Synthesis and Optimization of Poly(nickel-ethylenetetrathiolate) for High Performance n-Type Thermoelectric Polymers

Thermoelectric (TE) generators directly convert heat into electricity via the Seebeck effect, which creates a voltage in response to an applied temperature gradient. Thermoelectric generators have been limited to niche applications (namely space exploration) due to their high system costs. Electrically conducting polymers are an attractive alternative class of materials, particularly for low-grade waste heat recovery applications (< 200 °C). Furthermore, they are inexpensive owing to their abundance and potential to process from solution via printing techniques, and they have an inherently low thermal conductivity. Thermoelectric polymers are often compared with their power factor (PF = S²σ) which is determined by the electrical conductivity (σ) and the Seebeck coefficient (S). While significant progress has been made with organic p-type materials, n-type TE’s have trailed behind due to their instability in air. In this work, we investigate the thermoelectric properties of metallo-organic poly[K_x(Ni-ett)], which is one of the highest performing organic n-type TE materials. However, it is produced as a powder that is insoluble in common solvents such as methanol and water; previous attempts to solution process the material have resulted in significantly reduced thermoelectric properties. In this work, we optimize the synthesis of this polymer and fabricate a composite film by suspending poly[K_x(Ni-ett)] in a polyvinylidene fluoride matrix. This is achieved by optimizing the air exposure time and reducing the amount of polymer matrix needed to form a film. Characterization suggests that a short exposure time is beneficial and results in a larger amount of potassium counter-ion that aids charge transport. The obtained thin-film properties show a room temperature power factor of 3.1 μWm^-1K^-2, which is 7 times higher than that reported in literature and shows excellent stability in air. Additionally, the n-type composite material was used in a TE device based on a novel radial architecture to generate small amounts of power from a temperature gradient. This performance for a solution processed n-type material is encouraging and could find applications in waste heat recovery and flexible devices.