Lithium Manganese Oxide in an Aqueous Electrochemical System for Low-Grade Thermal Energy Harvesting

Abstract

Low-grade heat (<100 °C) is abundant but mostly wasted because its utilization requires efficient energy harvesting systems with low cost and high efficiency. The thermally regenerative electrochemical cycle is a promising strategy to harvest low-grade heat, which exploits the dependence of electrochemical potential on temperature. In each cycle between low and high temperature, the electrochemical cell is charged at a lower voltage and discharged at a higher voltage, therefore converting heat to electricity. The temperature coefficient (α) of the system is a key parameter to determine the conversion efficiency, and the α value of the full cell will be maximized if those of the cathode and the anode have opposite signs. Previously, most studies have worked on electrode materials with negative α. In this study, we focused on lithium manganese oxide (LMO), a widely used lithium-ion battery cathode material, showing a positive α of 0.62 mV K −1 and stable performance in an aqueous electrolyte. We demonstrate an electrochemical system consisting of an LMO cathode and a copper hexacyanoferrate anode in the Li + and K + hybrid electrolyte for low-grade heat harvesting. The α of the full cell is 1.061 mV K −1 and the heat-to-electricity conversion efficiency can reach 1.8% in the temperature range of 10−40 °C. The relative efficiency reaches 18.8% of the theoretical Carnot limit, which is enhanced to 27% with the assumption of 50% heat recuperation. This work may open new opportunities for studies on the electrode materials with positive α and hybrid electrolyte systems with both a positive-α and a negative-α material for efficient low-grade heat harvesting.

Efficient Low‐Grade Heat Harvesting Enabled by Tuning the Hydration Entropy in an Electrochemical System

“…The calculated energy density of the TREC demonstrated in Figure 3d is 7.81 J g −1 , which is the largest value reported in TREC systems so far. [ 21,34 ] The calculated efficiency η e is 4.34% without heat recuperation (η HR = 0%), which is 35.05% of Carnot efficiency. The η e can be improved to 6.21% and 7.49% when 50% and 70% of heat recuperation η HR are utilized, respectively.…”

Progress on the performances and applications of supercapacitors for thermoelectric conversion

“…Mu 等人 [38] 制备出以氧化石墨烯为正极，聚苯胺为负极，1 mol/L KCl 溶液作为电解质的超级电容器，测试装 示了一种极具前景的低品位热回收利用的新技术。Liu 等人 [39] 制备的超级电容器正极材料为锂锰氧化物，负极材 料是铜基普鲁士蓝类似物，电解液为 KNO 3 和 LiNO 3 混合溶液，通过充电、升温、放电、降温将热能转换为电能， 示意图如图 5(b)所示。超级电容器的温度系数为 1.061 mV/K，热电转换效率为 1.8%，相对卡诺循环效率为…”

“…图 5 (网络版彩色)无温差下需外加电源的系统示意图 (a)用热锅充电的超级电容器 [38] ;(b)超级电容器热电转换过程 [39] Fig. 5 (Color online) Schematic diagram of the system that requires an external power supply without temperature difference (a)…”

“…supercapacitor charged by a hot pot [38] , (b) process of thermoelectric conversion in a supercapacitor [39] 通过优化电极材料的结构，特别是制备复合电极材料，利用不同材料本身的优点，进一步提高超级电容器的 热电转换效率。Jiang 等人 [42] 进一步将螺旋碳纳米管与钴基普鲁士蓝类似物进行复合，形成多孔的网状结构，增 大了比表面积，使得复合材料与纯钴基普鲁士蓝类似物相比具有更高的温度系数、比电容以及更低的比热、电阻， 大大提高了超级电容器的热电转换性能。…”

Copper Hexacyanoferrate Thin Film Deposition and Its Application to a New Method for Diffusion Coefficient Measurement

“…Because PBAs can function in the presence of various alkaline ions and even multivalent ions, various ion-based batteries have been developed [ 10 ]. In addition, PBA-based batteries can harvest low-grade heat energy because of their high energy efficiency [ 5 , 11 , 12 ].…”