Ti-MXene (Ti3C2Tz) is the most common member of a larger family of 2D materials widely explored due to a variety of possible applications. MXenes are mostly synthesized using strong acids like HF and HCl or using procedures that require elevated temperatures. Here, we present a new method for Ti3C2Tz preparation with a weak acid solution, which is more beneficial for mass production with reduced environmental impact. It is demonstrated that aluminum can be etched from titanium aluminum carbide (Ti3AlC2) using ammonium fluoride (NH4F) dissolved in an aqueous solution of acetic acid (CH3COOH). Optimization of the balance between amounts of water and acetic acid in the etching solution allows for complete etching of Al atoms yielding partially nitrogen terminated MXene in addition to common -O/-OH and -F termination. The mechanism of MXene formation was investigated by the in situ synchrotron radiation X-ray diffraction (XRD), allowing characterization of "pristine" MXene structure forming directly in the process of Ti3AlC2 reaction with NH4F/CH3COOH. In situ XRD analysis also enables identification of the reaction byproducts, thus providing information about the mechanism of MXene formation.

Swelling in polar solvents is a fundamental property of graphite oxide (GO). Using in situ synchrotron X-ray diffraction (XRD), Brodie graphite oxide (BGO) swelling was studied in a series of primary amides with the number of carbon atoms in the alkyl chain ranging from one (acetamide) to ten (decanamide) and compared to GO swelling in primary alcohols of equivalent chain lengths. The uptake of solvent due to swelling was determined via Differential Scanning Calorimetry (DSC). Swelling of BGO in acetamide, propionamide, and butyramide was found to expand the interlayer distance d(001) by ∼3.5-3.7 Å, consistent with the intercalation of a single molecular layer with an orientation parallel to the GO sheets. Swelling in longer molten amides produced larger c-lattice expansions correlating with the c-unit cell parameter of the pure amides, suggesting two-layer intercalation in a tilted "stand-up" orientation. Reversible swelling transition was found in the BGO-formamide system. This transition corresponds to the change between BGO structures with one-layer and two-layer formamide intercalation and has an enthalpy of 0.01 kJ g-1 (BGO). No temperature-driven swelling transitions were observed for the other studied amides, acetamide through decanamide, in contrast to previously reported transitions in BGO-alcohol systems. These results demonstrate the wide tunability of interlayer spacing in BGO-amide systems and highlight the potential of controlled intercalation for applications such as molecular separation and sorption.

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 & Aring; and c = 18.577 & Aring;) 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.

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 similar to 7050 mu mol g-1 for super-oxidized porous carbon (SOPC) with a specific surface area (SSA) of similar to 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 (similar to 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 (similar to 7.5 & Aring;). 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.

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.

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.

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 Ti3C2Tx 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. Microsupercapacitors with MXene electrodes and gel electrolytes were painted on cylindrical capillaries and studied by synchrotron radiation XRD under conditions of charging and operation. XRF was demonstrated as a valid method for 2D element mapping of iodine in in-plane microsupercapacitors with gel KI electrolyte. image

AbstractGraphite 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).

Brodie Graphite Oxide Synchrotron radiation XRD provides unique opportunities for studies of temperature dependent phase transformations due to rapid recording of data. In article number 2300554, Alexander V. Talyzin and co-workers study ten different systems and analyze general trends for temperature dependent swelling of Brodie graphite oxide in molten long chain alcohols. Phase transformations related to incongruent melting of solid solvate phase are found in each system.

Abstract 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.

Intercalation of very long molecules into the structure of multi-layered graphene oxide (GO) was studied using example of 1-hexadecanol (C16), an alcohol molecule with 16 carbon atoms. Brodie graphite oxide (BGO) immersed in excess of liquid C16 just above the melting point shows expansion of c-unit cell parameter from -6 angstrom to -48.76 angstrom forming a structure with two densely packed layers of C16 molecules in a perpendicular orientation relative to the GO planes (alpha-phase). Heating of the BGO-C16 alpha-phase in excess of C16 melt results in reversible phase transition into 8-phase at 336-342K. The 8-phase shows much smaller unit cell parameter of 29.83 angstrom (363K). Analysis of data obtained using vacuum-driven evaporation of C16 from the 8-phase provides evidence for structure of 8-phase consisting of five layers of C16 molecules in parallel to GO plane orientation. Therefore, the transition from alpha-to 8-phase corresponds to change in orientation C16 molecules from perpendicular to parallel relative to GO planes and decrease in the amount of intercalated solvent. Cooling of the 8-phase in absence of C16 melt is found to result in the formation of gamma-phase with inter-layer distance of -26.5 angstrom corresponding to one layer of C16 molecules intercalated perpendicularly relative to the GO planes. Structures with one and two layers of C16 molecules parallel to GO planes were identified in samples with rather small initial loading of C16. Surprisingly rich variety of structures revealed in the BGO-C16 system provides opportunities to create materials with precisely controlled GO inter-layer distance.

Activated carbons have been previously produced from a huge variety of biomaterials often reporting advantages of using certain precursors. Here we used pine cones, spruce cones, larch cones and a pine bark/wood chip mixture to produce activated carbons in order to verify the influence of the precursor on properties of the final materials. The biochars were converted into activated carbons with extremely high BET surface area up to ∼3500 m2 g-1 (among the highest reported) using identical carbonization and KOH activation procedures. The activated carbons produced from all precursors demonstrated similar specific surface area (SSA), pore size distribution and performance to electrodes in supercapacitors. Activated carbons produced from wood waste appeared to be also very similar to "activated graphene" prepared by the same KOH procedure. Hydrogen sorption of AC follows expected uptake vs. SSA trends and energy storage parameters of supercapacitor electrodes prepared from AC are very similar for all tested precursors. It can be concluded that the type of precursor (biomaterial or reduced graphene oxide) has smaller importance for producing high surface area activated carbons compared to details of carbonization and activation. Nearly all kinds of wood waste provided by the forest industry can possibly be converted into high quality AC suitable for preparation of electrode materials.

Porous carbons are not favorable for sorption of heavy metals and radionuclides due to absence of suitable binding sites. In this study we explored the limits for surface oxidation of "activated graphene" (AG), porous carbon material with the specific surface area of ∼2700 m2/g produced by activation of reduced graphene oxide (GO). Set of "Super-Oxidized Activated Graphene" (SOAG) materials with high abundance of carboxylic groups on the surface were produced using "soft" oxidation. High degree of oxidation comparable to standard GO (C/O=2.3) was achieved while keeping 3D porous structure with specific surface area of ∼700-800 m2/. The decrease in surface area is related to the oxidation-driven collapse of mesopores while micropores showed higher stability. The increase in the oxidation degree of SOAG is found to result in progressively higher sorption of U(VI), mostly related to the increase in abundance of carboxylic groups. The SOAG demonstrated extraordinarily high sorption of U(VI) with the maximal capacity up to 5400 μmol/g, that is 8.4 - fold increase compared to non-oxidized precursor AG, ∼50 -fold increase compared to standard graphene oxide and twice higher than extremely defect-rich graphene oxide. The trends revealed here show a way to further increase sorption if similar oxidation degree is achieved with smaller sacrifice of surface area.

High surface area carbons are so far the best materials for industrial manufacturing of supercapacitor electrodes. Here we demonstrate that pine cones, an abundant bio-precursor currently considered as a waste in the wood industry, can be used to prepare activated carbons with a BET surface area exceeding 3000 m2 g-1. It is found that the same KOH activation procedure applied to reduced graphene oxide (rGO) and pine cone derived biochars results in carbon materials with a similar surface area, pore size distribution and performance in supercapacitor (SC) electrodes. It can be argued that "activated graphene" and activated carbon are essentially the same kind of material with a porous 3D structure. It is demonstrated that the pine cone derived activated carbon (PC-AC) can be used as a main part of aqueous dispersions stabilized by graphene oxide for spray deposition of electrodes. The PC-AC based electrodes prepared using a semi-industrial spray gun machine and laboratory scale blade deposition of these dispersions were compared to pellet electrodes.