Adsorption Of Electrolytes Research Articles

Electrolyte adsorption in graphene and hexagonal boron nitride nanochannels

Understanding the dynamics of confined electrolytes through graphene-based channels is essential in multiple fields of science including: (a) developing novel artificial membranes for drug delivery, DNA sequencing and water desalination and (b) developing quantum nanoelectronic devices. In this work, All-Atom molecular dynamics simulations are used to study the adsorption of confined NaCl electrolyte solutions in graphene (GR), hexagonal boron nitride (hBN) and combined GR-hBN-GR nano-channels over a wide range of electrolyte concentrations c∈(0.1,2) M. The behavior of electrolytes confined between these surfaces is analyzed by evaluating the ionic density profiles, Potential of Mean Force (PMF), net charge density and integrated charge. Although neither graphene nor hBN has net electric charge, we observed significant changes in the ionic structure including the enhanced accumulation of sodium and the depletion in chloride density close to the hBN interfaces at low salt concentration. However, increasing the salt concentration leads to accumulation of chloride close to hBN surfaces. Two different types of hBN-GR junctions are considered in this study: a) shuffle (zigzag bonds) and (b) glide (parallel bonds). High adsorption of sodium and chloride ions near shuffle GR-hBN-GR surfaces is observed due to electrostatic attraction forces between the sodium and nitrogen atoms and between chloride and boron atoms at hBN–graphene junctions across zigzag bonds. This study reveals that GR-hBN-GR with shuffle interface is highly advantageous in adsorbing sodium and chloride ions than graphene and hBN alone. To the best of our knowledge, this is the first study that elucidates the interaction between monovalent electrolytes and GR-hBN-GR heterostructures in a confined medium.

Thermotolerant separator of cross-linked polyimide fibers with narrowed pore size for lithium-ion batteries

Separators are of significance in the thermal safety and energy-storage performance of lithium-ion batteries (LIBs). Thermotolerant separators of electrospun polyimide (PI) fibers hold great promise as candidate to commercial polyolefin-based separators due to their high thermal and electrochemical stability. However, their applications in LIBs are limited by the undesirably large pore size (∼2 μm) that will induce dendrite growth and thermal runaway of batteries. Here, we demonstrate a facile and scalable strategy to prepare thermotolerant separators of cross-linked PI fibers (c-PI) with a narrowed pore size (0.78 μm) via a blended electrospinning of polyamic acid and polystyrene followed by a thermal imidization. The electrospun fibers in the c-PI separator are ribborn-like and feature adhesion structure between adjacent fibers, while no such structures are observed for PI separator obtained without adding polystyrene. In comparison with the PI separators, the c-PI separator exhibits higher porosity, better electrolyte adsorption and higher ionic conductivity. In Comparison to the batteries with PI and Celgard separators, the battery with c-PI separator shows greatly enhanced electrochemical performance with a high capacity of 100.1 mAh/g at 10 C after 1600 cycles. This work offers a feasible and effective route to develop advanced membranes of electrospun fibers for safe LIBs.

Effect of Pore Filling on Properties of Nanocomposites LiClO4-MIL-101(Cr) with High Ionic Conductivity.

Experimental data on nitrogen adsorption, pellets density and ionic conductivity of nanocomposite solid electrolytes (1−x)LiClO4–xMIL-101(Cr) were interpreted in frames of the model of the composite in which the lithium salt fills the pores of a metal-organic framework MIL-101(Cr). According to the model, the concentration of lithium salt located in the pores reaches a maximum at the concentration x = xmax which is defined by a ratio of the molar volume of LiClO4 and the total volume of accessible pores in the MIL-101(Cr) framework. The model allows one to describe the dependences of pore volume and pellet density on the concentration of MIL-101(Cr). Conductivity of the composites were successfully described by two separate mixing equations for concentration ranges x < xmax and x > xmax. In the first concentration region x < xmax, the composite may be regarded as a mixture of LiClO4 and MIL-101(Cr) with completely filled pores accessible for LiClO4. At x > xmax, the total amount of lithium perchlorate is located in the pores of MIL-101(Cr) and occupies only part of the volume of the accessible pores. It was found that xmax value determined from the concentration dependence of conductivity (xmax = 0.06) is noticeably lower than the corresponding value estimated from adsorption data (xmax = 0.085) indicating a practically complete filling the pores of MIL-101(Cr) in the composite pellets heated before conductivity measurements.

Core-shell structured Ni0.85Se@MoS2 nanosheets anchored on multi-walled carbon nanotubes-based counter electrode for dye-sensitized solar cells

The luxury price and scarcity of noble platinum (Pt) hamper its commercialization in dye-sensitized solar cells (DSSCs). Hence, developing cheap, earth-abundant, and easily available electrocatalyst in lieu of Pt is of great current priority for DSSCs. Herein, core-shell structured Ni0.85Se@MoS2 nanosheets anchored on carboxylic functionalized multi-walled carbon nanotubes (denoted as Ni0.85Se@MoS2@MWCNTs) were prepared via two-step hydrothermal method combined with probe-sonication process. The resultant of Ni0.85Se@MoS2@MWCNTs was successfully characterized by XRD, FESEM, TEM, Raman, and BET analysis. Benefiting from the larger surface areas and more electrolyte adsorption sites, the prepared Ni0.85Se@MoS2@MWCNTs exhibited excellent electrocatalytic performance, electrical conductivity, and electrochemical stability when applied as the counter electrode (CE) for DSSCs. More concretely, the DSSCs assembled with Ni0.85Se@MoS2@MWCNTs delivered remarkable power conversion efficiency (PCE) of 8.98%, exceeding the cell based on Pt CE (6.86%). The combination of Ni0.85Se@MoS2 core-shell structure and MWCNTs provided more active sites, remarkable electric conductivity, and an admirable catalytic property toward the reduction of triiodide. This research suggests a cost-effective strategy to exploring highly effective and robust non-precious-metal CE for their potential application in next-generation energy storage and conversion devices such as DSSCs, water splitting, fuel cells, and other electrochemical applications.

A high energy density supercapacitor fabricated with aqueous polymer electrolyte based on soybean protein isolate grafted by polyacrylic acid

Soybean protein isolate (SPI) has good reactivity, hydrophilicity and film-forming properties, but its electrochemical performance is poor when used as a polymer electrolyte matrix. The graft-modification of the SPI using polyacrylic acid (PAA) allows the introduction of strong hydrophilic carboxyl groups into the SPI molecular chains. This enables the SPI to effectively absorb electrolytes in the subsequent electrolyte adsorption process, resulting in a modified polymer electrolyte. The resulting constructed supercapacitor can operate stably in a voltage window of 0–2 V and exhibit excellent electrochemical behavior with a high single electrode specific capacitance up to 293.08 F g−1 and an energy density of 38.76 W h kg−1 at a power density of 734.38 W kg−1, which is much better than SPI-based supercapacitor. This simple, low-cost and environmentally friendly approach has a great potential for the development of high-performance supercapacitor with high voltage window, high specific capacitance and high energy density.

Powder metallurgical 3D nickel current collectors with plasma-induced Ni3N nanocoatings enabling long-life and dendrite-free lithium metal anode

Building three-dimensional (3D) current collectors is a promising strategy to surmount the bottlenecks of lithium metal anodes (LMAs), but the regulation methodology of a 3D current collector has seldom been considered comprehensively concerning both skeleton architectures and surface coatings. Herein, a robust porous 3D nickel skeleton (NS) with lithiophilic Ni3N nanocoatings (Ni3N@NS) is synthesized via an integrative route of powder metallurgy/plasma-enhanced nitridation technics. The facile powder metallurgical method facilitates the adjustment of NS architectures toward sufficient electrolyte adsorption and even current density distribution, while the followed plasma-enhanced chemical vapor deposition (PECVD) method can induce compact Ni3N nanocoatings on NS, which reduces the Li nucleation overpotential, accelerates the Li-ion transfer, and facilitates a highly reversible oriented texture of Li deposition morphology owing to the dense and homogenous deposition of Li into the pores. The optimized Ni3N@NS current collector shows a high averaged Coulombic efficiency (CE) of 98.8% over 350 cycles, a prolonged lifespan of 1000 h (at 2 mA cm−2) in symmetrical cells, together with the significant performance in full cells. The ingenious methodology reported in this work can also be broadly applicable for the controllable production of other 3D skeletons with nitride nanocoatings for various applications.

Controlled synthesis of NiCo2O4@Ni-MOF on Ni foam as efficient electrocatalyst for urea oxidation reaction and oxygen evolution reaction

Heterostructured materials with special interfaces and features give a unique character for much electrocatalytic process. In this work, the introduction of exogenous modifier Ni-MOF improved the reaction kinetics and morphology of the NiCo2O4@Ni-MOF/NF catalyst. As-obtained NiCo2O4@Ni-MOF/NF has excellent oxygen evolution reaction (OER) performance and urea oxidation reaction (UOR) performance. The catalyst need overpotential of 340 mV at a current density of 100 mA cm−2 for OER and a potential of 1.31 V at the same current density for UOR. The Tafel slopes of NiCo2O4@Ni-MOF/NF is 38.34 and 15.33 mV dec−1 for OER and UOR respectively, which is more superior than 78.58 and 66.73 mV dec−1 of NiCo2O4/NF. The nanosheets microstructure is beneficial to the adsorption and transport of electrolyte and the presence of a large number of mesoporous channels can also accelerate gas release, and then improves activity of the catalyst. Density functional theory calculation demonstrate that NiCo2O4 plays a role in absorbing water, while the existence of in situ generated NiOOH can promote the electron transfer efficiency. It is synergies of NiCo2O4 and in situ generated NiOOH that enhance the decomposition of water on the surface of the NiCo2O4@Ni-MOF/NF. This investigation provides a new strategy for the application of spinel oxide and MOF materials.

Electrochemical sensors for hydroquinone and catechol based on nano-flake graphite and activated carbon sensitive materials

The nano-flake graphite and bamboo activated carbon composites (NFG-BAC) were obtained by simple ball-milling and carbonization of bamboo powder and flake graphite at high temperature, which was modified on the surface of the electrode to obtain an electrochemical sensor for the simultaneous separation and determination of hydroquinone (HQ) and catechol (CC). NFG-BAC showed good wettability to electrolyte solution and adsorption capacity to target analytes. NFC-BAC composites possess both the rich porosity and high specific surface area of activated carbon and the good conductivity of nano-flake graphite. Moreover, NFG-BAC composites show good wettability to electrolyte solution and adsorption capacity to target analytes. Compared with the gold-standard methodology for water analysis, electrochemical sensing has fast, sensitive and other advantages. When the NFG-BAC/GCE sensor is used for quantitative analysis of HQ and CC, the linear range is wide (2–1000 μM for HQ and 1–1000 μM for CC), the detection limit is low (0.4 μM for HQ and 0.2 μM for CC), and the detection accuracy of these two pollutants in environmental water samples is high. Due to the sensor fabrication is simple, low-cost and efficient, the NFG-BAC/GCE sensor can be used for real-time monitoring of the pollution of catechol in environmental water samples.

First-Principles Study of Amorphous Al2O3 ALD Coating in Li-S Battery Electrode Design

The Li-S battery is exceptionally appealing as an alternative candidate beyond Li-ion battery technology due to its promising high specific energy capacity. However, several obstacles (e.g., polysulfides’ dissolution, shuttle effect, high volume expansion of cathode, etc.) remain and thus hinder the commercialization of the Li-S battery. To overcome these challenges, a fundamental study based on atomistic simulation could be very useful. In this work, a comprehensive investigation of the adsorption of electrolyte (solvent and salt) molecules, lithium sulfide, and polysulfide (Li2Sx with 2 ≤x≤ 8) molecules on the amorphous Al2O3 atomic layer deposition (ALD) surface was performed using first-principles density functional theory (DFT) calculations. The DFT results indicate that the amorphous Al2O3 ALD surface is selective in chemical adsorption towards lithium sulfide and polysulfide molecules compared to electrolytes. Based on this work, it suggests that the Al2O3 ALD is a promising coating material for Li-S battery electrodes to mitigate the shuttling problem of soluble polysulfides.

Aqueous Lithium Carboxymethyl Cellulose and Polyacrylic Acid/Acrylate Copolymer Composite Binder for the LiNi0.5Mn0.3Co0.2O2 Cathode of Lithium-Ion Batteries

Water-soluble green cathode binders are developed to increase the performance of 18650 type LiNi0.5Mn0.3Co0.2O2 (NMC532) lithium-ion batteries (LIBs). Using four basal substances to prepare the composite binders, it is indicated that the cathode with lithium carboxymethyl cellulose (CMCLi)-polyacrylic acid/acrylate copolymer (type 306 F) composite binder (Marked as Binder C) avoids the corrosion of aluminum substrate, and exhibits stronger adhesive force and better electrolyte adsorption capacity compared to other cathodes with PVDF binder and single aqueous binders. In particular, the electrochemical performance of the batteries with Binder C is also improved, initial specific capacity of 161.5 mAh g−1 at 0.2C and retention capacity of 88.9% at 1C after 1200 cycles are obtained. The batteries with Binder C also exhibit enhanced high-temperature storage performance, there is 97.9% residual capacity when the fully charged batteries are stored in 60 °C for 14 d. The enhanced performance is mainly attributed to the chemical stability and bonding ability of polyacrylic acid/acrylate copolymer and better conduction at the liquid-solid interface caused by CMCLi. These results indicate that Binder C has promising application prospects in the NMC532 cathode, and also provide a reference for the green production of NMC-based LIBs.

Water‐soluble Polyacrylate Copolymers as Green Binders of Graphite Anodes for High‐energy Density Lithium‐ion Pouch Cells with Enhanced Electrochemical and Safety Performance

AbstractWater‐soluble polyacrylate copolymers are developed as green functional binders of graphite anode to improve the performance of lithium‐ion batteries (LIBs). Using five basic binders, it is demonstrated that the graphite plate with the alkanol acrylate crosslinker (type U20)‐acrylic acid/acrylate copolymer (type LA138) composite binder (marked as Binder B) exhibits larger adhesive force, higher compaction density, and better electrolyte adsorption capacity compared to other plates with aqueous binders. In particular, the electrochemical performance of the batteries with Binder B is enhanced both at room temperature and low temperature, and retention capacity of 82.8 % at 1 C and 25 °C after 930 cycles and 91.3 % at 0.5 C and 0 °C are obtained. In addition, the batteries with Binder B also exhibit enhanced safety and heat stability. The enhanced performance is attributed to the good chemical stability of polyacrylate copolymers and unique film‐forming properties. These results indicate that the graphite anode with aqueous Binder B has promising prospects for practical application, and it also provides a reference for solving the poor low‐temperature performance of graphite anodes.

Fluorinated Graphite (FG)-Modified Li-S Batteries with Superhigh Primary Specific Capacity and Improved Cycle Stability.

Lithium-sulfur (Li-S) batteries have received extensive attention because of their high theoretical energy density and low cost. However, the low sulfur utilization and the shuttle effect of polysulfide cause low initial capacity and serious capacity decay. Herein, fluorinated graphite (FG) is introduced to the cathode to alleviate these issues. The results indicated that the FG could provide additional capacity during the first discharge process and increase the porosity and polarity of the cathode via in situ formation of lithium fluoride (LiF) nanocrystals, which can enhance the infiltration of electrolyte and polysulfide adsorption. As a result, the as-prepared cathode containing FG shows a high initial specific capacity of 1602 mA h g-1 and the reversible specific capacity is 650 mA h g-1 at 0.5C after 300 cycles. Moreover, its specific capacity remains at 860 mA h g-1 at 5C, which is 367% higher than that of the sample without FG. This paper provides a new strategy to improve the energy density and the cycle stability of Li-S batteries.

Ionic distribution of MgCl2 near the alcohol/water interface

An equation is developed to describe the surface potential at the diffuse plane for MgCl2 solutions. The equation, in combination with surface tension and surface potential data, was used to predict the Mg2+/Cl- distribution near the air/water surface of MIBC solution. It was found that net ionic adsorption has a negative charge, that it is more anionic than cationic. The result was contrasting to MIBC/NaCl system. The negative charge was confirmed by molecular simulations. Despite its small size, the divalent nature prevents Mg2+ from penetrating the anionic layer. The study provides important insights into the adsorption of electrolytes with unequal charges between anion and cation. One of the main applications of the study is predicting the bubble surface charge of foaming systems in the “hard” water.

The role of reversible and polarizable surface charge in the electro-sorption of NaCl electrolyte onto activated carbon-graphite electrode

An electrode made of activated carbon that had high-specific-surface area and low pHzpc supported on graphite sheet was fabricated. The electrode, NSA/G, was used to study the effect of reversible surface charge on the adsorption of simple electrolyte, i.e., NaCl, on polarizable electrode. Polarizable surface charge is originated from externally applied potential whereas reversible surface charge is derived from change of activity of potential-determining ions, i.e., H+ or OH– owning to hydrous surface, which undergoes protonation and deprotonation of surface hydroxyl groups. Ion adsorption density was obtained from direct analytical measurement of corresponding ion studied instead of conductivity measurement. Langmuir adsorption isotherm was applied to estimate the monolayer ion adsorption density. Electrosorption/desorption kinetics was fitted well by the first-order rate law. In the applied potential range of −0.3 to −1.0 V (vs. SCE) and pH of 3 to 10, the pH-determining surface, i.e., reversible charge, accounted for Na+ sorption intensity was estimated based on the Gouy-Chapman version of electrical double layer theory. Results revealed that the two major surface charges, i.e., polarizable (σE) and reversible (σpH), contributed equally to the total charge density (σT). At high pH (pH 10) and low applied voltage (-0.3 V vs. SCE), the fraction of pH-determining contribution to total surface charge with respect to Na+ sorption, i.e., σpH/σT, was around 0.50. Results reaffirmed the importance of reversible surface charge together with polarizable surface charge on the design and operation of CDI system for electrosorption of ions from water.

Electrolyte Competition Controls Surface Binding of CO Intermediates to CO2 Reduction Catalysts

Adsorbed CO is a critical intermediate in the electrocatalytic reduction of CO2 to fuels. The directed design of CO2RR electrocatalysts has centered on strategies to understand and optimize the differences in CO adsorption enthalpy across surfaces. This approach has largely ignored the role of competitive electrolyte adsorption in defining the CO surface population relevant for catalysis. Using in situ infrared spectroelectrochemistry and voltammetry, we uncover the contrasting influence of electrolyte competition on reversible CO binding to Au and Cu catalysts. Although reversible CO binding to Au surfaces is primarily driven by the adsorption processes associated with interfacial water, CO binding to Cu surfaces requires the reductive displacement of adsorbed carbonate anions. The divergent role of electrolyte competition for CO adsorption on Au versus Cu leads to a ∼600 mV difference in the potential region where CO accumulates on the two surfaces. The contrasting CO adsorption stoichiometry on Au and Cu also explains their disparate reactivity: interfacial water adsorption contributes to CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption contributes to CO accumulation on Cu surfaces, allowing for further reduction to hydrocarbons. These studies provide direct insights into how electrolyte constituents fine-tune CO surface populations and, thereby, CO2-to-fuel reactivity.

A porous ceramic separator prepared from natural minerals: Research on the mechanism of high liquid absorption and electrochemical properties of mineral material separator

Using three natural minerals (albite, quartz, and kaolinite) as raw materials, ammonium bicarbonate as a pore former, solid-phase sintering method is used to prepare porous ceramic separators for lithium-ion batteries. The porous ceramic separator has excellent thermal stability with the thermal stability temperature of 1100 °C. Rich three-dimensional pore structure and higher porosity (56.98%) of porous ceramic separator is beneficial to fast Li+ transport. Density functional theory calculation reveals that the lone pair electrons on the surface of albite and quartz (main components of porous ceramic) play an important role on the adsorption of electrolyte. The smaller electrolyte adsorption energies of albite and quartz (−2.48 eV and −0.18 eV) than commercial PP separator (−0.06 eV) promote the electrolyte affinity and the electrolyte uptake for porous ceramic separator. The cell assembled with the porous ceramic separator shows very good cycle performance and remarkable rate capability. When the charge/discharge rate is 10 C, the discharge specific capacity of the cell with porous ceramic separator can reach 91.31 mAh g−1, which is 46% higher than that of the cell with commercial PP separator (62.54 mAh g−1). Therefore, porous ceramic separators have broad application prospects in lithium-ion batteries that require high-power charging and discharging.

Electrochemical Properties of TiWN/TiWC Multilayer Coatings Deposited by RF-Magnetron Sputtering on AISI 1060

Nitride and carbide ternary coatings improve the wear and corrosion resistance of carbon steel substrates. In this work, Ti-W-N and Ti-W-C coatings were deposited on AISI 1060 steel substrates using reactive radio frequency (RF) magnetron sputtering. The coatings were designed as monolayers, bilayers, and multilayers of 40 periods. The coatings were obtained with simultaneous sputtering of Ti and W targets. The microstructure, composition, and electrochemical properties were investigated by techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization. XRD results shower a mix of binary TiN and W2N structures in the Ti-W-N layer, a ternary phase in Ti-W-C layers, in addition of a quaternary phase of Ti-W-CN in the multilayers. The analysis of the XPS demonstrated that the atomic concentration of Ti was more significant than W in the Ti-W-N and Ti-W-C layers. The lowest corrosion rate (0.19 mm/year−1) and highest impedance (~10 kΩ·cm2) out of all coatings were found in n = 40 bilayers. In the simulation of equivalent electrical circuits, it was found that the Ti-W-N coating presented three processes of impedance (Pore resistance + Coating + Inductance). However, the multilayer (n = 40) system presented a major dielectric constant through the electrolyte adsorption; therefore, this caused an increase in the capacitance of the coating.

Probing Specific Adsorption of Electrolytes at Kaolinite–Aqueous Interfaces by Atomic Force Microscopy

Adsorption of electrolytes (ions) at solid-liquid interfaces alters the physical and chemical properties of materials and hence plays a critical role in manufacturing and processing of nanomaterials featuring large surface or interfacial areas of desired structures and morphology. Many experiments and theoretical calculations using various electrical double layer (EDL) models have been conducted to understand how and where ions adsorb at charged surfaces in a liquid. However, conclusions from previous research remain inconclusive because of model-dependent approaches to studying ion adsorption at diverse solid-liquid interfaces. In this study, atomic force microscopy is used to image in liquids the surface lattice structure of two kaolinite basal planes in the presence and absence of monovalent and divalent cations. Distinct adsorption of ions through different mechanisms (such as electrostatic attraction and specific adsorption) is identified through atomic resolution imaging without the assumption of an EDL structure.

MOF composite fibrous separators for high-rate lithium-ion batteries

In this study, metal-organic frameworks (MOF)/polyacrylonitrile (PAN) composite fibrous separators were prepared for high-rate Li-ion batteries by anchoring MOF into PAN nanofibrous network. The MOF/PAN separators possess excellent dimensional, thermal stability, intrinsically high porosity, and good electrolyte adsorption, which contribute excellent Li-ion transport capacity. The Li/Li symmetric cells assembled with MOF/PAN separators demonstrate improved coulombic efficiency, inhibition of Li dendrites, and excellent long-term cycle stability with low-voltage hysteresis of around 46 mV after more than 1200 h Li plating/stripping process, in comparison with around 350 mV after 360 h for commercialized Celgard 2400 separator. The excellent ionic transport capacity reveals impressive performance of the LiFePO4-MOF/PAN-Li metal full cells under high rate and the capacity that could be up to 104 mAh g−1 at 5 C after 2000 cycles with a coulombic efficiency of almost 100%. This work provides a new prospect for the development of high-rate lithium-ion batteries.

Simple and effective strategy to synthesize porous carbon with controlled structures for supercapacitor

With remarkable electrochemical stability/conductivity and abundant porous structure, porous carbon electrodes can store considerable amount of charge electrostatically by reversible adsorption of electrolyte, thereby showing high capacity and cycle stability for supercapacitor. However, there was lack of simple and effective ways to synthesize porous carbon materials with controllable structures. Herein, porous carbon materials with controlled structures (copper coin, 3D framework with closed-packing order, and hollow sphere) have been synthesized by an extension of Stober method using phenolic resin as carbon sources. SiO2 nanoparticles (120 nm) were used as the macroporous templates and Pluronic F127/tetraethyl orthosilicate (TEOS) as mesoporous templates. Adjusting the amount of TEOS helped to explore the synthesis mechanism. As-synthesized porous carbon materials exhibit high surface area of 316–482 m2 g−1 and large pore volumes of 0.306–0.748 cm3 g−1. The materials thereby obtained when used as electrodes in capacitors demonstrate specific capacitance (up to 83 F g−1 at 5 mV s−1) and good robustness after 2000 cycles, which contributed by their abundant porous structure, large surface area/pore volume, morphology, and electrochemical conductivity.