N-type thermoelectric bulk composite based on poly(nickel-ethylenetetrathiolate): The positive effect of porosity on decreasing thermal conductivity

Recent progress in the research of n-type materials for organic thermoelectrics has drawn the attention to the metal coordination polymer poly(nickel-ethylenetetrathiolate) (poly(Ni-ett)). These polymers have excellent stability in air because their backbone structure is composed of air-stable ligands and exhibit good electronic properties when pressed into pellets or grown into crystals. However, due to their brittle nature and limited solubility, they are often blended with solution processable but electrically insulating polymers to produce composite films. Herein, we demonstrate the possibility of exploiting the 3D structuring ability of a polymer scaffolding matrix (based on polyvinylidene fluoride (PVDF)) to fabricate porous bulk structures. Porosity is introduced to reduce the lattice contribution to the thermal conductivity and used as a lever to increase the thermoelectric efficiency of the composite aerogel poly(Ni-ett): PVDF. For practical applications, these materials have great potential for vertical thermoelectric generators, as the low thermal conductivity and millimetre-thick samples would allow a thermal gradient to be maintained across it (without actively cooling one side, as in the case of certain thin-film technologies). In this manuscript, we present an original approach for the fabrication of novel n-type polymer aerogels resulting in lightweight and bulk size thermoelectric materials. The composite aerogels exhibit a low thermal conductivity of 52 mW m−1 K−1, and their figure of merit zT is comparable to the dense neat pellet with reasonable stability over six months.