High power density charging-free thermally regenerative electrochemical flow cycle for low-temperature thermoelectric conversion

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2024-10, Vol.497, p.154778, Article 154778
• Main Authors: Tian, Hua, Liu, Yusong, Song, Zicong, Wang, Weiguang, Shu, Gequn
• Format: Article
• Language: English
• Subjects: Flow battery; Low-grade thermal energy; Power density; Temperature coefficient; Thermally regenerative electrochemical cycle
• ISSN: 1385-8947
• DOI: 10.1016/j.cej.2024.154778

•Cs+ and Gdm+ are added to I−/I3−//Fe(CN)63−/4− pairs to form concentration gradient.•The charging-free TREC achieves record high temperature coefficient of −3.6 mV/K.•The power density of charging-free TREC increases from 0.7 W/m2 to 1.4 W/m2.•The modified membrane is used to inhibit the migration of I− and I3− ions.•The charging-free flow TREC with modified membrane realizes 36 h continuous operation. Low-temperature thermal energy widely exists in nature and is sustainable. The thermally regenerative electrochemical cycle (TREC) based on the Seebeck effect has attracted widespread attention due to its high thermoelectric efficiency and good cycle stability. However, it has a low power density and cannot sustain continuous operation for extended periods. This work proposes a continuous charging-free thermally regenerative electrochemically cycled flow battery (TREC-FB) system, which can discharge continuously at both high and low temperatures without the assistance of external power source. By adding guanidine hydrochloride (GdmCl) and inorganic salt ions (Cs+, Ca2+, Mg2+, etc.) to the K3Fe(CN)6/K4Fe(CN)6 and I2/KI electrolytes respectively, precipitates are formed in the electrolytes at low temperature and dissolve at high temperature, which creates the concentration gradient of ions. As a result, the temperature coefficient of the full cell increases from −2.24 mV/K to −3.6 mV/K, and both voltage and power density are doubled. Theoretical analysis of the temperature coefficient enhancement is conducted based on the Nernst equation, and the mechanism of the temperature coefficient enhancement is verified using UV–vis spectra. The discharging capacity and energy of the charging-free TREC-FB with high temperature coefficient reach 5 Ah/L and 1.9 kJ/L, respectively, and the thermoelectric efficiency reaches 8.9 % under sufficient heat recuperation. Two TREC-FBs at high and low temperatures are connected to carry out continuous operation experiments of the charging-free TREC-FB system. The use of charge-reinforced ion-selective membrane can effectively inhibit the migration of I− and I3− ions, achieving about 36 h of continuous operation, and the average power density achieves 0.6 W/m2.