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

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 mixedvalence (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 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 NiBH4 (S, 80 μVK; PF, 259 μWmK; 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.


Introduction
Current research and commercialization on renewable energy focus mainly on solar photovoltaic (PV), geothermal, and wind sources as viable energy sources for large-scale electricity production [1][2]. However, to a much lesser extent was the interest in utilizing waste heat through thermoelectric (TE) techniques . In this straight-forward technology, the temperature gradient (T) is directly related to the induced DC thermal voltage (V) and vice-versa as described by Seebeck power function (1) and Peltier refrigeration functions (2), where S, , Q, and I are the Seebeck coefficient, difference in Peltier factors, heat, and electric current, respectively [3,[11][12][13]. However, the ability of a material to produce TE power is determined by its power factor (PF), and the dimensionless figure of merit (ZT), Equations 3 and 4.
These findings also showed that the carrier mobility can be enhanced through microstructure where 1-D linear and 2-D layer structural arrays are preferred to 3-D bulk materials.

Copper Borohydride (Cu-BH4):
The Cu-BH4 was prepared and isolated as described above employing CuCl2

Formation and Characterization of Transition Metal Borohydrides (MT-BHs)
In general, MT-BHs are synthesized in various compositions, structures, and properties, as exemplified by their general chemical formula of Mx(BHn)y [36,38]. In this work, the desired MT-BHs were prepared by mixing the desired metal salt with fourfold excess of NaBH4 in a nonaqueous DMF/MeOH solvent system, Equation 5 [38][39][40][41][42][43][44][45]. This complex reaction process is generally affected by the MT:BH4¯ ratio, solvent, pH, and slow/rapid mixing of reagents. Due to the presence of traces of moisture, two main side reactions may be encountered during the reaction, the formation of metallic element (MT 0 ) and metal oxides /borates (MTO, MTBO3 Equation 6 [39,43,45]. To overcome these possible side reactions, our reactions were carried out in DMF/MeOH solvent mixtures using excess NaBH4.

Thermo-oxidation of the Prepared MT-BHs
where the mass gains represent the oxygen uptake. Upon further heating to 1100 ⁰C, the justformed MT-BO3 decomposes, eliminating dense white fumes of B2O3, leaving behind Fe2O3, Co2O3, and NiO residues. To ensure complete degradation of MT-BO3, the resulting residues were treated three times with deionized water and calcined to a constant weight (1100 °C, one hr.), leaving behind the oxide residues (Fe2O3, 78.6%; Co2O3, 72.0%; NiO, 61.9%). The obtained thermo-oxidative data in Table 1 allowed us to deduce their molecular formula in accordance with the general formula, Mx(BHn)y, reported in the literature [35,36,38].
On the other hand, a close look at the thermo oxidative data of Cu-BH4 shows that the early mass gain (5.6 %) observed at ~250 °C is attributed to the oxidation of the Cu-H phase forming Cu2O/CuO [35,39]. This step is followed by the dehydrogenation of Cu-BH4 phase forming Cu-B (250-500 ⁰C), which consequently oxidized (500-600 ⁰C) to generate the Cu-BO3 intermediate where it degrades upon further heating (600-900 ⁰C) by releasing B2O3 fumes leaving behind a total mass gain of 7.9% CuO residues. Considering the amounts of CuO formed due to the Cu-H oxidation early in the process (5.6%); thus, the amount corresponding to oxidation of the Cu-BH4 phase can be deduced from the difference (2.3%). The molar ratio of these two phases can be calculated from the obtained data and found to be 2.9 Cu I -H: 1.0 Cu II (BH4)2, which is consistent with the obtained magnetic data (vide infra).

Powder X-Ray Diffraction of the Prepared MT-BHs (p-XRD)
The p-XRD profiles in Figure 5 were analyzed by inspecting the diffraction patterns,  The diffraction parameters are summarized in Table 2. The crystalline phase in the structure was estimated from the area under the broad peaks in the P-XRD profiles. The corresponding interspatial distances (d-spacing) were calculated from the position of the most intense peak as given by Bragg's Law (n λ = 2 d sin θ) where n, λ, and θ are the diffraction order, wavelength of the X-ray beam, and diffraction angle, respectively. The interspatial distances were found to range from 2.04 to 2.47 Å. Furthermore, the crystallite size (t) was calculated as described in the Scherer formula (t = K λ / β cos θ) where K (Scherer constant, 0.95), λ (wavelength of the X-Ray), θ (Bragg's angle of the most intense peak) and β (peak width at half-height in radians). It is clear from the crystallite/grain size in Table 2 that the prepared MT-BHs are nano-scaled particles in the range of 6.4-26.2 nm. a. The relative peak heights (I/Io) are given in brackets. The most intense peak is given in bold.
Here, we report the lines that give a relative intensity of more than 20%. b. Estimated from the area under the diffraction peaks. c. Calculated from n λ = 2d sin θ. d. Calculated from Scherer formula (t = K λ / β cos θ).

Magnetic Characteristics of the Prepared MT-BHs
The magnetic characteristics of the prepared MT-BHs were investigated using a static magnetic susceptibility method at room temperature. The magnetic data in Table 3 shows that the determined magnetic moment (μeff) per molecular formula for Fe-BH4, Co-BH4, Ni-BH4, Ni-BH4 belong to multi-center systems while but not Cu-BH4 is an exception. The observed magnetic data for Ni II -BH4 demonstrates that the Ni II is situated in an octahedral environment (t2g 6 eg 2 ) rather than the diamagnetic square planar geometry (t2g 6 dz2 2 ). Applying the semiempirical scaling laws of µeff of clusters per magnetic site where their magnetic properties depend on different inherited structural factors, including cluster size, bonding, and geometry, we conclude that Fe-BH4 and Co-BH4 belong to trimetallic ferromagnetic aggregates, while the Ni-BH4 belongs to bimetallic ferromagnetic class [50,51]. This conclusion is supported by the fact that MT-BHs may adopt tetragonal clusters but not layer structures as supported by the P-XRD [26,35-38, 41,44].
On the contrary, the low μeff value of Cu-BH4 clusters (0.64 BM) compared to the μspin value of Cu II sites (d 9 , 1.73 BM) suggests that this material comprises diamagnetic Cu I (d 10 ) centers. This conclusion of biphasic Cu I -Cu II structure is consistent with the thermo-oxidative that the prepared Cu-BH4 material comprises Cu-H and Cu(BH4)2 phases. The molar ratio of Cu I : Cu II calculated from the magnetic data was found to be 2.7: 1, consistent with the molar ratio calculated in the thermo-oxidative section of 2.9: 1.

Electrical Conductivity (σ)
The electrical current (I) that develops between the hot and cold brass discs across the prepared MT-BHs was examined by plotting their I-V curves, Figure 6. The observed nonlinear trend suggests nonohmic behavior observed in diodes, thermistors, heated filaments, and photovoltaic cells where the dynamic resistance is reported to decrease with temperature.
This behavior can be attributed to various parameters, the most important of which are the temperature, current density, the time over which the electric field is applied, not to ignore the contribution of the induced ionic conduction [52,53]. Recently, Katsufuji etal. addressed the nonlinear behavior of resistance in various disordered systems and reported that the resistivity decreases as a function of current density /electric field but does not rely on the temperature or the materials. They discussed their observation in the light of the percolation conduction theory in disordered systems [53].
Here, the observed nonlinear curve indicates that the conduction in these MT-BHs is not purely electronic but being influenced by different possible factors such as temperature, induced ionization, magnetic field effects, and phonon migration. For that, the nonlinearity of the I-V curves was further analyzed by taking the logarithmic form as described by Equation 9 and 10 where I, V, α, and G are the current, voltage, ohmicity, and conductance (G = σ x area/thickness), respectively [54]. The α can be estimated from the slope of Ln (I) -Ln (V)

Ln(I) = α Ln(V) + Ln(G) (10)
linear plots in Figure 6.    Table 4. Electrical and thermal conductivity data for the investigated MT-BHs. The σ is further investigated as a function of temperature and found that the plot of  vs. T is linear, Figure 7. This increasing trend indicates that the ionic/mass diffusion becomes more prominent while phonon contribution becomes less prominent at higher temperatures.
However, the thermal heat transfer in TE semiconductors involves multi-transport pathways comprised of an electronic component due to charge carrier migration (e -& h + ) and phonon scattering due to lattice dynamics [4,55]. Thus, κtotal comprises two main contributions, an electronic (κel) and lattice (κlat) where κel is influenced by charge carrier concentration, mobility, their ratio, bipolar interactions, and scattering mechanisms similar to σ while κlat is generally influenced by lattice vibration, Equation 12 [4,55,56]. Fortunately, κel can be approximated from the σ values using Wiedemann-Franz law (κel = L σ T) where L is the Lorenz number (2.44 × 10 −8 WΩK −2 ). Although this law is best applied to metal, it is widely employed for semiconductors [4,55]. The calculated κel data in Table 4 demonstrates that the κtotal = κel + κlat (12) electronic contributions to κtotal are in the mW/Km range, indicating that the thermal conduction occurs essentially through lattice vibration. In other words, the thermal energy transport in these materials is basically phononic rather than electronic where κtotal ≈ κlat. In this respect, amorphous/ glassy semiconductors, as seen in our case, exhibit low κ as compared with crystalline semiconductors [55,56]. Furthermore, the multi-center clustering and nano-sized MT-BHs allow phonons scattering and thus suppress κlat in these semiconductors. Therefore, we attribute this observation to the limited mobility of the charge carriers, where they are scattered at the interfacial boundary of the amorphous/vitreous glassy structures.
To gain more insight into the relation between σ and κ in the examined MT-BHs, we plotted their ratio σ / κ versus T, Figure 8. The σ / κ quantitative data in the low and hightemperature ranges examined in this study are listed in Table 4

Thermoelectric Characteristics of the investigated MT-BHs
The key parameters that are generally used to characterize TE semiconductors include three parameters, S, PF, and ZT, where their interrelations have been the subject of several theoretical and experimental studies [3,4,16,21,22]. Here, it should be emphasized that the temperature limitation, 303 ≤ T ≤ 373, in this investigation is governed by the thermo-oxidative stability and our intention to prepare and examine semiconductors for waste heat-current conversion produced by body heat, sunshine, and domestic devices for small-scale TE applications.
The performance of TE semiconductors can be evaluated by examining their PF, which is highly influenced by  and S parameters (PF =  ⁎ S 2 ). Figure 10   The performance of TE semiconductors can also be deduced from their ZT values (ZT = (/) S 2 T), where this parameter relies directly on , S, and T but inversely on , Table 5.
The plots in Figure 10 show that the ZT values of Ni-BH4 increase rapidly in a nonlinear fashion while Fe-BH4, Co-BH4, and Cu-BH4 exhibit linear trends with relatively small temperature influence. However, extrapolation of the ZT -T plot of Ni-BH4 shows it approaches the value ZT = 1 that allows its use for practical application at about 400-450 K.
Furthermore, recent reports show that the TE efficiency (η%) can be calculated from average ZT values in the specified temperature range as in Equation 13. The η efficiency data in Table   5 shows that Ni-BH4 (2.56%) is far better than those of the other three MT-BHs (0.01-0.33%). Given the fact that TE measurements are sensitive to moisture, impurities, particleparticle interactions, and grains orientation within the examined cold-pressed discs, the reported TE values were found to be reproducible within 5-7%. In general, the S, PF, and ZT values for Ni-BH4 (S, 80 μVK -1 ; PF, 259 μW m -1 K -2 ; ZT 0.64; η, 2.56% ) were higher than those for the other three MT-BHs investigated in this study. Comparison with the TE values of n-doped ZnO nanoparticle at 375 K (S, -279 μVK -1 ; PF, 59 μW m -1 K -2 ; ZT ~0.075) shows that the Ni-BH4 offer a promising p-type candidate for TE applications.

Conclusions
The data in this study shows that using organic solvents rather than aqueous media is appropriate to minimize/avoid metal oxide side-products formation in the preparation of metal borohydrides nanoparticles. Unlike the generally observed trends where the thermal and electrical conductivities show similar trends on temperature, the reverse dependence is observed in the given MT-BHs; thus, MT-BHs and future MT-Bs semiconductors are promising candidates for TE applications. Our findings show that Ni-BH4 is a promising candidate to convert low-temperature waste heat to electric current resulting from body heat, sunshine, and domestic devices for small-scale applications at ambient conditions. Although the ZT values of the Ni-BH4 are less than those reported for the present late metal chalcogenides, it is advantageous for its simple preparation, moderate temperature dehydrogenation pathway to form Ni-Bs and applicability at ambient conditions. Our future endeavor is to investigate the influence of magnetic polarization and cold press compression on the TE behavior of Ni-BH4 and its fluoride analog (Ni-BF4). Figure 1 Device for thermoelectric properties.        PF and ZT dependence on temperature for obtained MT-BHs.