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smith2016--Na-Ion Desalination (NID) Enabled by Na-Blocking Membranes and Symmetric Na-Intercalation Porous-Electrode Modeling 《钠离子脱盐（NID）：基于钠阻挡膜和对称钠嵌入多孔电极的模型》本文介绍了一种新的设备概念——钠离子脱盐（NID），其利用钠离子电池（NIB）的嵌入剂来从水中去除NaCl。研究通过二维多孔电极模型预测了NID单元在海水和淡水盐度水平下的性能，模拟了电解质传输、膜极化和同时发生的电化学过程。NID单元中使用了阴离子选择性膜，因为模拟结果显示，相对于传统NIBs中使用的多孔隔膜（效率为22%），这种方法能显著降低进水盐度（达到63%）。为了减少能量消耗，文章中采用了对称的NIB嵌入剂和能量回收机制。例如，一个基于Na0.44MnO2的NID单元，其电极厚度仅为0.5毫米，能够在以C/2速率循环时，将700毫摩尔/升的进水盐度降低63%，而消耗的能量仅比热力学最小值高出50%（即0.74千瓦时/立方米）。NIB嵌入剂的高体积充电容量使得进水盐度可以降低59%-64%，对于700毫摩尔/升和70毫摩尔/升的进水，水回收率分别可达80%和95%。然而，当前对NID性能的预测较为乐观，忽略了如侧反应、嵌入剂分解、膜泄漏以及竞争性阳离子嵌入等实际操作中的问题。这些预测为NID的操作机制提供了洞察，未来将指导NID技术的发展和完善。该研究发表于《电化学学会期刊》（Journal of The Electrochemical Society），并遵循创作共用非商业无衍生4.0许可协议，允许非商业性的再使用、分布和复制，但需遵守协议条款。此研究的核心在于钠阻挡膜和对称钠嵌入多孔电极的设计，它不仅提高了脱盐效率，还降低了能源消耗。这一创新方法可能为海水淡化和淡水资源的可持续利用提供新的解决方案。同时，考虑到潜在的实际运行挑战，未来的研究需要深入探究上述未考虑的因素，以增强NID系统的稳定性和持久性。

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A530 Journal of The Electrochemical Society, 163 (3) A530-A539 (2016)

Na-Ion Desalination (NID) Enabled by Na-Blocking Membranes

and Symmetric Na-Intercalation: Porous-Electrode Modeling

Kyle C. Smith

∗ ,z

and Rylan Dmello

Department of Mechanical Science and Engineering and Computational Science and Engineering Program,

University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA

A new device concept, Na-Ion Desalination (NID), is introduced to desalinate NaCl from water using Na-ion battery (NIB)

intercalants. A two-dimensional porous-electrode model is used to predict the performance of NID cells operating at sea- and

brackish-water salinity-levels by simulating electrolyte transport, membrane polarization, and simultaneous electrochemistry. Anion-

selective membranes are used to isolate electrodes in NID cells because simulations showed substantial drop from inﬂuent salinity

(63%) compared to porous separators used in conventional NIBs (22%). Symmetric NIB intercalants and energy recovery were

used to minimize energy consumption. A Na

0.44

MnO

-based NID-cell with 0.5 mm-thick electrodes desalinated 700 mM inﬂuent

by 63% while consuming only 50% more energy (0.74 kWh/m

) than thermodynamic minimum when cycled at C/2 rate. The high

volumetric charge-capacity of NIB intercalants enabled 59–64% drop in inﬂuent salinity with water-recovery levels up to 80% and

95% for 700 mM and 70 mM inﬂuent, respectively. The present predictions of NID performance are optimistic (side reactions,

intercalant decomposition, membrane leakage, and competing cation-intercalation are neglected), providing mechanistic insights

into NID operation that will guide NID development in the future.

Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/),

which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any

way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0761603jes]

Manuscript submitted June 16, 2015; revised manuscript received November 16, 2015. Published January 5, 2016; publisher error

corrected January 21, 2016.

Widespread water scarcity makes cheap, high efﬁciency desali-

nation systems a global priority.

Worldwide installed seawater and

brackish water desalination capacity has been increasing at a rate

of ∼40% per year in the last decade

due to population growth,

aquifer shrinkage, and industrial utilization. This increase in world-

wide installed desalination capacity has been driven by reverse osmo-

sis, multi-stage ﬂash, and multiple-effect distillation technologies.

3,4

Electrochemical desalination systems, which include electrodialysis

and capacitive deionization, are also viable technologies that have

been limited to usage in high-recovery and brackish-water desalina-

tion respectively. Capacitive deionization (CDI) was ﬁrst developed

in the 1960s,

and uses porous carbon electrodes to store salt ions in

the electric double-layer to efﬁciently desalinate brackish water.

Re-

cent capacitive-deionization systems use membranes,

7,8

ﬂow-through

electrodes,

and hybrid CDI

systems to improve desalination per-

formance. Electrodialysis systems, used since the 1960s

as a com-

petitor to early reverse osmosis technologies, have improved cycling

efﬁciency using electrodialysis reversal (EDR),

and charge isolation

using porous separators instead of membranes for shielding.

13–15

Recently, devices that induce localized electric ﬁelds have

been used to desalinate water, including the ion-concentration

polarization

(ICP) and electrochemically mediated desalination

(EMD) methods. Similarly, the electric ﬁeld inside Li-ion batteries

can induce the simultaneous depletion of salt in one electrode and

accumulation in the opposing electrode when cycled at high rate. This

“salt depletion effect” can limit cycling capacity of Li-ion batteries,

and consequently energy-storage devices are engineered to prevent

it from occurring. For the present Na-Ion Desalination (NID) cell,

we exploit this effect by blocking Na transport between opposing

electrodes to maximize the degree of salt depletion.

Na-ion intercalation materials have historically been researched

for energy storage.

Recent successful demonstrations of Na-ion bat-

teries (NIBs) using NaTi

(PO

)

(NTP, Ref. 20), Na

0.44

MnO

(NMO,

Refs. 21,22), Na

CuFe(CN)

(Ref. 23), and Na

2.55

(Ref. 24)

have renewed interest in Na-ion intercalation materials.

25,26

While

capacity and energy density are not as high as in Li-ion batteries,

recent developments in NIBs suggest that they can be manufactured

cheaply and electroactive materials can be synthesized by various

∗

Electrochemical Society Active Member.

E-mail: kcsmith@illinois.edu

methods.

28–32

Improvements in the capacity and cycle life of NIBs

have also been obtained recently.

33,34,23,24

NIB intercalants have been

used for desalination in hybrid CDI cells in the past.

This work

demonstrated the stability of intercalants in seawater and corrobo-

rated earlier attempts to use Na

0.44

MnO

in a desalination cell.

electrochemical cell that uses Na-ion intercalation in both electrodes

(as in the NID cell introduced here) could provide enhanced volu-

metric desalination capacity over a hybrid CDI cell, because Na-ion

intercalants store charge inside electroactive particles while capacitive

desalination only stores charge in the Helmholtz double-layer.

To our

knowledge, previously there has been no device proposed to desali-

nate water that uses an electrochemical cell with symmetric electrodes

containing Na-ion intercalants.

We simulate the local electrochemical processes that occur in NID

cells to realistically account for nonlinear and non-uniform polariza-

tion and intercalation effects. With this approach we explore a variety

of cell conﬁgurations, comparing between materials, cell dimensions,

and operating parameters to determine the limits of performance

for NID cells. We ﬁrst describe the materials, system, and porous-

electrode model used presently. The electrochemical processes that oc-

cur during cell cycling are then elucidated by examining the transient

distributions of intercalated Na in electroactive particles and of salt in

the electrolyte. Subsequently, performance is quantiﬁed for a range of

applied current density and electrode thickness, after which the inﬂu-

ence of inﬂuent salt-concentration and water-recovery is explored.

Methodology

System deﬁnition and materials.—Figure 1 shows the two-

dimensional device simulated presently and its dimensions. The NID

device is a symmetric Na-ion cell, meaning that it contains the same

type of Na intercalant in both electrodes (rather than dissimilar in-

tercalants used in conventional NIBs). T hese electrodes are chosen

with common thickness w and length L (Fig. 1b). The electrodes

are porous composites, which enables them to conduct electrons and

ions along the x-direction while saline solution ﬂows through them

along the y-direction (Fig. 1b). The two electrodes can be isolated

(Fig. 1a) by either a polymeric separator (commonly used in NIBs) or

an anion-selective membrane that blocks Na ions.

Next, we describe the operating concept for the present cell. At the

beginning of the charge process electroactive material in the cathode

acts as a source of Na, while electroactive material in the anode

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 182.72.186.54Downloaded on 2016-02-04 to IP

Journal of The Electrochemical Society, 163 (3) A530-A539 (2016) A531

desalinated effluentconcentrated effluent

separator

membrane

influent

s,c

s,a

cathode

anode

(a) (b)

cell

Na intercalant Na intercalant

Figure 1. Schematic of the desalination device of interest during a charging cycle: (a) Na ions in the cathode (left) de-intercalate from electroactive material into

the electrolyte, while Na ions in the anode (right) intercalate into electroactive material. The solution-phase Na-ion concentration-difference between the electrodes

drives Cl-ion migration from the anode to the cathode, which in turn concentrates the cathode solution in both Na and Cl ions, while the anode solution is diluted

in both. (b) The coordinate system and cell dimensions are shown (x perpendicular to the current collector and y along the ﬂow direction).

acts as a sink. Both electrodes contain aqueous NaCl with a certain

initial concentration deﬁned by the water source of choice. When

charging starts, electrons are released into conductive carbon from the

electroactive material in the cathode, which de-intercalates Na ions

from the cathode electroactive material into the saline water that ﬂows

through the cathode (Fig. 1a, left inlay). This electrochemical reaction

induces solution-phase charge-imbalance that drives Cl ions from the

anode across the separator to the cathode and concentrates cathode

solution with Na and Cl ions. A diluted solution forms in the anode

(Fig. 1a, right inlay) in which electrons enter the anode electroactive

material, causing Na ions to intercalate into electroactive material

and forcing Cl ions to leave the anode. This process depletes anode

solution with Na and Cl ions. Due to the cathode solution being

concentrated and the anode solution being diluted simultaneously,

efﬂuent streams of two disparate salt concentrations are generated

(Fig. 1a, top). The situation is reversed during the discharge cycle,

where cathode solution depletes while anode solution concentrates

in Na and Cl ions. Since the accumulating and depleting streams

are switched during the discharge cycle, the concentrated efﬂuent

and desalinated efﬂuent tanks would be switched when the current

direction is switched in a practical, experimental device.

The simultaneous accumulation and depletion of electrolyte-phase

salt ions in opposing electrodes is known as the salt depletion effect,

which was ﬁrst observed in rocking-chair Li-ion batteries.

The pro-

posed desalination method resembles both conventional CDI (due to

porous electrodes) and electrodialysis (due to anion-exchange mem-

brane) processes, however it differs from both because it uses Na-ion

intercalants to store charge. In addition, the change in cell polarity

between the charge and discharge cycles is similar to electrodialysis

reversal,

which should decrease membrane fouling since there is no

buildup of Cl ions at the anion perm-selective membrane.

Finally,

the anion perm-selective membrane, using the Gibbs-Donnan effect,

can sustain a large concentration gradient between the electrodes, al-

lowing signiﬁcant desalination in the depleted electrode.

Electrochemical model.—A porous-electrode model

is used

here to model Na-ion intercalation, ion transport in ﬂowing NaCl

solution, and electron transport inside ﬂow-through electrodes. Over-

potential η drives Na-ion intercalation in solid electroactive particles

and is deﬁned as η = φ

−φ

,whereφ

, φ

,andφ

are the solid-

phase potential of the electronic conductor, solution-phase potential

of Na

, and the equilibrium potential of Na-ion intercalation, respec-

tively. The solid- and solution-phase potentials are coupled through

their respective current-conservation equations (described later). The

equilibrium potentials of NTP and NMO vary with the fraction of in-

tercalated Na, x

(deﬁned as the intercalated Na concentration divided

by the terminal value). The equilibrium potentials used here, shown

in Fig. 2, were estimated from the absolute electrode potentials (i.e.,

versus a Ag/AgCl reference) measured during charge and discharge

of an experimental NMO/NTP cell (Ref. 38) at 0.6 C rate. The termi-

nal concentration of intercalated Na (c

s,max

) by which intercalated-Na

fraction is normalized was taken as 14,687 mol-Na/m

and 8,095

mol-Na/m

for NTP and NMO respectively. These concentrations

were chosen to produce theoretical capacities of 133 mAh/g-NTP

and 50 mAh/g-NMO reported previously in the literature.

The reac-

tion current-density i

for intercalation at the surface of electroactive

particles is modeled using the Butler-Volmer equation:

= i



exp



0.5Fη



− exp



−

0.5Fη



, [1]

0 20 40 60 80 100

intercalated-Na fraction (%)

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

(V vs. Ag/AgCl)

initial anode

initial cathode

NTP cell

NaTi

(PO

)

(PO

)

NMO cell

0.44

MnO

0.26

MnO

0.455V

0.200V

Figure 2. NMO and NTP equilibrium potentials used in the present simu-

lations. These curves were extrapolated from absolute potentials measured

experimentally during charge and discharge of an NTP/NMO cell at 0.6 C in

Ref. 38. Data points show the conditions that electrodes in NTP and NMO

cells were initialized with in all simulations.

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 182.72.186.54Downloaded on 2016-02-04 to IP