A vast amount of low-grade heat (<100 °C) exists in industrial processes, the environment, biological entities, and solar-thermal and geothermal energy. Conversion of this low-grade heat to electricity is difficult due to the distributed nature of these heat sources and the low temperature differential. Different technologies, such as solid-state thermoelectric energy conversion and organic Rankine cycles, are being actively investigated but face their own challenges in energy conversion efficiency, cost, and system complexity. Thermally regenerative electrochemical cycle (TREC) is an alternative approach based on the temperature dependence of cell voltage of electrochemical systems.

—Yang et al.

TREC entails a four-step process:

1. heating up the cell with waste heat;
2. charging at the higher temperature;
3. cooling down the cell; and
4. discharging at low temperature.

Because the charging voltage is lower at high temperatures than at low temperatures, once the battery has cooled it delivers more electricity than what was used to charge it—i.e., converting heat to electricity.

The concept of TREC was developed a few decades ago, the researchers note, but focused on high-temperature applications (500−1500 °C) and showed efficiencies up to 40−50% of the Carnot limit. Low-temperature TREC did not received as much attention since electrode materials with low polarization and high charge capacity at low temperature were limited.

The low-temperature TREC system on which the research team earlier reported was based on a copper hexacyanoferrate (CuHCF) cathode and a Cu/Cu²⁺ anode. The low polarization of electrodes, moderate temperature coefficient, high charge capacity, and low heat capacity led to a high efficiency of 5.7% when the cell was operated between 10 and 60 °C, assuming a heat recuperation efficiency of 50%.

Schematic of thermally regenerative electrochemical cycle (TREC). (a) The voltage−capacity plot of a TREC. Net energy is generated because the discharge voltage is higher than charge voltage. (b) The corresponding temperature−entropy (T− S) plot. (c) A TREC with an ion-selective membrane to block certain ions to avoid side reactions (i.e., the earlier system). (d) The new membrane-free NiHCF/Ag/AgCl system with no unwanted reaction between electrodes and ions. Credit: ACS, Yang et al. Click to enlarge.

However, one potential issue they identified with their system was the use of an ion-selective membrane to allow NO₃⁻ anion passing through but not Cu²⁺ cations to avoid side reaction between CuHCF and Cu²⁺. Ion-selective membranes are currently expensive; further, it would be difficult to block completely the penetration of Cu²⁺ in long-term operation. A membrane-free systems would lower the cost and facilitate long-term operation, making the TREC battery more practical, they concluded.

To address this issue, we apply a criterion that any soluble chemical species in electrolyte should not induce adverse side reactions other than the desired two half-cell reactions. In this paper, a membraneless electrochemical system with a nickel hexacyanoferrate (NiHCF, KNi^IIFe^III(CN)₆) cathode and a silver/silver chloride anode is demonstrated, where no adverse side reaction is introduced due to solutes in electrolyte. … In this system, ions involved in each electrode do not have side reactions with each other, so the ion-selective membrane is unnecessary and can be replaced by an inexpensive porous separator.

…We believe that further optimization and searching for new systems will lead to new development and possibly practical deployment of TREC.

—Yang et al.

Resources

• Yuan Yang, James Loomis, Hadi Ghasemi, Seok Woo Lee, Yi Jenny Wang, Yi Cui, and Gang Chen (2014) “Membrane-Free Battery for Harvesting Low-Grade Thermal Energy” Nano Letters doi: 10.1021/nl5032106