Affordable combustion synthesis of V2AlC precursor for V2CTx MXene

Two-dimensional (2D) transition metal carbides and nitrides (MXenes) possess a unique combination of properties, such as metallic conductivity combined with hydrophilicity and surface redox activity, that are important for energy storage, printed electronics, biomedical, catalytic and other applications. However, the use of many MXene chemistries beyond titanium carbides is limited by the cost of MAX phase precursors, which are usually produced from pure elements, involving expensive transition metals. Herein, we demonstrate a low-cost rapid aluminothermic combustion synthesis of MAX phases from an inexpensive oxide precursor, producing V2AlC in seconds, with low energy input. A reactor for self-propagating high-temperature synthesis (SHS) was designed and manufactured for this study. The V2CTx MXene produced from the SHS MAX is similar to MXene from conventional pressureless sintered MAX in terms of oxidation resistance, environmental stability, conductivity, and electrochemical performance, but has a larger flake size. This work demonstrates an alternative, low-cost and scalable approach to the synthesis of MAX phases and, subsequently, MXenes without sacrificing their properties.


Introduction
MXenes represent a rapidly growing family of two-dimensional (2D) transition metal carbides and/or nitrides with a general chemical formula of M n+1 X n T x where M is an early transition metal (Ti, V, Nb, etc.), X is C and/or N, T x represents the surface groups (O, OH, F, etc.), and n is 1-4 [1,2].Their wide variety of structures and compositions, unique combination and tunability of properties, including metallic conductivity, ability to reversibly intercalate ions and molecules, and surface redox activity, lead to applications in electromagnetic interference shielding, communication, energy harvesting and storage, printable electronics, healthcare, etc. [3] In several of those applications, MXenes outperformed all other known materials [4].Those applications justified significant effort to scale up and improve MXene synthesis [5][6][7].Recently, it was also demonstrated that the defects, grain morphology, stoichiometry, and other characteristics of the precursor MAX phase strongly affect MXene properties [8][9][10].
Considering that the typical synthesis of MXenes by selective etching is a highly scalable process utilizing aqueous solutions of inexpensive chemicals (HCl or HF, LiF or NaF, etc.) at ambient or close-to-ambient temperature, a barrier to low-cost mass production of MXenes is the cost and availability of high-quality MAX phases, especially for chemistries beyond Ti [11].Typically, MAX phases are prepared by hot pressing or pressureless sintering of elemental mixtures, using expensive purified metals, and/ or carbide/nitride powders at temperatures above 1400 °C [12][13][14][15][16].There have been attempts to decrease the cost of precursors and/or synthesis temperature of MAX phases.Bärmann et al. used synthesis in molten salts to produce 1 kg of Ti 3 AlC 2 at 1250 °C over 5 h [12].Shao et al. decreased the molten salt synthesis temperature below 1000 °C, but the method required the use of expensive nanotubes of graphene.Jolly et al. used recycled carbon, aluminum scrap, and titanium oxide to produce Ti 3 AlC 2 [11].Pang et al. demonstrated the electrochemical synthesis of Ti 3 AlC 2 using titanium-rich slag [17].Li et al. used TiO 2 as a precursor during pressureless sintering of Ti 3 AlC 2 [18].The use of TiO 2 to make stoichiometric TiC prior to the MAX phase synthesis led to Ti 3 AlC 2 with low oxygen content and excellent properties [19].While the above studies targeted Ti 3 AlC 2 , Cuskelly et al. demonstrated that a variety of M n+1 AX n phases (M = Ti, V, Cr, Nb, or Ta; n = 1-3) can be produced employing aluminum reduction of oxide precursors at 1400-1600 °C for 3-12 h [20].While all of these methodologies demonstrate some improvements in cost/ energy, they still rely on an energy-consuming high-temperature synthesis.
The energy consumption can be decreased by using combustion synthesis, which is driven by the exothermic reaction.It is fast, occurring in less than a minute, and can be carried out under an inert atmosphere or even in air [21][22][23].MAX phases can be produced by self-propagating high-temperature synthesis (SHS) and volume combustion synthesis (VCS) (Fig. 1a); then processed into MXenes using established etching and delamination protocols (Fig. 1b) [24].In SHS, the reaction is initiated by local heating, followed by the propagation of a combustion front through the powder mixture.Among all reported synthesis techniques capable of producing MAX phases, SHS offers the highest energy efficiency (~ 0.1 kWh per ton of products) and productivity (10 tons/year for a 30-L SHS reactor) [22,25].In VCS, the entire reactive mixture is preheated to a temperature slightly above the melting point of the A element (~ 700 °C for aluminum).Once aluminum melts and spreads across the surface of oxide particles, the metallothermic reduction (Reaction 1) is initiated, and the reactive mixture ignites, forming the MAX phase.VCS requires lower temperatures (~ 700-800 °C vs ~ 1200-1600 °C) and shorter annealing durations (3-10 min vs 2-12 h) compared to pressureless sintering.The throughput and energy efficiency of VCS is lower than SHS, but VCS can be realized in a furnace, facilitating industrial adoption [22].Combustion synthesis of MAX phases can employ either elemental mixtures [26][27][28] or metal oxides [29][30][31][32][33][34] as precursors.While combustion synthesis of multiple MAX phases (Ti 3 AlC 2 [35][36][37], V 2 AlC [20,32,34], Ti 3 SiC 2 [38], Ti 2 AlC [35], Nb 2 AlC [39], as well as solid solutions (Ti,V) 2 AlC [30], (Cr,V) 2 AlC [30], (Cr,Mn) 2 AlC [32], etc.) from transitional metal oxides was reported, only Ti 3 AlC 2 produced by SHS of elemental mixtures (Ti + Al + C) was processed into MXene [40][41][42].The derived Ti 3 C 2 T x MXenes contained pinhole defects and oxide precipitates and exhibited a lower electrical conductivity (~ 3,300 S cm −1 ), as well as diminished electrochemical performance compared to conventionally synthesized Ti 3 C 2 T x (20,000 S cm −1 ) [40].No systematic investigation of the impact of the combustion mode (SHS vs VCS), the effect of oxide precursor, nor synthesis conditions on the quality of the produced MAX phases is available.Overall, insufficient data regarding reproducibility, scalability, and economics of combustion synthesis of MAX phases hinders its implementation for the synthesis of MXene precursors.
Among MXenes beyond Ti 3 C 2 T x , V 2 CT x attracts increasing attention since it possesses a large active area per mass and volume (three atomic layers for V 2 C vs five for Ti 3 C 2 ) Fig. 1 Synthesis process schematics.a Synthesis of V 2 AlC MAX phase by self-propagating high-temperature synthesis (SHS), volume combustion synthesis (VCS), and conventional pressureless sintering.b Synthesis of V 2 CT x MXene by MAX phase etching using aqueous HF + HCl solution followed by delamination using alkylammonium cations and a more chemically active transition metal (V) with multiple oxidation states, resulting in superior volumetric capacitance [24], optical transparency [43], and broadband microwave absorption [44].For large-scale manufacturing of V 2 CT x , combustion synthesis of MAX from oxides rather than elemental mixtures is desirable because the vanadium pentoxide (V 2 O 5 ) is > 25 times cheaper than metallic vanadium [45][46][47].
In this study, we investigate the combustion synthesis of V 2 AlC by SHS, using a research reactor designed for this purpose, and VCS, followed by etching of the MAX phase to produce V 2 CT x .Using V 2 O 5 as a starting material, we demonstrate that combustion synthesis is a reproducible and economical method to produce high-quality MXene with superior resistance to environmental degradation.We also show that the quality of the V 2 CT x MXene produced via combustion synthesis improves with increasing synthesis batch size, indicating the potential for effective process scaling.

MAX phase synthesis Combustion synthesis proceeds according to Reactions:
where the desired reaction 1 is accompanied by reaction 2, producing an undesired intermetallic phase.
Two grades of vanadium pentoxide (either 99.5%, − 325 mesh, Alfa Aesar, USA; or 99%, − 28 mesh, Thermo Fisher Scientific) were mixed with aluminum powder (99.5%, − 325 mesh, Alfa Aesar, USA) and graphite (99%, − 325 mesh, Alfa (1) Aesar, USA) in V 2 O 5 :Al:C atomic ratios of 1:(4.33-8.66):1.Two grades of vanadium pentoxide with vastly different particle sizes were used to ascertain the influence of oxide precursor size on the synthesis process.Al content was varied to investigate the influence of excess Al on the properties of the MAX phase and MXene.The mixtures were homogenized in a Y-mixer at 40 rotations per minute (rpm) using polypropylene bottles and zirconia balls (5 mm diameter), with a ball-to-mixture mass ratio of 10:1.Safety note: the mixture is pyrophoric and appropriate safety precautions must be taken to avoid spontaneous ignition or explosion, especially during and after ball milling [48,49].Rotational speeds above 40 rpm should be avoided, as it can lead to self-ignition of the reactive mixture in the mixer due to the dynamic impact of the milling bodies.Charges of the reactive mixture were either pressed using 13-60 mm steel dies at 0.3 MPa pressure or placed into alumina crucibles without pressing.Combustion synthesis through the SHS approach was carried out in a laboratory SHS reactor developed by the Materials Research Center, Ukraine (Figs. 2, S1).Each combustion synthesis took less than 1 min (see Supplementary Video-Combustion synthesis).To prevent a pressure build-up in the reactor during SHS, a mechanical pressure release valve (a rupture disc rated for a critical pressure of 7 atm) was used.
Combustion synthesis in the VCS mode (in air and argon flow) was performed by inserting a crucible with the reactive mixture into an alumina tube furnace pre-heated to 800 °C.After 10 min, the crucible was extracted from the furnace.CS in this mode is analogous to the previously explored sintering of oxide-based reactive mixtures but is conducted at lower temperatures (800 vs 1350 °C) and shorter annealing durations (10 min vs 1-2 h) [11].VCS was performed either in flowing argon or static air (1 atm) in a tube furnace.
SHS and VCS samples reported in Figs. 3, 4, 5 and 6 were produced using fine V 2 O 5 (Alfa Aesar), 50% excess Al, pressed 10 g pellets, and Ar atmosphere.For the parametric study (Figs.S5-S7, Tables S2-S4), the following Fig. 2 SHS reactor and its thermal model.a General view of the SHS reactor; b cross-section of the reactor; c thermal model of the SHS reactor with 60 g of the reaction mixture showing the temperature field in the reactor during the combustion synthesis.The reactive mixture is marked with red dotted line, crucible-with violet dotted line, graphite stand-with green dotted line, and reactor lidwith white dotted line parameters were varied: V 2 O 5 precursor (99.5%, − 325 mesh, Alfa Aesar, USA; 99%, − 28 mesh, Thermo Fisher Scientific), excess Al content (0-100 at.%), reactive mixture density (pressed vs loose powders), reactive mixture mass (3-60 g), ignition mode (SHS vs VCS), and synthesis atmosphere (air vs Ar).
Combustion products were crushed in a mortar and subsequently in a Y-mixer for 4 h at 100 rpm using polypropylene bottles and zirconia balls (5 mm diameter, ball-tomixture ratio of 10:1) and sieved using a 325 mesh sieve.To eliminate excess Al and intermetallic impurities, the crushed combustion powders were washed with 10 ml 12 M HCl per gram of powder in an ice bath for 24 h, washed with DI water through a vacuum filtration setup (Celgard 2500 membrane, pore size 200-300 nm), and dried in a vacuum oven at 80 °C overnight.It is noted that the HCl washing step is highly exothermic and releases significant gas.Care should be taken to slowly add the powder.
V 2 AlC was also produced through a conventional pressureless sintering approach as described previously [24].V (99%, − 325 mesh, Alfa Aesar, USA), Al, and C powders were mixed in a 2:1.1:0.9 atomic ratio by ball-milling using 10 mm yttria-stabilized zirconia balls (2:1 ball:powder mass ratio) in polypropylene bottles at 60 rpm for 18 h to ensure a homogeneous mixture of the powders.The powder mixture was annealed under Ar flow in a high-temperature tube furnace (Carbolite Gero) at 1550 °C for 2 h with a heating and cooling rate of 3 °C min −1 .Afterward, the sintered compact was crushed with a mortar and pestle, sieved using a 325 mesh sieve, and washed with HCl using the above-described conditions.

Synthesis of MXenes
MXenes were synthesized by selective etching of Al from the V 2 AlC MAX (Fig. 1b) [24].Typically, an etchant containing 12 ml of 48 wt% HF and 8 ml of 37 wt% HCl was prepared and 1 g of the precursor MAX was gradually added.The mixture was stirred for 48 h at 40 °C and 300 rpm, followed by washing with deionized water through  4 SEM micrographs of a, c ML V 2 CT x and e, g, i delaminated V 2 CT x MXene flakes supported on porous anodic aluminum oxide; XRD patterns of the corresponding b, d ML V 2 CT x and f, h, j free-standing V 2 CT x films, produced from MAX synthesized using the e, f SHS, g, h VCS, i, j conventional pressureless sintering approaches.Insets on the right show the visual appearance of corresponding films repeated cycles of centrifugation at 3500 rpm for 3 min, and decantation of the supernatant, until the pH of the mixture was neutral.Then a mixture of 10 ml of 35 wt% tetramethylammonium hydroxide (TMAOH) solution and 10 ml of DI water was added to the resultant multilayer MXene, followed by stirring at 30 °C for 6 h at 300 rpm.The mixture was then centrifuged at 10,000 rpm for 10 min, the supernatant decanted, DI water added, and the precipitate redispersed via shaking.This procedure was repeated 4 times to remove excess TMAOH.To separate the delaminated MXene from

Combustion synthesis of V 2 AlC and its topochemical conversion to V 2 CT x MXene
Figure 2 shows the photograph and the cross-sectional view of the SHS reactor designed and built for SHS synthesis of MAX phases.The reaction is initiated by resistance heating of a metallic coil, which is pressed against the body of the reactive powder mixture (Fig. 2b).The sample is placed in a ceramic crucible, which is in turn placed in a graphite holder separated from the reactor's wall by a ceramic gasket to prevent overheating of the reactor (Fig. 2c).During the synthesis, a vast amount of heat is quickly released, so mismanagement of thermal load can compromise the structural integrity of the reactor.Thermal modeling of the SHS process (Fig. 2c) shows the temperature distribution as a result of the combustion of 60 g of reactive mixture.This amount was determined to be close to the maximum amount for the safe operation of the reactor for the given SHS process.A video recording of the SHS process is available in Supplementary Video 1, and additional drawings of the SHS reactor are provided in Fig. S1.V 2 AlC MAX samples produced using the SHS, VCS, and pressureless sintering exhibit different morphologies, as shown by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses (Fig. 3).Pronounced phase separation occurs during SHS, with the MAX phase precipitating from spheroidized melt droplets and forming spherical beads (Fig. S2 a,d,e,f).VCS showed no tendency for phase separation, with elongated V 2 AlC grains and Al 2 O 3 closely intermixed in the combustion products (Fig. S2b).Furnace synthesis produced nearly equiaxial MAX phase grains (Fig. S2c).HCl washing reduced the mass of combustion products by 20-25% due to the dissolution of excess Al and V 3 Al intermetallic (Fig. S2g).The microstructure of milled powders was similar for all synthesis routes (Fig. 3a, c, e).
Depending on the synthesis conditions, the HCl-washed combustion products contained up to 43% V 2 AlC, the rest being Al 2 O 3 .No significant differences in the MAX phase yield were found between SHS and VCS products.If necessary, the alumina content in the combustion products can be reduced by the substitution of Al as a reducing agent by Mg or Ca [34] and by using the difference between the densities of Al 2 O 3 and V 2 AlC to separate the phases during combustion synthesis (gravity-assisted separation) [39,52].However, previous reports did not find any negative effects of Al 2 O 3 by-product on the properties of MXenes [18].No binary carbide impurities were present in the combustion products (discussed in more detail in the "Parametric study" section in the SI).This corresponds to 75 ± 5% conversion of V from the initial V 2 O 5 to V 2 AlC MAX phase, which is above the previously reported 65% conversion in metallothermic combustion synthesis (mainly due to the absence of VC x impurities present in previous work) [52].
For Ti 3 C 2 T x , the properties of MXene are known to depend significantly on the synthesis of the parent MAX phase [8,9].Similarly, the synthesis of V 2 AlC with different morphology, stoichiometry, and particle size is expected to affect the quality of the resultant V 2 CT x MXene.Mixed-acid (HF + HCl) etching of combustion products yielded multilayer MXenes (ML-V 2 CT x ) with a typical accordion-like structure (Fig. 4a-d).Delamination of the ML-V 2 CT x with TMAOH and subsequent washing produced a colloidally stable solution of single-and few-layer MXene flakes.SEM imaging of flakes (Fig. 4e, g, i) suggests that SHS-V 2 CT x flakes have fewer pinholes compared to VCS and conventional V 2 CT x .Vacuum filtering of delaminated V 2 CT x flakes produced films with shiny-bronze color (Fig. 4f, h, j), good flexibility, and conductivity of 590-650 S cm −1 (detailed conductivity data is provided in SI).

Flake size and environmental stability of V 2 CT x in solutions.
DLS measurements (Fig. 5a) show that SHS V 2 CT x has a slightly larger flake size (D avg = 1770 nm) and narrower size distribution compared to VCS V 2 CT x (D avg = 1600 nm) and conventional V 2 CT x produced at optimal conditions (D avg = 1100 nm).Optimal conditions for conventional V 2 AlC etching involved longer etching duration (72 h vs 48 h) and higher temperature (50 vs 40 °C) as compared to combustion-derived V 2 AlC.When conventional V 2 AlC was etched for 48 h at 40 °C, the flake size and yield decreased significantly (400 nm, yield ~ 21%), suggesting incomplete etching of the MAX phase particles.Conversely, etching of combustion-derived MAX phase for 72 h at 40 °C resulted in over-etching of the MXenes.Previous works produced V 2 CT x with D avg from 280 to ~ 1200 nm [24,43,[53][54][55][56][57].The increased flake size for SHS V 2 CT x can be related both to the morphology of SHS-V 2 AlC precursor and milder etching conditions.UV-Vis spectra of fresh solutions of V 2 CT x (0.01 mg/ml; low concentration accelerated degradation) produced from SHS, VCS, and conventional precursors were identical, with an absorbance peak at 270 nm and a broad "tail" at 500-1000 nm (Fig. 5b).Oxidation/hydrolysis of colloidal V 2 CT x results in a decay of these features, as well as the emergence and sharp rise of a peak at 200 nm corresponding to vanadium oxides.In situ UV-Vis spectroscopy measurements demonstrated notably improved stability of SHS-derived V 2 CT x compared to VCS-derived and conventional V 2 CT x (Fig. 5c) in dilute colloids.In VCS and conventional V 2 CT x (0.01 mg/ml), the 270 nm peak completely decayed after 5 h at room temperature, whereas SHS-derived V 2 CT x still retained a distinct 270 nm peak after 9 h, and the rise in the 200 nm peak was less pronounced.
Long-term environmental stability tests of concentrated V 2 CT x colloids (12 mg/ml) prepared by etching of MAX phase precursors for 48 h at 40 °C revealed improved solution stability of SHS-V 2 CT x , which showed only minor signs of degradation in its UV-Vis absorption spectra after 114 days (Fig. 5d).All tested MXene samples were stored in filled vials with no headspace.VCS MXene after 114 days had a substantially lower V 2 CT x peak intensity, and a higher oxide peak intensity compared to SHS V 2 CT x (Fig. 5e).Conventional V 2 CT x degraded completely after 67 days (Fig. 5f).Accelerated degradation of conventional V 2 CT x as compared to combustion-derived analogs is presumably caused by suboptimal etching conditions which resulted in a smaller flake size (400 nm).After 114 days of storage, SHS V 2 CT x flakes (Fig. 5g) have formed MXene scrolls, whereas VCS V 2 CT x contained mostly oxide precipitates with a few partially oxidized MXene flakes (Fig. 5h, j).

Environmental and thermal stability
of vacuum-filtered V 2 CT x films.
The environmental stability of vacuum-filtered V 2 CT x films was ascertained after 120 days of storage in a desiccator and open air.The surface chemistry of the MXene films was investigated using XPS where the probing depth is on the order of 5-10 nm.XPS indicates that pristine V 2 CT x MXene films produced using the SHS and VCS techniques exhibit similar surface chemistry.In both cases, the metallic C-V component at ~ 282 eV in the C 1s spectra confirms MXene.
There is also minimal surface oxidation, as shown by the strong contribution from the V 1+ component at ~ 513 eV and the weak contribution from the V 5+ component at ~ 517 eV in the V 2p spectra (Fig. 6 a,b).The SHS and VCS-derived MXenes had similar compositions, V 2 C 0.86 O 1.19 F 0.57 and V 2 C 0.82 O 1.12 F 0.55 (normalized to V = 2), based on quantitative XPS analysis.The sub-stoichiometry in the carbon-tovanadium ratio may result from the presence of oxygen in the carbon sublattice, defects introduced during the etching process, and uncertainty in the curve-fitting procedure including the shape of asymmetric and satellite components, among other factors [10,58,59].Furthermore, XPS indicates that while the surface of MXene films was oxidized after storage in a desiccator for 120 days, SHS-derived MXene was the most stable of all three materials under study, as shown by the smaller decrease in the C-V and increase in the V 5+ components (Fig. 6 c,d, Table S1).
The SHS-V 2 CT x was the only film that retained its metallic bronze shine after storage both in the desiccator and in the air (Fig. 6e), with no visible signs of oxidation, while other films showed oxidation-associated discoloration.This effect might have been caused by higher carbon stoichiometry in the MAX phase produced by SHS, as indicated by the slightly increased value of the c-lattice parameter (c = 13.1729Å) for SHS-V 2 AlC as compared to VCS-V 2 AlC (c = 13.1422Å) and conventional V 2 AlC (c = 13.1455Å), as well as the largest flake size.More rapid degradation of conventional V 2 CT x might be caused by a smaller flake size (400 nm), as well as the presence of residual water in the tested films (samples were not vacuum dried before testing).In any case, the environmental stability of the MXene samples produced from SHS MAX was the same or better than the best V 2 CT x reported to date [24].
Since SHS-V 2 CT x showed the best properties, we further tested the temperature stability of vacuum-filtered SHS-V 2 CT x films by thermal analysis in air and argon flow (Fig. S4).The onset of degradation in argon and oxidation in the air was somewhat higher compared to V 2 CT x reported in previous studies (650 vs 600 °C in argon, 450 °C vs 300-332 °C in the air) [60,61].

Electrochemical properties
Since vanadium-based MXenes have been extensively studied in energy storage applications, the energy storage capability of SHS-V 2 CT x was evaluated by cyclic voltammetry (Fig. 7).The potential window for SHS-V 2 CT x in the sulfuric acid electrolyte was determined to be from − 0.3 to − 0.9 V (vs Hg/HgSO 4 electrode).Cyclic voltammetry (CV) tests of the SHS-V 2 CT x electrodes (Fig. 7a) showed a pair of broad and highly reversible redox peaks at ≈ − 0.45 V and a highly capacitive response up to a very high scan rate of 1 V s −1 .The highest specific capacitance of ≈ 400 F g −1 was found at a scan rate of 2 mV s −1 .This gravimetric capacitance is 70-150 F g −1 higher than the capacitance of freestanding Ti 3 C 2 T x films [62] and about two times higher than the specific capacitance of 1 T MoS 2 electrodes [63].At a scan rate of 5 mV s −1 , the gravimetric capacitance of SHS-V 2 CT x was approximately 375 F g −1 (Fig. 7b), which is similar to the capacitance of state-of-the-art conventional V 2 CT x films restructured through ion exchange (420 F g −1 at 5 mV s −1 ) 1 3 [53,64].Importantly, such performance for SHS-V 2 CT x was achieved without capacitance-boosting ion exchange procedure and at a higher mass loading (2.35 vs 1.2-1.4mg cm −2 ) [63,64], which suggests that SHS-V 2 CT x is well-suited for pseudocapacitive energy storage.

Reproducibility and scalability of combustion synthesis
We performed a parametric study coupled with an analysis of variance (ANOVA), which ascertained the reproducibility of the measurements and statistical significance of ignition mode (SHS vs VCS), size of V 2 O 5 precursor powders, amount of excess aluminum in the reactive mixture (0-100 at.%), the density of reactive mixture (loose powder vs pressed pellets), synthesis environment (argon vs air), and the mass of reactive mixture charge (3-60 g).The results and their detailed discussion can be found in Supplementary Information (Figs.S4-7, Tables S2-4).The best results were achieved by conducting combustion synthesis in SHS mode, using fine oxide powders and a 50% excess Al.Scaling up of the SHS process produced larger MXene flakes and improved the conductivity of the films.Flake size increased from 1170 to 1770 nm and conductivity increased from 340 to 560 S cm −1 with the transition from 10 to 60 g batches: etching for 48 h at 40 °C.The environment did not significantly affect combustion products at sufficiently high excess aluminum contents (≥ 50%).While the synthesis of MAX can be conducted in the open air, if appropriate heat protection measures are taken (see Fig. 1), the effect of the environment of oxygen content in the MXene lattice will need a separate study.The use of VCS was complicated by the partial scattering of hot combustion products, leading to thermal shock and cracking of the alumina tube.The problem is exacerbated when the mass of the reactive mixture increases, thus hindering the scaling of the process.Therefore, the synthesis of MAX phases in SHS mode can be scaled up more easily.For scaling up of VCS, we advise using a box furnace or other more accommodating furnace, rather than a tube furnace.Since combustion synthesis of multiple MAX phases from transition metal oxides has been reported, we anticipate that other precursors for MXenes can be produced from inexpensive oxides using energy-saving SHS.This will allow a significant decrease in the cost of MXenes by eliminating the need for expensive metal powders (Mo, V, Nb, etc.) and long high-temperature treatments.It's important to mention that aluminum oxide formed during the aluminothermic reduction cannot be easily separated from the MAX by chemical methods.It can only be removed completely during the MXene delamination.Thus, SHS precursors can be used for the synthesis of MXene inks, fibers, aerogels and films, but not MXene powders that are used without delamination.

Conclusions
Two variations of aluminothermic oxide reduction, selfpropagating high-temperature synthesis (SHS) and volume combustion synthesis (VCS), were explored for V 2 AlC MAX phase synthesis from inexpensive vanadium pentoxide precursors.Samples produced by both combustion synthesis methods, as well as conventional high-temperature pressureless sintering had very similar structure, composition, and almost identical UV-Vis, Raman, and XPS spectra.While both processes are viable and offer cost and time savings compared to conventional high-temperature pressureless sintering of V 2 AlC MAX, SHS under the studied conditions produced a higher-quality V 2 CT x MXene.This method offers better productivity and economy of scale, utilizing an order of magnitude cheaper vanadium source, compared to pressureless sintering.It is important that the increase in the batch size from 10 to 60 g only increased the flake size and improved MXene properties, suggesting good scalability of the process.The reactor size needs to be adjusted to the amount of the material reacted to control the heat released during the reaction.Among the V 2 AlC precursors tested in this study, the SHS MAX yielded V 2 CT x with the largest flake size (D avg = 1770 nm), best stoichiometry, and the highest environmental stability.The electrical and electrochemical properties of SHS-V 2 C T x were similar to the MXenes derived from the conventional MAX phase, suggesting that the scale-up and cost reduction of MXene manufacturing can be achieved without sacrificing the material quality and properties.

Fig. 3
Fig. 3 SEM micrographs and XRD patterns showing the microstructure and phase composition of MAX phase powders prepared by a, b SHS, c, d VCS, and e, f pressureless sintering

Fig. 5 Fig. 6
Fig. 5 Characterization of V 2 CT x MXenes derived from MAX phases produced by SHS, VCS, and pressureless sintering.a Flake size distribution, b UV-Vis spectra of as-synthesized dispersions, off-set for clarity (they overlap completely at the same MXene concentration), c absorbance over initial absorbance (A/A 0 ) of the 270 nm peak tracked

Fig. 7
Fig. 7 Electrochemical characterization of SHS-V 2 CT x .a Cyclic voltammograms (CVs) and b gravimetric capacitance of TMAOH-delaminated and vacuum-filtered SHS-V 2 CT x electrodes in 3 M H 2 SO 4 electrolyte in different potential windows