PVDF-HFP/SWCNT radiation-modulated thermoelectric fibrous membranes enabling synergistic photothermal- thermoelectric conversion

Technical Summary: PVDF-HFP/SWCNT Radiation-Modulated Thermoelectric Fibrous Membranes

Problem Statement
The development of flexible thermoelectric materials for wearable electronics faces two primary challenges: balancing high electrical conductivity with mechanical flexibility, and establishing effective temperature gradients in open or wearable environments. Traditional thermoelectric materials are typically rigid, failing to meet the lightweight and flexible requirements of textiles. Furthermore, while flexible polymer substrates offer mechanical advantages, their inherent insulating nature hinders the construction of continuous conductive networks necessary for efficient carrier transport. Additionally, thermoelectric output relies on sustained temperature differences, which are often limited in natural wearable settings, restricting continuous energy generation. Existing strategies often struggle to simultaneously achieve continuous conductive networks, efficient solar absorption, and stable temperature gradient regulation within a single flexible fibrous material.

Methodology
To address these challenges, the authors developed a synergistic electrostatic air-blow spinning strategy that combines electrospinning with solution air spraying. This approach was used to construct poly(vinylidene fluoride-co-hexafluoropropylene)/single-walled carbon nanotube (PVDF-HFP/SWCNT) thermoelectric fibrous membranes.

• Fabrication Process: A PVDF-HFP solution was electrospun to form a continuous, three-dimensional porous nanofibrous framework. Simultaneously, a dispersion of SWCNTs was deposited onto the fibrous network via air-assisted atomization spraying. This decoupled the formation of the polymer skeleton from the deposition of conductive nanomaterials.
• Sample Optimization: By varying the feeding rate of the SWCNT dispersion (0.5, 2, 3, and 4 mL h⁻¹), a series of composite membranes (PVDF-HFP/SWCNT-0.5 to -4) were produced to systematically investigate the effects of SWCNT loading on microstructure, mechanical strength, electrical conductivity, and thermoelectric performance.
• Radiation-Modulated Design: To generate a temperature gradient without external heating, an asymmetric photothermal structure was designed. This utilized the contrasting optical properties of the components: the SWCNTs provided strong solar absorption and photothermal conversion, while the PVDF-HFP matrix exhibited high solar reflectance. By covering one side of the membrane with a reflective PVDF-HFP layer and exposing the other to simulated sunlight, an in-plane temperature difference was established.

Key Contributions and Results

• Microstructural Advantages: The electrostatic air-blow spinning strategy resulted in SWCNTs preferentially depositing on fiber surfaces and junctions. This created an interconnected conductive network without significantly disrupting the flexible polymer skeleton or the solution's rheological properties during spinning.
• Mechanical and Electrical Properties:
  • The optimized membranes demonstrated significant mechanical reinforcement. The PVDF-HFP/SWCNT-2 sample achieved a tensile strength of 34.71 MPa (a ~12.2-fold increase over pristine PVDF-HFP) and an elastic modulus of 992 MPa, though elongation at break decreased.
  • Electrical conductivity increased with SWCNT loading, reaching a maximum of 46 S m⁻¹ for the PVDF-HFP/SWCNT-4 sample. The material exhibited typical semiconducting behavior, with conductivity increasing in the 280–320 K temperature range.
• Thermoelectric Performance:
  • The PVDF-HFP/SWCNT-3 membrane was identified as the optimal balance of properties. It exhibited a Seebeck coefficient of 42.3 μV K⁻¹.
  • At a temperature difference of 20 K, the membrane achieved an output power density of 11.25 pW cm⁻².
  • COMSOL simulations confirmed the establishment of a stable electric potential distribution driven by the temperature gradient.
• Photothermal-Thermoelectric Synergy:
  • Under 1 sun irradiation, the asymmetric design generated an in-plane temperature difference of approximately 10 °C (36 °C on the exposed side vs. 26 °C on the covered side).
  • This self-established gradient enabled stable thermoelectric power generation without external heating devices. The system demonstrated reproducible output voltage responses during repeated on/off illumination cycles, confirming excellent operational stability and reversibility.

Significance
The paper claims that this work provides a structural strategy for integrating photothermal and thermoelectric conversion within flexible fibrous materials. By leveraging the specific optical contrast between SWCNTs and PVDF-HFP, the study demonstrates a viable pathway for solar-driven thermoelectric energy harvesting in flexible wearable applications. The electrostatic air-blow spinning technique offers a solution to the trade-off between constructing efficient conductive networks and maintaining the mechanical flexibility of fibrous frameworks. The authors conclude that this approach enables the functional integration of solar light capture, photothermal conversion, temperature gradient construction, and thermoelectric output in a single membrane, offering a reference for the design of future flexible energy conversion materials.