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 CO₂capture 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 (ZnCl₂:KCl, SnCl₂:KCl, AlCl₃: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 ZnCl₂: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, Zn²⁺, 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)|ZnCl₂:KCl(l, eut.)||SnCl₂: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 discharge of these cells can be subsequently captured via heat-transfer fluids (e.g. steam) for injection into the CCS stripper tower. Our studies are fine-tuning the chemistry and engineering necessary to control the balance of heat/electricity generation in these secondary cells; we ultimately envision a molten salt-based hybrid thermal/electrical energy storage medium. It is anticipated from Figure 1(b) that the electrochemical cell can self-heat to desired operating temperatures (achieving molten phase transition) with the excess electricity available during the charging phase.

Molten salt electrolytes for electric-to-thermal energy transduction from this study may lead to further progress in niche battery applications since few commercial products of this type operate in the 250-400 °C range.

References

1.   E.S. Rubin, et al., Progr. Energy Combust. Sci., 38(5), 630 (2012).

2.   J.K. Neathery, et al., A Method for Energy Storage to Utilize Intermittent Renewable Energy and Low-Value Electricity for CO2 Capture and Utilization. 2014.

3.   H. Kim, et al., J. Power Sources, 241, 239 (2013).

4.   D.J. Bradwell, et al., J. Am. Chem. Soc., 134(4), 1895 (2012).

Figure 1. (a) Current density (black curve, primary axis) and heat generated (red curve, secondary axis) during 1 h of 0 V discharge of the Zn-Zn(Sn) cell with ZnCl₂:KCl eutectic electrolyte. (b) OCV sampled during cooling of the second Zn-Sn cell type with frit-separated ZnCl₂:KCl and SnCl₂:KCl eutectics; OCV was steady at 0.22 V and T=377 °C when heater switched off at t=0.