In May, researchers at MIT and Stanford University reported the development of new battery technology for the conversion of low-temperature waste heat into electricity in cases where temperature differences are less than 100 ˚Celsius. The thermally regenerative electrochemical cycle (TREC) uses the dependence of electrode potential on temperature to construct a thermodynamic cycle for direct heat-to-electricity conversion. By varying the temperature, an electrochemical cell is charged at a lower voltage than discharged; thus, thermal energy is converted to electricity. (Earlier post.)
Now, in a paper in the ACS journal Nano Letters, the team reports a refinement of the earlier Prussian blue analog-based system system, which although it operated with high efficiency, used an ion-selective membrane which, in turn, raised concerns about the overall cost. The refined system is a membrane-free battery with a nickel hexacyanoferrate (NiHCF) cathode and a silver/silver chloride anode. When the battery is discharged at 15 °C and recharged at 55 °C, thermal-to-electricity conversion efficiencies of 2.6% and 3.5% are achieved with assumed heat recuperation of 50% and 70%, respectively.
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:
- heating up the cell with waste heat;
- charging at the higher temperature;
- cooling down the cell; and
- 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%.
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, KNiIIFeIII(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.
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