with Transistor‐Like Gating Characteristics (G‐Cap)

A novel three‐electrode electrolyte supercapacitor (electric double‐layer capacitor [EDLC]) architecture in which a symmetrical interdigital “working” two‐electrode micro‐supercapacitor array (W‐Cap) is paired with a third “gate” electrode that reversibly depletes/injects electrolyte ions into the system controlling the “working” capacity effectively is described. All three electrodes are based on precursor‐derived nanoporous carbons with well‐defined specific surface area (735 m² g^?1). The interdigitated architecture of the W‐Cap is precisely manufactured using 3D printing. The W‐Cap operating with a proton conducting PVA/H₂SO₄‐hydrogel electrolyte and high capacitance (6.9 mF cm^?2) can be repeatedly switched “on” and “off”. By applying a low DC bias potential (?0.5 V) at the gate electrode, the AC electroadsorption in the coupled interdigital nanoporous carbon electrodes of the W‐Cap is effectively suppressed leading to a stark capacity drop by two orders of magnitude from an “on” to an “off” state. The switchable micro‐supercapacitor is the first of its kind. This general concept is suitable for implementing a broad range of nanoporous materials and advanced electrolytes expanding its functions and applications in future. The integration of intelligent functions into EDLC devices has extensive implications for diverse areas such as capacitive energy management, microelectronics, iontronics, and neuromodulation.     

Organo‐metal Diodes Based on a Nanoparticulate Poly(3,4‐ethylenedioxythiophene) Composite

Diodes composed of a nanoparticulate composite of poly(3,4‐ethylenedioxythiophene) and a Cu–Cu²⁺ redox couple in a poly(ethylene oxide)–LiBF₄ polymer‐electrolyte matrix between Ag and Zr electrodes show rectifications in excess of 50 000 at applied fields of 4 V. These large changes are considered to arise from both rectification at the Zr/ZrO₂ composite interface and from the switching of the composite material between two conductivity states by the application of a low potential field. The preparation and electrochemical characterisation of these novel active devices are discussed.     

Development of Nano‐TiO2 dye sensitised solar cells on high mobility transparent conducting oxide thin films

The optical transmission of dye‐sensitised solar cells (DSCs) can be tuned by altering the dye and/or particle size of the mesoporous TiO₂ layers, to allow their application as the top device in tandem solar cells. To benefit from this semi‐transparency, parasitic optical losses by the transparent electrodes must be minimised. This work investigates the influence of using two different transparent conductors, namely, the high mobility material titanium doped indium oxide (ITiO) and fluorine doped tin oxide (FTO) as electrodes for semi‐transparent DSCs. The overall NIR transparency through the DSCs increased significantly as each FTO electrode was replaced by an ITiO electrode. This increase was from 20–45% in the 1300–700 nm wavelength range for fully FTO‐based cells, to about 60% for fully ITiO‐based cells, across the same spectrum. DSCs prepared on these electrodes exhibited short circuit currents ranging from 14·0–14·9 mA/cm². The conversion efficiency of the cell with ITiO as both the front and rear electrodes was 5·8%, which though significant, was lower than the 8·2% attained by the cell using FTO electrodes, as a result of a lower fill factor. Improvements in the ITiO thermal stability and in the processing of the TiO₂ interfacial layer are expected to improve the cell efficiency of such single DSC devices. The high current density and optical transparency of ITiO‐based DSCs make them an interesting option for tandem solar cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Fabrication of a “Soft” Membrane Electrode Assembly Using Layer‐by‐Layer Technology

A new type of thin‐film electrode that does not utilize conducting polymers or traditional metal or chemical vapor deposition methods has been developed to create ultrathin flexible electrodes for fuel cells. Using the layer‐by‐layer (LbL) technique, carbon–polymer electrodes have been assembled from polyelectrolytes and stable carbon colloidal dispersions. Thin‐film LbL polyelectrolyte–carbon electrodes (LPCEs) have been successfully assembled atop both metallic and non‐metallic, porous and non‐porous substrates. These electrodes exhibit high electronic conductivities of 2–4 S cm^–1, and their porous structure provides ionic conductivities in the range of 10^–4 to 10^–3 S cm^–1. The electrodes show remarkable stability towards oxidizing, acidic, or delaminating basic solutions. In particular, an LPCE consisting of poly(diallyldimethyl ammonium chloride)/poly(2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid)/carbon–platinum assembled on a porous stainless steel support yields an open‐circuit potential similar to that of a pure platinum electrode. With LbL carbon–polymer electrodes, the membrane‐electrode assembly (MEA) in a fuel cell can be made several times thinner, assume multiple geometries, and hence be more compact. The mechanism for LPCE deposition, electrode structure, and miniaturization will be presented and discussed, and demonstrations of the LbL electrodes in a traditional Nafion‐based proton fuel cell and the first demonstration of a thin‐film hydrogen–air “soft” fuel cell fully constructed using multilayer assembly are described.     

The Imprint of Electropolymerized Polyphenol Films on Electrodes by Donor‐Acceptor Interactions: Selective Electrochemical Sensing of N,N′‐dimethyl‐4,4′‐bipyridinium (Methyl Viologen)

Polyphenol is electropolymerized on a Au electrode in the presence of N,N′‐dimethyl‐4,4′‐bipyridinium, methyl viologen, MV²⁺, to yield an imprinted film for MV²⁺. The association and dissociation of MV²⁺ to and from the imprinted sites is studied by electrochemical means and compared to the interactions of MV²⁺ with the non‐imprinted polymer. Donor‐acceptor interactions provide the driving force for the formation of the imprinted sites. The imprinted polymer reveals selectivity toward the association of MV²⁺, and the polymer‐bound MV²⁺ enables vectorial electron transfer between the electrode and redox label dissolved in the bulk electrolyte solution.     

A High‐Potential Anion‐Insertion Carbon Cathode for Aqueous Zinc Dual‐Ion Battery

Aqueous dual‐ion batteries (DIBs) are promising for large‐scale energy storage due to low cost and inherent safety. However, DIBs are limited by low capacity and poor cycling of cathode materials and the challenge of electrolyte decomposition. In this study, a new cathode material of nitrogen‐doped microcrystalline graphene‐like carbon is investigated in a water‐in‐salt electrolyte of 30 m ZnCl₂, where this carbon cathode stores anions reversibly via both electrical double layer adsorption and ion insertion. The (de)insertion of anions in carbon lattice delivers a high‐potential plateau at 1.85 V versus Zn²⁺/Zn, contributing nearly 1/3 of the capacity of 134 mAh g^?1 and half of the stored energy. This study shows that both the unique carbon structure and concentrated ZnCl₂ electrolyte play critical roles in allowing anion storage in carbon cathode for this aqueous DIB.     

Design of conduction band structure of TiO2 electrode using Nb doping for highly efficient dye‐sensitized solar cells

A barrier layer of undoped TiO₂ was deposited on the Nb‐doped TiO₂ electrode to suppress the recombination at the Nb‐doped TiO₂/dye–electrolyte interface for highly efficient dye‐sensitized solar cells (DSCs). The Nb content in TiO₂ was varied in a range of 0.7–3.5 mol% to modify the TiO₂ energy‐band structure. Nb‐doped TiO₂/dye interfaces were characterized by a combination of ultraviolet photoemission spectroscopy and optical absorption spectroscopy measurements, allowing the determination of the conduction band minimum (CBM) of the TiO₂ electrode and the lowest unoccupied molecular orbital of the N719 dye. The lowering of TiO₂ CBM by Nb doping induced the increase in short‐circuit current of DSCs. However, open‐circuit voltage and fill factor are decreased, and this result was ascribed to the enhanced recombination at the Nb‐doped TiO₂/dye–electrolyte interface. The effect of doping on charge transport in DSCs was analyzed using electrochemical impedance spectroscopy. We have shown that by introducing of TiO₂ barrier layer, the Nb doping content, which results in DSC highest efficiency, can be increased because of the suppression of the dopant‐induced recombination. The energy conversion efficiency of the solar cells increased from 7.8% to 9.0% when undoped TiO₂ electrode is replaced with electrode doped with 2.7 mol% of Nb because of the improvement of the electron injection and collection efficiencies. The correlation between the electronic structure of the TiO₂ electrode, charge transfer characteristics, and photovoltaic parameters of DSCs is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Multicolored Electrochromism of a Poly{1,4‐bis[2‐(3,4‐ethylenedioxy)thienyl]benzene} Derivative Bearing Viologen Functional Groups

1,4‐Bis[2‐(3,4‐ethylenedioxy)thienyl]‐2,5‐bis[6‐(1′‐butyl‐4,4′‐bipyridyl)hexyloxy]benzene tetrahexafluorophosphate (BEDOTPh‐2V) was synthesized and electropolymerized to form a viologen‐bearing conductive polymer [P(BEDOTPh‐2V)] film on an electrode surface. This polymer exhibits multicolor electrochromic behavior: it is highly transparent light‐blue P(BEDOTPh^1.7+‐2V²⁺) at 1.0 V (vs. Ag/Ag⁺), pale‐bisque P(BEDOTPh^0.6+‐2V²⁺) at 0.1 V, magenta P(BEDOTPh⁰‐2V²⁺) at –0.5 V, purple P(BEDOTPh⁰‐2V ^•+) at –0.9 V, and crimson P(BEDOT⁰‐2V⁰) at –1.4 V. Since both the viologen pendant and the polymer backbone have cathodic coloring characteristics and their redox‐potential ranges do not overlap each other, the dual electrochromic action provides more plentiful electrochromic colors compared to simple poly{1,4‐bis[2‐(3,4‐ethylenedioxy)thienyl]‐2,5‐dialkoxybenzene}.     

Enhanced Interfacial Stability of Hybrid‐Electrolyte Lithium‐Sulfur Batteries with a Layer of Multifunctional Polymer with Intrinsic Nanoporosity

The use of lithium‐ion conductive solid electrolytes offers a promising approach to address the polysulfide shuttle and the lithium‐dendrite problems in lithium‐sulfur (Li‐S) batteries. One critical issue with the development of solid‐electrolyte Li‐S batteries is the electrode–electrolyte interfaces. Herein, a strategic approach is presented by employing a thin layer of a polymer with intrinsic nanoporosity (PIN) on a Li⁺‐ion conductive solid electrolyte, which significantly enhances the ionic interfaces between the electrodes and the solid electrolyte. Among the various types of Li⁺‐ion solid electrolytes, NASICON‐type Li_1+xAl_xTi_2‐x(PO₄)₃ (LATP) offers advantages in terms of Li⁺‐ion conductivity, stability in ambient environment, and practical viability. However, LATP is susceptible to reaction with both the Li‐metal anode and polysulfides in Li‐S batteries due to the presence of easily reducible Ti⁴⁺ ions in it. The coating with a thin layer of PIN presented in this study overcomes the above issues. At the negative‐electrode side, the PIN layer prevents the direct contact of Li‐metal with the LATP solid electrolyte, circumventing the reduction of LATP by Li metal. At the positive electrode side, the PIN layer prevents the migration of polysulfides to the surface of LATP, preventing the reduction of LATP by polysulfides.     

Sputtered Titanium Carbide Thick Film for High Areal Energy on Chip Carbon‐Based Micro‐Supercapacitors

The areal energy density of on‐chip micro‐supercapacitors should be improved in order to obtain autonomous smart miniaturized sensors. To reach this goal, high surface capacitance electrode (>100 mF cm^?2) has to be produced while keeping low the footprint area. For carbide‐derived carbon (CDC) micro‐supercapacitors, the properties of the metal carbide precursor have to be fine‐tuned to fabricate thick electrodes. The ad‐atoms diffusion process and atomic peening effect occurring during the titanium carbide sputtering process are shown to be the key parameters to produce low stress, highly conductive, and thick TiC films. The sputtered TiC at 10^?3 mbar exhibits a high stress level, limiting the thickness of the TiC‐CDC electrode to 1.5 µm with an areal capacitance that is less than 55 mF cm^?2 in aqueous electrolyte. The pressure increase up to 10^?2 mbar induces a clear reduction of the stress level while the layer thickness increases without any degradation of the TiC electronic conductivity. The volumetric capacitance of the TiC‐CDC electrodes is equal to 350 F cm^?3 regardless of the level of pressure. High values of areal capacitance (>100 mF cm^?2) are achieved, whereas the TiC layer is relatively thick, which paves the way toward high‐performance micro‐supercapacitors.     

Electrochemical Activation of Manganese‐Based Cathode in Aqueous Zinc‐Ion Electrolyte

Low‐cost and highly safe zinc‐manganese batteries are expected for practical energy storage and grid‐scale application. The electrolyte adjustment is further combined to boost their performance output; however, the mechanism behind the electrochemical contrast caused by electrolyte composition remains unclear, which has held back the development of these systems. Hence, new insight into the electrochemical activation of manganese‐based cathodes, which is induced by the aqueous zinc‐ion electrolyte, is provided. The relationship between the desolvation of Zn²⁺ from [Zn(OH₂)₆]²⁺‐solvation shell and the electrolyte/electrode interfacial reaction to form Zn₄SO₄(OH)₆·4H₂O phase or its analogues is established, which is the key for the electrochemical activation. Further electrolyte optimization promotes the cycling stability of Zn/LiMn₂O₄ battery with a long life span over 2000 cycles. This work illuminates the confused direction in exploring electrolyte for zinc‐manganese batteries.     

A Novel Moisture‐Insensitive and Low‐Corrosivity Ionic Liquid Electrolyte for Rechargeable Aluminum Batteries

Rechargeable aluminum batteries (RABs) are extensively developed due to their cost‐effectiveness, eco‐friendliness, and low flammability and the earth abundance of their electrode materials. However, the commonly used RAB ionic liquid (IL) electrolyte is highly moisture‐sensitive and corrosive. To address these problems, a 4‐ethylpyridine/AlCl₃ IL is proposed. The effects of the AlCl₃ to 4‐ethylpyridine molar ratio on the electrode charge–discharge properties are systematically examined. A maximum graphite capacity of 95 mAh g^?1 is obtained at 25 mA g^?1. After 1000 charge–discharge cycles, ≈85% of the initial capacity can be retained. In situ synchrotron X‐ray diffraction is employed to examine the electrode reaction mechanism. In addition, low corrosion rates of Al, Cu, Ni, and carbon‐fiber paper electrodes are confirmed in the 4‐ethylpyridine/AlCl₃ IL. When opened to the ambient atmosphere, the measured capacity of the graphite cathode is only slightly lower than that found in a N₂‐filled glove box; moreover, the capacity retention upon 100 cycles is as high as 75%. The results clearly indicate the great potential of this electrolyte for practical RAB applications.     

Carbon‐Stabilized High‐Capacity Ferroferric Oxide Nanorod Array for Flexible Solid‐State Alkaline Battery–Supercapacitor Hybrid Device with High Environmental Suitability

Iron oxides are promising to be utilized in rechargeable alkaline battery with high capacity upon complete redox reaction (Fe³⁺ Fe⁰). However, their practical application has been hampered by the poor structural stability during cycling, presenting a challenge that is particularly huge when binder‐free electrode is employed. This paper proposes a “carbon shell‐protection” solution and reports on a ferroferric oxide–carbon (Fe₃O₄–C) binder‐free nanorod array anode exhibiting much improved cyclic stability (from only hundreds of times to >5000 times), excellent rate performance, and a high capacity of ≈7776.36 C cm^?3 (≈0.4278 C cm^?2; 247.5 mAh g^?1, 71.4% of the theoretical value) in alkaline electrolyte. Furthermore, by pairing with a capacitive carbon nanotubes (CNTs) film cathode, a unique flexible solid‐state rechargeable alkaline battery‐supercapacitor hybrid device (≈360 μm thickness) is assembled. It delivers high energy and power densities (1.56 mWh cm^?3; 0.48 W cm^?3/≈4.8 s charging), surpassing many recently reported flexible supercapacitors. The highest energy density value even approaches that of Li thin‐film batteries and is about several times that of the commercial 5.5 V/100 mF supercapacitor. In particular, the hybrid device still maintains good electrochemical attributes in cases of substantially bending, high mechanical pressure, and elevated temperature (up to 80 °C), demonstrating high environmental suitability.     

Effectively Utilizing NIR Light Using Direct Electron Injection from Up‐Conversion Nanoparticles to the TiO2 Photoanode in Dye‐Sensitized Solar Cells

TiO₂/NaYF₄:Yb³⁺,Er³⁺ nano‐heterostructures are prepared in situ on the TiO₂ photoanode of dye‐sensitized solar cells (DSCs). Transmission electron microscopy (TEM) and high‐resolution (HR)‐TEM confirm the formation of TiO₂/NaYF₄:Yb³⁺,Er³⁺ nano‐heterostructures. The up‐converted fluorescence spectrum of the photoanode containing the nano‐heterostructure confirms electron injection from NaYF₄:Yb³⁺,Er³⁺ to the condution band (CB) of TiO₂. When using a photoanode containing the nano‐heterostructure in a DSC, the overall efficiency (η) of the device is 17% higher than that of a device without the up‐conversion nanoparticles (UCNPs) and 13% higher than that of a device containing mixed TiO₂ and UCNPs. Nano‐heterostructures of TiO₂/NaYF₄:Yb³⁺,Tm³⁺ and TiO₂/NaYF₄:Yb³⁺,Ho³⁺ can also be prepared in situ on TiO₂ photoanodes. The overall efficiency of the device containing TiO₂/NaYF₄:Yb³⁺,Ho³⁺ nano‐heterostructures is 15% higher than the control device without UCNPs. When nano‐heterostructures of TiO₂/NaYF₄:Yb³⁺,Tm³⁺ are used, the open‐circuit voltage (V_oc) and the short‐circuit current density (J_sc) are all slightly decreased. The effect of the different UCNPs results from the different energy levels of Er³⁺, Tm³⁺, and Ho³⁺. These results demonstrate that utilizing the UCNPs with the apporpriate energy levels can lead to effective electron injection from the UCNPs to the CB of TiO₂, effectively improving the photocurrent and overall efficiency of DSCs while using NIR light.     

High‐Performance Mesostructured Organic Hybrid Pseudocapacitor Electrodes

The electrodes of a hybrid electrochemical capacitor which utilize the quinone (Q)‐hydroquinone (QH₂) couple, a prototypical organic redox system known to provide fast and reversible proton‐coupled electron‐transfer reactions, are deterministically mesostructured via a colloidal templating strategy to provide good ion and electron transport pathways, enabling a high rate performance. Specifically, a conducting polymer, polypyrrole (PPy), is functionalized with a pseudocapacitive material, a Q/QH₂‐containing catechol derivative, by noncovalent interactions. The mesostructure of this hybrid material is formed into an ordered 3D porous structure by a polystyrene colloidal crystal template‐assisted electrosynthesis. The catechol derivative is sufficiently bound to the PPy through noncovalent interactions to provide a volumetric capacitance as high as ≈130 F cm^?3 and a capacitance retention of ≈75% over 10 000 charging/discharging cycles. When compared with a randomly structured electrode, the deterministically structured electrode exhibits an improved rate performance due to the mesostructure facilitated electron and ion transport.     

Next‐Generation Activated Carbon Supercapacitors: A Simple Step in Electrode Processing Leads to Remarkable Gains in Energy Density

The global supercapacitor market has been growing rapidly during the past decade. Today, virtually all commercial devices use activated carbon. In this work, it is shown that laser treatment of activated carbon electrodes results in the formation of microchannels that can connect the internal pores of activated carbon with the surrounding electrolyte. These microchannels serve as electrolyte reservoirs that in turn shorten the ion diffusion distance and enable better interaction between the electrode surfaces and electrolyte ions. The capacitance can be further increased through fast and reversible redox reactions on the electrode surface using a redox‐active electrolyte, enabling the operation of a symmetric device at 2.0 V, much higher than the thermodynamic decompostion voltage of water. This simple approach can alleviate the low energy density of supercapacitors which has limited the widespread use of this technology. This work represents a clear advancement in the processing of activated carbon electrodes toward the next‐generation of low‐cost supercapacitors.     

Thermodynamic Activation of Charge Transfer in Anionic Redox Process for Li‐Ion Batteries

Anionic redox processes are vital to realize high capacity in lithium‐rich electrodes of lithium‐ion batteries. However, the activation mechanism of these processes remains ambiguous, hampering further implementation in new electrode design. This study demonstrates that the electrochemical activity of inert cubic‐Li₂TiO₃ is triggered by Fe³⁺ substitution, to afford considerable oxygen redox activity. Coupled with first principles calculations, it is found that electron holes tend to be selectively generated on oxygen ions bonded to Fe rather than Ti. Subsequently, a thermodynamic threshold is unravelled dictated by the relative values of the Coulomb and exchange interactions (U) and charge‐transfer energy (Δ) for the anionic redox electron‐transfer process, which is further verified by extension to inactive layered Li₂TiS₃, in which the sulfur redox process is activated by Co substitution to form Li_1.2Ti_0.6Co_0.2S₂. This work establishes general guidance for the design of high‐capacity electrodes utilizing anionic redox processes.