# Electrochemical Ion Pumping Device for Blue Energy Recovery: Mixing Entropy Battery
## 7 Conclusions and Outlooks
The proof-of-concept for MEB studies tested with a combination of different electrolytes such as real wastewater effluent, fresh water or artificial recovery solutions; real or artificial seawater or brine; with Sodium Manganese Oxide (NMO) or Lithium Iron Phosphate (LFP) as a cationic electrode and _AgIAgCl_ as anionic electrode have been shown to have high energy recovery efficiency values (68 to 75%). Unfortunately, some drawbacks are that (i) the _AgIAgCl_ electrode is costly and partially soluble in seawater or hypersaline solutions such as Atacama brines, (ii) the NMO and LFP electrodes are also costly with a moderate specific capacity and (iii) the MEB operation requires a charging step with an upfront energy investment, increasing the operational complexity. A very convenient attribute is the open-framework structure belonging to PBA materials, that allows fast insertion/extraction of cations into/from the interstitial sites or crystal structure, evidenced by over 1000 charge/discharge cycles and lower cost (<1 USD/kg) [40]. It is interesting that anionic materials in electrochemical devices such as _BiBiOCl_, _AgIAgCl_ and _PPy_ have not only been tested in MEB batteries but also in chloride sensors, deionization devices and Lithium recovery from brines. The initial high adsorption/desorption activity for _Cl_- ions exhibited by _AgIAgCl_ and _BiOCl_ degrades during the cycles, as higher thermodynamic and kinetic energy consumptions gradually increase, whereby the recovered blue energy is reduced over time. The _PPy_ that is the cheapest material (<3 USD/kg) [40] as anode electrode presents the best behavior when capturing _Cl_- ions during cycling, this being manifested as a lower energy consumption. In this potential range the _PPy_ electrode also showed an excellent cycling performance that was close to 100% coulombic efficiency (ranging from 95.7 to 97.8%) and 92% capacity retention through 50 cycles [40]. Other reported electrodes made of inorganic materials such as _MgMn\({}_{2}\)O\({}_{4}\)_[69; 70; 71], _Na\({}_{2}\)FeP\({}_{2}\)O\({}_{7}\)_[72], _NaTi\({}_{2}\)_(PO\({}_{4}\))\({}_{3}\)_[73], _Na\({}_{0.44}\)MnO\({}_{2}\)_[74], _FeFe_(CN)\({}_{6}\)_[75], _K\({}_{4}\)Fe_(_CN)\({}_{7}\)_[76], _K\({}_{0.03}\)Cu_[_Fe_(_CN)\({}_{6}\)_]\({}_{0.65}\cdot 0.43\)_H\({}_{2}\)_O_[77], _NaFeHCF_[78], _MnHCF_[79], _Na\({}_{3}\)V\({}_{2}\)_(PO\({}_{4}\))\({}_{3}\)_[80], _NaMnO\({}_{2}\)_[81] and _NaNiHCF_[78] have been explored as active materials for Faradaic ions storage in electrochemical desalination devices. Recently new anode materials have been reported and are under development, such as _TiOCl_[82], _VOCI_[83; 84], _VOCI_[83; 84], _SnOCl\({}_{2}\)_[85], _FeOCI_[86; 87; 88], _PbCl\({}_{2}\)_[89] and _Bi_2O\({}_{3}\)[90]. Thus, at present development of the MEB technology, _PBA_ and _PPy_ seem to be the ideal candidates to be used as cationic and anionic electrode materials. Both have lower costs, overlapping potential range, sensitivity to _NaCl_ and _LiCl_ concentration changes and excellent cycling performance and stability in aqueous solutions. **Author Contributions:** Conceptualization, methodology, and writing-original daft preparation, F.G.; visualization, writing review and editing, L.C. ; sources and editing, L.M. ; visualization, writing review and editing, A.S. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Acknowledgments:** The author would like to the Programa de Doctorado en Ingenieria de Procesos de Minerales of the University of Antofagasta, Chile. **Conflicts of Interest:** The authors declare no conflict of interest.