Green energy harvesting has been actively researched in the last few decades to achieve carbon neutrality. Triboelectric nanogenerator or TENG is a promising green energy harvester that uses low-frequency mechanical energy wasted from ordinary motion to produce many things. Since its development in 2012, TENG has been considered very useful. Recently, a team of researchers worked on their previous concept with a new approach and generated an efficient and sustainable sulfur-rich composite with MXene.
Aim of the Study – To develop new tribo materials for creating high-performance TENGs.
Efficient and Sustainable Sulfur-Rich Composite with Mxene
However, concerning state-of-the-art tribo-materials for TENGs, over 50% of current research has utilized fluoropolymers, including polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), and polyvinylidene fluoride (PVDF).
According to the periodic table, fluorine has the highest electron affinity (1328.2 kJ mol-1) and electron negativity (4.0). This means they can effectively withdraw electrons from other materials and generate high negative surface charge density.
Due to this, fluoropolymers have been widely used as negatively chargeable contact layers for making TENGs. Despite being beneficial for the material, national organizations and contemporary research have issued several warnings against using fluoropolymers due to their polluting nature.
Highlights
- Triboelectric nanogenerators or TENGs use fluoropolymers as chargeable materials in triboelectric series.
- Poly- and per-fluoroalkyl substances (PFAS) present in TENGs are released into the environment during their lifecycle and cause environmental pollution.
- SRP or sulfur-rich polymer/MXene composite is a sustainable alternative which offers high performance.
- Sulfur is an abundant waste from petrol refining and among the polymerizable atoms has the highest electron affinity of around −200 kJ mol−1.
- Less than 0.5% MXene is added to SRP for even distribution without causing electrical percolation. This leads to an increase in the dielectric constant without much increase in dielectric loss.
- With homogeneous MXene distribution TENG enhancement in peak voltage (about 2.9%) and peak current (about 19.5%) in comparison to previous SRP-based TENGs.
- Due to its dynamic exchangeable disulfide bonds, it also shows its reusability without reducing the modulus and TEG performance.
- Once the wafer size is increased to 4 inches, there is around an 8.4 times increase in peak power density in SRP/MXene composite-based TENG. This reaches 3.80 W/m² in comparison to previous TENGs based on SRP.
- For the first time, researchers also established a closed-loop recycling system among SRP-based TENGs.
To mitigate bad impacts on the environment and human health sulfur rich polymers need to be developed. They are primarily composed of 7 million tons of elemental sulfur derived from the hydrodesulfurization process of petrol oil refining.
During the process, elemental sulfur is extracted from hydrogen sulfide (H2S) gas and results in exceptionally pure than other wastes. With its highest electron affinity of -200 kJ mol-1 and electron affinity of -122 kJ mol -1 they become a promising element for constructing high-performance TENG. Moreover, end-used SRP can be reused by thermal reprocessing without critical deterioration of mechanical properties due to their dynamically exchangeable disulfide bonds.
MXene – the New Nanomaterial
MXene is from a new family of 2D nanomaterials. It has a 2D sheet-like structure with a high aspect ratio. It possesses metallic electrical conductivity (5000-20,000 S cm-1). MXene also has a metallic core and oxide- and fluorine-based surface terminal groups, which give it negatively charged surfaces.
Negatively charged surfaces provide stable dispersion of MXene nanosheets in aqueous media. This further proves advantageous in environmental applications and coating processes for objects with varying 3D topography. To achieve stable dispersion in aqueous media and electrical conductivity, MXene does not require additional reduction or oxidation processes.
TENG Output Performance Enhancements with MXene
Electrical conductivity and negative surface charge of MXene is responsible for inducing microscopic dipoles at the interface between the polymer matrix and MXene. This boosts the dielectric constant of polymer nanocomposites. From various research, it has been evident that improving the dielectric constant of polymer nanocomposites enhances their performance.
Further, the performance of TENG was improved by adjusting the amount of MXene which controlled the connectivity of MXene structure. This way the electrical percolation was not affected either. Moreover, only 0.4 wt% MXene was required to achieve the highest output performance in TENG.
Furthermore, researchers implemented the scaling-up process and corona discharging for them. It resulted in an increase in the peak power density of SRP/MXene TENG. This enables them to efficiently power commercial electronics like charge capacitors and LEDs.
According to KIST researchers, Cost-Effective Green Hydrogen Production with Active MXenes.
Process of Generating Ti3C2Tx MXene Nanosheets
The following figure shows the synthesis of a Ti3C2Tx MXene aqueous solution to exfoliate MXene at the single layer level and then dispersed in delonized water. It is easy to stably disperse MXene nanosheets into aqueous media due to electrostatic repulsion among negatively charged MXene nanosheets.
SRP Matrix preparation includes inverse vulcanization of 75 wt% elemental sulfur (S) with 25 wt% 1,3-diisopropenylbenzene (DIB) as a comonomer. This enables obtaining solidified SRP chunks as shown in the image below. Furthermore, to prevent the recrystallization of elemental sulfur, researchers performed post-baking on SRP chunks for 10 minutes at 160° C. This helps in achieving further reactions of the unreacted species.
Thus, chemically stable SRP chunks were obtained and pulverized into powder form under the glass transition temperature (Tg) of ≈17° C. Researchers used a commercial blender and liquid nitrogen for the process. The average projected area radius of SRP powder measured from SEM images is 18.9 ± 14.4 µm.
Further, SRP powder was completely immersed in MXene aqueous solution via vigorous shaking. Then researchers used it to effectively coat MXene nanosheets by self-assembly via evaporation of aqueous media under vacuum conditions at room temperature 25° C for 72 hours.
Then to make an AI-electrode integrated SRP/MXene composite film, researchers placed MXene-coated SRP powder on AI foil and hot pressed at 140° C for 2 minutes. Since the disulfide bonds in SRP matrix can exchange dynamically at 140° C, it allows them to form a stable film when they come in physical contact along their boundaries.
Thus, MXene layers are tightly surrounded and have no voids by an adjacent SRP matrix with newly formed disulfide bonds. This ensures uniformity and stability. For achieving the target thickness of the composite film, researchers systematically investigated temperature’s effect on film thickness.

Strain Sweep Tests
Additionally, researchers need to check if thermo-mechanical properties were maintained during pulverizing and hot passing. For this strain sweep tests were performed on the composite films using 0.8 wt% MXene (which was the highest MXene in the content). 7 consecutive strain sweep tests were performed with 2-minute pauses in between.
To explore beyond linear viscoelastic regions inducing structural destruction, strains ranging from 0.01% to 100% were applied at each test. This was necessary for the recovery of thermo-mechanical properties through dynamic bond exchange.
The SRP/MXene composite films’ modulus was successfully recovered during these 2-minute pauses. Then these composite films were used to develop the TENG device that can be used for vertical contact and separation.
The Tyndall Effect of Ti3C2Tx MXene
Researchers also confirmed the Tyndall effect with a concentration of 0.02 mg mL-1 irradiated under a generally available green laser. The Tyndall effect occurs by even distribution of MXene nanosheets in an aqueous solution, which further leads to light scattering.
Find more about Giant Energy Storage in Carbon Nanotube Ropes than Lithium Batteries, says research.
Characteristics of Synthesized MXene Nanosheets
The synthesized nanosheet has the following features:
- Electric conductivity – 8,381 ± 319 S cm−1 (measured by a four-point probe)
- Lateral size – 3.0 ± 2.3 µm (measured via scanning electron microscopy)
- Height – less than 2 nm (measured by atomic force microscopy), which indicates a high aspect ratio of monolayer-level exfoliation.
The following table presents a summarized view of the atomic composition of MXene from X-ray photoelectron spectroscopy (XPS) measurement.
Atomic Percentage (%) | |
C 1s | 22.47 |
F 1s | 17.21 |
O 1s | 25.33 |
Ti 2p | 34.56 |
As the following figure shows, the larger surface area of SRP powder was achieved by a higher concentration of MXene aqueous solution. Low MXene content leads to no percolation in the composite films as there is a substantial distance between assembled MXene layers.
An increase in MXene content triggers rheological percolation as the distance between sporadically distributed layers is less. During hot-pressing, polymer chains with high-stiffness fillers at the interface align partially. This results in more rigid amorphous fractions.
At this stage, it is easy to enhance the elastic modulus of SRP/MXene composites. Also, electric percolation can also be increased to interconnect through physical contact forming a 3D network-like structure in SRP matrix. It leads to the development of conductive pathways extending from top to bottom through the thickness of SRP/MXene composite films.

Uniform Distribution with Segregated Structure
- The dark color of the MXene-coated SRP powder is due to increased MXene content.
- Next, as MXene content increased there was an increase in the intensity of hydroxyl group (O-H) peak (3,430 cm−1) which was confirmed by Fourier transform infrared (FT-IR) spectra.
- Also, no peak shift was observed which clearly suggests interactions between MXene and SRP. Cross-sectional SEM images (bright section) show the increased connectivity between layers of the MXene as its content increases.
- Figure e demonstrates a linear correlation between a weight fraction of Ti atoms and applied MXene contents. The next figure shows the segregated structures, lacunarity, and fractal dimension of distributed states of MXene.

The description of the following image is as follows (alphabetical order):
- Digital images of MXene-coated SRP powder (i) and SRP/MXene composite film with varied MXene content (ii)
- Cross-sectional SEM images of SEP/MXene films with varying MXene content.
- Cross-sectional EDS atomic mapping images with 0.8 wt% MXene.
- The high magnitude SEM cross-sectional and atomic mapping images to show composite films with 0.4% MXene.
- Weight fraction of Ti atom in SRP/MXene film as MXene content functions.
- Fractional dimension and lacunarity of segregated structure vs content (MXene).
- Storage modulus at rubbery plateau region and tan max of the composite film with varying MXene contents.
Dielectric Properties of SRP/MXene Composite Film
From the following figure, we can see different polarization mechanisms of neat SRP films and SRP/MXene composite films under external e-field. Unlike neat SRP films, here polarization charges accumulate at the he interface between the SRP and MXene.
Higher electronegativity was witnessed in terminal groups of the layer. It is more than sulfur and carbon present in the SRP matrix. Due to this electron density from the SRP matrix is withdrawn to the surface of MXene layers. Thus, the composite films can generate a higher total net charge under the external e-field due to additionally accumulated charge.

Have you read how MOF Can Boost Photocatalytic Hydrogen Production with 10% Quantum Yield, says OSU research?
Output Performance of SRP/MXene Composite Films (12.5 cm2)
The following figure demonstrates the output performance based on SRP/MXene composite films. Researchers measure the peak voltage and peak current of the TENGs to systematically investigate their output performance.
The composite films made from applied SRP/MXene have the following features.
- Average thickness – 150 μm
- Active area – 2.5 cm × 5.0 cm (12.5 cm2)
- Frequency of contact and separation – 30N and 0.65 Hz
- But as the content of MXene rises from 0 to 4 wt% there was an increase in both peak voltage from 68.8 ± 4.5 and Ipeak from 2.5 ± to μA to 161.0 ± 20.0V and 8.1 ± 1.0 μA.

As expected, the triboelectric output performance was high in SRP/MXene composite-based TENG with 0.4 wt%. Next, a parameter in TENG output performance was the thickness of the contact layer. When it was lesser than the optimized value and the charged surface was too close to AI electrode, it generated negative charges on the composite films.
Whereas, when the thickness was more than optimal, a positive charge wasn’t generated because the electric field decayed with distance. Thus, the optimal thickness was measured to be ≈500 μm.
Long-Term Operational Stability
With 24-hour power generation tests, researchers checked the long-term operational stability of the SRP/MXene composite-based TENGs. Stable power generation without significant reductions in Vpeak and Ipeak were recorded during 24 hours.
Moreover, uniform MXene distribution induces a large interfacial area between MXene and SRP which enables the notable TENG performances.
Deciphering Sustainability of MXene/SRP Composite-based TENG
Researchers experimentally demonstrated it using an entire closed-loop recycling process.
- They first removed the physically attached AI foil electrode from the composite film
- Then the film was pulverized below Tg with liquid nitrogen.
- It was then vacuum-dried to prevent moisture condensation.
- Then they reprocessed the repulverized SRP/MXene composite powder into a film by hot-pressing it under the initial preparational conditions.
- Again, the recycled film was used to develop TENG devices.
Characteristics of Recycled MXene/SRP Composite Film
- Negligible changes in appearance and color
- Rubber plateau remained constant and there was no deterioration (after recycling the film 4 times)
- Also, the Vpeak and Ipeak were retained.

To Enhance the Output Performance: Corona Discharge and Scaling up MXene/SRP (81.1 cm2)
After corona discharging, the surface potential of the composite films with 0.4 wt% was significantly improved. Researchers artificially injected the generated electrons into the surface of the contact layer. The SRP/MXene composite film was scaled up to 4-inch wafers to demonstrate its scope for various large-scale applications.
Scaled up TENG has Vpeak (1,717.7 V) and Ipeak (129.0 μA), respectively, which is around ≈3.6 making it 4.4 times higher than the ones measured before corona discharging.
Researchers noticed that at the 8 mΩ load resistance, the peak power density reached 3.80 W m-2. This is an 8.4-fold increase than the previous recorded set by SRP/PPFS blend-based TENGs. Moreover, the required load assistance was reduced by a factor of 12.5 in comparison to old records.
When we compared to previously reported vertical contact and separation mode MXene-based TENG, SRP/MXene-based TENG exhibited superior or comparable TENG output performance including power density, Vpeak, and Ipeak despite utilizing a significantly small amount of 0.4 wt% MXene. Particularly, the density per frequency of SRP/MXene-based TENG was around 5.86 W m−2 Hz−1.

A research disclosed that it is possible to develop eco-friendly air fertilizer from mashed purple marine bacteria.
Previous Reports on SRP Construction
2019 – Researchers introduced the concept of using SRP for constructing a high-performing TENG by directly fluorinating SRP film surface.
Drawbacks – This approach uses F2 gas which is flammable and highly toxic which endangers human safety and does not offer environmental sustainability.
2022 – A polymer blend incorporating non-volatile poly (pentafluoropolystyrene) or PPFS was developed into the SRP matrix to cater to the dangers associated with F2 gas. This 2nd generation SRP-based TENG also showed good output performance in comparison to the 1st generation one. Also, it showed long-term stable power generation.
This improvement came by a new processing design that made PPFS-rich surfaces to localize through phase separation during thermal film processing.
Drawbacks – The use of PPFS is restricted to 7.5 wt%. Benefits of sustainable SRP-based TENGs are detracted when PPFS is used. Environmental pollution by PPFS still remains in the loop without any attempts made to demonstrate the reusability. Since fluorine-based parts should be on the surface for TENG to perform better, it made reprocessing hard.
2024 – SRP composite system incorporating a minute amount of Ti3C2Tx MXene as a nanofiller for developing sustainable and high-performing TENGs. It effectively addresses the limitations of the previous SRP-based TENGs.
Conclusion
This way it became clearer that this approach recorded efficient and sustainable sulfur-rich composite with MXene. It demonstrated high power density and demonstrated closed-loop recycling without any compromise on the performance of the TENG device. TENG is considered environmentally friendly because it utilizes wasteful elements, sulfur. Also, these films have dynamically exchangeable disulfide bonds due to which they can be reused by repulverizing and thermal processing them. This innovative approach is expected to overcome the limitations of the previous system.
Source Content: High-Performance Yet Sustainable Triboelectric Nanogenerator Based on Sulfur-Rich Polymer Composite with MXene Segregated Structure
Source: Supporting Information