Next-generation Electrochemical Capacitors Research Articles

Electrochemical characterization and structural analysis of (In2O3)/(Fe2O3) nanocomposites for high-performance supercapacitors

This study presents a comprehensive investigation of the electrochemical characteristics and structural properties of novel nanocomposites with varying compositions of (In2O3)x/(Fe2O3)1-x, where x ranges from 1 to 0. These nanocomposites were synthesized using a versatile and cost-effective co-precipitation method. The crystallographic structure and morphology of the synthesized samples were thoroughly analyzed using Powder X-ray diffraction (PXRD) and Tunneling Electron Microscopy (TEM). Advanced analytical techniques were employed including Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening Spectroscopy (CDBS) to uncover critical insights into the structural and molecular properties of these nanocomposites. PALS analysis revealed valuable insights into the pore characteristics of the nanocomposites, while CDBS identified crucial molecular interactions within the materials. Electrochemical characterization of the nanocomposites is carried out using cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical-impedance-spectroscopy (EIS) measurements. The (In2O3)0.3/(Fe2O3)0.7 nanocomposite exhibits a remarkable specific capacitance of 945 F g-1 at 1 A g-1 and exceptional rate performance, retaining 93.6% of its specific capacitance at a six-fold higher current density. Moreover, this nanocomposite electrode demonstrates outstanding cyclic stability, maintaining 92.1% of its specific capacitance even after 3000 GCD cycles at 8 A g-1. These findings suggest that the novel composition and integrated electrochemical properties of the (In2O3)0.3/(Fe2O3)0.7 nanocomposite hold great promise for enhancing the performance of next-generation electrochemical capacitors. This research contributes valuable insights into the design and development of advanced energy storage materials with applications in various high-performance energy storage devices.

Fabrication of nanostructured iron-cobalt layered double hydroxide: An innovative approach for the facile synthesis

Studying semiconductive materials-based supercapacitors is significant for developing next-generation energy storage devices. However, the fabrication of a novel semiconductive material-based electrode with self-supporting design, flexible support, and nanoarchitecture is a challenge for the material researcher of 21st century. We present a facile method to fabricate three-dimensional (3D), porous, conductive electrodes based on semiconductive materials with high electrochemical performance. We fabricate the FeCo-LDH sample via hydrothermal route and decorate it on the flexible carbon cloth (CC) to get a flexible and self-supporting electrode. Textural, morphological, elemental, thermal, and functional group studies of the hydrothermally generated powder were performed using XRD, SEM, EDX, TGA, and FTIR methods. Well-known EIS, GCD, and CV tests were used to evaluate the electrochemical possessions of the FeCo-LDH/CC electrode. The performed electrochemical experiments indicated that our manufactured FeCo-LDH/CC electrode has an excellent specific capacity of 1041F/g at 0.5 A/g. Our self-supporting and flexible electrode has excellent cycle stability (it loses only 7.3 percent of its capacity after 5000 GCD cycles). It retains 79.2 percent of its capacitance when the current density increases from 0.5 A/g to 9 A/g. The superior electrochemical activity observed is attributed to the electrode's novel structure and logical design. According to the electrochemical experiment findings, our manufactured FeCo-LDH/CC electrode has great promise for practical applications in next-generation electrochemical capacitors.

Synergistic effect of hydrothermal and physical activation approaches to fabricate activated carbon for energy storage applications

Carbon is the main constituent of all waste biomass. The extraction of activated carbonaceous materials from the waste biomass and their utilization in energy-storage applications is a green, cost-effective strategy. Herein, honeycomb-like activated carbon (AC) with a large surface area (2344 m2g-1) and improved conductivity was derived from palm seeds via hydrothermal followed by CO2 activation approaches. The AC sample was decorated over a carbon cloth (CC) current collector to form a flexible working electrode for next-generation electrochemical capacitors. As-prepared palm seed-derived activated carbon electrode delivered a good specific capacitance (Cs) of 294.9 Fg−1 @ 01 Ag−1, with exceptionally high cycling stability, showing 97.36% retention of its initial capacitance, even after 10,000 charge-discharge cycles in an alkaline electrolyte. The porous structure with microsized cavities enables the activated carbon-based electrode to show excellent rate capability (93.1% capacitance retention) even at nine-fold higher current density. As-prepared flexible electrode exhibits excellent electrochemical performance due to their unique three-dimensional network structure combined with porous architecture and good conductivity. This work provides a facile way to enhance activated carbon's capacitive performance and opens up new avenues for developing high-performance capacitive materials using waste biomass to reduce environmental concerns.

Pre-Sodiated Ti3C2Tx MXene Structure and Behavior as Electrode for Sodium-Ion Capacitors.

Layered titanium carbide (Ti3C2Tx) MXene is a promising electrode material for use in next-generation electrochemical capacitors. However, the atomic-level information needed to correlate the distribution of intercalated cations with surface redox reactions, has not been investigated in detail. Herein we report on sodium preintercalated MXene with high sodium content (up to 2Na per Ti3C2Tx formula) using a solution of Na-biphenyl radical anion complex (E0 ≈ -2.6 SHE). Multiple sodiation sites and formation of a two-dimensional sodium domain structure at interfaces/surfaces is identified through combined computational simulations with neutron pair distribution function analysis. The induced layer charges and the redox process characterized by the density-functional tight-binding method on a local scale are found to greatly depend on the location of sodium ions. Electrochemical testing of the pre-sodiated MXene as an electrode material in a sodium-ion capacitor shows excellent reversibility and promising performance, indicating the feasibility of chemical preintercalation as an approach to prepare MXene electrodes for ion capacitors.

Impact of phase segregation on optical and electrochemical property of BiPO4 nanostructures for energy storage applications

In the present paper, we report the influence of phase segregation (hexagonal and monoclinic phase) of facile microwave-assisted BPO (BiPO4) nanostructures on optical and electrochemical properties for supercapacitor applications. The XRD patterns reveal pure phase formation of both the samples of BiPO4 synthesized at 300 °C and 600 °C (JCPDS #15-0766 and JCPDS #43-0637). Phase transition of BiPO4 from hexagonal to monoclinic phase obtained by sintering the sample (BP600) at 600 °C. The FWHM values of the X-ray diffraction peaks observed to decrease with an increase in sintering temperature, which can be ascribed to the enhancement of the crystallite size. The synthesized samples of BiPO4 are in the nanometer dimension, which also revealed through FESEM examination. FTIR, Raman, and PL spectroscopy also revealed the crystal structure symmetry distinction among the hexagonal and monoclinic BiPO4 phase. Specific capacity calculated from GCD (galvanostatic charge and discharge) analysis for BP300 and BP600 samples is 1074 Cg−1 and 451 Cg−1, correspondingly, at the current density of 1 Ag−1. The investigation divulges that the phase segregation plays a considerable utility in tuning the electrochemical performance of BiPO4 nanostructures as active electrode material for next-generation electrochemical capacitors application.

Pyrolytic Carbon Films with Tunable Electronic Structure and Surface Functionality: A Planar Stand-in for Electroanalysis of Energy-Relevant Reactions

Nanostructured carbons are critical components within many of the electrodes used for energy-storage and –conversion systems, where they can provide pathways for electron conduction to enable high-rate operation, act as supports for other active materials (e.g., electrocatalytic metals and oxides), and contribute to the mechanical properties of the electrode. Yet often overlooked when selecting a particular carbon is the nature of electronic/chemical/physical interactions at the junction of carbon with nanoscale phases that impart desired storage or electrocatalytic functionality. Our own research with otherwise high-performing carbon-based electrode architectures suggests that sub-optimal interface design ultimately hinders energy and power capability. In order to address fundamental questions regarding interfacial processes, we step back from the complex structure of practical carbon-containing electrodes to a planar electrode configuration designed to mimic those same interfaces. For example, pyrolytic carbon (pyC) films prepared by chemical vapor deposition are planar analogs of the disordered carbon blacks used in many battery and fuel-cell electrodes, and thus serve as an electroanalytical substrate on which we deposit charge-storing metal oxides (e.g., MnO2), electroactive polymers, and metal colloids [1]. We use classical electroanalytical methods to determine fundamental properties such as electron-transfer rate constants and impedance-derived response times, absent the complexities imposed by 3D electrodes. By moving to 2D model surfaces we are also able to interrogate interfacial processes by scanning probe microscopy, including in situ monitoring of conductivity and surface morphology under potential/current control. Lessons learned from these model 2D interfaces are readily applied to the redesign of practical 3D electrode structures to improve next-generation electrochemical capacitors and batteries. [1] J. F. Parker, G. E. Kamm, A. D. McGovern, P. A. DeSario, D. R. Rolison, and J. W. Long, Languir, 33 (2017) 9415–9425.

Interfacial Phenomena at the Junction of Conductive Carbon with Pseudocapacitive Metal Oxides and Polymers: Implications for Electrochemical Capacitors

The technological push to dramatically shorten the charge/discharge times of electrochemical energy-storage (EES) devices while still providing significant energy content requires rethinking how we design and assemble EES, from the choice of particular types of charge-storing materials to how those materials are spatially translated into a high-performance, yet still practical, electrode architecture. Materials that exhibit “pseudocapacitance,” with fast faradaic charge-storage mechanisms that mimic the current–voltage characteristics of a conventional capacitor, promise to satisfy both energy and power needs when incorporated into electrochemical capacitor (EC) devices. The poor electronic conductivity of most pseudocapacative materials (e.g., manganese oxides, MnOx) dictates that they must be integrated with a better electronic conductor, typically porous, nanostructured carbons, in order to minimize ohmic losses and to reasonably utilize innate faradaic capacity. The past decade has seen great advancements in the development of new electrode architectures that incorporate oxides or polymers at carbon substrates, accompanied by promising electrochemical results. Yet our experience with such multifunctional materials shows that we may still be “leaving on the table” too much energy and power performance, due to suboptimal electronic/chemical/physical interactions at the junction of carbon (with its semimetal character) and nanoscale overlayers of pseudocapacitive materials (often semiconductors or hopping conductors). In order to explore these fundamental questions, we step back from the complexity of 3D electrode architectures to planar electrode configurations that otherwise mimic the material characteristics of practical electrode structures. Carbon films prepared by chemical vapor deposition (CVD) that are based on the pyrolysis of benzene and related monomers serve as model substrates on which we deposit pseudocapacitive metal oxides and electroactive polymers. The resulting 2D interfaces are characterized by classical electroanalytical methods to determine fundamental properties such as electron-transfer kinetics. In conjunction with electrochemical characterization, we apply surface-sensitive techniques such as photoelectron spectroscopy and scanning probe microscopy to query electronic, chemical, and physical structure as those properties are tuned by variations in synthesis/deposition of each phase: carbon and pseudocapacitive coating. Lessons learned from these model 2D interfaces are readily applied to the redesign of practical 3D electrode structures in next-generation electrochemical capacitors and batteries.

Hierarchical architecture of PANI@TiO2/Ti3C2Tx ternary composite electrode for enhanced electrochemical performance

A hierarchical structure of PANI@TiO2/Ti3C2Tx ternary composite for high performance electrochemical capacitors was prepared by a hydrothermal treatment combining with in situ polymerization process. Layered Ti3C2Tx was used as both an active material with large surface area and a framework providing more paths for ion insertion/extraction. The introduction of PANI nanoflakes and TiO2 nanoparticles can offer an enhanced performance due to their good pseudocapacitance behavior and increased surface area of active materials. The hierarchical PANI@TiO2/Ti3C2Tx ternary composite exhibits high specific capacitance of 188.3Fg−1 at 10mVs−1, which is about twice higher than that of TiO2/Ti3C2Tx, and 435.4Fcm−2 at 0.5Ag−1 in KOH solution. Furthermore, it also shows remarkable cycle stability, remaining at 94% of the initial value even after 8000 cycles at a current density of 1Ag−1. The enhanced electrochemical performance can be attributed to the hierarchical architecture and the synergistic effect of combining PANI nanoflakes with TiO2/Ti3C2Tx composite. This work propels a novel way of using MXene matrix to prepare hierarchical structure composite for next-generation electrochemical capacitors electrode.

Understanding Pseudocapacitive Charge Storage in Transition Metal Oxides from Fundamental Interfacial Processes to 3D Electrode Design

Transition-metal oxides that exhibit “pseudocapacitance” are promising alternatives to high-surface-area carbons as charge-storing materials in next-generation electrochemical capacitors (ECs). Hydrous ruthenium oxides remain the state-of-the-art for pseudocapacitive materials due to their fortuitous combination of high electronic and ionic conductivity. Lower-cost alternatives are being vigorously pursued, yet the low electronic conductivity of most other metal oxides of interest (e.g., MnOx) necessitates that they be thoughtfully incorporated with a conductive carbon support. We have developed an electrode design in which pseudocapacitive oxides, such as MnOx and FeOx, are applied as nanoscale coatings onto ultraporous carbon nanofoam substrates that define the macroscale-to-nanoscale structure of the resulting electrode architecture [1,2]. In addition to their practical advantages for device fabrication, these nanofoam paper-based materials have also provided a designer platform with which to investigate the interplay of pore structure and electrochemical performance [3] and for in situ analysis of the mechanisms responsible for pseudocapacitance [4].In the background of these studies, we have accumulated evidence that the physicochemical nature of the carbon–metal oxide interface can have a significant impact on electrochemical performance. While continuing to develop and transition our 3D electrode architectures, we are refocusing our research efforts on investigating fundamental charge-transfer properties at nanoscale carbon–metal oxide interfaces. Shifting from the characterization complexities of the nanofoam-based architectures, we use planar pyrolytic-carbon substrates to mimic the surface properties of 3D carbon, but in forms that are more readily characterized by conventional surface spectroscopy and scanning-probe microcopy techniques. We apply nanoscale pseuodocapacitive oxides to these planar carbon films using redox-deposition protocols previously demonstrated at 3D carbons [5], and explore how the physical, chemical, and electronic structure of the resulting carbon–metal oxide interface impacts electrochemical properties. Lessons learned from these model interfaces are readily translated to improved performance in practical 3D electrode architectures. 1. Fischer, A.E.; Saunders, M.P.; Pettigrew, K.A.; Rolison, D.R.; Long, J.W. J. Electrochem. Soc. 2008, 155, A246. 2. Sassin, M.B.; Mansour, A.N.; Pettigrew, K.A.; Rolison, D.R.; Long, J.W. ACS Nano 2009, 4, 4505. 3. Sassin, M.B.; Hoag, C.P.; Willis, B.T.; Kucko, N.W.; Rolison, D.R.; Long, J.W. Nanoscale, 2013, 5, 1649. 4. Beasley, C.A.; Sassin, M.B.; Long, J.W. J. Electrochem. Soc. 2015, 162, A5060. 5. Sassin, M.B. Chervin, C.N.; Rolison, D.R.; Long, J.W. Acc. Chem. Res. 2013, 46, 1062.

Vertically Stacked Bilayer CuCo2O4/MnCo2O4 heterostructures on Functionalized Graphite Paper for High-Performance Electrochemical Capacitors

Cobaltite system with spinel structures is a promising cathode material for next-generation high-performance electrochemical capacitor because of its high stability in electrochemistry. However, increasing the mass loading of active materials without sacrificing the geometry of the nanostructures remains a challenge. In this study, we propose vertically stacked bilayer spinel heterostructures constructed from hierarchical CuCo2O4/MnCo2O4 on graphite paper as highly capable supercapacitor electrode. A two-step hydrothermal method with post annealing treatment is used in the preparation of heterostructures. CuCo2O4/MnCo2O4 electrode delivers remarkable specific capacitance of 1434 F g−1 at 0.5 A g− 1, considerable high-rate capability (810 F g−1 at 15 A g−1), and an excellent cycling stability maintaining 81.4% at 10 A g− 1 after 5000 cycles. An electrochemical capacitor device operating at 1.6 V is constructed using CuCo2O4/MnCo2O4 and graphene as positive and negative electrodes, respectively. The device shows high energy density of 42.1 Wh kg−1 at a power density of 400 W kg−1, as well as good cycling stability (88.4% retention after 10000 cycles). The stacking concept of heteronanostructures can potentially enrich the electrochemical performance of metal oxides for next-generation electrochemical capacitors. Figure 1

Invited: Probing Pseudocapacitance Mechanisms in Metal-Oxide–Functionalized Carbon Nanoarchitectures with an Electrochemical Quartz Microbalance

Transition-metal oxides that exhibit “pseudo-capacitance” are challenging high-surface-area carbons as the active material of choice for next-generation electrochemical capacitors (ECs).1 The electrochemical performance of such EC-relevant metal oxides as MnOx has been extensively reported,2 yet the underlying mechanisms responsible for pseudocapacitance are still a subject of debate in the scientific literature. Spectroscopic techniques have been used to verify reversible oxidation-state cycling of metal sites in these oxides (Mn(III/IV) in MnOx; Fe(II/III) in FeOx),3,4 but less is known about the participation of charge-compensating cations from the contacting electrolyte. In the case of mild aqueous electrolytes, redox reactions that give rise to pseudocapacitance may be coupled with insertion/ deinsertion of cations from the electrolyte salt (e.g., Li+, Na+, K+), protons that are available from the H2O solvent, or combinations thereof. Electrochemical quartz crystal microbalance (EQCM) techniques provide information on changes in electrode mass during electrochemical cycling that can address questions regarding ion insertion.5 Until recently the use of EQCM has been limited to well-defined thin-film electrodes, but advancements in EQCM methods now permit analysis of a wider range of substrates, including powder-composite electrodes of EC relevance.6,7 In this paper, we will describe the application of EQCM analysis to a class of electrode architectures comprising carbon nanofoam papers8 whose exterior and interior surfaces are exhaustively coated with nanometers-thick metal oxides that exhibit pseudocapacitance.4 ,9,10,11 Carbon nanofoam papers are freestanding, device-ready electrode architectures of controllable macroscale dimensions (1–102 cm2 in area; 100–500 mm thickness) in which nanoscale dimensions of pore and solid can also be readily adjusted over a wide range (10 nm to several mm). The addition of a nanoscale metal-oxide coating to nanofoam substrates amplifies charge-storage capacity via pseudocapacitance, while the high-frequency response inherent to the nanofoam architecture is retained by ensuring that the metal oxide coating is conformal and self-limiting and that the pore network stays open in three dimensions.For EQCM analysis of these nanofoam-based electrode architectures, we attach disk-shaped pieces of native or oxide-coated carbon nanofoam paper to Au-coated quartz crystals using a carbon-based conductive epoxy. Voltammetric cycling of the resulting electrodes in an EQCM cell yields the expected pseudocapacitance response in terms of both shape and magnitude, demonstrating effective electrical contact to the nanofoam paper. Coupled to the electrochemical response, we monitor mass changes during electrochemical cycling while also varying such parameters as aqueous electrolyte composition (Li+ vs. Na+ vs. K+), average pore size of the nanofoam (10–200 nm), and the type of metal-oxide coating (MnOx vs. FeOx). Preliminary EQCM experiments show that “depletion” effects6 are observed in the presence of Li+-containing electrolytes and for MnOx-coated nanofoams with pores <40 nm, in which the pseudocapacitance mechanism switches from Li+ to NO3 – insertion/deinsertion over the most positive regions of the active potential window of MnOx. The EQCM-observed depletion effect corresponds with the slowest time response of this series of MnOx–carbon nanofoams, as previously reported.12 We compare EQCM results obtained from nanofoam-based electrode architectures with those for thin-film versions of the respective metal oxides. Monitoring mass changes during electrochemical cycling will also allow us determine the conditions under which deleterious dissolution reactions may occur.

Redox Deposition of Nanoscale Metal Oxides on Carbon for Next-Generation Electrochemical Capacitors

Transition metal oxides that mix electronic and ionic conductivity are essential active components of many electrochemical charge-storage devices, ranging from primary alkaline cells to more advanced rechargeable Li-ion batteries. In these devices, charge storage occurs via cation-insertion/deinsertion mechanisms in conjunction with the reduction/oxidation of metal sites in the oxide. Batteries that incorporate such metal oxides are typically designed for high specific energy, but not necessarily for high specific power. Electrochemical capacitors (ECs), which are typically composed of symmetric high-surface-area carbon electrodes that store charge via double-layer capacitance, deliver their energy in time scales of seconds, but at much lower specific energy than batteries. The fast, reversible faradaic reactions (typically described as "pseudocapacitance") of particular nanoscale metal oxides (e.g., ruthenium and manganese oxides) provide a strategy for bridging the power/energy performance gap between batteries and conventional ECs. These processes enhance charge-storage capacity to boost specific energy, while maintaining the few-second timescale of the charge-discharge response of carbon-based ECs. In this Account, we describe three examples of redox-based deposition of EC-relevant metal oxides (MnO2, FeOx, and RuO2) and discuss their potential deployment in next-generation ECs that use aqueous electrolytes. To extract the maximum pseudocapacitance functionality of metal oxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures. Expressing the metal oxide in a nanoscale form often enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge. The "wiring" of the metal oxide, in terms of both electron and ion transport, when fabricated into a practical electrode architecture, is also a critical design parameter for achieving characteristic EC charge-discharge timescales. For example, conductive carbon must often be combined with the poorly conductive metal oxides to provide long-range electron pathways through the electrode. However, the ad hoc mixing of discrete carbon and oxide powders into composite electrodes may not support optimal utilization or rate performance. As an alternative, nanoscale metal oxides of interest for ECs can be synthesized directly on the surfaces of nanostructured carbons, with the carbon surface acting as a sacrificial reductant when exposed to a solution-phase, oxidizing precursor of the desired metal oxide (e.g., MnO4(-) for MnO2). These redox deposition methods can be applied to advanced carbon nanoarchitectures with well-designed pore structures. These architectures promote effective electrolyte infiltration and ion transport to the nanoscale metal oxide domains within the electrode architecture, which further enhances high-rate operation.