Why MXenes Matter

The world is racing to find materials that can solve several big problems at once:

• Producing clean hydrogen as a green fuel

• Storing electrical energy quickly and efficiently

• Capturing CO₂ to help reduce climate change

MXenes are a new family of two-dimensional (2D) materials that are attracting a lot of interest for exactly these kinds of applications. They are made by selectively removing certain atoms from layered ceramic compounds called MAX phases. The result is an ultra-thin, electrically conductive material with a lot of surface area and many active sites on its surface.

In this study, the focus is on one specific MXene: Ti₃C₂Tₓ.

• Ti = titanium

• C = carbon

• Tₓ = surface terminations (like –F, –OH, =O) that form during synthesis

The researchers wanted to do three things at once with a single batch of Ti₃C₂Tₓ MXene:

1. Use it as a catalyst for the hydrogen evolution reaction (HER) – the reaction that produces hydrogen gas from water.

2. Use it as an electrode material in supercapacitors – devices that can store and release energy very quickly.

3. Use it as a CO₂ adsorbent – a material that can capture and hold CO₂ gas.

On top of that, they also carried out a techno-economic analysis to estimate how much it would cost to produce this MXene on a larger scale.

The key result is very important:

The same Ti₃C₂Tₓ MXene, made in one batch, can successfully work in all three applications — hydrogen generation, energy storage, and carbon capture — at a realistic production cost of about 2.83 €/g.

This shows Ti₃C₂Tₓ is not just a niche material but a multifunctional platform for future clean-energy and environmental technologies.

2. What Are MXenes and What Is Special About Ti₃C₂Tₓ?

MXenes are 2D materials obtained from MAX phases. MAX phases have the general formula:
Mₙ₊₁AXₙ, where:

• M = early transition metal (e.g., Ti, Nb, V, Mo)

• A = group 13 or 14 element (e.g., Al, Si)

• X = C and/or N (carbon and/or nitrogen)

To get a MXene, the A-layer (like Al) is selectively etched away, leaving behind a layered carbide or nitride sheet (the MXene) with surface terminations.

For Ti₃C₂Tₓ:

• The starting MAX phase is Ti₃AlC₂.

• After chemical etching, the Al atoms are removed.

• The remaining layered structure is Ti₃C₂, and its surface becomes functionalized with groups like –F, –OH, and =O. These are denoted collectively as Tₓ.

Why Ti₃C₂Tₓ is so attractive:

• High electrical conductivity – good for catalysis and electrodes.

• Layered structure with tunable spacing – good for ion transport and gas adsorption.

• Surface functional groups – provide active sites for reactions or adsorption.

• Good chemical stability in many environments (especially after proper synthesis and treatment).

Because of these features, Ti₃C₂Tₓ has already been explored for:

• Batteries and supercapacitors

• Electromagnetic shielding

• Water treatment

• Sensors and catalysis

• CO₂ capture and gas separation

This study takes a more integrated approach: Can one properly prepared Ti₃C₂Tₓ MXene batch serve all three purposes – HER, supercapacitors, and CO₂ capture – effectively?

The answer turns out to be yes.

3. How Ti₃C₂Tₓ MXene Was Synthesized

The researchers used the Ti₃AlC₂ MAX phase powder as a starting material and applied a modified HF (hydrofluoric acid) etching method.

Key steps of the synthesis:

1. Etching

   • Ti₃AlC₂ powder is treated with HF solution.

   • Conditions: 40 °C, 6 hours.

   • The HF selectively dissolves the Al layers in Ti₃AlC₂.

   • This opens up the layered structure and creates the Ti₃C₂Tₓ MXene.

2. Washing and Neutralization

   • After etching, the mixture is centrifuged several times.

   • It is washed repeatedly with deionized water until the liquid (supernatant) reaches neutral pH.

   • This step removes excess acid and soluble byproducts like AlF₃.

3. Separation and Recovery

   • The solid Ti₃C₂Tₓ MXene is collected from the suspension.

   • It can be used either as dried powder or as an ink or dispersion, depending on the application.

This HF-based process is relatively mild (40 °C, 6 h) compared to some traditional methods and is designed to be more reproducible and scalable.

4. How the Material Was Characterized

To understand what they actually made, the researchers performed extensive physicochemical characterization using many techniques:

4.1 Surface Area and Porosity (BET, BJH, DFT)

• For the starting Ti₃AlC₂ MAX phase:

  • Surface area ~ 1.06 m²/g (very low).

  • Average pore size ~ 32.8 nm (large mesopores).

  • Some narrow micropores below 1 nm.

• For the final Ti₃C₂Tₓ MXene:

  • Surface area ~ 26.7 m²/g (a big increase).

  • Average pore size ~ 16.2 nm (smaller mesopores).

  • Significant pore volume in the ultramicropore range (0.5–0.8 nm), which is important for CO₂ adsorption.

What this means:
After etching, the MXene has a much larger surface area and a more favorable pore structure, with both micro- and mesopores. This is ideal for:

• Catalytic reactions (more active sites)

• CO₂ adsorption (more accessible pores)

• Charge storage in supercapacitors (better ion access)

4.2 Crystal Structure (XRD)

X-ray diffraction (XRD) was used to compare Ti₃AlC₂ and Ti₃C₂Tₓ.

• In the Ti₃AlC₂ MAX phase, the XRD pattern shows sharp peaks at specific angles typical of a well-ordered layered structure.

• After HF etching, in Ti₃C₂Tₓ MXene:

  • Several peaks shift to lower angles, meaning the interlayer spacing has increased (the layers are further apart).

  • Peak intensities become broader and weaker, showing that the structure is more disordered.

  • The disappearance of certain peaks (like the one at 38.7°) confirms that the Al layers have been removed.

  • A new peak at around 27° indicates the formation of Ti₃C₂(OH)₂ and supports the presence of surface –OH groups.

  • Small additional peaks correspond to AlF₃ residues, indicating some remaining Al-containing byproducts.

Conclusion: XRD clearly demonstrates the transformation from the dense MAX phase to a layered MXene with expanded interlayer spacing and surface functionalization.

4.3 Raman Spectroscopy

Raman analysis provides further confirmation:

• In Ti₃AlC₂, certain peaks are associated with vibrations involving Ti and Al atoms.

• After etching, in Ti₃C₂Tₓ:

  • Peaks related to Al vibrations are strongly reduced or disappear, again confirming Al removal.

  • New broad peaks appear in the 1000–1800 cm⁻¹ region, corresponding to D and G bands of graphitic carbon.

  • These features suggest:

    • Increased exposure of carbon on the surface

    • Some structural disorder

    • A graphene-like character in the thin MXene layers

This is consistent with a 2D, carbon-rich, Ti-based structure with defects and functional groups — ideal for electrochemical applications.

4.4 Morphology (SEM and HRTEM)

SEM (Scanning Electron Microscopy):

• Ti₃AlC₂ shows tightly packed, dense layers – classic MAX structure.

• Ti₃C₂Tₓ MXene shows:

  • More open, well-spaced parallel layers.

  • Interlayer spacing increases from ~0.16 μm (MAX) to ~0.38 μm (MXene).

  • Small spherical particles and thin fragments attached to the layers, likely AlF₃.

HRTEM (High-Resolution TEM):

• Reveals thin, flexible, layered nanosheets, similar to other 2D materials like graphene.

• Fast Fourier Transform (FFT) patterns confirm the presence of specific crystal planes of Ti₃C₂Tₓ (e.g., (101) planes), meaning the MXene retains a well-defined crystalline structure even after treatment.

• The layered structure, defects, and surface groups together contribute to better gas adsorption and faster electron transport.

4.5 Surface Chemistry (XPS)

X-ray photoelectron spectroscopy (XPS) was used to analyze which elements and chemical bonds exist on the surface:

• C 1s region:

  • Peaks for Ti–C (carbidic carbon) confirm the carbide backbone.

  • Peaks for C–O, C=O, and O–C=O indicate surface oxidation and oxygen-containing groups.

• Ti 2p region:

  • Peaks for Ti–C, confirming the MXene core.

  • Signals from Ti²⁺, Ti³⁺, Ti⁴⁺, as well as Ti–O and Ti–F, showing partial oxidation and fluorine terminations.

• O 1s region:

  • Peaks for Ti=O (titanium oxides) and oxygen-containing carbon species.

  • Higher binding-energy peaks for adsorbed H₂O and –OH groups, reflecting hydrophilicity.

• F 1s region:

  • Peaks for Ti–F terminations.

  • Peaks for AlFₓ species (residual fluorinated aluminum compounds).

Altogether, XPS shows that Ti₃C₂Tₓ is not a perfectly clean carbide surface. Instead, it has:

• Mixed terminations: –F, –OH, =O

• Some oxidized titanium

• Residual AlFₓ species

This is typical for HF-etched MXenes and is actually beneficial, as these terminations:

• Increase hydrophilicity

• Provide redox-active sites

• Enhance electrochemical performance

5. Application 1: Hydrogen Evolution Reaction (HER)

For HER testing, Ti₃C₂Tₓ MXene was deposited onto carbon felt to make a composite electrode. The same was done for Ti₃AlC₂ for comparison.

Key parameters:

• Electrolyte: 1 M H₂SO₄

• Reference electrode: Ag/AgCl (converted to RHE scale)

• Counter electrode: graphite felt

• Method: Linear sweep voltammetry (LSV), Tafel analysis, EIS, chronoamperometry

Performance comparison (MXene vs MAX):

• Onset potential (HER starts):

  • MAX (Ti₃AlC₂): ~ −698 mV

  • MXene (Ti₃C₂Tₓ): ~ −511 mV (much better; lower overpotential)

• Current density at −760 mV:

  • MAX: ~ 20 mA cm⁻²

  • MXene: 190 mA cm⁻² (almost 10× higher)

• Tafel slope:

  • MXene: 184 mV dec⁻¹

  • MAX: 210 mV dec⁻¹

A lower Tafel slope and lower onset potential mean the MXene catalyst is more efficient and faster than the MAX phase, although still not as good as platinum.

Electrochemical impedance spectroscopy (EIS):

• MXene shows:

  • Lower series resistance (better conductivity).

  • Steeper low-frequency slope (faster charge transfer and better ion accessibility).

The removal of Al and the formation of a layered, conductive MXene structure significantly improve charge transport.

Double-layer capacitance (Cdl):

• MXene composite: ~108.8 mF cm⁻²

• MAX composite: ~12.4 mF cm⁻²

This almost 9× increase in Cdl suggests many more electrochemically active sites on MXene, which explains its higher HER activity.

Durability:

• Chronoamperometry at −490 mV vs RHE for 10 hours:

  • Current density increases from 5 to 11 mA cm⁻².

  • This shows stable performance in acidic conditions.

  • MAX phase is not stable in acid, so long-term tests were not done for it.

Conclusion for HER:
Ti₃C₂Tₓ MXene is a promising, robust non-precious catalyst for hydrogen evolution in acidic media, significantly outperforming the parent MAX phase and showing suitable stability for practical use.

6. Application 2: Supercapacitor Electrode

The same MXene was tested as an electrode material in 3 M KOH using a three-electrode setup with a flexible graphite substrate.

Key findings:

• Cyclic voltammetry (CV):

  • Ti₃C₂Tₓ curves show large enclosed areas (high capacitance) and acceptable shape at scan rates from 2 to 5 mV s⁻¹.

  • The shape is not perfectly rectangular, indicating a mix of:

    • Electric double-layer capacitance (EDLC)

    • Pseudocapacitance from surface redox reactions (via –O and –OH groups).

• Quantitative analysis of contributions:

  • At 5 mV s⁻¹, about 86.9 % of the capacitance comes from EDLC, and 13.1 % from pseudocapacitance.

  • This combination allows both high rate capability and enhanced capacitance.

• Galvanostatic charge–discharge (GCD):

  • Ti₃C₂Tₓ has longer discharge times than Ti₃AlC₂ at the same current density, meaning it stores more charge.

  • GCD curves deviate from perfect triangles, again indicating pseudocapacitive behavior.

• Specific areal capacitance (from CV and GCD):

  • From CV at 2 mV s⁻¹:

    • Ti₃C₂Tₓ electrode: 218.17 mF cm⁻²

    • Graphite foil: 57.25 mF cm⁻²

  • From GCD at 2 mA cm⁻²:

    • Ti₃C₂Tₓ: 411.1 mF cm⁻²

    • Ti₃AlC₂: ~68.0 mF cm⁻²

So, Ti₃C₂Tₓ offers roughly 6× higher capacitance than Ti₃AlC₂ under the same conditions.

• Rate dependence:

  • Capacitance decreases as scan rate or current density increases, which is normal since ions have less time to penetrate deeper pores at high rates.

• Charge storage mechanism:

  • Analysis of log(current) vs log(scan rate) (the “b-value” method) and diffusion/capacitive separation shows that:

    • For Ti₃C₂Tₓ, diffusion-controlled contributions dominate at low scan rates (over 90 % at 2 mV s⁻¹).

    • At higher scan rates, capacitive contributions increase, but diffusion still plays a major role.

  • For Ti₃AlC₂, the diffusion contribution is lower at comparable scan rates.

• Impedance (Nyquist plots):

  • Ti₃C₂Tₓ shows the steepest slope at low frequencies, reflecting the best ion transport and lowest interfacial resistance.

Conclusion for supercapacitors:
Thanks to its higher surface area, wider interlayer spacing, and abundant functional groups, Ti₃C₂Tₓ MXene delivers excellent capacitive performance, high areal capacitance, and fast kinetics, making it a strong candidate for high-rate supercapacitors and energy storage devices.

7. Application 3: CO₂ Adsorption and Carbon Capture

The same Ti₃C₂Tₓ MXene was also tested for CO₂ capture in gas-phase adsorption experiments.

CO₂ uptake (at 1 bar):

• Ti₃C₂Tₓ MXene:

  • 0.80 mmol g⁻¹ at 0 °C

  • 0.66 mmol g⁻¹ at 25 °C

• Ti₃AlC₂ MAX:

  • Only about 0.05 mmol g⁻¹

So, the MXene shows more than an order of magnitude higher CO₂ capacity than its precursor MAX phase.

Why MXene performs better:

• Higher surface area

• More ultramicropores that fit CO₂ molecules

• Rich surface terminations (–F, –O, –OH) offering multiple interaction types

Types of interactions involved:

1. Physisorption

   • Van der Waals forces between CO₂ and the MXene surface.

   • Particularly relevant at lower energies and in micropores.

2. Chemisorption

   • Stronger interactions at specific sites, primarily oxygen and hydroxyl groups acting as Lewis bases or acids.

   • These sites allow CO₂ to bind more strongly.

3. Intercalation

   • The layered structure and increased interlayer spacing (especially due to bulky –F groups) allow CO₂ molecules to enter between layers.

The isosteric heat of adsorption (Qₛₜ) is in the range of 25–33 kJ mol⁻¹, which:

• Is higher than purely physical adsorbents like activated carbons (<20 kJ mol⁻¹)

• Is lower than strongly chemisorptive materials (>50 kJ mol⁻¹)

This indicates a mixed physisorption–chemisorption mechanism.

Isotherm modeling:

Several three-parameter isotherm models were applied, and the best fits were obtained with:

• Radke–Prausnitz model

• Sips model

These models are well-suited for heterogeneous surfaces with a mix of strong and weak adsorption sites, exactly what Ti₃C₂Tₓ MXene has.

Conclusion for CO₂ capture:
Ti₃C₂Tₓ MXene is an effective CO₂ adsorbent with a balance of strong and weak binding sites, good capacity at both 0 °C and 25 °C, and a realistic mechanism where oxygen-based groups drive stronger binding and fluorine terminations help with physisorption and interlayer access.

8. Techno-Economic Analysis: Can We Scale This?

A crucial part of the study is the cost analysis of producing Ti₃C₂Tₓ MXene on an industrial scale.

The estimated production cost is about 2.83 €/g, taking into account:

• Raw materials:

  • Ti₃AlC₂ MAX phase

  • HF

  • Water and other basic chemicals

• Energy consumption (heating, stirring, centrifugation, drying)

• Labor costs

• Equipment depreciation

• Facilities and infrastructure

• Waste handling and disposal

• Basic characterization steps

This cost is reasonable for a high-performance multifunctional material and suggests that scaling up MXene production is economically feasible, especially if the same material can be used in multiple application areas (as shown here).

9. Overall Significance and Outlook

This work demonstrates something very powerful:

A single batch of Ti₃C₂Tₓ MXene, prepared via optimized HF etching, can be used as-is for three very different but critical applications:

• Hydrogen evolution reaction (HER)

• Supercapacitor electrodes

• CO₂ adsorption for carbon capture

Key performance highlights:

• HER:

  • Onset potential: −511 mV vs RHE

  • Current density: 190 mA cm⁻² at −760 mV

  • Tafel slope: 184 mV dec⁻¹

  • Good stability in acidic media

• Supercapacitors:

  • Areal capacitance: 411.1 mF cm⁻² (GCD, 2 mA cm⁻²)

  • High EDLC contribution with additional pseudocapacitance

  • Fast kinetics and low interfacial resistance

• CO₂ capture:

  • Uptake: 0.80 mmol g⁻¹ (0 °C), 0.66 mmol g⁻¹ (25 °C) at 1 bar

  • Mixed physisorption–chemisorption behavior

  • Heterogeneous adsorption sites, well described by Radke–Prausnitz

• Cost and scalability:

  • Estimated production cost ~2.83 €/g

  • Process uses relatively mild conditions (40 °C, 6 h) and standard equipment.

Why this matters for energy and environment:

• For hydrogen production, Ti₃C₂Tₓ offers a realistic non-precious alternative to platinum in some systems.

• For energy storage, it supports fast charging/discharging and high power density, ideal for supercapacitors and possibly hybrid devices.

• For carbon capture, it provides a tunable, regenerable solid sorbent suitable for various CO₂ separation processes.

Because MXenes like Ti₃C₂Tₓ are 2D, conductive, chemically tunable, and relatively scalable, they are excellent platform materials. They can be further:

• Functionalized with other atoms or molecules

• Combined with oxides, carbons, or polymers to make composites

• Used as building blocks in membranes, electrodes, or catalysts

This study shows that we do not necessarily need separate materials for each function. A well-designed MXene can support multiple roles in the clean energy and environmental technology chain – from generating clean fuels to storing energy and capturing greenhouse gases.