Reply to “note on a method to interrelate inner and outer electrode areas” by H. Vogt

High-Performance Aqueous Supercapacitors Based on a Self-Doped n-Type Conducting Polymer

Environmentally-benign materials play a pivotal role in advancing the scalability of energy storage devices. In particular, conjugated polymers constitute a potentially greener alternative to inorganic- and carbon-based materials. One challenge to wider implementation is the scarcity of n-doped conducting polymers to achieve full cells with high-rate performance. Herein, this work demonstrates the use of a self-doped n-doped conjugated polymer, namely poly(benzodifurandione) (PBDF), for fabricating aqueous supercapacitors. PBDF demonstrates a specific capacitance of 202 ± 3 F g-1, retaining 81% of the initial performance over 5000 cycles at 10 A g-1 in 2 m NaCl( aq ). PBDF demonstrates rate performances of up to 100 and 50 A g-1 at 1 and 2 mg cm-2, respectively. Electrochemical impedance analysis reveals a surface-mediated charge storage mechanism. Improvements can be achieved by adding reduced graphene oxide (rGO), thereby obtaining a specific capacitance of 288 ± 8 F g-1 and high-rate operation (270 A g-1). The performance of PBDF is examined in symmetric and asymmetric membrane-less cells, demonstrating high-rate performance, while retaining 83% of the initial capacitance after 100 000 cycles at 10 A g-1. PBDF thus offers new prospects for energy storage applications, showcasing both desirable performance and stability without the need for additives or binders and relying on environmentally friendly solutions.

Molecular-Level Pore Tuning in 2D Conductive Metal-Organic Frameworks for Advanced Supercapacitor Performance

Two-dimensional (2D) electrically conductive metal-organic frameworks (MOFs) have emerged as viable candidates for active electrode materials in supercapacitors due to their high electrical conductivity, high specific surface area, and intrinsic redox-active sites. Despite their promising electrochemical performance, their pseudocapacitive behavior via fast and reversible charge transfer reactions remains yet to be fully exploited. Here, we investigate the electrochemical energy storage mechanism of Cu3(HHTATP)2 (HHTATP = 2,3,6,7,10,11-hexahydroxy-1,5,9-triaminotriphenylene), a 2D conductive MOF featuring characteristic redox-active pendant aromatic amines. Cu3(HHTATP)2 exhibited pseudocapacitive charge storage with an average gravimetric capacitance of 340 ± 15 F g-1 at a discharge rate of 0.2 A g-1 and maintained a capacitance retention over 90% after 7000 galvanostatic cycles at 5 A g-1. The polar pendant amines not only enhanced capacitance via additional amine/imine redox activity but also reduced interfacial charge transfer resistance through modified electrode-electrolyte interactions. These results highlight the potential of molecular-level pore environment tuning as a strategic approach in materials design for energy storage applications.

Solvent Co-Intercalation-Induced Activation and Capacity Fade Mechanism of Few-/Multi-Layered MXenes in Lithium Ion Batteries

MXenes attract tremendous research efforts since their discovery in 2011 due to their unique physical and chemical properties, allowing for application in various fields. One of them is electrochemical energy storage due to their pseudocapacitive (=redox) behavior, high electronic conductivity, and charge storage versatility regarding the cationic species (e.g., Li⁺ ). MXenes typically display stable charge/discharge cycling behavior over hundreds of cycles in numerous electrolytes, however, a drastic loss of reversible capacity is detectable during the initial cycles. Furthermore, an electrochemical "activation" is also reported in the literature, especially for free-standing electrodes. Here, these electrochemical phenomena are investigated by electrochemical and analytical means to decipher the responsible mechanism by comparing few-layered and multi-layered Ti₃ C₂ T_x . A change in the pseudocapacitive behavior of MXenes during cycling can be explained by in situ X-ray diffraction studies, revealing solvent co-intercalation in the first cycle for the morphologically different MXenes. This co-intercalation is responsible for the capacity decay detected in the first cycles and is also responsible for the ongoing "activation" occurring in later cycles.

High-performance organic pseudocapacitors via molecular contortion

Pseudocapacitors harness unique charge-storage mechanisms to enable high-capacity, rapidly cycling devices. Here we describe an organic system composed of perylene diimide and hexaazatrinaphthylene exhibiting a specific capacitance of 689 F g^-1 at a rate of 0.5 A g^-1, stability over 50,000 cycles, and unprecedented performance at rates as high as 75 A g^-1. We incorporate the material into two-electrode devices for a practical demonstration of its potential in next-generation energy-storage systems. We identify the source of this exceptionally high rate charge storage as surface-mediated pseudocapacitance, through a combination of spectroscopic, computational and electrochemical measurements. By underscoring the importance of molecular contortion and complementary electronic attributes in the selection of molecular components, these results provide a general strategy for the creation of organic high-performance energy-storage materials.

Niobium pentoxide based materials for high rate rechargeable electrochemical energy storage

The demand for high rate energy storage systems is continuously increasing driven by portable electronics, hybrid/electric vehicles and the need for balancing the smart grid. Accordingly, Nb₂O₅ based materials have gained great attention because of their fast cation intercalation faradaic charge storage that endows them with high rate energy storage performance. In this review, we describe the crystalline features of the five main phases of Nb₂O₅ and analyze their specific electrochemical characteristics with an emphasis on the intrinsic ion intercalation pseudocapacitive behavior of T-Nb₂O₅. The charge storage mechanisms, electrochemical performance and state-of-the-art characterization techniques for Nb₂O₅ anodes are summarized. Next, we review recent progress in developing various types of Nb₂O₅ based fast charging electrode materials, including Nb₂O₅ based mixed metal oxides and composites. Finally, we highlight the major challenges for Nb₂O₅ based materials in the realm of high rate rechargeable energy storage and provide perspectives for future research.

Tin Oxide Encapsulated into Pyrolyzed Chitosan as a Negative Electrode for Lithium Ion Batteries

Tin oxide is one of the most promising electrode materials as a negative electrode for lithium-ion batteries due to its higher theoretical specific capacity than graphite. However, it suffers lack of stability due to volume changes and low electrical conductivity while cycling. To overcome these issues, a new composite consisting of SnO2 and carbonaceous matrix was fabricated. Naturally abundant and renewable chitosan was chosen as a carbon source. The electrode material exhibiting 467 mAh g−1 at the current density of 18 mA g−1 and a capacity fade of only 2% after 70 cycles is a potential candidate for graphite replacement. Such good electrochemical performance is due to strong interaction between amine groups from chitosan and surface hydroxyl groups of SnO2 at the preparation stage. However, the charge storage is mainly contributed by a diffusion-controlled process showing that the best results might be obtained for low current rates.

Boosting the capacitive property of nickel cobalt aluminum layered double hydroxide in neutral electrolyte

Layered double hydroxide (LDH) has shown great potential for energy storage due to their high theoretical specific capacitance, relatively low cost and eco-friendliness. LDH, however, always works in alkali aqueous electrolyte for supercapacitors, which brings serious environmental pollution. In this work, a reduced graphite oxide/Fe(CN)₆^3-- nickel cobalt aluminum LDH (RGO/Fe(CN)₆^3--LDH) composite has been prepared via ion-exchange reaction using RGO/LDH as precursor. RGO/Fe(CN)₆^3--LDH electrode provides a specific capacitance of 221 F g^-1 in a wide potential window of -1 ~ 0.8 V vs. SCE in Na₂SO₄ aqueous electrolyte, and which is much higher than that of LDH electrode (3.56 F g^-1). Owing to the wide potential window of RGO/Fe(CN)₆^3--LDH electrode, a symmetrical solid supercapacitor device (RGO/Fe(CN)₆^3--LDH//RGO/Fe(CN)₆^3--LDH) with a high voltage of 2.0 V can deliver a high specific energy of 25.2 Wh kg^-1 at a specific power of 250 W kg^-1, and a capacitance retention of 75% after galvanostatic charging/discharging at 5 A g^-1 for 5000 times. This work supplies enlightenment for boosting the capacitive performance of LDHs in neutral electrolyte.

How to measure and report the capacity of electrochemical double layers, supercapacitors, and their electrode materials

Relevant fundamentals of the electrochemical double layer and supercapacitors utilizing the interfacial capacitance as well as superficial redox processes at the electrode/solution interface are briefly reviewed. Experimental methods for the determination of the capacity of electrochemical double layers, of charge storage electrode materials for supercapacitors, and of supercapacitors are discussed and compared. Intrinsic limitations and pitfalls are indicated; popular errors, misconceptions, and mistakes are evaluated. The suitability of available methods is discussed, and practical recommendations are provided.

Comparison of Charge Storage Properties of Prussian Blue Analogues Containing Cobalt and Copper

Prussian blue analogues are of great interest as alternative battery materials because of their long life cycle and potential use of earth-abundant elements. In this work, thin film mixed-metal hexacyanoferrates (HCFs) based on NiCo and NiCu alloys were fabricated in an all electrochemical process. The structure and composition of the samples were characterized, along with the charge storage capacity and kinetics of the charge transfer reaction. For both NiCo-HCF and NiCu-HCF samples, the total charge capacity increased with the substitution of Ni with more Co or Cu, and the increase was larger for Cu samples than for Co samples. On the other hand, the charge storage kinetics had only a modest change with substituted metal, and these effects were independent of the amount of that substitution. Thus, the mixed-metal HCFs have promise for increasing overall storage capacity without negatively influencing the rate capability when used in battery applications.

Electrochemical oxidation of gaseous benzene on a Sb-SnO2/foam Ti nano-coating electrode in all-solid cell

An all-solid cell with a solid polymer electrolyte was applied to electrochemical oxidation of low-concentration indoor gaseous aromatic pollution. Antimony-doped tin dioxide nanocoatings deposited on a titanium foam substrate (Ti/Sb-SnO₂) with different Sb/Sn ratios (4.8-14.0 mol%) and loading weight of Sb-SnO₂ (4.4-7.7 mg cm^-2) were used as dimensionally stable anodes. Sn and Sb were homogeneously dispersed on the substrate, and a crack-free nanocoating was built when the loading of nanocoating was increased to 6.3 mg cm^-2. The activity tests for oxidation of benzene showed that 40 ppm gaseous benzene was converted to CO₂ with high selectivity (85%) at the low cell voltage of 2.0 V in this all-solid cell. The conversion of benzene was greatly increased from 30% to 100% upon increasing the Sb/Sn ratio of the nanocoating from 4.7 mol% to 14.0 mol%. With the increase of nanocoating loading (Sb/Sn = 14.0 mol%) from 6.3 to 7.7 mg cm^-2, the conversion of 100 ppm benzene was increased from 70% to 100%. Cyclic voltammetry revealed that high Sb content in the oxide nanocoating increased the overpotential and current intensity of the oxygen evolution reaction. The large outer charge q_o^∗ related to the electroactive surface of the SS-7.7/Ti3 electrode was up to 305.3 mC cm^-2, which were responsible for its excellent electrochemical performance in the benzene oxidation process. Our studies provide a potential method for removal of indoor VOCs at ambient temperature.

Ultrathin Ti2 Nb2 O9 Nanosheets with Pseudocapacitive Properties as Superior Anode for Sodium-Ion Batteries

Sodium-ion batteries are emerging as promising candidates for grid energy storage because of the abundant sodium resources and low cost. However, the identification and development of suitable anode materials is far from being satisfactory. Here, it is demonstrated that the Ti₂ Nb₂ O₉ nanosheets with tunnel structure can be used as suitable anode materials for sodium-ion batteries. Ti₂ Nb₂ O₉ nanosheets are synthesized by liquid exfoliation combined with topotactic dehydration, delivering a high reversible capacity of 250 mAh g^-1 at 50 mA g^-1 at a suitable average voltage of ≈0.7 V. It is found that a low energy diffusion barrier, enlarged interlayer spacing, and exceptional nanoporosity together give rise to high rate performance characterized by pseudocapacitive behavior. The observed high reversible capacity, excellent rate capability, and good cyclability of Ti₂ Nb₂ O₉ nanosheets make this material competitive when compared to other sodium insertion anode materials.

Hybrid composite polyaniline-nickel hydroxide electrode materials for supercapacitor applications

Pristine and nanocomposite (NC) hybrid electrodes of polyaniline (PANI)-nickel hydroxide [Ni(OH)₂] have been prepared by single and two-step electrodeposition processes, respectively, onto stainless-steel (SS) substrates. Enhanced reversibility and stability of amorphous PANI- Ni(OH)₂ NC electrodes compared to single electrode materials have been explored. PANI has a nanofibrous morphology, Ni(OH)₂ has nanoplatelet-type morphology, and the NC electrodes retain an overall nanofibrous morphology. The maximum specific capacitance (SC), obtained from integrated charge under voltammetric conditions, for PANI (electro-deposited for 5 min), NC (electrodeposition of Ni(OH)₂ for 10 min and 20 min onto PANI electrode surface) and Ni(OH)₂ (electrodeposited for 10 min) electrodes, are 0.59, 39.06, 32.36, and 113.8 F/g, respectively, suggesting higher electrochemical performance of Ni(OH)₂ electrode compared to PANI and NC electrodes. The retention in SC values with faster scan rates from 10 to 100 mV/s for PANI, NC (10 min), NC (20 min) and Ni(OH)₂ are 38.7, 61.1, 52.4, and 29.0 %, respectively, explicitly confirming a higher reversibility in NC electrodes. The retention in SC values with increase of cycle number up to 1000 for PANI, NC (10 min), NC (20 min) and Ni(OH)₂ electrodes are 34.9, 61.5, 67.5, and 40.7 % respectively, demonstrating higher electrochemical stability of NC electrodes over pure-phase electrodes. Nearly 2.15, 79.36, 66.66 and 406.83 mC/cm² charges on PANI, NC (10 min), NC (20 min) and Ni(OH)₂ electrodes, respectively, are obtained. Inner to total charge and outer to total charge ratios have been used to explain contributing sites to total charge in pristine and NC electrodes.

Modifying Current Collectors to Produce High Volumetric Energy Density and Power Density Storage Devices

We develop zirconium-templated NiO/NiOOH nanosheets on nickel foam and polypyrrole-embedded in exfoliated carbon fiber cloth as complementary electrodes for an asymmetric battery-type supercapacitor device. We achieve high volumetric energy and power density by the modification of commercially available current collectors (CCs). The modified CCs provide the source of active material, actively participate in the charge storage process, provide a larger surface area for active material loading, need no additional binders or conductive additives, and retain the ability to act as the CC. Nickel foam (NF) CCs are modified by use of a soft-templating/solvothermal treatment to generate NiO/NiOOH nanosheets, where the NF is the source of Ni for the synthesis. Carbon-fiber cloth (CFC) CCs are modified by an electrochemical oxidation/reduction process to generate exfoliated core-shell structures (ECFC). Electropolymerization of pyrrole into the shell structure produces polypyrrole embedded in exfoliated core-shell material (PPy@rECFC). Battery-type supercapacitor devices are produced with NiO/NiOOH@NF and PPy@rECFC as positive and negative electrodes, respectively, to demonstrate the utility of this approach. Volumetric energy densities for the full-cell device are in the range of 2.60-4.12 mWh cm^-3 with corresponding power densities in the range of 9.17-425.58 mW cm^-3. This is comparable to thin-film lithium-ion batteries (0.3-10 mWh cm^-3) and better than some commercial supercapacitors (<1 mWh cm^-3).¹ The energy and power density is impressive considering that it was calculated using the entire cell volume (active materials, separator, and both CCs). The full-cell device is highly stable, retaining 96% and 88% of capacity after 2000 and 5000 cycles, respectively. These results demonstrate the utility of directly modifying the CCs and suggest a new method to produce high volumetric energy density and power density storage devices.

Frequent Pitfalls in the Characterization of Electrodes Designed for Electrochemical Energy Conversion and Storage

Focus on the importance of energy conversion and storage boosted research interest in various electrocatalytic materials. Characterization of solid-liquid interfaces during faradaic and non-faradaic processes is routinely conducted in many laboratories worldwide on a daily basis. This can be deemed as a very positive tendency. However, careful insight into modern literature suggests frequent misuse of electroanalytical tools. This can have very negative implications and postpone overall development of electrocatalytic materials with the desired properties. This work points out some of the frequent pitfalls in electrochemical characterization, suggests potential solutions, and above all encourages comprehensive analysis and in-depth thinking about electrochemical phenomena.

Molybdenum-Tungsten Mixed Oxide Deposited into Titanium Dioxide Nanotube Arrays for Ultrahigh Rate Supercapacitors

A novel method involving the fabrication of Mo-W mixed oxide (Mo_xW_1-xO₃) is proposed to modify the modest reaction kinetics and poor cycling stability of MoO₃ material. By a simple coelectrodeposition method, a series of Mo_xW_1-xO₃ oxides is deposited on a TiO₂ nanotube array substrate. Because of the differences between Mo⁶⁺ and W⁶⁺ in nature, there is significant distortion existing in the mixed oxides, leading to their decreased crystallite size and enlarged lattice space, which facilitates ion diffusion in the solid. As results, the mixed oxides show much better balance between specific capacitance and cycling stability than the bare MoO₃ or WO₃ sample, which suffers from either poor cycling stability or low electrochemical activity. Impressively, the optimal Mo-W mixed oxide exhibits a high specific capacitance of 517.4 F g^-1 at 1 A g^-1, and, moreover, it retains 89.3% of the capacitance even at a high current density of 10 A g^-1, demonstrating ultrahigh rate capability. These findings reveal the potential of the Mo-W mixed oxide for constructing advanced ultrahigh power supercapacitors.

Proton-insertion-enhanced pseudocapacitance based on the assembly structure of tungsten oxide

The capacitances of supercapacitors with carbon and metal oxides as electrodes are usually associated with the available surface areas of the electrode materials. However, in this paper, we report that proton insertion, an unusual capacitive mechanism, may effectively enhance the capacitance of metal oxides with low surface area but specific structures. Tungsten trioxide (WO3) as the electrode material for supercapacitors has always suffered from low capacitance. Nevertheless, enhanced by the proton insertion mechanism, we demonstrate that electrodes fabricated by an assembly structure of hexagonal-phase WO3 (h-WO3) nanopillars achieve a high capacitance of up to 421.8 F g(-1) under the current density of 0.5 A g(-1), which is the highest capacitance achieved with pure WO3 as the electrodes so far, to the best of our knowledge. Detailed analyses indicate that proton insertion dominates the electrochemical behavior of h-WO3 and plays the key role in reaching high capacitance by excluding other mechanisms. In addition, a thorough investigation on the temperature-dependent electrochemical performance reveals excellent performance stability at different temperatures. This study provides a new approach to achieving high capacitance by effective proton insertion into ordered tunnels in crystallized metal oxides, which is primarily important for the fabrication of compact high-performance energy storage devices.

Construction of Hybrid Supercapacitor-Batteries with dual-scale shelled architecture

Pseudocapacitors bridge the gap between supercapacitors and batteries. Controllable microstructures grown on substrates have achieved success with regard to energy storage. However, traditional designs have only focused on the surface of scaffolds, which results in high specific capacitance values for the electroactive material rather than the electrodes. Inspired by slurry-casting, a dual-scale shell-structured NiCo2 O4 on nickel foam was assembled by using a simple and flexible solution-based strategy. First, NiCo2 O4 nanosheets covering the Ni foam skeleton surface loosely (the sample is denoted as 'pasted') is obtained by a solution-grown and 'dip-and-dry' process (in a cobalt-nickel hydroxide solution) followed by annealing. Secondly, the NiCo2 O4 nanosheets are filled in the pores of the Ni scaffold (the obtained material is denoted as 'tailored') through chemical bath deposition process followed by annealing. The capacitance per weight of electroactive materials is not outstanding (1029 F g(-1) at 10 mA cm(-2) ), but is competitive with regard to area (3.23 F cm(-2) at 10 mA cm(-2) ). However, features in the cycling performance imply that the electrode exhibits a hybrid supercapacitor-battery behavior and that thermodynamic hysteresis promotes the 'breaking' and 'fusing' behavior of the material. The overall design highlights a new pathway to step out from surface to space.

High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with RuO2·xH2O in an acidic electrolyte. However, we recently demonstrated that a pseudocapacitive mechanism occurs when lithium ions are inserted into mesoporous and nanocrystal films of orthorhombic Nb2O5 (T-Nb2O5; refs 1,2). Here, we quantify the kinetics of charge storage in T-Nb2O5: currents that vary inversely with time, charge-storage capacity that is mostly independent of rate, and redox peaks that exhibit small voltage offsets even at high rates. We also define the structural characteristics necessary for this process, termed intercalation pseudocapacitance, which are a crystalline network that offers two-dimensional transport pathways and little structural change on intercalation. The principal benefit realized from intercalation pseudocapacitance is that high levels of charge storage are achieved within short periods of time because there are no limitations from solid-state diffusion. Thick electrodes (up to 40 μm thick) prepared with T-Nb2O5 offer the promise of exploiting intercalation pseudocapacitance to obtain high-rate charge-storage devices.

High-performance sodium-ion pseudocapacitors based on hierarchically porous nanowire composites

Electrical energy storage plays an increasingly important role in modern society. Current energy storage methods are highly dependent on lithium-ion energy storage devices, and the expanded use of these technologies is likely to affect existing lithium reserves. The abundance of sodium makes Na-ion-based devices very attractive as an alternative, sustainable energy storage system. However, electrodes based on transition-metal oxides often show slow kinetics and poor cycling stability, limiting their use as Na-ion-based energy storage devices. The present paper details a new direction for electrode architectures for Na-ion storage. Using a simple hydrothermal process, we synthesized interpenetrating porous networks consisting of layer-structured V(2)O(5) nanowires and carbon nanotubes (CNTs). This type of architecture provides facile sodium insertion/extraction and fast electron transfer, enabling the fabrication of high-performance Na-ion pseudocapacitors with an organic electrolyte. Hybrid asymmetric capacitors incorporating the V(2)O(5)/CNT nanowire composites as the anode operated at a maximum voltage of 2.8 V and delivered a maximum energy of ∼40 Wh kg(-1), which is comparable to Li-ion-based asymmetric capacitors. The availability of capacitive storage based on Na-ion systems is an attractive, cost-effective alternative to Li-ion systems.

Controlling size, amount, and crystalline structure of nanoparticles deposited on graphenes for highly efficient energy conversion and storage

A facilitated electrochemical reaction at the surface of electrodes is crucial for highly efficient energy conversion and storage. Herein, various nanoparticles (NPs) including Au, Pt, Pd, Ru, and RuO(2), were synthesized in situ and directly deposited on the ionic liquid (IL)-functionalized reduced graphene oxides (RGOs) in a controlled manner. The size, amount, and crystalline structures of discrete NPs were readily controlled, giving rise to enhanced methanol oxidation and pseudocapacitance. The well-defined nanostructure of decorated NPs and the favorable interaction between ILs and RGOs (or NPs) facilitated the electrochemical reaction, where NPs acted as electrocatalysts for energy conversion and played the role of redox-active electrodes for energy storage.