Enhanced thermoelectric properties of Bi₂S₃ polycrystals through an electroless nickel plating process

Bi₂S₃ is an eco-friendly alternative compound for thermoelectric devices. However, the low electrical conductivity of the pristine Bi₂S₃ hinders the improvement of its ZT value, which further restricts its application in the field of thermoelectricity. In this work, we report the first attempt to optimize the thermoelectric properties of Bi₂S₃ by electroless nickel plating. A nickel plated Bi₂S₃ powder sample was synthesized by electroless nickel plating on the precursor Bi₂S₃ powder prepared by mechanical alloying. Then, the powder was sintered to a bulk material by spark plasma sintering. The relationships between the composition, microstructure and thermoelectric properties of the bulk samples were investigated. The XRD results showed a AgBi₃S₅ second phase which was formed by the reaction of Ag residues with the Bi₂S₃ substrate during the sintering process. The nickel element and AgBi₃S₅ second phase introduced in the nickel plating process directly affect the electronic conductivity and Seebeck coefficient of the nickel plating sample, resulting in the relatively high power factor of 244 μW m^-1 K^-2 at 628 K. What's more, the thermal conductivity of the sample was also reduced moderately, obtaining a low value of 0.40 μW m^-1 K^-1 at 628 K. Therefore, a maximum ZT value of 0.38 was obtained at 628 K for the nickel plated sample, which is three times higher than that (0.12 at 628 K) of pristine Bi₂S₃ materials.