Evaluating the Electrochemical Capacitance of Surface-Charged

Mar 19, 2012 - IMDEA Energy Institute, Móstoles 28933, Spain. •S Supporting Information. ABSTRACT: While transition metal oxides have been thoroughly ...
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Evaluating the Electrochemical Capacitance of Surface-Charged Nanoparticle Oxide Coatings Kevin C. Leonard,*,† Wendy E. Suyama,‡ and Marc A. Anderson§,∥ †

Materials Science Program, §Environmental Chemistry and Technology Program, University of Wisconsin-Madison, 660 North Park Street, Madison, Wisconsin 53706, United States ‡ SolRayo, Inc., 4005 Felland Road #107, Madison, Wisconsin 53718, United States ∥ IMDEA Energy Institute, Móstoles 28933, Spain S Supporting Information *

ABSTRACT: While transition metal oxides have been thoroughly investigated as coatings for electrochemical capacitors due to their pseudocapacitance, little work has been done investigating other oxide coatings. There exists a whole class of nanoporous oxides typically synthesized by sol− gel chemistry techniques that have very high differential capacitance. This high differential capacitance has been attributed to the surface potential of these materials and the close approach of counterions near the surface of these oxides. This study focuses on investigating the electrochemical capacitance of non-transition metal oxide nanoparticle coatings when deposited on supporting electrodes. Here, we show that, by adding coatings of SiO2, AlOOH, TiO2, and ZrO2 nanoparticles to graphite support electrodes, we can increase the electrochemical capacitance. We also show that the measured electrochemical capacitance of these oxide-coated electrodes directly relates to the electrophoretic mobility of these materials with the lowest values in capacitance occurring at or near the respective isoelectric pH (pHIEP) of each oxide.

1. INTRODUCTION It is possible to synthesize suspensions of various types of oxide nanoparticles (colloids) by sol−gel chemistry techniques.1−4 A unique feature of many of these oxide particles (e.g., SiO2, TiO2, ZrO2, and others) is that they have an inherent surface potential that is a function of pH and other potential determining ions. The surface potential of the oxide is affected by potential determining ions (such as the proton or protolyzable anions) and can change from positive to negative by varying the pH of the suspension. When this happens, there exists a pH at which there is a net neutral charge on the surface of the oxide. This pH value is typically called the isoelectric pH (pHIEP) or the point of zero charge. More formally, the isoelectric pH is the pH at which the electrophoretic mobility or zeta potential is zero.5 Information about this surface potential can be obtained by measuring the electrophoretic mobility or zeta potential of these particles when placed in an electric field. Over fifty years of research has been devoted to exploring the electrical double-layer properties of these oxide suspensions.6 In traditional double-layer theory (i.e., the Gouy−Chapman− Stern Model), there are two layers of ions and solvent molecules surrounding the electrode: the compact layer which is the plane of closest approach of the ions to the surface of the electrode (i.e., specifically adsorbed ions) and the diffuse layer which is the ion gradient extending into the bulk of the solution (i.e., nonspecifically adsorbed ions).7 However, the inherent © 2012 American Chemical Society

surface potential of these oxide particles creates a unique electrical double-layer. Colloid chemists have explored this unique double layer noting the anomalously high differential capacitance (i.e., the ratio of the surface charge to the surface potential) of various oxide particles.5,6,8 Surprisingly, these inorganic oxides exhibited differential capacitances greater than 120 μF/cm2 6 in various aqueous media. This is more than twice the values reported in classic double-layer studies involving Hg or AgI.8 These values also greatly exceed the typical double-layer capacitance per unit surface area of carbon electrodes (10−20 μF/cm2) in aqueous electrolytes.9 Increasing the capacitance per unit surface area can occur either by having the ions closer to the surface of the electrode or by increasing the dielectric constant as shown in eq 1 ε εA C= 0 (1) d where A is the surface area of the electrode, d is the separation between the surface of the electrode and the ions in the electrolyte, ε is the dielectric permittivity, and ε0 is the permittivity of free space (permittivity of a vacuum). Several attempts have been made in the literature to explain this high differential capacitance. One of the suggested Received: October 24, 2011 Revised: March 7, 2012 Published: March 19, 2012 6476

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electrochemical capacitor electrodes having higher energy storage.

mechanisms was surface complexation of the charged oxide surface, which results in a close association of counterions to the surface of the oxide.5,6 This led to “triple-layer” models describing the electrochemical interface in three layers: the surface charge on the oxide, the specifically adsorbed ions, and the diffuse layer.6,8 In addition, it has been suggested that there exists a gel or porous layer at the interface which may be penetrable by the supporting electrolyte making the inner Helmholtz plane become embedded in the porous layer of the oxide.8 It has also been proposed that a partial charge transfer may exist between the charged surface of the oxide and an ion separated from the surface by a water molecule in its hydration sphere.8 Finally, it has been suggested that interfacial water at the surface of the oxide is highly structured resulting in a higher dielectric constant in the compact zone compared to the diffuse zone.8 Both mechanistic models and experimental data have shown that the differential capacitance of these oxide particles is a function of surface potential which increases with increasing surface potential.6,10 However, many of the previous measurements of surface capacitance for these oxide materials have been made by potentiometric titration.6,11,12 Very little if any work has been done to evaluate the electrochemical capacitance of these materials when deposited on a supporting electrode. In previous studies, we have deposited oxide particles on carbon electrodes for use in capacitive deionization13 and electrochemical capacitors.14 We have found that adding an oxide coating to porous carbon electrodes can increase the electrochemical capacitance normalized to specific surface area14 and improve ion removal for capacitive deionization.13 We also found that adding a SiO2 coating to high-surface-area carbon electrodes, similar to those used in commercial electrochemical capacitors, can increase the energy density even under high power loads.14 Even though we have had success in using these oxide particles for capacitive deionization and electrochemical capacitors, the role of the surface potential of these oxides is not yet fully understood. In terms of utilizing oxide coatings, electrochemical capacitors and capacitive deionization systems could benefit from research conducted in the related field of lithium ion battery cathodes. Here, significant attention has been devoted to the solid electrolyte interface (SEI) layer and, in particular, to the coating of lithium ion battery cathodes with various oxide materials.15−18 It should be noted that, in addition to double-layer capacitors, a class of capacitors called “pseudocapacitors” have been shown to have very high capacitance because they store charge by electron transfer reactions that produce chemical or oxidation state changes in the electroactive materials.19 These are typically composed of transition metal oxides20−26 including ruthenium dioxide,27,28 manganese oxide,26,29 vanadium oxide,30,31 and nickel oxide.32,33 However, in this study, we are focusing on oxide coatings that do not undergo traditional redox reactions, thus are not transition metal oxides. Here, we attempt to modify the interfacial chemistry at the electrode/ electrolyte interface by adding a surface-charged oxide coating to a capacitor electrode. If one can improve electrochemical capacitance by adding oxide coatings to a supporting electrode, and one can understand which properties of the oxide coating have the greatest effect on the capacitance, one should be able to produce higher efficiency capacitive deionization electrodes and

2. MATERIALS AND METHODS SiO2, AlOOH, TiO2, and ZrO2 nanoparticles were synthesized in suspension using known sol−gel chemistry techniques reported elsewhere.1,3,4 These four oxides were chosen because of their range in pHIEP values and because they are not traditional pseudocapacitors that undergo redox reactions. The SiO2 sol−gel was synthesized by mixing tetraethylorthosilicate (TEOS) with concentrated ammonium hydroxide and Milli-Q water in a volume ratio of 4.5:1:30, respectively. The suspension was stirred on a magnetic stirrer for at least 1 h or until the suspension became clear. The AlOOH sol was created by mixing aluminum tri-sec-butoxide (ATSB), 2-butanol, Milli-Q water, and 1.6 M HNO3 in a volume ratio of 1:1:8.6:2. First, the mixture of water, ATSB, and 2-butanol was mixed for at least 2 h at 85 °C. Then, the HNO3 was added and the suspension was stirred for another 6 h. Finally, the suspension was allowed to reflux for at least 12 h and then filtered through a 0.45 μm filter. The TiO2 sol was created by mixing titania isopropoxide, Milli-Q water, and concentrated HNO3 in a 11.5:140:1 volumetric ratio. The suspension was stirred on a magnetic stirrer for at least 72 h or until the suspension became clear. The resulting sol−gel was dialyzed through a 3500 MWCO membrane until the pH of the suspension reached ca. 2.5. The ZrO2 sol was created by mixing zirconium propoxide, Milli-Q water, and concentrated HNO3 in a 3.7:50:1 volumetric ratio. The suspension was stirred on a magnetic stirrer for at least 24 h or until the suspension became clear. The resulting sol−gel was dialyzed through a 3500 MWCO membrane until the pH of the suspension reached ca. 2.5. In addition to the main study using flat graphite electrodes, we also report data on coating two different high-surface-area carbon materials. The first was an activated carbon powder (YP-17 Kuraray Chemical). The particles were coated onto the powder by tumbling an aqueous suspension with a solids content of 5 wt % SiO2 nanoparticles and 95 wt % powder for 1 h. The nanoparticle/carbon powder was allowed to dry for at least 12 h at 80 °C. Both SiO2 coated and uncoated electrodes were created by mixing the active material with 10 wt % graphite (Acros Chemical), 5 wt % acetylene black (Cabot), and 5 wt % PTFE (Acros Chemical) binder using a mortar and pestle. The electrode mixture was then pressed into a stainless steel mesh screen. The second high-surface-area carbon material was an activated carbon cloth electrode (Hollingsworth & Vose). Nanoparticles were deposited on this substrate using a dip-coating technique whereby the electrodes were withdrawn from a stable suspension of SiO 2 nanoparticles at a controlled velocity of 24 cm/min. For the activated carbon cloth, a conductive carbon agent (HSF54 Y-Shield) was applied to both the uncoated and SiO2-coated electrodes to improve conductivity. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were conducted on the uncoated and SiO2-coated activated carbon powder and activated carbon cloth electrodes using a LEO 1530 FESEM. TEM images were also obtained for the SiO2 nanoparticle suspension using a Phillips CM200 UT. For the electrochemical analysis of the high-surface-area carbons, CR2032 coin cells were used to assemble symmetric electrochemical capacitors. In all cases, the positive and negative electrodes were the same. The electrodes were placed into the CR2032 coin cells and a 1.59 cm (5/8 in.) diameter porous separator (Nippon Kodoshi Corporation) was used as an ion-conducting but electronically insulating membrane to avoid contact between the positive and negative electrodes. The coin cells were assembled with appropriate orings, spacers, and springs and filled with 1 M tetraethylammonium tetrafluoroborate in acetonitrile (TEABF4/ACN) electrolyte inside a glovebox under an argon atmosphere with an oxygen content less than 6477

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Figure 1. Ragone plot showing the energy density as a function of power density using the raw data from the cyclic voltammetry experiments for activated carbon powder (A) and activated carbon cloth (C). Also shown is the energy increase with the addition of the SiO2 nanoparticles as a function of power density in both absolute [Wh/kg] and relative [%] terms for activated carbon powder (B) and activated carbon cloth (D). 1 ppm and moisture content less than 0.5 ppm. Each coin cell was crimped with a Hohsen CR2032 coin cell crimper. The electrochemical impedance spectroscopy (EIS) tests were conducted on the coin cells using a Princeton Applied Research VMP2 potentiostat with no (0 V) DC bias and applying a 20 mV sine wave at frequencies varying from 100 kHz to 10 mHz. For the flat graphite electrode tests, each of the four nanoparticles was deposited as a porous film on flat conductive graphite support electrodes by a dip-coating technique whereby the electrodes were withdrawn from a stable suspension at a controlled velocity of 24 cm/ min. Graphite was chosen as a supporting electrode instead of a more typical electrochemical capacitor material like activated carbon so as to eliminate any effects of the supporting electrode’s surface area and pore size distribution. The electrochemical capacitance of each coated electrode and an uncoated electrode was determined using a three-electrode technique with a Pt cage counter electrode and a saturated calomel reference electrode in solutions of 0.1 M Na2SO4. These measurements were performed over a pH range from pH 2 to pH 10 to determine the effect of changing surface potential on electrochemical capacitance of these electrodes. A SolRayo ETS-2003 potentiostat using cyclic voltammetry at scan rates between 5 and 200 mV/s was used to characterize the electrochemical performance of these electrodes. The cyclic voltammetry experiments were conducted between voltage of −0.5 and 0.5 V with respect to the saturated calomel reference electrode. The instantaneous capacitance of the cell was determined from the current measured and the scan rate of the cyclic voltammetry experiments as shown in eq 2 C=

The percent improvement in capacitance between each of the coated electrodes versus the uncoated electrode was measured for each coating at each pH. The percent loading of each of the nanoparticles was determined by thermogravimetric analysis (Mettler Toledo TGA/SDTA 851). While measuring the sample mass, the samples were heated to 1050 °C to burn off any carbon-based material. The weight differential between the coated and uncoated samples was used to determine the percent loading of the nanoparticles. Electrochemical capacitance measurements were also performed on an uncoated and SiO2-coated activated carbon cloth (Hollingsworth and Vose) in aqueous electrolytes. The same coating techniques and electrochemical analysis were performed as was done with the graphite electrodes.

3. RESULTS AND DISCUSSION 3.1. Coatings on High-Surface-Area Carbons. In previous research, our group reported the increase in electrochemical capacitance of high-surface-area carbon materials by the addition of SiO2 nanoparticles in organic electrolytes.14 Specifically, we have shown our ability to increase the energy density of electrochemical capacitors (ECs) at power densities above 500 W/kg,14 which is the critical power density where the performance of ECs begins to exceed that of Li-Ion batteries.34,35 Figure 1 shows example Ragone plots comparing uncoated high-surface-area carbons to carbons coated with SiO2 nanoparticles. Figure 1A is a Ragone plot of the uncoated and SiO2-coated activated carbon powder, a material very similar to active material used in commercial electrochemical capacitors. Figure 1B shows the energy increase as a function of power density when the SiO2 coating is added in both absolute terms (Wh/ kg) and as a percent increase. As Figure 1A and B

I dV dt

( )

(2)

where C is the capacitance in Farads, I is the current, and dV/dt is the cyclic voltammetry scan rate. The total cell capacitance was determined by averaging the instantaneous capacitance measurements. 6478

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Figure 2. SEM images and EDS spectra of the activated carbon cloth. Uncoated activated carbon cloth (A and B) and SiO2-coated activated carbon cloth (C and D).

present in addition to the carbon substrate (Figure S1C). However, the individual SiO2 particles are too small to be visible at this magnification; Figure S1D shows the porous nature of the coated substrate. Figure 2 shows that the activated carbon cloth is composed of long porous fibers that are approximately 5−20 μm in diameter and several hundred micrometers long. The EDS spectrum in Figure 2A also shows that the uncoated activated carbon cloth is composed of only carbon and its porosity can be seen in Figure 2B. For the SiO2-coated activated carbon cloth, we see that the coating covers the carbon fibers and in some cases extends beyond the fiber. These observations are supported by the EDS spectrum shown in Figure 2C. The spectrum that is over the carbon fiber shows both Si and O present with the carbon. However, the EDS spectrum of the material next to the fiber shows a much higher proportion of Si compared to carbon. In addition, the porous morphology of the SiO2 coating can be seen in Figure 2D. Figure S2 shows a TEM image of the raw SiO2 nanoparticles that were synthesized by sol−gel chemistry techniques. Here, we see that the primary particle diameter of the SiO2 nanoparticles is approximately 3−5 nm. 3.1.2. Electrochemical Impedance Spectroscopy Modeling of SiO2-Coated Carbon Electrodes. Since SiO2 is electrically insulating, it is interesting that we are able to increase the energy density under high power loads. To further investigate the frequency response of these coatings, we characterized both the coated and uncoated electrodes with EIS. Nyquist plots of the EIS data for the uncoated and SiO2-coated activated carbon powder and activated carbon cloth are shown in Figure 3 and show analogous behavior to similar carbons reported in the literature.36 We modeled the EIS data using a traditional circuit model for electrochemical capacitors37 (Figure 3A) and the parameters for the circuit elements are shown in Table 1. Several items should be noted when comparing the values for the two coated to the two uncoated samples. First and

demonstrates, the SiO2 coating provides significantly increased energy density when the power exceeds 500 W/kg. At 500 W/ kg, the SiO2 coating provides an increase in energy density of 10% over the uncoated electrodes. This improvement in energy density reaches 20% by 800 W/kg, 30% by 1000 W/kg, and over 50% at a power density of 1400 W/kg. In addition to the activated carbon powder, we also show Ragone plots of both an uncoated and SiO2-coated activated carbon cloth (Figure 1C,D). We see that the SiO2 coating has the same effect on the activated carbon cloth as it does on the activated carbon powder. By adding the SiO2 coating, we can significantly increase the energy density at power densities exceeding 500 W/kg. In fact, at 500 W/kg the SiO2 coating provides a 35% improvement in energy density and increases even further with increasing power density. Due to the fact that SiO2 is considered electrically insulating, one would not normally expect SiO2-based devices to be capable of storing and delivering electrical energy, or that the addition of SiO2 to a carbon substrate would increase the energy density, particularly at high power. Also, unlike pseudocapacitors, which employ transition metal oxides that undergo faradaic reactions, Si is not a transition metal and neither direct oxidation nor reduction occurs with SiO2. The goal of this manuscript is to try to explain how the surface potential of the SiO2 coating is contributing to the increase in electrochemical capacitance. 3.1.1. Mircoscopy of Coatings. To understand the morphology of these coatings, SEM and EDS were conducted on the uncoated and SiO2-coated activated carbon powder (Figure S1) and activated carbon cloth (Figure 2) electrodes. The activated carbon powder is composed of porous particles that are several micrometers across. The EDS spectrum in Figure S1A shows that only carbon is present in the uncoated activated carbon powder and the porous structure of this material can be seen in Figure S1B. The EDS spectrum for the SiO2-coated activated carbon powder shows silicon and oxygen 6479

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tor.42−44 In addition, the constant phase element has been attributed to the capacitance of the geometric surface area of the electrode. Due to its low exponential value, this element is not proportional to the electrochemical capacitance.37 However, it is interesting that the SiO2 coating does not increase this resistance in either sample despite its being considered an electronic insulator. Finally, REL corresponds to the bulk resistance associated with the system.37 Here, we have demonstrated that a non-pseudocapacitor oxide coating can increase the capacitance of a carbon electrode in an organic electrolylte. However, our goal is to investigate how the surface potential of the oxide coating contributes to electrochemical capacitance. Because of the complexity of the high-surface-area carbon materials’ morphologies, we chose to investigate these coatings on flat graphite electrodes to eliminate all surface area effects. Also, since the surface potential of the oxide is pH-dependent in aqueous media we chose to investigate the graphite electrodes in aqueous electrolytes. Finally, we investigated three additional oxide materials (TiO2, ZrO2, and AlOOH) in addition to the SiO2 used on the high-surface-area materials. 3.2. Investigating Coatings on Flat Graphite Electrodes. Figure 4 shows the cyclic voltammograms for the

Figure 3. Electrochemical impedance spectroscopy data along with an equivalent circuit model. Equivalent circuit model (A), uncoated and SiO2-coated activated carbon powder (B), uncoated and SiO2-coated activated carbon cloth (C).

foremost, the SiO2-coated samples produce higher C1 values as compared to their respective uncoated counterparts. The capacitance of C1 represents the electrochemical capacitance of the electrodes,37 and the larger values for the SiO2-coated samples correspond to data reported previously.14 Second, RLeak corresponds to the small leakage current at the electrode/ electrolyte interface37 and large values for both the SiO2-coated and uncoated samples relate to the small leakage current for these cells. Third, the Warburg element, ZW, models the diffusion of ions at high frequency into the active material.38,39 For both electrode materials, the SiO2 coating results in a smaller Warburg coefficient, which could suggest that the coating provides less diffusion limitations for the ions and/or is decreasing the ionic charge-transfer resistance.40,41 Fourth, the semicircle region that corresponds to the parallel connection of R1 and ZCPE has several reported meanings in the literature. This connection could be due to the electrical charge transfer in the electrode material (possibly between electrode particles), resistance across the separator due to ion depletion, or passivation layer between the electrode and current collec-

Figure 4. Cyclic voltammograms for an uncoated graphite electrode and graphite electrodes coated with SiO2, AlOOH, TiO2, and ZrO2 in 0.01 M Na2SO4 at pH 4 with a scan rate of 200 mV/s.

uncoated graphite electrode and the graphite electrodes coated with SiO2, AlOOH, TiO2, and ZrO2 at pH 4 with a scan rate of 200 mV/s. From eq 2, the area of the cyclic voltammograms is directly proportional to the electrochemical capacitance. Thus, it is visually apparent from Figure 4 that the electrochemical capacitance of each of the coated electrodes is higher than that of the uncoated electrode. Using eq 2 to calculate the capacitance at 200 mV/s, we find the geometric area capacitance of the uncoated electrode to be 1.1 mF/cm2. The capacitance for each of the coated electrodes was determined to

Table 1. Best-Fit Values for the Equivalent Circuit Model for the Uncoated and SiO2-Coated Activated Carbon Cloth and Activated Carbon Powder REL (Ω) Uncoated Powder SiO2 Coated Powder Uncoated Cloth SiO2 Coated Cloth

2.471 2.00 2.05 0.44

R1 (Ω) 6.04 3.137 2.18 1.794

AW (Ω s−1/2) 6.74 5.19 90.6 65.94 6480

QCPE (F sn‑1) 2.85 1.83 2.63 10.1

× × × ×

−3

10 10−3 10−3 10−3

n

C1 (F)

RLeak (Ω)

0.48 0.55 0.30 0.31

0.779 1.11 0.041 0.095

108 7 × 1012 6300 10500

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be 4.0 mF/cm2, 3.1 mF/cm2, 4.0 mF/cm2, and 3.6 mF/cm2, respectively, for the SiO2, AlOOH, TiO2, and ZrO2 electrodes. The cyclic voltammetry experiments demonstrate that the electrochemical capacitance of each of these coated electrodes is significantly higher than that of the uncoated electrode. We would like to determine if the surface potential of these oxide materials is contributing to the increased electrochemical capacitance. For this, we report the percent improvement of the coated samples over the uncoated sample (measured as the difference in capacitance between the coated and uncoated samples, divided by the capacitance of the coated sample multiplied by 100) as a function of pH for the four different oxide coatings tested. To determine if there is a correlation between surface potential and electrochemical capacitance, we compare the electrochemical capacitance as a function of pH to the reported electrophoretic mobility values as a function of pH for SiO2,2 AlOOH,1 TiO2,4 and ZrO2.4 A correlation between the percent improvement in capacitance and the electrophoretic mobility would suggest that surface potential of these oxides contributes to this increase in electrochemical capacitance. The results of these experiments are shown in Figures 5−8.

Figure 6. Percent improvement in electrochemical capacitance as measured by cyclic voltammetry between the AlOOH-coated and uncoated graphite electrodes as a function of pH. The error bars reported are the standard deviation in percent improvement in capacitance as measured at 6 different scan rates ranging from 5 to 200 mV/s. Also shown are the reported values of electrophoretic mobility as a function of pH.1

improvement in capacitance at pH 10, which is near the isoelectric point of AlOOH. As the pH is decreased to either 8 or 6, we see an increase in the improvement in capacitance. This corresponds to an increase in the electrophoretic mobility in the positive direction as pH decreases. However, we also notice a dip in performance once the pH reaches 4. As opposed to SiO2 , AlOOH becomes more soluble under acidic conditions.46,47 For the TiO2 (Figure 7) and ZrO2 (Figure 8), we also see a direct correspondence between the improvement in capacitance and the electrophoretic mobility. TiO2 has an isoelectric point around pH 5, and ZrO2 has an isoelectric point around pH 6. At these pH values, we observed a dramatic decrease in the percent improvement for the TiO2-coated and ZrO2-coated Figure 5. Percent improvement in electrochemical capacitance as measured by cyclic voltammetry between the SiO2-coated and uncoated graphite electrodes as a function of pH. The error bars reported are the standard deviation in percent improvement in capacitance as measured at 6 different scan rates ranging from 5 to 200 mV/s. Also shown are the reported values of electrophoretic mobility as a function of pH.2

Figure 5 shows the percent improvement in capacitance as a function of pH as well as the electrophoretic mobility for SiO2. SiO2 has an isoelectric pH around 2, with electrophoretic mobility becoming more negative as pH increases. From Figure 5, we see that at pH 2 the SiO2-coated electrode has the lowest percent improvement in electrochemical capacitance. At pH 4 and pH 6, the percent improvement in capacitance is significantly higher as electrophoretic mobility increases in the negative direction. It should be noted that after pH 8 there is a dip in percent improvement in capacitance. However, SiO2 becomes more soluble under basic conditions,45 and this decrease may be attributed to the increase in solubility of SiO2 at high pH. A similar but reversed behavior is found for the electrochemical capacitance as a function of pH for AlOOH (Figure 6). Here, we observed the lowest value for the percent

Figure 7. Percent improvement in electrochemical capacitance as measured by cyclic voltammetry between the TiO2-coated and uncoated graphite electrodes as a function of pH. The error bars reported are the standard deviation in percent improvement in capacitance as measured at 6 different scan rates ranging from 5 to 200 mV/s. Also shown are the reported values of electrophoretic mobility as a function of pH.4 6481

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curves of the oxide materials. There is a sharp change in the electrophoretic mobility about 1 pH unit around the isoelectric point. However, as pH moves farther away from the isoelectric point, electrophoretic mobility becomes more constant. Figure 9 also shows that once the pH is more than one unit away from pHIEP there is very little difference between the percent improvements for the four oxide coatings. The results from these four oxide coatings suggest that the surface potential of oxide coatings can contribute to electrochemical capacitance when these oxides are coated on supporting electrodes. However, we also want to determine if any correlation exists between the dielectric constant of the materials and the improvement in capacitance. Table 2 shows Table 2. Bulk Dielectric Constant and the Maximum Measured Capacitance at 5 mV/s for TiO2, ZrO2, AlOOH, and SiO2 Coated Graphite Electrodes

Figure 8. Percent improvement in electrochemical capacitance as measured by cyclic voltammetry between the ZrO2-coated and uncoated graphite electrodes as a function of pH. The error bars reported are the standard deviation in percent improvement in capacitance as measured at 6 different scan rates ranging from 5 to 200 mV/s. Also shown are the reported values of electrophoretic mobility as a function of pH.4

TiO2 ZrO2 AlOOH SiO2

electrodes. For both coatings, the percent improvement away from the isoelectric point was similar whether or not the electrophoretic mobility for these materials was positive or negative. To determine if there are similarities between the oxide coatings, we examined the percent improvement as a function of absolute value of the reduced variable |pH − pHIEP| for each of the four oxide coatings (Figure 9). Two interesting trends

dielectric

max capacitance (mF/cm2)

86−170 12.5 6−11.5 4.4−4.6

23.87 17.83 30.07 17.52

the maximum capacitance value on a per gram basis of the coating compared to the bulk dielectric constant. As Table 2 demonstrates, there is no direct comparison between the dielectric constant and the capacitance of the oxide coatings. This strongly suggests that the surface potential is the main contributor to the electrochemical capacitance just as it is with the differential capacitance of the oxide materials. As stated above, we have performed studies using these types of coatings on electrochemical capacitor electrodes. However, we wanted to determine if this pH effect is also observed when using a high-surface-area carbon. For this, we used an activated carbon cloth (Hollingsworth and Vose) having a BET surface area of 1466 m2/g and a modal pore width of 0.71 nm. We measured the improvement in specific capacitance normalized to BET surface area of SiO2-coated and uncoated activated carbon cloth electrodes in an aqueous electrolyte at pH values of 2, 4, and 6 before the solubility of SiO2 becomes an issue (Figure 10). We normalized the capacitance to BET surface area to remove any effects of the coating on surface area, and it should be noted that the BET surface area decreased to 1102 m2/g after the SiO2 was added. Also included in Figure 10 is the percent improvement of the SiO2-coated graphite electrode at pH values of 2, 4, and 6 on a second y-axis. Even though the absolute values of the percent improvement were different for the activated carbon and graphite electrodes, we observed a nearly identical trend for the two electrodes as we changed the pH from 2 to 6. At pH 2, the electrophoretic mobility of the SiO2 is low, and at this pH, we observed our lowest improvement in specific surface area capacitance of 16%. However, as we increased pH to 4 or 6, the electrophoretic mobility of the SiO2 nanoparticles became more negative indicating a more negative surface potential on these nanoparticles. When we forced the surface potential to become negative, we saw an increase in the improvement of specific capacitance normalized to BET surface area to 36%. The fact that we were able to change the percent improvement in capacitance through alterations in solution pH strongly suggests that the surface potential of the SiO2 affects capacitance.

Figure 9. Percent improvement as a function of |pH − pHIEP| for SiO2, AlOOH, TiO2, and ZrO2 coated graphite electrodes. Also shown is a trend line displaying the sharp increase in electrochemical capacitance within one pH unit of the isoelectric point and a constant improvement farther away from the isoelectric point.

are apparent for all four of the oxide coatings. First, increasing from pHIEP to one pH unit away from pHIEP, there exists a strong increase in the percent improvement. The first segment of the trend line in Figure 9 illustrates this phenomenon. However, when |pH − pHIEP| becomes larger than 1, the percent improvement is relatively constant. The second segment of the trend line in Figure 9 shows this effect. These two observations correspond to the electrophoretic mobility 6482

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coatings, which provide higher electrochemical capacitance, can be utilized to create higher efficiency capacitive deionization electrodes and higher energy storage electrochemical capacitors.

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ASSOCIATED CONTENT

S Supporting Information *

Additional SEM and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. Notes

The authors declare no competing financial interest.

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Figure 10. Percent improvement in electrochemical capacitance as measured by cyclic voltammetry between the SiO2-coated and uncoated activated carbon cloth as a function of pH along with the percent improvement in electrochemical capacitance of the SiO2 coated graphite electrode on a second y-axis.

ACKNOWLEDGMENTS We thank SolRayo, Inc. and Enable IPC Corp. for their financial support; Dr. M. I. Tejedor-Anderson of UW-Madison for numerous insightful discussions, assistance, and guidance; Dr. W. Zeltner of UW-Madison for helpful assistance; Dr. M. Daugherty of SolRayo, Inc. for valuable comments; and D. Walker of Enable IPC for numerous discussions and support.

We have previously reported using these coatings on highsurface-area carbons with organic electrolytes.14 To determine if the improvement in capacitance as a function of surface potential also translates to these organic electrolytes, we measured the electrophoretic mobility of the SiO2 nanoparticles suspended in acetonitrile. It should be noted that others have also reported that it is possible to measure the ζ-potential of similar materials in acetonitrile.48 At pH 6 in water, the electrophoretic mobility of the SiO2 nanoparticles was −1.6 mobility units (μm cm V−1 s−1), and the percent improvement in capacitance normalized to specific surface area for carbon cloth was 36%. We determined the electrophoretic mobility of the SiO2 nanoparticles suspended in acetonitrile to be −2.8 mobility units. As reported earlier,14 the capacitance normalized to the specific surface area of the uncoated activated carbon cloth was 10.6 μF/cm2. This increased to 14.7 μF/cm2 when the SiO2 coating was added and results in a percent improvement in capacitance normalized to specific surface area of 39%. This suggests that, since the electrophoretic mobility of the SiO2 nanoparticles in the organic electrolyte is in the range of the electrophoretic mobility in the aqueous media, and the percent increase in capacitance is similar in both cases, the surface potential of SiO2 plays a contributing role in the effect of these coatings for both aqueous and organic electrolytes.

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REFERENCES

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4. CONCLUSION We have shown that by adding a thin-film coating of insulating or nonpseudocapacitor oxides to a supporting electrode one can increase the electrochemical capacitance of the electrode. We have also shown that this increase in electrochemical capacitance is related to the surface potential of the oxides, but not to the bulk dielectric constant of the material. The main portion of these experiments was performed on a flat graphite electrode to determine the fundamental behavior of the oxide particles, but we have also demonstrated that these nanoparticle coatings can be used on higher-surface-area materials. Finally, we have demonstrated that these oxide particles can exhibit a surface potential in organic electrolytes, which means they could serve as coatings for electrochemical capacitors in organic electrolytes as well. We believe that these surface-charged 6483

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