Quantitative Examination of Aqueous Ferrocyanide Oxidation in a

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Quantitative Examination of Aqueous Ferrocyanide Oxidation in a Carbon Nanotube Electrochemical Filter: Effects of Flow Rate, Ionic Strength, and Cathode Material Mary H. Schnoor and Chad D. Vecitis* School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States S Supporting Information *

ABSTRACT: The unique conductivity, small diameter, and high specific surface area of carbon nanotubes (CNTs) make them suitable for a range of nontraditional electrode structures. For example, an electrochemically active CNT microfilter has been shown to be effective for water treatment. The forced convection through the 3D CNT electrode improves mass transfer relative to conventional 2D electrodes, but the effects of several solution and reactor parameters in this system are poorly understood because previous works focused on strongly sorbing organic species capable of undergoing several multielectron transfer reactions. Here, the use of Fe(CN)64−/3− as a model soluble and nonadsorbing redox system enabled a near complete accounting of the predominant electron transfer reactions of anodic Fe(CN)64− oxidation and cathodic water reduction to H2. Subsequently, the effects of liquid flow rate, electrolyte concentration, target molecule concentration, anode potential, and cathode material on anodic Fe(CN)64− oxidation kinetics and thermodynamics in an electrochemical filter were investigated. Electrochemical kinetics were observed to increase linearly with increasing flow rate, and electrolyte concentration had negligible effects in the flow configuration due to convective replenishment of Fe(CN)64− and electrolyte to the CNT surface. In the electron-transfer-limited regime ([Fe(CN)64−]0 = 15 mM), a rate constant for the oxidation of Fe(CN)64− was calculated to be in the range ket = 2332− 3705 s−1 or (6−10) × 10−3 cm s−1. The use of a CNT network cathode furthermore enabled the cathodic reaction to proceed at a lower cell potential, indicating that the CNT network catalyzed water reduction. The total cell potential required for facile water reduction decreased from 1.6 V with the Ti cathode to 0.8 V with the CNT cathode, resulting in a total cell energy efficiency (>75%) that approached the current efficiency. Overall, the results quantitatively exemplify some of the advantages of using a 3D CNT electrode in the flow-through configuration.

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INTRODUCTION The electronic, physical, and chemical properties of carbon nanotubes (CNTs) make them suitable for a variety of electrochemical applications.1 In the past several years, CNTs have been used as electrodes in Li-ion batteries,2 as catalyst substrates in fuel cells,3−6 as supporters of charge transfer and charge transport in photoelectrochemical cells,7,8 and as electrodes for electroanalytical devices.9−11 The unique conductivity of CNTs, due to their density of electronic states, distinguishes them from conventional carbon fibers, and their small diameters, high aspect ratio, and high specific surface area allow for a range of nontraditional electrode structures.1,12−14 One such electrode configuration is a flow-through porous electrode composed of multiwalled CNTs (MWNTs) randomly oriented in a two-dimensional plane. A randomly oriented CNT mat used as a flow-through electrode has been shown to be an effective and efficient electrochemically active microfilter for water treatment purposes including virus removal and inactivation15,16 and oxidation of dyes17,18 and phenol.19,20 A flow-through electrode is an efficient design for carrying out bulk electrolysis, for example, in drinking water treatment or the recovery of metals from industrial waste streams.21−25 © 2013 American Chemical Society

Both the high total surface area of a three-dimensional electrode and the forced electrolyte convection through the electrode improve mass transfer relative to conventional twodimensional bipolar electrodes. In turn, the improved mass transfer of the target molecule to the electrode surface enables the efficient electrolytic treatment of both dilute and concentrated solutions via direct electron transfer. Therefore, studies of flow-through porous electrodes produced from a variety of materials have been undertaken to quantify the effects of increased mass transfer on electrochemical kinetics. The material types and textures of interest have shifted over time, generally in the direction of increasing specific surface area: from granular fixed beds to stacks of nets or grids, to stacks of grids of expanded metal, to graphite felts and cloths.22 Among the several types of electrode materials investigated, including metal (nickel or platinum) felts, foams, or wire gauze and carbon-based felt or fibers, the most efficient were found to be elemental carbon-based felts.22 The electrochemical efficiency of these felts was attributed to their small fiber dimension Received: November 12, 2012 Revised: January 15, 2013 Published: January 16, 2013 2855

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Table 1. Material Properties of Flow-Through Porous Electrodes authors

ref

electrode material

electrode length (cm)

specific surface area (cm2 cm−3)

porosity

linear flow rate (cm s−1)

fiber diameter (μm)

conductivity (S cm−1)

Oren and Soffer, 1983 Marracino et al., 1987

23

graphite felt

0.35

0.95

n/a

0.1−1.5

9

2.86

22

nickel felt

0.13

0.95

5000

0.46−5.3

n/a

1.96 × 107

Delanghe et al., 1990

24

nickel foam carbon felt

0.14−0.195 0.13−0.4

0.95−0.96 n/a

1300 150−310

0.46−5.3 0.3−6

n/a 10

1.61 × 107 n/a

Vilar and Coeuret, 1995

52

graphite felt nickel foam

0.12−0.35 0.21−3

n/a 0.975

185−325 35−92.5

0.3−6 1.8−24

9 n/a

n/a n/a

nickel felt sintered nickel platinum gauze reticulated vitreous carbon (RVC) graphite felt multiwalled carbon nanotubes

0.13 0.26 0.032 1.2

0.95 0.34 0.97 0.97

360 690 6 66

1.8−24 1.8−24 1.8−24 0.05−0.25

n/a n/a n/a n/a

n/a n/a n/a n/a

1.2 4.1 × 10−3

0.97 0.85

517 3.63 × 105

0.05−0.25 4.7 × 10−4 to 9.4 × 10−3

n/a 1.5 × 10−2

n/a 102.0

Nava et al., 2009

53

Schnoor and Vecitis, 2012

this work

(diameter of 9−10 μm), which corresponded to the largest electrode area per volume unit. However, even these fibrous carbon electrodes were found to have limited potential beyond the laboratory scale.23 The combination of the large void space between fibers and low porosity within the fiber bundles resulted in the target species passing through the large pores without encountering much of the electrode specific surface area, i.e., only the immediate surface of the fiber bundles was effective for electrochemistry. Additionally, at longer industrial length scales and large electrode depths, the carbon fibers were not conductive enough to sustain applied potentials, due to significant ohmic drop across the electrode. Conductive and porous CNT networks with a diameter approximately 2−3 orders of magnitude smaller than carbon fibers, a completely accessible electrode specific surface area,17 higher conductivity (CNT, 102.0 S cm−1; graphite felt, 2.86 S cm−1), and lesser depth may offer a solution for large-scale industrial applications (Table 1). Despite the fact that the porous CNT electrode characteristics align well with the requirements for an efficient flowthrough electrode material, the fundamental physical chemical properties of the flow-through CNT electrode have yet to be compared to the earlier three-dimensional electrodes documented in the literature. These requirements are the following: (1) chemical inertness over a wide range of potentials,5 (2) high specific surface area and porosity with surface sites easily accessible to the electrochemically active species,26 (3) high fluid permeability,27 (4) convenient shape to meet cell design considerations,28 (5) high electronic conductivity and continuity of the electronic contact throughout the electrode bed,29 and (6) low cost.23,30 Although once an expensive material, the cost of manufacturing CNTs, and in particular MWNTs similar to those used in this study, has dropped substantially in recent years; e.g., MWNTs cost >$100 per gram in the 1990s, and their price had decreased to around $100 per kilogram by 2009.30 Obtaining a quantitative understanding of the electrochemical CNT filter performance will enable a comparison to previous flow-through porous electrodes and provide a basis for evaluating the current filter design in regards to CNT properties, such as diameter and length, and in turn CNT

network electrode properties such as pore size, tortuosity, total surface area, and number of reactive sites. In order to quantify the CNT electrode’s performance, it is necessary to account for all possible electron transfer reactions occurring in the system. However, previous applications of the electrochemical CNT filter were focused on the oxidation and degradation of organic contaminants such as dyes and phenols.17−19 In these studies, the target molecule could undergo several different multielectron transfers, so steady-state current and species conversion could not be directly related and assumptions were made about the number of electrons transferred per molecule. The effect of target molecule adsorption and product desorption on electron transfer processes was also not well understood. Here, these issues are avoided by using the ferrocyanide/ferricyanide (Fe(CN)64−/3−) couple as a model soluble and nonadsorbing redox system and sodium sulfate (Na2SO4) as an electrolyte that is inert at relevant potentials. The Fe(CN)64−/3− redox system is commonly used to evaluate electrode performance,31 since Fe(CN)64− can undergo only a single electron transfer to form Fe(CN)63− and thus this reaction can be directly monitored both by spectrophotometry and chronoamperometry. Herein, we investigate how liquid flow rate, electrolyte concentration, target molecule concentration, anode potential, and cathode material and surface area affect anodic Fe(CN)64− oxidation kinetics in a flow-through electrochemical CNT filter. First, both a cathodic and anodic electron balance is produced by current, pH, dissolved O2, inorganic carbon, and Fe(CN)63− measurements. Next, the effects of the aforementioned parameters are examined. The anodic CNT filter properties are compared to previous reports, and potential improvement in regards to CNT selection are discussed.

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MATERIALS AND METHODS Chemicals and Materials. Potassium ferrocyanide trihydrate (K4Fe(CN)6·3H2O; 98.5−102.0%), sodium sulfate (Na2SO4; ≥99%), sodium chloride (NaCl; ≥99%), and ethyl alcohol (EtOH; ≥95%) were purchased from Sigma-Aldrich (St. Louis, MO). The multiwalled carbon nanotubes (CNTs, ⟨d⟩ = 15 nm, ⟨l⟩ = 100 μm, and 300 °C) in the absence of oxygen, indicating that there is a large energetic barrier to the reduction of these groups.32,33 Positive evidence of gas being evolved can be observed visually, particularly at the 15 mM concentration of influent K4Fe(CN)6. The majority of the initial gas produced is trapped in the top section of the filtration device (Figure S5, Supporting Information). Thus, the H2 production rate calculated from influent/effluent measurements inf ra (at 3.0 mL min−1 and 15 mM influent K4Fe(CN)6 the calculated H2 production rate is 21 μmol min−1) can be roughly compared to the approximate volume of gas bubble evolution. An approximate volume of a single “slice” of the top of the filtration device (shown in Figure S5A, Supporting Information) was estimated to be 170 mm2. Assuming the ideal gas law, a pressure after gas evolution of 1.5 atm,34 and all of the gas in the bubble to be H2, each “slice” of bubble would contain 10 μmol of H2. As can be observed in Figure S5B (Supporting Information), the electrochemically produced gas bubbles make up one to two of these “slice” volumes after approximately 1 min of applied potential or between 10 and 20 μmol of H2 by visual estimation. This value is clearly on the same order of magnitude as the 21 μmol of H2 calculated from the H2 production rate after 1 min. Thus, all evidence points toward H2 production being the most likely and dominant cathodic reaction. Further discussion of this system’s potential application to the generation of hydrogen from wastewater can be found in the Supporting Information. Under the conditions of the experiment, the standard potentials for eqs 1−3 can be adjusted by the Nernst equation, eq 4, to account for the nonstandard pH and concentrations of chemical species:

Information). The nearly identical results obtained with either the CNT or the titanium cathode (Figure 2A and B) indicate that reduction of CNT surface groups is not a significant source of the cathodic current, since the titanium cathode lacks any such groups. This is supported by an extended experiment (2 h) in the presence of Fe(CN)64− (Figure S4, Supporting Information) that shows no decrease in steady-state current with time, which would be expected if the CNT surface groups

E = E0 −

0.059 log10(K ) n

(4)

where K is the chemical equilibrium constant, i.e., the ratio of the activities of the products (reduced state) to the activities of the reactants (oxidized state) and n is the number of electrons transferred in the reaction. For the purposes of this discussion, approximate potentials have been calculated for two conditions that will span the range of conditions observed: (1) where current is first observed (0.15 V anode potential; see Figure 2: pH 9.5, [Fe(CN)63−] = 0.16 mM, PH2 = 0.002 atm) and (2) where near-complete conversion occurs (0.3 V vs Ag/AgCl anode potential; pH 11, [Fe(CN)63−] = 0.95 mM, PH2 = 0.01 atm. Under the first condition, the approximate potentials are as follows: Eeq 1 = 0.092 V vs Ag/AgCl, Eeq 2a/2b = −0.72 V vs Ag/AgCl, Eeq 3 = 0.45 V vs Ag/AgCl. Under the latter condition, the approximate potentials are as follows: Eeq 1 = 0.21 V vs Ag/AgCl, Eeq 2a/2b = −0.83 V vs Ag/AgCl, and Eeq 3 = 0.37 V vs Ag/AgCl. Four measurements were made of the influent and effluent to monitor the occurrence of these reactionspH, TIC, dissolved O 2 , and [Fe(CN) 6 3−]and these measurements were compared to the steady-state current over a range of anode potentials to complete an electron balance as displayed in Figure 2. The TIC measurement was used to account for the fact that CO2 dissolved from the atmosphere into the high pH solution would reduce the pH. For this adjustment, it was assumed that, under the elevated pH conditions observed in

Figure 2. Electron transfer balance. Steady-state rates in μmol of electrons min−1 of total electrons transferred as measured by current (black squares), primary anodic (Δ420 nm; red circles), and cathodic (ΔpH; blue triangles) reactions in the electrochemical filtration system as a function of applied anode potential. Experimental conditions are J = 1.5 mL min−1, [K4Fe(CN)6]in = 1 mM, and [Na2SO4] = 100 mM. (A) Perforated Ti cathode, (B) CNT filter cathode, and (C) depiction of primary electrode surface reactions. 2859

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experiments, each carbonic acid molecule dissociates to CO32− and releases two H+. The oxidation of Fe(CN)64− was monitored by the change in influent to effluent absorption at 420 nm, and for every Fe(CN)63− molecule formed, there is one electron transferred to the anode, eq 1. Thus, the extent of conversion of Fe(CN)64− to Fe(CN)63− is plotted as the μmol of electrons transferred due to eq 1 per minute for comparison to steadystate current measurements. Similarly, the production of H2 was measured by the increase in pH of the effluent, which corresponds to the increase in concentration of OH− and decrease in concentration of H+ in the aqueous solution, eq 2a/ 2b. It was ascertained that the oxygen reduction did not significantly contribute to proton consumption, since dissolved oxygen levels, as measured by DO probe, remained approximately constant until the higher potentials measured, where the current starts to rise due to the onset of anodic water oxidation to O2 (eq 3). As in the case of the anodic reaction, for every H+ molecule removed from solution, one electron is transferred from the cathode. Thus, the adjusted change in pH could be converted to the μmol of electrons transferred per minute. At each potential, the current was also measured, to obtain a total rate of electron transfer in the system. It was found that the hypothesized reactions did indeed account for the majority of the electron transfer processes at both the anode and the cathode and that the two reactions were in balance across the range of potentials; i.e., the electrons transferred for eq 1 was roughly equivalent to eq 2a/2b, and the current efficiency for both processes was >75% under all conditions. The electrochemical filtration mechanism was not affected by the specific cathode material, as the results are quite similar for a perforated Ti cathode, Figure 2A, and a CNT filter cathode, Figure 2B. At the higher anode potentials measured (>0.5 V), it is hypothesized that the additional current is due to the oxidation of CNT surface oxide groups. These redox-active surface groups can be observed by cyclic voltammetry, Figure 3,

dominant reactions occurring at the cathode and the anode, regardless of cathode material, are those depicted in Figure 2C. Electrochemical Filtration Kinetics as a Function of Flow Rate. The Fe(CN)64−/3− redox reactions are known to occur rapidly at carbon electrodes.31 Thus, the extent of Fe(CN)64− oxidation and current density during electrochemical filtration as a function of anode potential, 0.0−0.7 V, and flow rates J, 0.2−4.0 mL min−1, was examined using [K4Fe(CN)6]in = 1 mM, [Na2SO4] = 100 mM, and the titanium cathode. The onset of Fe(CN)64− electrooxidation is observed at 0.15 V anode potential, indicating that there is minimal overpotential to this process, eq 1. Sufficient anode potentials of ≥0.3 V resulted in near complete (>90%) oxidation of Fe(CN)64− at all flow rates tested (Figure 4A). At these potentials, the system is mass-transfer-limited; i.e., the

Figure 3. Cyclic voltammetry of the two electrode materials as a working electrode. CV recorded for CNT electrode (red) and for Ti electrode (black), [Na2SO4] = 100 mM. Flow configuration, J = 1.5 mL min−1, scan rate = 0.1 V s−1. Potential in volts vs 1 M Ag/AgCl. Figure 4. Electrochemical filtration kinetics as a function of flow rate. Steady-state oxidation during electrochemical filtration as a function of anode potential (0−0.7 V) and flow rate (J; 0.2−4.0 mL min−1). [K4Fe(CN)6]in = 1 mM, [Na2SO4] = 100 mM and Ti cathode. (A) Fraction of Fe(CN)64− oxidized in %, (B) bulk oxidation rate (μmol of Fe(CN)64− min−1) as a function of J at 0.3 V anode potential, (C) current efficiency of Fe(CN)64− oxidation and of H2 production at 0.3 V anode potential.

as the small anodic peaks at −0.1 and 0.4 V vs Ag/AgCl and corresponding cathodic peaks at −0.3 and −0.5 V vs Ag/AgCl. The lack of a corresponding increase in hydrogen production observed with the onset of CNT surface group oxidation is an analytical artifact, since this process will release protons, e.g., CHOH → CO + 2e− + 2H+, negating the reductive proton consumption, resulting in no change in pH. In summary, the 2860

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electrooxidation rate is limited by the flow rate of Fe(CN)64− into the system. (Below the 0.3 V anode potential, the system is under mixed mass transfer and reaction control. Influent concentrations of Fe(CN)64− that are high enough to result in reaction-limited conditions even above 0.3 V will be considered inf ra.) Although the same percent conversion was achieved in all cases, faster flow rates led to greater total Fe(CN)64− mass transport to the electrode surface, and in turn to greater absolute rates of Fe(CN)64− oxidation to Fe(CN)63− (Figure 4B). Therefore, in this mass-transfer-limited regime, the absolute electrooxidation kinetics can be linearly increased by simply increasing the liquid flow rate. A simple calculation can confirm that the electrochemical filter conversion should be insensitive to flow rate over the range of flow rates tested. A characteristic minimum time for diffusion, tD = l2/D, can be calculated, in which l is the characteristic diffusion length. In this case, the minimum length is the average radius of a pore in the CNT mat, l = 57.5 nm,17 which is the maximum linear distance a molecule within the CNT network must diffuse in order to reach the electrode surface. The approximate diffusion coefficient of ferrocyanide is D = 0.7 × 10−5 cm2 s−1,35 so the characteristic minimal time for diffusion is tD = 4.7 × 10−6 s. Once the molecule has reached the electrode surface, the empirical electron transfer rate constant ket = 2332 s−1, calculated inf ra, can be used to determine a characteristic time of reaction, tR = 1/ket = 4.3 × 10−4 s. The fastest flow rate tested, J = 4.0 mL min−1, corresponds to a characteristic electrode liquid residence time of τ = V/J = 0.42 s, where V, the volume of the CNT electrode, is 2.8 × 10−8 m3.17 Therefore, the characteristic time of the diffusion and reaction processes, (tD + tR), is 3 orders of magnitude less than the minimal residence time in the electrode, ∼0.4 s, under the stated conditions and will remain less unless there are significant (3 orders of magnitude) increases to flow rate, decreases to CNT filter depth, or increases to CNT diameter, i.e., increased pore diameter and diffusion length. From this comparison, it is clear that the factor that limits the bulk electrooxidation rate under these conditions is the flow rate, so the system is mass-transfer-limited not by the diffusional mass transfer of Fe(CN)64− from the bulk to the electrode surface but by the convective flow rate through the electrode and subsequent replenishment of the target molecule. Higher flow rates were not evaluated here due to pressure limitations (∼1 bar) of the current filtration casing, but future flow-through electrochemical reactor designs would allow for order of magnitude increases in electrochemical kinetics by simply increasing the driving pressure and liquid flow rate. Current efficiency was found to be slightly dependent on liquid flow rate (Figure 4C). The current efficiency of Fe(CN)64− oxidation increases slightly with increasing flow rate from J = 0.5 to 4.0 mL min−1, reaching a plateau at the higher flow rates. This is likely due to the higher flow rates increasing mass transfer of Fe(CN)64− to the anode surface such that it more effectively outcompetes other reductants, e.g., H2, for the electrochemically active CNT anode sites. The current efficiency of H2 production decreases slightly over the same range of flow rates and also levels off at the higher flow rates. The decrease in the efficiency of this cathodic reaction can be attributed to greater volumes of H2 gas evolution at higher flow rates and subsequent bubble formation that blocks active cathode sites. The flow-rate-dependent H2 production current efficiency is increased when a CNT network is used as the cathode to 79−96% with a corresponding anodic current

efficiency of 77−88%. These high and similar current efficiencies indicate that >90% of the electrons oxidized from Fe(CN)64− are used to reduce water to H2. Effects of Electrolyte Concentration on Electrochemical Filtration. The influence of the inert electrolyte concentration on Fe(CN)64− oxidation was investigated to further characterize the system. The influent and flow conditions were similar to those for Figure 2, and the concentration of the electrolyte Na2SO4 spanned 3 orders of magnitude: 1, 10, and 100 mM. As discussed previously, the conductivity of each of these solutions was measured, and it was determined that the solution potential drop between the electrodes was negligible in all cases (Table S2, Supporting Information). The Na2SO4 concentration had minimal effect on the Fe(CN)64− oxidation in the flow configuration, as nearly identical voltage-dependent conversion profiles were observed (Figure 5A). This result yields insight into the mechanism of Fe(CN)64− oxidation, which occurs by direct electron transfer at the electrode surface and does not involve reaction with any oxidizing species derived from the electrolyte, as expected.16 Furthermore, this finding indicates that there is little to no influence from shielding of electrode active sites by SO42− ions or from effects of ionic strength on solution conductivity. The lack of electrolyte concentration effects on electrooxidation kinetics is promising because many waters to be treated, such as surface, ground, and many waste waters, typically have low dissolved ion contents, 1−10 mM.36 This result is in contrast to several previous studies of aqueous electrochemical oxidation, which report strong dependence on electrolyte type and concentration.37−41 In particular, greatly reduced rates of oxidation have been observed when Na2SO4 is used as the electrolyte instead of NaCl, and rates in both cases are correlated to salt concentration. For systems showing this result, the primary chemical oxidant is a Cl-radical species and indirect oxidation of the target species in the bulk solution is the primary mechanism.37 Whereas in the electrochemical CNT filtration system here, whether the target species is Fe(CN)64−, an organic compound, or a microorganism, direct oxidation of that target at the CNT surface is observed to be dominant.16,18,19 This is in agreement with Comninellis et al., who employed a Ti/IrO2 anode that acted primarily by direct oxidation and observed that the effect of electrolyte type (Na2SO4 vs NaCl) was negligible.40 These previous studies were all completed with the classical bipolar batch electrode configuration, so the lack of observed electrolyte concentration effects may be due to the flow-through configuration employed here where the convective flow rapidly replenishes the electrode surface with target molecules. The impact of electrolyte concentration on a flow-through porous electrode system, in contrast to the batch systems cited above, has not been well-studied. Therefore, the effect of electrolyte concentration (1−100 mM Na2SO4) was probed by electrochemical impedance spectroscopy (EIS) in both the flow (Figure 5B) and batch (Figure 5C and Figures S6, Supporting Information; the latter with equal scaled axes) configurations, so as to further elucidate the effect of the liquid flow through the electrode on the overall electrochemical process. Note the difference in x-axis scales for Figure 5B and C. For the purposes of this analysis, qualitative trends in the data provide the requisite information; therefore, only a representative spectrum is shown for each experimental condition. Figure 5B shows that, for each of the electrolyte concentrations tested, the flow 2861

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electrode prevents the additional resistances caused by mass transfer limitations that would otherwise be expected in a relatively large porous electrode. For example, the Nyquist plot of the batch system displays additional arcs, a feature of porous electrodes in batch systems, which arise from the internal concentration gradients produced due to the balance between slow diffusion into and fast charge transfer within the porous electrode.42 Thus, the flow-through CNT electrode constantly replenishes the entire network depth and surface area, preventing strong diffusion gradients that can lead to additional resistances to electron transfer in porous electrodes. In the flow configuration, the electrochemical impedance spectra were nearly identical for the three electrolyte concentrations ([Na2SO4] = 100, 10, 1 mM) with only slight increases in solution resistance (the distance between the y-axis and the left-most point on the spectrum in Figure 5B) and with minimal electrolyte effects on charge transfer resistance, Rct (the width of the arc), and double-layer capacitance, Cdl (obtained by ω = 1/RctCdl, where ω is the frequency corresponding to the peak of the arc). In stark contrast, in the batch configuration, the charge transfer resistance (the width of the largest low frequency arc) increases significantly with decreasing electrolyte concentration. Thus, the relative difference in resistance magnitude between the flow and batch systems increases with decreasing electrolyte concentration. Finally, it is of note that the total flow resistance is always less than the total batch resistances under all electrolyte conditions, e.g., total resistance 1 mM Na2SO4 flow (dashed line 5C) is less than total resistance 100 Na2SO4 mM batch, again indicating that the electrochemical processes in the flow system are hydrodynamically enhanced. In the batch system, a lower concentration of electrolyte results in fewer ions to neutralize the negative charges accumulating at the anode and in turn significantly slower charge neutralization, i.e., charge diffusion rate scales with concentration. In the batch configuration, the dominant product of the anodic reaction, Fe(CN)63−, is only transported away from the anode surface by diffusion. Therefore, there is more resistance to Fe(CN)64− diffusing in from the bulk, because it is repelled by the Fe(CN)63− that is already present, an effect that is exacerbated at lower concentrations of Na+ electrolyte ions, since at lower ionic strength the charge−charge interactions are stronger and fewer positive ions are present to neutralize the solution near the anode.21 The build-up of product near the electrode surface will also increase the potential required for further oxidation; see eqs 1 and 4. On the other hand, in the flow system, fresh electrolyte and reactant Fe(CN)64− are continually renewed at the CNT surface, while simultaneously produced Fe(CN)63− is swept away from the electrode/electrolyte interface by the convective flow. The constant convective renewal of the solution near the CNT surface in the flow configuration results in faster charge neutralization compared to the batch condition. The convection, therefore, prevents any significant increase in charge transfer resistance with decreasing ionic strength by hydrodynamically reducing the diffusion film thickness near the CNT surface. In summary, convection causes the electrochemical filtration system to be much less sensitive to the concentration of electrolyte than a comparable batch system due to increases in near-CNT surface mass transport, resulting in hydrodynamic reductions of charge transfer resistances and thus demonstrating the advantages of using the flow-through configuration.

Figure 5. Effect of electrolyte concentration on electrochemical filtration. [K4Fe(CN)6]in = 1 mM, J = 1.5 mL min−1, and the Ti cathode were used for all experiments. In all cases, [Na2SO4] = 1 mM (blue triangles), 10 mM (red circles), and 100 mM (black squares). (A) Fraction of Fe(CN)64− oxidized during steady-state electrochemical filtration as a function of anode potential (0.0−0.6 V) over a range of electrolyte concentrations, (B) electrochemical impedance spectrum for a range of electrolyte concentrations in the flow configuration, and (C) electrochemical impedance spectrum for a range of electrolyte concentrations using the same electrodes in the batch configuration. The red dashed vertical line indicates the maximum Rs + Rct measured in the flow configuration. Electrochemical impedance spectroscopy was completed using a potential amplitude of 5 mV over a frequency range of 0.1−25 000 Hz, 0.6 V anode potential.

configuration Nyquist plot matches the shape of a Randles cell equivalent circuit with a Warburg impedance observable at low frequency and for all three ionic strengths in the flow conditions; Rct = 4.0 ± 0.2 Ω and Cdl = 260 ± 30 μF. The Randles cell equivalent circuit is commonly used to model a simple situation in which the impedance at the electrode interface is caused by a combination of a resistance due to a Faradaic charge-transfer process and a resistance due to the diffusive mass transfer associated with double-layer charging. The fact that this simple spectrum is obtained for the flow configuration indicates that the convective flow through the 2862

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Figure 6. Effect of cathode material on electrochemical filtration. (A) Fraction of Fe(CN)64− oxidized during steady-state electrochemical filtration at 0.3 V anode potential for [K4Fe(CN)6]in = 1 mM and [K4Fe(CN)6]in = 15 mM for both cathode materials. (B) Fraction of Fe(CN)64− oxidized during steady-state electrochemical filtration with either CNT or titanium cathode and with unadjusted pH, influent pH 11.23, or influent pH 11.58, as a function of total cell potential applied. Cathode and anode potential in V as a function of total cell potential applied in V: (C) Ti cathode and [K4Fe(CN)6]in = 1 mM, (D) CNT cathode and [K4Fe(CN)6]in = 1 mM, (E) Ti cathode and [K4Fe(CN)6]in = 15 mM, and (F) CNT cathode and [K4Fe(CN)6]in = 15 mM. J = 1.5 mL min−1 and [Na2SO4] = 100 mM for all experiments. Red dashed lines represent a range of Fe(CN)64− oxidation potentials under experimental conditions, and black dashed lines represent a range of water reduction potentials under experimental conditions, as calculated in the Discussion.

Effect of Cathode Material on Electrochemical Filtration. As described in the text, experiments were conducted both with the previously reported perforated titanium shim (Ti) cathode and with a new design utilizing a second CNT network as the cathode. The addition of a cathodic CNT network could increase the specific surface area of the cathode by up to nearly 3 orders of magnitude, from 1.4 × 10−3 to 1.05 m2, if the entire depth is electrochemically active as observed for phenol electropolymerization,20 nearly 20 times, from 1.4 × 10−3 to 2.6 × 10−2 m2, even if only the top 1 μm is active, and also introduces a material that may have a different electrochemical activity. Subsequent discussion assumes that the entire CNT depth is electroactive for the purposes of calculating electrode surface area, reactor volume, and current densities, and a more detailed discussion of the reactive depth of the CNT electrode may be found in the Supporting Information. Electrochemical filtration experiments with the two cathodes were initially completed with [K4Fe(CN)6]in = 1 mM, 0.3 V anode potential, J = 1.5 mL min−1, and

[Na2SO4] = 100 mM, as depicted in Figure 6A. However, there was no discernible difference between the conversion or current observed at a given applied anode potential, since in both cases the Fe(CN)64− oxidation was near complete, >95%. Thus, the influent Fe(CN)64− concentration was increased ([K4Fe(CN)6]in = 15 mM) such that the system was approaching the electron-transfer-limited regime, and in this case, the different cathodes resulted in significantly different extents of oxidation. Only 50% of the influent Fe(CN)64− was oxidized at 0.3 V anode potential with Ti cathode, while over 80% of the influent Fe(CN)64− was oxidized under the same conditions with the CNT cathode. Since the experiments in Figure 6A were completed at a set anode potential, which may yield minimal insight into the cathodic process, flow electrolysis experiments were completed over a range of applied voltages or total cell potentials using both the Ti and CNT cathodes with [K4Fe(CN)6]in = 1 mM, J = 1.5 mL min−1, and [Na2SO4] = 100 mM, as depicted in Figure 6B. It is quite obvious from the applied voltage2863

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dependent Fe(CN)64− conversion at unadjusted pH that the CNT cathode results in near complete conversion at 0.8 V, nearly 1 V lower total cell voltage than required for the Ti cathode. A similar result is observed for the higher pH conditions (pH 11.23 and pH 11.58) in Figure 6B that displays a potential shift of ∼0.3 V as compared to the lower, unadjusted pH condition (pH 6.5), in agreement with expectations from the Nernst equation for the pH-dependent cathodic water reduction reaction, eq 2a/2b. These results suggest that the CNT cathode catalyzes water reduction and in turn reduces the total energy required for the overall electrochemical process. To support this hypothesis, open circuit potential measurements at different applied voltages were made for the Ti and CNT cathodes with [K4Fe(CN)6]in = 1 and 15 mM (Figure 6C−F). At 1 mM Fe(CN)64−, it is apparent that the CNT cathode causes the potential distribution to shift more toward the anode. For example, with the CNT cathode, the anode potential obviously moves into the Fe(CN)64− standard potential range calculated with eq 4 at 0.2 V total applied potential, whereas it takes 0.6 V applied potential to achieve a similar anode potential with the Ti cathode. At high influent concentrations ([K4Fe(CN)6]in = 15 mM), the difference in anode potential between the two cathodes is less apparent due to the lower increase in anode potential with applied voltage as a result of the increased Fe(CN)64− concentration. Even so, with the CNT cathode, the anode potential increases into the Fe(CN)64− standard potential range at 0.4 V applied potential, whereas it takes 0.8 V total applied potential to achieve a similar anode potential with the Ti cathode. As a result of this shift in potential distribution toward the anode with the CNT cathode due to lower cathodic requirements, the total energy input required by the system is greatly decreased for the CNT cathode as compared to the Ti cathode. The mechanism of CNT-catalyzed water electroreduction may be explained by a few possible mechanisms that are likely acting simultaneously. First, the CNT electron-transfer active sites are most likely the surface oxy-group defect sites (−OH, −COOH) that possess dissociable protons and tend to be concentrated at the ends of the tubes.43 As stated previously, the presence of these groups is indicated by Figure 3. Therefore, unlike the Ti cathode, the CNT cathode may have a significant and renewable near-surface concentration of dissociable H+ for H2 production, in agreement with CNT cathode potentials near that for proton reduction, eq 2a, producing hydrogen. Evidence for the surface oxy-group mechanism can be observed in Figure 6B where upon going from a bulk pH of 11.23 to 11.58 there is a large voltage shift for the CNT cathode as compared to that of 6.5 to 11.23 as well as compared to the Ti cathode. This shift may be representative of increasing the pH past the pKa and deprotonation of the surface oxy-groups, resulting in higher necessary potentials for H2 production, and suggests these groups are aromatic hydroxyls or hydroquinones. Although the pKa of dissolved aqueous phenols and hydroquinones is typically in the range 9−10, a recent study observed that anthrahydroquinone near a CNT surface had a pKa three units higher than expected, in agreement with the results presented here.44 In line with the first possible mechanism, previous studies have observed that the pH near an anodic CNT electrode surface is 2−3 pH units lower than expectations from bulk solution measurements.34 A greater near-CNT-surface proton concentration than expected from bulk measurements would promote H2 production from water and its dissociation

products, as both reactions have a pH-dependent potential, eq 2a/2b. This mechanism would also help explain why water reduction (Figure 6B−F; eq 2a/2b) occurs at the CNT cathode at a significantly lower potential than predicted by the Nernst equation using the bulk pH and near that for proton reduction. For this reaction to occur at the potentials observed, the local environment near the CNT surface would need to be significantly enriched (5 orders of magnitude) in H+ as compared to the bulk. A third mechanistic explanation for CNT-catalyzed water reduction is that nanostructured elemental carbon has been reported to have the ability, like noble metals, to reduce protons or water by a single electron transfer, the Volmer reaction. The produced H-atoms are stabilized by sorption to the extended sp2 carbon by sharing of electron density with the conjugated π-bonded structure:45−48 H+ + C + e− → CH•ad

(5)

The adsorbed H-atoms (H•ad) can then react to form H2 by the Heyrovsky (Eley−Rideal; H•ad + H+ + e− → H2) or Tafel (Langmuir−Hinshelwood; 2H•ad → H2) pathways.48 The H•ad stabilization mechanism may contribute to the CNT cathode’s reduced H2 production potential relative to the Ti cathode as well as the increased kinetics. Further examination will be needed to determine the true mechanism of enhancement, or more likely the relative contributions of the plausible mechanisms presented here. The presence of catalyzed H+ reduction, which permits H2 production to occur at a lower total cell potential than H2O reduction, is the primary factor for increased electrooxidative efficacy and overall energy efficiency with the CNT cathode, as compared to the Ti cathode. The increase in energy efficiency with the CNT cathode can be quantified by taking into account the applied power (I × V) and comparing this input to the ideal thermodynamic energy requirements. The overall energy efficiency is determined by the following equation, where the thermodynamic power required to drive both the Fe(CN)64− oxidation and H2 production is divided by the net power input assuming the produced hydrogen can be used as fuel: η= F(E FeCN[Fe(CN)6 4 − ox. rate] − E H2O[2 × H 2 prod. rate]) 1 Iobs × Vapp − ΔHH2 comb⎡⎣ 4 × H 2 prod. rate⎤⎦ (6)

In eq 6, F = 96 485 C mol−1 is Faraday’s constant, EFeCN = 0.092 V vs Ag/AgCl is the standard potential for Fe(CN)64− oxidation (eq 1), EH2O = −0.72 V vs Ag/AgCl is the standard potential for reduction of water to H2 (eq 2a/2b), and ΔHH2 comb = 286 kJ mol−1 is the heat of combustion for hydrogen. The potentials used for the calculation are adjusted from their standard values by the Nernst equation, as described in the section Electron Transfer Balance; the condition with lower potentials was used for this calculation as a conservative estimate. The net power input is the product of the total cell potential applied and the corresponding observed current minus the energy that could be recaptured by combustion of the produced H2, assuming 25% efficiency of the overall process, i.e., H2 capture and combustion. For both cathode designs, efficiency is calculated at the total cell potential where Fe(CN)64− conversion plateau is achieved, 1 V applied in the case of the CNT cathode and 2 V applied in the case of the Ti cathode. The values used for the calculation of the Fe(CN)64− 2864

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since at all flow rates tested the same near-complete conversion was obtained. In contrast, in the high concentration regime ([K4Fe(CN)6]in = 15 mM), the electrooxidation rate begins to plateau at higher flow rates, indicating that the oxidation kinetics have become electron-transfer-limited. In the electron-transfer-limited regime, since the electron transfer reaction at the anode is the rate-determining step, a pseudo-first-order rate of electron transfer ket can be calculated. Here, the electrochemical filter kinetics will be modeled assuming an ideal plug-flow reactor equation, which has been shown to be a valid approximation for this system17

oxidation rate and the H2 production rate at these cell potentials can be found in Table S3 (Supporting Information). Due to the lower applied cell potential needed for nearcomplete conversion because of catalyzed H+ reduction, the overall energy efficiency is significantly higher for the CNT cathode. Quantitatively, the CNT cathode design is found to have ∼78% energy efficiency at 1 V applied voltage, while the Ti cathode design has ∼33% energy efficiency at 2 V applied voltage. The maximum possible efficiency for the CNT cathode as determined by the current efficiency (Figure 2A and B) is in the range 80−90%. Therefore, it is of note that the overall energy efficiency of the electrochemical filter with CNT networks for both the anode and the cathode is >75% and near the current efficiency. Electron-Transfer-Limited Electrooxidation Regime. Dependence of steady-state electrooxidation rate (μmol min−1) on flow rate J at 0.3 V anode potential with the Ti cathode and [K4Fe(CN)6]in = 1 mM (blue circles) and [K4Fe(CN)6]in = 15 mM (black squares) is depicted in Figure 7A. In the low concentration condition ([K4Fe(CN)6]in = 1 mM), the electrooxidation rate increases linearly with flow rate,

V=

∫ −k

F0

4− et[Fe(CN)6 ]0 (1 − R )

dR (7)

where V is the volume of the reactor (V = A × L = 2.89 × 10−2 cm3), F0 is the molar flow rate in mol s−1, and R is the fractional conversion. Solving the integral, eq 7 becomes ⎛1 ⎞ F0 V =⎜ ⎟ ln(1 − R ) ⎝ ket ⎠ [Fe(CN)6 4 − ]0

(8)

V/ln(1 − R) can be plotted against F0/[Fe(CN)6]in, and if linear, then the inverse of the slope of the resulting plot is the pseudo-first-order electron-transfer rate ket (Figure 7B). Using the Ti cathode, ket = 2332 s−1, and using the CNT cathode, ket = 3705 s−1; these similar values indicate that, at the conditions studied, the cathode material (and its accompanied surface area) does not have any significant effect on anodic reaction kinetics, although the cathodic current density is much higher with the titanium cathode than with the CNT cathode and it could be expected that, at high enough cathodic current densities, the cell current and hence the anodic reaction kinetics could be limited by the stability of the cathode material. Under the experimental conditions, with a typical current of 25 mA under the electron-transfer-limited conditions ([Fe(CN)64−]in = 15 mM, J = 1.5 mL min−1), the Ti cathode alone (SA ∼1.4 × 10−3 m2) will have a current density of 17.8 A m−2 (178 mA cm−2), whereas with the CNT cathode (SA ∼1.05 m2), the cathodic current density may be reduced to 2.4 × 10−2 A m−2 (0.24 mA cm−2) if all of the surface area is electrochemically active. Calculating ket permits a quantitative comparison between different electrode designs and configurations. The values of ket calculated here for the direct transfer of an electron from Fe(CN)64− to the anodic CNT surface accord well with the value of the electron transfer rate constant for the same reaction occurring at a film composed of aligned multiwalled CNT calculated from both EIS and absolute electrochemical rates determined by Tsierkezos and Ritter.49 Their rate constant, ks, was expressed in units of cm s−1 to facilitate calculation of the rate of electron transfer per unit area of electrode. The conversion from the calculated ket values here to this linear form is completed using eq 9:

Figure 7. Electron-transfer-limited electro-oxidation regime. (A) Dependence of steady-state electro-oxidation rate on flow rate J at 0.3 V anode potential with titanium cathode. Linear dependence in the low concentration regime ([K4Fe(CN)6]in = 1 mM) and exponential dependence in the electron-transfer-limited high concentration regime ([K4Fe(CN)6]in = 15 mM). (B) Effect of cathode material and surface area on oxidation reaction pseudo-first-order rate constant in the electron-transfer-limited high concentration regime. Assuming an ideal plug-flow reactor, the slope is 1/ket. V is the volume of the reactor (2.8 × 10−8 cm3), R is the fraction oxidized, and F0 is the molar flow rate (mol min−1 flowing into the reactor). 0.3 V is the anode potential, J = 1.5 mL min−1, and [Na2SO4] = 100 mM.

⎡ cm ⎤ ⎡1⎤ V ks ⎢ ⎥ = ket ⎢ ⎥ × ⎣ s ⎦ ⎣ s ⎦ SA

(9)

where V is the volume of the electrode and SA is the electrode surface area. Using the specific electrode surface area, our ks values are 10 × 10−3 and 6 × 10−3 cm s−1 for the CNT and Ti cathodes, respectively. This is similar to the rate constants calculated by Tsierkezos and Ritter, which ranged between 3.4 and 5.8 × 10−3 cm s−1, depending on which method was used to calculate ks. It should be noted that Tsierkezos and Ritter’s 2865

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potential applications. For example, changes in both the CNT diameter and length will affect the electrochemical filtration process and there is likely an optimal size for both depending on the desired application. The smaller the CNT diameter, the greater the specific surface area and number of electron transfer sites. On the other hand, a smaller CNT diameter will increase the CNT packing and in turn reduce the pore size and porosity, thereby increasing the driving pressure and energy required. Similarly, decreasing the CNT length will increase the number of edge sites that contain the majority of electron transfer sites but will reduce the mechanical stability of the film as a whole.

CNT electrode was considered to be two-dimensional (geometric SA = 2.28−2.83 cm2) and the ks rate constants calculated from the ket results here used the CNT electrode’s specific surface area of 1.05 m2. If instead the geometric surface area of the CNT electrode, 7.1 cm2, was used, the ks values here are increased to 14.3 and 8.6 cm s−1 for the CNT and Ti cathodes and are >3 orders of magnitude greater than the previously reported values. Once again, this result demonstrates the benefit of using three-dimensional electrodes in the flowthrough configuration. Comparison to Previous Flow-Through Porous Electrodes. Compared to previously studied flow-through porous electrodes, the CNT electrochemical filter both operates under different conditions and has unique material properties (Table 1). The system studied here has an electrode length approximately 2 orders of magnitude shorter and a linear flow rate approximately 3 orders of magnitude slower than what has been previously studied in the literature. Taken together, the liquid residence time in the CNT filter system is ∼10 times greater. The works referenced in Table 1 all operate in the pore diffusion-limited regime, and therefore can compare themselves to other flow-through porous electrodes on the basis of a mass transfer coefficient. However, although it is possible that the residence time within the CNT electrode could be reduced such that it was in the pore diffusion-limited regime (see the discussion in the section Electrochemical Filtration Kinetics as a Function of Flow Rate), even then electron transfer may become limiting before diffusion. For a quantitative example, consider the graphite felt electrode of Oren and Soffer.23 Assuming the graphite felt electrode has approximately the same fiber diameter to average pore diameter ratio as the CNT electrode, the graphite electrode’s pore size would be ∼70 μm, compared to the CNT filter’s pore size of ∼115 nm. Thus, a characteristic time of diffusion from the center of a pore to the graphite felt electrode surface is tD = l2/D; calculated the same way as above for the CNT electrode, tD for the graphite felt is ∼1.75 s, compared to tD = 4.7 × 10−6 s for the CNT network. Therefore, the small diameter of CNT, which permits smaller pores while maintaining high porosity, greatly reduces the effect of mass transfer limitations within pores. These favorable mass transfer properties, combined with the high conductivity of CNT as compared to graphite (Table 1), the ease of incorporating the CNT network into a modified commercial filtration device, and the results presented here indicate that the electrochemical CNT filter is an effective and efficient flowthrough porous electrode that may have potential for industrial applications. The use of CNTs as three-dimensional or porous electrodes has been recognized in the literature. Li et al. first reported the use of CNT as three-dimensional electrodes, demonstrating capacitance and electron-transfer reactions not only on the exterior surface but also within a film composed of aligned multiwalled CNT “towers”.13 Since then, three-dimensional CNT electrode structures have been used for a wide range of applications, including microbial fuel cells,50 supercapacitors,29 Li-ion batteries,2 and fuel cells.6 However, all of these previous studies have utilized the CNT electrodes in the classical batch configuration. Even though their effectiveness has been demonstrated, porous flow-through CNT electrode structures have been less studied.16,17,51 An increased understanding of the fundamental physical chemical processes underlying CNT flow-through porous electrodes will guide further developments in the design of future hydrodynamic CNT electrodes and their

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CONCLUSIONS In summary, this study has used the Fe(CN)64−/3− redox system to delineate the electron transfer reactions taking place during electrochemical filtration and to quantitatively examine oxidation kinetics at an anodic CNT porous flow-through electrode. Due to the experimental setup, the produced H2 could not be measured directly; however, control experiments and indirect evidence demonstrated water reduction to H2 to be the most likely and dominant cathodic reaction. An electron balance indicated that anodic Fe(CN)64− oxidation and cathodic H2 production were the primary electrochemical reactions. When a CNT network was used for both the cathode and anode, the current efficiency for anodic Fe(CN)64− oxidation and cathodic H2 production was flow rate dependent, ranging from 77 to 88% and 79 to 96%, respectively. Thus, >90% of the electrons oxidized from Fe(CN)64− went toward H2 production, indicating a strong potential of the electrochemical filter for waste-to-fuel processes. The use of both a CNT network cathode and anode also resulted in overall energy efficiencies that approached the current efficiency. The CNT cathode displayed a significant improvement in energy efficiency relative to the Ti cathode, suggesting CNT-catalyzed water reduction. Possible mechanisms by which the CNT cathode catalyzes water reduction were discussed, and future studies will aim to better understand the CNT surface chemistry that leads to these results. It was also determined that the Fe(CN)64− electrooxidation kinetics scaled linearly, with a slope near 1, with filtration flow rate and were unaffected by electrolyte concentration, due to rapid convective replenishment of both Fe(CN)64− and electrolyte within the CNT network electrode. Overall, the results presented here quantitatively exemplify some of the advantages of using a 3D electrode in the flow-through configuration and demonstrate the potential of a CNT electrochemical filter for environmental and energy applications.

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ASSOCIATED CONTENT

S Supporting Information *

Relevant dimensions of the experimental system (Table S1), conductivity of solutions used and corresponding solution potential drops (Table S2), values used in efficiency calculations (Table S3), determination of steady-state effluent conversion (Figure S1), control experiments in the absence of K4Fe(CN)6 and under anoxic conditions (Figure S2), control experiments with NaCl electrolyte (Figure S3), investigation of time dependence (Figure S4), observation of cathodic gas production (Figure S5), Figure 5C with equal scaled axes (Figure S6), Discussion of the reactive depth of the CNT electrode, and discussion of electrochemical CNT filter use for hydrogen generation from wastewater. This material is available free of charge via the Internet at http://pubs.acs.org. 2866

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(30) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Science 2009, 323, 760−764. (31) Nugent, J. M.; Santhanam, K. S. V.; Rubio, A.; Ajayan, P. M. Nano Lett. 2001, 1, 87−91. (32) Langley, L. A.; Fairbrother, D. H. Carbon 2007, 45, 47−54. (33) Kundu, S.; Wang, Y.; Xia, W.; Muhler, M. J. Phys. Chem. C 2008, 112, 16869−16878. (34) Gao, G.; Vecitis, C. D. Environ. Sci. Technol. 2011, 45, 9726− 9734. (35) Konopka, S. J.; McDuffie, B. Anal. Chem. 1970, 42, 1741−1746. (36) Nazaroff, W. W.; Alvarez-Cohen, L. Environmental Engineering Science; John Wiley & Sons, Inc: New York, 2001. (37) Park, H.; Vecitis, C. D.; Hoffmann, M. R. J. Phys. Chem. C 2009, 113, 7935−7945. (38) Kim, S.; Choi, S. K.; Yoon, B. Y.; Lim, S. K.; Park, H. Appl. Catal., B 2010, 97, 135−141. (39) Murugananthan, M.; Yoshihara, S.; Rakuma, T.; Uehara, N.; Shirakashi, T. Electrochim. Acta 2007, 52, 3242−3249. (40) Comninellis, C.; Nerini, A. J. Appl. Electrochem. 1995, 25, 23− 28. (41) Gonzalez, T.; Dominguez, J. R.; Palo, P.; Sanchez-Martin, J. J. Chem. Technol. Biotechnol. 2010, 86, 121−127. (42) Lasia, A. In Electrochemical Impedance Spectroscopy and Its Applications. Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J., White, R. E., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999. (43) Wildgoose, G. G.; Abiman, P.; Compton, R. G. J. Mater. Chem. 2009, 19, 4875−4886. (44) Masheter, A. T.; Abiman, P.; Wildgoose, G. G.; Wong, E.; Xiao, L.; Rees, N. V.; Taylor, R.; Attard, G. A.; Baron, R.; Crossley, A.; Jones, J. H.; Compton, R. G. J. Mater. Chem. 2007, 17, 2616−2626. (45) Conway, B. E.; Bockris, J.O’M. J. Chem. Phys. 1957, 26, 532− 541. (46) Lee, S. M.; An, K. H.; Lee, Y. H.; Seifert, G.; Frauenheim, T. J. Am. Chem. Soc. 2001, 123, 5059−5063. (47) Gundiah, G.; Govindaraj, A.; Rajalakshmi, N.; Dhathathreyan, K. S.; Rao, C. N. R. J. Mater. Chem. 2003, 13, 209−213. (48) Jurewicz, K.; Frackowiak, E.; Beguin, F. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 981−987. (49) Tsierkezos, N. G.; Ritter, U. J. Chem. Thermodyn. 2012, 54, 35− 40. (50) Xie, X.; Hu, L.; Pasta, M.; Wells, G. F.; Kong, D.; Criddle, C. S.; Cui, Y. Nano Lett. 2011, 11, 291−296. (51) Yang, J.; Wang, J.; Jia, J. Environ. Sci. Technol. 2009, 43, 3796− 3802. (52) Vilar, E. O.; Coeuret, F. Elechrochim. Acta 1995, 40, 585−590. (53) Nava, J. L.; Recendiz, A.; Gonzalez, L. G.; Carreno, G.; Martinez, F. Port. Electrochim. Acta 2009, 27, 381−396.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (617) 496-1458. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.H.S. was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. M.H.S. thanks Dr. Guandao Gao for his assistance with electrochemical techniques and Peggy Mativo for her assistance in conducting experiments.

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REFERENCES

(1) Gooding, J. J. Electrochim. Acta 2005, 50, 3049−3060. (2) Morris, R. S.; Dixon, B. G.; Gennett, T.; Raffaelle, R.; Heben, M. J. J. Power Sources 2004, 138, 277−280. (3) Jeng, K. T.; Hsu, N. Y.; Chien, C. C. Int. J. Hydrogen Energy 2011, 36, 3997−4006. (4) Jeng, K. T.; Chien, C. C.; Hsu, N. Y.; Yen, S. C.; Chiou, S. D.; Lin, S. H.; Huang, W. M. J. Power Sources 2006, 160, 97−104. (5) Wang, X.; Li, W.; Chen, Z.; Waje, M.; Yan, Y. J. Power Sources 2006, 158, 154−159. (6) Hasche, F.; Oezaslan, M.; Strasser, P. Phys. Chem. Chem. Phys. 2010, 12, 15251−15258. (7) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 25477−25484. (8) Dillon, A. C. Chem. Rev. 2010, 110, 6856−6872. (9) Takase, M.; Yoneyama, Y.; Murata, M.; Hibi, K.; Ren, H.; Endo, H. Anal. Bioanal. Chem. 2012, 403, 1187−1190. (10) Sotiropoulou, S.; Chaniotakis, N. A. Anal. Bioanal. Chem. 2003, 375, 103−105. (11) Lu, T. L.; Tsai, Y. C. Sens. Actuators, B 2011, 153, 439−444. (12) McCreery, R. L. Chem. Rev. 2008, 108, 2646−2687. (13) Li, J.; Cassell, A.; Delzeit, L.; Han, J.; Meyyappan, M. J. Phys. Chem. B 2002, 106, 9299−9305. (14) Sherigara, B. S.; Kutner, W.; D’Souza, F. Electroanalysis 2003, 15, 753−772. (15) Rahaman, M. S.; Vecitis, C. D.; Elimelech, M. Environ. Sci. Technol. 2012, 46, 1556−1564. (16) Vecitis, C. D.; Schnoor, M. H.; Rahaman, M. S.; Schiffman, J. D.; Elimelech, M. Environ. Sci. Technol. 2011, 45, 3672−3679. (17) Vecitis, C. D.; Gao, G.; Liu, H. J. Phys. Chem. C 2011, 115, 3621−3629. (18) Liu, H.; Vecitis, C. D. J. Phys. Chem. C 2012, 116, 374−383. (19) Gao, G.; Vecitis, C. D. ACS Appl. Mater. Interfaces 2012, 4, 1478−1489. (20) Gao, G.; Vecitis, C. D. ACS Appl. Mater. Interfaces 2012, 4, 6096−6103. (21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. (22) Marracino, J. M.; Coeuret, F.; Langlois, S. Electrochim. Acta 1987, 32, 1303−1309. (23) Oren, Y.; Soffer, A. Electrochim. Acta 1983, 28, 1649−1654. (24) Delanghe, B.; Teller, S.; Astruc, M. Electochim. Acta 1990, 35, 1369−1376. (25) Coeuret, F.; Oliveira Vilar, E.; Bezerra Cavalcanti, E. J. Appl. Electrochem. 2002, 32, 1175−1182. (26) Niu, C.; Sichel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys. Lett. 1997, 70, 1480−1482. (27) Brady-Estevez, A. S.; Schnoor, M. H.; Vecitis, C. D.; Saleh, N. B.; Elimelech, M. Langmuir 2010, 26, 14975−14982. (28) Wildgoose, G. G.; Banks, C. E.; Leventis, H. C.; Compton, R. G. Microchim. Acta 2006, 152, 187−214. (29) Dai, L.; Chang, D. W.; Baek, J. B.; Lu, W. Small 2012, 8, 1130− 1166. 2867

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