Brunauer-Emmett-Teller BET Method

BET is an important tool for ES characterization because surface area and pore size are both important parameters in determining material capacitance [59]. It is used to determine specific surface areas of electrode materials such as activated carbon and graphene [60]. Pore size measurements enable the estimation of electrochemical effectiveness of an active material when matched to a specific electrolyte [61-63]. Pore size and surface area are also good indicators of structural changes after chemical or heat treatment [64]. 

Modern BET instrument from Micrometries. (Source Micrometries. ASAP 2020 (online) http // www.micromeritics.com/ [accessed April 22,2012]. With permission.) 

In this chapter, both electrochemical and physical instrument characterizations for ES materials, components, and performance are discussed. Conventional three-electrode and two-electrode testing cells and their associated design and fabrication techniques for electrochemical characterization of supercapacitors in terms of equivalent series resistance, capacitance, and pseudocapacitance are presented. 

Three common methods [cyclic voltammetry (CV), charging-discharging curve (CDC), and electrochemical impedance spectroscopy (EIS)] are briefly introduced. For fast screening of electrode materials, the conventional ex situ three-electrode cell is the choice test method. For in situ characterization of materials and supercapacitor performance, the two-electrode test cell can more closely represent the real conditions encountered during operation. 

For equivalent series resistance and capacitance measurements, CV and CDC may be more simple or reliable than EIS in terms of data treatment. Although EIS may yield more information about the processes of supercapacitor operation, data simulation can induce arbitrary conclusions due to the equivalent circuit construction and the complexity of the simulation. Regarding physical instrument characterization of ES materials, components, development, and structure optimization, important methods such as SEM, TEM, XRD, EDX, XPS, RS, FTIR, and BET are briefly reviewed. 

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A vast amount of research has been undertaken on adsorption phenomena and the nature of solid surfaces over the fifteen years since the first edition was published, but for the most part this work has resulted in the refinement of existing theoretical principles and experimental procedures rather than in the formulation of entirely new concepts. In spite of the acknowledged weakness of its theoretical foundations, the Brunauer-Emmett-Teller (BET) method still remains the most widely used procedure for the determination of surface area similarly, methods based on the Kelvin equation are still generally applied for the computation of mesopore size distribution from gas adsorption data. However, the more recent studies, especially those carried out on well defined surfaces, have led to a clearer understanding of the scope and limitations of these methods furthermore, the growing awareness of the importance of molecular sieve carbons and zeolites has generated considerable interest in the properties of microporous solids and the mechanism of micropore filling. 

In writing the present book our aim has been to give a critical exposition of the use of adsorption data for the evaluation of the surface area and the pore size distribution of finely divided and porous solids. The major part of the book is devoted to the Brunauer-Emmett-Teller (BET) method for the determination of specific surface, and the use of the Kelvin equation for the calculation of pore size distribution but due attention has also been given to other well known methods for the estimation of surface area from adsorption measurements, viz. those based on adsorption from solution, on heat of immersion, on chemisorption, and on the application of the Gibbs adsorption equation to gaseous adsorption. 

Specific surface area (SSA), total pore volume and average pore diameter were measured by N2 adsorption-desorption isotherms at 77K using Micromeritics ASAP 2020. The pore size was calculated on the adsorption branch of the isotherms using Barrett-Joyner-Helenda (BJH) method and the SSA was calculated using the Brunauer-Emmett-Teller (BET) method. 

Nitrogen adsorption was performed at -196 °C in a Micromeritics ASAP 2010 volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3.10 3 Torr static vacuum was reached. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Micropore volume and external surface area were evaluated by the alpha-S method using a standard isotherm measured on Aerosil 200 fumed silica [8]. Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Broker AXS D-8 diffractometer with Cu Ka radiation. Thermogravimetric analysis was carried out in air flow with heating rate 10 °C min"1 up to 900 °C in a Netzsch TG 209 C thermal balance. SEM micrographs were recorded on a Hitachi S4500 microscope. 

Multilayer adsorption and the popular Brunauer-Emmett-Teller (BET) method of analysis are described in Section 9.5. This section also describes the determination of specific areas by gas adsorption. Low-temperature N2 adsorption and the BET method of analysis are so widely used for this purpose that these topics will receive special attention. 

The most common method used for the determination of surface area and pore size distribution is physical gas adsorption (also see 1.4.1). Nitrogen, krypton, and argon are some of the typically used adsorptives. The amount of gas adsorbed is generally determined by a volumetric technique. A gravimetric technique may be used if changes in the mass of the adsorbent itself need to be measured at the same time. The nature of the adsorption process and the shape of the equilibrium adsorption isotherm depend on the nature of the solid and its internal structure. The Brunauer-Emmett-Teller (BET) method is generally used for the analysis of the surface area based on monolayer coverage, and the Kelvin equation is used for calculation of pore size distribution. 

In principle, isotherms at low partial pressures of the sorbate may be used to determine specific surface areas by the Brunauer-Emmett-Teller (BET) method (G64). In this method, it is assumed that molecules of the sorbate are adsorbed on surfaces that can include the walls of pores, provided that the distance between molecules on opposing walls is large compared with molecular dimensions. From a plot derived from the isotherm, and given the effective cross-sectional area of the sorbate molecule, the specific surface area of the sorbent and the net heat of adsorption are obtained. Using water as sorbate, specific surface areas of about 200 m per g of D-dried paste have typically been obtained for mature cement pastes of normal w/c ratios... 

The nitrogen adsorption-desorption isotherms for specific surface area and porosity assessment were recorded at -196 C in a Gemini instrument from Micromeritics. The specific surface areas were determined by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were obtained from the desorption branch, and the micropore volume was determined by the t-plot method, using literature software [14]. 

Brinker and Scherer (8) pointed out that the area of a surface is defined largely by the method of surface area measurement. Many of the measurements of surface areas in work reported before the 1980s were based on the method of determining monolayer capacity of an adsorbent molecule of known cross-sectional area. In the Brunauer-Emmett-Teller (BET) method (45) the apparent surface area is determined from nitrogen adsorption. However, because the nitrogen molecule surface area is 16.2 A2, this definition of the surface excludes microporosity that is accessible, for example, to water molecules. 

The size of the specific surface area, which is often determined according to the Brunauer-Emmett-Teller (BET) method (30), is easily reproducible, but gives no indication as to the existence of an internal or external surface. The submicroscopic (and microscopic) pore volume is determined, for instance, according to the de Boer t-curve method (31, 32)... 

The surface areas of common support materials and other solids can be readily measured by the Brunauer-Emmett-Teller (BET) method which relies on the adsorption of a monolayer of an inert gas such as argon or nitrogen. Commercial automatic surface area analysers and porosimeters can accurately measure surface areas ranging from < 1 m2 g 1 to > 1000 m2 g. It is possible to measure surface areas manually using gas burettes but this is a very slow, labour-intensive method. 

In order to obtain true surface area specific rate constants for the surface kinetics, the real (microscopic) surface area must be taken into account. For powders, the Brunauer, Emmett, Teller (BET) method may be used to determine the true surface area. Eor dense solid bodies, however, the BET method may be too insensitive, since the overall surface area is relatively low. In such cases, measurements of the surface roughness, for example by atomic force microscopy (AFM),... 

Crystallite sizes were estimated from X-ray diffraction broadening using the Scherrer equation and/or TEM. Surface areas were calculated by the Brunauer-Emmett-Teller (BET) method. Pore sizes were calculated by the Barrett-Joyner-Halenda (BJH) method. 

Surface Area and Porosity. The specific surface area of a catalyst or support (in m /g) is determined by measuring the volume of gas, usually N2, sufficient to form a monomolecular layer (5). In the Brunauer-Emmett-Teller (BET) method approach, the determination of the monolayer capacity is based on the... 

The Brunauer Emmett Teller (BET) method based on nitrogen adsorption (Bmnnauer et al 1940)... 

The adsorption of N2 gas is often used to evaluate the surface-accessible area and pore size distribution by the Brunauer-Emmett-Teller (BET) method (353). The accessible surface is generally that of the internal pores within the crystallites and the external surface between the crystallites. Correspondingly, the measured pores are those inside of and between the crystallites. Within most common organic and inorganic LDHs, the interlayers are full of the anions as well as water, and thus only the external surface of the crystallites contributes to the accessible surface area. Values normally range from 10 to 200 g (4,354),... 

Later Kaganer observed that the constant A can be identified with In Vm, Vm being the monolayer capacity measured using the standard Brunauer-Emmett-Teller (BET) method. Bearing in mind this result, equation (1) can be written in the following way... 

In physical gas adsorption, when an inert gas (such as nitiogen or argon) is used as an absorbent gas, the adsorption isotherm indicates the surface area and/or the pore size distribution of the objective material by applying experimental data to the theoretical adsorption isotherm for gas adsorption on the polymer surface (eg, Brunauer— Emmett—Teller (BET)" method). 

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