Sustainable one-step strategy to highly graphitized capacitive carbons with hierarchical micro-meso-macro porosity

Large micropore surface area, superior electrical conductivity and suitable pore size are simultaneously desired characteristics for high-performance capacitive carbons. However, these desired features tend to be mutually competing, and are...

Efficient hyperparameter-tuned machine learning approach for estimation of supercapacitor performance attributes

Recent years have witnessed the rise of supercapacitor as effective energy storage device. Specifically, carbon-based electrodes have been experimentally well studied and used in the fabrication of supercapacitors due to their excellent electrochemical properties. This work reports the development and utilization of highly tuned and efficient Machine Learning (ML) models that give insights into correlation between structural features of electrodes and supercapacitor performance metrics namely specific capacitance, power density and energy density. Artificial Neural Networks (ANN) and Random Forest (RF) models have been employed to predict the various in-operando performance metrics of carbon-based supercapacitors based on three input features such as mesopore surface area, micropore surface area and scan rate. Experimentally measured values of these parameters used for training and testing these two models have been extracted from a set of research papers reported in literature. The optimization techniques and various tuning methodologies adopted for identifying model hyperparameters are discussed in this paper. The authors demonstrate the importance of hyperparameter tuning and optimization in building accurate and reliable computational models.

Application of design of experiments (DoE) for optimised production of micro- and mesoporous Norway spruce bark activated carbons

In this work, Norway spruce (Picea abies (Karst) L.) bark was employed as a precursor to prepare activated carbon using zinc chloride (ZnCl2) as a chemical activator. The purpose of this study was to determine optimal activated carbon (AC) preparation variables by the response surface methodology using a Box–Behnken design (BBD) to obtain AC with high specific surface area (SBET), mesopore surface area (SMESO), and micropore surface area (SMICR). Variables and levels used in the design were pyrolysis temperature (700, 800, and 900 °C), holding time (1, 2, and 3 h), and bark/ZnCl2 impregnation ratio (1, 1.5, and 2). The optimal conditions for achieving the highest SBET were as follows: a pyrolysis temperature of 700 °C, a holding time of 1 h, and a spruce bark/ZnCl2 ratio of 1.5, which yielded an SBET value of 1374 m2 g−1. For maximised mesopore area, the optimal condition was at a pyrolysis temperature of 700 °C, a holding time of 2 h, and a bark/ZnCl2 ratio of 2, which yielded a SMESO area of 1311 m2 g−1, where mesopores (SMESO%) comprised 97.4% of total SBET. Correspondingly, for micropore formation, the highest micropore area was found at a pyrolysis temperature of 800 °C, a holding time of 3 h, and a bark/ZnCl2 ratio of 2, corresponding to 1117 m2 g−1, with 94.3% of the total SBET consisting of micropores (SMICRO%). The bark/ZnCl2 ratio and pyrolysis temperature had the strongest impact on the SBET, while the interaction between temperature and bark/ZnCl2 ratio was the most significant factor for SMESO. For the SMICRO, holding time was the most important factor. In general, the spruce bark AC showed predominantly mesoporous structures. All activated carbons had high carbon and low ash contents. Chemical characterisation indicated that the ACs presented disordered carbon structures with oxygen functional groups on the ACs’ surfaces. Well-developed porosity and a large surface area combined with favourable chemical composition render the activated carbons from Norway spruce bark with interesting physicochemical properties. The ACs were successfully tested to adsorb sodium diclofenac from aqueous solutions showing to be attractive products to use as adsorbents to tackle polluted waters. Graphical abstract

Efficient toluene adsorption/desorption on biochar derived from in situ acid-treated sugarcane bagasse

Carbon-based materials with great adsorption performance are of importance to meet the needs of industrial gas adsorption. Being massive agricultural wastes of sugarcane bagasse, China could use this waste into wealth. However, the comprehensive utilization of sugarcane bagasse as precursors for biochar that can be used as adsorbent has not been extensively explored. In this study, a series of in-situ sulfuric acid modified biochar were prepared by hydrothermal carbonization process. The prepared biochar (SBAC-7) is combined of two main advantages that are high microporosity (micropore surface area = 1106 m 2 /g) and rich in S-containing functional groups on the surface. In particular, SBAC-7 showed an excellent adsorption capacity of toluene (771.1 mg/g) at 30 o C, which is nearly 3 times as high as the commercial activated carbons. Meanwhile, it showed great stability and cyclic regeneration performance with five toluene adsorption-desorption test cycles. This study provides a high-performance biochar for adsorption-desorption cycle in practical engineering applications, and would contribute to the sustainable “sugarcane production - bagasse utilization” circular economy.

Effect of using regenerated combined FAU and MOR zeolites as catalysts during the pyrolysis of kraft lignin

The SiO2/Al2O3 mole ratio, pore size, and acid sites are the key parameters of zeolite’s activity in lignin pyrolysis. In this study, the comparison of individual Y and M zeolites, the combined ‘Y + M’ sample after regeneration, and their effect on lignin pyrolysis were studied in five cycles (regeneration and reuse). The results were explained using Brunauer, Emmet, and Teller (BET), micropore surface area (MSA), and total acid sites (TAS) analyses. In comparison with the individual Y or M zeolite sample, the consistent higher catalytic activities of the combined ‘Y + M’ sample in repeated cycles were observed. Pyrolysis heavy oils were characterized by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The NMR analyses revealed that with increased zeolite regeneration cycles, p-hydroxy phenyl and methoxyl groups increased. Decreases in guaiacyl phenolic hydroxyl were less for the combined ‘Y + M’ sample than the individual Y and M zeolites. Lower weight average (Mw) of heavy oil for the combined ‘Y + M’ sample indicated the enhanced cleavage of lignin structures in pyrolysis. These results support the higher catalytic activity of regenerated zeolites for the combined ‘Y + M’ sample compared with individual Y and M zeolites due to the improved MSA and TAS.

CO2 Adsorption Investigation on an Innovative Nanocomposite Material with Hierarchical Porosity

A NaX nanozeolite-geopolymer monolith, with hierarchical porosity, has been produced by a one-pot hydrothermal synthesis using metakaolin as alluminosilicate source and a sodium silicate solution as activator. Its final composition, reported in terms of oxides, is 1.3–Na2O–3.0SiO2–1Al2O3–12H2O. Its microstructural and chemical features and CO2 adsorption performance have been investigated. The microstructure of the composite is characterized by NaX zeolite nanocrystals glued by the geopolymeric binder to form a complex three-dimensional network of pores. Overall porosity resulted ~23.5%, whereas compressive strength is 16±0.7 MPa. Monolith showed BET surface area of 350 m2/g, a micropore surface area of 280 m2/g and a mesopore volume, due to the geopolymeric binder, of 0.09 cm3/g. Its CO2 adsorption capacity has been measured at the temperatures of 7, 25 and 42 °C up to 15 bar using an optimized Sievert-type (volumetric) apparatus. All the adsorption data were evaluated by Toth/Langmuir isotherm model and commercial pure NaX zeolite was used as reference. CO2 adsorption isotherms show a maximum uptake value around 21 wt% at (~7 °C) that decrease to 18 wt% at high temperature (~42 °C) passing through 19 wt% at room temperature (~25 °C). The homogeneity grade of the surface, as obtained using Toth analysis performed on the adsorption isotherm, is close to t ≅ 0.40, lower than the 0.61 obtained for pure commercial NaX zeolite, as a consequence of the binder formation. Monolith exhibits a notably higher K values and quicker saturation with respect to reference that can be ascribed to the presence of mesoporosity that provides an easier and faster transport of CO2 in the NaX nanozeolite framework. The produced composite is a potential solid adsorbent candidate in industrial process.

Synthesis of an excellent MTP catalyst: hierarchical ZSM-5 zeolites with great mesoporosity

A unique organosiloxane-polyether amine (OPA) was produced and used as mesoporogen to efficiently synthesize hierarchical ZSM-5 zeolites with great mesoporosity. We have employed silica sol and tetraethylorthosilicate, respectively, to investigate the influence of different silicon sources on hierarchical zeolites in the presence of OPA. The mesopores of synthesized samples focused on 6–15 nm, and the external surface area varied from 185 to 463 m 2 g −1 where the micropore surface area was maintained at 245–334 m 2 g −1 . Benefiting from the superior structure properties, these samples were used as catalysts in the reaction of methanol to propylene, and the optimal one catalysed for 180 h with methanol conversion above 95%. The as-produced OPA could connect steadily with zeolite frameworks through covalent bonds (–Si–O–Si–) during the hydrothermal crystallization process. This type of connection mode could effectively avoid the formation of amorphous phase and the special molecular structure of OPA could efficiently introduce abundant mesopores with few micropores being consumed. The samples synthesized with silicon sol were made up of quasi-circular particles of about 800 nm in size and further consisted of nanocrystals of 40 nm, and the samples produced with TEOS have a particle size of about 1–2 µm aggregated with nanocrystals of 300 nm.

Chemical Activation of Durian Shell Activated Carbon: Effects of Activation Agents

Selection of suitable activation agent is important in order to produce high surface area of activated carbon. The present study was undertaken to develop high surface area of durian shell activated carbon (DSAC) using different chemical activation agents which were potassium hydroxide (KOH) and phosphoric acid (H3PO4). Surface porosity and surface area were directly measured from scanning electron microscopy (SEM) and surface area analyzer, respectively. For the optimum condition, it showed that H3PO4 treated DSAC had the highest surface area which was 257.50 m2/g compared to KOH treated DSAC which was 13.10 m2/g. H3PO4 treated DSAC also showed the highest micropore surface area, external surface area and total pore volume with 191.22 m2/g, 66.28 m2/g and 0.149 cm3/g, respectively. SEM result showed that H3PO4 treated DSAC had a well pronounce porosity than durian shell char. Surface area and surface porosity were important in an adsorption process.

Sustainable activated carbon fibers from liquefied wood with controllable porosity for high-performance supercapacitors

The combination of the high micropore surface area and the controlled mesopore size and mesopore/micropore ratio is responsible for high specific capacitance and excellent rate capability.

Preparation and Characterization of Microporous Hemp Stem-Based Activated Carbons

Activated carbons are prepared from hemp stem with KOH as activating agent under different ratio of KOH to carbon conditions. The BET(Brunauer Emmett and Teller) specific surface area of the hemp stem-based activated carbons first increases and then decreases with the increasing ratio of KOH to carbon. The specific surface area, micropore surface area and volume of the activated carbons reach a maximum of 1589.27m2/g 1420.52m2/g, 89% of the total area, 0.751m3/g at the ratio of 4.5:1. The micropore size distribution shows the activated carbons contain a large number of ultramicropore and supermicropore.