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Porous carbons based on activated reduced graphene oxide (rGO) have been demonstrated as excellent sorbents for U(vi), with their sorption capacity correlating with the degree of their oxidation. Herein, we demonstrate an extraordinarily high U(vi) sorption of ∼7050 μmol g−1 for super-oxidized porous carbon (SOPC) with a specific surface area (SSA) of ∼970 m2 g−1 and an extremely high degree of oxidation (C/O = 2.1), similar to graphene oxide. The SOPC materials were prepared using an oxidation treatment applied to activated carbon produced from spruce cones. The extremely high SSA of the precursor activated carbon (∼3400 m2 g−1) as well as its microporous structure and mild oxidation treatment allowed for the preservation of a significant part of the surface area, providing materials with rather narrow pore size distribution (∼7.5 Å). The SOPC prepared from spruce cone biochar is similar to defective graphene oxide but with a significantly higher surface area, resulting in superior U(vi) sorption. Analysis of EXAFS and XPS data shows that U(vi) likely binds to carboxylic groups on the opposite sides of the micropores. The small size of the micropores and irregular pore wall structure are the main factors affecting pore sorption. The spruce-cone biochar has a strong advantage compared with earlier used rGO as a precursor for the preparation of SOPC.

Using molten salts for etching aluminum (Al) away from the MAX phase for MXene synthesis is an attractive alternative method that allows one to avoid the use of toxic hydrofluoric acid (HF) solutions. However, the mechanism of the MAX phase reaction with molten salts remains to date unclear due to the lack of in situ data. Here, we present a detailed in situ time-resolved synchrotron radiation X-ray diffraction study of the MAX phase annealing in molten ZnCl₂ and SnCl₂. The reaction of salts with the MAX phase is found to occur in two stages. The initial period of annealing results in the delamination of two-dimensional (2D) Ti₃C₂ layers, vigorous evolution of AlCl₃ bubbles, and dissolution of Zn in a ZnCl₂ melt. The chlorine-terminated Ti₃C₂ sheets formed in the delaminated state are restacked into a relatively well-ordered MXene structure (P6₃/mmc, a = 3.071 Å and c = 18.577 Å) during the prolonged annealing in molten salts. Surprisingly, the data recorded directly in molten salts at temperatures up to 873 K demonstrate that Ti₃C₂Cl_x MXene shows no swelling in both liquid ZnCl₂ and SnCl₂. The structure of MXene studied directly in the molten salts is found to be the same as in ex situ experiments performed after cooling and water washing under ambient conditions. The absence of the “pristine” melt-swollen phase indicates a rather different mechanism of MXene formation compared to HF-based solution methods. Formation of MXene by gradually removing Al from the MAX phase starting at the edges of flakes and propagating into the deeper parts of interlayers is not possible, since the molten salt is not capable of penetrating between Cl-terminated Ti₃C₂ layers.

Many emerging industry applications demand electronic systems with reliable operation at temperatures >300 °C. To date, the most promising on-chip power sources, micro-supercapacitors (MSCs), can only operate at temperatures up to 250 °C for a short period as limited by the vulnerability of their electrolyte frameworks at high temperatures. Here, a strategy is proposed to use liquids to lock the phase transformations of bassanite microrods for scalable on-chip printing of interlocking ceramic frameworks with high thermal stability. The robust ceramic frameworks enable simple yet scalable fabrication of MSCs to work at 300 °C with an areal capacitance of up to >60 mF cm−2 and only ≈3% performance degradation after 1000 cycles during a test period of ≈3 h. A large-scale MSC array, consisting of 20 cells within a footprint area of 4 cm × 8 cm, has been able to supply a power of 7.2 mW at 300 °C. These break through the present limit of 250 °C of almost all high-temperature energy storage devices and pave the way for on-chip MSCs for high-temperature electronics.

Ti-MXene is a promising electrode material for supercapacitors. The layered structure of MXene expands due to swelling in electrolytes allowing the penetration of ions into the interlayers. A study of effects related to the match between the size of cations in hydrated or dehydrated state and the interlayer distance of MXene is performed here using operando X-ray diffraction (XRD) in capillary-size supercapacitors with alkali metal chloride electrolytes. The supercapacitors are studied during charging and discharging over several cycles revealing structural changes at both MXene electrodes. Experiments reveal an expansion of the MXene c-lattice in LiCl, NaCl, and KCl electrolytes (compared to the expansion in pure water) under an increase of applied voltage from 0 to 1 V and structural oscillations related to a change of polarity. The interlayer spacing of MXene remains close to the water-swollen state in RbCl, CsCl, and NH4Cl electrolytes showing no further expansion as a function of applied voltage. Only rather small variations of interlayer spacing are found in H2SO4 electrolyte during tens of charge–discharge cycles. Analysis of the match between the sizes of ions and the width of MXene interlayers demonstrates that some cations and anions could be inserted into MXene interlayers only in dehydrated state.

Graphite oxides (GO) swell in liquid alcohols with significant expansion of c-lattice. However, temperature-dependent swelling of Hummers GO (HGO) has so far been reported only for methanol and ethanol. Here, HGO swelling in liquid 1-alchohols (C1 to C22 according to the number of carbons) is studied as a function of temperature using in situ synchrotron radiation XRD. Swelling transitions never previously observed for HGO in any kind of polar solvents are found, enthalpy of these transition and compositions of HGO-Cx solid solvates near the point of solvent melting reported. Swelling transitions from low temperature to high-temperature phase are found for HGO in C10–C22 alcohols, similarly to earlier reported transitions in Brodie graphite oxide (BGO). The transitions correspond to a strong change of inter-layer distance correlating with the alcohol molecules length and change in molecules orientation from perpendicular to parallel to GO planes (Type II transitions). However, Type I swelling transitions (related to insertion/removal of one layer of alcohol molecules) reported earlier for BGO are not found in HGO. Continuous changes of the d(001) spacing are revealed for HGO immersed in all smaller alcohols in the range C1 (methanol) to C9 (nonanol).

Swelling is the most fundamental property of graphite oxides (GO). Here, a structural study of Brodie graphite oxide (BGO) swelling in a set of long chain 1-alcohols (named C11 to C22 according to the number of carbons) performed using synchrotron radiation X-ray diffraction at elevated temperatures is reported. Even the longest of tested alcohols (C22) is found to intercalate BGO with enormous expansion of the interlayer distance from ≈6Å up to ≈63Å, the highest expansion of GO lattice ever reported. Swelling transitions from low temperature α-phase to high temperature β-phase are found for BGO in all alcohols in the C11–C22 set. The transitions correspond to decrease of inter-layer distance correlating with the length of alcohol molecules, and change in their orientation from perpendicular to GO planes to layered parallel to GO (Type II transitions). These transitions are very different compared to BGO swelling transitions (Type I) found in smaller alcohols and related to insertion/de-insertion of additional layer of alcohol parallel to GO. Analysis of general trends in the whole set of 1-alcohols (C1 to C22) shows that the 1-alcohol chain length defines the type of swelling transition with Type I found for alcohols with C<10 and Type II for C>10. 

Very strong difference in many properties is well documented for graphite oxides (GtO) synthesized by Brodie (BGO) and Hummers (HGO) methods. The difference is typically assigned to the type of oxidant (chlorates or KMnO4, respectively). However, not only oxidants but also acids used in these methods are different. It is still unclear which of the different properties of GtO are dependent on the oxidant or acid used in the synthesis. Here we synthesized a new type of graphite oxide using an oxidation agent typical for the Brodie method (KClO3) in combination with acids so far used only in modified Hummers' method (H2SO4+H3PO4). The GtO synthesized by this method (MGO) demonstrates some properties similar to BGO (higher temperature of exfoliation and less defected structure) but also similarity to some other properties of HGO (absence of sharp swelling transitions). Comparing MGO, BGO, and HGO allows us to distinguish the effects of acids and oxidants on the properties of graphite oxides. The new procedure proposed in this study allows preparation of GtO nearly free from hole/vacancy defects (similarly to BGO) but avoids dangerous HNO3. MGO is suggested as a favorable precursor for the preparation of graphene films by thermal or chemical reduction methods.

Edible white-rot fungi are commonly cultivated on wood-based substrates and selectively degrade lignin to a larger extent during their growth. Spent mushroom substrate (SMS) is produced in huge amounts by the mushroom industry and today there is a lack of proven methods to valorize this kind of biomass waste, which in most cases is landfilled or used as fuel. This study demonstrates that birch wood-based SMS from the cultivation of oyster mushrooms can be converted into high-quality activated carbon (AC) with an extremely high surface area of about 3000 m²/g. These activated carbons showed good performance when used in electrodes for supercapacitors, with energy storage parameters nearly identical to AC produced from high-quality virgin birch wood. Moreover, AC produced from SMS showed high potential as an adsorbent for cleaning reactive orange-16 azo dye from aqueous solutions as well as contaminants from synthetic effluents and from real sewage water. The kinetics of adsorption were well represented by the Avrami fractional order model and isotherms of adsorption by the Liu model. The theoretical maximum reactive orange-16 adsorption capacities were approximately 519 mg/g (SMS-based carbon) and 553 mg/g (virgin birch-based carbon). The removal of contaminants from synthetic effluents made of different dyes and inorganic compounds was around 95% and 83% depending on the effluent composition. The removal of contaminants from raw sewage water was around 84%, and from treated sewage water was around 68%. Overall, the results showed that activated carbon prepared from waste generated during cultivation of white-rot fungi is as good as activated carbon prepared from high-quality virgin wood.

Synchrotron radiation X-ray diffraction (XRD) with nanoscale beam size was used here for in situ and in operando study of micro-supercapacitors (MSC) with gel electrolyte and MXene Ti₃C₂T_x electrodes. The electrode structure was characterized as a function of applied voltage and distance from the gap separating electrodes using microscopic cells with cylindrical shape designed for transmission mode XRD. The devices with gel electrolytes based on H₂SO₄ (with H₂O/PVA and DMSO/PVA) showed stable performance with no changes in MXene structure under voltage swaps between positive and negative values. Experiments with KI-based electrolytes demonstrated changes of MXene structure correlated with decrease of energy storage parameters under conditions of increased operation voltage starting from 0.8 V. The optimal performance of the MSCs was observed when the MXene structure remained unchanged upon switching the applied voltage polarity. The changes of inter-layer distance of MXene upon swap of applied voltage correlate with decrease of device performance and are undesirable for stable operation of MSC's. We also tested feasibility of X-ray fluorescence (XRF) for characterization of electrolyte ion migration in MSCs using 2D element mapping. Irreversible sorption of iodine by MXene was found using XRF mapping of charged electrodes using standard in-plane MSC device and KI electrolyte.

Super-oxidized porous carbons produced from reduced graphene oxide have been demonstrated recently to show unique sorption properties. Here we report experiments with identical oxidative treatment applied to four very different types of porous carbon materials with extremely broad range of specific surface area (SSA) ∼350–3400 m²g⁻¹. It is demonstrated that KOH-activated carbons can be oxidized to the same degree as graphene oxide (GO) while preserving a larger part of SSA. Prolonged oxidation with ammonium persulfate resulted in extraordinary high degree of oxidation with C/O ratio reaching 2.1. Precursor carbons with largest share of micropores are found to be more stable against oxidative treatment, while single-walled carbon nanotubes (SWCNT's) are oxidized to much less extent. While mesopores collapse, micropores survive strong oxidation treatment providing materials with rather narrow pore size distribution and high (for given oxidation degree) BET SSA up to ∼1150 m²g⁻¹. Super-oxidized porous carbons (SOPC's) show a high abundance of hydroxyl, epoxide, carbonyl, and carboxyl functional groups (similar to GO). As a result of oxidation, the hydrophobic carbons are converted into hydrophilic materials. Characterization shows that many properties of SOPC (e.g. degree of oxidation, type of functional groups, thermal stability etc.) are rather similar to GO, except for three-dimensional porous structure which can be considered an advantage for sorbent applications. SOPC's demonstrate superior sorption capacity for Cu(II) (up to ∼105 mgg⁻¹), which is 8-fold higher than the value for non-oxidized precursor.