Enhanced Thermopower In Thermogalvanic Cells Using D2O As a Solvent

Thermogalvanic cells have emerged as promising devices for direct heat-to-electricity conversion. In this study, we investigate the utilization of heavy water $\left(\text{D}_{2} \text{O}\right)$ as a solvent in thermogalvanic cells, with a focus on two systems: $\text{K}_{3}\left[\text{Fe}(\text{CN})_{6}\right] / \text{K}_{4}\left[\text{Fe}(\text{CN})_{6}\right] \quad$ and guanidinium chloride (GdmCl) added $\text{K}_{3}\left[\text{Fe}(\text{CN})_{6}\right] / \text{K}_{4}\left[\text{Fe}(\text{CN})_{6}\right]$. For the $\text{K}_{3}\left[\text{Fe}(\text{CN})_{6}\right] / \text{K}_{4}\left[\text{Fe}(\text{CN})_{6}\right]$ system, D2O consistently exhibits a higher Seebeck coefficient compared to regular water $\left(\text{H}_{2} \text{O}\right)$ across a wide range of concentration levels, which could be attributed to the dielectric constant difference of D2O. However, the higher internal resistance resulting from D2O 's higher viscosity offsets the improved Seebeck coefficient, leading to a diminished power output. For the GdmCl added $\text{K}_{3}\left[\text{Fe}(\text{CN})_{6}\right] / \text{K}_{4}\left[\text{Fe}(\text{CN})_{6}\right]$ system, the cell utilizing D2O not only shows a higher Seebeck coefficient but also demonstrates a higher power output, unlike the system without GdmCl addition. UV-Vis measurements and quantification of precipitated crystals provide evidence of a stronger interaction between $\text{Gdm}^{+}$ and $\text{Fe}(\text{CN})_{6}{ }^{4-}$ in D2O. The higher reaction equilibrium constant between $\text{Gdm}^{+}$and $\text{Fe}(\text{CN})_{6}{ }^{4}$ in D2O supports the observed enhancement in $S_{\text{e}}$, consistent with our previous studies. This investigation highlights the promising potential of D2O as a solvent in thermogalvanic cells, offering valuable insights into the intricate interplay between solvent properties, ion interactions, and thermoelectric performance.