Side Reactions in Capacitive Deionization (CDI) Processes: The Role of Oxygen Reduction

Electrochimica Acta 220 (2016) 285–295

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Side Reactions in Capacitive Deionization (CDI) Processes: The Role of Oxygen Reduction Barak Shapira, Eran Avraham* , Doron Aurbach Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel

A R T I C L E I N F O

Article history: Received 7 September 2016 Received in revised form 17 October 2016 Accepted 18 October 2016 Available online 19 October 2016 Keywords: capacitive deionization (CDI) activated carbon electrodes oxygen reduction reaction (ORR) electro-adsorption

A B S T R A C T

This paper aims at analyzing the impact of irreversible faradaic side reactions and, in particular, oxygen reduction on capacitive deionization (CDI) processes. As opposed to electrochemical supercapacitors, the presence of dissolved oxygen in the feed stream is unavoidable and requires appropriate attention. By constructing simple two- and three-electrode cells, comprising untreated activated carbon cloth, aerogel, and hydrogen-treated activated carbon cloth, we show that the rate at which oxygen is reduced at the active site of the electrodes has significant impact on the cell parameters and, in particular, the potential distribution with respect to its initial state. This has impact on the endurance of the electrodes during long-term cycling. The rate and mechanism of the oxygen catalytic reaction on the carbon electrode is evaluated by adopting the self-discharge model suggested by Conway [1]. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Capacitive deionization (CDI) is as an energy-efficient technique for the removal of salt from water [2–7]. Briefly, the charged species are electro-adsorbed onto high-surface carbon electrodes, and a potential difference is generated between the carbon electrodes. The charged species can be released back into the flow solution when the potential difference is altered or canceled. Capacitive deionization as a sustainable method for brackish water desalination has been extensively investigated over the last years. CDI has been a subject for the development of new carbonbased electrode materials [8–10], new cell architectures [11–13], the modification of electrode surfaces and cell structures to obtain higher charge efficiency, the development and modification of time-dependent double-layer models for porous carbon [14], energy recovery in CDI processes and the endurance of carbon electrodes during charge-discharge cycling [15–17]. Many efforts have been made in recent years to improve the performance of CDI processes in terms of efficiency. For instance, the fabrication of spherical activated carbon as electrodes’ material in Flow Electrodes CDI (FCDI) architecture for desalination of water containing high salt concentration [18], fabrication of nitrogen doped porous carbon using metal-organic framework for high

* Corresponding author. E-mail address: [email protected] (E. Avraham). http://dx.doi.org/10.1016/j.electacta.2016.10.127 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

specific capacity of salt adsorption [19], functionalization of three dimensional graphene electrodes with sulfonic and amine groups for improved charge efficiencies [20], hybrid constant voltage and current operation modes in order to obtain ultrapure water [21] and fabrication of three dimensional graphene electrodes with hierarchical porous structure for fast ion adsorption and high adsorption capacities [22]. However, irreversible faradaic side reactions on carbon electrodes has attracted attention mainly in the context of supercapacitors. Side reactions can occur even if the cell potentials are kept within the thermodynamic stability window of the electrolyte solution. Side reactions may lead to low coulombic efficiency and energy loss, and may be a concern for the shelf-life issue of capacitors. Newman [23] attributes the side reactions in aqueous media to the electrolysis of water arising from hydrogen and oxygen partial pressures. Following the galvanostatic charge-discharge cycling of a symmetric porous carbon capacitor in aqueous media, he introduced the concept “faradaic rectification”. When asymmetry exists between the side-reaction kinetics of the cathode and the anode, it follows that as the cycle number increases, the losses decrease, eventually reaching a stationary state at which the efficiency and potentials become constant and the coulombic efficiency is maximized. “Faradaic rectification” is followed by reorganization of the potentials of the anode and cathode during charge (in relation to the initial state) and by a shift in the discharge potential (E0) of

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both electrodes. Faradaic side reactions can be attributed, besides water electrolysis, to organic and inorganic contaminations in the water (such as iron), irreversible surface reactions (electrooxidation of the electrode surface) and dissolved oxygen reduction. Although the presence of oxygen in the electrolyte solution can be eliminated in supercapacitors, the removal of oxygen from the feed stream in the CDI system is unrealistic, because the energy required for oxygen removal from the water stream can constrain the desalination of brackish water by CDI. The aim of this paper is to raise the issue of the effects of the faradaic side reaction and, in particular, the dissolved oxygen reduction, on CDI processes. Moreover, in order to assess the direct and indirect influence of the oxygen-reduction reaction on CDI processes, a simple analytical method is suggested. 2. Experimental Section 2.1. CDI cell configuration The investigated electrodes (untreated activated carbon cloth (ACF), treated ACF and aerogel) were clamped together, with a glass paper separator mounted between them to each side of a plastic mesh which served as a structural support. Each sheet of the investigated carbon electrodes was 2.5
2.5 cm. Each of the electrodes was attached to a platinum wire which served as a current collector. A reference electrode (standard saturated calomel electrode (SCE)) was mounted adjacent to the working electrode, which was designated to function as the positive electrode, with the aid of a lugging capillary holder. The CDI cell used was a static cell. In each experiment the cell was submerged into a large batch of electrolyte solution (de-areated with nitrogen bubbling or non de-areated solution depending on the experiments’ purpose). Depending on the purpose of the experiment, the cells functioned as two- or three-electrode cells. In the case of a twoelectrode cell, a step of current, or potential, was applied between the two electrodes, where the individual potential of the positive electrode was continuously reordered vs. the reference electrode, with the aid of a standard voltmeter. Prior to any potentio-dynamic application, the carbon electrodes were submerged in boiling distilled water in order to remove air bubbles entrapped in the micropores. The carbon electrodes were soaked in the investigated electrolyte solution for 1 hour before the experiment. 2.2. Electrochemical Measurements The electrochemical measurements were conducted using a PGSTAT Autolab electrochemical measuring system from Ecco Chemie, Inc. (The Netherlands). For the self-discharge experiment, the electrodes were kept at a certain potential for more than 2000 seconds in order to eliminate wrong interpretation of the voltage decline with time, arising from the self-discharge, as the cause of the charge-re-distribution phenomenon within the carbon micropores [24,25,1]. 2.3. Materials Two types of commercial carbon electrodes were used in this work. One is activated carbon cloth, a product [Acc-507-15] of Nippon Kynol, Japan. Its measured BET surface area is 1440 m2/g. The second one was aerogel carbon (Reade Advanced Materials), characterized with mesoporous structure, made by resorcinolformaldehyde pyrolysis, with a BET surface area of 450 m2/g. The

analytical-grade sodium chlorides were purchased from Frutarom, Israel. 3. Results and Discussion During the charge step, the negative electrode in the CDI cell is brought into a state of positive high free energy relative to its initial state. The high-energy state drives the equilibrium of the oxygenreduction reaction towards oxygen reduction on the carbon surface. The potential on the negative electrode is low enough to promote oxygen reduction Oxygen reduction can be followed by a direct four-electron or two-electron pathway (peroxide pathway) [26]. The direct four-electron pathway for oxygen reduction in alkaline solutions is given by O2 þ H2 O þ 4e ! 4OH ; E0 ¼ 0:401V :

ð1Þ

The formula for oxygen reduction in acidic solution is O2 þ 4Hþ þ 4e ! 2H2 O; E0 ¼ 1:229V:

ð2Þ

0

Note that if the pH is neutral, E = 0.81 V. The peroxide pathway in alkaline solution is O2 þ H2 O þ 2e ! HO2  þ OH ; E0 ¼ 0:065V:

ð3Þ

This reaction is followed by either the reduction HO2  þ H2 O þ 2e ! 3OH ; E0 ¼ 0:867V

ð4Þ

or decomposition via the disproportionation reaction 2HO2  ! 2OH þ O2 :

ð5Þ

In acidic solutions, the peroxide pathway is O2 þ 2Hþ þ 2e ! H2 O2 ; E0 ¼ 0:67V:(6) After that, either a further reaction occurs, according to the equation or the disproportionation 2H2 O2 ! 2H2 O þ O2 :

ð7Þ

Mostly, the oxygen-reduction reaction on the carbon electrodes proceeds predominantly as a two-electron process [26]. The dissolved oxygen content can be calculated using Henry's Law and a typical atmospheric oxygen content of 0.21 atm. The concentration of oxygen in an unstirred solution is about 5.15
105 M. The oxygen content in water, at the same concentration, shifts the equilibrium potential by only about 40 mV. Given that the typical zero-charge potential of carbon electrodes falls in the range of 150–400 mV vs. SHE (Standard Hydrogen Electrode), any application of a potential to a CDI cell comprising carbon electrodes leads to sufficient polarization of the negative electrode and promotes trace oxygen reduction. 4. The rate at which oxygen is reduced on a given carbon electrode The separation of faradaic reactions from non-faradaic interactions is not easy, especially when the electrode possesses a very large surface area, although some theoretical and semi-empirical techniques that efficiently separate the faradaic current from the non-faradaic current have been suggested [27–29]. Diagnostic kinetic models have been developed by Conway et al. [1] to aid in the identification of self-discharge mechanisms. The self-discharge mechanism model can identify whether the selfdischarge goes through an activation-controlled or diffusioncontrolled process. The advantage of this technique, i.e., monitoring the voltage drop of a capacitor system as a function of time after

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polarization, is that the electrode possesses a large surface area and the decline of the potential is governed partially by the high capacitance. We distinguish between two mechanisms through which the capacitive electrodes can be self-discharged (assuming that the self discharge is not a cause of a physical short-circuit between the electrodes). For instance, if the electrode is overcharged beyond a certain decomposition potential limit of the electrolyte solution, the self-discharge process should follow a spontaneous decline of the overpotential, h, until h ! 0. The self-discharge process correlates with faradaic charge transfer reactions. Analogous to an equivalent circuit, with parallel faradic resistance, R, the faradaic resistance should increase with the decline of the electrode’s potential. However, if the electrolyte solution contains traces or low concentration of impurities of electroactive species (for instance, Fe2+ ions), it is likely that the self-discharge mechanism follows a diffusion controlled pathway. In CDI systems, oxygen is usually presents in the range of tens of ppm. Due to such low concentrations, self-discharge processes related to trace oxygen reduction are likely to follow the diffusion controlled mechanism as is further elaborated. In fact, the self-discharge of the electrolytic capacitor occurs when the electrode is almost overcharged, via a passage of faradaic currents that can oxidize or reduce moieties in the electrolyte solution or in the electrode itself. The potential decline with time is expected to follow a certain behavior, according to the reaction rate [1]. Whatever the nature of the reaction rate (activation- or diffusion-controlled), the self-discharge potential-time derivative is expected to obey the equation C dl

dE ¼ I: dt

ð8Þ

If the kinetics of the self-discharge are determined by an activation-controlled electrochemical reaction, Eq. (8) can be written as C dl ðdE=dtÞ ¼ I0 eðaEF=RTÞ ;

ð9Þ

where I0 is the exchange current, a is the transfer coefficient and F is the Faraday constant. Cdl is not necessarily the actual electrode differential double-layer capacitance, but rather the interfacial capacitance that represents the active sites where these side reactions occur. Assuming that the capacitance does not change with changes in the electrode potential, the integration of Eq. (9) gives   RT aFI0 RT Ct  ln t þ ð10Þ ; E ¼  ln aF RTC aF i0 where C is the interfacial capacitance and t is the integration constant. When the faradaic reaction is governed by activation control, a plot of E vs. log(t) is expected to follow a linear decline after a certain plateau. Note that the slope is the opposite of the Tafel plot. The self-discharge is governed by diffusion control according to C dl ðdE=dtÞ ¼ zFDðdC=dxÞ:

ð11Þ

Assuming a planar electrode, this equation converges to CðEi  Et Þ ¼ 2zFAD0:5 p0:5c0 t0:5

ð12Þ 2

where Ei is the initial potential, A is given in cm , D is given in cm2 s1, and c0 is the bulk concentration of the electroactive species in mol cm3. In the case of a diffusion-controlled reaction, a plot of V vs. t0.5 is expected to follow a straight line.

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Oickle et al. [30] postulated that although this equation is derived for a planar electrode, a small amount of oxygen is present in the water, resulting in rapid reactant depletion in the pores. Therefore, the pores deep within the carbon do not contribute to the self-discharge, and the electrode acts as a planar electrode. The oxygen reduction is expected to exhibit a diffusion-controlled profile due to the low concentration of dissolved oxygen in the electrolyte. In fact, the plot we present of the potential decline with time can provide, in addition to an understanding of the rate mechanism of the side reactions, an excellent perspective on the rate at which side reactions take place at any given potential of the positive and negative electrodes in the CDI cells. The rate at which oxygen reduction on carbon occurs eventually determines the position of the CDI cell parameters (E1, E2 and E0, as described below) and may have a significant effect on the stability of practical CDI cells. In previous publication [31] we have discussed the concept of symmetry in CDI cells. Moreover, extensive and intensive parameters were evaluated and the connections among them were explained. We mention below main the topics that were thoroughly discussed in our previous publications, based on which the present study was established: a) We showed how the differential capacitance of each electrode in the desalination cell (Cd1 and Cd2) can be obtained. The capacitance depends strongly on the carbon-pore structure [32]. b) We described how the carbon electrodes’ PZC can be determined [33]. c) The overall potential applied to the capacitive deionization cell (E), can be distributed asymmetrically between the electrodes although they are physically similar, because possible side reactions. d) The ions transport in the carbon-electrode pores (adsorption/ þ

desorption) is related to the potential difference applied (G ðEÞ  for cation adsorption and G ðEÞ for anion adsorption). In previous publications [32], it has been demonstrated that the behavior of the cations and anions may be quite symmetric with respect to the electrodes’ PZC if the electrodes have an openpore structure (molecular sieving effect does not exist) that accommodates the negative and positive ions in a similar manner. e) The carbon-electrodes’ potential in CDI cells when the cell is short-circuited (E0), is an important parameter that can be defined independently vs. any stable reference electrode included in the CDI cell. f) The individual potential which falls on the electrodes: E1 for the positively polarized electrode and E2 for the negatively polarized electrode, can be also assesses independently vs. reference potentials (e.g. E0., E referenceelectrode). The initial electrodes’ PZC, their specific capacities and their capability to adsorb ions as a function of their potentials are intrinsic features of the electrodes. E0 and the distribution of potentials between the electrodes depend on the operational conditions of the CDI cell. Note that the only fully controllable parameter is the total potential applied to the entire desalination cell (E). As Newman et al. [23] demonstrated, originally capacitive systems with side reactions tend to be “rectified” in such a way that the steady state in the potentials of the electrodes is reached by the rearrangement of the electrode potentials in the working potential window, and by a shift in the discharge potential (E0). In this way, the necessary charge balance is achieved in the cell.

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The first implication for faradaic side reactions in CDI cells is the decrease in the energy required for a certain level of desalination, measured as kWh per a cubic meter of desalinated water, because the electrical charge invested drives the parasitic reactions instead of the capacitive interactions that lead to ions separation. In fact, the latter parameter can be defined also in terms of kWh per moles of salt extracted by a CDI periodic process from each m3 of desalinated water (i.e. product of the CDI process). Hence, when a parasitic/side reaction takes place, the cell reaches a charge balance on the expense of the efficiency of the CDI process, which is the charge invested in each cycle divided by the amount of salt removed, translated to the charge included in the adsorbed ions. It has previously been shown [31,34,35] that losing the symmetry along the discharge potential may result in a decrease in the salt removal capabilities and charge efficiency. In Fig. 1, we present illustrations of situations in which the charge efficiency declines, with a positive shift in E1, E2 and E0. The durability of the carbon electrodes (mainly the positive electrode) may be critical for the sustainability of the CDI cells and their effective operation [17,31]. The electrochemical stability of the positive electrode for the electro-oxidation processes is a critically important factor. The durability of the CDI cells is shown to be significantly affected by the potential applied to the cell (e.g. 0.9 V vs. 0.7 V). It was postulated that the rate at which the positive electrode is oxidized is accelerated as the potential becomes higher. In fact, the oxygen-reduction reaction at the negative electrode may be the trigger for the parasitic surface electrooxidation of the positive carbon electrode surface.

In order to demonstrate how the parameters E1, E2 and E0 of a capacitive system consisting of two untreated ACF electrodes can be affected, a cell composed of identical ACF electrodes was submerged into an unstirred non-de-aerated batch (0.1 M NaCl). A galvanostatic charge-discharge cycling (70 mA/g for charge and discharge) was applied to the cell. The cutoff potentials for the charge-discharge cycles were 0.7 V and 0 V. The individual potential of the positive electrode (vs. SCE) was recorded over time. In Fig. 2, the potentials of the electrodes tend to converge to a steady state where the potential values (E1, E2 and E0) are positively shifted. This is the first clue to the dominance of the oxygenreduction reactions in the CDI system. Fig. 3 shows the self-discharge behavior (E vs. t) of a cell composed of untreated ACF electrodes upon polarization (both positive and negative electrodes are involved). The self-discharge experiment was conducted as follows (in the same manner for all the inspected electrodes): the electrode was brought to a certain potential vs. the SCE reference electrode, as described in the Experimental Section. The electrodes were kept at this potential for 2000 seconds in order to avoid the mistaken interpretation of a possible voltage decline as the cause of the charge re-distribution in the carbon micropores[24]. The y-axis is the potential of the electrode vs. a reference electrode. The PZC (immersion potential [33]) is shown in the y-axis. As in the observations of Oickle et al. [30], the oxygen-reduction reaction is in the form of a diffusioncontrolled mechanism, as expected, where E vs. t0.5 follows a linear plot. The reaction of the positive electrode, however, is in the form of an activation-controlled mechanism (Fig. 3, insets). The faradaic

Fig. 1. Illustration of working potential domains of symmetrical positive and negative electrodes (the symmetry is in the capacitance and the ion characteristic, with respect to the PZC) at the initial state and after positive shifting at E1, E2 and E0. The shift, in this illustration, results in a decrease in the salt removal capability and charge efficiency, accordingly.

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Fig. 2. The potential of the positive electrode vs. SCE along the galvanostatic charge-discharge cycling. The shift at E1 and E0 is clearly observed.

Fig. 3. Self-discharge profile of untreated ACF. According to the insets, the self-discharge mechanism of the positive potentials follows the activation-controlled mechanism, whereas that of the negative electrodes is governed by diffusion.

reactions of the positive electrode may be detrimental to its stability. However, the study of the degradation of the positive electrodes in the CDI cells due to overly high polarization is beyond the scope of this work. When inspecting the rate of reaction (dE/dt in the plot) with respect to the initial state (i.e. E0 = PZC), it seems that the faradaic side reactions at the negative electrode are the dominant ones. Similarly, to what we discussed above, a constant voltage chargedischarge cycling was applied to the untreated ACF cells in the following experiment (the cell configuration was the same as described in the previous experiment). When the cutoff values of the CDI processes are 0-0.7 V, some of the parasitic faradaic reactions may be attributed to water electrolysis. Thereby, in order to avoid any complications that may arise from water electrolysis, the voltage applied was only 0.5 V. In this experiment, the

individual potential of the positive electrode (vs. SCE) was continuously recorded. Fig. 4a shows the potential of the positive electrode vs. the reference electrode during charge-discharge cycling with 0-0.5 V between electrodes for the untreated ACFs. As expected, the discharge potential (E0) is positively shifted and the system rearranges itself into a state where the faradaic side reactions on both sides (dE/dt in the plot (highlighted in Fig. 5)) are the same (with E1  0.404, E2  0.096). Note that E2 (the potential of the negative electrode) = E–E1. This makes sense because it enables the system to converge to a steady state and minimizes energy loss. The only cases in which the system does not reach a steady state are when the properties of the carbon, such as its capacitance and resistance (RC constant, (R- resistance (in ohms) and C- capacitance (in farads)), change as a result of oxidation, or when oxidation is followed by degradation (decline in the capacitance).

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Fig. 4. a—The potential of the positive electrode (ACF) vs. SCE upon constant voltage cycling between 0–0.5 V. b—llustration of capacitive system with side reaction at steady state.

Although the carbon electrodes possess the same weight, the potential distribution is uneven. This can be explained by the fact that the symmetry of the differential capacitance is around the PZC, and E0 is no longer equal to the PZC.

As we postulated, the untreated ACF electrodes tend to reach a steady state by rearranging E1 and E2 so that they reach values in which the faradaic reaction rates (dE/dt in Fig. 2) for both electrodes are the same (Fig. 4b). In this case, there is a positive shift in E1, E2 and E0.

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Fig. 5. Corresponding dE/dt curves, highlighted for positive and negatitrode (ACF) vs. SCE upon constant voltage cycling between 0–0.5 Vve electrodes. The potentials E1 (+0.4) and E2 (0.1) have similar reaction rates (dE/dt).

Fig. 6. The potential of the positive electrode (aerogel) vs. SCE after constant voltage cycling between 0-0.5 V.

The same procedure (i.e., constant voltage cycling between 00.5 V and a self-discharge profile at different potentials) was applied to these cells (the cell configuration was the same as described in the previous experiments). In Fig. 5, the faradaic reaction rates (dE/dt) with respect to the PZC (recall that E0initial = PZC) is much higher for the negative electrode, whereas the reaction rate for the positive electrode is much more moderate (Fig. 7). The reaction mechanism for the negative electrode seems to be diffusion-controlled and, surprisingly, so is the behavior of the positive electrode. In this case, E1, E2 and E0 exhibits higher (positive) shifting in comparison to the cells that comprise untreated ACF electrodes (Fig. 6). However, given the high faradaic activity on the negative electrode, the fact that the differential shift (E1, E2 and E0) for the untreated ACF electrodes is positive may imply a strong tendency of the positive electrode to become electro-oxidized. It is important to mention that the experiment took place in a stationary two- (and three-) electrode system. In realistic CDI cells,

the dynamics in which oxygen is brought by convection to the electrode active sites may be more complex and depend strongly on the cell structure and the flow conditions. In general, convection results in a reduction in the diffusion layer thickness and higher reaction rates. It has been shown [17,36] that alternating the polarity of the electrodes during cycling may induce stability in the system, resulting in greater endurance. This seems logical, according to the results shown above, because alternating the polarization every few cycles induces reverse “rectification”, and the average at E1, E2 and E0 is sustained. 5. Oxygen Reduction on Carbon Electrodes Oickle et al. [30] postulated that, during the charge step, the amount of oxygen in the pores is insufficient to account for the degree of self-discharge (based on the concentration of dissolved

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Fig. 7. Self-discharge profile of aerogel. The insets demonstrate the diffusion-controlled mechanism for both the negative and positive electrodes. Corresponding dE/dt for the positive and negative electrodes are highlighted. The potentials E1 (+0.54) and E2 (0.4) have similar reaction rates (dE/dt).

oxygen in the water), and the oxygen must diffuse from the bulk to the electrode surface. When the capacitance system was admitted into the nitrogen bubbled solution, upon negative polarization of the investigated electrode, the self-discharge was much less significant. Since the oxygen concentration in the bulk decreased as a result of the nitrogen bubbling, the system may have become more diffusion-controlled, as expected. In order to provide evidence for O2 reduction as the main cause of the parasitic side reaction during negative polarization, the cell with carbon aerogel electrodes was submerged in a pre-bubbled solution (i.e., de-aerated with nitrogen). Prior to the admittance of the cell with aerogel carbon electrodes into the solution (0.1 M NaCl), nitrogen gas was purged through the solution for 1 hour. The differences between the self-discharge profiles of the cells working with nitrogen bubbled solution and cells working with regular (untreated) solutions are significant for the cells with aerogel carbon electrodes (Fig. 8). Indeed, previous publications [17,31] report better CDI performance, in terms of electrode endurance, when nitrogen is continuously purged to the feed stream. Yeager et al. investigated the nature of the carbon to reduce dissolved oxygen [37]. It was hypothesized that O2 reduction on carbon is involved in strong interactions of O2 with functional

groups on the surface. This thesis was supported by Tammeveski et al. [38], where quinone groups were pre-adsorbed to the basal plane of graphite, causing enhanced oxygen reduction activity. Evans and Kuwana [39] suggest that oxygen-containing groups can facilitate oxygen reduction by serving as mediators between the electrode and the electroactive species. In addition, surface oxidative treatments were shown to increase the coverage of quinonoid-type functional groups, making the carbon matrix more active toward oxygen reduction. The efficient removal of the oxidative functional surface group from the activated carbon cloth can be achieved by a prolonged reaction of the carbon with a hydrogen-containing atmosphere at an elevated temperature (1000 C ) [35]. The removal of oxidative groups was confirmed by XPS(X-ray photoelectron spectroscopy) analysis. The following experiment was an attempt to eliminate, at least partially, the active sites of oxygen reduction. For this purpose, an activated carbon cloth was kept in a rotary oven at an elevated temperature (1000 C ), followed by a flow of Ar/H2 (5% w/w), for 4 hours. As shown in Fig. 9a, the self-discharge profile of the hydrogen-treated ACF, upon negative polarization, exhibits a much more moderate decline in the potential with time, indicating a decrease in the catalytic active sites on the carbon surface and

Fig. 8. A comparison of the self-discharge profiles of the nitrogen-bubbled and not bubbled solution for the carbon aerogel cell.

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Fig. 9. a—A comparison of self-discharge profiles for untreated ACF and hydrogen-treated ACF (negative polarization). b—A comparison of self-discharge profiles for untreated ACF and hydrogen-treated ACF (positive polarization).

complete oxygen-reduction reaction. The self-discharge experiment lasted only 850 seconds because of the high tendency of the KynolTM ACF to re-oxidize under ambient atmosphere [40]. In contrast (Fig. 9b), the decline in the potential of the positive electrode is much more significant than in the untreated ACF. This may be attributed to the high tendency of the hydrogen-treated carbon surface toward oxidation [40]. Following the different behaviors of the hydrogen-treated ACFs toward faradaic side reactions, it is expected that CDI cells with treated electrodes, under charge-discharge cycling (0-0.5 V), will demonstrate lower positive shift (of E1, E2 and E0) than the untreated ACF system. During the very first cycles for the treated ACF electrodes, the system, indeed, tends to acquire lower values of E1, E2 and E0 (Fig. 10a). However, since the treated ACF electrodes undergo surface re-oxidation (reflected in Fig. 10b, where the selfdischarge profile after 10 h of cycling shows much greater decline from its initial self-discharge profile), a gradual positive shift in E1, E2 and E0 during cycling is observed. This is a result of the irreversible re-oxidation of the electrodes, rather than the capacitance system rectification (Fig. 10c). In order to provide another evidence for the tendency of the reduced carbon to undergo re-oxidation processes, the following experiment was performed: an hydrogen treated ACF electrode was immersed in non de-areated NaCl electrolyte solution. The solution was exposed to ambient atmosphere, and the immersion potential (vs. reference electrode) was continuously monitored (zero current). A gradual increase in the potential (with respect to a stable reference electrode) was observed (Fig. 10d). The increase in the immersion potential with time is attributed to incremental increase in the

concentration of oxidative functional surface groups on the carbon, in ambient atmosphere. However, in the CDI system, the electrodes are polarized, and thus, the rate at which the electrodes undergo re-oxidation processes is expected to be accelerated (compared to OCV conditions).

6. Conclusions In this work, the significance of the faradaic side reactions and, in particular, oxygen reduction, on the desalination process in CDI processes, is discussed. We show that the rate at which oxygen reacts at the negative electrode has a detrimental effect on the cell parameters in the steady state (E1, E2 and Edischarge). The consequence is a positive shift in these parameters, driving the positive electrode into accelerated unwanted oxidative side reactions. Treating the carbon at an elevated temperature under a reductive environment may aid in lowering the catalytic active sites and, finally, assist in stabilizing the system (lowering positive shift in E1, E2 and Edischarge). Moreover, faradaic side reactions on the investigated positive electrode (activated carbon cloth, [Acc507-15] of Nippon Kynol) were shown to be significant. This work was performed in a stationary typical three-electrode cell. The dynamic in which electro-active species are brought to active sites on the carbon surface in a real-flow CDI cell is more complex. Further research is needed on the effects of the faradaic side reactions in a real-dynamic-flow CDI system and on the CDI performance.

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[4]

[5]

[6]

[7] [8]

[9]

[10]

[11]

[12]

[13]

[14]

[15] [16]

[17]

[18]

[19]

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Fig. 10. a—The potential of the positive electrode (Hydrogen treated ACF and untreated ACF) vs. SCE upon constant voltage cycling between 0-0.5 V. The very first cycles are shown. The changes at the PZC for the hydrogen-treated ACF are insignificant (50 mV). b—A comparison of self-discharge profiles for untreated ACF and hydrogen – before charge-discharge cycling and after 10 h of cycling. c—The potential of the positive electrode (hydrogen-treated ACF electrodes) vs. SCE upon constant voltage cycling between 0-0.5 V. d—Immersion potential/time profile of hydrogen treated carbon electrode versus a S.C.E. reference electrode.

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