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. Fresh drinking water is becoming increasingly scarce around the 25 globe, intensifying the need for energy efficient desalination meth-26 ods that could be powered with renewable energy sources. 1 The need 27 for fresh drinking water tops the list of 50 Breakthroughs: Critical 28 scientific and technological advances needed for sustainable global 29 development compiled by the Institute of Globally Transformative 30 Technologies at Lawrence Berkeley National Lab. 2 Capacitive deion-31 ization (CDI) is a promising water desalination technique based on 32 the reversible electrosorption of ions. Unlike other desalination tech-33 niques, CDI requires only a nominal voltage, and therefore could 34 easily be coupled with solar power or other renewable energy sources. 35 During CDI operation, the electrical potential across two elec-36 trodes is cycled between two modes, a 'desalination' half cycle and 37 a 'regeneration' half cycle. During the desalination half cycle of a 38 traditional CDI cell an electrical potential is applied to the CDI cell, 39 causing ion sorption to the electrodes and producing fresh water. Dur-40 ing the regeneration half cycle of a traditional CDI cell the polarization 41 is reduced or reversed and ions desorb from the electrodes, thereby 42 regenerating the electrodes and producing brine. Conventionally, CDI 43 electrodes are composed of inert carbon. In these carbon electrodes, 44 energy is stored during the desalination step by ion sorption in the 45 electric double layer (EDL), and some of this energy can be recovered 46 during regeneration. This unique behavior lowers the overall net en-47 ergy consumption of CDI, particularly for desalination of low salinity 48 feed waters. 3 49 To compete with commercial desalination technologies used for 50 higher salinities (e.g. reverse osmosis), CDI costs must be reduced. At 51 present CDI is viable at saltwater concentrations below ∼ 0.05 M, well 52 below the salinity levels of seawater. 3,4 Carbon electrodes currently 53 used in CDI have low salt sorption capacity (SSC) (units in mg NaCl 54 (g electrode) −1), limited to the available surface area for EDL ion 55 sorption. Low salt sorption capacity leads to larger devices with higher 56 capital costs. Additionally, low salt sorption capacity materials require 57 larger composite electrode thicknesses, which introduce ohmic and 58 diffusion losses and limit the efficiency of conventional CDI. 59 By increasing the capacity of CDI electrode materials, smaller 60 devices can be constructed using thinner electrodes, which will re-61 duce capital costs and provide lower operating costs through greater 62 energy efficiency. Consequently, hybrid CDI (HCDI) electrodes that 63 z E-mail: Matthias.Young@anl.gov; Steven.George@colorado.edu incorporate ion intercalation materials commonly used in batteries 64 and supercapacitors have been explored and have shown to improve 65 performance. 5-7 These materials often have higher charge storage 66 capacities (CSC) (with units of F g −1 or F cm −2) than carbon elec-67 trodes, and exhibit low self-discharge rates. 8 The use of ion-selective 68 intercalation materials is expected to increase charge efficiency 69 (∧ = mol NaCl (mol e-) −1) by reducing the energetic contribution 70 of co-ion desorption during the charging half-cycle, and to increase 71 the coulombic efficiency (η = mol e regeneration /mol e desalination) due to 72 the low self-discharging properties of these materials. 73 Manganese oxide, used as a reversible Na + intercalation material 74 in Na-ion batteries, 9-11 is a promising candidate for increasing the 75 SSC of HCDI electrodes. Recent studies have confirmed that the CSC 76 of MnO 2 in aqueous Na + solutions is largely due to cation (i.e. Na +) 77 sorption and intercalation. 12-17 Several reports have investigated the 78 incorporation of sodium manganese oxide (NMO) particles into HCDI 79 electrodes, demonstrating modest improvements in SSC. 6,7,18-20 Us-80 ing an NMO electrode, Lee et al. achieved the highest HCDI SSC at 81 31.2 mg (salt) /g (electrode) , more than twice that of the highest SSC re-82 ported for a conventional CDI system. 4,7 This increase in SSC was 83 achieved by adding Na 0.44 MnO 2 particles into the cell's carbon cath-84 ode. However, this factor of two increase falls well short of the factor 85 of ten improvement expected when comparing NMO CSC versus car-86 bon CSC, suggesting that the advantages of NMO may not have been 87 fully realized in these studies. 21,22 88 Here we describe the study of Na + intercalation and charge stor-89 age in nanoscale thin films of NMO formed with a controlled two-step 90 process-atomic layer deposition (ALD) of MnO, followed by elec-91 trochemical oxidation to form NMO-and provide an initial study 92 of its potential application to enhance HCDI. This work expands on 93 previous work from our group, where NMO was formed from MnO 94 in Na 2 SO 4(aq). 13 Here, we electrochemically convert MnO to NMO in 95 NaCl (aq) to mimic industrially relevant feed water for eventual sim-96 plified deployment in HCDI devices. By measuring the CSC versus 97 thickness after conversion to NMO, we probe the depth to which MnO 98 is oxidized to NMO in NaCl (aq). We use these results to produce HCDI 99 cathodes with a starting MnO thickness tuned to produce the highest 100 possible capacity. 101 We compare cycling performance of a conventional CDI config-102 uration consisting of two carbon nanotube (CNT) electrodes and 103 an HCDI configuration where the CNT cathode is coated with 104 NMO, as described above, and the CNT anode is decorated with Ag 105