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The effect of physical adsorption on the capacitance of activated carbon electrodes
Carbon 150 (2019) 334e339
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Carbon journal homepage: www.elsevier.com/locate/carbon
The effect of physical adsorption on the capacitance of activated carbon electrodes Nathan L. Tolman a, Jason M. Mukai a, Shuqing Wang b, *, Alessandra Zito a, 1, Tianyi Luo a, Haitao Liu a, ** a b
University of Pittsburgh, Department of Chemistry, Pittsburgh, PA, 15260, USA SINOPEC Research Institute of Petroleum Processing, Beijing, 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2019 Received in revised form 1 May 2019 Accepted 3 May 2019 Available online 8 May 2019
This work reports the effect of physisorption of organic compounds on the double layer capacitance of activated carbon electrodes. Exposure of activated carbon electrodes to toluene or chloroform vapor for less than 10 minutes resulted in a capacitance loss of 77% and 84%, respectively. Even adsorbates, such as acetone and ethanol, miscible with the aqueous Li2SO4 electrolyte caused 20e30% losses in capacitance. It was also found that there was an adsorbate size dependence: above a certain threshold, a larger adsorbate could have more than twice the impact on capacitance than a slightly smaller one. The results were consistent with the hypothesis that volatile organic contaminants (VOC) block access of the aqueous electrolyte to the carbon electrode surface. Porous activated carbon is currently the material of choice for supercapacitor electrodes in both research laboratories and commercial supercapacitor applications. These facilities also often house and use many volatile organic solvents either for research and development or simply for cleaning purposes. Because of this, our work has significant implications to the research and development of carbon-based supercapacitors. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Contamination of carbon materials and its effect on their surface properties has received a lot of attention recently. It has been recently discovered that graphitic carbons, such as graphene and highly-ordered pyrolytic graphite (HOPG), are more hydrophilic than previously thought due to the masking of surface properties caused by adsorption of airborne hydrocarbon contamination from the ambient environment. For example, Li et al. found that the water contact angle (WCA) of freshly prepared, copper-supported, chemical vapor deposition (CVD)-grown graphene was 44 . This value increased to 60 within 20 minutes of air exposure and finally to 80 within one day [1]. Similarly, Kozbial et al. found for HOPG that the WCA was ~64 within 10 s of exfoliation followed by a steady increase over time to 90 [2]. Both studies attributed this rise in hydrophobicity to hydrocarbon contamination from ambient air.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (H. Liu). 1 Present Address: University of California Irvine, Department of Chemistry, Irvine, California, 92697, USA. https://doi.org/10.1016/j.carbon.2019.05.005 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
The capacitance changes of HOPG as a function of air aging have been studied by Zou et al. They showed a capacitance decrease from 6.0 mF cm2 to 4.3 mF cm2 between freshly exfoliated HOPG and 24-h air-aged HOPG respectively. Using X-ray photoelectron spectroscopy, they showed that this was most likely the result of the presence of a 75% monolayer of adventitious carbon adsorbed on the electrode basal plane. They also showed that the capacitance of the electrode could be partially restored by heating in a vacuum to desorb the contaminants [3]. Expanding on Zou's work, Hurst et al. showed that contamination of the HOPG electrode surface can also occur in the electrolyte solution [4]. For a freshly exfoliated HOPG electrode immersed in ~1 M NaCl (>99.0% purity salt), the capacitance degraded by 33% over a 24-h period. Most of the capacitance loss occurred within the first 250 minutes however. Using spectroscopic ellipsometry, they calculated the contamination thickness to be approximately 0.7 nm which corresponds to a monolayer of contamination. Activated carbon is another graphitic material whose capacitance is particularly sensitive to surface properties. Unlike graphene and graphite, activated carbon is a much more complex material, having ordered graphitic domains of winding short basal plains as well as disordered sp2 cluster domains often found in
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amorphous carbon [5]. The material is further complicated by its porous nature, which can include networks of macropores (>50 nm), mesopores (2e50 nm), or micropores (<2 nm) or combinations of all three. The porosity of activated carbon gives rise to its high specific surface area (SSA) which is typically found to be between 500 and 3000 m2/g [6]. High SSA, good conductivity, processability, and low cost make activated carbon currently the most commonly used electrode material for supercapacitors, or electric double layer capacitors (EDLCs) [7]. The effect of hydrocarbon contamination and physical adsorption on the capacitance of activated carbon electrodes would be of particular concern to both the scientific and industrial communities alike because both have an interest in maximizing the efficiency of energy storage devices. Wang et al. have studied the effect of water contamination of organic electrolyte solutions on the performance of an activated carbon/graphite supercapacitor [8]. Their work primarily focused on the effect contamination had on the degradation of the negative electrode at the high potential windows accessible to organic electrolytes. A cause of capacitance decay, he suggested, was that the degradation products of the carbon electrodes blocked electrolyte ions from pores resulting in losses in capacitance [8]. Duignan and Zhao through quantum mechanical calculations showed that the lower than expected capacitance of HOPG was due to a dielectric dead layer of adsorbed hydrocarbons on the electrode surface and speculated that activated carbon electrodes could suffer the same capacitance losses [9]. Fic studied the effect of contaminant surfactant molecules on activated carbon capacitance, however, and found in some cases a significant increase in capacitance, purportedly due to increased wettability of the electrode and ion-channeling effects, while in other cases he found modest losses of capacitance [10]. In this study, we aim to show that physical adsorption of contaminants either from air or electrolyte solvents in an industrial or laboratory environment can also reduce the capacitance of activated carbon electrodes by similar means. While capacitance values for porous electrodes can greatly vary depending on many factors, they typically are as low as ~30 F/g and as high as ~150 F/g [11]. In some exceptional cases the capacitances can be as high as 300 F/g [12]. Unknown and unaccounted for contaminant adsorption could reduce a high performance material's capacitance to that of a low performing one. This issue is of significant importance to commercial and research interests alike for the maximization of energy storage. 2. Experimental 2.1. Activated carbon fiber cloth (ACFC) Flexzorb knitted activated carbon fiber cloth (Chemviron Cloth Division of Calgon Carbon, Inc.) was used as the electrode material for all experiments. The cloth electrodes were first cut with a 1 cm diameter hole punch. They were then placed in a quartz tube with stainless steel fittings on a quartz boat in a Thermo Scientific Lindberg Blue M furnace. The tube was pumped down to 70 mtorr pressure before H2 gas was flown through the tube at 2 standard cubic centimeters per minute. The tube was then heated to 1050 C for 1 h followed by cooling to room temperature under H2 gas. Cloth was then stored in a glass vacuum desiccator with a bed of anhydrous calcium chloride. Activated carbon Brunauer-Emmett-Teller (BET) surface area was calculated from nitrogen adsorption isotherms (Quantachrome Instruments Autosorb-1). The sample was subjected to outgassing under dynamic vacuum at 120 C until the pressure change rate was less than 2 mmHg/min (about 3.5e5 h) followed by nitrogen
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adsorption at 77 K. Isotherms were recorded in 58 steps at a relative pressure range from 0.00001 to 0.995. Total pH was measured to be 8.28 by addition of 0.1125 g of ACFC to 50.0 mL of 0.01 M NaNO3. The solution was gently stirred while sparged with N2 gas. After 24 h, the pH was measured with a pH meter (Mettler Toledo). X-ray photoelectron spectroscopy was carried out using a Thermo Fisher ESCALAB 250 Xi X-ray photoelectron spectrometer (Al Ka monochromatic X-rays, 650 mm spot size coupled with charge compensation, Thermo Avantage software). Spectra and activated carbon surface functional group identification can be found in the supporting information. Thermogravimetric analysis (TGA) and thermogravimetric analysis/mass spectrometry (TGA-MS) were carried out on a Netzsch STA 409CD-QMS Skimmer instrument using a heating rate of 10 K/min under He. 2.2. Ambient air vs. inert air aging Electrodes were subjected to ambient laboratory air or N2 atmosphere inside a glovebox (<15 ppm O2, <0.1 ppm H2O) as a function of time by first recording their mass and then placing them on labeled boats in the the environment. No attempt was made to protect them from particulate matter or light. 2.3. Contaminant adsorption For vapor phase contamination, electrodes were exposed to adsorbate vapor by placing them on a 5 cm watch glass with an adsorbate reservoir also on a 5 cm watch glass under an overturned already vapor-saturated 14 2 cm Pyrex dish as a function of time. Vapor adsorbates included acetone (99.5%, ACS Reagent, Sigma Aldrich), chloroform (99.8%, ACS Reagent, Sigma-Aldrich), diethyl ether (anhydrous, 99.9%, Fisher Chemical), ethanol (anhydrous, histological grade, 0.5% water maximum, Fisher Chemical), and toluene (99.5%, ACS Reagent, Sigma-Aldrich). For solution phase adsorption, cloth electrodes were added to a vial containing 5 mL diethyl ether (anhydrous, 99.9%, Fisher Chemical) and known amounts of adsorbate. They were then stirred gently for 30 min at 320 rpm followed by 40 min of air drying of ether on a bed of CaCl2 in a 150 75 mm crystalizing dish covered with aluminum foil punctured with small holes to provide a dry, covered but ventilated environment. Adsorbates included 1-decene (97.0%, Sigma-Aldrich), 1-dodecene (95%, Fisher Chemical), 1eicosene (99.2%, MP Biomedicals), 1-hexadecene (98.5%, Sigma-Aldrich), 1-octadecene (99.5%, Sigma-Aldrich), 1tetradecene (97.0%, GC Grade, Sigma-Aldrich), anthracene (99.0%, Fluka Analytical), coronene (97%, Sigma-Aldrich), naphthalene (reagent grade, Fisher Chemical), and pyrene (98%, SigmaAldrich). 2.4. Cyclic voltammetry A Gamry Reference 600 potentiostat was used to collect cyclic voltammetry data using a 3-electrode custom-made Swagelok-type symmetrical coin cell setup modelled after Amatucci et al. [13] Perfluoroalkoxy (PFA plastic) tee ¾” compression fittings and Gardner Compression Springs (302 stainless steel with closed flat ends, 0.06 .045 1.500”) were purchased from McMaster-Carr Inc. Plungers and coin-shaped current collectors (both 316-grade stainless steel) were custom made to fit the PFA fitting inner diameter by the University of Pittsburgh Dietrich School of Arts and Sciences machine shop. Glass microfiber filter papers (Whatman 934-Alt™ RTU, 90 mm) were used as porous separators. The reference electrode was a Ag/AgCl reference electrode (Accumet
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13-620-53) and was inserted into the electrolyte reservoir through the top hole of the fitting tee. Cyclic voltammograms were carried out at 1 or 2 mV/s from 0 V to 0.9 V in 1 M aqueous Li2SO4 (monohydrate, 99þ%, Acros Organics). The potentiostat preconditioned the cell at 0 V for 60 s before each voltammogram. Data analysis was carried out using Gamry's Echem Analyst software. Capacitance was calculated from 0 V to 0.9 V using the following equation:
1 C¼ 2wðDVÞ
t2 ð
iðtÞ dt ; dt ¼ t1
dV v
where t is time, i is current, V is potential, w is electrode mass, and n is scan rate. 3. Results and discussion 3.1. Characterization of the activated carbon material We used Flexzorb knitted activated carbon fiber cloth in this study. The cloth as received showed redox peaks in the cyclic voltammograms for some areas of the cloth but not others. Annealing the cloth in H2 (1050 C for 1 h, 70 mtorr) before use corrected this problem. All electrodes were therefore annealed before use and then stored in a vacuum desiccator. 3.1.1. TGA-MS ̄ By TGA-MS analyses, the as received sample showed ca. 10% weight loss upon heating to 995 C, compared to ca. 5% weight loss for the annealed sample under the same condition (Fig. 1A). The TGA-MS data of the as recevied sample shows a noticeable peak for þ þ m/z ¼ 17 (NHþ 3 ) and 18 (H2O ) at around 130 C. For m/z ¼ 28 (CO ), there was a significant increase above 600 C in the ion current. For m/z ¼ 44 (COþ 2 ), a broad plateau was observed between ca. 140 and
750 C. For the annealed samples, the m/z ¼ 17 and m/z ¼ 44 ion current curves become featureless. The m/z ¼ 18 and m/z ¼ 28 traces still resemble those of the as-received sample (Fig. 1B and C). 3.1.2. Specific surface area (SSA) and pore size distribution (PSD) The SSA of the cloth electrodes was calculated from nitrogen adsorption isotherms at 77K to be 1198 ± 33 m2/g. Total pore volume was calculated to be 0.545 ± 0.012 cm3/g. Quenched Solid Density Functional Theory (QSDFT) assuming slit-shaped pores showed that most pore diameters ranged from 0.6 to 1.5 nm (Fig. 2). 3.2. Volatile organics contamination of actived carbon electrodes We first show that intentional exposure to VOC vapor decreases capacitance of activated carbon materials. For these experiments, we thermally annealed activated carbon fiber electrodes and then exposed them to saturated VOC vapor environments for set periods of time before measurement of capacitance in 1 M aqueous Li2SO4. Fig. 3A shows that 10 minutes of saturated VOC vapor resulted in capacitance losses of 24, 28, 43, 77, and 84% for ethanol, acetone, diethyl ether, toluene, and chloroform respectively (top to bottom in Fig. 3A). Fig. 3B shows the effect physical adsorption of toluene had on the cyclic voltammogram (1 M aqueous Li2SO4, 1 mV/s). The blue line is from a sample not exposed to toluene (clean) while the green and red lines represent 2 and 5 minutes of toluene vapor exposure respectively. We have conducted TGA-MS on the solvent vapor saturated activated carbon electrodes. The desorption of the adsorbed molecules were detected by monitoring the molecular ion or its major fragments (e.g., CH3CH2OHþ for ethanol, (CH3CH2)2Oþ for diethyl þ ether, CH3COþ for acetone, C7Hþ 8 for toluene, and CCl2H for chloroform). A single desorption peak was observed in all cases except ether, in which case a closely-spaced two-peak pattern was observed (Figure S1A). The peak desorption temperature increases in the order of ethanol (95 C), acetone (111 C), ether (118 C and
Fig. 1. (A) TGA and TGA-MS analysis of (B) annealed and (C) as received activated carbon samples. (A colour version of this figure can be viewed online.)
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Fig. 2. Sample QSDFT pore size distributions calculated from N2 adsorption isotherms at 77 K and assuming slit-shaped pores of Chemviron Carbon's Flexzorb 100 activated carbon fiber cloth. Figures are from two separately taken adsorption isotherms. (A colour version of this figure can be viewed online.)
Fig. 3. (A) The effect of small volatile organic contaminant vapor adsorption on the double layer capacitance of activated carbon fiber electrodes. Voltammograms were taken a 2 mV/s in 1 M Li2SO4 against a Ag/AgCl reference electrode. (B) The effect of physical adsorption of toluene vapor on cyclic voltammogram shape as a function of time. Cyclic voltammograms done at 1 mV/s in aqueous 1 M Li2SO4 for activated carbon fiber electrodes not subjected to toluene (blue) and electrodes subjected to 2 (green) and 5 (red) minutes of saturated toluene vapor are shown. (A colour version of this figure can be viewed online.)
140 C), chloroform (124 C), and toluene (146 C). 3.3. Contaminant size effects To probe the effect of adsorbate size on electrode capacitance, polycyclic aromatic hydrocarbons (PAHs) of increasing size were adsorbed to activated carbon electrodes from 2.5 mM solutions in diethyl ether during a 24-h period. After the adsorption, the carbon materials were then dried on a bed of CaCl2 in a 150 75 mm crystalizing dish covered with Al foil and ventilated with small holes for another 24 h before assembled into a coin cell for capacitance measurement. Upon adsorption, the mass of the carbon materials increased and their capacitance decreased. The capacitance of the electrodes and the percent mass increase are shown in Fig. 4A. Percent mass increase is an indicator of how much adsorbate stayed adsorbed to the cloth after the drying period. It should be noted that not all of the ether desorbs in the drying
Fig. 4. (A) The capacitance measurements (green) after physical adsorption of PAHs of differing sizes and percent mass increase (blue) of activated carbon electrodes are shown. Adsorbates were adsorbed over 24 h from gently stirred 2.5 mM adsorbate/ diethyl ether solutions and then dried for 24 h at atmospheric pressure. (B) The effect of 1-alkene adsorption from diethyl ether on the capacitance of activated carbon electrodes. Electrodes were exposed to 10 mL of 5 mM adsorbate dissolved in diethyl ether, stirred, and dried in a dry, covered but ventilated environment. (A colour version of this figure can be viewed online.)
process, as can be seen by comparing control versus clean in Fig. 4A. Clean refers to an electrode that has not been subjected to anything other than the annealing procedure that all electrodes were subjected to and storage in a vacuumed desiccator while the control has been stirred in pure ether and dried. The capacitance of the control was higher than that of ether vapor adsorbed cloth shown in Fig. 3A but lower than that of clean cloth suggesting that not all of the ether desorbed. TGA-MS data showed that the desorption temperature increases with increasing molecular weight of the
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adsorbate, as expected (Figure S1B). The desorption of naphthalene, anthracene, and pyrene occurred at ca. 340 C, 434 C, and 443 C, respectively. Complete desorption of ether requires heating of the electrode to more than 300 C (Figure S1A), which will result in some desorption of the larger hydrocarbon contaminants. As expected, an increased amount of adsorbed contaminant resulted in a larger decay in capacitance of the electrode. The ether solvent and naphthalene showed a modest loss of capacitance of approximately 20% compared to clean cloth while the largest molecule, coronene, caused a 61% loss of capacitance compared to the control and 70% compared to clean electrodes although the standard deviation was quite high. In a similar experiment, activated carbon electrodes were immersed in diethyl ether solutions of 5 mM 1-alkene adsorbate, stirred gently for 24 h, and then dried in the same manner mentioned above for the PAH adsorption study. Again, not all solvent desorbed and it was possible that some of the smaller adsorbates desorbed with the solvent although likely in far lesser quantities. The effect of 1-alkene adsorption on electrode capacitance is shown in Fig. 4B. For most of the adsorbates 17e25% capacitance loss occurred compared to the control until 1-eicosene, the only solid and twice as big as 1-decene, where 67% capacitance loss occurred compared to the control (75% when compared to a clean electrode).
of adsorbed water on the surface. The latter is supported by the TGA-MS data (Fig. 1), which shows broad peaks in the m/z ¼ 18 (H2Oþ) ion current traces at ca. 130 C for the as-received and annealed samples, consistent with desorption of physisorbed water. Activated carbon containing polar functional groups such as the one studied herein (Tables S1-2, Figure S2-3); they adsorb significant amounts of water rather quickly when left out in the open air. Li and coworkers have shown that for graphitic surfaces water can block hydrocarbon contaminant adsorption [14]. Hydrocarbons at lower concentrations in the air than water would lose the competition for surface area on the carbon adsorbent and essentially be blocked from adsorption by the water. To test this hypothesis, we stored the activated carbon electrodes in a dry (<0.1 ppm H2O), nitrogen-filled glovebox and observed that they did show a decrease in capacitance of up to 16% after a month (Fig. 5). We note that similar results have been reported by Zou [3] and coworkers on the capacitance of basal plane of HOPG surface. In their study, the possible source of hydrocarbon contamination in the glovebox was not identified. In this study, the glovebox we used housed several bottles of toluene and chloroform, among other chemicals. Again, it should be noted that the glovebox environment contained less than 0.1 ppm water unlike the laboratory air. Thus the surface of carbon materials would not likely be covered by water when stored in a glovebox [14]. 3.5. HOPG model vs. activated carbon porous electrodes
3.4. Airborne contaminant effects After establishing that intentional VOC exposure decreases the capacitance of active carbon electrode, we then tested if airborne VOC exposure can produce the same effect. For flat HOPG electrodes, previous studies by Zou et al. [3] and Hurst et al. [4] showed that its capacitance is greatly reduced by airborne and waterborne contamination. To our surprise, activated carbon electrodes initially showed no significant change in electrode capacitance upon exposure to ambient air (Fig. 5). Given enough time, the electrodes stored on a shelf in laboratory air actually increased in capacitance by about 6%. The cause of this capacitance increase is not well understood. In the TGA-MS data (Fig. 1), there was a rapid increase of ion current of m/z ¼ 28 (COþ) at above 600 C, consistent with the formation of CO from chemisorbed oxygen species on the surface. Such chemisorbed oxygen species could increase the capacitance of the material through redox reactions (pseudo capacitance). However, there was no obvious redox peaks in the CV curve, which may have been due to the small contribution (6%) and heterogeneity of the oxygenated species. We hypothesize that the adsorption of airborne VOC is limited by the slow diffusion in the porous carbon material and/or presence
Fig. 5. The effect of ambient air aging versus inert (N2) environment aging on the capacitance of activated carbon fiber electrodes. The data points are single measurements and therefore do not include error bars. (A colour version of this figure can be viewed online.)
HOPG has been often used as a model electrode for carbon materials. However, our data shows that their responses to airborne VOC contamination are very different. HOPG experiences significant loss of double layer capacitance upon adsorption of trace amount of VOC in ambient air and water (e.g., 33% over 24 h). In contrast, activated carbon materials do not show performance degradation in ambient air; loss of capacitance was observed only after long term storage in a dry environment or intentional exposure to high concentration of VOC vapors or solutions. Several trends emerged from our intentional VOC exposure experiments. For the small molecules shown in Fig. 3A, the capacitance degradation of the electrodes increased with decreasing water solubility (e.g. 24% loss for ethanol vs. 77% loss for toluene, Fig. 3A) of the VOC. In addition, the magnitude of the capacitance loss also correlates very well with the peak desorption temperature of the VOC, with the higher desorption temperature generally correlated with higher capacitance loss (Fig. S1A). Chloroform is the only exception; its desorption profile is similar to that of ether but its impact on capacitance is significantly larger. It is likely that the solubility of the VOC in water plays the major role here: the solubility of ether in water (ca. 6.5 g/100 mL at 25 C) is much larger than that of chloroform (ca. 0.81 g/100 mL at 20 C), facilitating the displacement of ether by the aqueous electrolyte. For the size series shown in Fig. 4, increasing adsorbate size increased final adsorbent mass and also increased capacitance losses. The increase in the final adsorbent mass is consistent with stronger binding of the larger adsorbates, as can be seen in the TGAMS data (Fig. S1B) and is also consistent with previous theoretical calculations [15]. However, for the two series of molecules we studied, there also appeared to be a critical size of the molecule, above which a sudden increase in the capacitance loss was observed (i.e., coronene for the PAH series and 1-eicosene for the 1alkene series). This observation can be explained by a pore blocking model as detailed below. The adsorption of small VOCs occurs through gradual pore filling followed by capillary condensation according to Dubinin [16]. In porous materials literature, kinetic diameter is often used as the size descriptor when size exclusion from pores is being studied
N.L. Tolman et al. / Carbon 150 (2019) 334e339
[17e19]. For the activated carbon sample used here, most pore diameters ranged from 0.6 to 1.5 nm (Fig. 2). Assuming that coronene has a kinetic diameter roughly three times that of benzene, perhaps slightly smaller (5.85 Å for benzene and therefore 17.55 Å for coronene) [20], size exclusion of coronene from pores should have occurred in pores smaller than 1.8 nm in diameter. Despite the expected size exclusion, coronene resulted in the largest mass increase among the PAHs (Fig. 4A), suggesting that in addition to coronene, the other PAHs within this series are not filling the pores as well (Note that we expect complete pore filling for the smaller VOCs shown in Fig. 3). The same conclusion can be made for 1alkene series, where a similar degree of mass increase was observed for the whole series. Based on this data, we suggest that the sudden increase in the capacitance loss for coronene and 1eicosene may have been due to the adsorbate sizes reaching a point where blocking of the entrances of the pores or bottlenecks in the pores occurred [21]. Such pores are not filled by the large adsorbates but are accessible by water and ions. However, with pore entrances or bottlenecks blocked by the largest adsorbates, the accessible surface area for water/ions significantly decreased. In the cases of both 1-eicosene and coronene, the standard deviation of the capacitance of the electrode was somewhat larger than that of the other adsorbates, possibly due to difference in the pore size from one electrode to the other. Activated carbon and other porous adsorbents are widely used as supercapacitor electrodes in both academic laboratory research and commercial supercapacitor applications. Many of the adsorbates studied in this work are common solvents in research laboratories and manufacturing centers. Given the significant impact of VOC contamination demonstrated here, this work should be of interest to the scientific and industrial community alike as both are concerned with maximizing the efficiency of energy storage devices. Our study also showed that the presence of water may inhibit the negative impact of VOC adsorption [14]. For activated carbon electrodes having a high affinity to water, adsorption of hydrophobic adsorbates could be blocked by already adsorbed water monolayer coverage or pore filling. Water adsorption would not be a concern if the activated carbon material will be eventually used in an aqueous environment. However, there are also cases where water is undesired, e.g., supercapacitors using organic or ionic liquid electrolytes [8]. In these cases, the carbon materials need to be dried and VOC adsorption could occur as a result. Finally, since the activated carbon materials could be contaminated by VOC in solution phase, the long-term performance of carbon-based capacitors may be impacted by a slow leaching of hydrocarbon contaminants from plastic components (e.g., containers, glue) within the devices. 4. Conclusion We showed that physisorption of VOC in vapor and liquid phases had a dramatic effect on the capacitance of activated carbon materials. Within 1e10 minutes of exposure to water-immiscible VOC vapor, electrode capacitance decayed by approximately 70e80% in an aqueous electrolyte. Even VOCs miscible with the aqueous electrolyte exhibited capacitance losses ca. 20e30%. Capacitance losses were attributed to blocking access of the electrolyte to the carbon surface. The results further showed that the size of the adsorbate can greatly affect the loss of capacitance as
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well. In both a PAH series and a 1-alkene series of increasing adsorbate molecule size, the largest molecule in the series, coronene for the PAHs and 1-eicosene for the 1-alkenes, exhibited a capacitance loss more than twice that of the other adsorbates in the series at a similar level of adsorption. This phenomenon was attributed to blocking of pore entrances or bottlenecks by the large molecules. Acknowledgement This work is supported in part by ONR (N000141812555). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.05.005. References [1] Z. Li, Y. Wang, A. Kozbial, G. Shenoy, F. Zhou, R. McGinley, P. Ireland, B. Morganstein, A. Kunkel, S.P. Surwade, L. Li, H. Liu, Effect of airborne contaminants on the wettability of supported graphene and graphite, Nat. Mater. 12 (10) (2013) 925e931. [2] A. Kozbial, Z. Li, J. Sun, X. Gong, F. Zhou, Y. Wang, H. Xu, H. Liu, L. Li, Understanding the intrinsic water wettability of graphite, Carbon 74 (2014) 218e225. [3] Y. Zou, A.S. Walton, I.A. Kinloch, R.A.W. Dryfe, Investigation of the differential capacitance of highly ordered pyrolytic graphite as a model material of graphene, Langmuir 32 (2016) 11448e11455. [4] J.M. Hurst, L. Li, H. Liu, Adventitious hydrocarbons and the graphite-water interface, Carbon 134 (2018) 464e469. [5] N. Shimodaira, A. Masui, Raman spectroscopic investigations of activated carbon materials, J. Appl. Phys. 92 (2) (2002) 902e909. [6] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (1) (2006) 11e27. [7] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (11) (2008) 845. [8] H. Wang, M. Yoshio, Effect of water contamination in the organic electrolyte on the performance of activated carbon/graphite capacitors, J. Power Sources 195 (1) (2010) 389e392. [9] T.T. Duignan, X.S. Zhao, Impurities limit the capacitance of carbon-based supercapacitors, J. Phys. Chem. C 123 (7) (2019) 4085e4093. [10] K. Fic, G. Lota, E. Frackowiak, Effect of surfactants on capacitance properties of carbon electrodes, Electrochim. Acta 60 (2012) 206e212. guin, Carbon materials for the electrochemical storage of [11] E. Frackowiak, F. Be energy in capacitors, Carbon 39 (6) (2001) 937e950. [12] V.V.N. Obreja, On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated materialda review, Phys. E Lowdimens. Syst. Nanostruct. 40 (7) (2008) 2596e2605. [13] G.G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, An asymmetric hybrid nonaqueous energy storage cell, J. Electrochem. Soc. 148 (8) (2001) A930eA939. [14] Z. Li, A. Kozbial, N. Nioradze, D. Parobek, G.J. Shenoy, M. Salim, S. Amemiya, L. Li, H. Liu, Water protects graphitic surface from airborne hydrocarbon contamination, ACS Nano 10 (1) (2016) 349e359. [15] W. Wang, Y. Zhang, Y.-B. Wang, Noncovalent p,,, p interaction between graphene and aromatic molecule: structure, energy, and nature, J. Chem. Phys. 140 (9) (2014), 094302. [16] M. Dubinin, E. Zaverina, V. Serpinsky, The sorption of water vapour by active carbon, J. Chem. Soc. (1955) 1760e1766. [17] L. Li, P.A. Quinlivan, D.R.U. Knappe, Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution, Carbon 40 (12) (2002) 2085e2100. [18] Z. Hu, N. Maes, E. Vansant, Molecular probe technique for the assessment of the carbon molecular sieve structure, J. Porous Mater. 2 (1) (1995) 19e23. [19] M. Helmich, M. Luckas, C. Pasel, D. Bathen, Characterization of microporous activated carbons using molecular probe method, Carbon 74 (2014) 22e31. [20] P. Wu, A. Debebe, Y.H. Ma, Adsorption and diffusion of C6 and C8 hydrocarbons in silicalite, Zeolites 3 (2) (1983) 118e122. [21] J. Chmiola, G. Yushin, R. Dash, Y. Gogotsi, Effect of pore size and surface area of carbide derived carbons on specific capacitance, J. Power Sources 158 (1) (2006) 765e772.
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Carbon journal homepage: www.elsevier.com/locate/carbon
The effect of physical adsorption on the capacitance of activated carbon electrodes Nathan L. Tolman a, Jason M. Mukai a, Shuqing Wang b, *, Alessandra Zito a, 1, Tianyi Luo a, Haitao Liu a, ** a b
University of Pittsburgh, Department of Chemistry, Pittsburgh, PA, 15260, USA SINOPEC Research Institute of Petroleum Processing, Beijing, 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2019 Received in revised form 1 May 2019 Accepted 3 May 2019 Available online 8 May 2019
This work reports the effect of physisorption of organic compounds on the double layer capacitance of activated carbon electrodes. Exposure of activated carbon electrodes to toluene or chloroform vapor for less than 10 minutes resulted in a capacitance loss of 77% and 84%, respectively. Even adsorbates, such as acetone and ethanol, miscible with the aqueous Li2SO4 electrolyte caused 20e30% losses in capacitance. It was also found that there was an adsorbate size dependence: above a certain threshold, a larger adsorbate could have more than twice the impact on capacitance than a slightly smaller one. The results were consistent with the hypothesis that volatile organic contaminants (VOC) block access of the aqueous electrolyte to the carbon electrode surface. Porous activated carbon is currently the material of choice for supercapacitor electrodes in both research laboratories and commercial supercapacitor applications. These facilities also often house and use many volatile organic solvents either for research and development or simply for cleaning purposes. Because of this, our work has significant implications to the research and development of carbon-based supercapacitors. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Contamination of carbon materials and its effect on their surface properties has received a lot of attention recently. It has been recently discovered that graphitic carbons, such as graphene and highly-ordered pyrolytic graphite (HOPG), are more hydrophilic than previously thought due to the masking of surface properties caused by adsorption of airborne hydrocarbon contamination from the ambient environment. For example, Li et al. found that the water contact angle (WCA) of freshly prepared, copper-supported, chemical vapor deposition (CVD)-grown graphene was 44 . This value increased to 60 within 20 minutes of air exposure and finally to 80 within one day [1]. Similarly, Kozbial et al. found for HOPG that the WCA was ~64 within 10 s of exfoliation followed by a steady increase over time to 90 [2]. Both studies attributed this rise in hydrophobicity to hydrocarbon contamination from ambient air.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (H. Liu). 1 Present Address: University of California Irvine, Department of Chemistry, Irvine, California, 92697, USA. https://doi.org/10.1016/j.carbon.2019.05.005 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
The capacitance changes of HOPG as a function of air aging have been studied by Zou et al. They showed a capacitance decrease from 6.0 mF cm2 to 4.3 mF cm2 between freshly exfoliated HOPG and 24-h air-aged HOPG respectively. Using X-ray photoelectron spectroscopy, they showed that this was most likely the result of the presence of a 75% monolayer of adventitious carbon adsorbed on the electrode basal plane. They also showed that the capacitance of the electrode could be partially restored by heating in a vacuum to desorb the contaminants [3]. Expanding on Zou's work, Hurst et al. showed that contamination of the HOPG electrode surface can also occur in the electrolyte solution [4]. For a freshly exfoliated HOPG electrode immersed in ~1 M NaCl (>99.0% purity salt), the capacitance degraded by 33% over a 24-h period. Most of the capacitance loss occurred within the first 250 minutes however. Using spectroscopic ellipsometry, they calculated the contamination thickness to be approximately 0.7 nm which corresponds to a monolayer of contamination. Activated carbon is another graphitic material whose capacitance is particularly sensitive to surface properties. Unlike graphene and graphite, activated carbon is a much more complex material, having ordered graphitic domains of winding short basal plains as well as disordered sp2 cluster domains often found in
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amorphous carbon [5]. The material is further complicated by its porous nature, which can include networks of macropores (>50 nm), mesopores (2e50 nm), or micropores (<2 nm) or combinations of all three. The porosity of activated carbon gives rise to its high specific surface area (SSA) which is typically found to be between 500 and 3000 m2/g [6]. High SSA, good conductivity, processability, and low cost make activated carbon currently the most commonly used electrode material for supercapacitors, or electric double layer capacitors (EDLCs) [7]. The effect of hydrocarbon contamination and physical adsorption on the capacitance of activated carbon electrodes would be of particular concern to both the scientific and industrial communities alike because both have an interest in maximizing the efficiency of energy storage devices. Wang et al. have studied the effect of water contamination of organic electrolyte solutions on the performance of an activated carbon/graphite supercapacitor [8]. Their work primarily focused on the effect contamination had on the degradation of the negative electrode at the high potential windows accessible to organic electrolytes. A cause of capacitance decay, he suggested, was that the degradation products of the carbon electrodes blocked electrolyte ions from pores resulting in losses in capacitance [8]. Duignan and Zhao through quantum mechanical calculations showed that the lower than expected capacitance of HOPG was due to a dielectric dead layer of adsorbed hydrocarbons on the electrode surface and speculated that activated carbon electrodes could suffer the same capacitance losses [9]. Fic studied the effect of contaminant surfactant molecules on activated carbon capacitance, however, and found in some cases a significant increase in capacitance, purportedly due to increased wettability of the electrode and ion-channeling effects, while in other cases he found modest losses of capacitance [10]. In this study, we aim to show that physical adsorption of contaminants either from air or electrolyte solvents in an industrial or laboratory environment can also reduce the capacitance of activated carbon electrodes by similar means. While capacitance values for porous electrodes can greatly vary depending on many factors, they typically are as low as ~30 F/g and as high as ~150 F/g [11]. In some exceptional cases the capacitances can be as high as 300 F/g [12]. Unknown and unaccounted for contaminant adsorption could reduce a high performance material's capacitance to that of a low performing one. This issue is of significant importance to commercial and research interests alike for the maximization of energy storage. 2. Experimental 2.1. Activated carbon fiber cloth (ACFC) Flexzorb knitted activated carbon fiber cloth (Chemviron Cloth Division of Calgon Carbon, Inc.) was used as the electrode material for all experiments. The cloth electrodes were first cut with a 1 cm diameter hole punch. They were then placed in a quartz tube with stainless steel fittings on a quartz boat in a Thermo Scientific Lindberg Blue M furnace. The tube was pumped down to 70 mtorr pressure before H2 gas was flown through the tube at 2 standard cubic centimeters per minute. The tube was then heated to 1050 C for 1 h followed by cooling to room temperature under H2 gas. Cloth was then stored in a glass vacuum desiccator with a bed of anhydrous calcium chloride. Activated carbon Brunauer-Emmett-Teller (BET) surface area was calculated from nitrogen adsorption isotherms (Quantachrome Instruments Autosorb-1). The sample was subjected to outgassing under dynamic vacuum at 120 C until the pressure change rate was less than 2 mmHg/min (about 3.5e5 h) followed by nitrogen
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adsorption at 77 K. Isotherms were recorded in 58 steps at a relative pressure range from 0.00001 to 0.995. Total pH was measured to be 8.28 by addition of 0.1125 g of ACFC to 50.0 mL of 0.01 M NaNO3. The solution was gently stirred while sparged with N2 gas. After 24 h, the pH was measured with a pH meter (Mettler Toledo). X-ray photoelectron spectroscopy was carried out using a Thermo Fisher ESCALAB 250 Xi X-ray photoelectron spectrometer (Al Ka monochromatic X-rays, 650 mm spot size coupled with charge compensation, Thermo Avantage software). Spectra and activated carbon surface functional group identification can be found in the supporting information. Thermogravimetric analysis (TGA) and thermogravimetric analysis/mass spectrometry (TGA-MS) were carried out on a Netzsch STA 409CD-QMS Skimmer instrument using a heating rate of 10 K/min under He. 2.2. Ambient air vs. inert air aging Electrodes were subjected to ambient laboratory air or N2 atmosphere inside a glovebox (<15 ppm O2, <0.1 ppm H2O) as a function of time by first recording their mass and then placing them on labeled boats in the the environment. No attempt was made to protect them from particulate matter or light. 2.3. Contaminant adsorption For vapor phase contamination, electrodes were exposed to adsorbate vapor by placing them on a 5 cm watch glass with an adsorbate reservoir also on a 5 cm watch glass under an overturned already vapor-saturated 14 2 cm Pyrex dish as a function of time. Vapor adsorbates included acetone (99.5%, ACS Reagent, Sigma Aldrich), chloroform (99.8%, ACS Reagent, Sigma-Aldrich), diethyl ether (anhydrous, 99.9%, Fisher Chemical), ethanol (anhydrous, histological grade, 0.5% water maximum, Fisher Chemical), and toluene (99.5%, ACS Reagent, Sigma-Aldrich). For solution phase adsorption, cloth electrodes were added to a vial containing 5 mL diethyl ether (anhydrous, 99.9%, Fisher Chemical) and known amounts of adsorbate. They were then stirred gently for 30 min at 320 rpm followed by 40 min of air drying of ether on a bed of CaCl2 in a 150 75 mm crystalizing dish covered with aluminum foil punctured with small holes to provide a dry, covered but ventilated environment. Adsorbates included 1-decene (97.0%, Sigma-Aldrich), 1-dodecene (95%, Fisher Chemical), 1eicosene (99.2%, MP Biomedicals), 1-hexadecene (98.5%, Sigma-Aldrich), 1-octadecene (99.5%, Sigma-Aldrich), 1tetradecene (97.0%, GC Grade, Sigma-Aldrich), anthracene (99.0%, Fluka Analytical), coronene (97%, Sigma-Aldrich), naphthalene (reagent grade, Fisher Chemical), and pyrene (98%, SigmaAldrich). 2.4. Cyclic voltammetry A Gamry Reference 600 potentiostat was used to collect cyclic voltammetry data using a 3-electrode custom-made Swagelok-type symmetrical coin cell setup modelled after Amatucci et al. [13] Perfluoroalkoxy (PFA plastic) tee ¾” compression fittings and Gardner Compression Springs (302 stainless steel with closed flat ends, 0.06 .045 1.500”) were purchased from McMaster-Carr Inc. Plungers and coin-shaped current collectors (both 316-grade stainless steel) were custom made to fit the PFA fitting inner diameter by the University of Pittsburgh Dietrich School of Arts and Sciences machine shop. Glass microfiber filter papers (Whatman 934-Alt™ RTU, 90 mm) were used as porous separators. The reference electrode was a Ag/AgCl reference electrode (Accumet
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13-620-53) and was inserted into the electrolyte reservoir through the top hole of the fitting tee. Cyclic voltammograms were carried out at 1 or 2 mV/s from 0 V to 0.9 V in 1 M aqueous Li2SO4 (monohydrate, 99þ%, Acros Organics). The potentiostat preconditioned the cell at 0 V for 60 s before each voltammogram. Data analysis was carried out using Gamry's Echem Analyst software. Capacitance was calculated from 0 V to 0.9 V using the following equation:
1 C¼ 2wðDVÞ
t2 ð
iðtÞ dt ; dt ¼ t1
dV v
where t is time, i is current, V is potential, w is electrode mass, and n is scan rate. 3. Results and discussion 3.1. Characterization of the activated carbon material We used Flexzorb knitted activated carbon fiber cloth in this study. The cloth as received showed redox peaks in the cyclic voltammograms for some areas of the cloth but not others. Annealing the cloth in H2 (1050 C for 1 h, 70 mtorr) before use corrected this problem. All electrodes were therefore annealed before use and then stored in a vacuum desiccator. 3.1.1. TGA-MS ̄ By TGA-MS analyses, the as received sample showed ca. 10% weight loss upon heating to 995 C, compared to ca. 5% weight loss for the annealed sample under the same condition (Fig. 1A). The TGA-MS data of the as recevied sample shows a noticeable peak for þ þ m/z ¼ 17 (NHþ 3 ) and 18 (H2O ) at around 130 C. For m/z ¼ 28 (CO ), there was a significant increase above 600 C in the ion current. For m/z ¼ 44 (COþ 2 ), a broad plateau was observed between ca. 140 and
750 C. For the annealed samples, the m/z ¼ 17 and m/z ¼ 44 ion current curves become featureless. The m/z ¼ 18 and m/z ¼ 28 traces still resemble those of the as-received sample (Fig. 1B and C). 3.1.2. Specific surface area (SSA) and pore size distribution (PSD) The SSA of the cloth electrodes was calculated from nitrogen adsorption isotherms at 77K to be 1198 ± 33 m2/g. Total pore volume was calculated to be 0.545 ± 0.012 cm3/g. Quenched Solid Density Functional Theory (QSDFT) assuming slit-shaped pores showed that most pore diameters ranged from 0.6 to 1.5 nm (Fig. 2). 3.2. Volatile organics contamination of actived carbon electrodes We first show that intentional exposure to VOC vapor decreases capacitance of activated carbon materials. For these experiments, we thermally annealed activated carbon fiber electrodes and then exposed them to saturated VOC vapor environments for set periods of time before measurement of capacitance in 1 M aqueous Li2SO4. Fig. 3A shows that 10 minutes of saturated VOC vapor resulted in capacitance losses of 24, 28, 43, 77, and 84% for ethanol, acetone, diethyl ether, toluene, and chloroform respectively (top to bottom in Fig. 3A). Fig. 3B shows the effect physical adsorption of toluene had on the cyclic voltammogram (1 M aqueous Li2SO4, 1 mV/s). The blue line is from a sample not exposed to toluene (clean) while the green and red lines represent 2 and 5 minutes of toluene vapor exposure respectively. We have conducted TGA-MS on the solvent vapor saturated activated carbon electrodes. The desorption of the adsorbed molecules were detected by monitoring the molecular ion or its major fragments (e.g., CH3CH2OHþ for ethanol, (CH3CH2)2Oþ for diethyl þ ether, CH3COþ for acetone, C7Hþ 8 for toluene, and CCl2H for chloroform). A single desorption peak was observed in all cases except ether, in which case a closely-spaced two-peak pattern was observed (Figure S1A). The peak desorption temperature increases in the order of ethanol (95 C), acetone (111 C), ether (118 C and
Fig. 1. (A) TGA and TGA-MS analysis of (B) annealed and (C) as received activated carbon samples. (A colour version of this figure can be viewed online.)
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Fig. 2. Sample QSDFT pore size distributions calculated from N2 adsorption isotherms at 77 K and assuming slit-shaped pores of Chemviron Carbon's Flexzorb 100 activated carbon fiber cloth. Figures are from two separately taken adsorption isotherms. (A colour version of this figure can be viewed online.)
Fig. 3. (A) The effect of small volatile organic contaminant vapor adsorption on the double layer capacitance of activated carbon fiber electrodes. Voltammograms were taken a 2 mV/s in 1 M Li2SO4 against a Ag/AgCl reference electrode. (B) The effect of physical adsorption of toluene vapor on cyclic voltammogram shape as a function of time. Cyclic voltammograms done at 1 mV/s in aqueous 1 M Li2SO4 for activated carbon fiber electrodes not subjected to toluene (blue) and electrodes subjected to 2 (green) and 5 (red) minutes of saturated toluene vapor are shown. (A colour version of this figure can be viewed online.)
140 C), chloroform (124 C), and toluene (146 C). 3.3. Contaminant size effects To probe the effect of adsorbate size on electrode capacitance, polycyclic aromatic hydrocarbons (PAHs) of increasing size were adsorbed to activated carbon electrodes from 2.5 mM solutions in diethyl ether during a 24-h period. After the adsorption, the carbon materials were then dried on a bed of CaCl2 in a 150 75 mm crystalizing dish covered with Al foil and ventilated with small holes for another 24 h before assembled into a coin cell for capacitance measurement. Upon adsorption, the mass of the carbon materials increased and their capacitance decreased. The capacitance of the electrodes and the percent mass increase are shown in Fig. 4A. Percent mass increase is an indicator of how much adsorbate stayed adsorbed to the cloth after the drying period. It should be noted that not all of the ether desorbs in the drying
Fig. 4. (A) The capacitance measurements (green) after physical adsorption of PAHs of differing sizes and percent mass increase (blue) of activated carbon electrodes are shown. Adsorbates were adsorbed over 24 h from gently stirred 2.5 mM adsorbate/ diethyl ether solutions and then dried for 24 h at atmospheric pressure. (B) The effect of 1-alkene adsorption from diethyl ether on the capacitance of activated carbon electrodes. Electrodes were exposed to 10 mL of 5 mM adsorbate dissolved in diethyl ether, stirred, and dried in a dry, covered but ventilated environment. (A colour version of this figure can be viewed online.)
process, as can be seen by comparing control versus clean in Fig. 4A. Clean refers to an electrode that has not been subjected to anything other than the annealing procedure that all electrodes were subjected to and storage in a vacuumed desiccator while the control has been stirred in pure ether and dried. The capacitance of the control was higher than that of ether vapor adsorbed cloth shown in Fig. 3A but lower than that of clean cloth suggesting that not all of the ether desorbed. TGA-MS data showed that the desorption temperature increases with increasing molecular weight of the
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adsorbate, as expected (Figure S1B). The desorption of naphthalene, anthracene, and pyrene occurred at ca. 340 C, 434 C, and 443 C, respectively. Complete desorption of ether requires heating of the electrode to more than 300 C (Figure S1A), which will result in some desorption of the larger hydrocarbon contaminants. As expected, an increased amount of adsorbed contaminant resulted in a larger decay in capacitance of the electrode. The ether solvent and naphthalene showed a modest loss of capacitance of approximately 20% compared to clean cloth while the largest molecule, coronene, caused a 61% loss of capacitance compared to the control and 70% compared to clean electrodes although the standard deviation was quite high. In a similar experiment, activated carbon electrodes were immersed in diethyl ether solutions of 5 mM 1-alkene adsorbate, stirred gently for 24 h, and then dried in the same manner mentioned above for the PAH adsorption study. Again, not all solvent desorbed and it was possible that some of the smaller adsorbates desorbed with the solvent although likely in far lesser quantities. The effect of 1-alkene adsorption on electrode capacitance is shown in Fig. 4B. For most of the adsorbates 17e25% capacitance loss occurred compared to the control until 1-eicosene, the only solid and twice as big as 1-decene, where 67% capacitance loss occurred compared to the control (75% when compared to a clean electrode).
of adsorbed water on the surface. The latter is supported by the TGA-MS data (Fig. 1), which shows broad peaks in the m/z ¼ 18 (H2Oþ) ion current traces at ca. 130 C for the as-received and annealed samples, consistent with desorption of physisorbed water. Activated carbon containing polar functional groups such as the one studied herein (Tables S1-2, Figure S2-3); they adsorb significant amounts of water rather quickly when left out in the open air. Li and coworkers have shown that for graphitic surfaces water can block hydrocarbon contaminant adsorption [14]. Hydrocarbons at lower concentrations in the air than water would lose the competition for surface area on the carbon adsorbent and essentially be blocked from adsorption by the water. To test this hypothesis, we stored the activated carbon electrodes in a dry (<0.1 ppm H2O), nitrogen-filled glovebox and observed that they did show a decrease in capacitance of up to 16% after a month (Fig. 5). We note that similar results have been reported by Zou [3] and coworkers on the capacitance of basal plane of HOPG surface. In their study, the possible source of hydrocarbon contamination in the glovebox was not identified. In this study, the glovebox we used housed several bottles of toluene and chloroform, among other chemicals. Again, it should be noted that the glovebox environment contained less than 0.1 ppm water unlike the laboratory air. Thus the surface of carbon materials would not likely be covered by water when stored in a glovebox [14]. 3.5. HOPG model vs. activated carbon porous electrodes
3.4. Airborne contaminant effects After establishing that intentional VOC exposure decreases the capacitance of active carbon electrode, we then tested if airborne VOC exposure can produce the same effect. For flat HOPG electrodes, previous studies by Zou et al. [3] and Hurst et al. [4] showed that its capacitance is greatly reduced by airborne and waterborne contamination. To our surprise, activated carbon electrodes initially showed no significant change in electrode capacitance upon exposure to ambient air (Fig. 5). Given enough time, the electrodes stored on a shelf in laboratory air actually increased in capacitance by about 6%. The cause of this capacitance increase is not well understood. In the TGA-MS data (Fig. 1), there was a rapid increase of ion current of m/z ¼ 28 (COþ) at above 600 C, consistent with the formation of CO from chemisorbed oxygen species on the surface. Such chemisorbed oxygen species could increase the capacitance of the material through redox reactions (pseudo capacitance). However, there was no obvious redox peaks in the CV curve, which may have been due to the small contribution (6%) and heterogeneity of the oxygenated species. We hypothesize that the adsorption of airborne VOC is limited by the slow diffusion in the porous carbon material and/or presence
Fig. 5. The effect of ambient air aging versus inert (N2) environment aging on the capacitance of activated carbon fiber electrodes. The data points are single measurements and therefore do not include error bars. (A colour version of this figure can be viewed online.)
HOPG has been often used as a model electrode for carbon materials. However, our data shows that their responses to airborne VOC contamination are very different. HOPG experiences significant loss of double layer capacitance upon adsorption of trace amount of VOC in ambient air and water (e.g., 33% over 24 h). In contrast, activated carbon materials do not show performance degradation in ambient air; loss of capacitance was observed only after long term storage in a dry environment or intentional exposure to high concentration of VOC vapors or solutions. Several trends emerged from our intentional VOC exposure experiments. For the small molecules shown in Fig. 3A, the capacitance degradation of the electrodes increased with decreasing water solubility (e.g. 24% loss for ethanol vs. 77% loss for toluene, Fig. 3A) of the VOC. In addition, the magnitude of the capacitance loss also correlates very well with the peak desorption temperature of the VOC, with the higher desorption temperature generally correlated with higher capacitance loss (Fig. S1A). Chloroform is the only exception; its desorption profile is similar to that of ether but its impact on capacitance is significantly larger. It is likely that the solubility of the VOC in water plays the major role here: the solubility of ether in water (ca. 6.5 g/100 mL at 25 C) is much larger than that of chloroform (ca. 0.81 g/100 mL at 20 C), facilitating the displacement of ether by the aqueous electrolyte. For the size series shown in Fig. 4, increasing adsorbate size increased final adsorbent mass and also increased capacitance losses. The increase in the final adsorbent mass is consistent with stronger binding of the larger adsorbates, as can be seen in the TGAMS data (Fig. S1B) and is also consistent with previous theoretical calculations [15]. However, for the two series of molecules we studied, there also appeared to be a critical size of the molecule, above which a sudden increase in the capacitance loss was observed (i.e., coronene for the PAH series and 1-eicosene for the 1alkene series). This observation can be explained by a pore blocking model as detailed below. The adsorption of small VOCs occurs through gradual pore filling followed by capillary condensation according to Dubinin [16]. In porous materials literature, kinetic diameter is often used as the size descriptor when size exclusion from pores is being studied
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[17e19]. For the activated carbon sample used here, most pore diameters ranged from 0.6 to 1.5 nm (Fig. 2). Assuming that coronene has a kinetic diameter roughly three times that of benzene, perhaps slightly smaller (5.85 Å for benzene and therefore 17.55 Å for coronene) [20], size exclusion of coronene from pores should have occurred in pores smaller than 1.8 nm in diameter. Despite the expected size exclusion, coronene resulted in the largest mass increase among the PAHs (Fig. 4A), suggesting that in addition to coronene, the other PAHs within this series are not filling the pores as well (Note that we expect complete pore filling for the smaller VOCs shown in Fig. 3). The same conclusion can be made for 1alkene series, where a similar degree of mass increase was observed for the whole series. Based on this data, we suggest that the sudden increase in the capacitance loss for coronene and 1eicosene may have been due to the adsorbate sizes reaching a point where blocking of the entrances of the pores or bottlenecks in the pores occurred [21]. Such pores are not filled by the large adsorbates but are accessible by water and ions. However, with pore entrances or bottlenecks blocked by the largest adsorbates, the accessible surface area for water/ions significantly decreased. In the cases of both 1-eicosene and coronene, the standard deviation of the capacitance of the electrode was somewhat larger than that of the other adsorbates, possibly due to difference in the pore size from one electrode to the other. Activated carbon and other porous adsorbents are widely used as supercapacitor electrodes in both academic laboratory research and commercial supercapacitor applications. Many of the adsorbates studied in this work are common solvents in research laboratories and manufacturing centers. Given the significant impact of VOC contamination demonstrated here, this work should be of interest to the scientific and industrial community alike as both are concerned with maximizing the efficiency of energy storage devices. Our study also showed that the presence of water may inhibit the negative impact of VOC adsorption [14]. For activated carbon electrodes having a high affinity to water, adsorption of hydrophobic adsorbates could be blocked by already adsorbed water monolayer coverage or pore filling. Water adsorption would not be a concern if the activated carbon material will be eventually used in an aqueous environment. However, there are also cases where water is undesired, e.g., supercapacitors using organic or ionic liquid electrolytes [8]. In these cases, the carbon materials need to be dried and VOC adsorption could occur as a result. Finally, since the activated carbon materials could be contaminated by VOC in solution phase, the long-term performance of carbon-based capacitors may be impacted by a slow leaching of hydrocarbon contaminants from plastic components (e.g., containers, glue) within the devices. 4. Conclusion We showed that physisorption of VOC in vapor and liquid phases had a dramatic effect on the capacitance of activated carbon materials. Within 1e10 minutes of exposure to water-immiscible VOC vapor, electrode capacitance decayed by approximately 70e80% in an aqueous electrolyte. Even VOCs miscible with the aqueous electrolyte exhibited capacitance losses ca. 20e30%. Capacitance losses were attributed to blocking access of the electrolyte to the carbon surface. The results further showed that the size of the adsorbate can greatly affect the loss of capacitance as
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well. In both a PAH series and a 1-alkene series of increasing adsorbate molecule size, the largest molecule in the series, coronene for the PAHs and 1-eicosene for the 1-alkenes, exhibited a capacitance loss more than twice that of the other adsorbates in the series at a similar level of adsorption. This phenomenon was attributed to blocking of pore entrances or bottlenecks by the large molecules. Acknowledgement This work is supported in part by ONR (N000141812555). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.05.005. References [1] Z. Li, Y. Wang, A. Kozbial, G. Shenoy, F. Zhou, R. McGinley, P. Ireland, B. Morganstein, A. Kunkel, S.P. Surwade, L. Li, H. Liu, Effect of airborne contaminants on the wettability of supported graphene and graphite, Nat. Mater. 12 (10) (2013) 925e931. [2] A. Kozbial, Z. Li, J. Sun, X. Gong, F. Zhou, Y. Wang, H. Xu, H. Liu, L. Li, Understanding the intrinsic water wettability of graphite, Carbon 74 (2014) 218e225. [3] Y. Zou, A.S. Walton, I.A. Kinloch, R.A.W. Dryfe, Investigation of the differential capacitance of highly ordered pyrolytic graphite as a model material of graphene, Langmuir 32 (2016) 11448e11455. [4] J.M. Hurst, L. Li, H. Liu, Adventitious hydrocarbons and the graphite-water interface, Carbon 134 (2018) 464e469. [5] N. Shimodaira, A. Masui, Raman spectroscopic investigations of activated carbon materials, J. Appl. Phys. 92 (2) (2002) 902e909. [6] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (1) (2006) 11e27. [7] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (11) (2008) 845. [8] H. Wang, M. Yoshio, Effect of water contamination in the organic electrolyte on the performance of activated carbon/graphite capacitors, J. Power Sources 195 (1) (2010) 389e392. [9] T.T. Duignan, X.S. Zhao, Impurities limit the capacitance of carbon-based supercapacitors, J. Phys. Chem. C 123 (7) (2019) 4085e4093. [10] K. Fic, G. Lota, E. Frackowiak, Effect of surfactants on capacitance properties of carbon electrodes, Electrochim. Acta 60 (2012) 206e212. guin, Carbon materials for the electrochemical storage of [11] E. Frackowiak, F. Be energy in capacitors, Carbon 39 (6) (2001) 937e950. [12] V.V.N. Obreja, On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated materialda review, Phys. E Lowdimens. Syst. Nanostruct. 40 (7) (2008) 2596e2605. [13] G.G. Amatucci, F. Badway, A. Du Pasquier, T. Zheng, An asymmetric hybrid nonaqueous energy storage cell, J. Electrochem. Soc. 148 (8) (2001) A930eA939. [14] Z. Li, A. Kozbial, N. Nioradze, D. Parobek, G.J. Shenoy, M. Salim, S. Amemiya, L. Li, H. Liu, Water protects graphitic surface from airborne hydrocarbon contamination, ACS Nano 10 (1) (2016) 349e359. [15] W. Wang, Y. Zhang, Y.-B. Wang, Noncovalent p,,, p interaction between graphene and aromatic molecule: structure, energy, and nature, J. Chem. Phys. 140 (9) (2014), 094302. [16] M. Dubinin, E. Zaverina, V. Serpinsky, The sorption of water vapour by active carbon, J. Chem. Soc. (1955) 1760e1766. [17] L. Li, P.A. Quinlivan, D.R.U. Knappe, Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution, Carbon 40 (12) (2002) 2085e2100. [18] Z. Hu, N. Maes, E. Vansant, Molecular probe technique for the assessment of the carbon molecular sieve structure, J. Porous Mater. 2 (1) (1995) 19e23. [19] M. Helmich, M. Luckas, C. Pasel, D. Bathen, Characterization of microporous activated carbons using molecular probe method, Carbon 74 (2014) 22e31. [20] P. Wu, A. Debebe, Y.H. Ma, Adsorption and diffusion of C6 and C8 hydrocarbons in silicalite, Zeolites 3 (2) (1983) 118e122. [21] J. Chmiola, G. Yushin, R. Dash, Y. Gogotsi, Effect of pore size and surface area of carbide derived carbons on specific capacitance, J. Power Sources 158 (1) (2006) 765e772.