# Thermodynamics of Ion Separation by Electrosorption

**Authors:** Ali Hemmatifar, Ashwin Ramachandran, Kang Liu, Diego I. Oyarzun, Martin Z. Bazant, Juan G. Santiago  
**arXiv:** [1803.11532](https://arxiv.org/abs/1803.11532)  
**Date:** 2018-03-30  
**Summary page:** https://www.summarizepaper.com/en/paper/1803.11532v1/

## Key points

- The paper presents a study on minimum energy consumption in electrosorptive ion separation
- Focuses specifically on electrostatic capacitive deionization (CDI)
- Develops a top-down approach using the variational form of the Gibbs free energy
- Theoretical framework can be applied to any thermodynamically consistent model
- Minimum required electric work energy is equivalent to the free energy of separation under certain assumptions
- Defines thermodynamic efficiency for CDI using their theoretical framework
- Current experimental CDI systems have very low efficiencies, typically less than 1%
- Constructed and operated an optimized CDI cell with significantly higher thermodynamic efficiency of 9%
- Achieved highest ever demonstrated thermodynamic efficiency for CDI
- Further improvements in efficiency can be achieved by reducing CDI cell series resistances and optimizing operational parameters
- Provides insights into understanding and improving energy consumption in electrosorptive ion separation processes, particularly CDI
- Offers practical implications for designing more efficient CDI systems.

## Summary

The paper titled "Thermodynamics of Ion Separation by Electrosorption" presents a comprehensive study on the calculation of minimum energy consumption in electrosorptive ion separation. The authors focus specifically on electrostatic capacitive deionization (CDI) and develop a top-down approach using the variational form of the Gibbs free energy. The theoretical framework proposed in this study is independent of the specific details of the double-layer charge distribution and can be applied to any thermodynamically consistent model, including the Gouy-Chapman-Stern (GCS) and modified Donnan (mD) models. The authors demonstrate that, under certain assumptions, the minimum required electric work energy is equivalent to the free energy of separation. One key contribution of this research is the definition of thermodynamic efficiency for CDI using their theoretical framework. The authors explore the thermodynamic efficiency of current experimental CDI systems and find that these systems have very low efficiencies, typically less than 1%. To improve upon this, they construct and operate a CDI cell with judicious selection of materials, geometry, and process parameters. This optimized CDI cell achieves a significantly higher thermodynamic efficiency of 9%, corresponding to 4.6 kT energy per removed ion. This achievement represents by far the highest thermodynamic efficiency ever demonstrated for CDI. Based on their findings, the authors hypothesize that further improvements in efficiency can be achieved by reducing CDI cell series resistances and optimizing operational parameters. This suggests potential avenues for future research in enhancing the performance of CDI systems. Overall, this paper provides valuable insights into understanding and improving the energy consumption in electrosorptive ion separation processes, particularly focusing on electrostatic capacitive deionization. The results presented here contribute to advancing our knowledge in this field and offer practical implications for designing more efficient CDI systems.