Abstract:

Objective As widely used thermoelectric materials, ionogels offer advantages such as high ionic conductivity, excellent chemical stability, and a broad operating temperature range, but their mechanical properties are relatively poor. Due to their unique structure, outstanding mechanical properties, and mature industrialization, three-dimensional spacer fabrics exhibit distinct advantages for the development of high-performance and low-cost flexible materials. In this study, by integrating ionogels with knitted spacer fabrics, a double-layer structured device was designed to simultaneously achieve photothermal evaporation and thermoelectric power generation, thereby enhancing the solar energy utilization efficiency.

Method Using Fe²⁺/Fe³⁺ as the redox pair, polydimethylsiloxane (PDMS) and silicon dioxide (SiO₂) were doped into polyvinyl alcohol (PVA) via physical blending method, respectively. By employing spacer fabric as the skeleton, ionogel (thermoelectric layer) was prepared by freeze-thaw. Different spacer fabrics, electrostatic-flocked with graphene/carbon nanotubes/carbon powder (photothermal layer) and integrated with cotton strip water channels, formed a bilayer structure (upper fabric, lower ionogel). Its Seebeck coefficient, output power, conductivity, photothermal and evaporation performances were studied.

Results The study focused on three types of ionogels, namely, PVA-PDMS, PVA-SiO₂, and pure PVA. Comprehensive characterization through Seebeck coefficient measurements, output power testing, and electrical conductivity assessments revealed that the incorporation of specific fillers into the PVA matrix significantly alters the thermoelectric properties. The doping with PDMS and SiO₂ respectively demonstrated a pronounced impact on the ionogels' performance metrics. Among the three formulations, PVA-PDMS ionogel exhibited superior thermoelectric characteristics, achieving a Seebeck coefficient of 1.62 mV/K, a maximum output power of 156.0 nW, and an electrical conductivity of 1.73 S/m. Concurrently, the photothermal and evaporation capabilities were examined using a simulated solar irradiation system, consisting of a xenon lamp equipped with an AM 1.5 optical filter. A comparative analysis was performed on spacer fabrics of varying specifications that were functionalized with different electrostatic flocking materials, namely graphene, carbon nanotubes, and carbon powder. This evaluation aimed to determine the influence of both the fabric pore structure and the carbon-based coating material on the photothermal conversion efficiency and water evaporation rate. Under standardized testing conditions at an illumination intensity of 1 kW/m², the medium-pore-sized spacer fabric modified with carbon powder electrostatic flocking demonstrated optimal performance. This particular configuration yielded enhanced photothermal response and the highest evaporation efficiency, reaching a notable evaporation rate of 1.26 kg/(m²·h). The research successfully integrated these components into a coherent bilayer architecture, comprising an upper spacer fabric layer for photothermal processes and a lower ionogel layer for thermoelectric conversion. This strategically designed photothermal-thermoelectric dual-layer composite structure demonstrates the feasibility of simultaneous and efficient solar energy harvesting for two distinct purposes, i.e., generating power through the thermoelectric effect and producing clean water via photothermal evaporation. This configuration confirms the feasibility of simultaneous and efficient solar energy harvesting for two distinct applications: thermoelectric power generation and photothermal water evaporation, thereby providing a viable approach for enhancing overall solar energy utilization efficiency.

Conclusion Through structural design, a spacer fabric and an ionogel were integrated to develop a photothermal-thermoelectric bilayer composite structure with an upper spacer fabric layer and a lower ionogel layer. Test results indicated that the PVA-PDMS ionogel exhibited superior thermoelectric performance, while the medium-pore-size spacer fabric with carbon powder electrostatic flocking demonstrated enhanced photothermal and evaporation performance. This integrated configuration enables simultaneous photothermal evaporation and thermoelectric power generation under solar irradiation, providing a novel strategy for the integration of photothermal and thermoelectric structures.

Key words: knitting, ionogel, photothermal-electric performance, photothermal evaporation, composite material, 3-D spacer fabric, solar energy utilization