Simultaneous enhancement of thermopower and ionic conductivity for N-type Fe(III/II) thermocell

Emerging thermocells (TECs) are promising for low-grade heat harvesting, but the lack of high-performance n-type TECs remains a significant bottleneck for the practical application of p-n integrated TEC devices. Due to the well-known inherent interdependence between thermodynamics and kinetics, simultaneously improving the thermopower ( Se , a thermodynamics parameter) and ionic conductivity ( σ i , a kinetics parameter) of TECs is challenging. Here, a simple perchloric acid (HClO 4 ) incorporation method is developed to enhance the charge density of the oxidant Fe(III) ions in the state-of-the-art n-type Fe(III/II)-ClO 4 redox pair (0.8M), thereby improving the Se and σ i simultaneously. In Fe(III/II)-ClO 4 electrolyte, the addition of HClO 4 composed of protons and weakly coordinating anions suppresses the deprotonation of [Fe(H 2 O) 6 ] 3+ (small amount of monomeric [Fe(OH)(H 2 O) 5 ] 2+ and predominantly dimeric [(H 2 O) 4 Fe(OH) 2 Fe(H 2 O) 4 ] 4+ ) without inducing severe Fe(III)-anion combination. As a result, the deprotonated Fe(III) complexes in the electrolyte were converted to [Fe(H 2 O) 6 ] 3+ , and the ionic conductivity of the electrolyte increased by 127% to 37.1 S/m. The n-type TEC using HClO 4 -acidified Fe(III/II)-ClO 4 as electrolyte and hydrophilic carbon fiber cloth as the electrode was characterized and demonstrated a Se of 1.5 mV/K (comparable to -1.4 mV/K of benchmark p-type Fe(CN) 6 3- /Fe(CN) 6 4- TECs) and an excellent temperature normalized power density of 1.19 mW/m 2 /K 2 (2.64 times higher than that of the state-of-the-art n-type TECs using carbon electrodes), overcoming barriers for practical p-n integrated TEC applications. • A simple HClO 4 incorporation method was designed to increase the charge density and mobility of Fe(III) ions. • Thermopower (thermodynamics) and ionic conductivity (kinetics) of Fe(III/II) are improved simultaneously. • The normalized power density of HClO 4 -acidified Fe(III/II)-ClO 4 TECs increased by 73% to 1.19 mW/m 2 /K 2 . • The thermo-electric conversion performance of the n-type TEC is comparable to that of the p-type benchmark TEC.