Changing the pore structure and surface chemistry of hard carbon by coating it with a soft carbon to boost high-rate sodium storage

Various electrode materials have been developed over the years for use in the electrochemical double layer capacitors (EDLCs) or supercapacitors. Among all, activated carbon materials are widely studied and used due to their pore structure, very high specific surface area, low cost and ease of processing. It is known that the electrochemical performance of supercapacitors highly depends upon surface area, pore size/shape, overall pore volume, pore size distribution, surface functionality and electrical conductivity. In addition, microstructure and surface chemistry have significant influence on their electrochemical performance. In order to enhance the performance, stability and cycle life of the supercapacitors fabricated from activated carbon materials, their pore structure, surface area, microstructure and surface chemistry need to be controlled and modified via various processing routes. This study investigated the influence of processing routes on the properties of the activated carbon materials, and also electrochemical performance and stability of the fabricated supercapacitors. Lignocellulosic biomass can be easily, and cost effectively processed to produce activated carbon materials with unique microstructure and surface properties as electrodes. The aim of this study was to utilize different processing routes to understand their effect on the pore structure, microstructure, surface chemistry and final electrochemical performance. Therefore, three major processing techniques, which are air torrefaction (or oxidative thermostabilization), pyrolysis and chemical activation, were used in this study. The air torrefaction pretreatment was performed at low temperature regime (200°-300°C) and in air to stabilize and control the microstructure at the initial stage. Selected samples were also pyrolyzed at 450°C under nitrogen to achieve biochar samples. Chemical activation process was conducted using potassium hydroxide (KOH) to further modify the pore structure and surface chemistry of the biomass-derived carbon. Prior to fabrication of the supercapacitors, properties of the activated biomass and biochar samples were characterized by BET, SEM, TGA, XPS and Raman spectroscopy. Activated carbon materials with ultra-high specific surface area (up to 3265 m2/g), high cumulative pore volume (1.99 cm3/g) and controlled microstructure were successfully obtained. The results also demonstrated significant changes in the relative concentration of oxygen- and carbon-containing surface functional groups depending upon the processing route. To fabricate supercapacitors, activated biomass and biochar samples were mixed with polyvinylidene fluoride (PVDF) binder and carbon black for 24 h to prepare electrode inks. The electrodes were then casted on a stainless steel sheet using a doctor blade. The supercapacitors were then fabricated using a typical CR-2032 parts, KOH as an electrolyte, and nafion as a separator. Constant-current charge-discharge and self-discharge tests were conducted at the range of 0.1-1.0 V and 0.1 A/g, up to 1000 and 50 cycles, respectively. The activated biochar samples showed higher specific capacitance (107-123 F/g) compared to activated biomass (70-92 F/g), presenting the positive influence of the pyrolysis process. Initial air torrefaction process substantially increased the electrochemical performance, since there was a 22-25 F/g increase in the specific capacitance. This may be related to better microstructural control and further change in surface chemistry. All supercapacitors fabricated from the activated biomass and biochar samples showed high electrochemical stability up to 1000 cycles. The average capacitive loss after 1000 cycles was found to be around 2.0%. Acknowledgements: The U.S. Department of Agriculture (USDA), McIntire Stennis Grant Program supported this work under accession number of 1007044 and project number of WVA00118. The authors also would like to acknowledge the West Virginia University Shared Research Facilities for support through materials characterization. Dr. Yumak acknowledges financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) BIDEB-2219 Postdoctoral Research Program.

Comparison of melamine resin and melamine network as precursors for carbon electrodes

Novel approach to the characterization of the pore structure and surface chemistry of porous carbon with Ar, N2, H2O and CH3OH adsorption

A facile and green preparation of durian shell-derived carbon electrodes for electrochemical double-layer capacitors

A series of follow‐up investigations were performed to produce data for improving the four‐indicator carbon selection method that we developed to identify high‐potential activated carbons effective for removing specific organic water pollutants. The carbon's pore structure and surface chemistry are dependent on the raw material and the activation process. Coconut carbons have relatively more small pores than large pores; coal and apricot nutshell/walnut shell fruit carbons have the desirable pore structures for removing adsorbates of all sizes. Chemical activation, excessive activation, and/or thermal reactivation enlarge small pores, resulting in reduced phenol number and higher tannic acid number. Activated carbon's phenol, iodine, methylene blue, and tannic acid numbers are convenient indicators of its surface area and pore volume of pore diameters <10, 10–15, 15–28, and >28 Å, respectively. The phenol number of a carbon is also a good indicator of its surface acidity of oxygen‐containing organic functional groups that affect the adsorptive capacity for aromatic and other small polar organics. The tannic acid number is an indicator of carbon's capacity for large, high‐molecular‐weight natural organic precursors of disinfection by‐products in water treatment. The experimental results for removing nitrobenzene, methyl‐tert‐butyl ether, 4,4‐bisphenol, humic acid, and the organic constituents of a biologically treated coking‐plant effluent have demonstrated the effectiveness of this capacity–indicator‐based method of carbon selection. A comprehensive table of iodine, phenol, methylene blue, and tannic numbers of common activated carbons is a useful database to environmental professionals for selecting few high‐potential activated carbons to be evaluated in an efficient water/wastewater treatability study. © 2007 American Institute of Chemical Engineers Environ Prog, 2007

The adsorbability on active carbons of substances with different physicochemical properties has been investigated and a comparative analysis of the pore structure parameters of the active carbons as estimated from the sorption of substances with various molecular diameters has been made. The influence of the surface chemistry of carbon sorbents on the adsorption of water vapour has also been studied. The need for a quantitative analysis of the pore structure parameters and the surface chemistry of active carbons as a basis for their universal characterization is discussed.

To understand the nature of H2S adsorption onto carbon surfaces under dry and anoxic conditions, the effects of carbon pore structure and surface chemistry were studied using activated carbon fibers (ACFs) with different pore structures and surface areas. Surface pretreatments, including oxidation and heattreatment, were conducted before adsorption/desorption tests in a fixed-bed reactor. Raw ACFs with higher surface area showed greater adsorption and retention of sulfur, and heat treatment further enhanced adsorption and retention of sulfur. The retained amount of hydrogen sulfide correlated well with the amount of basic functional groups on the carbon surface, while the desorbed amount reflected the effect of pore structure. Temperature-programmed desorption (TPD) and thermal gravimetric analysis (TGA) showed that the retained sulfurous compounds were strongly bonded to the carbon surface. In addition, surface chemistry of the sorbent might determine the predominant form of adsorbate on the surface.

In order to address open questions concerning the surface chemistry and pore structure characterization of nanoporous carbons, we performed extensive experiments by combining various experimental techniques on a series of commercially available activated carbons which exhibit diverse surface chemistry characteristics. Pore size analysis was performed on Ar (87 K), N2 (77 K) and CO2 (273 K) adsorption isotherms using state-of-the art methods based on density functional theory, including the recently developed quenched solid density functional theory (QSDFT). A detailed study of the surface chemistry was obtained by applying temperature programmed desorption coupled with mass spectrometry (TPD-MS) as well as XPS (X-Ray-Photoelectron Scattering). This information together with the pore structure information leads to a reliable interpretation of systematic water adsorption measurements obtained on these materials. Our results clearly suggest that water adsorption is indeed a sensitive tool for detecting differences in surface chemistry between chemically and physically activated active carbon materials with comparable ultramicropore structure. The occurrence of sorption hysteresis associated with the filling of micro- and narrow mesopores (in a range where nitrogen and argon isotherms are reversible) provides additional structural information, complementary to the insights from argon/nitrogen/carbon dioxide adsorption.

Activated carbon fibers (ACFs) were oxidized using both aqueous and nonaqueous treatments. As much as 29 wt % oxygen can be incorporated onto the pore surface in the form of phenolic hydroxyl, quinone, and carboxylic acid groups. The effect of oxidation on the pore size, pore volume, and the pore surface chemistry was thoroughly examined. The average micropore size is typically affected very little by aqueous oxidation while the micropore volume and surface area decreases with such a treatment. In contrast, the micropore size and micropore volume both increase with oxidation in air. Oxidation of the fibers produces surface chemistries in the pore that provide for enhanced adsorption of basic (ammonia) and polar (acetone) molecules at ambient and nonambient temperatures. The adsorption capacity of the oxidized fibers for acetone is modestly better than the untreated ACFs while the adsorption capacity for ammonia can increase up to 30 times compared to untreated ACFs. The pore surface chemical makeup was analyzed using elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS).

Nanoporous carbons were prepared from rice husk (RH) according to a three-step procedure that includes pre-carbonization, silica removal and chemical activation. The surface area and pore structure of the nanoporous carbons are characterized intensively using N2 adsorption technique. The as-prepared carbons have specific surface area of up to 3283 m2/g, and total pore volume of up to 1.8 cm3/g. The effect of activating agent dosage on the pore structure of carbons was investigated and elucidated clearly. As evidenced by electrochemical measurements, the as-prepared carbons have good capacitive performances and their specific capacitances are much higher than ordered mesoporous carbon, thus highlighting the success of preparing high performance electrode material for EDLC from a biomass waste, rice husk.

Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution

The surface chemistry and pore structure of porous carbons determine its application. The surface chemistry could be modified by various methods, such as, acid treatment, oxidization, ammonization, plasma, microwave treatment, and so on. In this paper, the modification methods were illustrated and compared, some new methods also reviewed. The surface chemical functional groups were determined by the treatment methods, the amminization could increase its basic property while the oxidization commonly improved its acids. In the end, the commonly characterization methods were also mentioned. Some interesting patents are also discussed in this article. Keywords: Porous carbon, surface chemical groups, modification, characterization

Highly porous carbons were prepared from sunflower seed shell (SSS) by chemical activation and used as electrode material for electrochemical double layer capacitor (EDLC). The surface area and pore structure of the porous carbons are characterized intensively using N2 adsorption technique. The results show that the pore-structure of the carbons is closely related to activation temperature. Electrochemical measurements show that the carbons have excellent capacitive behavior and high capacitance retention ratio at high drain current, which is due to that there are both abundant macroscopic pores and micropore surface in the texture of the carbons. More importantly, the capacitive performances of these carbons are much better than ordered mesoporous carbons, thus highlighting the success of preparing high performance electrode material for EDLC from SSS.

Mechanochemistry induced pore regulation and pyridinic nitrogen doping in anthracite derived carbon for sodium storage

Amorphous carbon, particularly hard carbon (HC), is widely considered as the most promising anode material for sodium-ion batteries (SIBs) due to its high reversible capacity and cost-effectiveness. However, the complex and poorly defined structural properties of HC present challenges in understanding the underlying sodium storage mechanisms. To facilitate the rational design of high-performance HC anodes, a comprehensive understanding of the correlation between microstructure and sodium storage behavior is critical. This Review critically examines the interplay between the structural features of HC and its sodium storage capabilities, focusing on two key factors: pore structure and surface functional groups. It begins by outlining the fundamental sodium storage mechanisms in HC, followed by an in-depth discussion of how pore structure and surface chemistry influence sodium-ion storage. Finally, strategic insights are provided on how to manipulate these structural factors to optimize sodium storage performance. This Review aims to drive the development of next-generation high-performance HC anodes and support the commercialization of SIBs.