Machine-learning-assisted material discovery of oxygen-rich highly porous carbon active materials for aqueous supercapacitors

Porous carbons are the active materials of choice for supercapacitor applications because of their power capability, long-term cycle stability, and wide operating temperatures. However, the development of carbon active materials with improved physicochemical and electrochemical properties is generally carried out via time-consuming and cost-ineffective experimental processes. In this regard, machine-learning technology provides a data-driven approach to examine previously reported research works to find the critical features for developing ideal carbon materials for supercapacitors. Here, we report the design of a machine-learning-derived activation strategy that uses sodium amide and cross-linked polymer precursors to synthesize highly porous carbons (i.e., with specific surface areas > 4000 m²/g). Tuning the pore size and oxygen content of the carbonaceous materials, we report a highly porous carbon-base electrode with 0.7 mg/cm² of electrode mass loading that exhibits a high specific capacitance of 610 F/g in 1 M H₂SO₄. This result approaches the specific capacitance of a porous carbon electrode predicted by the machine learning approach. We also investigate the charge storage mechanism and electrolyte transport properties via step potential electrochemical spectroscopy and quasielastic neutron scattering measurements.

Analysis of the Influence of Activated Carbons' Production Conditions on the Porous Structure Formation on the Basis of Carbon Dioxide Adsorption Isotherms

The results of a study of the impact of activation temperature and the mass ratio of the activator to the carbonised precursor on the porous structure of nitrogen-doped activated carbons obtained from lotus leaves by carbonisation and chemical activation with sodium amide (NaNH₂) are presented. The analyses were carried out via the new numerical clustering-based adsorption analysis, the Brunauer-Emmett-Teller, the Dubinin-Raduskevich, and the density functional theory methods applied to carbon dioxide adsorption isotherms. Carbon dioxide adsorption isotherms' analysis provided much more detailed and reliable information about the pore structure analysed. The analyses showed that the surface area of the analysed activated carbons is strongly heterogeneous, but the analysed activated carbons are characterised by a bimodal pore structure, i.e., peaks are clearly visible, first in the range of pore size from about 0.6 to 2.0 nm and second in the range from about 2.0 to 4.0 nm. This pore structure provides optimal adsorption performance of carbon dioxide molecules in the pore structure both for adsorption at atmospheric pressure, which requires the presence of narrow pores for the highest packing density, as well as for adsorption at higher pressures, which requires the presence of large micropores and small mesopores. However, there are no micropores smaller than 0.5 nm in the analysed activated carbons, which precludes their use for carbon dioxide adsorption for processes conducted at pressures less than 0.01 MPa.

Stoichiometric carbocatalysis via epoxide-like C-S-O configuration on sulfur-doped biochar for environmental remediation

Heteroatom doping is a promising technique to enhance biochar for effective environmental remediation. However, development of electroactive heteroatom-doped biochars, e.g., sulfur-doped biochar, has been hindered due to complex nature of non-stoichiometric biomass-derived carbon and changeable electrochemical state of dopants. Herein, we produced a series of wood waste-derived biochars with customized levels of minerals and redox-active moieties, aiming to unravel the crucial factors for sulfur doping. Calcium (Ca) in biochar was found to preferentially coordinate with sulfur to form inactive inorganic sulfur minerals (i.e., CaSO₄ and CaS) with inferior catalytic reactivity. After diminishing the inherent Ca minerals beforehand, we could introduce surface phenoxyl-type radicals (C-O^•) and vacancy defects on the biochar to develop an electrophilic C-S-O bonding configuration, which guaranteed a high affinity towards peroxymonosulfate (PMS, 2.08 mM g^-1, 30 min) and efficient removal of bisphenol A (BPA, 91.1%, 30 min). Scavenging experiments and in-situ Raman analyses indicated that the epoxide-like C-S-O structure induced nucleophilic addition of PMS to generate surface-bound singlet oxygen (¹O₂, major) and hydroxyl radicals (^•OH, minor) through a preservative and stoichiometric interfacial reaction. Overall, the proposed approach overcomes the major hurdles in science-informed fabrication of sulfur-doped biochar and advances its development for environmental remediation.

In Situ Dry Chemical Synthesis of Nitrogen-Doped Activated Carbon from Bamboo Charcoal for Carbon Dioxide Adsorption

In this work, nitrogen-doped bamboo-based activated carbon (NBAC) was in situ synthesized from simply blending bamboo charcoal (BC) with sodamide (SA, NaNH₂) powders and heating with a protection of nitrogen flow at a medium temperature. The elemental analysis and X-ray photoelectron spectra of as-synthesized NBAC showed quite a high nitrogen level of the simultaneously activated and doped samples; an abundant pore structure had also been determined from the NBACs which has a narrow size distribution of micropores (<2 nm) and favorable specific surface area that presented superb adsorption performance. The fcarbon dioxide (CO₂) adsorption of the NBACs was measured at 0 °C and 25 °C at a pressure of 1 bar, whose capture capacities reached 3.68–4.95 mmol/g and 2.49–3.52 mmol/g, respectively, and the maximum adsorption could be observed for NBACs fabricated with an SA/BC ratio of 3:1 and activated at 500 °C. Further, adsorption selectivity of CO₂ over N₂ was deduced with the ideal adsorbed solution theory ((IAST), the selectivity was finally calculated which ranged from 15 to 17 for the NBACs fabricated at 500 °C). The initial isosteric heat of adsorption (Qst) of NBACs was also determined at 30–40 kJ/mol, which suggested that CO₂ adsorption was a physical process. The results of ten-cycle adsorption-desorption experimentally confirmed the regenerated NBACs of a steady CO₂ adsorption performance, that is, the as-synthesized versatile NBAC with superb reproducibility makes it a perspective candidate in CO₂ capture and separation application.

H2O-prompted CO2 capture on metal silicates in situ generated from SBA-15

A series of metal silicates, NaMSi₁₀Ox (M = Cu, Mn and Ni), were prepared by in situ doping of metals into mesoporous SBA-15 under a hydrothermal process, displaying a continuous framework of SiO₄ structure with a narrow pore size distribution. These metal silicate materials were tested for CO₂ adsorption behavior in the absence and presence of water. The results exhibited that the effect of H₂O on the CO₂ capture capability of metal silicates depends on the types of metal inserted into SBA-15. Compared to the dry condition, H₂O addition enhances CO₂ uptake dramatically for NaCuSi₁₀Ox by 25%, and slightly for NaNiSi₁₀Ox (∼10%), whereas little effect is shown on NaMnSi₁₀Ox. The metal silicate materials are stable after adsorption of CO₂ under wet conditions, which is benefited from their synthesis method, hydrothermal conditions. The improvement of CO₂ uptake on metal silicates by H₂O is attributed to the competitive and synergistic adsorption mechanism on the basis of IR investigations, where initially adsorbed H₂O acts as a promoter for further CO₂ capture through a hydration reaction, i.e., formation of bicarbonate and carbonates on the surface of the samples. These observations provide new possibilities for the design and synthesis of porous metal silicate materials for CO₂ capture under practical conditions where moisture is present.

Porous metal silicates prepared by an in situ doping strategy of metals into SBA-15 under hydrothermal conditions display efficient CO₂ capture performances in the absence and presence of moisture.

Ordered mesoporous carbon with enhanced porosity to support organic amines: efficient nanocomposites for the selective capture of CO2

Highly ordered mesoporous carbon with a much expanded porosity was impregnated with PEI to act as an efficient composite material for the highly selective capture of CO₂ from flue gas.

Controlling Pore Shape and Size of Interpenetrated Anion-Pillared Ultramicroporous Materials Enables Molecular Sieving of CO2 Combined with Ultrahigh Uptake Capacity

The separation of carbon dioxide (CO₂) from hydrocarbons is a critical process for the production of clean energy and high-purity chemicals. Adsorption based on molecular sieving is an energy-saving separation process; however, most of molecular sieves with narrow and straight pore channels exhibit low CO₂ uptake capacity. Here, we report that a twofold interpenetrated copper coordination network with a consecutive pocket-like pore structure, namely, SIFSIX-14-Cu-i (SIFSIX = hexafluorosilicate, 14 = 4,4'-azopyridine, i = interpenetrated) is a remarkable CO₂/CH₄ molecular sieving adsorbent which completely blocks the larger CH₄ molecule with unprecedented selectivity, whereas it has excellent CO₂ uptake (172.7 cm³/cm³) under the ambient condition. The exceptional separation performance of SIFSIX-14-Cu-i is attributed to its unique pore shape and functional pore surface, which combine a contracted pore window (3.4 Å) and a relatively large pore cavity decorated with high density of inorganic anions. Dispersion-corrected density functional theory calculation and neutron powder diffraction were performed to understand the CO₂ binding sites. The practical feasibility of SIFSIX-14-Cu-i for CO₂/CH₄ mixtures separation was validated by experimental breakthrough tests. This study not only demonstrates the great potential of SIFSIX-14-Cu-i for CO₂ separation but also provides important clues for other gas separations.

Role of Alumina Basicity in CO2 Uptake in 3-Aminopropylsilyl-Grafted Alumina Adsorbents

Oxide-supported amine materials are widely known to be effective CO₂ sorbents under simulated flue-gas and direct-air-capture conditions. Most work has focused on amine species loaded onto porous silica supports, though potential stability advantages may be offered through the use of porous alumina supports. Unlike silica materials, which are comparably inert, porous alumina materials can be tuned to have substantial acidity and/or basicity. Owing to their amphoteric nature, alumina supports play a more active role in CO₂ sorption than silica supports, potentially directly participating in the adsorption process. In this work, primary amines associated with 3-aminopropyltriethoxysilane are grafted onto two different mesoporous alumina materials having different levels of basicity. Adsorbent materials with different amine loadings are prepared, and the CO₂ -adsorption behavior of similar amines on the two alumina supports is demonstrated to be different. At low amine loadings, the inherent properties of the support surface play a significant role, whereas at high amine loadings, when the alumina surface is effectively blocked, the sorbents prepared on the two supports behave similarly. At high amine loadings, amine-CO₂ -amine interactions are shown to dominate, leading to adsorbed species that appear similar to the species formed over silica-supported amine materials. The sorbent properties are comprehensively characterized using N₂ physisorption analysis, in situ FTIR spectroscopy, and adsorption microcalorimetry.

Controllable synthesis of nitrogen-doped hollow carbon nanospheres with dopamine as precursor for CO2 capture

Nitrogen-doped hollow carbon nanospheres are synthesized by using dopamine as carbon and nitrogen sources and tetraethyl orthosilicate as structure-assistant agent.

N-, P- and B-doped mesoporous carbons for direct dehydrogenation of propane

N-, P- and B-doped mesoporous carbons were used as metal-free catalysts in propane dehydrogenation, and their catalytic performances were influenced critically by the chemical structures and oxygen-containing groups in these carbons.

Facile synthesis of mesoporous melamine-formaldehyde spheres for carbon dioxide capture

Greenhouse effect and excessive carbon dioxide (CO₂) emissions have caused widespread public concern in recent years.

New nitrogen-rich azo-bridged porphyrin-conjugated microporous networks for high performance of gas capture and storage

Synthesis of a series of nitrogen-rich azo-bridged porphyrin conjugates and its application for excellent CO₂ adsorption.