Electroactive Materials For Supercapacitors Research Articles

Redox-Active Boron Clusters.

ConspectusIn this Account, we discuss our group's research over the past decade on a class of functionalized boron clusters with tunable chemical and physical properties, with an emphasis on accessing and controlling their redox behavior. These clusters can be thought of as three-dimensional aromatic systems that have distinct redox behavior and photophysical properties compared to their two-dimensional organic counterparts. Specifically, our lab has studied the highly tunable, multielectron redox behavior of B12(OR)12 clusters and applied these molecules in various settings. We first discuss the spectroscopic and electrochemical characterization of B12(OR)12 clusters in various oxidation states, followed by their use as catholytes and/or anolytes in redox flow batteries and chemical dopants in conjugated polymers. Additionally, the high oxidizing potential and visible light-absorbing nature of fluoroaryl-functionalized B12(OR)12 clusters have been leveraged by our group to generate weakly coordinating, photoexcitable species that can promote photooxidation chemistry.We have further translated these solution-phase studies of B12(OR)12 clusters to the solid state by using the precursor [B12(OH)12]2- cluster as a robust building block for hybrid metal oxide materials. Specifically, we have shown that the boron cluster can act as a thermally stable cross-linking material, which enhances electron transport between metal oxide nanoparticles. We applied this structural motif to create TiO2- and WO3-containing materials that showed promising properties as photocatalysts and electroactive materials for supercapacitors. Building on this concept, we later discovered that B12(OCH3)12, the smallest of the B12(OR)12 family, could retain its redox behavior in the solid state, a previously unseen phenomenon. We successfully harnessed this unique behavior for solid-state energy storage by implementing this boron cluster as a cathode-active material in a Li-ion prototype cell device. Recently, our group has also explored how to tune the redox properties of clusters other than B12(OR)12 species by synthesizing a library of vertex-differentiated clusters containing both B-OR and B-halogen groups. Due to the additive qualities of different functional groups on the cluster, these species allow access to a region of electrochemical potentials previously inaccessible by fully substituted closo-dodecaborate alkoxy-based derivatives.Lastly, we discuss our research into smaller-sized redox-active polyhedral boranes (B6- and B10-based cluster cores). Interestingly, these clusters show significantly less redox stability and reversibility than their dodecaborate-based counterparts. While exploring the functionalization of closo-hexaborate to create fully substituted derivates (i.e., [B6R6Hfac]-), we observed unique oxidative decomposition pathways for this cluster system. Consequently, we leveraged this oxidative instability to generate useful alkyl boronate esters via selective chemical oxidation. We further explored a closo-decaborate cluster as a platform to access electrophilic [B10H13]+ species capable of directly borylating arene compounds with unique regioselectivity. Upon chemical oxidation of the arylated decaborate clusters, we successfully synthesized various aryl boronate esters, establishing the generality of the oxidative cluster deconstruction concept.Overall, our work shows that boron clusters are an appealing class of redox-active molecules, and this fundamental and understudied property can be leveraged for constructing novel materials with tunable physical and electrochemical properties, as well as producing unique chemical reagents for small molecule synthesis.

Sonochemically synthesized porous V2CTx MXene/red-black phosphorus composite: A promising electrode for supercapacitors

Densely packed structure of MXene leads to less utilization of active material and thus show the mediocre performance. Fabrication of electrolyte-accessible, porous MXene composites with high rate capability and improved cycle life still needs to be looked upon. Herein, we've synthesized a red/black phosphorus (RBP) hybrid via an efficient and sustainable sonochemical route using low-cost red phosphorus (RP) in different solvents. RBP hybrid was then incorporated between V2CTx MXene layers to fabricate hetero-structured 3D V2CTx MXene-red/black phosphorus nanohybrid (MXRBP) as an electroactive material for supercapacitors. Red/black phosphorus prevented the MXene sheet from restacking and the resulted highly porous structure offered large electrochemically active surface area (ECSA ∼2125 m2 g−1). The MXRBP nanocomposite electrode blends the superior electrochemical characteristics of V2CTx MXene and the mechanical resilience of RBP. Therefore, the synergy in MXRBP||MXRBP symmetric cell result in energy density of 55 Wh Kg−1 @452 W Kg−1. This study, which is based on the design and fabrication of MXene based materials, is anticipated to offer a broad foundation for the next generation energy storage devices.

Electrochemical performance of SrMoO4 as electrode material for energy storage systems

In this work, strontium molybdenum oxide (SrMoO4) is reported as electrode material for supercapacitors (SCs). The physico-chemical properties of SrMoO4 were investigated and tested for capacity performance by employing cyclic voltammetry (CV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS). SrMoO4 showed significant specific capacity of 384 C g−1 at 5 mVs−1 with good cyclic stability and capacity retention of 91.1% upto 3000 GCD cycles at current density 1 Ag−1. Also, the as-fabricated SrMoO4 showed the coulombic efficiency and energy efficiency of 81.2% and 82.6% respectively over 5000 GCD cycles. The substantial by active surface area and increased abundance of reactive sites, revealed by the morphological analysis contributed for the higher specific capacity. The lowest values of Rs and Rct exhibited by SrMoO4 such as 2.21 Ω and 1.31 Ω, respectively predicted the faster reaction kinetics. Furthermore, symmetric supercapacitor device (SSD) was also assembled and it exhibited a wide operating voltage window of 1.6 V, with an energy density of 67.3 Wh kg−1 at a power density of 3150 Wkg−1. The pseudocapacitive performance of SrMoO4 was attributed to its unique structure, which enabled fast ion /electron transport and low solution resistance. The electrochemical results demonstrated that SrMoO4 could be utilized as electro-active material for SCs.

NaCl-assisted triethylene glycol combustion preparation of lithium manganese oxides with hierarchical mesopores for energy storage

The development of electroactive materials with affordable prices, strong activity, and outstanding cyclic life are desirable for energy storage. Herein, a novel NaCl-assisted triethylene glycol (TEG) combustion strategy was adopted to prepare lithium manganese oxides (LMO) with hierarchical mesopores for energy storage. The electrochemical activity of LMO strongly depends on the dosage of NaCl. The LMO-200 shows optimal electrochemical activity, which can deliver 120.1 C g−1 of discharge capacity at 0.25 A g−1. When employed as electroactive material for supercapacitors, Li-ion batteries, and LMO//Zn batteries, LMO-200 delivered outstanding energy storage performance. The LMO-200 with hierarchical mesopores can offer a good deal of active sites and suppress the volume change during charge/discharge cycles, thus boosting the specific capacity, rate capability, and cyclic performance. The present work not only developed a convenient route to LMO with hierarchical mesopores but also displayed its wide applications for electrochemical energy storage.

Improving energy storage ability of Universitetet i Oslo-66 as active material of supercapacitor using carbonization and acid treatment

The zirconium-based metal organic framework, Universitetet i Oslo-66 (UIO-66), has attracted much attention as electroactive material for supercapacitors. The carbonization and acid treatment are applied to enhance the energy storage ability of UIO-66. The detail physical and electrochemical comparison are firstly discussed in this work to understand the carbonization and acid treatment effects. The octahedron morphology is obtained for UIO-66 and the derivatives with carbonization and acid treatments. The largely reduced size and rougher surface are obtained for the UIO-66 with carbonization and acid treatments (C-UIO-66-AT) due to the escape of carbon gas and removal of ZrO2. A higher specific capacitance (CF) of 117.7 F/g is obtained for the C-UIO-66-AT electrode than those of 0.61 and 32.78 F/g for UIO-66 and C-UIO-66, respectively, owing to the carbon nature with higher porosity and rougher surface for the former case. The solid-state supercapacitor (SSC) composed of C-UIO-66-AT electrodes and gel electrolyte shows a potential window of 0.9 V and a maximum specific energy of 2.133 Wh/kg at the specific power of 200 W/kg. The CF retention of 85% and Coulombic efficiency of 100% are also achieved for the SSC after 10,000 times repeated charge/discharge cycling process. This work proposes a simple method to largely enhance the energy storage ability of UIO-66. More discussion of carbonization and acid treatment parameters will be proposed in near future.

Synthesizing novel NH4CoxNi1-xF3 as electroactive material for supercapacitors using 2-methylimidazole: Study of reaction durations

Zeolite imidazolate framework-67 (ZIF67) with high surface area is one of promising metal–organic frameworks as electroactive material for supercapacitors (SC). Energy storage ability of electroactive material is primarily influenced by morphology, which is confined via structure-directing agent (SDA). It is the first time to use NH4F as SDA to develop novel bimetallic ZIF67 derivative, NH4CoxNi1-xF3 (NCNF), as electroactive material for SC. The polyhedron structure of ZIF67 is greatly evolved to particle-aggregated sphere (PAS) when NH4F is incorporated. Different reaction times are used for synthesizing NCNF to understand PAS formation mechanism. The NCNF electrode prepared using 24 h shows the highest specific capacitance (CF) of 490.4 F/g at 20 mV/s, due to less grain boundaries between smooth particles for assembling huge sphere, while bimetallic ZIF67 synthesized without NH4F merely shows CF value of 2.7 F/g. The largely enhanced CF value for NCNF electrode with NH4F incorporation is benefitted from novel PAS formation and high electronegativity of fluorine. The SC composed of optimized NCNF electrodes shows CF value of 150 F/g at 1 A/g, maximum energy density of 19.7 Wh/kg at 500 W/kg, and excellent cycling stability with CF retention of 100% and Coulombic efficiency of 76% after 5000 times charge/discharge process.

Preparation, characterization, and electrochemical properties of KOH-activated carbon from cassava root

This work reports the electrochemical properties of activated carbon prepared from cassava starch activated using KOH. The effect of the impregnation ratios by weight on the surface area and pore structure of the prepared carbon was characterized by N2 adsorption-desorption techniques. By increasing the impregnation ratio by weight from 1:1 to 1:5, the specific surface area increased from 1049 to 1350 m2 g−1 and mean pore diameter also increased from 1.89 to 2.15 nm, respectively. The decrease of mean pore diameter enhanced the specific capacitance value. The carbon electrode with the impregnation ratio of 1:1 reached the maximum specific capacitance of 175 ± 12 F g−1 at the current density of 2 A g−1 due to the smaller mean pore size. Moreover, the specific capacitance retained about 98% after 1000 cycles. The excellent electrochemical performance of activated carbon prepared from cassava starch could be considered as an electroactive material for supercapacitors.

Hexylsulfanyl-substituted CoPc and GO-CoPc on Ni foam as electroactive material for supercapacitors

For energy storage applications, hexylsulfanyl-substituted CoPc and GO-CoPc electrodes were prepared on Ni foam, and then electrochemical properties of the materials were investigated. The GO-CoPc nanocomposite electrode exhibits 56 mFcm[Formula: see text] at 1.4 mAcm[Formula: see text] current density. The results show that the GO-CoPc nanocomposite electrode is a promising electroactive material for supercapacitors.

Enhanced electrochemical properties of manganese-based metal organic framework materials for supercapacitors

Novel nanorod-like manganese-based metal–organic frameworks (Mn-MOFs) were prepared through a facile solvothermal method using 1,3,5-benzentricarboxylic acid as organic linker and applied as electroactive material for supercapacitors. The Mn-MOFs exhibited a high-specific capacitance of 371 F g−1 at 0.5 A g−1 and an excellent rate capability even if the current density was promoted to 10 A g−1. Furthermore, the designed asymmetric supercapacitors with Mn-MOFs as cathode, graphene as anode and 6 mol L−1 KOH as electrolyte delivered a specific capacitance of 89 F g−1 in the working potential range of 0 to 1.6 V and the energy density can reach 31.6 Wh kg−1. Notably, the specific capacitance of the fabricated hybrid supercapacitors retained 91.8% of the initial capacitance after successive 10,000 charge–discharge cycles at a current density of 1 A g−1. These results suggest that the as-synthesized nanorod-shaped Mn-MOFs can act as promising electrode materials for high-performance supercapacitors. Nanorod-like Mn-MOFs were prepared and investigated as promising electroactive nanomaterials for supercapacitors, which exhibited a high-specific capacitance of 371 F g-1, excellent rate capability and long cycle life with an eminent energy density of 31.6 Wh kg-1 for the designed Mn-MOFs//graphene supercapacitor.

Stamen-petal-like CeO2/NiMn layered double hydroxides composite for high-rate-performance supercapacitor

Layered double hydroxides are widely used as electroactive materials for supercapacitors. However, they exhibit poor rate performances at high current densities. To improve the rate performance of layered double hydroxides, a stamen-petal-like CeO2/NiMn layered double hydroxides composite is synthesized successfully by a one-step hydrothermal method. During the hydrothermal reaction, stamen-like CeO2 precipitates first, followed by the crystallization of petal-like NiMn layered double hydroxides sheets around the CeO2 stamen-like particles. By adjusting the amount of Ce(NO3)3 in the reaction mixture, an optimal composite electrode is obtained, which exhibits maximum specific capacities of 1956.0 F g−1 (1173.6C g−1) and 1261.4 F g−1 (756.8C g−1) at 1 A g−1 and 20 A g−1, respectively, and a much better rate performance compared with the traditional NiMn layered double hydroxides and CeO2 composite. A hybrid capacitor assembled using the optimal composite and activated carbon shows a high specific capacitance of 145.7 F g−1, and a capacitance retention rate of 91.9% after 5000 cycles at 10 A g−1. The maximum energy density is 51.8 Wh kg−1 at a power density of 809 W kg−1. The excellent performance is attributed to the special petal-like structure of the layered double hydroxides sheets, which provides a higher specific surface area and abundant pathways for ion transport. In addition, the CeO2 particles among the petal-like layered double hydroxides sheets effectively reduce the internal electrochemical resistance of the composite electrode, thereby according a high rate performance.

Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets

Two-dimensional (2D) nanostructures with their high surface area and large in-plane conductivity have been regarded as promising materials for supercapacitors (SCs). Tungsten disulfide (WS2) is highly suitable for charge accumulation with its abundant active sites in the interspacing between the 2D structures and the intraspacing of each atomic layer, as well as on the tungsten centers with the charges generated by the Faradaic reactions. This study proposes the preparation of well-constructed WS2/reduced graphene oxide (RGO) nanosheets using a simple molten salt process as the electroactive material for SCs, which presents a high specific capacitance (CF) of 2508.07 F g−1 at the scan rate of 1 mV s−1, because of the synergic effect of WS2 with its large charge-accumulating sites on the 2D planes and RGO with its highly enhanced conductivity and improved connections in the WS2 networks. The excellent cycling stability of 98.6% retention after 5000 cycles charge/discharge process and the Coulombic efficiency close to 100% for the entire measurement are also achieved for the WS2/RGO-based SC electrode. The results suggest the potential for the combination of the 2D metal sulfide and carbon materials as the charge storage material to solve the energy problems and attain a sustainable society.

Investigation of the electroactive capability for the supercapacitor electrode with cobalt oxide rhombus nanopillar and nanobrush arrays

Well-defined nanostructures of the cobalt oxide are designed intensively to pursue large surface area and high conductivity as the electroactive material for supercapacitors (SCs). Instead of merely fabricating effective electroactive materials, two kinds of the cobalt oxide nanostructures synthesized directly on nickel foam to achieve good contact between the material and the substrate are compared to clarify the growth mechanism and the structure-dependent SC performance. The nanobrushes are completely composed of nanoparticles while the rhombus nanopillars present integrated structure by the recrystallization of single nanopillars. A higher specific capacitance (CF) of 509 F/g is obtained for the SC electrode with CoO rhombus nanopillar array at a scan rate of 10 mV/s, comparing to that of 169 F/g for the CoO nanobrush array-based SC electrode, due to the larger electroactive surface area and less recombination sites for the former case. The CF value is enhanced by 38% as compared with the initial value after 3000 cycles of repeated charge/discharge process for the CoO rhombus nanopillar array-based SC electrode due to the activation of the material. The results provide a blue print for achieving highly efficient SC electrode by carefully designing the well-established electroactive material to attain facile and long-lasting faradic reactions.

UV Light-assisted Electropolymerization of Pyrrole on TiO2 for Supercapacitors: Investigating the Role of TiO2

One-dimensional (1D) TiO2 nanorod array (TiNA) is used as the support for controllablly growing polypyrrole (PPy) on fluorine-doped tin oxide glasses as the electroactive material for supercapacitors (SCs). Ultraviolet (UV) light is applied during pyrrole electropolymerization on TiNA to provide photoholes for oxidation and improve charge-transfer between pyrrole and TiO2. The light condition and pyrrole monomer concentration for the electropolymerization play important roles on the deposition extent and the performance of the SC electrode. The specific capacitance (CF) value of 200.45F/g was obtained for the PPy/TiNA-based SC electrode at a scan rate of 5mV/s. The role of TiO2 is found to be significant in the electrode as the underlayer for the effective 1D electron transfer other than being the electroactive material for Faradic reactions. The CF retention of 72% is attained after 1,500 repeated charge/discharge processes at 2A/g for the optimized PPy/TiNA-based SC electrode. The result provides new sight to use poor electrocapacitive material as the highway for electron transfer to enhance the SC electrochemical performance and verifies the position to place the nanomaterial in the electrode is of great importance to affect the SC performance.

Carbon quantum dots/nickel oxide (CQDs/NiO) nanorods with high capacitance for supercapacitors

Novel CQDs/NiO nanorods have been prepared via a facile complexation method followed by a thermal treatment process and used as electroactive materials for supercapacitors, which deliver a high specific capacitance of 1858 F g−1 at 1 A g−1.

Facilely synthesized α phase nickel–cobalt bimetallic hydroxides: Tuning the composition for high pseudocapacitance

A facile and novel precursor conversion method is successfully developed to grow Ni–Co hydroxides with α phase structure. The method is robust and versatile enough to be employed to prepare α phase Ni–Co hydroxides with similar interconnected flower-like nanostructures even at different Ni and Co contents. When used as the electroactive materials for supercapacitors, the bimetallic Ni–Co hydroxides deliver much higher specific capacitance with improved rate capability and cycling stability than monometallic Ni hydroxide. Especially, the Ni0.5Co0.5 hydroxide exhibits the highest specific capacitance of 1600Fg−1 at a current density of 1Ag−1. Meanwhile, the mechanism for better cycling stability of the bimetallic Ni–Co hydroxides is studied by FT-IR measurement. It is found that the introduction of Co ions contributes to the stable existence of ethylene glycol molecules in the interlayer space of the α phase Ni–Co hydroxides, resulting in superior cycling stability of the bimetallic Ni–Co hydroxides.

Synthesizing highly conductive cobalt sulfide hydrangea macrophylla using long carbon-chain sulfur source for supercapacitors

Novel cobalt sulfide hydrangea macrophylla with conductive carbon layer on the surface was synthesized in a one-pot method using 1-dodecanethiol as the sulfur source connected with long carbon chains as the electroactive material for supercapacitors.

Electrochemical synthesis of reduced graphene oxide/TiO2 nanotubes/Ti for high-performance supercapacitors

RGO/TiO2NTs/Ti electrodes with high surface area and good capacitive characteristics were prepared by simple electrochemical reduction of graphene oxide (GO) onto the previously formed TiO2 nanotubes. Microstructure studies show that reduced graphene oxide (RGO) having high surface area has been deposited onto the TiO2NT arrays. The electrochemical capacitive behaviors of the obtained electrodes were investigated by cyclic voltammetry (CV), galvanostatic charge–discharge studies, and electrochemical impedance spectroscopy (EIS) in 1 M H2SO4 solution. The electrochemical data demonstrated that the electrodes displayed specific capacitance of 410 F g−1 at the current density of 50 A g−1 and specific capacitance retention of ca. 85 % after 10 days of being used in electrochemical experiments, indicating to be promising electroactive materials for supercapacitors.

A 3D hierarchical porous α-Ni(OH)2/graphite nanosheet composite as an electrode material for supercapacitors

Supercapacitors are the most promising energy storage devices by virtue of high power density, long cycle life, short charging time and environmental benignity. In order to enhance the energy density, rate capability and cycle stability for supercapacitors, a α-Ni(OH)2/graphite nanosheet composite is prepared via a homogeneous precipitation method. The morphology and microstructure of the as-prepared composite are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. It is demonstrated that after introducing the graphene oxide nanosheets into α-Ni(OH)2, a 3D hierarchical porous structure of fine α-Ni(OH)2 nanocrystals as building blocks is formed directly on the matrix of graphite nanosheets in the presence of urea as a mild reducing agent. The electrochemical performance of the as-prepared α-Ni(OH)2 and α-Ni(OH)2/graphite nanosheet composites as electro-active materials for supercapacitors is investigated by a galvanostatic charge–discharge method. As expected, the as-prepared α-Ni(OH)2/graphite nanosheet composite exhibits large specific capacitance, good rate capability and long cycle stability as compared to the pure α-Ni(OH)2. Apparently, the unique structure of fine α-Ni(OH)2 nanocrystals fabricated on the matrix of graphite nanosheets is responsible for the improvement of the reaction kinetics and subsequent electrochemical performance of the composite.

Synergistic effect of Fe3O4/reduced graphene oxide nanocomposites for supercapacitors with good cycling life

Fe3O4/reduced graphene oxide (rGO) composites were prepared through a facile hydrolysis route and subsequent annealing process and used as the electroactive materials for supercapacitors. The as-prepared Fe3O4/rGO composites exhibit much higher specific capacitances than either pure rGO or pure Fe3O4, meanwhile, the electrochemical performance of Fe3O4/rGO composites with different amount of RGO has been compared. The optimized Fe3O4/rGO composites (FG-0.8) exhibit the highest specific capacitance of 350.6Fg−1 at 1mVs−1, 157.6Fg−1 of the specific capacitance can still be retained when the scan rate increases 100 times to 100mVs−1, indicating a good rate capability. These Fe3O4/rGO composites also show excellent cycling performance without any decrease of specific capacitance after 10,000 cycles. These fascinating performances can be attributed to the positive synergistic effect between Fe3O4 and rGO.