Zn-Sn Electrochemical Cells with Molten Salt Eutectic Electrolytes and Their Potential for Energy Storage Applications

The energy required to run carbon capture systems (CCS) constitutes a huge fraction (30% or more) of that which is produced by coal-fired power plants (1). This parasitic consumption is a major impediment to CCS implementation and retrofitting. The most energy-intensive element of CCS is the reboiler which provides thermal energy for the regeneration of amine-based CO 2 capture solvents. Our efficiency-boosting method stores excess electrical energy (produced during off-peak hours at baseload power plants) as chemical free energy. This device, resembling a secondary battery, is then optimized to preferentially produce thermal energy in its discharge phase for on-demand, solvent-regenerating heat when required during peak electricity use. We have developed several embodiments of the technology utilizing low cost electrode materials (Zn, Al, Sn, Bi and/or Pb) and molten salt eutectic electrolytes (ZnCl 2 :KCl, SnCl 2 :KCl, AlCl 3 :NaCl:KCl) (2). Figure 1(a) shows heat-generation for a Zn-Zn(Sn) system where a Zn anode is reversibly plated and stripped during charge and discharge, respectively (0 V discharge shown). This uses an air-stable ZnCl 2 :KCl eutectic electrolyte and molten Sn cathode where Zn reversibly forms an alloy. A ΔT max = 6 °C, steady current density of 10 mA cm -2 , and 64% capacity retention after the first cycle demonstrates the proof-of-concept of this novel system which utilizes only ~50 g of earth-abundant active materials in this embodiment. The simple design – one active charge carrying species, Zn 2+ , in a one-compartment, separator-free cell – has shown promise as a battery technology with other materials, albeit at higher temperatures (3, 4). In an alternate, traditional galvanic cell design, Figure 1(b) demonstrates the temperature dependency of the open circuit voltage (OCV) during cooling (at t =0, cell T= 377 °C and heater is switched off) of a fritted H-cell containing Zn(s)|ZnCl 2 :KCl(l, eut.)||SnCl 2 :KCl(l, eut.)|Sn(l) (m.p. = 230, 176, 232 °C). As the molten materials freeze (around 2 h), there is an optimal operating temperature for this binary compartment cell. Interestingly, the OCV is poorer at higher temperatures (an opportunity for energy/cost reduction) and is still high and fluctuating significantly well after all materials are frozen (>3 hours). Ongoing experiments are exploring the mechanism behind this behavior and investigating lower temperature charge/discharge capacities. The thermal energy dissipated upon