Programmable Electrochemical Thermopower via Cation Storage Mode and Structural Order

Abstract

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 LixTiO2 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 NaxTiO2 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 LixTiO2 electrode shows a continuous change in 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 αFaradaic and αEDL can be independently designed within a single anatase TiO2 material system, providing a platform for electrochemical thermocells with thermopower of several mV K−1.