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Pseudocapacitive Coating for Effective Capacitive Deionization Meng Li, and Hyung Gyu Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14643 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Applied Materials & Interfaces

Pseudocapacitive Coating for Effective Capacitive Deionization

Meng Li and Hyung Gyu Park*

Nanoscience for Energy Technology and Sustainability, Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule (ETH) Zürich, Tannenstrasse 3, Zürich CH-8092, Switzerland

ABSTRACT: Capacitive deionization (CDI) features a low-cost and energy-efficient desalination approach based on electrosorption of saline ions. To enhance the salt electrosorption capacity of CDI electrodes, we coat ion-selective pseudocapacitive layers (MnO2 and Ag) onto porous carbon electrodes (activated carbon cloth) with only minimal use of a conductive additive and a polymer binder (300 F/g) and great cell stability (70% retention after 500 cycles). A CDI cell out of these pseudocapacitive electrodes yields as high charge efficiency as 83% and a remarkable salt adsorption capacity up to 17.8 mg/g. Our finding of outstanding CDI performance of the pseudocapacitive electrodes with no use of costly ionexchange membranes highlights the significant role of a pseudocapacitive layer in the electrosorption process.

KEYWORDS: capacitive deionization, electrosorption, pseudocapacitive coating, charge efficiency, salt adsorption capacity

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■ INTRODUCTION Fresh water shortage is an increasing concern humanity shares today related to municipal and industrial growth.1 Along with the size of the shortage problem, a vast amount of seawater and brackish water resources lends desalination cruciality as a technology to research and develop. Attributed to the lower energy consumption (~1-10 kWh/m3 of treated water)2 than other techniques such as multi-stage flashing or multi-effect distillation,3 membrane-based reverse osmosis contributes to a majority (~65%) of the global desalination capacity.4 It is predicted, however, that capacitive deionization (CDI), a low-pressure and ideally membrane-free technology, could partly outperform reverse osmosis in energy efficiency at brackish concentrations (< 3000 ppm of NaCl).5 In conventional CDI, ions in saline water are stored capacitively in an electric double layer developing on porous carbon electrodes, and so a specific surface area of the electrode plays a significant role in determining the adsorption performance. Whereas a prototype was realized in early 1960s,6 standard characterization metrics such as maximal salt adsorption capacity (mSAC, also abbreviated as SAC) was proposed much later and reached merely ~0.5 mg/g in 1996.7 The development of carbon nanomaterials has greatly promoted the performance of CDI in the recent two decades.8-20 It is seen that SAC values of sheer carbon-based capacitive electrodes plateau at ~16 mg/g in a standard test condition (at 1.2 V).16,

21-23

Recently, inspired by the field of

supercapacitors where pseudocapacitive metal oxides can help boost specific capacitance of porous carbon,24 MnO2 25-26 and TiO2 27-28 have been added to activated carbon (AC) to construct composite CDI electrodes, yielding SAC values at 10-17 mg/g. Faradaic electrode materials, such as Na2FeP2O7

29

and Na4Mn9O18,30 commonly used as cathode in a Na ion battery, could


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further increase SAC values up to ~30 mg/g. On the other hand, engineering of anodic material has been reported rarely because of concerns around stability and Cl- capture capability.31 For instance, Ag nanoparticles (NPs)32 and coating33 turned out to be useful, yet at the expense of degradation to AgCl or AgOH precipitates.32 As mSAC reflects the electrode material property in relation to capital cost of a CDI unit, equally important is to consider operation cost related to energy consumption or charge efficiency. An ion exchange membrane (IEM) with a high intrinsic charge density34 can help limit physical adsorption of coions at an initial unbiased state and thus mitigate wasted charge to repel them.10 Nearly 100% of charge efficiency has been reported by using porous carbon electrodes with IEMs in the CDI process, called MCDI.12 Polymeric ion-exchange coating could be directly applied to the porous carbon surface, though yielding charge efficiency lower than 75%.11, 35-36 Specific ion-selective coating, Ca-alginate for example, is developed to control the hardness of water.37 Apart from improving electrode wettability, charged functional groups are also shown to have effect on charge efficiency.17, 19 It is noteworthy that pursuits of extreme SAC and ultimate charge efficiency are mutually exclusive to a certain extent, ascribable to highly resistive nature of faradaic electrodes. External IEM attachment cannot provide a way to tackle this problem. To balance the capacitance and resistance, we employ activated carbon cloth (ACC) as a conductive platform that can facilitate charge conduction between charge collector and pseudocapacitive layer. This architecture allows minimal use of conductive additives and binders, as opposed to 15-25% in other CDI electrodes. Except those electroadsorbed onto ACC, ions can interact with the pseudocapacitive coatings selectively (Na+ with MnO2 and Cl- with Ag, for example). With no use of costly IEMs, this design can address both SAC and charge efficiency concurrently.



■ Experimental Material synthesis. An activated carbon cloth (ACC, CarboClothTM) with a large specific surface area (1000-2000 m2/g) purchased from Charcoal House, LLC. was cut into desired size and washed with 0.1 M H2SO4 and then DI water (Milli-Q®) overnight, in order to remove residual contaminants. The sample was dried via vacuum annealing at 65 °C for 1 h. To synthesize MnO2 on ACC, an aqueous KMnO4 solution at different concentrations (1 mM or 100 mM) was added and maintained at 65 °C for different duration (1-24 h). After the treatment, the sample was thoroughly washed with DI water and vacuum dried at 65 °C overnight. 100-nmthick Ag was evaporated onto both sides of the ACC using e-beam evaporation (Evatec BAK 501LL) at a constant deposition rate of 0.1 nm/s. Chamber pressure remained below 3×10-6 mbar during the deposition. Conductive wrapping of pseudocapacitively coated ACC samples with single-walled carbon nanotubes (SWCNTs, TimesdisperserTM TNWDSR) or poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, CleviosTM PH 1000) follows similar dip-coating approach as reported by previous publication.38 Mass loading of the additives was carefully controlled to be ~0.1 mg/cm2. Material Characterization. Structure and morphology of the ACC samples were characterized using scanning electron microscope (SEM, Hitachi SU 8230) at 5 kV electron accelerating voltage. Phase information was identified with Raman spectroscopy (Renishaw, inVia) at 785-nm laser irradiation. X-ray diffraction (XRD) spectra were measured by use of an X-ray diffractometer from


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Bruker XRD D8 Discover with a Cu Kα radiation source (1.5418 Å wavelength). Samples were weighed with an analytical microbalance (Mettler Toledo, XPE 105) after vacuum drying. Electrochemical characterization. Electrochemical characterization was carried out using electrochemical workstation (CH Instruments, CHI 760E). To fabricate the working electrode, the ACC samples were mounted atop thin graphite-foil current collectors and sealed with polyimide (Kapton) insulating tape to expose an effective area of 1×1 cm2. Cyclic voltammetry (CV) of this sample was then conducted in a three-electrode configuration in a 1-M NaCl electrolyte at various scanning rates (v). For reference and counter electrodes, we used Ag/AgCl (saturated KCl) and a large-area Pt foil, respectively. Electrochemical impedance spectroscopy (EIS) was taken within 0.01-100 kHz at 5 mV oscillation amplitude. Nyquist plot was fitted to a Randles circuit model using EIS spectrum Analyser software, with less than 6% error. Specific capacitance (Cm) can be calculated with the following equation, where m is the working electrode mass (including ACC), I the current, and Vt the voltage window:

=

( 1 )

Supercapacitor cell was a two-electrode system, made with a pair of ACC electrodes separated by a piece of Celgard® 3501 separator (25 µm thickness, soaked with 1-M NaCl electrolyte). Galvanostatic charging and discharging (GCD) tests were carried out in the voltage range of 01.2 V at various current densities. Cm of the cell can be obtained with eq (2), where tc is the charging time, td the discharging time, VIR the initial voltage drop (1 s after fully charging), and mtotal the total mass of the two electrodes. And the ratio between td and tc defines the Coulombic efficiency η, as listed in eq (3). 5 ACS Paragon Plus Environment


=

=

( )

× 100(%)

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( 2 )

( 3 )

Desalination characterization. CDI measurements were performed in a single pass (SP) mode and a flow-through geometry (Figure S1 in the Supporting Information). The fixture comprises a pair of polycarbonate ending plates and a piece of rubber plate with 3 cm × 3 cm × 0.2 cm opening as gasket. Two ACCs were cut into 2.9 cm × 2.9 cm and glued to graphite current collectors with a trace amount of silver epoxy. A 2 cm × 2 cm hole was punched in the middle of the graphite foil beforehand, in order to allow the flow passage. Along with one piece of separator (Celgard® 3501) as a spacer, the ACC-graphite electrodes were sealed into the fixture and tightened with 6 bolts. Typical mass of the two electrodes is ~0.2 g. A flow-through conductivity probe (eDAQ, ET908) was calibrated (Figure S2) and placed at the exit to monitor the real-time concentration change (∆c). pH of the effluent could be additionally determined with a pH meter (WTW, Multi 3420). Feed electrolyte (1-20 mM NaCl) was degassed with pure N2 for more than 1 h to lower the content of dissolved O2 and CO2. Flow rate (Φ) was controlled constant (~0.13 mL/s) with a peristaltic pump during the CDI operation. The CDI cell was first flushed with a feed solution for at least 30 min for the physical adsorption to reach saturation. Measurement was initiated with constant biasing (1.2 V) for charging (desalination) and with short-circuiting for discharging (regeneration). SAC of the cell was calculated using eq (4), where MNaCl equals 58.44 g/mol.


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=

!"# ∅ ∆&

( 4 )

The charge efficiency was extracted by use of eq (5), where the Faradaic constant F is taken as 96485 C/mol. ℎ()*+ +--./.+0/1 =

2∅ ∆&

× 100 (%)

( 5 )

Average salt adsorption rate (ASAR) is defined as:

3 =

456 789:&;&9< =
10 mg/cm2), yet still exceeding that of typical supercapacitors (O(1) mg/cm2) and approaching that of Li ion batteries (20 mg/cm2).48 To match of the capacitance of the optimized MnO2/ACC cathode, Ag was deposited as the anodic coating on ACC (Figure 1(e)). Except the adjunct regions between carbon fibers, Ag coating is achieved in decent quality with densely packed NPs in tens of nanometers in diameter (Figure 1(i)). CV scanning found out that most of the Ag had detached from the ACC surface due likely to lack of chemical bonding with carbon (Figure 1(f)), an expectable outcome from fully occupied d-electron orbitals of Ag.49 Even worse, Ag itself as a fcc metal without large crystalline vacancies does not allow easy Cl- intercalation and can crack upon huge volume expansion. Enlightened by the use of a conductive polymer binder on Si NPs in a recent report about the Li ion battery,50 PEDOT:PSS was applied here to encapsulate and protect the Ag NPs (Figure 1(h)), so that the Ag-NP-fixated ACC anode (Ag/ACC) can sustain CV scanning. As seen in Figure 3(b), Faradaic peaks of Ag/Ag+ take place at around 0 V with respect to the Ag/AgCl reference at low scanning rates, whereas they tend to shift outside the voltage window at increased scanning rates, or in the ion diffusion limited regime. No visible precipitation of Ag or an Ag compound was detected during the CV scanning even at as fast a rate as 20 mV/s, proving good stability of the coating.



Figure 3. Cyclic voltammetry of (a) ACC treated with KMnO4 at 1 mV/s, (b) ACC coated with Ag and PEDOT:PSS at various scanning rates. (c) Specific capacitance and (d) High-frequency Nyquist plot of ACC samples treated variously, with equivalent circuit model inset and double arrows marking the resistances.


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Figure 4. (a) Galvanostatic charging and discharging, (b) IR drop with linear fit, (c) specific capacitance of the MnO2/ACC||Ag/ACC supercapacitor cell with respect to current density. (d) Lifetime stability of the cell cycled at 5 mA/cm2.

An asymmetric pseudocapacitive supercapacitor cell (MnO2/ACC||Ag/ACC) was assembled with the MnO2-coated ACC (MnO2/ACC) as cathode and the Ag-NP-fixated ACC (Ag/ACC) as anode. The cell reveals triangle-like (capacitive) and nearly symmetrical charging and discharging curves in Figure 4(a), with a Cm of 71 F/g at lowest current density. Compared with the theoretical Cm of symmetric ACC||ACC cell (168 F/g × 1/4 = 42 F/g), enhancement is about 70%. Fitting of the IR drop (an example shown in Figure S4) tells a cell resistance of 6.9 Ohm, slightly larger than the series resistance of MnO2/ACC cathode (6.3 Ohm) from the horizontal intercept in the Nyquist plot. Stability evaluation at 5 mA/cm2 displays that the cell can retain 70%



of the initial capacitance without losing Coulombic efficiency after 500 cycles. As known, MnO2 pertains good cyclability in supercapacitors, possibly overcoming 50,000 GCD cycles.51 The decay of our cell, therefore, happens mainly with Ag. Considering a typical MnO2/Ag battery with 87% retention in 20 cycles at very small current density,52 our cell demonstrates reasonably good stability. It is noticed that a recent study suggests to replace Ag with conductive polymer polypyrrole as Cl- capturer,53 which might also solve the stability problem of the anode.


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Figure 5. CDI performance characterizations: (a) conductance and current change during the CDI process at a NaCl feed concentration of 2 mM (marked with a dashed line); (b) salt adsorption capacity, with Langmuir isotherm fittings and charge efficiency; (c) lifetime stability evaluation with 1 mM NaCl feed; (d) average salt adsorption rate of the ACC||ACC and MnO2/ACC||Ag/ACC cells with respect to NaCl feed concentration; (e) evolution of conductance at a NaCl feed concentration of 20 mM in an ACC||ACC cell during charging, with an inset (f) magnifying the overshooting period; (g) pH evolution during the charging process, with linear fittings, for various CDI conditions. 15 ACS Paragon Plus Environment


The deionization capacity of our pseudocapacitive MnO2/ACC||Ag/ACC cell (8.6 mg/g) exceeds the symmetric ACC||ACC cell (6.0 mg/g) by 42% at a feed concentration of 2 mM, as exemplified in Figure 5(a). As a slight trade-off, the pseudocapacitive cell consumes longer time to complete a deionization cycle, which comes likely from the increased resistance. At higher feed concentration such as 20 mM, it is noteworthy that conductance reading during deionization can exceed the feed (Figure 5(e),(f)), which likely originates from parasitic reactions.54-55 To understand such an effect, pH of the effluent was monitored by holding 1.2 V during an operation for sufficient time. The resulting pH-evolution data (Figure 5(g)) demonstrates that higher feed concentration with less Ohmic drop in the electrolyte can promote side reactions and create a basic environment more easily. Possible mechanisms have been previously reported that O2 dissolved in water can be reduced into OH- at the carbon cathode.56 Hence, higher O2 content can turn to even more OH-, as shown by the result without N2 bubbling. Also, with time, pH drops in an approximately linear fashion to the feed level, meaning an exponential decrease of the OH- concentration, probably due to curtailed catalytic activity of carbon after salt adsorption. Most importantly, it is observed that the initial pH of the symmetric and pseudocapacitive cells nearly overlaps with each other, hinting that the pseudocapacitive coating does not suppress parasitic reactions effectively. Unlike cation-exchange membranes,57 the use of the highly porous structure of MnO2 may not be able to completely block the O2 diffusion access to the carbon cathode. To obtain the actual desalination performance (Na+ and Cl-), conductivity from additional H+ or OH-, or σw (in the unit of mS/cm), needs to be carefully subtracted from the apparent conductivity reading. A simplified form from the literature58 is adopted in the form of eq (8) with


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the mobilities of H+ and OH-, µH+ and µOH-, as 36.23×10-8 cm2 V-1 s-1 and 20.64×10-8 cm2 V-1 s-1, respectively.

HI = J(KL × 10ML + KNLO × 10MLPA )

( 8 )

Upon this correction, ASAR of the symmetric CDI cell is correlated with the salt concentration linearly, suggesting a first-order kinetics (Figure 5(d)). This kinetics is likely caused by the limited amount of adsorption sites on ACC and ample supply of ions. The pseudocapacitive cell, on the other hand, displays a sublinear ASAR trend, perhaps attributable to transition from fast double layer adsorption at low concentrations to slow faradaic reaction on MnO2 and Ag at high concentrations. A detailed mechanistic understanding calls for further investigation. Despite the lower rate at higher concentration, our pseudocapacitive cell turns out to outperform the symmetric ACC cell in both SAC and charge efficiency (Figure 5(b)). SAC can be nicely fitted with Langmuir isotherm (eq (9)), where k is the equilibrium constant of salt adsorption. At 1 mM, the two cells reveal the similar result, very likely limited by the large cell resistance. At higher concentrations, on the other hand, the pseudocapacitive cell shows a clear advantage, with a SAC value up to 17.8 mg/g, higher than the symmetric cell by 56%. The improvement agrees roughly with the increase of Cm in the supercapacitor counterpart. Charge efficiency is also enhanced and saturated at 83% for large feed concentrations, a rarely obtainable value for CDI without IEMs. If charges wasted in parasitic reactions were prevented, charge efficiency could further increase up to 94%, almost approaching the MCDI record.12 Therefore, not only introducing additional capacitance for salt removal, the pseudocapacitive 17 ACS Paragon Plus Environment


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coating also functions as if a good IEM, thanks to its great ion selectivity. In terms of SAC and charge efficiency, our pseudocapacitive cell excels most of the CDI cells reported so far, as evidenced in the figure-of-merit chart (Figure 6). Interestingly, the pseudocapacitive cell also demonstrates higher lifetime stability over the symmetric cell in terms of SAC (Figure 5(c)), primarily explained by the inversion effect in a single-pass mode associated with oxidation of the carbon electrodes.55 Sheer ACC electrodes with larger area of carbon exposed to the electrolyte could be more influenced by this effect. Exhibiting higher inversion spikes at the beginning of each desalination cycle (Figure S6), ACC electrodes are likely to result in salt adsorption lower than the pseudocapacitive counterpart.

Figure 6. Comparison of reported values of salt adsorption capacity and charge efficiency, all obtained using solid electrodes under constant voltage (1.2 V), including our data marked in red. Abbreviations include: reduced graphene oxide (rGO), metal-organic-framework derived carbon (MOFC), CO2 activated ACC (ACC-CO2), and carbide derived carbon (CDC).

=

Q& PRQ&

S


( 9 )

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■ CONCLUSION Pseudocapacitive coating of MnO2 and Ag is applied onto an ACC matrix in a facile way to promote both selective electrosorption of ions and charge access. As the result, the pseudocapacitive CDI cell exhibits 17.8 mg/g of mSAC and 83% of charge efficiency, a state-ofthe-art achievement in CDI cell performance. With demonstrating themselves as economic alternatives to costly IEMs, these pseudocapacitive coatings lend the CDI electrode design herein proposed eligibility for an effective desalination option in the future.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXXX. CDI schematic setup, conductance calibration, additional electrochemical data, and lifetime stability evaluation of the CDI cells (PDF)

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Meng Li: 0000-0002-7474-688X



Hyung Gyu Park: 0000-0001-8121-2344 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS We appreciate the support from Binnig and Rohrer Nanotechnology Center of ETH Zürich and IBM Zürich. We thank the NRP 70 Program “Energy Turnaround” (407040_153978), and Sinergia project (CRSII2-147615/1) of the Swiss National Science Foundation for financial support. We also acknowledge the SCCER Heat & Electricity Storage in collaboration with the CTI Energy Programme for provision of forum for discussion.

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