In the past decades, the energy and environment issues have become increasingly urgent and attracted a great deal of attention. New energy technologies, aiming to harvest ambient energy in clean and sustainable way, have emerged and developed rapidly to become important alternatives to fossil fuels. Recently, nanogenerator (NG), which utilizes nanomaterials for converting environmental mechanical energy into electricity based on piezoelectric and triboelectric effects, has been demonstrated as an effective and efficient approach with huge practically-applicable potentials since 2006 [1-5]. In addition to mechanical energy, heat energy also exists ubiquitously and usually goes to waste in our living environment. If waste heat could be efficiently harvested, it could not only increase energy efficiency in many areas, but also power portable electronic devices independently and sustainably. In this regard, NG based on pyroelectric effect has been developed for harvesting thermal energy and self-powered sensors [6-7]. However, the energy output of pyroelectric NG is very low and requires further improvement for practical applications. Besides pyroelectric effect, thermoelectrics is another significant option for recovery of waste heat and has been used in the field of space power generation because of its high reliability and simplicity [8].

Thermoelectric effect refers to a phenomenon in which a temperature gradient across a material will produce an electric potential to drive electric current, usually called the Seebeck effect [9]. Based on the thermoelectric effect, thermoelectric devices can generate electricity from the hot exhaust stream of cars, solar energy, wood stoves, and many other situations. Nevertheless, today thermoelectric devices are not yet in common use. The most critical reason is low efficiency, represented by the dimensionless figure of merit (ZT) of thermoelectric materials [8]. To improve ZT, we need materials with high electrical conductivity, high Seebeck coefficient, and low thermal conductivity. Unfortunately, the strong interdependence of these three parameters severely limits maximizing ZT in homogeneous bulk materials [10]. Recently, thermoelectric nanocomposite has been developed to resolve the above issue by combining different nanomaterials with respective excellent electrical and thermal properties to simultaneously improve the ZT [10-12].

In addition, two-dimensional (2D) nanomaterials have attracted a host of attention because of their unique electronic structures and excellent physical properties since 2004 [13-14]. Among the 2D nanomaterials, MoS₂ is a promising thermoelectric material due to its low thermal conductivity and theoretically-predicted high Seebeck coefficient and ZT [15-16]. However, for pristine MoS₂, it has a large direct bandgap of 1.2-1.9 eV, which renders it insulating and therefore requiring a strong external electrical field to realize higher electrical conductivity [17]. Unlike MoS₂, graphene is a gapless semimetal with high electrical conductivity [18-19]. Yet, unfortunately, it also has high thermal conductivity. A nanocomposite based on the above two materials would be favorable for high-performance thermoelectric devices by integrating the advantages of both materials and avoiding the weaknesses of each.

In this work, we report a flexible thermoelectric nanogenerator (TENG) based on MoS₂/graphene nanocomposite. The nanocomposite TENG shows an enhanced energy output compared with that of pristine MoS₂ TENG, possibly resulting from the improved electrical conductivity which is confirmed by I-V characteristics. The as-developed TENG can be further applied as a self-powered sensor for temperature measurement.