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Half-Cell Ion Concentration Polarization on Nafion-Coated Electrode Rhokyun Kwak, and Jongyoon Han J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01214 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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The Journal of Physical Chemistry Letters

Half-cell Ion Concentration Polarization on Nafion-coated Electrode

Rhokyun Kwaka* and Jongyoon Hanb,c,d*

a

Department of Mechanical Engineering, Hanyang University, Seoul 04763, Republic of Korea b c

Department of Electrical Engineering and Computer Science,

Department of Biological Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA d

BioSystems and Micromechanics (BioSyM) IRG,

Singapore-MIT Alliance for Research and Technology (SMART) Centre, Singapore, Singapore.

*Correspondence

should be addressed to Rhokyun Kwak and Jongyoon Han E-mail: [email protected]; phone: +82-2-2220-2900 E-mail: [email protected]; phone: +1-617-253-2290; fax: +1-617-258-5846;

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The Journal of Physical Chemistry Letters 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

Abstract On ion selective membranes, cation/anion selective transport under electric field initiates ion concentration polarization (ICP); ion concentration increases at one side of the membrane (ion enrichment), while it decreases at the other side (ion depletion). This polarization always occurs as the pair of ion enrichment and ion depletion. Here, departing from such pair generation, we demonstrate that only half of ICP (either ion enrichment or ion depletion) can be solitary on a Nafion-coated electrode. Currentvoltage-time responses and conductance measurement capture this half-cell ICP with qualitative in situ pH / ion concentration visualization. In this half-cell, ion depletion hinders an ion flux whereas ion enrichment facilitates the flux, so a diode-like current rectification is observed even in high voltage regime (< ±200V) with a rectification factor up to 500. The results in this work give us deeper understanding about ICP on the electrodes, and also open the possibility to use half-cell ICP as a high-voltage ionic diode and related sensing/energy applications.

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Ion concentration polarization (ICP) is the result of selective ion transport or selective ion consumption, often found in ion exchange membranes or electrodes1-3. Characterized by significant perturbations in ion concentration profile in either side of the selective transport barrier, such as ion depletion or enrichment, ICP is therefore one of the most critical factors determining operating efficiencies in various electrochemical systems, including electrodialysis4-8, redox flow battery9-10 (with selective ion transport) and ion selective electrodes11-13, fuel cells14, membrane capacitive deionization15-16 (with selective ion consumption or adsorption) and many others. Recently, the study of this fascinating yet fundamental electrochemical process have been advanced by the new experimental works in microfluidic platforms, providing microscopic details of the phenomenon in terms of flow profile17-19, ion concentration profile6, 18, 20, and pH changes21. In addition, several devices that utilize ICP in microfluidic environment have emerged for applications such as biomolecule preconcentration22-25, small-scale desalination26-28, and ionic diodes29-31. In spite of these renewed interests and research efforts, there is still much to be desired in our understanding and modeling of ICP, which in turn will facilitate better engineering of relevant electrochemical devices and systems. While ICP from ion selective membrane has been extensively studied, ICP originating from bare or membrane-covered electrodes, in spite of its similarity, received less attention. In general, ion transport near electrodes are more complicated than that of ion selective membranes, with various Faradaic reactions as well as water dissociation (also known as water splitting) further complicates the picture. Previously, several groups described ICP on bare electrodes, but they neglected any Faradaic reactions under relatively small voltages32 or used reversible Faradaic reactions33-34.



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In this paper, we study ICP on Nafion-coated electrodes in a microfluidic channel without additional control of Faradaic reactions in general electrolytes. Being widely used for electrochemical sensing applications12-13,

35

, electrochemical

performances of Nafion-coated electrodes have been characterized in the nearequilibrium (reaction-limited) regime. However, ICP-dominant (diffusion-limited) regime (under relatively high voltage/current bias) has not been investigated previously. We verify that only the half of ICP (either ion depletion or ion enrichment zone) is developed in contrast to the pairwise generation of ion depletion and enrichment zones in ion selective membranes2-3. Nafion-coated electrode consumes or produces Nafion-transferable protons with water splitting, whereas blocks hydroxide ions and related reactions; bare electrodes cannot generate ICP with full H+/OHreactions of water splitting36-38. We then discuss the possible applications of half-cell ICP as a high voltage diode with an extremely low reverse current. With inert electrodes (e.g. Pt, Au, and carbon), electrolysis of water produces current flow (red and blue boxes in Figure 1a). Under a sufficient voltage (>1.23V), water reduction (oxidation) at the cathode (anode) generates hydroxide ions and hydrogen gas (protons and oxygen gas); and those protons and hydroxide ions obtain or lose their charges at the other side of the electrodes, and become hydrogen / oxygen gases1, 39: Cathode:

2H+ + 2e− → H2(g) 2H2O +2e− → H2(g) + 2OH−

E0=0.00V E0=-0.83V

(1) (2)

Anode:

2H2O → O2(g) + 4H+ + 4e− 4OH− → O2(g) + 2H2O + 4e−

E0=1.23V E0=0.40V

(3) (4)

Overall:

2H2O → 2H2(g) + O2(g)

(5)

where E0 is a standard reduction potential at 298.15 K and 101.325 kPa39. In addition, there are the electrophoretic movements of cation α+ and anion β− under electric field 4 ACS Paragon Plus Environment

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in bulk electrolyte (green box in Figure 1a). If α+ or β− is electroactive proton or hydroxide ion, it contributes to current flux by supplying the source of Faradaic reactions (eqs 1 and 4). If the ions are inert (i.e. supporting electrolytes), they contribute to reduce the electrical resistance of bulk solution1, 39. In this electrolytic cell, electrolysis (eqs 1-5) could change pH near the electrodes by enriching hydroxide ions or protons, but it is hard to generate ICP. For example, to generate ion depletion on the cathode, β− should be depleted away as consuming H+ for satisfying electroneutrality. In this scenario, if αβ is a supporting electrolyte (e.g. NaCl), there is not enough proton ions (~10-7 M at pH 7) to be consumed, so OH- generation by water splitting (eqn 2) leads ion enrichment with pH shift. Similarly, on the anode, H+ generation leads ion enrichment with pH shift. As a result, bare electrodes in supporting electrolytes show (Butler-Volmer like) current-voltage curves with only H+/OH- productions39. If α+ is H+ (e.g. αβ is HCl), ion depletion may be able to occur on the bare cathode by consuming enough proton ions, as like ion depletion with reversible Faradaic reactions34. However, the bare electrode is effectively binary ion conductors (i.e. at cathode; OH- can be generated and conducted away, while incoming H+ can be consumed). Much faster mobility of OH- than that of supporting anions (e.g. Cl-) would hinder the depletion of those anions by supplying hydroxide ions. Of course, the exact transport numbers for H+ and OH- (from electrode reaction) may not match those of bulk conduction. It is still possible to have ICP in bare electrodes due to this mismatch, but it is tend to be weak in general because the electrode Faradaic reaction is not selective to any one ionic species. In bulk electrolyte, ICP has been initiated by ‘blocking’ (via ion selective membranes) either cation or anion movements2-3 (Figure 1b). When a cation selective membrane (e.g. Nafion) is positioned between two electrodes, on the anodic side of 5 ACS Paragon Plus Environment


the membrane, anions β− depart from the anode while Nafion film blocks anion supply, rendering anodic Faradaic reaction selective and resulting in ion depletion zone (dde). On the cathodic side of the membrane, β− are matched with cations α+ passing through Nafion, resulting in ion enrichment zone (den). If we use an anion selective membrane, the membrane blocks cation transport; so, the location of ion depletion and ion enrichment zones are reversed. It is noteworthy that this ICP by selective ion transport always brings ion depletion and ion enrichment zones together for all kinds of α+ and β−. Often the development of ICP induces pH shifts on either side of the membrane, as a result of complex transport and generation of H+ / OHnear the membrane21. Such pH shifts become significant only under relatively high current, and limited ranging in pH 5-8.5. Then, how can we make ICP on electrodes with just supporting electrolytes? When the electrodes are coated with Nafion or other ion selective materials, Faradaic reactions are modified. Nafion blocks hydroxide ion transport to the electrodes, so only proton reactions are available (eqs 1 and 3) (Figure 1c-d). Hydroxide ions are blocked by Nafion, and therefore electrode Faradaic reactions made ‘unipolar’ because of the Nafion film. Since the bulk electrolyte support bipolar ion conduction, this ion carrier mismatch results in ICP. On the Nafion-coated cathode, H+ is consumed continuously (regardless of whether its concentration is low or high) and OH- is not supplied from the cathode, inducing ion depletion and lowering pH value (Figure 1c). On the Nafion-coated anode, H+ is generated by water splitting and OHor any other anion is not consumed, resulting ion enrichment and lifting pH value (Figure 1d). Introduction of Nafion brings about another complication, since they can initiate water dissociation near membrane-electrolyte interface. Sulfonic groups in 6 ACS Paragon Plus Environment

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Nafion could promote this dissociation reaction significantly (Figure 1e-f)36-38, leading to significant water dissociation on the surface of Nafion: R-SO3H + H2O → R-SO3− + H3O+(=H+) R-SO3− + H2O → R-SO3H + OH−

(6) (7)

Overall: 2H2O → H3O+(=H+) + OH−

(8)

Even with so-called current-induced membrane discharging, the unipolar conduction of Nafion allows us to consume H+ and push OH- and supporting anions (without OH- production) on the Nafion-coated cathode. On the anode, OH- by this membrane discharging still cannot be consumed, so it would be matched with H+ from the anode. In the end, water dissociation on Nafion-coated electrodes (eqs 6-8) still can initiate ion depletion on the cathode and ion enrichment on the anode. Interestingly on the Nafion-coated electrode, protons through Nafion are changed to neutral hydrogen gas (water) on the cathode (anode), instead of maintaining cation transfer across the membrane. Consequently, ion enrichment on the anode and ion depletion on the cathode can occur independently, which we named as ‘half ICP’. This is a departure from the notion that one always generate both depletion and enrichment zones in ICP by selective ion transfer2-3. The half ICP on the Nafion-coated electrode also induces pH shifts (pH raise on the cathode and pH drop on the anode). This pH shift is similar to that of ICP by selective ion transport with pH-regulatable ions; but it will happen by water dissociation even without those ions. The Nafion-coated electrodes in this study were composed of one linepatterned electrode and one microchannel (Figure 2a). We fabricated gold electrodes (width: 100 µm, height: 200 nm, length: 1.5 cm) on a glass substrate with a typical life-off process. Nafion was patterned on the gold electrode by micro-flow patterning40 with poly(dimethylsiloxane) (PDMS) mold (width: 200 µm, height: 50 7 ACS Paragon Plus Environment


µm, length: 1.5 cm). 20 wt% Nafion perfluorinated resin (Sigma-Aldrich, St. Louis, MO) was used. Then, the Nafion-coated electrode was bonded with the PDMS microchannel (width: 200 µm, height: 50 µm, length: 1.5 cm) by oxygen plasma treatment. As described in Figure 2a, the electrode was located at the middle of the channel, and this channel has 2 mm diameter reservoirs (height: 5-7 mm) at the both ends. The height of the patterned Nafion was ~100nm40, so no significant leakage happened between the glass substrate and the flexible PDMS channel. The similar systems were fabricated with a bare electrode and a bare Nafion pattern (Figure 5). With the Nafion pattern on glass substrate (without the electrode), we mimicked halfcell ICP by selective ion transport with an asymmetric geometry (microchannel vs. millimeter reservoir)29-31. The cathode was on a large reservoir to buffer ICP with massive supporting ions. To visualize ICP phenomenon, negatively charged fluorescent dye (0.78 µM Alexa Fluor 488, Invitrogen, Carlsbad, CA) was added in deionized water and 1-10 mM disodium phosphate buffer (Na2HPO4). Fluorescence images were captured using an inverted epifluorescence microscope (Olympus, IX-71) with a charged-coupled device (CCD) camera (Hamamatsu Co., Japan). In situ pH was visualized with universal indicator in organic solution (UI-100, Micro Essential Laboratory Inc, Brookyn, NY) and litmus aqueous solution (Sigma-Aldrich, St. Louis, MO). The pH images were recorded with iphone 5s (Apple Inc, Cupertino, CA) by connecting the phone to the eyepiece of the microscope. The dc voltage was applied on reservoirs or electrodes, and current response was measured simultaneously with a source measurement unit (Keithley 236, Keithley Instruments, Inc., Cleveland, OH). Figure 2b shows half-cell ICP on Nafion-coated electrodes in various ionic strengths (deionized water, 1mM and 10mM Na2HPO4) with the fluorescent dye (0.78 8 ACS Paragon Plus Environment

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µM Alexa Fluor 488). Similar with well-documented ICP by selective ion transport2-3, we observe ion depletion at V+ = 5V with the flat low concentration zone (dark regions) and dye preconcentration at the zone boundary (see SI Figure 1 for fluorescent profiles). In this depletion zone, a pair of vortex, which is the representative characteristics of nonlinear ICP, is also initiated by electroconvection with overlimiting conductance (Figure 2c and SI Figure 2)6, 41. In reverse bias, at V+ = −5V, ion enrichment with smooth increase of fluorescent intensity is visualized. When applied voltage is below the reaction overpotential of water splitting (1.23V), there is a weak linear ion depletion or enrichment probably because of ion adsorption in electric double layer on electrodes1, 11 (SI Figure 1). It is noted that half ICP is also available in deionized water. This supports the fact that half-cell ICP can be initiated / maintained even with low concentration of proton ions and water dissociation on Nafion. As only the half of ICP is developed, a unique diode-like current rectification is shown in current-voltage responses (Figure 2d). If we neglect ICP, current flux I is described by electromigration and the rate of water dissociation on Nafion; current by electromigration is linearly proportional to the electric field E1, and water dissociation is exponentially proportional to E36-37. Assuming fast reactions on electrodes, current flux depends on proton transfer by electromigration and proton supply by water dissociation: I=A⋅exp(BV)+CV,

(9)

where A, B and C are representing the degree of water dissociation, the rate constant of water dissociation on Nafion, and the degree of electromigration, respectively. The second term of eq 9 is well-known Ohm’s law with elecrolyte’s conductance C1. With the global (shared) constant B for three cases (deionized water, 1mM and 10mM



Na2HPO4), the theoretical currents follow the experimental values in the case of ion enrichment mode (V+0. The dropped current is then exponentially recovered to the reference value (> 5V in Figure 3). This trend represents that ion depletion zone has a flat, significant low concentration profile and sharp concentration gradient at the boundary, as like the fluorescent profiles show (Figure 2b and SI Figure 1). When we


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cut the voltage on the electrode (step 2), such deionization shock (which is the term quoted from Mani et al.27) dissipates quickly with strong concentration gradient. Then, the dissipation speed (current recovery speed) slows down as the concentration gradient becomes blunt, resulting exponential recovery of current. On the other hand, increased currents with ion enrichment (at V+

Half-cell Ion Concentration Polarization on Nafion ... - ACS Publications

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