Ultrahigh Nitrogen Doping of Carbon Nanosheets for High Capacity and Long Cycling Potassium Ion Storage

Potassium‐based energy storage devices (PESDs) are promising candidates for large‐scale energy storage applications owing to potassiums abundant in nature, the low standard redox potential (?2.93 V for K/K⁺ vs the standard hydrogen electrode) of potassium (K), and high ionic conductivity of K‐ion based electrolytes. However, lack of proper cathode and anode materials hinder practical applications of PESDs. In this work, carbon nanosheets doped with an ultrahigh content of nitrogen (22.7 at%) are successfully synthesized as an anode material for a K‐ion battery, which delivers a high capacity of 410 mAh g^?1 at a current density of 500 mA g^?1, which is the best result among the carbon based anodes for PESDs. Moreover, the battery exhibits an excellent cycling performance with a capacity retention of 70% after 3000 cycles at a high current density of 5 A g^?1. In situ Raman, galvanostatic intermittent titration, and density functional theory calculations reveal that the ultrahigh N‐doped carbon nanosheet (UNCN) simultaneously combines the diffusion and pseudocapacitive mechanisms together, which remarkably improves its electrochemical performances in K‐ion storage. These results demonstrate the good potential of UNCNs as a high‐performance anode for PESDs.     

The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis,Advanced Characterization,and Fundamentals

The incorporation of atomic scale defects, such as cation vacancies, in electrode materials is considered an effective strategy to improve their electrochemical energy storage performance. In fact, cation vacancies can effectively modulate the electronic properties of host materials, thus promoting charge transfer and redox reaction kinetics. Such defects can also serve as extra host sites for inserted proton or alkali cations, facilitating the ion diffusion upon electrochemical cycling. Altogether, these features may contribute to improved electrochemical performance. In this review, the latest progress in cation vacancies‐based electrochemical energy storage materials, covering the synthetic approaches to incorporate cation vacancies and the advanced techniques to characterize such vacancies and identify their fundamental role, are provided from the chemical and materials point of view. The key challenges and future opportunities for cation vacancies‐based electrochemical energy storage materials are also discussed, particularly focusing on cation‐deficient transition metal oxides (TMOs), but also including newly emerging materials such as transition metal carbides (MXenes).     

Anthraquinone Derivatives in Aqueous Flow Batteries

Anthraquinone derivatives are being considered for large scale energy storage applications because of their chemical tunability and rapid redox kinetics. The authors investigate four anthraquinone derivatives as negative electrolyte candidates for an aqueous quinone‐bromide redox flow battery: anthraquinone‐2‐sulfonic acid (AQS), 1,8‐dihydroxyanthraquinone‐2,7‐disulfonic acid (DHAQDS), alizarin red S (ARS), and 1,4‐dihydroxyanthraquinone‐2,3‐dimethylsulfonic acid (DHAQDMS). The standard reduction potentials are all lower than that of anthraquinone‐2,7‐disulfonic acid (AQDS), the molecule used in previous quinone‐bromide batteries. DHAQDS and ARS undergo irreversible reactions on contact with bromine, which precludes their use against bromine but not necessarily against other electrolytes. DHAQDMS is apparently unreactive with bromine but cannot be reversibly reduced, whereas AQS is stable against bromine and stable upon reduction. The authors demonstrate an AQS‐bromide flow cell with higher open circuit potential and peak galvanic power density than the equivalent AQDS‐bromide cell. This study demonstrates the use of chemical synthesis to tailor organic molecules for improving flow battery performance.     

Fabrication of glucose oxidase/polypyrrole biosensor by galvanostatic method in various pH aqueous solutions

The pH effect of pyrrole electropolymerization in the presence of glucose oxidase (GODx) on the performance and characteristic of galvanostatically fabricated glucose oxidase/polypyrrole (Ppy) biosensor is reported. Preparing the GODx/Ppy biosensors in 0.1 M KCl saline solution with various pH containing 0.05 M pyrrole monomer and 0.5 mg/ml GODx at 382 microA/cm2 current density for 100 mC/cm2 film thickness, both the galvanostatic responses and characteristics of these resulted biosensors were obtained. The results revealed that the galvanostatic glucose biosensor fabricated at neutral pH condition exhibited much higher sensitivity than those fabricated at lower or higher pH conditions, and had a good linearity form zero to 10 mM glucose with the sensitivity of 7 nA/mM. Finally, the long-term stability and the kinetic parameters, Michaelis constant and maximum current, of this biosensor were also reported.     

Recent Progress in Flexible Electrochemical Capacitors: Electrode Materials,Device Configuration,and Functions

With increasing demand for portable, flexible, and even wearable electronic devices, flexible energy storage systems have received increasing attention as a key component in this emerging field. Among the options, supercapacitors, commonly referred to as ultracapacitors or electrochemical capacitors, are widely recognized as a potential energy storage system due to their high power, fast charge/discharge rate, long cycling life‐time, and low cost. To date, considerable effort has been dedicated to developing high‐performance flexible supercapacitors based on various electrode materials; including carbon nanomaterials (e.g., carbon nanotubes, graphene, porous carbon materials, carbon paper, and textile), conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), and hybrid materials. A brief introduction to the field is provided and the state‐of‐the‐art is reviewed with special emphasis on electrode materials and device configurations.     

Synthesis of polypyrrole within the cell wall of yeast by redox-cycling of [Fe(CN)6]3−/[Fe(CN)6]4−

Yeast cells are often used as a model system in various experiments. Moreover, due to their high metabolic activity, yeast cells have a potential to be applied as elements in the design of biofuel cells and biosensors. However a wider application of yeast cells in electrochemical systems is limited due to high electric resistance of their cell wall. In order to reduce this problem we have polymerized conducting polymer polypyrrole (Ppy) directly in the cell wall and/or within periplasmic membrane. In this research the formation of Ppy was induced by [Fe(CN)₆]³⁻ions, which were generated from K₄[Fe(CN)₆], which was initially added to polymerization solution. The redox process was catalyzed by oxido-reductases, which are present in the plasma membrane of yeast cells. The formation of Ppy was confirmed by spectrophotometry and atomic force microscopy. It was confirmed that the conducting polymer polypyrrole was formed within periplasmic space and/or within the cell wall of yeast cells, which were incubated in solution containing pyrrole, glucose and [Fe(CN)₆]⁴⁻. After 24 h drying at room temperature we have observed that Ppy-modified yeast cell walls retained their initial spherical form. In contrast to Ppy-modified cells, the walls of unmodified yeast have wrinkled after 24 h drying. The viability of yeast cells in the presence of different pyrrole concentrations has been evaluated.     

Conducting polymer based fluorescence quenching as a new approach to increase the selectivity of immunosensors

Polypyrrole (Ppy) has been shown to be a superior matrix for fluorescence detection based immunosensors: (i) the fluorescence of polypyrrole and polypyrrole modified by entrapped proteins was almost not detectable when this polymer was excited by near UV 325 nm light; (ii) polypyrrole quenched the fluorescence of such fluorescence agents as fluoresceine 5(6)-isothiocyanate, rhodamine B and enzyme-horseradish peroxidase (HRP) by almost 100% if they were deposited in the solution as a drop at the Ppy surface followed by evaporation of the solvent. According to our knowledge, this work is first application of Ppy in the design of a fluorescence-based immunosensor, where low Ppy fluorescence background and Ppy induced fluorescence quenching were exploited. These sensors were devoted to the detection of antibodies against bovine leukemia virus (BLV) protein gp51 (anti-gp51-Ab). A biological recognition system of this fluorescence immunosensor model was based on polypyrrole with entrapped BLV proteins gp51 (gp51/Ppy). This gp51/Ppy layer was applied for the detection of anti-gp51-Ab. Secondary antibodies against anti-gp51-Ab labeled with HRP (Ab*) were applied as fluorescence-detectable labels that are able to recognize specifically and interact with the complex of gp51 proteins and anti-gp51-Ab antibodies (gp51/anti-gp51-Ab). It was demonstrated that fluorescence of non-specifically adsorbed Ab* was almost completely quenched by the Ppy substrate. In addition, enzymatic activity of HRP was exploited as a traditional reference method for verification of the formation of the immune complex gp51/anti-gp51-Ab/Ab*.     

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS‐based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS‐based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS‐based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS‐based electrode materials for next‐generation electrochemical energy storage and conversion systems.     

From Ionogels to Biredox Ionic Liquids: Some Emerging Opportunities for Electrochemical Energy Storage and Conversion Devices

Ionic liquids (ILs) continue to receive attention for applications in electrochemistry because of their unique properties as charge carriers (electrolytes) and redox shuttles (solar cells) and their ability to promote energy storage either electrostatically (supercapacitors) or chemically (secondary batteries). More specifically, the confinement of ILs in nanopores or the adsorption at surfaces, are considered a promising strategy for all solid‐state energy storage and conversion devices. Upon such immobilization, one benefits from the specific properties of ILs (large electrochemical window, relatively high ionic conductivity, task‐specific functionalities, etc.) combined with surface and confinement effects that can be tuned by playing with the porosity and chemical nature of the host. Here, some emerging applications of ILs in electrochemistry are first discussed: silica‐based ionogels as solid electrolytes and systems that involve carbon host substrates, as typical electrode materials in electrical double layer capacitors and lithium secondary batteries. Also, a non‐exhaustive, yet a comprehensive picture of the confinement and surface effects at play in such applications is presented. Then, the confinement of task‐specific ILs such as protonic ILs, IL lithium salts, and biredox ILs, is discussed, which paves the way for promising perspectives. Finally, some concluding remarks are reported and directions for future work are suggested.     

Spatially Confined MnO2 Nanostructure Enabling Consecutive Reversible Charge Transfer from Mn(IV) to Mn(II) in a Mixed Pseudocapacitor‐Battery Electrode

Mn oxides are highly important electrode materials for aqueous electrochemical energy storage devices, including batteries and supercapacitors. Although MnO₂ is a promising pseudocapacitor material because of its outstanding rate and capacity performance, its electrochemical instability in aqueous electrolyte prevents its use at low electrochemical potential. Here, the possibility of stabilizing MnO₂ electrode using SiO₂‐confined nanostructure is demonstrated. Remarkably, an exceptionally good electrochemical stability under large negative polarization in aqueous (Li₂SO₄) electrolyte, usually unattainable for MnO₂‐based electrode, is achieved. Even more interestingly, this MnO₂–SiO₂ nanostructured composite exhibits unique mixed pseudocapacitance‐battery behaviors involving consecutive reversible charge transfer from Mn(IV) to Mn(II), which enable simultaneous high‐capacity and high‐rate characteristics, via different charge‐transfer kinetic mechanisms. This suggests a strategy to design and stabilize electrochemical materials that are comprised of intrinsically unstable but high‐performing component materials.     

Two‐Dimensional Materials for Beyond‐Lithium‐Ion Batteries

Lithium‐ion batteries (LIBs) have dominated the portable electronics industry and solid‐state electrochemical research and development for the past two decades. In light of possible concerns over the cost and future availability of lithium, sodium‐ion batteries (SIBs) and other new technologies have emerged as candidates for large‐scale stationary energy storage. Research in these technologies has increased dramatically with a focus on the development of new materials for both the positive and negative electrodes that can enhance the cycling stability, rate capability, and energy density. Two‐dimensional (2D) materials are showing promise for many energy‐related applications and particularly for energy storage, because of the efficient ion transport between the layers and the large surface areas available for improved ion adsorption and faster surface redox reactions. Recent research highlights on the use of 2D materials in these future ‘beyond‐lithium‐ion’ battery systems are reviewed, and strategies to address challenges are discussed as well as their prospects.     

Exploiting the Condensation Reactions of Acetophenone to Engineer Carbon‐Encapsulated Nb2O5 Nanocrystals for High‐Performance Li and Na Energy Storage Systems

Efficient synthetic methods to produce high‐performance electrode‐active materials are crucial for developing energy storage devices for large‐scale applications, such as hybrid supercapacitors (HSCs). Here, an effective approach to obtain controllable carbon‐encapsulated T‐Nb₂O₅ nanocrystals (NCs) is presented, based on the solvothermal treatment of NbCl₅ in acetophenone. Two separate condensation reactions of acetophenone generate an intimate and homogeneous mixture of Nb₂O₅ particles and 1,3,5‐triphenylbenzene (TPB), which acts as a unique carbon precursor. The electrochemical performance of the resulting composites as anode electrode materials can be tuned by varying the Nb₂O₅/TPB ratio. Remarkable performances are achieved for Li‐ion and Na‐ion energy storage systems at high charge–discharge rates (specific capacities of ≈90 mAh g^?1 at 100 C rate for lithium and ≈125 mAh g^?1 at 20 C for sodium). High energy and power densities are also achieved with Li‐ and Na‐ion HSC devices constructed by using the Nb₂O₅/C composites as anode and activated carbon (YPF‐50) as cathode, demonstrating the excellent electrochemical properties of the materials synthesized with this approach.     

Rationalizing the Molecular Design of Hole‐Selective Contacts to Improve Charge Extraction in Perovskite Solar Cells

Two new hole selective materials (HSMs) based on dangling methylsulfanyl groups connected to the C‐9 position of the fluorene core are synthesized and applied in perovskite solar cells. Being structurally similar to a half of Spiro‐OMeTAD molecule, these HSMs (referred as FS and DFS) share similar redox potentials but are endowed with slightly higher hole mobility, due to the planarity and large extension of their structure. Competitive power conversion efficiency (up to 18.6%) is achieved by using the new HSMs in suitable perovskite solar cells. Time‐resolved photoluminescence decay measurements and electrochemical impedance spectroscopy show more efficient charge extraction at the HSM/perovskite interface with respect to Spiro‐OMeTAD, which is reflected in higher photocurrents exhibited by DFS/FS‐integrated perovskite solar cells. Density functional theory simulations reveal that the interactions of methylammonium with methylsulfanyl groups in DFS/FS strengthen their electrostatic attraction with the perovskite surface, providing an additional path for hole extraction compared to the sole presence of methoxy groups in Spiro‐OMeTAD. Importantly, the low‐cost synthesis of FS makes it significantly attractive for the future commercialization of perovskite solar cells.     

Phase Transformation Induced Capacitance Activation for 3D Graphene‐CoO Nanorod Pseudocapacitor

Development of a pseudocapacitor over the integration of metal oxide on carbonaceous materials is a promising step towards energy storage devices with high energy and power densities. Here, a self‐assembled cobalt oxide (CoO) nanorod cluster on three‐dimensional graphene (CoO‐3DG) is synthesized through a facile hydrothermal method followed by heat treatment. As an additive‐free electrode, CoO‐3DG exhibits good electrochemical performance. Compared with CoO nanorod clusters grown on Ni foam (i.e., CoO‐Ni, ≈680 F g^?1 at 1 A g^?1 and ≈400 F g^?1 at 20 A g^?1), CoO‐3DG achieves much higher capacitance (i.e., ≈980 F g^?1 at 1 A g^?1 and ≈600 F g^?1 at 20 A g^?1) with excellent cycling stability of 103% retention of specific capacitance after 10 000 cycles. Furthermore, it shows an interesting activation process and instability with a redox reaction for CoO. In addition, the phase transformation from CoO nanorods to Co₃O₄ nanostructures was observed and investigated after charge and discharge process, which suggests the activation kinetics and the phase transformable nature of CoO based nanostructure. These observations demonstrate phase transformation with morphological change induced capacitance increasement in the emergent class of metal oxide materials for electrochemical energy storage device.     

Integrated Conductive Hybrid Architecture of Metal–Organic Framework Nanowire Array on Polypyrrole Membrane for All‐Solid‐State Flexible Supercapacitors

Metal–organic frameworks (MOFs) with intrinsically porous structures are promising candidates for energy storage, however, their low electrical conductivity limits their electrochemical energy storage applications. Herein, the hybrid architecture of intrinsically conductive Cu‐MOF nanowire arrays on self‐supported polypyrrole (PPy) membrane is reported for integrated flexible supercapacitor (SC) electrodes without any inactive additives, binders, or substrates involved. The conductive Cu‐MOFs nanowire arrays afford high conductivity and a sufficiently active surface area for the accessibility of electrolyte, whereas the PPy membrane provides decent mechanical flexibility, efficient charge transfer skeleton, and extra capacitance. The all‐solid‐state flexible SC using integrated hybrid electrode demonstrates an exceptional areal capacitance of 252.1 mF cm^?2, an energy density of 22.4 µWh cm^?2, and a power density of 1.1 mW cm^?2, accompanied by an excellent cycle capability and mechanical flexibility over a wide range of working temperatures. This work not only presents a robust and flexible electrode for wide temperature range operating SC but also offers valuable concepts with regards to designing MOF‐based hybrid materials for energy storage and conversion systems.     

Molecularly imprinted polypyrrole-based synthetic receptor for direct detection of bovine leukemia virus glycoproteins

Preparation and basic characterization of polypyrrole-based molecularly imprinted polymer (MIP) for label-free detection of bovine leukemia virus (BLV) glycoprotein gp51 (gp51) is firstly described. Polypyrrole (Ppy) was selected as a matrix for preparation of MIP. Polypyrrole doped by gp51 (gp51/Ppy) was prepared by electrochemical deposition of this polymer on the surface of platinum-black electrode. Then, molecules of gp51 were removed from polymeric backbone and molecularly imprinted polypyrrole (mPpy) was ready for recognition of gp51 in the aqueous solution. Pulsed amperometric detection (PAD) was applied for label-free detection of gp51 in the samples. Anti-gp51 antibodies and secondary antibodies labeled with horseradish peroxidase (HRP) were involved as markers for the control of mPpy preparation procedures. Control experiments were also simultaneously performed by spectrophotometrical detection of HRP activity. Application of anti-gp51 and HRP labelled secondary antibodies confirmed that generation of analytical signal was based on redoping of mPpy by gp51. During our experiments, only few mPpy redoping/dedoping cycles were effective, but generally this method seems to be very effective for the future development of mPpy-based MIPs. Preparation, electrochemical investigation and control procedures are described in the current paper.     

Graphitic Carbon Nanocage as a Stable and High Power Anode for Potassium‐Ion Batteries

As an emerging electrochemical energy storage device, potassium‐ion batteries (PIBs) have drawn growing interest due to the resource‐abundance and low cost of potassium. Graphite‐based materials, as the most common anodes for commercial Li‐ion batteries, have a very low capacity when used an anode for Na‐ion batteries, but they show reasonable capacities as anodes for PIBs. The practical application of graphitic materials in PIBs suffers from poor cyclability, however, due to the large interlayer expansion/shrinkage caused by the intercalation/deintercalation of potassium ions. Here, a highly graphitic carbon nanocage (CNC) is reported as a PIBs anode, which exhibits excellent cyclability and superior depotassiation capacity of 175 mAh g^?1 at 35 C. The potassium storage mechanism in CNC is revealed by cyclic voltammetry as due to redox reactions (intercalation/deintercalation) and double‐layer capacitance (surface adsorption/desorption). The present results give new insights into structural design for graphitic anode materials in PIBs and understanding the double‐layer capacitance effect in alkali metal ion batteries.     

Controlled Synthesis and Energy Applications of One‐Dimensional Conducting Polymer Nanostructures: An Overview

The past decade has witnessed increasing attention in the synthesis, properties, and applications of one‐dimensional (1D) conducting polymer nanostructures. This overview first summarizes the synthetic strategies for various 1D nanostructures of conjugated polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), poly(p‐phenylenevinylene) (PPV) and derivatives thereof. By using template‐directed or template‐free methods, nanoscale rods, wires/fibers, belts/ribbons, tubes, arrays, or composites have been successfully synthesized. With their unique structures and advantageous characteristics (e.g., high conductivity, high carrier mobility, good electrochemical activity, large specific surface area, short and direct path for charge/ion transportation, good mechanical properties), 1D conducting polymer nanostructures are demonstrated to be very useful for energy applications. Next, their applications in solar cells, fuel cells, rechargeable lithium batteries, and electrochemical supercapacitors are highlighted, with a strong emphasis on recent literature examples. Finally, this review ends with a summary and some perspectives on the challenges and opportunities in this emerging area of research.     

Recent Progress and Perspective in Electrode Materials for K‐Ion Batteries

The development of rechargeable batteries using K ions as charge carriers has recently attracted considerable attention in the search for cost‐effective and large‐scale energy storage systems. In light of this trend, various materials for positive and negative electrodes are proposed and evaluated for application in K‐ion batteries. Here, a comprehensive review of ongoing materials research on nonaqueous K‐ion batteries is offered. Information on the status of new materials discovery and insights to help understand the K‐storage mechanisms are provided. In addition, strategies to enhance the electrochemical properties of K‐ion batteries and computational approaches to better understand their thermodynamic properties are included. Finally, K‐ion batteries are compared to competing Li and Na systems and pragmatic opportunities and future research directions are discussed.     

Sulfur‐Impregnated,Sandwich‐Type,Hybrid Carbon Nanosheets with Hierarchical Porous Structure for High‐Performance Lithium‐Sulfur Batteries

Sandwich‐type hybrid carbon nanosheets (SCNMM) consisting of graphene and micro/mesoporous carbon layer are fabricated via a double template method using graphene oxide as the shape‐directing agent and SiO₂ nanoparticles as the mesoporous guide. The polypyrrole synthesized in situ on the graphene oxide sheets is used as a carbon precursor. The micro/mesoporous strcutures of the SCNMM are created by a carbonization process followed by HF solution etching and KOH treatment. Sulfur is impregnated into the hybrid carbon nanosheets to generate S@SCNMM composites for the cathode materials in Li‐S secondary batteries. The microstructures and electrochemical performance of the as‐prepared samples are investigated in detail. The hybrid carbon nanosheets, which have a thickness of about 10–25 nm, high surface area of 1588 m² g^?1, and broad pore size distribution of 0.8–6.0 nm, are highly interconnected to form a 3D hierarchical structure. The S@SCNMM sample with the sulfur content of 74 wt% exhibits excellent electrochemical performance, including large reversible capacity, good cycling stability and coulombic efficiency, and good rate capability, which is believed to be due to the structure of hybrid carbon materials with hierarchical porous structure, which have large specific surface area and pore volume.