Pseudocapacitive Energy Storage Research Articles

Low-Temperature pseudocapacitive energy storage in Ti3C2Tx MXene

The use of pseudocapacitive electrode materials can enable devices to store more energy than electrical double-layer capacitors (EDLCs). However, only a few pseudocapacitive materials can maintain excellent performance at low temperatures, which limits their application in harsh climate conditions. Here we demonstrate that a pseudocapacitor with two-dimensional transition metal carbide (MXene) electrode can exhibit excellent low-temperature performance like EDLC. The MXene electrodes contain electrolyte between 2D sheets, and the electrolyte ions can unimpededly reach redox-active sites and interact with surface oxygen groups rapidly, even at low temperatures. With a combination of 40 wt.% sulfuric acid solution as the electrolyte, the working temperature of the MXene electrode extends to -60 °C. The electrode exhibits temperature-insensitive performance at a low scan rate, and the capacity of MXene (88 mAh g−1 at 5 mV s−1) stays almost constant when the temperature decreases from 20 to -50 °C. Moreover, at -50 °C, MXene electrodes show a high capacity retention of > 75% at 100 mV s−1, indicating good low-temperature rate performance. Interestingly, a broad working potential window of 1.5 V is achieved at -60 °C. Such an excellent low-temperature performance demonstrates that MXene is a promising electrode candidate for low-temperature pseudocapacitive energy storage applications.

Probing consequences of anion-dictated electrochemistry on the electrode/monolayer/electrolyte interfacial properties

Altering electrochemical interfaces by using electrolyte effects or so-called “electrolyte engineering” provides a versatile means to modulate the electrochemical response. However, the long-standing challenge is going “beyond cyclic voltammetry” where electrolyte effects are interrogated from the standpoint of the interfacial properties of the electrode/electrolyte interface. Here, we employ ferrocene-terminated self-assembled monolayers as a molecular probe and investigate how the anion-dictated electrochemical responses are translated in terms of the electronic and structural properties of the electrode/monolayer/electrolyte interface. We utilise a photoelectron-based spectroelectrochemical approach that is capable of capturing “snapshots” into (1) anion dependencies of the ferrocene/ferrocenium (Fc/Fc+) redox process including ion-pairing with counter anions (Fc+–anion) caused by differences in Fc+–anion interactions and steric constraints, and (2) interfacial energetics concerning the electrostatic potential across the electrode/monolayer/electrolyte interface. Our work can be extended to provide electrolyte-related structure-property relationships in redox-active polymers and functionalised electrodes for pseudocapacitive energy storage.

Polyaniline-filled carbonized wood membrane as an advanced self-supported electrode for superior pseudocapacitive energy storage

Integrating electrochemical redox-active materials with 3D conductive scaffolds holds high promise to the development of high-performance energy storage devices. Herein, a self-supported, hierarchically porous polyaniline (PANI)-filled carbonized wood membrane (CWM@PANI) electrode is rationally designed and fabricated for high-performance pseudocapacitors. The synthetic strategy, engaging a controllable in situ electropolymerization, enables a uniform deposition of PANI particles on the porous wall of the aligned microchannels of the CWM to form a structurally integrated heterostructure. The aligned open microchannels inside the CWM favor unimpeded and continuous supply of electrolyte to the anchored PANI particles. Moreover, the uniformly distributed PANI particles contribute maximum volume to participate in the redox reactions. As a result, measured with three-electrode configuration at 0.5 A g−1, the as-fabricated CWM@PANI electrode with a high PANI loading (6.0 mg cm−2) delivers gravimetric and areal capacitances of 639.5 F g−1 and 3837 mF cm−2, respectively, as well as good rate capacity (369.8 F g−1 at 10 A g−1) and excellent cycling stability (88.5% capacitance retention after 5000 cycles). Furthermore, a maximum energy density of 43.11 Wh kg−1 at a power density of 350 W kg−1 is also achieved. This work presents an effective strategy to fabricate a high-performance self-supported electrode as a promising alternative of its powdery counterparts for practically applicable supercapacitor devices.

Enhanced pseudocapacitive energy storage of oxides grown on nanoporous alloys by solid solution

Development of transition-metal-oxides (TMO) based high-energy density aqueous pseudocapacitors requires simultaneously high charge storage capacity, high electronic conductivity and large redox potential window. Engineering the interfacial electronic structure among electrolyte, oxides and current collectors and thus modifying the corresponding multi-redox reactions displays a great potential to address the above challenges. Herein, a series of nanoporous alloy@oxide composite electrodes are prepared by self-oxidation of the nanoporous NiMMn alloys, where various dopants of 3d transition metal elements (M = Ti, V, Cr, Fe, Co, Cu) are introduced to tune the electronic structures prior to oxidation of the alloys. Within the alloy-core and oxide-shell structure, it is found that all the dopants effectively suppress the hydrogen production to ensure the high operation voltage. Additionally, Co dopant displays the largest energy density (145 mWh cm−3) comparing to others. The hydrogen suppression stems from the tuning of the work function of NiMn via doping different 3d elements so that the out-layer oxide electronic structures downshift accordingly. Further DFT calculation on the doped active surfaces (001) and (111) illustrates that Co dopant results in the large amounts of density of states within the external scan voltage −0.9 V ~ 0.6 V (vs. Ag/AgCl electrode), yielding the high capacitance to accommodate the electrons. This work provides insights into the design of the promising capacitive materials with a high energy density via interface engineering of the core alloy and the out-layer oxides.

Self-assembly synthesis of SnNb2O6/amino-functionalized graphene nanocomposite as high-rate anode materials for sodium-ion batteries

A two-dimensional (2D) SnNb2O6/amino-functionalized graphene (En-RGO) nanocomposite with a representative 2D–2D architecture has been constructed by an easy self-assembly approach and firstly investigated as anode materials for secondary sodium-ion batteries. The SnNb2O6 nanosheets are evenly anchored with the amino-functionalized graphene through electrostatic attractive interplay between the negatively charged SnNb2O6 and positively charged En-RGO after modification. As a result, a remarkable reversible capacity of 300 mAh·g−1 was obtained at 50 mA·g−1, and significantly, the En-RGO electrode could also deliver ultra-long calendar life up to 1900 cycles with a high reversible capacity of 200 mAh·g−1 at current of 500 mA·g−1. Such excellent electrochemical characteristics can be mainly ascribed to its fast pseudo-capacitive energy storage mechanism, and the capacitive contribution can even reach up to 90% at 1.2 mV·s−1.

Assembling free-standing and aligned tungstate/MXene fiber for flexible lithium and sodium-ion batteries with efficient pseudocapacitive energy storage

Free-standing aligned fiber is capable of providing sufficient electrochemical properties for fiber electrode without sacrificing flexibility, which is highly desirable for fiber batteries that are expected to be flexible and efficient energy storage. Here, we propose a free-standing and aligned tungstate/MXene fiber spun from their mixed liquid crystals colloid for fiber-shaped lithium/sodium-ion batteries. Benefiting from the 3D interconnected ion transport tunnels, fast charge transfer frameworks and minimum ion-path tortuosity provided by the aligned structures of 2D tungstate and MXene nanosheets, the fiber is endowed with both efficient pseudocapacitive energy storage and flexibility. As expected, the fiber delivers high reversible capacity, excellent rate capability and outstanding long-term cycling performance. Even under mechanical deformations, the fiber batteries can power the LED, digital timer, temperature-humidity meter, and smart wristband. This work may promote the further development of practical fiber batteries for wearable electronics.

Rational design of cobalt–nickel double hydroxides for flexible asymmetric supercapacitor with improved electrochemical performance

Rational design of micro-nano morphology and suitable crystalline structure are highly desired for metal hydroxides to achieve overall high-performance in the advanced electrodes for flexible supercapacitors. Herein, a novel wisteria flower-like microstructure of cobalt–nickel double hydroxide (CoNi-DH) is successfully constructed on carbon cloth (CC) using an in-situ hydrolysis-induced exchange process between hydroxide ions and organic ligands of the Co-MOF in four different kinds of solutions containing Ni2+. The as-prepared wisteria flower-like microstructure grown on CC shows vertically aligned arrays with high specific area and abundant active sites, which not only guarantee the CoNi-DH active materials to be thoroughly exposed in the electrolyte, resulting in highly effective pseudocapacitive energy storage, but also are beneficial to rapid and reversible redox kinetics and thus give rise to high-rate capability. In addition, compared to Ni(NO3)2, NiCl2, and Ni(CH3COO)2 solutions, the Ni2SO4 solution is found to facilitate the formation of the most regular morphology and the largest interlayer spacing on (003) plane of the layered nickel hydroxide phase in the resultant CoNi-DH. As a result, the optimal CoNi-DH-S@CC (CoNi-DH prepared in Ni2SO4) serves as an advanced electrode to show high-rate capability (only 13% Cs decay after a 15-fold current elevation) and a superior specific capacity (Cs) of 929.4 C g−1, which remarkably exceeds those of CoNi-DH-N (823.1 C g−1, in Ni(NO3)2), CoNi-DH-Cl (798.4 C g−1, in NiCl2), CoNi-DH-C (803.8 C g−1, in Ni(CH3COO)2), and other similar metal hydroxides. Moreover, with this CoNi-DH-S electrode as the positive electrode, the as-prepared asymmetric supercapacitor (ASC) delivers an impressive capacity of 204.8 C g−1, a superior energy density of 42.5 Wh kg−1, and satisfactory cycle life (81.5% reservation after 7500 cycles). As a proof-of-concept application, a quasi-solid-state ASC is further successfully fabricated based on the CoNi-DH-S electrode to exhibit encouraging application potential.

Self-standing star-shaped tri-metallic oxides for pseudocapacitive energy storage electrode materials

Novel star-shaped trimetallic oxides (CuMnCoO4) have been synthesized employing hydrothermal method and favorably compared to the Co3O4 and bimetallic (MnCo2O4) oxides. The star-shaped CuMnCoO4 consisted of bundles of orderly layered and loosely interconnected nanoneedles with a hole in the center; each nanoneedle was composed of single-crystalline nanoparticles. Its distinctive structural features significantly increased the electrochemically active sites and facilitated the mass and charge transports. Furthermore, the three transition metal cations in the trimetallic oxide contributed to the multiple redox reactions. Consequently, CuMnCoO4 exhibited a significantly enhanced capacitance (1,715F g−1 at 1 A g−1) and outstanding cycling stability, compared to those of Co3O4 (742F g−1) and MnCo2O4 (1,305F g−1). Additionally, an asymmetric supercapacitor (AS) (assembled with CuMnCoO4, as the positive electrode, and activated carbon (AC), as the negative electrode) exhibited a high energy density (E) of 40.1 W h kg−1, at a power density (P) of 799 W kg−1, thereby highlighting CuMnCoO4 as a promising candidate for energy storage. This research can also pave the way for the study of new spinel-trimetallic compounds with EES applications.

Electrochromic and pseudocapacitive behavior of hydrothermally grown WO3 nanostructures

We report the direct synthesis of various WO3 nanostructures (nanoplates, nanobricks, and stacked nanosheets) on fluorine-doped tin oxide conducting substrates for electrochromic and pseudocapacitive energy storage applications. These nanostructures were formed by varying the pH of the hydrothermal solution, which led to monoclinic and triclinic crystal structures. Among these structures, vertically aligned WO3 nanoplates showed good electrochromic properties, with rapid and reversible response of the colored and bleached states in 0.5 M H2SO4 electrolyte. Moreover, the vertically aligned WO3 nanoplates exhibited promising energy storage behavior as a negative electrode material with a higher areal capacitance of 72.6 mF cm−2 in 0.5 M Na2SO4 electrolyte and better electrochemical performance than the nanobricks and stacked nanosheets. The two-dimensional WO3 nanoplates exhibit strong potential for use in smart windows and negative-electrode pseudocapacitors.

Catalyst-free solvothermal synthesis of ultrapure elemental N- and B-doped graphene for energy storage application

Even though atomically thin graphene has excellent electronic mobility, is flexible and promptly removes unwanted heat from device interfaces, the wonder material is not yet been able to be employed in energy sector due to unavailability of electrochemically active sites. Doping graphene not only is expected to generate electrochemically active sites, will enhance inter-layer distance as a consequence of doping (up to 6 Å). On-demand tunable high level of doping of graphene is thus desirable to enable it for energy sector applications. While Raman spectroscopy of doped graphene attained via solvothermal method, attests to chemical fingerprints of doped graphene systems, X-ray photoelectron spectroscopy establishes that doping of graphene (as high as ~19.8%) by nitrogen and boron has been attained. While pristine as well as B-doped graphene exhibited EDLC behaviour, N doped graphene exhibited excellent signatures of pseudo-capacitive energy storage behaviour with energy density of 170 Wh/kg and power density as high as 8.3 kW/kg. Solvothermal technique of doping graphene being single step, facile and economic; seems to have great potential for pseudo-capacitive energy storage applications where scalable production of doped graphene is needed.

Co(OH)2/MXene composites for tunable pseudo-capacitance energy storage

In our study, Co(OH)2/Ti3C2Tx MXene materials with layered 2D/2D heterojunction structures were synthesized as model systems, and their performance for pseudocapacitive energy storage was inverstigate. The results demonstrated that the coupling of Co(OH)2 and Ti3C2Tx can increase both the redox and intercalation pseudocapacitance of the as-prepared composite materials. Consquently, a total pseudo-capacitance of 153.0 F g−1 can be achieved for the Co(OH)2/Ti3C2Tx sample with a Ti3C2Tx loading of 25 wt%, which is twice of that of the bare Co(OH)2, and an excellent stablility with only 1% decrease in capacitance after 1000 cycles has also been accomplished. Moreover, the quantitative kinetic analysis demonstrated that the dominant pseudocapacitive contribution for the as-prepared Co(OH)2/Ti3C2Tx materials can be tuned from surface capacitance into intercalation pseudocapacitance by adjusting the Ti3C2Tx loading. This work may provide a new approach for designing MXene based pseudo-capacitive electrode materials with better rate capability and stability.

Constructing three-dimensional ordered porous MoS2/C hierarchies for excellent high-rate long-life pseudocapacitive sodium storage

Pseudocapacitive energy storage offers short charging time, long-term cycling stability and high-rate capability, but developing nanostructured electrodes with high pseudocapacitance is still challenging. Herein, we present a facile template-assisted nanocasting method to fabricate three-dimensional (3D) ordered porous MoS2/C hierarchical nanostructures through simultaneous thermal decomposition of the (NH4)2MoS4/(C6H9NO)n/SiO2 mixture by annealing and subsequent template removal. The interplanar spacing along (0 0 2) plane of MoS2 is greatly expanded owing to the incorporation of the PVP-derived carbon species, which also effectively enhance the structural stability of the overall hierarchies composing of interconnected hollow MoS2/C spheres with ultrathin shells and vast macro/mesoporous structure. These novel structure characteristics endow the as-prepared MoS2/C hierarchies with many advantages for sodium storage, ex., lower Na+ intercalation barrier, higher penetration of electrolyte and larger surface active sites. When applied as anode materials for sodium ion batteries (SIBs), the as-prepared MoS2/C exhibits outstanding electrochemical properties, delivering a high reversible capacity of 412 mAh/g after 500 cycles at 2 A/g, and remarkably excellent rate capacities of 156/128/76 mAh/g at 10/20/50 A/g after 5000 cycles, respectively. The excellent cycle stability and rate capability are attributed to the unique 3D ordered porous nanostructure, in which the conductive carbon matrices and the enlarged van der Waals gaps in the layered MoS2 synergistically result in the ultrahigh pseudocapacitance contribution in sodium storage.

Ultra-High-Rate Pseudocapacitive Energy Storage in Three-Dimensional Nanoporous Gold/Silver Oxide Composites through Oxidation/Reduction of Silver(I) Oxide to Silver (III) Oxide in Non-Aqueous Electrolytes

Charge storage in pseudocapacitive materials occurs through electron transfer (i.e. faradaic current) involving oxidation and reduction reactions. This process is usually extremely fast because the reactions take place near the surface of the material, meaning the species involved diffuse over short distances. Ionic species such as H+, OH-, Li+, and Na+ commonly used to store charge in pseudocapacitive materials are usually “foreign” elements in the materials. In this talk, I will demonstrate a pseudocapacitive charge storage approach where the ionic species is part of the material. Here we use Ag+ as the working ion in a non-aqueous electrode and silver (I) oxide grown on a three-dimensional nanoporous gold scaffold as the storage medium. Pseudocapacitive charge storage occurs through electrochemical oxidation/reduction of silver (I) oxide to silver (III) oxide at scan rates as high as 1000 mV/s. These high rates are justified by (i) the intrinsic nature of the working ion (Ag+), which is better accommodated by the host silver oxide material as opposed to foreign ions, and (ii) the excellent electronic conductivity provided by the 3D metallic scaffold current collector.

Designing Nanoparticles for Optimized Pseudocapacitive Energy Storage

Power harvesting from intermittent renewable energy sources requires high charge storage capacity without compromising charging speed. These properties are achievable within hybrid capacitors by enhancing the pseudocapacitive (Faradaic) charge storage relative to the double-layer (non-Faradaic) charge storage, via the overlapping of equally-spaced redox reactions to reach a near ideal voltammetry profile. In this work, we examine how the structure of nanoparticles may be utilized to tune the pseudocapacitive response. This is accomplished by utilizing Gerischer-Hopfield electron transfer theory to capture the effects of redox species solvation through the solvent reorganization energy. Our model translates the total-energy picture by Marcus-Hush theory into single-particle picture for electron transfer in Faradaic charge storage processes, thereby arriving at a general phenomenological description of how pseudocapacitive properties can be engineered at the nanoscale. We also explore the design limitations of such nanoparticle-derived pseudocapacitance through a fundamental consideration of electron transfer energetics and kinetics. In general, our analysis is intended to aid the development enhanced pseudocapacitive properties in hybrid capacitor systems.

Quantifying Intercalation Pseudocapacitance Using Single Nanoparticle Electro-Optical Imaging

Understanding the correlated dynamics between atoms, electrons, and ions in solids is critical to the holistic design of energy storage materials. A critical requirement for electrochemical energy storage is to transport electrons and guest ions (e.g., Li+, Na+) in and out of a host lattice. Intercalation pseudocapacitors exhibit unusual charge dynamics in that they store charge throughout the bulk, improving energy density, but do so via a fast capacitor-like intercalation process that is not limited by solid-state diffusion, improving rate. The physical principles and materials chemistry properties that govern and promote fast ion transport in pseudocapacitors is not entirely understood. One major challenge is to quantify the surface and/or bulk sites that participate in the pseudocapacitive charge storage process. Here we show how single particle electro-optical imaging and theoretical analyses can distinguish diffusion-limited and pseudocapacitive energy storage processes in single nanoparticles. Electro-optical measurements quantify redox changes in the solid electrode that have the same scaling relations in time, potential, etc. as the electrochemical scaling relations for pseudocapacitance. We quantify the fraction of surface and bulk sites that charge/discharge with pseudocapacitive signatures and defind structure/property relationships that give rise to enhanced pseudocapacitive contributions. Figure 1

Energetic Cost for Being "Redox-Site-Rich" in Pseudocapacitive Energy Storage with Nickel-Aluminum Layered Double Hydroxide Materials.

Defining the energetic landscape of pseudocapacitive materials such as transition metal layered double hydroxides (LDHs) upon redox-site enrichment is essential to harnessing their power for effective energy storage. Here, coupling acid solution calorimetry, in situ XRD, and in situ DRIFTS, we demonstrate that as the Ni/Al ratio increases, both as-made (hydrated) and dehydrated NiAl-LDH samples are less stable as evidenced by their enthalpies of formation. Moreover, the higher specific capacity at an intermediate Ni/Al ratio of 3 is enabled by effective water-LDH interactions, which energetically stabilize the excessive near-surface Ni redox sites, solvate intercalated carbonate ions, and fill the expanded vdW gap, paying for the "energetic cost" of being "redox-site-rich". Thus, from a thermodynamic perspective, engineering molecule/solid-LDH interactions on the nanoscale with confined guest species other than water, which energetically impose stronger stabilization, may help us to achieve their specific capacitance potential.

Corrigendum to “Stable high-voltage aqueous pseudocapacitive energy storage device with slow self-discharge” [Nano Energy 64 (2019) 103961

Corrigendum to “Stable high-voltage aqueous pseudocapacitive energy storage device with slow self-discharge” [Nano Energy 64 (2019) 103961

Construction of Sn–Mo bimetallic oxide nanoparticle-encapsulated P-doped 3D hierarchical porous carbon through an in-situ reduction and competitive cross-linking strategy for efficient pseudocapacitive energy storage

Efficient pseudocapacitive energy storage devices have attracted considerable attention due to the growing demand of electrical power supply. Still, the design and fabrication of electrode materials with high power and energy densities is challengeable. Herein, an in-situ reduction and competitive cross-linking strategy is presented to construct a novel Sn–Mo bimetallic oxide nanoparticle-grown on P-doped hierarchical porous carbon (SnMo–O@P-HPC). The attractive electrochemical performance of SnMo–O@P-HPC composite with an ultrahigh specific capacitance of 2121.3 F g−1 and excellent cycle stability of 92.5% over 5000 cycles is resulted from unique active component, abundant active sites, and well-organized 3D porous structure. An asymmetric supercapacitor composed of activated carbon and SnMo–O@P-HPC displays an amazing specific energy density of 114.9 W h kg−1 at 515.5 and 98.8 W h kg−1 even at 20,900 W kg−1. Importantly, it also displays a wonderful stability with excellent capacitance retention of 92.5% after 5000 cycles. The work will provide an effective and novel method to obtain the promising electrode materials for supercapacitors.

Constructing expanded ion transport channels in flexible MXene film for pseudocapacitive energy storage

2D transition metal carbide (MXene) based flexible films have received increasing attention in the application of energy storage for portable and wearable electronic devices. However, the self-restacking caused by Van der Waals interactions between the layers lead to insufficient access to allow the electrolyte ions to contact the active material. Herein, we rationally utilized the freeze-drying technique to prepare MXene film electrode for alleviating its self-restacking and improving the electrochemical performances. Through freeze-drying treatment, the frozen solvent molecules were removed by sublimation, and alleviating the adverse effect of van der Waals forces and contributing to the enlarged layer space. The constructed freeze-dried MXene (f-MXene) films generate unique porous architecture, which can provide highly efficient ion diffusion and transport channels. Consequently, the typical f-MXene-10 film exhibit good specific capacitance (341.5 F g−1/922.1 F cm−3 at 1 A g−1) and desired capacitance retention of 60.4% from 1 to 10 A g−1. Moreover, the assembled asymmetric device delivered a maximal gravimetric energy density of 6.1 Wh Kg−1, volumetric energy density of 16.4 Wh L−1 and a good capacitance retention of 89.3% after 10,000 cycles. The rationally designed synthesizing route has attractive promise to promote MXene materials for diverse energy and environmental applications.

Enhanced pseudocapacitive energy storage properties of budding-branch like MoO2@C/CNT nanorods.

Despite its high theoretical capacitance, molybdenum dioxide still cannot be well applied in supercapacitors due to its low conductivity and structural instability. Generally, compositing molybdenum dioxides with carbon materials can provide new possibilities. In this work, we successfully fabricated a budding-branch like MoO2@C composite wrapped by carbon nanotubes (CNTs). Different from those MoO2 nanodots synthesized by traditional approaches, the MoO2 nanodots in this composite were distributed uniformly inside the carbon rods, which effectively alleviated their self-aggregation and benefited their electron transportation. The introduction of the CNTs further provided the composite with better contact with the electrolyte and increased charge transfer. The prepared MoO2@C/CNT electrode exhibits a superior specific capacitance of 1667.2 F g-1 at 1 A g-1 and an excellent reversibility of 92.8% capacitance retention after 3000 cycles. Furthermore, asymmetric supercapacitor devices based on the MoO2@C/CNT composite and active carbon were assembled, which showed promising electrochemical properties at an extended operating voltage of 1.4 V and could light a green LED device for 15 minutes after charging for 30 s.