Aqueous electrochemical system of Prussian blue analogue for low-grade thermal energy harvesting

• Main Author: Liu, Yezhou
• Other Authors: Lee Seok Woo
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering::Energy conservation; Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/151432
• Institution: Nanyang Technological University

With the growing demand for green energy to replace fossil fuel, the explosion of clean energy will never stop. Usually, clean energy includes solar energy, wind energy, geothermal heat, hydropower, bioenergy, and so on. A lot of research has been done on solar energy, wind energy, and hydrogen energy and their applications have been established all over the world. But low-grade thermal energy (<100 °C) is an abundant and reversible energy source but mostly wasted, in lack of low-cost and high-efficiency systems. The thermally regenerative electrochemical cycle (TREC) is a powerful candidate in low-grade heat harvesting by utilizing the relationship between electrode potential and temperature. In a TREC cycle, the charging voltage of the electrochemical cell is lower than the discharging voltage, therefore, converting thermal energy to electrical energy. The temperature coefficient (α) is the ability to change voltage with the changing temperature. Previous research focused on the applications of electrode materials with negative α. Lithium manganese oxide is a cathode material widely used in energy storage. It has stable performance in aqueous electrolyte and a positive α of 0.617 mV K-1. In this thesis, I demonstrate an electrochemical cell for low-grade thermal energy harvesting. This cell includes a lithium manganese oxide (LMO) cathode and a copper hexacyanoferrate (CuHCF) anode in LiNO3 and KNO3 hybrid electrolyte. The full cell has a temperature coefficient of 1.161 mV K-1 and heat-to-electricity conversion efficiency of 0.4% in the temperature range of 10- 50 °C. This work may start the opportunities for positive α material in low-grade heat harvesting. Electric Energy Storage (EES) is as important as the energy conversion devices in the energy harvesting area. But 10-60% of the energy will be wasted in EES systems. Energy loss limits the development of EES. TREC can be applied to EES to reduce energy loss. A battery with a negative α may work with Solar Panel, utilizing the day and night temperature fluctuation. An electrochemical full cell consisting of copper hexacyanoferrate (CuHCF) cathode and Zinc metal anode in Zn2+ and K+ hybrid electrolyte is demonstrated as a temperature coefficient assisted energy storage battery. The battery has a voltage of 1.78 V, a temperature coefficient of 1.09 mV K-1 and reduces energy loss from 4.12% to 0% when operating between 10 to 30 ℃. With the help of thermal energy harvesting, the energy efficiency of the solar cell system can be further improved. The temperature coefficient of the TREC system is the most important parameter because it determines the ability of energy conversion. Previously, many studies have worked on electrodes with higher temperature coefficient in aqueous electrolytes. But few of them have focused on the influence of the solvents. Here, the temperature coefficient differences of the same electrodes in different solvents, water and acetonitrile are demonstrated. The relation between the temperature coefficient and entropy change is discussed. For the first time, the concept of the temperature coefficient difference between solvents are utilized on thermal energy harvesting. The results of this study may provide a fundamental understanding of the temperature coefficient in different solvents and a new application of the large temperature offset between different solvents on low-grade heat harvesting.