Enhanced anion electroadsorption into carbon molecular sieve electrodes in acidic media

We previously showed that, for neutral electrolytes of small cations and relatively larger anions, it is possible to design certain pore sizes in active carbons that are large enough to electroadsorb cations but too small to allow anion electroadsorption. This situation leads to an electrical double-layer (EDL) capacitance that is significant only at potentials that are negative to the potential of zero charge (PZC); hence, much smaller capacitance is measured at potentials positive to the PZC. It was found that when the electrolyte is a strong acid (e.g., H(2)SO(4), HCl), a considerable capacitance is observed at positive potentials, even when the average pore size is too small to allow the insertion of large anions in neutral electrolyte solutions. This effect disappears when the pore size becomes considerably larger than the size of the ions. In this case, the EDL capacitance at positive potentials for both neutral and acidic solutions is comparable. The following four-step mechanism was found to comply best with the experimental data: (1) By acid catalysis, the protons form carbonium species within the conjugated carbon network. (2) The anions react with the carbonium ions, providing uncharged species in an activated state, which are chemibound as surface groups to the walls of the pores. (3) Because these surface groups are effectively much smaller in size than are the charged ions, they can migrate by chemical bond exchange within the carbon skeleton via constrictions (known to exist in microporous and molecular sieving carbons), which are too narrow to accommodate hydrated charged species. (4) Upon reaching wider spaces, the uncharged species are reionized and solvated by water molecules, which can fill small pores. The justification for the above mechanism is thoroughly discussed and demonstrated by the experimental results.     

Carbon/carbon supercapacitors

Supercapacitors, or electrochemical capacitors, are a power storage system applied for harvesting energy and delivering pulses during short periods of time. The commercially available technology is based on charging an electrical double-layer (EDL), and using high surface area carbon electrodes in an organic electrolyte. This review first presents the state-of-the-art on EDL capacitors, with the objective to better understand their operating principles and to improve their performance. In particular, it is shown that capacitance might be enhanced for carbons having subnanometric pores where ions of the electrolyte are distorted and partly desolvated. Then, strategies for using environment friendly aqueous electrolytes are presented. In this case, the capacitance can be enhanced through pseudo-faradaic contributions involving i) surface functional groups on carbons, ii) hydrogen electrosorption, and iii) redox reactions at the electrode/electrolyte interface. The most promising system is based on the use of aqueous alkali sulfate as electrolyte allowing voltages as high as 2 V to be reached, due to the high overpotential for di-hydrogen evolution at the negative electrode.     

Preparation and performance of polyvinyl alcohol-based activated carbon as electrode material in both aqueous and organic electrolytes

Porous carbon materials with high surface area and different pore structure have been successfully prepared by phenolic resin combined with polyvinyl alcohol (PVA) and KOH as activation agents. The surface morphology, structure, and specific surface area of the carbon materials were studied by scanning electron microscopy, X-ray diffraction, and nitrogen sorption measurement, respectively. Furthermore, the effects of specific surface area, pore structure, and electrolyte on electrochemical properties were investigated by galvanostatic charge–discharge measurement. The results show that KOH–PVA-activated carbon materials display specific capacitance as high as 218 F?g^?1 in 30 wt.% KOH aqueous electrolyte, 147 F?g^?1 in 1 M LiPF₆/(ethylene carbonate (EC) + dimethyl carbonate) (1:1?v/v), and 115 F?g^?1 in 1 M Et₃MeNBF₄/propylene carbonate organic electrolyte, respectively. In addition, the carbon materials demonstrate long-term cycle stability, especially the AK3P-0.30 in aqueous electrolyte and the AK2P-0.30 with excellent rate capability in organic electrolyte. These reveal that the existence of a micro-mesoporous structure of activated carbon is beneficial to store energy in an aqueous supercapacitor and broad pore size distribution of activated carbon is favorable to energy storage in an organic supercapacitor. The carbon materials with pore size distribution in different ranges improve the electrochemical performance of supercapacitor in different electrolytes. A new pore-expand agent (PVA combining with KOH) was used to obtain porous carbons with enhanced properties for supercapacitor.     

含磷活性炭作为双电层电容器电极材料的电化学性能

采用磷酸活化和磷酸改性制备了不同种类的含磷活性炭,采用元素分析、X射线光电子能谱(XPS)和氮气吸附等手段分析了活性炭的元素含量、表面化学性质和孔隙结构,采用恒电流充放电、循环伏安和交流阻抗分别考察了活性炭在KOH和H₂SO₄电解质溶液中作为超级电容器电极材料的电化学性能,采用自由截距多元线性回归拟合统计分析研究了活性炭电极比电容量的影响因素,应用三电极体系分析了磷元素对活性炭电化学性能的影响机理。研究结果表明,活性炭掺杂的磷引入了赝电容,提高了活性炭电极的比电容量,磷元素含量为5.88%(w)的活性炭的比电容量在0.1 A·g^-1下达到185 F·g^-1。统计分析结果显示,活性炭的中孔有利于电解质离子向微孔内的扩散。在6 mol·L^-1 KOH电解质溶液中,孔径在1.10-1.61 nm、2.12-2.43nm及3.94-4.37 nm范围内是电解质离子在活性炭孔隙内部形成双电层的主要场所;在1 mol·L^-1 H₂SO₄电解质溶液中,孔径在0.67-0.72 nm范围内有利于双电层电容的形成。     

Relation between the ion size and pore size for an electric double-layer capacitor

The research on electrochemical double layer capacitors (EDLC), also known as supercapacitors or ultracapacitors, is quickly expanding because their power delivery performance fills the gap between dielectric capacitors and traditional batteries. However, many fundamental questions, such as the relations between the pore size of carbon electrodes, ion size of the electrolyte, and the capacitance have not yet been fully answered. We show that the pore size leading to the maximum double-layer capacitance of a TiC-derived carbon electrode in a solvent-free ethyl-methylimmidazolium-bis(trifluoro-methane-sulfonyl)imide (EMI-TFSI) ionic liquid is roughly equal to the ion size (approximately 0.7 nm). The capacitance values of TiC-CDC produced at 500 degrees C are more than 160 F/g and 85 F/cm(3) at 60 degrees C, while standard activated carbons with larger pores and a broader pore size distribution present capacitance values lower than 100 F/g and 50 F/cm(3) in ionic liquids. A significant drop in capacitance has been observed in pores that were larger or smaller than the ion size by just an angstrom, suggesting that the pore size must be tuned with sub-angstrom accuracy when selecting a carbon/ion couple. This work suggests a general approach to EDLC design leading to the maximum energy density, which has been now proved for both solvated organic salts and solvent-free liquid electrolytes.     

LiTFSI/2-噁唑烷酮电解液与不同微结构碳材料的电化学兼容性

二（三氟甲基磺酸酰）亚胺锂（LiTFSI）与1,3-氮氧杂环戊-2-酮（OZO）形成的离子液体具有良好的物理和电化学性能,表现出宽的液相温度范围和高的离子电导率,可满足超级电容器的应用需求。本文制备的LiTFSI-OZO离子液体体系中,各种离子的结构组成（如自由离子、离子对、积聚离子）及其之间的相互作用对离子液体的电化学性能具有较大的影响,将其作为电解液应用于不同微结构特性（孔径、比表面积等）的炭材料（碳纳米管(CNTs)、中孔活性炭(MEACs)和微孔活性炭(MIACs)）作为电极的电化学双层电容器中,电化学兼容性研究表明,由于中孔活性炭电极材料有最大的比表面积及最适宜的孔径分布,相应的模拟电容具有最高的比容量184.6 F?g-1。该研究表明,对电极材料的微结构特性与离子液体离子尺度进行优化匹配是实现离子液体作为电解液应用于超级电容器的关键。     

Recent Advanced Applications of Ionic Liquid for Future Iontronics

Recently, electronic devices that make use of a state called the electric double layers (EDL) of ion have opened up a wide range of research opportunities, from novel physical phenomena in solid-state materials to next-generation low-power consumption devices. They are considered to be the future iontronics devices. EDLs behave as nanogap capacitors, resulting the high density of charge carriers is induced at semiconductor/electrolyte by applying only a few volts of the bias voltage. This enables the low-power operation of electronic devices as well as new functional devices. Furthermore, by controlling the motion of ions, ions can be used as semi-permanent charge to form electrets. In this article, we are going to introduce the recent advanced application of iontronics devices as well as energy harvesters making use of ion-based electrets, leading to the future iontronics research.     

The dependence of the electronic conductivity of carbon molecular sieve electrodes on their charging states

The dependence of the electronic conductivity of activated carbon electrodes on their potential in electrolyte solutions was examined. Kapton polymer films underwent carbonization (1000 degrees C), followed by a mild oxidation process (CO(2) at 900 degrees C) for various periods of time, to obtain carbons of different pore structures. A specially designed cell was assembled in order to measure the conductivity of carbon electrodes at different potentials in solutions. When the carbon electrodes possessed molecular sieving properties, a remarkable dependence of their conductivity on their charging state was observed. Aqueous electrolyte solutions containing ions of different sizes were used in order to demonstrate this phenomenon. As the average pore size of the activated carbons was larger, their molecular sieving ability was lower, and the dependence of their conductivity on their charging state regained its classical form. This behavior is discussed herein.     

电化学双电层电容器用新型炭材料及其应用前景

活性炭是目前使用最为广泛的一种电化学双电层电容器（EDLC）的电极材料,但其固有的缺点制约了EDLC性能的进一步提高。用新型高性能炭电极材料可使EDLC比能量和比功率性能进一步提高。这些新型炭材料包括基于石墨层状结构的纳米门炭,基于碳纳米管阵列结构的毛皮炭,通过高温置换反应制备的骨架炭以及电极可整体成型的纳米孔玻态炭。本文介绍了这些炭材料的电化学特性及其在电化学双电层电容器中的应用,指出用这4种新型炭材料制备EDLC的比能量或比功率性能远高于目前活性炭基EDLC,具有良好的应用前景。     

Saturation of subnanometer pores in an electric double-layer capacitor

Improvements in the energy density of electric double-layer capacitors (EDLCs) can in particular be gained by enhancing their capacitance. Recent findings suggest that the specific capacitance can be increased by matching the sizes of pores and desolvated ions. However, on such matching, we evidenced that charge storage saturation can occur in organic electrolyte before reaching the maximum voltage, e.g. 2.7 V, due to the insufficiently developed porosity. The experimental charge is larger than the calculated on account of the size of rigid cations, because of the intercalation-like behaviour and/or distortion of ions. The experimental and calculated values are less coincident for the higher sweep rates, which reveals that the optimal average pore size should depend on the current load, tending to shift to higher values in the normal usage conditions of supercapacitors.     

Biomass-derived activated carbon materials with plentiful heteroatoms for high-performance electrochemical capacitor electrodes

Activated carbons for electrochemical capacitor electrodes are prepared from soyabean using chemical activation with KOH. The pore size is easily controllable by changing the mass ratio between KOH and carbonized product. The as-prepared materials possess a large specific surface area, unique structure, well- developed hierarchical porosity and plentiful heteroatoms(mainly O and N). Thus resulted in its high specific capacitance,good rate capacity and cycling stability. Moreover, attributing to worldwide availability, renewable nature and low-cost, activated carbon prepared from soyabean has a good potential in energy conversion and storage devices.     

Facile Self-templating Melting Route Preparation of Biomass-derived Hierarchical Porous Carbon for Advanced Supercapacitors

Biomass-derived porous carbons show great potential as electrode materials for supercapacitors due to the environmental friendliness. However, most of the carbonaceous electrode materials suffer from low specific capaci-tance and rate capacity because of the poor porosity. Here, we reported a simple and effective approach to prepare micro/nano-hierarchical structured carbon materials derived from rice husk by NaOH-KOH molten salt co-activation. The as-prepared activated carbons exhibit high porosity and suitable pore size distributions for more electrolyte ion adsorption, which are all beneficial for achieving remarkable electrochemical performances, such as high specific capacitance(194.6 F/g), excellent rate capability(retention of 85.9%) and outstanding cycling stability. Thus, the above biomass-derived carbon materials with high porosity and micro/nano structures obtained by co-activation method offered a new insight into novel electrode material for the use in energy storage systems with high energy density and excellent rate performance.     

Biomass‐Derived Carbon Materials as Prospective Electrodes for High‐Energy Lithium‐ and Sodium‐Ion Capacitors

Biomass‐derived carbon materials have received special attention as efficient, low‐cost, active materials for charge‐storage devices, regardless of the power system, such as supercapacitors and rechargeable batteries. In this Minireview, we discuss the influence of biomass‐derived carbonaceous materials as positive or negative electrodes (or both) in high‐energy hybrid lithium‐ion configurations with an organic electrolyte. In such hybrid configurations, the electrochemical activity is completely different to conventional electrical double‐layer capacitors; that is, one of the electrodes undergoes a Faradaic reaction, whilst the counter electrode undergoes a non‐Faradaic reaction, to achieve high energy density. The use of a variety of biomass precursors with different properties, such as surface functionality, the presence of inherent heteroatoms, tailored meso‐/microporosity, high specific surface area, various degrees of crystallization, calcination temperature, and atmosphere, are described in detail. Sodium‐ion capacitors are also discussed, because they are an important alternative to lithium‐ion capacitors, owing to the low abundance and high cost of lithium. The electrochemical performance of carbonaceous electrodes in supercapacitors and rechargeable batteries are not discussed.     

Pseudo-capacitance of nanoporous carbons in pyrrolidinium-based protic ionic liquids

Protic ionic liquids (PILs) were used as novel electrolyte for carbon-based supercapacitors. The cyclic voltammograms in three-electrode cells show reversible redox humps, revealing pseudo-faradaïc charge transfer. Oxidative treatment of activated carbon enriches the surface functionality and leads to a higher capacitance owing to a stronger pseudo-faradaïc contribution. The capacitors using PILs demonstrate a higher voltage window than with aqueous H₂SO₄, while keeping the same values of capacitance, and being able to operate at lower temperature. A combination of activated carbons and PILs holds promise for improving the energy characteristics of supercapacitors.     

Mesoporous Carbons Templated by PEO-PCL Block Copolymers as Electrode Materials for Supercapacitors

In this study, samples of activated mesoporous carbon are fabricated with pore structures with cylinder and gyroid nanostructures through the templating effect of amphiphilic poly(ethylene oxide-block-caprolactone) (PEO-PCL) and by using specific resol/PEO-PCL weight ratios (e.g., 60:40 for cylinders; 55:45 for gyroids). After carbonization and KOH activation, the activated mesoporous carbons were tested as electrode materials for electric double-layer capacitor (EDLC) supercapacitors. The electrochemical properties were examined by using three-electrode (6 m KOH_(aq) as electrolyte) and CR2032 coin-cell (1 m tetraethylammonium tetrafluoroborate (TEABF₄)/CN as the electrolyte) systems. The gyroid carbon samples provided specific capacitances higher than those of the cylinder carbon samples in both aqueous and organic systems: 155 F g⁻¹ compared with 135 F g⁻¹ in 6 m KOH_(aq), and 105.6 compared with 96 F g⁻¹ in 1 m TEABF₄/MeCN, after 100 charge/discharge cycles. It is suspected that the bi-continuous mesochannels of the gyroid-type activated mesoporous carbons provided a relatively higher effective adsorption surface area; in other words, the greater surface area for energy storage originated from a moderate pore size and an interconnected pore structure.     

Simple fabrication of a phosphorus-doped hierarchical porous carbon via soft-template method for efficient CO2capture

Hierarchical porous carbons are widely used as adsorbents, catalyst supports, electrode materials, and other applications because of their high specific surface area (SSA), varied pore structure, adjustable porosity, and excellent physicochemical stability. Introducing heteroatoms such as N, P, or S, with electronegativities different from that of carbon, into the carbon skeleton can change the chemical properties of the surface and the density of the electron cloud around the carbon matrix, thus altering interactions of CO₂molecules with the surface and improving CO₂adsorption capacity. Therefore, doping heteroatoms in carbon materials has attracted a great amount of attention. In this paper, the template method was used with F108 (polyethylene glycol–polypropylene glycolpolyethylene glycol) as the template, resorcinol and formaldehyde solutions as the carbon sources, phosphoric acid as the phosphorus source, and KOH as the activator to prepare phosphorus-doped hierarchical porous carbons. Through a series of characterization and CO₂adsorption experiments, the influence of the amount of KOH and template agent on the pore structure of carbon materials was studied. We conclude that these phosphorus-doped hierarchical porous carbon materials are promising CO₂adsorbents.     

Monte Carlo simulation of electrical double-layer formation from mixtures of electrolytes inside nanopores

The formation of the electrical double layer (EDL) in the presence of trivalent and monovalent ions inside a slit-type nanopore was simulated via the canonical Monte Carlo method using a primitive model. In large pores, the distribution of ionic species is similar to that observed in an isolated planar double layer. Screening of surface charge is determined by the competitive effects between ion size and charge asymmetry of the counterions. On the other hand, as the pore size approaches the dimension of the ionic species, phenomena such as EDL overlapping become enhanced by ion-size effects. Simulation results demonstrate that EDL overlapping is not only a function of such parameters as ionic strength and surface charge density, but also a function of the properties of the ionic species involved in the EDL. Furthermore, charge inversion can be observed under certain conditions when dealing with mixtures of asymmetric electrolytes. This phenomenon results from strong ion-ion correlation effects and the asymmetries in size and charge of ionic species, and is most significant in the case of trivalent counterions with larger diameters. The simulation results provide insights into the fundamental mechanisms behind the formation of EDL within nanopores as determined by pore size and by the properties of ionic species present in solution. The findings of this work are relevant to ion sorption and transport within nanostructured materials.     

Electrochemical behavior of activated carbon electrodes in electric double layer capacitors with tetrametylammonium bis(oxalato)borate electrolyte synthesized by microwave irradiation

The electrochemical behavior of electric double layer capacitors (EDLCs) with tetramethylammonium bis(oxalato)borate electrolyte and electrodes based on various activated carbons (ACs) was studied. Tetraalkylammonium bis(oxalate)borate salts were synthesized by means of microwave (MW) irradiation. The specific conductivities of salt solutions were determined. It was shown that the efficiency of electric double layer capacitors increases with an increase in specific surface area and a decrease in the purity of carbon materials.     

由微孔棒状羟基磷灰石为模板合成的新型层次孔炭材料的电化学电容性能

电化学电容器已经成为极具潜力的可满足高功率需求的储能系统器件. 多孔炭具有大比表面积、高导电性、化学惰性、廉价及可调孔结构等优势, 因此成为电化学电容器最为常用的电极材料. 本文报道由微孔棒状羟基磷灰石为模板及蔗糖为碳源合成的新型具有层次孔道结构的孔炭材料的电化学电容器的性能. 采用X射线衍射分析仪、扫描电子显微镜、透射电子显微镜、X射线光电子能谱及BET表面分析仪表征了合成的多孔炭的形貌及表面特性. 采用循环伏安法、交流阻抗图谱分析及恒流充放电评价多孔炭材料在1 mol·L^-1硫酸中的电化学电容性能. 多孔炭具有高的比表面积(719.7 m²·g^-1)和大的孔容(1.32 cm³·g^-1), 其无序的孔道由任意分布的微孔、坍塌的中孔及类模板形状的相互交织的棒状中孔组成. 随着炭化温度的增加, 微孔及棒状中孔的密度随之降低, 在炭化温度高达900℃时, 孔径分布图上出现了三个峰. 正是由于这些特殊的结构特征, 由900℃炭化得到的多孔炭制成的电极展示出很好的电化学电容性能.     

金属-有机框架衍生的中空碳材料及其在电化学能源存储与氧还原领域中的应用

金属-有机框架材料(metal-organic frameworks,MOFs)是一类由金属离子或金属团簇与有机配体通过配位键连接形成的具有周期性网络结构的多孔配位聚合物。这类材料通常具有孔道规整、比表面积大、孔隙率高、结构可设计及孔壁易修饰等特点,诸多的优点使得MOFs的研究从配位化学跨越到多个学科领域,成为当前多学科交叉前沿热点之一。近来的研究发现,以MOFs为前驱体碳化后制得的碳材料可保留MOFs的大比表面积和多孔结构,同时可以实现均匀的杂原子(如N、P、S、B等)掺杂,而且通过选择合适的MOFs前驱体可调控产物的组成和形貌尺寸,这些显著的结构特征使其具备了成为高性能功能性材料的潜力。最近,以MOFs为模板或前驱体制备的中空碳材料引起了人们的广泛关注,这主要是因为中空结构可有效缓解材料在电化学过程中产生的体积变化及受到的冲击,而且中空结构可暴露出更多的活性位点,具有快速的传质过程,使得材料发挥出最优性能,故而此类材料可被用在二次电池、电容器、电催化等多种电化学器件和多个领域中。基于此,本文综述了MOFs衍生的中空碳材料在储能器件及电催化领域的研究进展,主要包括锂离子电池、锂硫/硒电池、钠离子电池、超级电容器、电催化氧还原等领域,并对这类材料当前面临的挑战及未来的发展趋势进行了阐述。