(Invited) Recent Advances in Thermally Regenerative Batteries: A New Approach for Generating Electrical Power from Low-Grade Heat

Abstract

Thermally regenerative flow batteries (TRBs) seek to tap into the unused terawatt-hours of low-grade heat that are currently being expelled from industrial plants and stationary power sources each year. Unlike typical flow batteries, TRBs can be recharged using thermal energy, therefore increasing grid efficiency by using an energy stream that is already widely available. This unique feature is accomplished through a thermally recoverable energy carrier that, when added or removed from an electrolyte, dramatically alters the standard electrode potential of an electrode reaction. To date, most TRBs use ammonia as the energy carrier, which lowers the standard electrode potential of their anode reaction by complexing with transition metal ions such as copper, silver, or zinc. Despite the early successes of TRBs relative to other emerging electrochemical technologies trying to harness low-grade thermal energy sources, they still lacked the energy efficiency and power densities needed to be economically viable. The first iterations of TRBs had peak power densities up to 3 mW cm-2 with competitive theoretical energy efficiencies (up to 1 %), but they suffered from extreme coulombic efficiency issues that limited realizable energy production values. Herein, we will discuss recent changes that have greatly improved the performance possible from these systems and discuss persistent challenges that remain to be solved. Coulombic inefficiencies were resolved by replacing unstable deposition-based reactions with all-aqueous alternatives that provided larger cell potentials and coulombic efficiencies for single metal and bimetallic-TRB systems. Electrolyte composition, operating temperature, and membrane transport all strongly influenced energy efficiency and power density limits from these new electrolyte chemistries. Peak power densities up to 100 mW cm-2 and theoretical energy efficiencies up to 12 % were identified through a preliminary survey of possible design choices. However, tradeoffs between attainable power density and energy efficiency limited cell performance values to a fraction of these values. Future work to better control diffusive transport without conductivity losses will be crucial for approaching theoretical battery performance limits.