In recent years, the demand for better energy storage has exploded. From electric vehicles to portable electronics and smart grids, we need devices that can charge fast, deliver high power, and last a long time. Batteries are great for storing a lot of energy, but they often charge slowly and degrade over time. Supercapacitors, on the other hand, charge and discharge very quickly and are highly durable, but their energy storage is usually lower than that of batteries.

One of the most active research topics today is finding new electrode materials that can push supercapacitors to the next level—higher capacitance (energy storage), better cycling stability, and good conductivity. Among the many candidates, MXenes have become a star material family.

This blog explains a research study that focuses on a specific MXene, Nb₂C (niobium carbide MXene), and how its performance can be greatly improved by doping it with nickel (Ni). The paper combines experimental work and computational calculations to understand why Ni-doped Nb₂C works so well as an electrode for supercapacitors.

We’ll walk through:

• What MXenes are and why Nb₂C was chosen

• How the researchers synthesized Nb₂C and Ni-doped Nb₂C

• What the structural and surface analyses show

• What the computer simulations (DFT) reveal about electronic structure

• How the electrochemical tests prove enhanced energy storage performance

• Why this work matters for future energy storage devices

All in simple, logical language, without assuming you’re an expert.

1. Background: MXenes and the Need for Better Electrodes

1.1. Why supercapacitors matter

Supercapacitors are electrochemical devices that store energy in two major ways:

• Electrochemical double-layer capacitors (EDLCs):
  Energy is stored by the physical adsorption of ions from the electrolyte on the surface of the electrode (no chemical reaction, mainly surface charging).

• Pseudocapacitors:
  Energy is stored through fast, reversible redox reactions at or near the electrode surface (chemically driven charge storage, but still fast).

Compared with batteries, supercapacitors:

• Charge and discharge much faster

• Deliver very high power

• Have long cycle life

However, they typically store less energy per gram. Improving their specific capacitance (how much charge they can store per gram of material) is one of the main goals in this field.

1.2. Why 2D materials and MXenes?

Two-dimensional materials—like graphene, MoS₂, and others—are attractive for energy storage because ions can easily enter between layers, and the large surface area offers many active sites for storing charge.

But many 2D materials have drawbacks:

• Limited conductivity

• Narrow interlayer spacing (harder for ions to move in and out)

• Sometimes hydrophobic (not very friendly to aqueous electrolytes)

MXenes, discovered in 2011, are a newer class of 2D materials. They are made by selectively etching “A” layers from a parent material called a MAX phase, which has the general formula:

Mₙ₊₁AXₙ

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

• A = element like Al, Si, Ga (group IIIA or IVA)

• X = C (carbide), N (nitride), or both (carbonitride)

After etching away the A-layer, what remains are thin layers of M–X (e.g., Nb–C), with surface terminations like –O, –OH, and –F. The resulting MXenes are written as:

Mₙ₊₁XₙTₓ

where Tₓ indicates these surface terminations.

Key advantages of MXenes for supercapacitors:

• High electrical conductivity

• Large surface area

• Hydrophilic surfaces (good contact with aqueous electrolytes)

• Easy ion intercalation between layers

1.3. Why niobium carbide (Nb₂C) and why nickel doping?

The studied MXene here is Nb₂C, derived from the MAX phase Nb₂AlC. Nb₂C already has good potential as an electrode material, but the researchers aim to enhance its energy storage performance further.

A common strategy to improve 2D materials is doping—inserting foreign atoms to tune the:

• Electronic structure

• Surface chemistry

• Interlayer spacing

• Number of active sites

In this work, they choose nickel (Ni) as a dopant because:

• Ni has very good electrochemical activity (rich redox behavior)

• Ni has good electronic conductivity

• The ionic radius of Ni²⁺ (~0.07 nm) is very close to Nb²⁺ (~0.08 nm), making it structurally compatible

The idea is that Ni will introduce more active sites and modify the electronic density of states in a way that improves charge storage.

2. How Nb₂C and Ni–Nb₂C MXene Were Prepared

2.1. Synthesis of Nb₂C MXene

The starting material is Nb₂AlC MAX phase. To convert it into Nb₂C MXene, the researchers:

1. Took 1 g of Nb₂AlC powder (with a certain mesh size).

2. Immersed it in a solution of 50 wt% hydrofluoric acid (HF).

3. Kept it under magnetic stirring at around 55 °C for about 30 hours.

4. After etching, they repeatedly washed the reaction mixture with deionized water, using centrifugation, until the pH of the supernatant reached about 6.

5. Finally, the MXene powder was dried under vacuum at 40 °C for 24 hours.

During this etching process, the Al layers are removed from Nb₂AlC, leaving layered Nb₂CTₓ (where Tₓ includes surface groups like –O, –OH, and –F).

2.2. Synthesis of Ni-doped Nb₂C (Ni–Nb₂CTₓ)

Next, the team produced Ni-doped Nb₂C MXene using a hydrothermal method:

• They prepared an aqueous suspension of Nb₂CTₓ powder in deionized water.

• They dissolved Ni(NO₃)₂·6H₂O (nickel nitrate hexahydrate) in water to create a Ni precursor solution.

• They mixed the MXene suspension and the Ni precursor and stirred them.

• They added ammonia solution dropwise to adjust the pH to about 9.

• This mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 90 °C for 16 hours.

• After cooling, the product was washed with deionized water and dried at 60 °C for 24 hours.

They repeated this process for different Ni loadings (2.5–10 wt%). This allows them to see how different Ni contents affect structure and performance.

3. What the Structural and Surface Analyses Show

The researchers used several standard materials characterization techniques. We’ll only describe what the paper explicitly reports.

3.1. X-ray diffraction (XRD)

XRD was used to verify:

• The structure of the starting MAX phase

• The formation of MXene

• The effect of Ni doping

Key observations:

• The original Nb₂AlC MAX phase shows characteristic diffraction peaks consistent with a hexagonal structure.

• After HF treatment, a peak that is associated with the Al-containing layer disappears, confirming that aluminum has been successfully etched out.

• The main MXene (002) peak shifts to lower angle and broadens, indicating:

  • An increase in the interlayer spacing between MXene sheets

  • An increase in the c-lattice parameter

When Ni is introduced:

• The (002) peak of Ni–Nb₂C MXene shifts even further to a lower angle compared to undoped Nb₂C, consistent with expansion of the interlayer spacing due to Ni intercalation or surface interaction.

• Some peaks corresponding to other planes decrease in intensity with increasing Ni content, suggesting a reduction in crystallite size and increased lattice distortion.

Overall, XRD confirms:

• Successful conversion of MAX to MXene

• Successful incorporation of Ni and structural changes (increased spacing, possible defects)

3.2. Raman spectroscopy

Raman spectroscopy helps reveal bonding and structural changes.

Comparing MAX and MXene:

• Several Raman peaks broaden and shift in MXene relative to MAX, indicating changes in bonding and local environment.

• The changes are consistent with removal of Al and strengthening of Nb–C bonds.

• The ratio of the D-band to G-band intensity (ID/IG) slightly decreases from MAX to MXene, suggesting that the MXene has slightly fewer structural defects compared to the starting MAX phase.

This indicates that the etching produced relatively ordered Nb₂C layers rather than heavily damaged structures.

3.3. Surface area (BET analysis)

They used nitrogen adsorption to determine the specific surface area.

• Undoped Nb₂CTₓ MXene: about 5.21 m²/g

• Ni–Nb₂CTₓ (5 wt% Ni): about 18.02 m²/g

So, the surface area more than triples upon Ni doping. The isotherm pattern indicates a type typical of solids with increasing gas uptake at higher pressure.

Why is this important?

• Larger surface area means more active sites for ion adsorption and redox reactions.

• This directly helps increase the specific capacitance of the electrode.

3.4. Morphology: SEM and TEM

Scanning electron microscopy (SEM) shows:

• Nb₂CTₓ MXene: layered morphology with sheet-like structures, somewhat like exfoliated graphite.

• Ni–Nb₂CTₓ MXene: still layered, but with more finely split nano-sheets. This suggests that Ni doping helps to break up and open the structure, which can help ion access.

Transmission electron microscopy (TEM) of Ni–Nb₂CTₓ:

• Reveals sheets and flakes with hexagonal symmetry preserved.

• High-resolution images show folded edges and crumpled areas, reflecting flexible 2D sheets.

• The accompanying elemental mapping (using EDS/EELS) confirms that Nb and Ni are distributed throughout the sample, indicating that Ni is homogeneously incorporated rather than forming large separate particles.

These results support the idea that Ni is well integrated into the MXene structure and that the morphology is favorable for electrochemical applications.

4. What the Computational (DFT) Study Reveals

To better understand why Ni doping improves performance, the authors performed density functional theory (DFT) calculations.

4.1. Model and approach

They modeled Nb₂C MXene with typical surface terminations (O and F) and then simulated Ni adsorption/doping.

They then calculated:

• The total density of states (TDOS)

• The contribution of each element (Nb, C, O, F, Ni) to the electronic states

• The band structure

4.2. Key findings from density of states (DOS)

For undoped Nb₂C MXene:

• The density of states spans across the Fermi level, indicating metallic behavior (no band gap).

• The main contributions near the Fermi level come from Nb d-orbitals and C p-orbitals.

For Ni-doped Nb₂C MXene:

• The TDOS near the Fermi level increases significantly.

• The DOS becomes more continuous and more intense around the Fermi level.

• Ni d-orbitals contribute strongly near the Fermi level.

What does that mean in simple terms?

• More electronic states available around the Fermi level generally mean better electrical conductivity and richer electronic activity, which is beneficial for fast redox reactions and charge transport in electrochemical devices.

• So, Ni doping improves the electronic structure in a way that helps for energy storage.

4.3. Band structure

The calculated band structure shows:

• The MXene remains essentially metallic after Ni doping (no meaningful band gap opens).

• The valence and conduction bands meet at the same k-point, consistent with metallic or near-zero-gap behavior.

• The strong contributions of Nb and Ni d-orbitals across the Fermi level reflect strong hybridization and bonding interactions.

This supports the idea that Ni–Nb₂C MXene is a good electronic conductor, suitable for use as an electrode material where low resistance is essential.

5. Electrochemical Performance: How Well Does Ni–Nb₂C Store Energy?

The final and most practically important part of the study is the electrochemical testing.

5.1. Experimental setup

The authors used a three-electrode system:

• Working electrode: Ni foam (Ni-foam) coated with either pristine Nb₂CTₓ or Ni–Nb₂CTₓ MXene

• Reference electrode: Ag/AgCl

• Counter electrode: Platinum

• Electrolyte: 1 M PVA–H₂SO₄ gel polymer electrolyte

Why a gel polymer electrolyte?

• PVA–H₂SO₄ gel is conductive, stable, and mechanically robust.

• PVA contains –OH groups, which absorb water and help ion conduction.

• Gel electrolytes also allow for flexible or solid-state device designs.

5.2. Cyclic voltammetry (CV)

The team recorded cyclic voltammograms for both undoped and Ni-doped MXene.

Observations:

• The Ni–Nb₂CTₓ electrode shows larger CV loops (bigger area under the curve) compared to pristine Nb₂CTₓ at the same scan rate, indicating higher charge storage.

• Redox peaks appear in the CV curves, indicating pseudocapacitive behavior (involving fast redox reactions, not just pure double-layer charging).

• At 5 mV/s, Ni–Nb₂CTₓ shows a clearly enhanced current response.

This indicates:

• Higher ionic conductivity

• More electrochemically active sites

• Enhanced interaction with the electrolyte

5.3. Specific capacitance

Using the area under the CV curves, the researchers calculated the specific capacitance.

Results:

• Pristine Nb₂CTₓ MXene: ~258.6 F/g

• Ni–Nb₂CTₓ MXene: ~666.67 F/g at 5 mV/s in PVA–H₂SO₄

This is a very large improvement—more than 2.5 times higher capacitance after Ni doping.

This increase is consistent with:

• Higher surface area (from ~5.21 to ~18.02 m²/g)

• Enhanced electronic DOS near the Fermi level

• Additional electro-active sites introduced by Ni

• Better ion access due to expanded interlayer spacing and modified morphology

They also compare their result to other MXene-based electrodes reported in the literature (like Ti₃C₂, Mo₂C, Nb-doped Ti₃C₂, etc.) and show that Ni–Nb₂C MXene reaches higher specific capacitance than many of these previous systems.

5.4. Effect of scan rate

As usual in supercapacitors:

• At low scan rates, ions have more time to penetrate into pores and fully interact with the electrode surface. Capacitance is higher.

• At higher scan rates, there is less time for ion diffusion deep into the structure, so capacitance appears lower.

The Ni–Nb₂CTₓ sample follows this typical trend: specific capacitance decreases as scan rate increases, but the values remain significantly higher than those of pristine Nb₂CTₓ over the whole range.

5.5. Cycling stability

One of the strongest points of supercapacitors is their cycle life.

In this work:

• Ni–Nb₂CTₓ MXene was cycled 10,000 times.

• After these cycles, it retained about 81% of its initial capacitance.

This shows:

• Good structural stability of Ni-doped MXene

• No severe degradation of the electrode despite repeated charge/discharge

• Potential for real-world applications where long lifetimes are needed

6. Why This Work Matters

This study shows a clear and logical story:

1. Nb₂C MXene is a promising 2D material for supercapacitor electrodes.

2. Nickel doping significantly modifies its structure and electronic properties:

   • Larger surface area

   • Expanded interlayer spacing

   • Higher density of electronic states near the Fermi level

   • More active sites for charge storage

3. These changes lead to:

   • Much higher specific capacitance (up to 666.67 F/g)

   • Good cycling stability (~81% retention after 10,000 cycles)

4. The combination of experimental electrochemistry and DFT calculations gives a consistent explanation: Ni doping enhances conductivity, increases active sites, and improves interaction with the electrolyte, all of which support strong pseudocapacitive behavior.

Beyond this specific system, the work sends a broader message:

• MXenes can be fine-tuned through metal doping to achieve tailored properties.

• Different dopant metals (Ni, Co, Gd, Mn-based species, etc.) and different electrolytes can be explored to design optimized electrodes for various energy storage technologies.

• Using a Ni-foam-supported MXene electrode and gel polymer electrolytes points toward more practical, flexible, and possibly solid-state devices.

In other words, Ni-doped Nb₂C MXene is not just a laboratory curiosity—it demonstrates a general strategy for designing high-performance supercapacitor electrodes:

Take a highly conductive, 2D MXene and engineer its surface and electronic structure with metal dopants to unlock better energy storage.