Construction of an MXene/Organic Superlattice for Flexible Thermoelectric Energy Conversion

Abstract

Construction of an inorganic/organic superlattice-based film has been proven effective in enhancing thermoelectric (TE) performance as well as flexibility by a variety of mechanisms, typically for two-dimensional (2D) TiS2-based flexible TEs. MXenes, typically, Ti3C2T x , are a type of 2D material widely investigated in fields of flexible batteries and electromagnetic shielding, among others. However, they have rarely been reported in flexible TEs. One of the key factors is that the surface termination groups (−T) on an MXene could trap electrons, restricting the electronic transport. Herein, −T groups were tailored and substituted by organic ions (−HA) by facile preannealing, exfoliation, and reassembly. The intercalation of −HA introduced Ti–N bonding, forming a flexible MXene/organic superlattice film. The electrical conductivity of the superlattice film was increased by 5 times to 1.6 × 105 S m–1 due to defect reduction as well as the electron injection effect. While the Seebeck coefficient was maintained, the power factor was increased from 4 to 18 μW m–1 K–2. The TE module based on the superlattice film revealed an output power of 7.6 nW at a temperature gap of 50 K. This work opens up an avenue of fabricating flexible MXene-based TE films by tailoring the surface termination group and constructing inorganic/organic superlattice structures.

Robustly Enhanced Seebeck Coefficient in the MXene/Organics/TiS2 Misfit Structure for Flexible Thermoelectrics

“…As shown in Figure e, the MT-0.5 film revealed a power factor of 77.2 μW m –1 K –2 , which was 18 times that of the MXene film. This result is the current maximum of the TE properties of Ti 3 C 2 T x -based films (Figure f). ,,, …”

“…Figure b shows the deconvolution results of the Ti 2p region, where the Ti 2p spectra were split into peaks assigned to Ti–C (455.9 eV; 461 eV), C–Ti–O (457.2 eV; 463 eV), C–Ti–OH (456.1 eV; 461.5 eV), TiO 2 (458.5 eV; 464.2 eV), Ti–S (455.1 eV), and C–Ti–S (457.6 eV). , The presence of C–Ti–S implied the chemical bonding between TiS 2 and MXene, facilitating carrier migration between MXene and TiS 2 . Figure c shows the deconvolution results of the N 1s region, where the N 1s spectra were split into peaks assigned to N–Ti (397.2 eV), N–H (401.1 eV), and N–C (399.8 eV). , The formation of the N–Ti bond was the result of the Lewis acid–base reaction between TiS 2 and HA, which led to the connection between negatively charged MXene and HA + under the electrostatic force. Figure d shows the deconvolution results of the S 2p region, where the S 2p spectra were split into peaks assigned to S–Ti–C (162.5 eV), S–Ti (159.2 eV), and S–O (164.5 eV) .…”

“…Figure 6c shows the deconvolution results of the N 1s region, where the N 1s spectra were split into peaks assigned to N−Ti (397.2 eV), N−H (401.1 eV), and N−C (399.8 eV). 48,49 The formation of the N−Ti bond was the result of the Lewis acid−base reaction between TiS 2 and HA, which led to the connection between negatively charged MXene and HA + under the electrostatic force. Figure 6d shows the deconvolution results of the S 2p region, where the S 2p spectra were split into peaks assigned to S−Ti−C (162.5 eV), S−Ti (159.2 eV), and S−O (164.5 eV).…”

“…This result is the current maximum of the TE properties of Ti 3 C 2 T x -based films (Figure 7f). 30,32,49,54 Flexibility and Stability. The flexibility is usually characterized by bending the MXene/organics/TiS 2 film and attaching it to a succession of glass tubes with decreasing radii (Figure 9a) and comparing the resistivity at the bent state (R) to that at the flat state (R 0 ).…”

Stretchable Thermoelectric Fibers with Three-Dimensional Interconnected Porous Network for Low-Grade Body Heat Energy Harvesting

“…Thus, the fabrication of flexible thermoelectric generators with the conventional, and often most practical, cross-plane π-shaped structure is quite challenging due to the lack of satisfactory n-type thermoelectric materials with stretchable characteristics. − The exploration of n-type materials for wearable bioelectronics is highly desired. Transition-metal carbides, nitrides, and carbonitrides (also called MXenes) have garnered attention for their outstanding properties such as high electrical conductivity and excellent Seebeck coefficients, especially Ti 3 C 2 T x MXene, which possesses scarce n-type thermoelectric characteristics. − MXenes are usually hybridized with other thermoelectric materials such as (Bi,Sb) 2 Te 3 or CNTs to compose flexible thermoelectric films. Although those composite films possess high electrical conductivity, they exhibit p-type properties rather than scarce n-type Seebeck coefficients and relatively high thermal conductivity. − Furthermore, the films are nonstretchable and unsuitable for wearable body heat energy harvesting …”