Ion Transport and Competition Effects on NaTi2(PO4)3 and

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Article 2

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Ion transport and competition effects on NaTi(PO) and NaMnO selective insertion electrode performance 4

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Sneha Shanbhag, Yousuf Bootwala, Jay F. Whitacre, and Meagan S Mauter Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02861 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Ion transport and competition effects on NaTi2(PO4)3 and Na4Mn9O18 selective insertion

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electrode performance

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S. Shanbhaga, Y. Bootwalaa, J.F. Whitacre*b,c,d, M.S. Mauter*a,c,d

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a

5

Ave, Pittsburgh PA 15213

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b

7

Ave, Pittsburgh PA 15213

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c

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Pittsburgh PA 15213

Department of Civil and Environmental Engineering, Carnegie Mellon University, 5000 Forbes

Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes

Department of Engineering and Public Policy, Carnegie Mellon University, 5000 Forbes Ave,

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d

11

Pittsburgh PA 15213

The Scott Institute for Energy Innovation, Carnegie Mellon University, 5000 Forbes Ave,

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*Authors to Whom Correspondence Should Be Addressed

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E-mail: [email protected]; [email protected]

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ABSTRACT

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We evaluate the efficiency and capacity of electrochemically reversible insertion electrodes for

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use in targeted ion removal applications in aqueous solutions. The relative attributes of

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insertion material chemistry are evaluated by comparing the performance of two different

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sodium insertion materials NaTi2(PO4)3 and Na4Mn9O18 in different electrolyte environments.

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We performed experiments over a range of solution compositions containing both sodium and

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other non-inserting ions, and we then developed mechanistic insight into the effects of solution

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concentration and composition on overpotential losses and round trip coulombic efficiency. In

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dilute aqueous streams, performance was limited by the rate of ion transport from the bulk

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electrolyte region to the electrode interface. This leads to slow rates of ion removal, large

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overpotentials for ion insertion, parasitic charge loss due to water electrolysis, and low round trip

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coulombic efficiencies. This effect is particularly large for insertion electrodes with redox

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potentials exceeding the water stability window. In solutions with high background

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concentrations of non-inserting ions, the accumulation of non-inserting ions at the electrode

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interface limits inserting ion flux and leads to low ion removal capacity and round trip coulombic

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efficiency.

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1 INTRODUCTION While ion removal at concentrations exceeding 2000 ppm total dissolved solids (TDS) is

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efficiently performed via membrane separation processes, the energy intensity of removing

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dilute species of concern remains a separations challenge.1 Capacitive deionization may reduce

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the energy intensity of low salinity separations, but the electrosorption process of ion removal is

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non-specific.2, 3 In contrast, ion exchange and chemical precipitation approaches can selectively

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remove dilute species, but are materially intensive. Furthermore, selectivity often depends on

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background solution composition and can be difficult to control in variable environmental

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systems.4, 5

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An alternative approach to selective ion removal of dilute species is to leverage the

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energy efficiency of electrochemical processes, but to do so with a selective insertion compound

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based electrode. Unlike capacitive materials such as activated carbons, insertion compounds

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exhibit well-defined redox potentials at which ion insertion or de-insertion occurs, enabling

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voltage-controlled selectivity. Further, crystalline insertion compounds often have internal lattice

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vacancies and diffusion pathways of a definite size, resulting in the materials exhibiting inherent

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selectivity to species with attributes that are amenable to interfacial exchange and internal

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transport. Finally, insertion compounds exhibit limited functional potential windows and high

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specific capacities,6 which manifests in a high electrode ion removal capacity per unit mass and

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volume.7

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Cation insertion electrodes have been widely used in electrochemical energy storage, 8, 9

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and more recently have been explored for electrochemical deionization applications.10-12

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Common cation insertion compounds include NaTi2(PO4)3 (NTP), Na4Mn9O18 (NMO),

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Na2FeP2O7 (NFPP), and K2NiFe(CN)6 (NIHCF),10, 13-15 which have commonly been investigated

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in single cation (sodium) solutions. In each case, the transition metal atoms, which are


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covalently bonded in a crystalline host structure, undergo a redox transition. Concurrently, a

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cation is absorbed or rejected to maintain charge neutrality. In the case of NTP and NMO, only

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the smaller monovalent Li+ and Na+ ions are inserted into the host compound’s crystal lattice,

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while the larger monovalent K+ or divalent Ca+2 ions are left behind in the solution.16-18

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Depending on lattice geometry, the inserting ions may lose their solvation shell during the

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insertion process.10, 13, 14


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Figure 1. Transport regions in sodium manganese oxide (NMO) electrode immersed in a mixed ion aqueous solution at open circuit (A), during sodium insertion (B) and sodium de-insertion (C). Region 1 is the bulk solid-state electrode, region 2 is the electrode-electrolyte interface, region 3 is the electrical double layer region, and region 4 is the bulk electrolyte. The fundamental transport equations that predict the ionic flux and current density for each of these regions are reported in Table 1.


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While the interfacial transport processes control selectivity in insertion electrodes, bulk

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transport of ions to the interface and solution composition often control ion removal rate, ion

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removal capacity, and the round trip coulombic efficiency of ion uptake. Indeed, the insertion-

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based ion removal process involves several transport processes in series that are illustrated in

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Figure 1.

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In absence of an applied potential (Figure 1A), diffusion and advection govern the

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transport of ions between the electrode interface (region 1) and the bulk electrolyte (region 4).

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However, in presence of an electric field (Figure 1B), an additional mode of ion transport,

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electromigration, results in the movement of ions between the electrode and the electrolyte.

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When the applied potential exceeds the equilibrium potential for the Faradaic reaction, the

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transition metal and covalently bonded atoms in the insertion compound undergo a change in

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redox state. The bulk electrode develops an excess charge that repels the co-ions in the

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adjacent electrolyte layer and attracts the counter-ions leading to the formation of an electrical

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double layer (region 3). Only the inserting ions among the counter-ions accumulated on the

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electrode surface, diffuse through the solid-state electrode (region 4), until reaching the empty

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lattice site. If inserting ions shed their hydration shell, the previously bound water remains at the

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electrode-electrolyte interface (region 2). When the polarity is reversed during ion de-insertion

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(Figure 1C), the same redox and ion transport processes proceed in the opposite direction.

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The commonly accepted fundamental transport equations that relate ion fluxes (JNa+) to

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current (i) in the bulk solid-state electrode (region 1), the electrode-electrolyte interface (region

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2), the electrical double layer (region 3), and the bulk electrolyte (region 4) are presented in

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Table 1. For solid state diffusive transport, DNa+, solid is the apparent diffusion coefficient for Na+

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in the solid state. The concentration of Na+ occupied crystal sites is given by CNa+,solid, and the

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diffusion coefficient, electrochemical potential. and mobility of Na+ in the electrolyte are denoted


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by D(Na+,water), μNa+, and uNa+ respectively. Similarly, CNa+ denotes the concentration of Na+ in the

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electrolyte phase, while the velocity of the electrolyte is denoted by v. Further, the diffusion

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coefficient, flux, mobility, concentration, electrochemical potential and charge number of any

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ionic species ‘j’ in the electrolyte are given by Dj , Jj, uj ,Cj, μj and zj respectively. The exchange

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current density or the equilibrium current for the faradaic reaction is given by i0 at a temperature

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T. The transfer coefficients for the anodic and cathodic redox reactions are denoted by αa and

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αc .

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Table 1. Governing equations for transport phenomena in insertion electrode devices. Region

Description

Governing Transport Equations ‫ܬ‬ே௔శ = −‫ܦ‬ே௔శ ௦௢௟௜ௗ ߘ‫ܥ‬ே௔శ

1

Bulk electrode

Surface*

Electrode-electrolyte

1-2

interface

3

Electrical Double Layer

݅ = ݅௢ ൤݁

103 104

Bulk electrolyte

−݁

ିఈ೎ ிఎ ோ் ൨

݅௢ = ݇‫ܥ(ܨ‬ே௔శ ) ఈೌ (‫ܥ‬ே௔శ ௦௢௟௜ௗ, ௠௔௫ − ‫ܥ‬ே௔ି௦௢௟௜ௗ ) ఈೌ (‫ܥ‬ே௔శ௦௢௟௜ௗ ) ఈ೎ ‫ܬ‬ே௔శ (ௗ௜௟௨௧௘) = − ‫(ܦ‬ே௔శ ,

௪௔௧௘௥) ߘ‫ܥ‬ே௔శ

‫ܬ‬ே௔శ (௖௢௡௖௘௡௧௥௔௧௘ௗ) = −

4

ఈೌ ிఎ ோ்

௦௢௟௜ௗ

‫(ܦ‬ே௔శ ,

− ‫ݑ‬ே௔శ ‫ܥܨ‬ே௔శ ߘ߶ + ‫ܥ‬ே௔శ ‫ݒ‬

௪௔௧௘௥)

ܴܶ

‫ܥ‬ே௔శ (ߘߤே௔శ ) + ‫ܥ‬ே௔శ ‫ݒ‬

‫ܬ‬ே௔శ (ௗ௜௟௨௧௘) = − ‫(ܦ‬ே௔శ ,

௪௔௧௘௥) ߘ‫ܥ‬ே௔శ − ‫ݑ‬ே௔ శ ‫ܥܨ‬ே௔శ ߘ߶ + ‫ܥ‬ே௔శ ‫ݒ‬ ‫(ܦ‬ே௔శ , ௪௔௧௘௥) ‫ܬ‬ே௔శ (௖௢௡௖௘௡௧௥௔௧௘ௗ) = − ‫ܥ‬ே௔శ (ߘߤே௔శ ) + ‫ܥ‬ே௔శ ‫ߘݒ‬. ݅ ܴܶ = ߘ. (‫ ܨ‬෍ ‫ݖ‬௝ ‫ܬ‬௝ ) = 0 ௝

* Interfacial charge transfer kinetics is undergoing several theoretical developments. This table only lists the Butler 19-21 Volmer equation, while acknowledging other kinetic theories being developed.

105 106

The overpotential or excess potential beyond the equilibrium reaction potential needed

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to drive the oxidative or reductive faradaic process depends on the electrolyte solution

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composition. The overpotential is the sum of the activation overpotential necessary to facilitate

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the faradaic reaction, the ohmic drop in the electrode-electrolyte system and the concentration

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overpotential needed to drive ions to the electrode-electrolyte interface. The activation

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overpotential is independent of electrolyte composition. However, the electrolyte composition


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affects its conductivity and therefore influences the ohmic drop in the electrolyte. Further, the

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concentration overpotential (ηc), which is the potential difference between the electrode surface

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and the bulk electrolyte is also dependent on the gradient in the concentration of the counter

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and co-ions between the electrode surface and the bulk electrolyte.

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The solution composition, specifically the concentrations of inserting and non-inserting

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ions also influence the transport of inserting ions, and in turn the ion removal capacity. Higher

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concentration of inserting ions lowers concentration overpotential, improves inserting-ion flux in

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the bulk electrolyte and at the interface, and an ion removal capacity approaching the theoretical

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capacity of the material. In contrast, depending on the relative ion mobilities, increasing

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concentration of non-inserting ions could reduce the inserting-ion flux and the ion removal

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capacity of the electrode. This interference effect results from the competing ions occupying the

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electrical double layer and restricting inserting ion transport, with species having higher charge

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migrating faster to the electrode surface and resulting in a larger reduction in the measured ion

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removal capacity.

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The selectivity of insertion compounds is reduced in presence of high concentrations of

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non-inserting ions. Ion removal and charge storage in insertion materials predominantly

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emerges from ions stored in the bulk electrode. However, a small fraction of it is also

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contributed by the electrical double layer or any pseudocapacitive effects. The double layer or

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pseudocapacitive contribution to capacity is the predominant mode of charge storage when high

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levels of non-inserting ions exist in solution resulting in reduced and non-specific ion removal.

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While previous literature on insertion electrodes has examined effects of inserting ion

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concentration on insertion potential and selectivity between inserting ions,22, 23 the effects of

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non-inserting ions and their concentration on the ion insertion potential, specific capacity and

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round trip efficiency have not been explored.


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The dependence of insertion electrode systems on interfacial transport, bulk transport,

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and solution composition has hindered the realistic assessment of their performance for

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selective ion removal. The present work addresses this gap by providing characterization

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methodology for insertion compound selection and by identifying the transport-limited step for a

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range of solution compositions. We study the uptake of Na+ by two different sodium insertion

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materials with different insertion/de-insertion potentials, solid-state diffusion coefficients, and

142

specific capacities. NaTi2(PO4)3 or NTP is a NASICON (sodium superionic conductor) material

143

with a cubic structure, while Na4Mn9O18 or NMO is an orthorhombic insertion compound with

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three biphasic transitions that correspond to three insertion/de-insertion voltage peaks. Both

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materials selectively and reversibly insert only Li+ and Na+ ions, while rejecting K+, divalent, and

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trivalent electrolytes.18 We assess the electrode insertion potential, specific ion absorption

147

capacity, the efficiency of the insertion process, and the round trip coulombic efficiency of the

148

selective ion removal process as a function of inserting and non-inserting ion concentrations.

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We hypothesize that solution conditions with dilute concentrations of inserting ions will

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be limited by the rate of ion transport from the bulk electrolyte region to the electrode interface.

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As a result, we expect large overpotentials for ion insertion, reduced rates of ion removal, high

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parasitic charge loss due to water electrolysis, and low round trip coulombic efficiency. We also

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expect that larger overpotentials will lead to the shifting of the insertion/de-insertion potential to

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more extreme voltages. In contrast, for solutions with high background concentrations of non-

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inserting ions, we hypothesize that the accumulation of non-inserting ions at the electrode

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interface will limit inserting ion flux and lead to low ion removal capacity and round trip

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coulombic efficiency. In testing these hypotheses, we advance the understanding of the

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applications of insertion compounds in electrochemical deionization systems, selective ion

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removal from aqueous solutions, and the performance of insertion compounds in energy

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storage electrodes.


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2 EXPERIMENTAL SECTION

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2.1 Active Material Selection and Synthesis

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We aim to study the uptake of Na+ by two different sodium insertion materials,

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NaTi2(PO4)3, a phase change type NASICON (Sodium superionic conductor) material and

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Na4Mn9O18, a layered intercalation compound, from electrolytes ranging in concentration in

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presence and absence of non-inserting ions. We selected these two materials for our study due

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to their previously reported sodium insertion capability in sodium-ion battery literature and their

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inherently different electrochemical properties.8, 18, 24 Both these materials have distinct insertion

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potentials and are typically unable to insert ions larger than sodium. In Table 2, we list the

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electrochemical properties of these two electrode materials. We recognize that the sodiated

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form of NTP is prone to self-discharge and undergoes chemical de-sodiation in presence of

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dissolved oxygen. NMO on the other hand is extremely stable at potentials above 0.1 V vs

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NHE. NMO however does undergo an irreversible crystal distortion if is cycled below this

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potential. Finally, we note that the solid-state sodium-ion diffusion coefficient for NTP are at

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least two orders of magnitude larger than that of NMO.

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Table 2. Electrochemical properties of two sodium insertion electrode materials, and

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NaTi2(PO4)3 and Na4Mn9O18.

Material Attributes

Electrode Material NaTi2(PO4)3

Ions accepted in crystal lattice Number of insertion voltage peaks Insertion potential or potential range vs NHE 17, 18 (V)

+

+ 18

+

Li , Na

+ 8, 24

Li , Na

25

Multiple (Four, three within water 6, 17 stability window)

26

0.29 V-0.74 V 6, 17 (0.29, 0.53 V, 0.74 V)

Single -0.6 V

Hydrogen evolution below pH 10 Possible parasitic reactions

Na4Mn9O18

Self-discharge due to chemical 27 oxidation by dissolved oxygen

Oxygen evolution above pH 11 Loss in capacity due to crystal distortion at low potentials (below 0.1 17 V v/s NHE)

+

Solid state Na diffusion coefficient 2

-1

-10

(10

-12

- 10 )

28

-14

(10

-16

- 10 )

26, 29

(DNa+ cm s ) Specific capacity (mAh/g)

133 (theoretical)

66 (theoretical*)

* Calculated using Faraday’s law for above defined voltage range, based on known phase composition of oxidized form (NaTi2(PO4)3 and Na1.8Mn9O18) and reduced form (Na3Ti2(PO4)3 and Na4 Mn9O18) of above insertion compounds.

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Titanium dioxide (MKN-TiO2-A050 Anatase, M K Impex Corp.), reagent

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grade(NH4)2HPO4 (Sigma-Aldrich), and Na2CO3 (Sigma-Aldrich) in a mole ratio of (2:3:0.5) were

181

weighed and mixed in a beaker. This mixture was further blended with 5 wt % carbon black

182

powder (Timcal C-65, Imerys) and 10 wt % graphite powder (Timcal KS-6, Imerys) and ball

183

milled for 10 hours. The resulting mixture was then calcined under an inert argon atmosphere

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at 850 ºC for 2 hours to prepare carbon coated NTP (C-NTP). To synthesize NMO, reagent

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grade Na2CO3 (Sigma-Aldrich) and Mn2O3 (American Elements) were weighed in a molar ratio

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of (0.55:1) and mixed in a mortar and pestle. NMO was synthesized by ball milling the above

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mixture for 1 hour, followed by calcining in air at 750 ºC for 8 hours.6 The as-synthesized NTP

188

and NMO powders were washed with deionized water, and dried overnight in an oven at 80 ºC.

189

Reagent grade NaCl, CaCl2.2 H2O, AlCl3, and KCl (Sigma-Aldrich) were used in the preparation

190

of electrolyte solutions of varying concentrations by mixing them with deionized water. The


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initial pH of the electrolyte solutions was noted to be between 6-7. The electrolytes were not

192

deaerated to ensure tests represent practical deionization environments. A platinum wire

193

(Sigma Aldrich, 99.99% purity) was used as a counter electrode and titanium mesh was used as

194

a current collector for electrodes through all electrochemical tests.

195

2.2 Material Characterization

196

The as-synthesized NTP and NMO powders were characterized using X-ray powder

197

diffraction (PANalytical X’Pert X-ray powder diffraction system) equipped with a Cu Kα radiation

198

tube (λ=1.54056 Å). We tested the as-synthesized powders to determine the crystalline phase

199

of the powders using the X-ray powder diffractometer between 2θ values of 10º to 60º with a

200

step size of 0.013º. In addition, the scanning electron microscopy (Philips XL30 Scanning

201

Electron Microscope) was employed to obtain surface morphology of the powders synthesized

202

in our study. We sputter coated the samples using a platinum target prior to imaging in order to

203

improve the conductivity of the sample surface and avoid surface charging. The sputter coating

204

time was 20 seconds. The images of the surface of the powders were taken using an

205

acceleration voltage of 20 kV and spot size 3 in the secondary electron imaging mode. The as-

206

synthesized materials were also characterized using Nitrogen gas adsorption to determine BET

207

surface area and pore size distribution using a Quantachrome Nova 2000 instrument as detailed

208

in SI Section S1.4.

209

2.3 NMO and NTP Electrode Preparation

210

The as-synthesized NTP was ball-milled for five minutes with polytetrafluoroethylene

211

(PTFE) powder (Alfa Aesar) in the weight ratio of (0.9:0.1). Similarly, the as-synthesized NMO

212

powder was ball milled for five minutes with PTFE and carbon black (Timcal C-65, Imerys) in a

213

weight ratio of (0.8:0.1:0.1). The final concentrations of conductive carbon black in the NTP and

214

NMO samples are equivalent, though introduced in different stages of electrode production.


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The ball milled mixtures were then sheeted using a mortar and pestle to produce sheets of

216

electrode material that were pressed onto one edge of a titanium mesh of size 0.5 cm x 4 cm to

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produce an electrode area of less than 0.4 cm2. Each electrode was pressed to a thickness of

218

0.25 +/- 0.04 mm. The mass of electrode material loaded onto the titanium wire-mesh was

219

determined for each test case by weighing the mesh before and after loading. A typical

220

electrode mass of 5 -10 mg was used for each measurement. Additionally, prior to testing, we

221

submerged each wire-mesh electrode into the test electrolyte and completely wetted it by

222

subjecting to vacuum for fifteen minutes.

223

2.4 Electrochemical Testing

224

Electrochemical tests were performed using a potentiostat (Bio-Logic Science

225

Instruments VMP3) and an Ag/AgCl reference electrode (0.2 V v/s NHE, Koslow Scientific

226

Company). The same experimental set-up was used for all electrochemical tests

227

(Supplementary Information / SI Section 1.1) and only the electrode materials and electrolytes

228

were changed. To remove any limitations from anion uptake at the counter electrode, we used

229

a platinum counter electrode that catalytically splits water to maintain charge balance, instead of

230

an electrosorptive electrode. Further, we utilized a large excess of electrolyte and located the

231

counter electrode 4 cm away from the working and reference electrodes to ensure any local

232

changes to solution composition will not alter the uptake of cations by the working electrode.

233

2.4.1 Cyclic Voltammetry

234

The specific capacity of the NTP and NMO active materials was determined by cyclic

235

voltammetry of the respective electrodes in 1 eq L-1 (gram equivalent per liter) NaCl, KCl, CaCl2

236

and AlCl3 solutions. The NTP electrode was tested between -1 V and 0 V vs. Ag/AgCl and the

237

NMO electrode was tested between 0 V and 0.7 V vs Ag/AgCl.


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2.4.2 Galvanostatic cycling We examined the insertion behavior of NTP and NMO in a range of NaCl

240

concentrations, by galvanostatically cycling them at a specific current of 25 mA/g between the

241

potential ranges of 0 to -1.1 V v/s Ag/AgCl (for NTP) and between -0.1 to 0.75 V v/s Ag/AgCl

242

(for NMO). The NaCl electrolyte concentrations chosen for this test included 0.017 eq L-1, 0.051

243

eq L-1, 0.085 eq L-1, 0.17 eq L-1, 0.4 eq L-1, 0.6 eq L-1, 0.8 eq L-1 and 1 eq L-1. The insertion and

244

de-insertion potentials were determined from the galvanostatic profiles of the respective

245

electrodes using differential charge or dQ/qV analysis as described in SI Section 1.3.

246

In addition, we evaluated the electrochemical performance of NTP and NMO electrodes

247

in electrolytes with varying concentrations of NaCl (0.017 eq L-1, 0.17 eq L-1 and 1 eq L-1). For

248

each concentration, the NTP electrodes were cycled at several different specific currents

249

between 25-1000 mA/g. We elect to use specific current (mA g-1) as our primary metric;

250

however areal current densities were in the range of 0.5 -20 mA cm-2. Similarly, the NMO

251

electrodes were cycled at several specific currents between 15-1000 mA/g for each electrolyte

252

concentration. The experimental matrix used to study the effect of NaCl concentration on

253

insertion potential and ion removal properties is presented in Table 1.2.1 of SI Section 1.2. The

254

repeatability of the method was validated prior to conducting the full set of experiments. Due to

255

the large experimental matrix, repeatability of the method was evaluated for a small number of

256

experimental conditions ranging over various ionic strengths of electrolytes and several current

257

densities. We found error bars to be small even at low concentrations (SI Section S2.5) and this

258

method was deemed suitable for a qualitative assessment of transport effects on thin electrodes

259

in a large excess of electrolyte.

260 261

We then examined the performance of NTP and NMO electrodes in solutions containing inserting ions, Na+ and one non-inserting ion, either K+, Ca+2 or Al+3. For two different NaCl


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262

concentrations (0.085 eq L-1 and 1 eq L-1), we investigated the effects varying equivalent

263

concentrations of CaCl2 on insertion behavior of NTP and NMO. Further, NTP and NMO

264

electrodes were tested in five different equivalent concentrations of NaCl and CaCl2 while

265

keeping the total ionic strength at 0.085 eq L-1. The equivalent concentrations of NaCl and

266

CaCl2 in each electrolyte used in this study is provided in Table 1.2.2 in SI Section 1.2. Further,

267

both NMO and NTP electrodes were galvanostatically characterized in electrolytes containing

268

0.085 eq L-1 of inserting ion, Na+ and 0.085 eq L-1 of a non-inserting ion, either K+, Ca+2 or Al+3.

269

Finally, in order to examine the effect of non-reacting ions on reacting ion flux

270

independent of porous electrode structure, we also performed copper electroplating

271

experiments on a flat copper disc in acidified CuSO4 electrolytes containing Na+, K+, Ca+2 or

272

Al+3. Experimental details are reported in SI Section 1.6.

273 274

3 RESULTS

275

3.1 Structural Characterization of Insertion Electrodes

276

Scanning electron micrographs of the as-synthesized powders reveal differences in the surface

277

morphologies of NTP and NMO. NTP coated carbon has cubic domains connected by flakes or

278

sheets of carbon (Figure 2A), while NMO has needle-like morphology (Figure 2B). These

279

surface morphologies of the as-synthesized powders are indistinguishable from the prepared

280

electrode. The morphological differences between NTP and NMO allude to different interfacial

281

and solid-state transport characteristics in these two materials


Langmuir

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282

283 284

Figure 2. Scanning electron micrographs of NTP (A) and NMO (B) powders collected at 20 keV

285 286

We note that there are small differences in the mass loading, thickness and geometric

287

surface area of each wire-mesh electrode. The wire-mesh electrodes prepared for

288

electrochemical testing weighed approximately 5-10 mg and were 0.25 +/- 0.04 mm thick.

289

However, our previous work confirms that the resulting differences in measured specific

290

capacity and insertion potentials for these electrodes are insignificant for any given electrode

291

material.27

292

3.2 Electrochemical Characterization of Insertion Materials via Cyclic Voltammetry

293

The insertion and de-insertion potentials of Na+ in NTP and NMO lattices were evaluated via

294

cyclic voltammetry in 1 eq L-1 solutions to avoid bulk transport limitations. For pure NaCl

295

electrolyte, the insertion and de-insertion potentials of NTP occur at -0.78 V and -0.42 V vs NHE

296

respectively (Figure 3A). This results in an average redox potential of -0.6 V, which is well

297

below the hydrogen evolution potential at neutral pH (SI Figure S2.1). NMO has three insertion

298

peaks occurring at 0.26 V, 0.48 V, and 0.71 V and de-insertion peaks occurring at 0.32 V, 0.55

299

V, and 0.76 V vs NHE (Figure 3B). The average redox potentials are 0.28 V, 0.53 V and 0.74 V


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Langmuir

300

vs NHE. The specific capacity for Na+ ions of NTP as calculated from cyclic voltammetry is 56

301

mAh g-1 and that of NMO is 48 mAh g-1.

302

Cyclic voltammetry was also performed in concentrated solutions of non-inserting

303

cations of increasing valency (1 eq L-1 solutions of KCl, CaCl2, and AlCl3). For NTP, the specific

304

capacity for Na+ insertion is nearly an order of magnitude higher than K+, Ca+2, or Al+3 insertion

305

(Figure 2A). The specific capacity of NMO for K+, Ca+2, or Al+3 is slightly higher than that of NTP

306

(Figure 2B). The absence of redox peaks for both electrode materials in CaCl2 suggests that

307

Ca+2 does not insert into the electrode and the associated specific capacity is due to the

308

electrochemical double layer capacitance or pseudocapacitance of the insertion materials

309

(Figure 2). The lower Ca+2 capacity of NTP relative to NMO is indicative of a lower electrical

310

double layer capacitance or pseudocapacitance of NTP. The BET surface area and pore

311

volume of NMO are slightly higher than NTP as detailed in SI Section S2.2, although the

312

differences between them are very small. A higher surface area may be related to higher

313

double layer capacitance.

314

315 316 317

Figure 3. Specific capacity measured using cyclic voltammetry of NTP (A) and NMO (B) in 1eq L-1 solutions of NaCl, KCl, CaCl2 and AlCl3 at 0.5 mV s-1 scan rate

318


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319

3.3 Effect of sodium ion concentration in electrolyte

320

At lower concentrations of inserting cations, we hypothesized that ion-insertion potential would

321

shift to more extreme values due to high overpotentials and ion depletion effects. We identified

322

the galvanostatic insertion and de-insertion potentials for NTP and NMO as a function of NaCl

323

concentration (Figure 4A and 4B) using differential capacity analysis (SI Section 2.2), observing

324

ion depletion effects at concentrations below 0.4 eq/L for both electrode materials. The

325

hysteresis between the insertion and de-insertion potentials is equivalent to the sum of the net

326

overpotentials for insertion and de-insertion processes (Figure 4 C and 4 D). Further, when a

327

current is applied or the direction of the current is changed, an instantaneous potential drop is

328

observed, which corresponds to the ohmic drop in the half-cell. We estimate the sum of ohmic

329

potential drop for the insertion (R1) and de-insertion (R2) process as a function of concentration

330

by adding the instantaneous potential drop during the insertion and the de-insertion processes

331

respectively (Details found in SI Section S2.5). The difference between the hysteresis and the

332

sum of ohmic drops should equal the sum of activation and concentration overpotentials for the

333

insertion and de-insertion steps. Based on our data, we note that with increasing equivalent

334

concentration of electrolyte, both ohmic drop and net overpotential decrease. Since activation

335

overpotential for a given electrode remains unchanged with increasing electrolyte concentration,

336

we can expect the reduced net overpotential results from a reduced concentration overpotential.

337

Further, at low concentrations, the net overpotential and ohmic drop is slightly higher for NTP

338

when compared with NMO. With increasing concentration, the ohmic drop for both NTP and

339

NMO becomes similar in magnitude. However, the sum of activation and concentration

340

overpotentials is higher for NTP than for NMO at high concentrations. Based on our results, we

341

can say that either or both activation and concentrations overpotentials for NTP are higher than

342

for NMO at any given concentration.

343


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344 345 346 347 348 349 350 351

Figure 4. Insertion and de-insertion potentials for NTP (A) and NMO (B) at various NaCl concentrations measured at 25 mA g-1. where dotted lines represent redox potentials for NTP and NMO; Hysteresis between insertion and de-insertion potentials at various NaCl concentrations for NTP (C) and NMO (D) with sum of ohmic drop in blue (R1, insertion and R2, de-insertion), and sum of activation and concentration overpotentials in red for both insertion and de-insertion processes. (Details of methods used to estimate ohmic drops and sum of overpotentials are provided in SI Section S1.5)

352

We also posited that ion uptake by insertion compounds in solutions with dilute

353

concentrations of inserting ions will be limited by the rate of ion transport from the bulk

354

electrolyte to the electrode. The transport limits of insertion electrodes are explained by

355

quantifying the specific capacities (Figures 5 A and B) and RTE (Figure 5C and D) for NTP and

356

NMO at various specific currents and NaCl concentrations. At low specific currents, the ion

357

removal capacity is limited by inserting slow ion transport from the bulk electrolyte to the

358

electrode-electrolyte interface. Alternatively, at high specific currents, ion removal is limited by

359

the difference in the rates at which the bulk electrolyte and the solid-state electrode can


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360

transport ions. As such low specific currents and low concentrations result in higher relative

361

parasitic water splitting currents, thereby reducing round trip coulombic efficiency (RTE). On the

362

other hand, operating at high specific currents can lower round-trip coulombic efficiency due to

363

the inability of the electrode-electrolyte system to sustain high ion conduction rates. As such,

364

for any concentration, there is an optimal specific current that maximizes ion removal capacity

365

and round trip coulombic efficiency (RTE). Further, the maximum ion removal capacity and

366

round trip efficiency for both NTP and NMO electrodes is observed at the highest equivalent

367

concentration of Na+.

368

While trends in insertion behavior of NTP and NMO electrodes under similar conditions

369

are similar, their ion removal capacity and RTE are quite different. Unlike NMO, the highest

370

RTE in case of NTP is far below 100% due to significant parasitic charge losses. Further, the

371

specific capacity and RTE of NTP sharply reduce with increasing specific current at even high

372

concentrations, whereas those of NMO only moderately decrease.


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373

374 375 376 377

Figure 5. Effect of specific current on specific capacity for NTP (A) and NMO (B); and round-trip coulombic efficiency for NTP (C) and NMO(D) at various NaCl concentrations at specific currents ranging between 25-1000 mA g-1(current densities ranging between 0.5 -20 mA cm-2).

378 379

3.4 Effect of non-inserting ions on insertion electrode performance

380

We continue the investigation into the effects of solution composition on the specific capacity

381

and round trip coulombic efficiency by evaluating the effect of non-inserting ion concentration on

382

electrode performance. Because the rate limiting steps for ion insertion are different at high vs

383

low inserting ion concentrations, we evaluate these trends for both 0.085 eq L-1 Na+ and 1 eq L-1


Langmuir

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384

Na+ ion concentrations. We select Ca2+ as an impurity or a non-inserting ion in these solutions

385

since it does not insert into either NTP or NMO electrode materials.

386

In solutions containing non-inserting Ca2+ ions, we observe a reduction in specific

387

capacity and round trip coulombic efficiency of both NTP and NMO. We attribute this reduction

388

in specific capacity and RTE to the occlusion of the electrode-electrolyte interface by Ca2+ ions,

389

thereby restricting Na+ transport. For NTP, increasing non-inserting Ca2+ ion concentration

390

moderately reduces both specific capacity and RTE of Na+ insertion (Figure 6) in high and low

391

concentrations of inserting ions. For NMO, however, increasing Ca2+ ion concentration leads to

392

very large decreases in the specific capacity of the electrode material, with the effect being

393

larger for low inserting ion concentrations. Interestingly, the RTE decreases only slightly at

394

concentrations up to 0.05 eq L-1 Ca2+ before dropping significantly in the range between 0.05

395

and 1 eq L-1. Additionally, at lower Na+ concentrations, RTE and specific capacity of both NTP

396

and NMO decrease with increasing Ca2+ concentration. However, for NMO at high Na+

397

concentrations, the RTE nearly remains unchanged while specific capacity decreases. It is

398

likely, that these differences in the interference effects of Ca2+ ions on ion insertion behavior of

399

NTP and NMO may be a result of the differences in their interfacial and charge storage

400

properties.

401 402


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403 404 405 406 407

Figure 6. Effect of equivalent concentration non-inserting Ca+2 ion on specific capacity of NTP (A) and NMO(B), and on round trip coulombic efficiency of NTP (C) and NMO (D) in electrolytes with inserting Na+ ion concentration of 0.085 eq L-1 and 1 eq L-1 respectively at 100 mA g-1. To test our hypothesis of the interference effects of Ca2+ and control for the effects of

408

increasing ionic strength, we perform similar mixed cation experiments in which the ratio of

409

Na+/Ca2+ concentrations changes, but the overall ionic strength is held constant. As illustrated

410

in Figure 7, for both NTP and NMO, increasing Ca2+ concentration reduces the specific capacity

411

to nearly the electrical double layer capacity of the insertion material. However, the round trip

412

coulombic efficiency is either enhanced or remains nearly the same. As such, the insertion

413

capacity appears to be strongly related to the interfacial flux and concentration of Na+ ions,

414

whereas RTE and water splitting are likely related to ion depletion at low ionic strengths.


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415

416 417 418 419 420

Figure 7. Effect of equivalent concentration of non-inserting calcium ions on specific capacity of NTP and NMO(A), and round trip coulombic efficiency of NTP and NMO (B) in electrolyte with ionic strength of 0.085 eq L-1 at 100 mA g-1 While each of the non-inserting ions decreases the specific capacity and influences the

421

round trip coulombic efficiency of insertion electrodes, the magnitude of the effect is different for

422

ions with different sizes or charge numbers. In Figure 8 A and B, we depict the specific capacity

423

of NTP and NMO in presence of varying levels of non-inserting ions, either K+, Ca2+ or Al3+, and

424

at two effective equivalent concentrations of Na+ (0.085 eq L-1 and 1 eq L-1). We observe that

425

the reduction in specific capacity is higher for ions with higher charge number; the effect being

426

most profound for Al+3 in case of both NTP and NMO. We expect that multivalent ions would

427

migrate faster than monovalent ions under the influence of an electric field. This would result in

428

a more pronounced occlusion of the interface, hindering the transport of Na+, and hence a more

429

significant reduction in specific capacity at equivalent concentrations. Further, the reduction in

430

specific capacity is more significant in case of NMO compared with NTP likely due to the

431

differences in interfacial interactions and resulting charge storage properties of NTP and NMO.


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432

In presence of non-inserting ions, RTE is reduced at low ionic strengths, while remaining

433

unchanged at higher ionic strengths as depicted in Figure 8 C and D, and discussed in previous

434

sections. However, even at high ionic strengths, the presence of non-inserting ions has a

435

profound reduction in specific capacity and ion removal; the electromigration of highly mobile

436

non-inserting ions greatly affects the ion removal capacity of insertion electrodes.

437

Further, through a set of electroplating experiments on a flat copper disc (SI Section

438

1.5), we validate that the observed effect of non-inserting/non-reacting ions does not result from

439

transport limitations of porous electrodes. During electroplating, the addition of non-reacting

440

ions reduces plating ion flux as reported in Section 2.3 of SI. Based on these observations, we

441

can expect that the presence of non-inserting ions reduces insertion ion flux, which results in a

442

reduced ion removal capacity and that this effect is not entirely a result of the porous electrode

443

structure.

444


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445

446 447 448 449

Figure 8. Specific Capacity and round trip coulombic efficiency of NTP (A,C) and NMO (B,D) in electrolyte + 2+ 3+ with similar levels of non-inserting ions (K , Ca and Al ) and various concentrations of inserting ion +) -1 (Na , tested at 100 mA g

450


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451 452

Langmuir

4 DISCUSSION We observed and compared multiple competing ion transport effects in Na+ selective

453

electrodes in electrolytes containing multiple ionic species. At low bulk electrolyte

454

concentrations, the electrical double layer is thicker and the surface concentration of inserting

455

ions is lower than in concentrated electrolytes.30 In such cases, we expect large overpotentials

456

and ohmic voltage losses for ion-insertion, and these become smaller with increasing ionic

457

strength. The sum of activation and concentration overpotentials for the insertion and de-

458

insertion processes are larger for NTP electrodes compared to NMO electrodes. We attribute

459

this difference to the variance in Na+ solid-state diffusion rates and the resulting depletion of Na+

460

at the electrode-electrolyte interface. The diffusion coefficient of NTP is larger than that of

461

NMO, resulting in faster ion depletion at the electrode-electrolyte interface, and a lower

462

interfacial Na+ concentration for NTP. Lower interfacial Na+ ion concentrations result in higher

463

concentration overpotential for NTP. Additionally, the larger hysteresis seen in NTP could also

464

be explained in part by the higher overpotential phase-change behavior of the NTP electrode

465

versus the solid solution behavior of the NMO electrode.31

466

Further, our results suggest that the limiting transport mechanism for insertion electrodes

467

operating in dilute electrolytes is the transport of ions from bulk through the electrode-electrolyte

468

interfacial region. The reduction in specific capacity and RTE with increasing specific current is

469

higher at low ionic strengths due to the existence of higher overpotentials, lower electrolyte

470

conductivity, higher ion depletion at electrode surface, and a higher proclivity for parasitic water

471

splitting. This limitation is further magnified in electrodes with insertion potentials that lie at or

472

outside the water stability window under typical pH conditions such as NTP (SI Section 2.1).

473

Operating electrodes based on insertion materials at potentials exceeding the water-

474

stability window results in parasitic reactions that reduce round-trip coulombic efficiency. For

475

NTP, even in a concentrated electrolyte, the round trip coulombic efficiency ranges between 10-


Langmuir

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476

70% (Figure 5). NTP is also known to encounter chemical de-sodiation when exposed to

477

dissolved oxygen.27 However, using chemical de-sodiation in similar dissolved oxygen

478

environments cannot explain the wide range of round trip coulombic efficiencies observed for

479

NTP. We therefore expect that an alternate source of parasitic charge loss must exist. Based

480

on previous reports of pH increase and gas evolution during reduction of NTP, we infer that the

481

parasitic charge loss results from hydrogen evolution.32 This phenomenon is similar to the

482

observed parasitic charge loss at a carbon anode exceeding oxygen evolution potentials in

483

capacitive de-ionization systems, due to carbon corrosion and oxygen evolution.33

484

The higher observed electrochemical double layer capacitance or pseudocapacitance of

485

NMO compared to NTP might also result in a relatively stronger alternative path for charge

486

storage, and this effect would reduce the impact of ion depletion in dilute solutions. While higher

487

electrochemical double layer capacitance or pseudocapacitance can be beneficial in improving

488

the RTE in dilute solutions, it also results in reduced Na+ ion removal capacities in solutions

489

containing both inserting and non-inserting ions. Accumulation of non-inserting ions at the

490

electrode-electrolyte interface and the electrical double layer region limits Na-ion flux and ion

491

removal capacity. The results presented in Figures 6 through 8 illustrate how non-inserting ions

492

bring about a substantial reduction in the specific capacity of the electrode, particularly at lower

493

total concentrations.

494

The presence of non-inserting ions results in a profound reduction in specific capacity

495

and ion removal at both high and low ionic strengths and influences the selectivity of the

496

insertion electrode. The specific capacity is reduced due to slower inserting ion transport in the

497

presence of the supporting electrolyte,30 along with faster migration and subsequent occupation

498

of the double layer by multivalent ions. As such, multivalent ions that are more mobile than Na+

499

occupy the double layer region, thereby restricting interfacial transport of Na+ and reducing

500

specific capacity. In our studies, the reduction in specific capacity followed a trend wherein


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Langmuir

501

larger hydrated ions (Al+3 (0.96 nm) > Ca+2 (0.82 nm) > Na+ (0.72 nm) > K+ (0.66 nm), ionic

502

diameters) that are more mobile than Na+ (Al3+ (11.7 x 10-8) > K+ (7.62 x 10-8)> Ca2+ (6.17 x 10-8)

503

> Na+ (5.19 x 10-8), ionic mobilities in m2 s-1 V-1), obstruct the electrode surface, and reduce

504

inserting ion flux to the interface.34-36

505

Through this work, we advance the understanding of the application of insertion

506

compounds by bridging the gap between their performance in energy storage systems, and that

507

in electrochemical deionization systems or during selective ion removal from aqueous solutions.

508

Unlike in energy storage devices where the electrolyte contains only one inserting counter ion,

509

dilute electrolytes with multiple ionic species in electrochemical deionization systems, impose

510

several limitations on ion removal performance of insertion compounds. Maximizing the

511

performance of insertion compounds in dilute and multi-ion electrolytes requires replenishing the

512

depleted surface or interfacial concentration of inserting ions. By tailoring electrode materials,

513

improving advective ion transport to the electrode surface and through proper selection of

514

applied specific current and electrode potentials, it may be possible to improve the specific ion

515

removal performance of insertion electrodes.

516

5 CONCLUSIONS

517

In this work, we have demonstrated that insertion electrodes provide a solution to preferentially

518

remove specific ions (monovalent cations) from aqueous streams ranging in concentration and

519

composition. For insertion-based electrodes, selectivity is based on ionic charge number and

520

ionic size consistent with the lattice vacancy in the host’s crystal lattice. When an insertion

521

electrode is used to selectively remove ions from a dilute aqueous stream, the transport of ions

522

from the bulk electrolyte to the electrode surface limits the rate of ion removal and the round trip

523

coulombic efficiency. Further, insertion electrodes with redox potentials exceeding the water

524

stability window may show reduced round trip coulombic efficiency due to parasitic water

525

splitting reaction. In such systems, ion depletion resulting from the insertion process and the


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526

slow transport of ions from the bulk to the electrode-electrolyte interface further promotes

527

parasitic water electrolysis. Additionally, in waters containing non-inserting ions, the

528

accumulation of non-inserting ions in the electrical double layer region constrains interfacial

529

transport of inserting ion, thereby reducing ionic flux and achievable ion removal capacity. In

530

spite of these challenges, optimizing the operating current density, cut-off voltage, and

531

advective ion transport from the bulk to the electrode surface will improve the ion removal

532

capacity and round trip coulombic efficiency of insertion electrodes in dilute aqueous streams.

533

With ion removal capacities that are 5 -10 times higher than carbon materials and high

534

preference to specific inserting ions, selective insertion electrodes remain a promising path

535

forward for challenging separations.

536 537 538

ASSOCIATED CONTENT

539

Supporting Information in the form of the following files is available free of charge.

540

Supplementary Information (SI) includes the following supplementary methods: experimental

541

set-up for electrochemical testing of insertion electrodes; experimental matrix for galvanostatic

542

cycling in various electrolytes; differential charge or dQ/dV analysis approach; experimental

543

details of electroplating experiment in supporting electrolyte; methods for estimating

544

overpotentials during galvanostatic cycling and the following supplementary results: Pourbaix

545

analysis of NTP and NMO electrodes; differential charge or dQ/dV analysis results; BET

546

analysis and pore size distribution of NaTi2(PO4)3 and Na4Mn9O18, effect of supporting

547

electrolyte during electroplating results. (PDF)

548 549

AUTHOR INFORMATION


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550

Corresponding Authors

551

[email protected]

552

[email protected]

553 554

ORCID

555

Sneha Shanbhag: 0000-0003-0424-8313

556

Jay F. Whitacre: 0000-0002-3439-4111

557

Meagan S. Mauter: 0000-0002-4932-890X

558

Author Contributions

559

The manuscript was written through contributions of all authors. All authors have given approval

560

to the final version of the manuscript.

561

Funding Sources

562

NSF Grant CBET 1403826

563

BSF Grant 2012142

564

Scott Institute for Energy Innovation at Carnegie Mellon, Seed Grant

565

Notes

566

The authors declare no competing financial interest.

567 568

ACKNOWLEDGMENTS

569

We acknowledge the National Science Foundation under the award number CBET-1403826

570

and the United States-Israel Binational Science Foundation (BSF) under the award number


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Page 32 of 45

571

2012142 for supporting this research. Further, we also acknowledge support in the form of a

572

Seed Grant from the Scott Institute for Energy Innovation at Carnegie Mellon.

573

ABBREVIATIONS

574

NTP, sodium titanium phosphate; NMO, sodium manganese oxide; SEM, scanning electron

575

microscopy; XRD, X-ray diffraction; PTFE, polytetrafluoroethylene; RTE, round-trip coulombic

576

efficiency.

577 578

LIST OF SYMBOLS

579

‫ܬ‬ே௔శ

580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600

+

‫ܦ‬ே௔శ௦௢௟௜ௗ ‫ܦ‬ே௔శ, ‫ܥ‬ே௔శ

௪௔௧௘௥

‫ܥ‬ே௔ି௦௢௟௜ௗ ݅

݅௢ ݇

ߙ௔ ߙ௖ ‫ܨ‬

ܴ ܶ ߟ

ߟ௖

‫ݑ‬ே௔శ ߤே௔శ ‫ݒ‬

‫ݖ‬௝

‫ܬ‬ே௔శ ‫ܥ‬௝

‫ݑ‬௝

-2

Na flux, mol m s

-1 +

2

Solid-state diffusion coefficient for Na , m s Diffusion coefficient for Na in water, m s

+

2

-1

+

-3

Concentration of Na in electrolyte, mol m

Concentration of Na in solid phase, mol m Current density, A m

-1

-3

-2

Exchange current density, A m-2 -1

Reaction rate constant for redox reaction, s m

-2

Anodic transfer coefficient Cathodic transfer coefficient Faraday constant, 96487 C/equivalent -1

-1

Universal gas constant, 8.314 J mol K Temperature, K Activation overpotential, V Concentration overpotential, V +

2

Mobility of Na in electrolyte, m s

−1

−1

V

-1

Electrochemical potential of Na+ in electrolyte, J mol Velocity of electrolyte or fluid, m s

-1

Charge number on ion j Flux of species j, mol m s ‫ܦ‬௝ -2

-1

2

Diffusion coefficient of ion j in electrolyte, m s

Concentration of ion j in electrolyte, mol m 2

Mobility of ion j in electrolyte, m s

−1

-3

−1

V


-1

Page 33 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

601

ߤ௝

Electrochemical potential of species j in electrolyte, J mol ߶

602

potential in solution, V

-1

Electric

603 604 605 606 607

REFERENCES

608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642

1. Zhao, R.; Porada, S.; Biesheuvel, P. M.; van der Wal, A., Energy consumption in membrane capacitive deionization for different water recoveries and flow rates, and comparison with reverse osmosis. Desalination 2013, 330, 35-41. 2. Porada, S.; Zhao, R.; van der Wal, A.; Presser, V.; Biesheuvel, P. M., Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58, (8), 1388-1442. 3. Anderson, M. A.; Cudero, A. L.; Palma, J., Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete? Electrochim. Acta 2010, 55, (12), 3845-3856. 4. Koon, J. H.; Kaufman, W. J., Ammonia Removal from Municipal Wastewaters by Ion Exchange. J. - Water Pollut. Control Fed. 1975, 47, (3), 448-465. 5. Du, Q.; Liu, S.; Cao, Z.; Wang, Y., Ammonia removal from aqueous solution using natural Chinese clinoptilolite. Sep. Purif. Technol. 2005, 44, (3), 229-234. 6. Whitacre, J. F.; Tevar, A.; Sharma, S., Na4Mn9O18 as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device. Electrochem. Commun. 2010, 12, (3), 463-466. 7. Lee, J.; Kim, S.; Kim, C.; Yoon, J., Hybrid capacitive deionization to enhance the desalination performance of capacitive techniques. Energy Environ. Sci. 2014, 7, (11), 36833689. 8. Doeff, M. M.; Peng, M. Y.; Ma, Y.; De Jonghe, L. C., Orthorhombic Na x MnO2 as a Cathode Material for Secondary Sodium and Lithium Polymer Batteries. J. Electrochem. Soc. 1994, 141, (11), L145-L147. 9. Lu, Y.; Wang, L.; Cheng, J.; Goodenough, J. B., Prussian blue: a new framework of electrode materials for sodium batteries. Chem. Commun. 2012, 48, (52), 6544-6546. 10. Lee, J.; Kim, S.; Kim, C.; Yoon, J., Hybrid capacitive deionization to enhance the desalination performance of capacitive techniques. Energy Environ. Sci. 2014, 7, (11), 36833689. 11. Ikeshoji, T., Separation of Alkali Metal Ions by Intercalation into a Prussian Blue Electrode. J. Electrochem. Soc. 1986, 133, (10), 2108-2109. 12. Rassat, S. D.; Sukamto, J. H.; Orth, R. J.; Lilga, M. A.; Hallen, R. T., Development of an electrically switched ion exchange process for selective ion separations. Sep. Purif. Technol. 1999, 15, (3), 207-222. 13. Porada, S.; Bukowska, P.; Shrivastava, A.; Biesheuvel, P.; Smith, K. C., Nickel Hexacyanoferrate Electrodes for Cation Intercalation Desalination. arXiv preprint arXiv:1612.08293 2016.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693

14. Kim, S.; Lee, J.; Kim, C.; Yoon, J., Na2FeP2O7 as a Novel Material for Hybrid Capacitive Deionization. Electrochim. Acta 2016, 203, 265-271. 15. Kim, S.; Yoon, H.; Shin, D.; Lee, J.; Yoon, J., Electrochemical selective ion separation in capacitive deionization with sodium manganese oxide. J. Colloid Interface Sci. 2017, 506, (Supplement C), 644-648. 16. Doeff, M. M.; Anapolsky, A.; Edman, L.; Richardson, T. J.; De Jonghe, L. C., A HighRate Manganese Oxide for Rechargeable Lithium Battery Applications. J. Electrochem. Soc. 2001, 148, (3), A230-A236. 17. Sauvage, F.; Laffont, L.; Tarascon, J.-M.; Baudrin, E., Study of the insertion/deinsertion mechanism of sodium into Na0. 44MnO2. Inorg. Chem. 2007, 46, (8), 3289-3294. 18. Delmas, C.; Nadiri, A.; Soubeyroux, J. L., The nasicon-type titanium phosphates Ati2(PO4)3 (A=Li, Na) as electrode materials. Solid State Ionics 1988, 28, 419-423. 19. Bazant, M. Z., Theory of Chemical Kinetics and Charge Transfer based on Nonequilibrium Thermodynamics. Acc. Chem. Res. 2013, 46, (5), 1144-1160. 20. Smith, R. B.; Khoo, E.; Bazant, M. Z., Intercalation kinetics in multiphase layered materials. arXiv preprint arXiv:1701.08858 2017. 21. Bai, P.; Bazant, M. Z., Charge transfer kinetics at the solid–solid interface in porous electrodes. Nat. Commun. 2014, 5. 22. Wessells, C. D.; Peddada, S. V.; McDowell, M. T.; Huggins, R. A.; Cui, Y., The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes. J. Electrochem. Soc. 2011, 159, (2), A98-A103. 23. Siperko, L. M.; Kuwana, T., Electrochemical and Spectroscopic Studies of Metal Hexacyanometalate Films: I . Cupric Hexacyanoferrate. J. Electrochem. Soc. 1983, 130, (2), 396-402. 24. Robert Armstrong, A.; Huang, H.; A. Jennings, R.; G. Bruce, P., Li0.44MnO2: an intercalation electrode with a tunnel structure and excellent cyclability. J. Mater. Chem. 1998, 8, (1), 255-259. 25. Wang, X.; Lee, J. S.; Tsouris, C.; DePaoli, D. W.; Dai, S., Preparation of activated mesoporous carbons for electrosorption of ions from aqueous solutions. J. Mater. Chem. 2010, 20, (22), 4602. 26. Xu, M.; Niu, Y.; Chen, C.; Song, J.; Bao, S.; Li, C. M., Synthesis and application of ultralong Na0.44MnO2 submicron slabs as a cathode material for Na-ion batteries. RSC Adv. 2014, 4, (72), 38140-38143. 27. Wu, W.; Shabhag, S.; Chang, J.; Rutt, A.; Whitacre, J. F., Relating Electrolyte Concentration to Performance and Stability for NaTi2(PO4)3/Na0.44MnO2 Aqueous Sodium-Ion Batteries. J. Electrochem. Soc. 2015, 162, (6), A803-A808. 28. Sagane, F., Synthesis of NaTi2(PO4)3 Thin-Film Electrodes by Sol-Gel Method and Study on the Kinetic Behavior of Na+-Ion Insertion/Extraction Reaction in Aqueous Solution. J. Electrochem. Soc. 2016, 163, (13), A2835-A2839. 29. Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J., Reversible Sodium Ion Insertion in Single Crystalline Manganese Oxide Nanowires with Long Cycle Life. Adv. Mater. 2011, 23, (28), 3155-3160. 30. Newman, J.; Thomas-Alyea, K. E., Electrochemical Systems. Wiley: 2012. 31. Huggins, R., Advanced Batteries: Materials Science Aspects. Springer US: 2008. 32. Mohamed, A. I. NaTi2(PO4)3 as an Aqueous Anode: Degradation Mechanisms and Mitigation Techniques. Carnegie Mellon University, 2017. 33. Shanbhag, S.; Whitacre, J. F.; Mauter, M. S., The Origins of Low Efficiency in Electrochemical De-Ionization Systems. J. Electrochem. Soc. 2016, 163, (14), E363-E371. 34. Nightingale Jr, E., Phenomenological theory of ion solvation. Effective radii of hydrated ions. J. Phys. Chem. 1959, 63, (9), 1381-1387. 35. Atkins, P.; Atkins, P. W.; de Paula, J., Atkins' Physical Chemistry. OUP Oxford: 2014.


Page 34 of 45

Page 35 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

694 695

36. Coury, L., Conductance Measurements Part 1: Theory. Current Separations 1999, 18, (3), 92.

696

LIST OF TABLES

697 698

Table 2. Governing equations for transport phenomena in insertion electrode devices.

699

Table 2. Electrochemical properties of two sodium insertion electrode materials, NaTi2(PO4)3

700

and Na4Mn9O18

701


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

702 703

TABLE OF CONTENTS/ABSTRACT GRAPHIC

704

705


Page 36 of 45

Page 37 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Cl-

Brackish Water

Na+

Na+

Na

Na+ Cl-

+

e-

Cl-

Cl-

Na+

Cl-

Na+

Ca+2

Cl-

Na+

Cl-

Na+

Cl-

Cl-

Na+

-

Cl Cl-

Cl-

+

-

e-

Na+ Na

Na+

+

Na+ Ca+2

Desalted Water

Na+

Na+

Na+

Insertion Based Negative Electrode


Langmuir

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A

Page 38 of 45

4

Current collector

i

B

Current collector

1

2

3

4

1

2

3

4

ee-

i

eeee-

C

Current collector

ee-

i

eeee-

Hydrated Sodium Ion

Hydrated Calcium Ion


Hydrated Chloride Ion

Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

A

B

10 μm


10 μm

Langmuir

800

50 40 30

400 200 0 -200 -400 -600 -800

20

-1

-0.8

-0.6

-0.4

-0.2

Potential vs NHE (V)

10 0

Na+

K+

Ca+2

Al+3

0

Specific capacity (mAh/g)

600

60

400 300

50

Current (mA/g)

B

60

Current (mA/g)

A

Specific capacity (mAh/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

40 30

200 100 0 -100 -200 -300 -400

20

0

0.2

0.4

0.6

0.8

Potential vs NHE (V)

10 0

Na+


K+

Ca+2

Al+3

1

Page 41 of 45

B Insertion Potential vs NHE (V)

-0.55

Insertion Potential vs NHE (V)

A

-0.6

-0.65 -0.7

-0.75 -0.8 0

C

0.2

De-insertion Insertion 0.2

0.4

0.6

0.8

NaCl Concentration (eq/L)

D

Concentration + Activation Overpotentials Net Ohmic Drop R1 + R2

0.6 0.5 0.4 0.3 0.2 De-insertion Insertion

0.1 0

0.2

0.2

0.4

0.6

0.8


1

Concentration + Activation Overpotentials Net Ohmic Drop R1 + R2

Potential (V)

0.05

0

0.7

0.15

0.1

0

0.8

0

1

0.15

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

0.1

0.05

0.2

0.4

0.6

0.8


1

0

0

0.2

0.4

0.6

0.8


1

Figure 3: Insertion and de-insertion potentials for NTP (A) and NMO (C); Sum of insertion and de-insertion overpotentials (eta1+ eta2) an ohmic


Langmuir

Specific Capacity (mAh/g)

C

1 eq/L NaCl 0.17 eq/L NaCl 0.085 eq/L NaCl 0.017 eq/L NaCl

Max capacity

50 40 30 20 10 0 10 1

10 2

10 3

Specific Current (mA/g)

120

B

60

D


40 30 20 10

10 2

10 3


120

10 4


110 100

RTE (%)

80 60

90

40

80

20

70

0 10 1

Max capacity

50

0 10 1

10 4


100

RTE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60


A

Page 42 of 45

10 2

10 3

Specific Current(mA/g)

10 4

60 10 1

10 2

10 3


10 4

Figure 3. Relationship between specific capacity with current density for NTP (A) and NMO(C); and round trip coulombic efficiency with increasing current density for NTP (B) and NMO(D)


Page 43 of 45

Specific Capacity - (mAh/g)

A

60

Langmuir

B

60


RTE (%)

RTE (%)

1 2 50 50 3 4 40 40 5 6 30 30 7 8 20 9 20 10 11 10 0.085 eq/L NaCl 0.085 eq/L NaCl 10 12 1 eq/L NaCl 1 eq/L NaCl 13 0 0 14 -3 -2 -1 0 1 -3 -2 -1 0 1 0 10 10 10 10 10 0 10 10 10 10 10 15 Equivalent concentration of Ca2+ (eq/L) Equivalent concentration of Ca2+ (eq/L) 16 17 C 100 D 100 18 19 20 80 80 21 22 60 60 23 24 25 40 40 26 27 28 20 20 29 0.085 eq/L NaCl 0.085 eq/L NaCl 1 eq/L NaCl 1 eq/L NaCl 30 31 0 0 -3 -2 -1 0 1 -3 -2 -1 0 1 32 0 10 10 10 10 10 0 10 10 10 10 10 33 2+ 2+ Equivalent concentration of Ca (eq/L) Equivalent concentration of Ca (eq/L) 34 35 Figure 3. Relationship between specific capacity with concentration of calcium ions in electrolyte for NTP (A) and NMO(B); and round trip coulom36bic efficiency with concentration of calcium ions in electrolyte for NTP (C) and NMO(D) 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

A

Langmuir B

Equivalent concentration of Na+ (eq/L) 0.10

0.08

0.06

0.04

0.02

0

Equivalent concentration of Na+ (eq/L) 0.10

0.08

0.06

0.04

0.02

Page 44 of 45 0

RTE (%)


NTP NTP 1 50 100 2 NMO NMO 3 4 40 80 5 6 30 60 7 8 9 20 40 10 11 10 20 12 13 14 0 0 0 0 0.02 0.04 0.06 0.08 0.10 15 0.02 0.04 0.06 0.08 0.10 2+ 2+ 16 Equivalent concentration of Ca (eq/L) Equivalent concentration of Ca (eq/L) 17 18 19Figure 4. Relationship between specific capacity with concentration of calcium ions in electrolyte for NTP and 20NMO(A) and round trip coulombic efficiency with concentration of calcium ions in electrolyte for NTP and 21NMO(B) in electrolyte with ionic strength of 0.085 eq/L 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 45 of 45

C

B

60



A

50 40 30 20 10 0

D

100

60 50 40 30 20 10 0

100 80

RTE (%)

80

RTE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

60 40

60 40

20

20

0

0 0.085 eq/L NaCl

1 eq/L NaCl

0.085 eq/L NaCl + 0.085 eq/L KCl

1 eq/L NaCl + 0.085 eq/L CaCl2

0.085 eq/L NaCl + 0.085 eq/L CaCl2

1 eq/L NaCl + 1 eq/L CaCl2

0.085 eq/L NaCl + 0.085 eq/L AlCl3

Figure 7: Specific Capacity and Round trip coulombic efficiency of NTP (A,B) and NMO (C,D) as a function of calcium ion concentration in electrolyte with different ionic species and effective ionic strength of 0.17 N


Ion Transport and Competition Effects on NaTi2(PO4)3 and

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