Programmable Electrochemical Thermopower via Cation Storage Mode and Structural Order

This work aims to decouple and quantify the origins of thermopower (α) by controlling structure and charge-storage mechanisms within a single anatase TiO₂ chemistry. Li⁺ and Na⁺ were inserted into anatase TiO₂ particles with sizes of 30 and 5 nm to construct three model electrodes: a bulk crystalline intercalation electrode (30 nm, Li), a nanoscale intercalation and electrical double-layer (EDL) hybrid electrode (5 nm, Li), and a Na-induced amorphous, surface-dominated electrode (5 nm, Na). The 30 nm Li_xTiO₂ electrode shows an almost constant α of -1.5 to -1.6 mV K^-1 in the biphasic region, corresponding to behavior dominated by lattice-intercalation entropy. In contrast, amorphous Na_xTiO₂ shows α = -4.8 mV K^-1 at 0% degree of sodiation and saturates near -2.0 mV K^-1 with sodiation, indicating that structural disorder and interfacial entropy strongly enhance, |α|. The 5 nm Li_xTiO₂ electrode shows a continuous change in $\alpha$ from + 2.19 to -1.6 mV K^-1 and acts as an intermediate design point that combines and weights Faradaic and EDL contributions. These results demonstrate that α_Faradaic and α_EDL can be independently designed within a single anatase TiO₂ material system, providing a platform for electrochemical thermocells with thermopower of several mV K^-1.