Solid-state Thermoelectric Characteristics of NiII, FeII, CoII, and CuII Borohydrides

Abstract Four transition metal borohydrides (MTBHs, MT = Ni, Fe, Co, and Cu) were prepared by sonicating a mixture of the desired MT salt with excess NaBH4 in a nonaqueous DMF/CH3OH media. The process afforded bimetallic (Ni-BH4), trimetallic (Fe-BH4, Co-BH4), and mixed-valence (Cu-H, Cu-BH4) amorphous, ferromagnetic nanoparticles as identified by thermal, ATR-IR, X-Ray diffraction, and magnetic susceptibility techniques. The electrical conductivity (σ) of cold-pressed discs of these MTBHs shows a nonlinear increase while their thermal conductivity (κ) decreases in the temperature range of 303 ≤ T ≤ 373 K. The thermal energy transport occurs through phonon lattice dynamics rather than electronic. The σ/κ ratio shows a nonlinear steep increase from 9.4 to 270 KV-2 in Ni-BH4, while a moderate-weak increase is observed for Fe-BH4, Co-BH4, and Cu-BH4. Accordingly, the corresponding thermoelectric (TE) parameters S, PF, ZT, and η were evaluated. All TE data shows that the bimetallic Ni-BH4 (S, 80 μVK-1; PF, 259 μWm-1K-2; ZT 0.64; η, 2.56%) is a better TE semiconductor than the other three MT-BHs investigated in this study. Our findings show that Ni-BH4 is a promising candidate to exploit low-temperature waste heat from body heat, sunshine, and small domestic devices for small-scale TE applications.