Efficient Capacitive Deionization Using Thin Film Sodium Manganese Oxide

Abstract

More energy efficient desalination methods are needed to address global water scarcity. Capacitive deionization (CDI) is an emerging electrochemical desalination technology that could outperform other desalination technologies if new electrode materials were developed with high salt sorption capacity and efficiency. In this paper, we report on the desalination performance of thin-film sodium manganese oxide (NMO). We deposit thin-film MnO via atomic layer deposition (ALD), and electrochemically convert the MnO to NMO in NaCl (aq). Charge storage capacity is tuned with NMO thickness, and the relationship between charge storage capacity and reversible salt sorption is probed. NMO coated electrodes exhibit increases in charge storage capacity up to 170 times higher than uncoated electrodes. Electrochemical quartz crystal microbalance (EQCM) measurements reveal that thin-film NMO leads to the efficient electrochemical removal of Na + ions. A hybrid CDI (HCDI) cell comprised of NMO-coated carbon nanotube (CNT) cathode and Ag nanoparticle-decorated CNT anode yields a ∼20-fold improvement in charge storage over bare CNT electrodes. The HCDI cell has an anomalously high reversible charging efficiency, which we study using ab initio modeling and EQCM. This is the first CDI report using thin film NMO, and the high desalination efficiency we identify promises to facilitate the development of HCDI devices with enhanced performance.

The Unified Electrochemical Band Diagram Framework: Understanding the Driving Forces of Materials Electrochemistry

“…The two regions with different slopes in Figure b correspond to two regions of different behavior predicted by ab initio modeling, as shown in Figure c. In this study, we determined that the top ≈15 nm of this Na x MnO y film is Na rich (Na 0.17 MnO 2 ), while the bottom ≈25 nm of this 40 nm thick Na x MnO y film is sodium deficient (Na 0.1 MnO 1.6 ), resulting from a competing process of Cl − oxidation during electrochemical conversion which limits the depth of conversion . Considering the composition profile of this film and that the majority of the charge storage arises from the film's top ≈15 nm, a Na‐rich description corrected for the surface band‐bending environment should properly describe the dominant behavior of the surface region of this graded film, while a Na‐deficient model with no surface corrections should capture the bulk behavior.…”

“…However, the UEB framework provides a means to establish a deep understanding of the principles that govern electrochemical mechanisms, which are transferable to other materials and readily provide a detailed guide for material development. Starting in 2014, we reported the use of this theoretical framework to understand charge storage in pseudocapacitive α‐MnO 2 , to provide new insights into the charge storage properties of LiMn 2 O 4 , to understand the desalination performance of NaMn 4 O 8 , and to understand electrochemistry in layered hydroxides . Here, we summarize the implications of our prior studies and provide new results that extend the UEB framework into additional areas of emerging importance.…”

“…We demonstrated that up to ≈1/4 of Mn centers in bulk lithium manganese oxide and sodium manganese oxide are able to acquire an extra unit of charge without cation insertion/removal due to stabilization of conduction band states by interstitial cations, giving rise to ultrafast pseudocapacitive charge storage. Further work demonstrates potential‐dependent cation sorption into thin‐film sodium manganese oxide without stoichiometric electron transfer. Together, these observations call into question the assumption of charge balance in transition metal semiconductor electrochemistry, e.g., for lithium insertion by , this work suggests that x and y in this equation are not necessarily equal, and that under certain conditions x < y or x > y .…”

“…As we described in the examples above, the driving forces of materials electrochemistry are affected by electrolyte conditions, interfacial band‐bending, and defect–defect interactions. Figure shows an excerpt from a study which explores the impact of these factors on the charging behavior of Na x MnO y , which was modeled as Na‐incorporated α‐MnO 2 . Figure a shows CV data for a 40 nm thick film of Na x MnO y measured using an EQCM with in situ measurement of mass changes shown in Figure b.…”

“…Evidence for electron‐decoupled ion transfer in Na x Mn 4 O 8 including a) cyclic voltammetry of thin‐film Na x MnO y measured during b) electrochemical quartz crystal microbalance measurements which show mass changes consistent with c) UEB charged defect calculations that predict electron‐decoupled ion transfer for the formation of V Na at the surface of NaMn 4 O 8 (solid blue line). Reproduced with permission . Copyright 2018, The Electrochemical Society.…”