Thermocells Driven by Phase Transition of Hydrogel Nanoparticles.

Thermoelectric conversion of low temperature, delocalized, and abundant thermal sources is crucial for the development of the Internet of Things (IoT) and/or a carbon-free society. Thermocells are of great interest in thermoelectric conversion of low-temperature heat due to the low cost and flexibility of components. However, significant improvement of the conversion efficiency is required for the practical use of the cells. Here, we report thermo-electrochemical cells driven by volume phase transition (VPT) of hydrogel nanoparticles (NPs). Entropically driven VPT of poly(N-isopropylacrylamide) NPs containing carboxylic acids and amines generates a pH gradient of up to 0.049 and -0.053 pH K-1, respectively, around physiological temperature. The pH gradient triggers the proton-coupled electron transfer (PCET) reactions of quinhydrone on the electrodes, resulting in the highly efficient thermoelectric conversion with a Seebeck coefficient (Se) of -6.7 and +6.1 mV K-1. Thermocells driven by phase transition of hydrogels provide a nontoxic, flexible, and inexpensive charger that harvests carbon-free energy from abundant energy sources such as solar, body and waste heat.

Constructing CoNi-LDH/Fe MOF/NF Heterostructure Catalyst for Energy-Efficient OER and UOR at High Current Density.

The sluggish kinetics of the oxygen evolution reaction (OER) always results in a high overpotential at the anode of water electrolysis and an excessive electric energy consumption, which has been a major obstacle for hydrogen production through water electrolysis. In this study, we present a CoNi-LDH/Fe MOF/NF heterostructure catalyst with nanoneedle array morphology for the OER. In 1.0 M KOH solution, the heterostructure catalyst only required overpotentials of 275 and 305 mV to achieve high current densities of 500 and 1000 mA/cm2 for OER, respectively. The catalytic activities are much higher than those of the reference single-component CoNi-LDH/NF and Fe MOF/NF catalysts. The improved catalytic performance of the heterostructure catalyst can be ascribed to the synergistic effect of CoNi-LDH and Fe MOF. In particular, when the anodic OER is replaced with the urea oxidation reaction (UOR), which has a relatively lower thermodynamic equilibrium potential and is expected to reduce the cell voltage, the overpotentials required to achieve the same current densities can be reduced by 80 and 40 mV, respectively. The cell voltage required to drive overall urea splitting (OUS) is only 1.55 V at 100 mA/cm2 in the Pt/C/NF||CoNi-LDH/Fe MOF/NF two-electrode electrolytic cell. This value is 60 mV lower compared with that required for overall water splitting (OWS). Our results indicate that a reasonable construction of a heterostructure catalyst can significantly give rise to higher electrocatalytic performance, and using UOR to replace the anodic OER of the OWS can greatly reduce the electrolytic energy consumption.

Rational Design of Thermocells Driven by the Volume Phase Transition of Hydrogel Nanoparticles.

Thermocells are thermoelectrochemical conversion systems for harvesting low-temperature thermal energy. Liquid-state thermocells are particularly desirable because of low cost and their high conversion efficiency at temperatures around physiological temperature, and they have, thus, been extensively studied. However, the performance of the thermocells has to be improved to utilize them as energy chargers and/or batteries. Recently, we reported that a liquid-state thermocell driven by the volume phase transition of hydrogel nanoparticles showed highly efficient thermoelectric conversion with Seebeck coefficient (Se) of -6.7 mV K-1. Here, we report the design rationale of the thermocells driven by the phase transition. A high Se of -9.5 mV K-1 was achieved at temperature between 36 and 40 °C by optimizing choice and amount of redox chemical species. The figure of merit (ZT) of the thermocell was improved by selecting appropriate electrolyte salt to increase the ionic conductivity and prevent the precipitation of nanoparticles. Furthermore, screening of nanoparticles revealed the high correlation between Se and the pH shift generated as a result of phase transition of the nanoparticles. After optimization, the maximum ZT of 8.0 × 10-2 was achieved at a temperature between 20 and 70 °C.