Thermoelectrochemical Cells Based on Ferricyanide/Ferrocyanide/Guanidinium: Application and Challenges

Ferricyanide/ferrocyanide/guanidinium-based thermoelectrochemical cells have been investigated under different loading conditions in this work. Compared with ferricyanide/ferrocyanide-based devices, the device with guanidinium-added electrolytes shows higher power and energy densities. We observed that the enhanced performance is not due to the ionic Seebeck effect of guanidinium but because of the configuration entropy change resulting from the selective binding of Gdm⁺ to Fe(CN)₆^4-. However, the device with guanidinium-added electrolyte does not show steady-state operation. The two possible reasons include (1) the difficult diffusion of Fe(CN)₆^3- into the crystal layer of (Gdm⁺)_n[Fe(CN)₆^4-] at the hot electrode and (2) the difficult precipitation of (Gdm⁺)_n[Fe(CN)₆^4-] formed at the cold side upon the binding of the reduced Fe(CN)₆^4- with Gdm⁺. Nevertheless, the performance recovers once the device is disconnected from the external loading. Due to the high thermopower after adding guanidinium, we successfully fabricate self-powered sensors by connecting four flexible cells in series. The sensors can transfer humidity, temperature, and air pressure data wirelessly using body heat. Therefore, ferricyanide/ferrocyanide/guanidinium is a promising electrolyte material for applications of low-grade energy harvesting.