DOI: 10.1002/cssc.201500156
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Peculiar Properties of Mesoporous Synthetic Carbon/ Graphene Phase Composites and their Effect on Supercapacitive Performance Mykola Seredych,[a] Enrique Rodrguez-Castellûn,[b] and Teresa J. Bandosz*[a] Composites of mesoporous synthetic carbon and the graphene phase were synthesized in aqueous suspension by employing dispersive interactions of both phases. The resulting carbonbased materials were further heat treated in air at 350 8C. The composites and their components were characterized by using adsorption of nitrogen, potentiometric titration, thermal analysis–mass spectrometry, X-ray photoelectron spectroscopy, SEM, high-resolution TEM, and XRD. Then, they were tested as supercapacitors in three-electrode cells and under visible-light irradiation. The composites and the initial carbon share exactly
the same pore-size distributions, but they exhibit significant differences in their surface chemistry, wettability, and conductivity. This allowed us to determine the extent of their effects on their capacitive/pseudocapacitive performance. The results showed that features other than the textural properties can increase the capacitive performance by more than 100 %. The synergistic properties of the composites and their sulfur functional group related photoactivity were linked to chemical interactions between the nanoporous carbon phase and graphite oxide during the formation of the composite.
Introduction For energy storage, two types of materials have been considered: carbon-based supercapacitors[1–6] and metal-oxide-based redox-type capacitors.[7, 8] If doped with N and S, the former materials can reach specific capacitance values of 450 F g¢1 in 1 m H2SO4.[9] Moreover, a high discharge capacitance in aqueous electrolyte was recorded for a polyaniline/mesoporous carbon composite (900 F g¢1) at a current density of 0.5 A g¢1.[10] The formation of an electrical double layer of ions on the surface of porous carbons has been identified as the main mechanism of charge storage.[2–5, 11–13] This electric double-layer capacitance (EDLC) is enhanced if the small pores,[11–14] similar in size to the electrolyte ions, are present in a high volume[13, 14] that is fully utilized by the electrolyte ions. That utilization of the pore space is affected by the wettability of the carbon,[9, 13–16] the direct current (DC) conductivity,[14, 16–19] and other factors such as surface charge delocalization,[3, 11] and even photoactivity.[9, 20] The right combination of important surface features, even with a small volume of pores similar in size to the electrolyte ions, can significantly enhance the capacitive performance of carbons.[3, 13, 14]
[a] Dr. M. Seredych, Prof. T. J. Bandosz The City College of New York Department of Chemistry 160 Convent Avenue, New York, NY 10031 (USA) E-mail: [email protected] [b] Prof. E. Rodrguez-Castellûn Departamento de Qumica Inorgnica Universidad de Mlaga Mlaga 29071 (Spain) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500156.
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Besides the EDLC mechanism, pseudocapacitance originating from Faradaic reactions on the functional groups located in the pore system of the carbon can significantly contribute to the capacitive performance.[5, 9, 10, 21–26] It has been demonstrated that certain oxygen, nitrogen, and sulfur functionalities enhance the capacitance through redox reactions. These groups include quinone/hydroquinones,[5] pyrrole and pyridine nitrogen atoms,[21, 22] and sulfones and sulfoxides.[9, 25, 26] Some of other functional groups of a carbon surface such as quaternary N atoms and pyridine N-oxides can also promote electron transfer through the carbon matrix.[22] Other heteroatom arrangements such as phosphorus groups can increase the stability of supercapacitors at high operation voltages in acidic electrolytes.[27] Given that these functional groups are located in pores larger than 1 nm, the mesopores of carbons in addition to the micropores are engaged in the energy-storage process.[24] Moreover, the doping of carbon matrices with nitrogen or sulfur affects the charge on the carbon atoms, because the electronegativities of sulfur and nitrogen are different than that of carbon,[28, 29] and this makes the pore walls attract more ions to the electric double layer.[24–26] These sulfur and nitrogen functionalities have been recently found to be photoactive[9, 20] and contribute to the charge-storage mechanism on nanoporous carbons.[9, 21, 22, 24–26] The objective of this paper is to provide an introduction to new mesoporous carbon/graphite oxide composites and to evaluate their performance as supercapacitors. The original intention of the addition of graphite oxide was to increase DC conductivity. Even though dispersive interactions of two carbonaceous phases are involved in building the composites, the data obtained showed that their surface chemistry plays a crucial role not only determining the final properties of the com-
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Full Papers posites but also in their electrochemical behavior. Given that, as a result of our synthetic procedure, two materials with exactly the same porosities were obtained with marked differences in conductivity, wettability, photoactivity, and surface chemistry, the effect of the last four features on the capacitive performance could be clearly differentiated.
Table 1. Parameters of the porous structures calculated from nitrogen adsorption measurements by using the 2 D-NLDFT model, the amounts of water adsorbed, and the conductivity (s) of the samples.[a] Sample
SBET SNLDFT [m2 g¢1] [m2 g¢1] total
V of pores [cm3 g¢1] Vmic/Vt H2O s mesopores < 0.7 nm < 1 nm micropores [wt %] [S m¢1]
CP CP-AO CGO CGO-AO GO-C[a] GO[b]
836 1128 450 853 350 6
0.489 0.585 0.232 0.503 1.339 –
712 1042 352 800 307 –
0.737 0.939 0.431 0.766 1.398 –
0.106 0.177 0.038 0.129 0.039 –
0.248 0.354 0.199 0.263 0.059 –
0.34 0.38 0.46 0.34 0.04 –
17.5 28.4 32.2 36.9 – 39.8
29.8 0.30 23.2 11.6 0.22 1.1 Õ 10¢3
[a] GO exposed to heating at 350 8C in air for 3 h. [b] GO is nonporous.
Results and Discussion Given that the synthesis method used in this work was original and used for the first time, before the capacitive performance was tested the surfaces of the composites were evaluated in detail. The measured nitrogen adsorption isotherms and poresize distributions calculated by using two-dimensional nonlocal density functional theory (2 D-NLDFT)[30] are presented in Figure 1. Whereas in the carbon/graphite oxide (CGO) sample
Figure 1. a) N2 adsorption isotherms at ¢196 8C and b) pore-size distribution for the materials studied (GO-C: GO exposed to heating at 350 8C in air for 3 h).
the nitrogen uptake significantly decreased relative to that of the parent porous synthetic carbon, after activation, the isotherms of the polymer-derived carbon (CP) and the hot-airtreated composite were almost identical leading to very similar pore-size distributions. Both materials, besides the small microChemSusChem 2015, 8, 1955 – 1965
0.070 0.118 0.016 0.089 0.000 –
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pores with sizes of approximately 0.6 and 1 nm, also have a marked volume of the mesopores that is smaller than 6 nm. Air treatment of CP to provide CP-AO (AO: air oxidized/activated) also significantly increased the surface area through the development of both micropores and mesopores. The parameters of the porous structure collected in Table 1 clearly show the similarity between the CP and CGO-AO samples. The only visible difference is that in the composite there is almost a 20 % higher volume of pores that are less than 0.7 nm in size than in the nanoporous carbon. Given that the surface area of the air-treated composite is higher than the hypothetical one calculated by assuming a physical mixture of the components, a synergy during the formation of the composite affects its porosity. The formation of the composite also significantly affected the surface chemistry. Owing to the high acidity of GO (pH 3.15),[31] the CGO sample is much more acidic than the carbon itself. This is visible in the marked proton release in the whole titration window (Figure 2 a). After hot-air treatment, the number of groups dissociating at pH > 7 significantly decreased. This treatment visibly increased the acidity of the CP carbon. The pKa distributions of the species detected on the surfaces of our materials are presented in Figure 2 b. The composites show a high degree of surface chemistry heterogeneity. Interestingly, hot-air treatment of CGO caused only a decrease in the number of basic groups with pKa > 7, whereas the acidic groups remained almost intact. Details on the numbers of groups in each pKa category are presented in Table S1 (Supporting Information). On this basis, the total number of groups on the surfaces of CP, CP-AO, CGO, and CGO-AO were 0.488, 1.080, 1.254, and 0.858 mmol g¢1, respectively. Notably, the number of groups of GO was 2.705 mmol g¢1, and after heating at 350 8C this number decreased to 0.534 mmol g¢1.[31] Taking into account that 50 wt % GO is used for the formation of the composites, these results suggest that dispersive interactions with the carbon surface cause reduction of the GO phase and air treatment enhances this process. The FTIR results presented in Figure 2 c confirm the trend in the surface chemistry of our materials upon the formation of the composite and its subsequent hot-air treatment. For graphite oxide, the bands at n˜ = 1720, 1620, 1360, 1225, 1040, and 960 cm¢1 represent carboxylic acids, the O¢H groups in water and/or cyclic ethers, hydroxyl groups, sulfonic acids and/
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Figure 2. Surface characterization: a) Proton binding curves, b) pKa distribution, and c) FTIR spectra of the materials studied.
or epoxides, C¢O vibration, and epoxy/peroxide groups, respectively.[32, 33] The carbon exhibits bands only at n˜ = 1720, 1560, and 1040 cm¢1 linked to C=O and C=C vibrations of the aromatic ring conjugated to carbonyl and carboxylate groups and C¢O vibrations, respectively.[34] In the FTIR spectrum for CGO, the same species as those on the carbon itself were detected, although the bands exhibit enhanced intensities. A visible feature is the presence of some ¢OH groups at n˜ = 3380 and 1360 cm¢1. Supporting the reduction of GO is the absence of the majority of its bands representing surface oxygen bonds. The FTIR spectra of the CP-AO and CGO-AO samples seem to be simpler than that of the CGO composite; however, the intensity of the bands at n˜ = 1720 and 1560 cm¢1 is greater than that for the latter sample. Thermal analysis results presented in Figure 3 show reduction of the epoxy groups of the GO component. The weight loss associated with the decomposition is less than 50 % of that in GO, and the epoxy groups of GO remaining in the composite are slightly more thermally stable than those in the parent GO. Differential thermal analysis (DTA) curves (Figure 3 b) for GO and CGO support this process by showing exothermic peaks at approximately T = 200 8C with slightly shifted maxima (T = 195 8C for GO and T = 220 8C for CGO). The differential thermogravimetric (DTG) curve for CP shows the presence of mainly carboxyl groups decomposing as a broad peak between T = 200 and 450 8C with well-defined maxima at T = 220 and 290 8C, which represent acidic species such as carboxylic acids.[35] We do not interpret the weight loss above the carbonization temperature of this material. On the surface of CPAO and CGO-AO, mainly high-temperature decomposing groups detected as a broad peak in the DTG curves between T = 400 and 800 8C are revealed. They might represent phenolic, carbonyl, anhydride, lactone, ether, and quinone functionalities.[35] The m/z thermal profiles collected in Figure S1 show the heterogeneity of the low-temperature decomposing groups containing oxygen (m/z = 16 and 17) and the peak at ChemSusChem 2015, 8, 1955 – 1965
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Figure 3. Surface characterization: a) Differential thermogravimetric curves and b) differential thermal analysis curves for the materials studied.
m/z = 44 in the thermal profile is broader and more intense than that at m/z = 28 for the air-treated composite, which indicates more CO2 is released than CO. This allows us to conclude that the majority of the groups on the surface of this sample are carboxyl, anhydride, and lactone groups.[35] The thermal profiles at m/z = 48 and 64 representing sulfur-containing groups in the form of SO and SO2 support the presence of sul-
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Full Papers fonic acids and sulfones in GO,[32, 33] CP, and CGO (low-temperature peak at T = 220 8C[24, 25]), whereas for the heat-treated composite only a very low intensity broad signal for SO and SO2 is detected with a maximum at T = 400 8C. This represents more thermally stable C¢S configurations.[36] Interestingly for the CPAO carbon, the m/z signal representing sulfur functionalities has a very low intensity, which suggests decomposition of sulfur-containing moieties during heat treatment. Apparently, the formation of the composite stabilizes these species. The change in the chemistry detected by potentiometric titration, FTIR spectroscopy, and thermal analysis–mass spectrometry (TA–MS) is supported by the X-ray photoelectron spectroscopy (XPS) results. The atomic concentration of the elements and deconvolution of the C 1s, O 1s, and S 2p core energy level spectra are presented in Table 2 and Figure 4. The contributions of each detected species are summarized in
Table 2. Atomic concentration of elements on the surfaces of the materials studied, as determined by XPS. Sample [a]
GO CP CP-AO CGO CGO-AO
Content [at %] O
C 65.9 93.2 86.1 79.9 85.6
33.2 4.9 13.6 18.6 13.5
S 0.9 1.9 0.3 1.5 0.9
[a] XPS results for GO taken from Ref. [32]
Table 3. As expected, the addition of GO increased the content of oxygen in the composite. Interestingly, even though the carbon and GO contain sulfur on their surfaces, its content in the composite is less than that expected if a physical mixture is assumed. This could be the result of uneven distribution of
Table 3. Deconvolution results of the C 1s, O 1s, and S 2p core energy levels. Energy [eV]
Bond assignment
GO[a]
C 1s 284.8 286.2 287.4 288.9 290.0
C¢(C, S) (graphitic carbon) C¢O, (phenolic, alcoholic, etheric) C=O (carbonyl or quinone) O¢C=O (carboxyl or ester) carbonate, occluded CO, p electrons in aromatic ring
57.1 38.5 – 4.4 –
O 1s 531.5 533.3 535.3
O=C/O=S (carboxyl/carbonyl or sulfoxides/sulfones) O¢C/O¢S (phenol/epoxy or thioethers/sulfonic) ¢O¢ (carboxyl, water or chemisorbed oxygen species)
10.3 89.7 –
S 2p3/2 164.1 165.1 167.5 168.6 169.6
R¢S¢S¢ (bisulfides configuration or in thiols) C¢S¢ (thiols) R2¢S=O/R¢SO2¢R (sulfoxides, sulfones) R¢SO3H (sulfonic acids)/SO42¢ RO2¢S¢S¢R or R¢SO3H (sulfonic acids)
– – – 100 –
[a] XPS results for GO taken from Ref. [32]
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sulfur on the surface or the result of the reduction of some sulfur-containing groups, likely sulfonic acids, with the carbon phase during formation of the composite, and this would lead to the release of sulfur groups as SO2. Treatment at 350 8C further decreased the amount of sulfur on the surface, and as the TA–MS results suggest, the sulfonic groups were removed. Given that GO contains some residual SO42¢ ions,[32] these species could also be removed by heating at 350 8C. The content of oxygen also decreased, which we associate with the removal of oxysulfur species and some carboxylic acids. On the surface of CP treated at 350 8C, the content of sulfur detected by XPS decreased significantly, which is in agreement with the TA–MS analyses, and much more oxygen is attached to the surface, which enhances its surface acidity. Whereas CP-AO and CGO-AO have similar carbon and oxygen contents, the formation of composites definitely stabilizes/preserves the sulfur content, as suggested above on the basis of the TA–MS results. Details on the surface chemistry obtained from deconvolution of the C 1s, O 1s, and S 2p core energy level spectra (Table 3 and Figure 4) indicate an increase in the acidity after the addition of GO, which is demonstrated by an increase in the contribution of all species having C¢O bonds. Interestingly, on the basis of the deconvolution of the C 1s core energy spectrum, the CGO composite has a marked contribution of carbonyl/quinone groups (at a binding energy of 287.4 eV), but the O 1s spectrum shows the predominance of oxygen in O¢C/ O¢S configurations (at a binding energy of 533.3 eV), which we link to the presence of epoxy and sulfonic groups. Their contribution significantly decreased after hot-air treatment of the composites. This treatment also resulted in an increase in the contribution of the species at a binding energy of 531.5 eV (carboxyl/carbonyl groups and/or sulfoxides/sulfones). A marked decrease in the contribution of sulfonic acids is also noticed in the S 2p spectrum (binding energies of 168.6 and 169.6 eV) of CGO-AO. These results fully support the changes in chemistry indicated by the results of other surface characterization methods and discussed above. Speciation of sulfur in the CGO composite also shows that a significant number of sulfonic CP CP-AO CGO CGO-AO acid groups of GO was reduced by the surface of the carbon 80.5 70.3 54.0 72.5 component during formation of 10.8 16.1 12.1 14.4 the composite. The CP-AO 4.4 5.9 24.7 6.25 2.6 5.1 5.8 4.68 sample has its sulfur (small con1.7 2.6 3.5 2.21 tent) mainly in the form of reduced species. The content of surface ele42.4 38.1 13.2 43.6 47.1 58.9 78.9 56.4 ments derived from energy-dis10.5 3.0 7.9 – persive X-ray spectroscopy (EDX) analysis is included in Table S2. Even though in the case of 73.9 76.8 51.3 77.6 – 16.8 – – sulfur the percentage of ele10.6 – 9.7 4.9 ments detected on the surface is 6.4 6.4 32.2 16.6 higher than that obtained from 9.1 – 6.8 0.9 XPS analysis, the trend in the changes is the same. The ele-
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Figure 4. C 1s, O 1s, and S 2p core energy level spectra for the materials studied.
ment maps presented in Figure S2 show a high distribution of sulfur on the surface of the carbon and its composites. ChemSusChem 2015, 8, 1955 – 1965
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Changes in the structure of the carbonaceous material upon formation of the composite are seen in Figure 5, in which the
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Figure 5. X-ray diffraction patterns of the materials studied.
X-ray diffraction patterns are collected. Although for CGO a d002 diffraction peak representing GO (0.82 nm) is clearly visible, it cannot be detected upon heat treatment, which suggests its exfoliation. In the case of CGO-AO, a well-defined peak at 2 q = 25.078 of the graphene component is seen, and it represents an interlayer spacing of 0.35 nm. This indicates that hot-air oxidation of the CGO composite resulted in a very heterogeneous structure, in which two phases of graphene and amorphous carbons are detected. Such a process is, in fact, expected,[37] and it might be associated with the observed shift in the exothermic peaks in the DTA curve assigned to the decomposition of the epoxy groups of GO (Figure 3 b and Figure S3). This process might contribute to the porosity changes (Figure 1 and Table 1). The textural features of the materials studied are presented in Figures 6 and 7 as SEM (SEM images at a high magnification of 300.00 kX also included in Figure S4) and high-resolution (HR)TEM images. The carbon from the polymer shows an interesting texture consisting of an outside thin “skin” covering the porous texture (Figure 6). That “skin” almost resembles wrinkled graphene layers.[38–40] The composite shows mainly aggregated GO layers, which might be attached through dispersive forces to the outer particles of carbon. After thermal treatment, exfoliation of these aggregates is seen. In the HRTEM images, both the organized layered structure of GO and the totally amorphous structure of CP carbon are seen (Figure 7). These GO layers are also visible on the surface of the CGO composite. On the other hand, interesting features are detected in the case of CGO-AO, for which “rings” of a layered structure of GO are detected within the amorphous phase of the carbon component. Visual analysis of the units resembling the graphite structure in CGO-AO indicates an interlayer distance of 0.35 nm, which is consistent with the XRD analysis. On the basis of the results obtained, a mechanism for the formation of the composite is proposed. During polymer carbonization in the presence of ZnCl2, which is a dehydrating agent/pore former,[41, 42] the micro-/mesoporous structure is developed in the carbon phase. Apparently, pores smaller than 10 nm are too small to accommodate dispersed GO layers. ChemSusChem 2015, 8, 1955 – 1965
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Figure 6. SEM images.
Therefore, those GO units were attached through dispersive interactions to the outside surface of the carbon particles. Specific chemical interactions through the involvement of functional groups cannot be ruled out, as the results suggest partial reduction of the GO phase. This process causes an apparent decrease in porosity because of the addition of a 50 % nonporous phase (dilution effect) and because the entrances of some pore at the surface of the granules are blocked by the GO phase. Heat treatment at 350 8C in air causes exfoliation of GO and further, but still partial, reduction of its surface. Moreover, some thermally unstable functional groups from the carbon phase decompose. Interestingly, that process leads to a pore texture that is very similar to that of the CP carbon with the development of an additional volume of small pores as a result of “air activation”. Apparently, chemistry and spatial constrains result in a synergistic effect on the porosity, as the surface area of the composite is higher than the hypothetical one consisting of a physical mixture of both composite components heated at 350 8C. The cyclic voltammetry (CV) curves of our materials measured after equilibration and with broadening of the potential window from ¢0.2 to 0.2–0.8 V are presented in Figure 8 a–d. Only the first cycle for CGO shows a marked reduction in functional groups. This feature is not visible in the CV curve of CGO-AO, owing to apparent removal of these groups during heat treatment. Then, all the CV curves show broad loops with
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Figure 7. HRTEM images.
well-marked oxidation/reduction humps at approximately 0.3 V versus Ag/AgCl, which represent quinone–hydroquinone redox transitions[5] and small humps at approximately 0.08 V versus Ag/AgCl. Given that the latter humps are not visible if the experiments were run in the dark and under visible light with oxygen removed from the electrolyte (Figure 8 e–h), we link them to the oxygen reduction reaction (ORR).[43] Notably, even though the effects are not very pronounced owing to the predominant capacitive behavior, the visible light and oxygen in the systems increase the intensity of the humps, which suggests photoactivity. The effect is most pronounced for the composite (i.e., CGO) with the lowest porosity. The CV curve for the air-oxidized carbon (i.e., CP-AO) shows some chargetransfer limitations, as demonstrated in its deviation from the rectangular shape. Interestingly, no clear response to visible light in the ORR was found. The lack of this feature in the CV curves can be linked to the very small sulfur content on the surface of this carbon. Sulfur species have been implicated in the enhancement of the catalytic activity of carbon for the ORR.[28, 37] The photoactivity is clearly seen in Figure 9 a–d in the current generated under visible-light exposure at a potential of 0.08 V. The photocurrent density is the highest for CGO and ChemSusChem 2015, 8, 1955 – 1965
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the smallest for the CP and CP-AO samples (Figure 9 e). We link this to the presence of GO conducting units in the composites, which help in charge separation, and to the presence of a significant amount of sulfur in sulfonic acid chromophore like moieties, which upon excitation by light can give electrons to oxygen.[9, 44] The highest photocurrent generation was found for the sample with the highest contribution of these species. Not without significance is also the presence of a marked amount of sulfur in the form of bisulfide and thiol. Sulfur incorporated into the carbon matrix has been found to create catalytic sites for oxygen reduction[28, 37, 45, 46] and to help increase the hydrophobicity in small pores, into which oxygen can be adsorbed.[46] A comparison of the measured capacitance is presented in Figure 9 f. Whereas only a small decrease in the capacitance for CGO is found relative to that for the CP sample, CGO-AO exhibits a capacitance that is almost two times higher than of the other two samples tested. The similar capacitance of CP and CGO, in spite of the fact that the volume of the pores is less than 0.7 nm in the latter (most active for EDLC[4, 11–14]), is only 25 % of the capacitance of the CP sample, which suggests that features other than a specific porosity, introduced by GO, determine the capacitive performance. On the basis of earlier studies,[5, 9, 13–19, 25, 26] these features include the conductivity of GO, higher wettability owing to the abundance of oxygen from GO, and the presence of groups participating in redox reactions such as quinones,[5] sulfoxides, and sulfones.[9, 25, 26] Whereas the first two factors increase the degree of utilization of the existing pore space,[13, 14] the latter factor contributes to the pseudocapacitive performance. Indeed, the results collected in Table 1 show that the composite samples have the highest water affinity. The conductivity (Table 1) should also play a role, and the presence of GO in CGO-AO significantly enhances its conductivity relative to that of the CP-AO carbon. The capacitive performance is even more interesting for the CGO-AO sample, especially if its electrochemical capacitance is compared to that of the CP carbon. As described above, these two samples have almost the same pore-size distributions. In this case, the physical mechanism of EDLC is expected to be the same for both materials provided that other factors do not play a role. Apparently, this is not a case, as the specific capacitance of the composite is two times larger than that of the CP carbon. This is a clear indication of the marked effect of surface chemistry, wettability, and conductivity. As seen from the results discussed above, CGO-AO has almost three times more oxygen (atomic concentration %) on the surface and double the amount of groups decomposing between pH 3 and 11 than the CP carbon. The XPS results indicate that CGO-AO also has a higher contribution of quinones, which are known for their pseudocapacitive performance.[5] A much higher sulfonic acid content than that in CP increases the wettability of the composite by providing high hydrophilicity (Table 1). Comparison of the capacitance values for CGO-AO and CP shows that factors other than the texture can increase the capacitive performance by even more than 100 %. The impedance spectroscopy results for all samples exhibit an arc at high frequency and that arc/semicircle is most pro-
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Full Papers Conclusions The results presented in this paper showed for the first time that mixing nanoporous carbon possessing rich surface chemistry with graphite oxide (GO) results in the formation of a new composite with unique properties. This composite shows synergistic effects in both porosity and surface chemistry. Whereas the latter is affected by reactions between both carbonaceous phases, for the former, spatial constrains and chemistry of GO exfoliation also play significant roles. The presence of the reduced GO phase increased the conductivity and also stabilized the presence of sulfur in the composites. Sulfur is important for the generation of photocurrent (enhancing the oxygen reduction reaction and capacitance), and in specific configurations (sulfones), it enhances the surface and thus increases the accessibility of ions to the small pores and the utilization of their species. Given that two of the samples obtained had the same porosity but different chemistry, the direct current conductivity and the wettability, the effect of features other than texture could be evaluated.
Experimental Section Materials
Figure 8. a–d) CV curves recorded in different potential windows and e–h) CV curves measured at a scan rate of 5 mV s¢1 in the dark (D) and under visible-light (VL) exposure.
nounced for the CP sample (Figure S5). On the other hand, it almost does not exist for CGO. The presence of these arcs might be linked to limitations in charge transfer to the small pores, which is affected not only by the sizes of the pores but also by the density of surface groups and the conductivity of the carbon matrix. The internal resistance is similar for all samples (0.67 W for CP, 0.87 W for CP-AO, 0.68 W for CGO, and 0.85 W for CGO-AO). ChemSusChem 2015, 8, 1955 – 1965
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The formula of the organic polymer used as a precursor for the preparation of the nanoporous carbon material is as follows {BES.DM = bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]sulfide, GMA = glycidyl methacrylate}:
The properties of this particular polymer were described in detail by Podkoscielna.[47] Reagent-grade ZnCl2 powder (> 98 %) from Aldrich was used as an activating agent. In the first step, the polymer was added to ZnCl2 dissolved in distilled water (100 mL) in a weight ratio of 1:2. The obtained mixture was dried at 120 8C. Then, the material was carbonized in a horizontal furnace under a flow of nitrogen (100 mL min¢1) at a heating rate of 10 8C min¢1 up to 800 8C with
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in a ratio of 50:50 wt %. GO was obtained by using Hummers method.[48] First, GO was well dispersed in water by sonication, and then the ground carbon powder was added to the GO suspension. The mixture was sonicated for 1 h more and then stirred overnight. Afterward, the carbon/GO suspension was filtered without washing and was dried at 120 8C. The composite obtained in this way is referred to as CGO. Then, the composite (i.e., CGO) was heated in air at 350 8C for 3 h to modify its structure[9, 49, 50] and chemistry.[9, 49, 50] This material is referred to as CGOAO. The same treatment was applied to the CP sample; it is referred to as CP-AO.
Methods Electrochemical measurements: Prior to electrochemical measurements, to avoid polarization of the working electrode the electrodes were wet in sulfuric acid (1 m) for 24 h. Then, the open circuit potential . Cyclic voltammetry (CV) experiments were run in a very narrow potential window (from 0.2 to ¢0.2 V) to confirm a lack of changes in the CV curve with an increase in the number of cycles. Photoelectrochemical measurements were performed in 1 m H2SO4 in a three-electrode cell with a Pt wire counter electrode and a saturated Ag/AgCl (3 m KCl) reference electrode. For the preparaFigure 9. a–d) Examples of current signals in the dark and under solar-light irradiation at + 0.08 V versus Ag/AgCl tion of the electrode, a slurry of for the carbon and composite materials; e) photocurrent response at various bias potentials; f) capacitance values the carbon or carbon/graphene calculated by CV in the dark and under solar-light irradiation (VL). oxide composite, polyvinylidene fluoride, and a carbon black conductive additive (ratio 80:10:10) in N-methyl-2-pyrrolidone was coated on a Ti foil collector with an a holding time of 40 min. The polymer-derived carbon sample was active area of 1 cm2. The electrodes were dried in air at 120 8C. A extensively washed with 1 m HCl to remove Zn from the carbon total mass of the active electrode material was approximately network and was then washed with distilled water in a Soxhlet ap7 mg. A solar simulator (Solar Light Co., INC, XPS-150TM) with paratus to remove all residual soluble impurities until a constant a 420 nm cut-off filter was used as the irradiation source. A VersapH was reached and no chloride ions were detected in the filtrate. STAT MC (Princeton Applied Research) electrochemical workstation The carbon obtained in this manner is referred to as CP. was employed to record the electrochemical behavior. The transiThe new carbonaceous composite was synthesized by mixing polyent photocurrent was obtained under a constant bias potential bemer-derived carbon with graphite oxide (GO) in aqueous solution tween 0.6 and ¢0.2 V versus Ag/AgCl under on/off illumination. ChemSusChem 2015, 8, 1955 – 1965
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Full Papers Dark current equilibrium at the applied potential was allowed before irradiation. Values of the specific capacitance [F g¢1] were estimated by cyclic voltammetry (scan rate of potential 5 mV s¢1). The specific capacitance (C) was calculated according to Equation (1): C¼
Q ðDE mÞ
ð1Þ
in which Q is the charge obtained after integrating the voltammogram, m is the total mass of the electrode, and DE is the potential window. Surface characterization: Sorption of nitrogen at ¢196 8C was performed by using an ASAP 2020 (Micromeritics, Surface Area and Porosity Analyzer) on the active materials/carbons. Before the experiments, samples were out-gassed at 120 8C to constant vacuum (0.13 Pa). The surface area (SBET), total pore volumes (Vt, from the last point of the isotherm at a relative pressure of 0.99), volume of the micropores (Vmic), volume of pores less than 0.7 and 1 nm (V < 0.7 nm and V < 1 nm), mesopore volumes (Vmeso), and pore-size distributions were calculated from the isotherms. The volume of the mesopores represents the difference between the total pore volume and the micropore volume. The surface area (SNLDFT), volume of pores, and pore-size distribution were calculated by using 2 D-NLDFT (www.NLDFT.com) by assuming heterogeneity of the pore sizes.[30] X-ray diffraction (XRD): XRD measurements were conducted on the active carbon components of the electrodes by using standard powder diffraction procedures analyzed by CuKa radiation (40 kV and 40 mA) generated in a Phillips X’Pert X-ray diffractometer. The scan rate used was 2.3 deg min¢1. X-ray photoelectron spectroscopy (XPS): XPS analysis was performed by using a Physical Electronics PHI 5700 spectrometer with non-monochromatic MgKa radiation (300 W, 15 kV, 1253.6 eV) for the analysis of the core level signals of C 1s, O 1s, and S 2p and with a multichannel detector. Spectra of powdered samples were recorded with constant pass energy values at 29.35 eV by using a 720 mm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV full width at half maximum at a binding energy of 84.0 eV. The spectrometer energy scale was calibrated by using Cu 2p3/2, Ag 3d5/2, and Au 4f7/2 photoelectron lines at binding energies of 932.7, 368.3, and 84.0 eV, respectively. The PHI ACCESS ESCA-V6.F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted by using Gauss–Lorentz curves to determine the binding energy of the different element core levels more accurately. The error in the binding energy was estimated to be approximately 0.1 eV. Fourier transform infrared (FTIR) spectroscopy: FTIR spectroscopy was performed on the active carbon components of the electrodes by using a Nicolet Magna-IR 830 spectrometer by using the attenuated total reflectance (ATR) method. The spectrum was generated and collected 64 times and corrected for background noise. The experiments were done on the powdered samples, without the addition of KBr.
were transformed into proton binding curves representing the total amount of protonated sites. From them, the pKa distributions and the numbers of groups represented by certain pKa values were calculated.[51, 52] Thermal analysis–mass spectrometry (TA–MS): Thermogravimetric (TG) curves were obtained by using a TA instrument thermal analyzer (SDT Q 600) that was connected to a gas analysis system (OMNI Star) mass spectrometer. The active carbon components were heated up to 1000 8C (10 8C min¢1) under constant helium flow (100 mL min¢1). From the TG curves, differential TG (DTG) curves were derived. The composition of gases was evaluated by MS, and gas evolution profiles as a function of temperature were obtained. High-resolution transmission electron microscopy (HRTEM): HRTEM was performed on the active carbon components of the electrodes with a JEOL 2100 LaB6 instrument operating at 200 kV. Analyses were performed after the carbon samples were resuspended in ethanol. Scanning electron microscopy (SEM): SEM images were performed at Zeiss Supra 55 VP. The accelerating voltage was 5.00 kV. Scanning was performed in situ on a sample powder without coating. Electron-dispersive X-ray spectroscopy (EDX) analysis was done at magnification 5 KX with an accelerating voltage 15.00 kV, and the contents of the elements on the surface were calculated. DC conductivity measurements: DC conductivity was measured by using the four-probe method on pellets with a composition of 90 wt % carbon-based materials and 10 wt % polytetrafluoroethylene as binder. The prepared composition was pressed by a Carver Press machine by applying 0.02 MPa pressure and disk-shaped well-packed pellets with diameter 8 mm were formed. The pellets were dried in oven for 12 h. The thickness of the pellets was measured by a spring micrometer. Conductivity measurements were performed by using a Keithley 2400 Multimeter. Water affinity evaluation: The changes in surface hydrophilicity were determined by measuring water adsorption affinity. The initial carbon-based samples were dried at 120 8C to constant mass and were then placed in a closed vessel with constant pressure of water vapor at ambient temperature. After 24 h, TA experiments were performed by using a TA instrument thermal analyzer (SDT Q 600). The weigh lost in nitrogen between 30 and 120 8C was assumed as an equivalent to the quantity of water adsorbed on the surface.
Acknowledgements The authors are grateful to Dr. Beata Podkos´cielna of Maria Curie-Sklodowska University (Lublin, Poland) for kindly providing the polymer sample. This research was partially funded by the Junta de Andaluca, Spain (P12-RNM-1565) and the European Fund for Economic and Regional Development (FEDER). Keywords: graphene · photoactivity supercapacitors · surface chemistry
Potentiometric titration: Potentiometric titration measurements were performed on the active carbon components of the electrodes with a 888 Titrando automatic titrator (Metrohm). Details of the experiments are presented in Ref. [22]. The experimental data ChemSusChem 2015, 8, 1955 – 1965
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[1] [2] [3] [4]
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porosity
·
A. Burke, Electrochim. Acta 2007, 53, 1083. A. G. Pandolfo, A. F. Hollenkamp, J. Power Sources 2006, 157, 11. P. Simon, Y. Gogotsi, Acc. Chem. Res. 2013, 46, 1094. G. Salitra, A. Soffer, L. Eliad, Y. Cohen, D. Aurbach, J. Electrochem. Soc. 2000, 147, 2486.
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers [5] H. A. Andreas, B. E. Conway, Electrochim. Acta 2006, 51, 6510. [6] H. Nishihara, T. Kyotani, Adv. Mater. 2012, 24, 4473. [7] M. B. Sassin, C. N. Chervin, D. R. Rolison, J. W. Long, Acc. Chem. Res. 2013, 46, 1062. [8] X. Zhao, B. M. Sanchez, P. J. Dobson, P. S. Grant, Nanoscale 2011, 3, 839. [9] M. Seredych, E. Rodriguez-Castellon, M. J. Biggs, W. Skinner, T. J. Bandosz, Carbon 2014, 78, 540. [10] Y. G. Wang, H. Q. Li, Y. Y. Xia, Adv. Mater. 2006, 18, 2619. [11] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P. L. Taberna, Science 2006, 313, 1760. [12] C. Largeot, C. Portet, J. Chmiola, P. L. Taberna, Y. Gogotsi, P. Simon, J. Am. Chem. Soc. 2008, 130, 2730. [13] M. Seredych, M. Koscinski, M. Sliwinska-Bartkowiak, T. J. Bandosz, ACS Sustainable Chem. Eng. 2013, 1, 1024. [14] M. Seredych, M. Koscinski, M. Sliwinska-Bartkowiak, T. J. Bandosz, J. Power Sources 2012, 220, 243. [15] D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu, T. J. Bandosz, Carbon 2010, 48, 1767. [16] H. Gûmez, M. K. Ram, F. Alvi, P. Villalba, E. Stafanakos, A. Kumar, J. Power Sources 2011, 196, 4102. [17] C. W. Huang, C. M. Chuang, J. M. Ting, H. Teng, J. Power Sources 2008, 183, 406. [18] K.-S. Kim, S.-J. Park, J. Electroanal. Chem. 2012, 673, 58. [19] Z. Tai, X. Yan, J. Lang, Q. Xue, J. Power Sources 2012, 199, 373. [20] K. Singh, M. Seredych, T. J. Bandosz, ChemElectroChem 2014, 1, 565. [21] D.-W. Wang, F. Li, L.-C. Yin, X. Lu, Z.-G. Chen, I. R. Gentle, G. Q. Lu, H.-M. Cheng, Chem. Eur. J. 2012, 18, 5345. [22] D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu, T. J. Bandosz, Adv. Funct. Mater. 2009, 19, 438. [23] H. Itoi, H. Nishihara, T. Kogure, T. Kyotani, J. Am. Chem. Soc. 2011, 133, 1165. [24] M. Seredych, T. J. Bandosz, J. Mater. Chem. A 2013, 1, 11717. [25] X. Zhao, Q. Zhang, C.-M. Chen, B. Zhang, S. Reiche, A. Wang, T. Zhang, R. Schlogl, D. S. Su, Nano Energy 2012, 1, 624. [26] M. Seredych, K. Singh, T. J. Bandosz, Electroanalysis 2014, 26, 109. [27] D. Hulicova-Jurcakova, A. M. Puziy, O. I. Poddubnaya, F. Suarez-Garca, J. M. D. Tascon, G. Q. Lu, J. Am. Chem. Soc. 2009, 131, 5026. [28] J. Liang, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2012, 51, 11496; Angew. Chem. 2012, 124, 11664.
ChemSusChem 2015, 8, 1955 – 1965
www.chemsuschem.org
[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]
W. Kicinski, M. Szala, M. Bystrzejewski, Carbon 2014, 68, 1. J. Jagiello, J. P. Olivier, Adsorption 2013, 19, 777. M. Seredych, T. J. Bandosz, J. Phys. Chem. C 2007, 111, 15596. C. Petit, M. Seredych, T. J. Bandosz, J. Mater. Chem. 2009, 19, 9176. C. Hontoria-Lucas, A. J. Lûpez-Peinado, J. de D. Lûpez-Gonzlez, M. L. Rojas-Cervantes, R. M. Martn-Aranda, Carbon 1995, 33, 1585. J. Zawadzki, M. Wisniewski, Carbon 2003, 41, 2257. J. L. Figueiredo, M. F. R. Pereira, M. M. A. Freitas, J. J. M. rf¼o, Carbon 1999, 37, 1379. H. C. Chin, Carbon 1981, 19, 175. M. Seredych, J.-C. Idrobo, T. J. Bandosz, J. Mater. Chem. A 2013, 1, 7059. H. P. Boehm, A. Clauss, G. O. Fischer, U. Hofmann, Z. Anorg. Allg. Chem. 1962, 316, 119. M. Seredych, J. A. Rossin, T. J. Bandosz, Carbon 2011, 49, 4392. J. Guo, J. R. Morris, Y. Ihm, C. I. Contescu, N. C. Gallego, G. Duscher, S. J. Pennycook, M. F. Chisholm, Small 2012, 8, 3283. M. Molina-Sabio, F. Rodrguez-Reinoso, Colloids Surf. A 2004, 241, 15. K. Kante, C. Nieto-Delgado, J. R. Rangel-Mendez, T. J. Bandosz, J. Hazard. Mater. 2012, 201 – 202, 141. D. Yu, Q. Zhang, L. Dai, J. Am. Chem. Soc. 2010, 132, 15127. J. E. Beecher, T. Durst, J. M. J. Fr¦chet, A. Godt, A. Pangborn, D. R. Robello, C. S. Willand, D. J. Williams, Adv. Mater. 1993, 5, 632. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 2012, 6, 205. M. Seredych, T. J. Bandosz, Carbon 2014, 66, 227. B. Podkoscielna, J. Appl. Polym. Sci. 2011, 120, 3020. W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339. T. J. Bandosz, E. Rodriguez-Castellon, J. M. Montenegro, M. Seredych, Carbon 2014, 77, 651. C. O. Ania, M. Seredych, E. Rodriguez-Castellon, T. J. Bandosz, Carbon 2014, 79, 432. J. Jagiello, Langmuir 1994, 10, 2778. J. Jagiello, T. J. Bandosz, J. A. Schwarz, Carbon 1994, 32, 1026.
Received: January 29, 2015 Revised: February 24, 2015 Published online on April 27, 2015
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Peculiar Properties of Mesoporous Synthetic Carbon/ Graphene Phase Composites and their Effect on Supercapacitive Performance Mykola Seredych,[a] Enrique Rodrguez-Castellûn,[b] and Teresa J. Bandosz*[a] Composites of mesoporous synthetic carbon and the graphene phase were synthesized in aqueous suspension by employing dispersive interactions of both phases. The resulting carbonbased materials were further heat treated in air at 350 8C. The composites and their components were characterized by using adsorption of nitrogen, potentiometric titration, thermal analysis–mass spectrometry, X-ray photoelectron spectroscopy, SEM, high-resolution TEM, and XRD. Then, they were tested as supercapacitors in three-electrode cells and under visible-light irradiation. The composites and the initial carbon share exactly
the same pore-size distributions, but they exhibit significant differences in their surface chemistry, wettability, and conductivity. This allowed us to determine the extent of their effects on their capacitive/pseudocapacitive performance. The results showed that features other than the textural properties can increase the capacitive performance by more than 100 %. The synergistic properties of the composites and their sulfur functional group related photoactivity were linked to chemical interactions between the nanoporous carbon phase and graphite oxide during the formation of the composite.
Introduction For energy storage, two types of materials have been considered: carbon-based supercapacitors[1–6] and metal-oxide-based redox-type capacitors.[7, 8] If doped with N and S, the former materials can reach specific capacitance values of 450 F g¢1 in 1 m H2SO4.[9] Moreover, a high discharge capacitance in aqueous electrolyte was recorded for a polyaniline/mesoporous carbon composite (900 F g¢1) at a current density of 0.5 A g¢1.[10] The formation of an electrical double layer of ions on the surface of porous carbons has been identified as the main mechanism of charge storage.[2–5, 11–13] This electric double-layer capacitance (EDLC) is enhanced if the small pores,[11–14] similar in size to the electrolyte ions, are present in a high volume[13, 14] that is fully utilized by the electrolyte ions. That utilization of the pore space is affected by the wettability of the carbon,[9, 13–16] the direct current (DC) conductivity,[14, 16–19] and other factors such as surface charge delocalization,[3, 11] and even photoactivity.[9, 20] The right combination of important surface features, even with a small volume of pores similar in size to the electrolyte ions, can significantly enhance the capacitive performance of carbons.[3, 13, 14]
[a] Dr. M. Seredych, Prof. T. J. Bandosz The City College of New York Department of Chemistry 160 Convent Avenue, New York, NY 10031 (USA) E-mail: [email protected] [b] Prof. E. Rodrguez-Castellûn Departamento de Qumica Inorgnica Universidad de Mlaga Mlaga 29071 (Spain) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500156.
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Besides the EDLC mechanism, pseudocapacitance originating from Faradaic reactions on the functional groups located in the pore system of the carbon can significantly contribute to the capacitive performance.[5, 9, 10, 21–26] It has been demonstrated that certain oxygen, nitrogen, and sulfur functionalities enhance the capacitance through redox reactions. These groups include quinone/hydroquinones,[5] pyrrole and pyridine nitrogen atoms,[21, 22] and sulfones and sulfoxides.[9, 25, 26] Some of other functional groups of a carbon surface such as quaternary N atoms and pyridine N-oxides can also promote electron transfer through the carbon matrix.[22] Other heteroatom arrangements such as phosphorus groups can increase the stability of supercapacitors at high operation voltages in acidic electrolytes.[27] Given that these functional groups are located in pores larger than 1 nm, the mesopores of carbons in addition to the micropores are engaged in the energy-storage process.[24] Moreover, the doping of carbon matrices with nitrogen or sulfur affects the charge on the carbon atoms, because the electronegativities of sulfur and nitrogen are different than that of carbon,[28, 29] and this makes the pore walls attract more ions to the electric double layer.[24–26] These sulfur and nitrogen functionalities have been recently found to be photoactive[9, 20] and contribute to the charge-storage mechanism on nanoporous carbons.[9, 21, 22, 24–26] The objective of this paper is to provide an introduction to new mesoporous carbon/graphite oxide composites and to evaluate their performance as supercapacitors. The original intention of the addition of graphite oxide was to increase DC conductivity. Even though dispersive interactions of two carbonaceous phases are involved in building the composites, the data obtained showed that their surface chemistry plays a crucial role not only determining the final properties of the com-
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Table 1. Parameters of the porous structures calculated from nitrogen adsorption measurements by using the 2 D-NLDFT model, the amounts of water adsorbed, and the conductivity (s) of the samples.[a] Sample
SBET SNLDFT [m2 g¢1] [m2 g¢1] total
V of pores [cm3 g¢1] Vmic/Vt H2O s mesopores < 0.7 nm < 1 nm micropores [wt %] [S m¢1]
CP CP-AO CGO CGO-AO GO-C[a] GO[b]
836 1128 450 853 350 6
0.489 0.585 0.232 0.503 1.339 –
712 1042 352 800 307 –
0.737 0.939 0.431 0.766 1.398 –
0.106 0.177 0.038 0.129 0.039 –
0.248 0.354 0.199 0.263 0.059 –
0.34 0.38 0.46 0.34 0.04 –
17.5 28.4 32.2 36.9 – 39.8
29.8 0.30 23.2 11.6 0.22 1.1 Õ 10¢3
[a] GO exposed to heating at 350 8C in air for 3 h. [b] GO is nonporous.
Results and Discussion Given that the synthesis method used in this work was original and used for the first time, before the capacitive performance was tested the surfaces of the composites were evaluated in detail. The measured nitrogen adsorption isotherms and poresize distributions calculated by using two-dimensional nonlocal density functional theory (2 D-NLDFT)[30] are presented in Figure 1. Whereas in the carbon/graphite oxide (CGO) sample
Figure 1. a) N2 adsorption isotherms at ¢196 8C and b) pore-size distribution for the materials studied (GO-C: GO exposed to heating at 350 8C in air for 3 h).
the nitrogen uptake significantly decreased relative to that of the parent porous synthetic carbon, after activation, the isotherms of the polymer-derived carbon (CP) and the hot-airtreated composite were almost identical leading to very similar pore-size distributions. Both materials, besides the small microChemSusChem 2015, 8, 1955 – 1965
0.070 0.118 0.016 0.089 0.000 –
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pores with sizes of approximately 0.6 and 1 nm, also have a marked volume of the mesopores that is smaller than 6 nm. Air treatment of CP to provide CP-AO (AO: air oxidized/activated) also significantly increased the surface area through the development of both micropores and mesopores. The parameters of the porous structure collected in Table 1 clearly show the similarity between the CP and CGO-AO samples. The only visible difference is that in the composite there is almost a 20 % higher volume of pores that are less than 0.7 nm in size than in the nanoporous carbon. Given that the surface area of the air-treated composite is higher than the hypothetical one calculated by assuming a physical mixture of the components, a synergy during the formation of the composite affects its porosity. The formation of the composite also significantly affected the surface chemistry. Owing to the high acidity of GO (pH 3.15),[31] the CGO sample is much more acidic than the carbon itself. This is visible in the marked proton release in the whole titration window (Figure 2 a). After hot-air treatment, the number of groups dissociating at pH > 7 significantly decreased. This treatment visibly increased the acidity of the CP carbon. The pKa distributions of the species detected on the surfaces of our materials are presented in Figure 2 b. The composites show a high degree of surface chemistry heterogeneity. Interestingly, hot-air treatment of CGO caused only a decrease in the number of basic groups with pKa > 7, whereas the acidic groups remained almost intact. Details on the numbers of groups in each pKa category are presented in Table S1 (Supporting Information). On this basis, the total number of groups on the surfaces of CP, CP-AO, CGO, and CGO-AO were 0.488, 1.080, 1.254, and 0.858 mmol g¢1, respectively. Notably, the number of groups of GO was 2.705 mmol g¢1, and after heating at 350 8C this number decreased to 0.534 mmol g¢1.[31] Taking into account that 50 wt % GO is used for the formation of the composites, these results suggest that dispersive interactions with the carbon surface cause reduction of the GO phase and air treatment enhances this process. The FTIR results presented in Figure 2 c confirm the trend in the surface chemistry of our materials upon the formation of the composite and its subsequent hot-air treatment. For graphite oxide, the bands at n˜ = 1720, 1620, 1360, 1225, 1040, and 960 cm¢1 represent carboxylic acids, the O¢H groups in water and/or cyclic ethers, hydroxyl groups, sulfonic acids and/
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Figure 2. Surface characterization: a) Proton binding curves, b) pKa distribution, and c) FTIR spectra of the materials studied.
or epoxides, C¢O vibration, and epoxy/peroxide groups, respectively.[32, 33] The carbon exhibits bands only at n˜ = 1720, 1560, and 1040 cm¢1 linked to C=O and C=C vibrations of the aromatic ring conjugated to carbonyl and carboxylate groups and C¢O vibrations, respectively.[34] In the FTIR spectrum for CGO, the same species as those on the carbon itself were detected, although the bands exhibit enhanced intensities. A visible feature is the presence of some ¢OH groups at n˜ = 3380 and 1360 cm¢1. Supporting the reduction of GO is the absence of the majority of its bands representing surface oxygen bonds. The FTIR spectra of the CP-AO and CGO-AO samples seem to be simpler than that of the CGO composite; however, the intensity of the bands at n˜ = 1720 and 1560 cm¢1 is greater than that for the latter sample. Thermal analysis results presented in Figure 3 show reduction of the epoxy groups of the GO component. The weight loss associated with the decomposition is less than 50 % of that in GO, and the epoxy groups of GO remaining in the composite are slightly more thermally stable than those in the parent GO. Differential thermal analysis (DTA) curves (Figure 3 b) for GO and CGO support this process by showing exothermic peaks at approximately T = 200 8C with slightly shifted maxima (T = 195 8C for GO and T = 220 8C for CGO). The differential thermogravimetric (DTG) curve for CP shows the presence of mainly carboxyl groups decomposing as a broad peak between T = 200 and 450 8C with well-defined maxima at T = 220 and 290 8C, which represent acidic species such as carboxylic acids.[35] We do not interpret the weight loss above the carbonization temperature of this material. On the surface of CPAO and CGO-AO, mainly high-temperature decomposing groups detected as a broad peak in the DTG curves between T = 400 and 800 8C are revealed. They might represent phenolic, carbonyl, anhydride, lactone, ether, and quinone functionalities.[35] The m/z thermal profiles collected in Figure S1 show the heterogeneity of the low-temperature decomposing groups containing oxygen (m/z = 16 and 17) and the peak at ChemSusChem 2015, 8, 1955 – 1965
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Figure 3. Surface characterization: a) Differential thermogravimetric curves and b) differential thermal analysis curves for the materials studied.
m/z = 44 in the thermal profile is broader and more intense than that at m/z = 28 for the air-treated composite, which indicates more CO2 is released than CO. This allows us to conclude that the majority of the groups on the surface of this sample are carboxyl, anhydride, and lactone groups.[35] The thermal profiles at m/z = 48 and 64 representing sulfur-containing groups in the form of SO and SO2 support the presence of sul-
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Full Papers fonic acids and sulfones in GO,[32, 33] CP, and CGO (low-temperature peak at T = 220 8C[24, 25]), whereas for the heat-treated composite only a very low intensity broad signal for SO and SO2 is detected with a maximum at T = 400 8C. This represents more thermally stable C¢S configurations.[36] Interestingly for the CPAO carbon, the m/z signal representing sulfur functionalities has a very low intensity, which suggests decomposition of sulfur-containing moieties during heat treatment. Apparently, the formation of the composite stabilizes these species. The change in the chemistry detected by potentiometric titration, FTIR spectroscopy, and thermal analysis–mass spectrometry (TA–MS) is supported by the X-ray photoelectron spectroscopy (XPS) results. The atomic concentration of the elements and deconvolution of the C 1s, O 1s, and S 2p core energy level spectra are presented in Table 2 and Figure 4. The contributions of each detected species are summarized in
Table 2. Atomic concentration of elements on the surfaces of the materials studied, as determined by XPS. Sample [a]
GO CP CP-AO CGO CGO-AO
Content [at %] O
C 65.9 93.2 86.1 79.9 85.6
33.2 4.9 13.6 18.6 13.5
S 0.9 1.9 0.3 1.5 0.9
[a] XPS results for GO taken from Ref. [32]
Table 3. As expected, the addition of GO increased the content of oxygen in the composite. Interestingly, even though the carbon and GO contain sulfur on their surfaces, its content in the composite is less than that expected if a physical mixture is assumed. This could be the result of uneven distribution of
Table 3. Deconvolution results of the C 1s, O 1s, and S 2p core energy levels. Energy [eV]
Bond assignment
GO[a]
C 1s 284.8 286.2 287.4 288.9 290.0
C¢(C, S) (graphitic carbon) C¢O, (phenolic, alcoholic, etheric) C=O (carbonyl or quinone) O¢C=O (carboxyl or ester) carbonate, occluded CO, p electrons in aromatic ring
57.1 38.5 – 4.4 –
O 1s 531.5 533.3 535.3
O=C/O=S (carboxyl/carbonyl or sulfoxides/sulfones) O¢C/O¢S (phenol/epoxy or thioethers/sulfonic) ¢O¢ (carboxyl, water or chemisorbed oxygen species)
10.3 89.7 –
S 2p3/2 164.1 165.1 167.5 168.6 169.6
R¢S¢S¢ (bisulfides configuration or in thiols) C¢S¢ (thiols) R2¢S=O/R¢SO2¢R (sulfoxides, sulfones) R¢SO3H (sulfonic acids)/SO42¢ RO2¢S¢S¢R or R¢SO3H (sulfonic acids)
– – – 100 –
[a] XPS results for GO taken from Ref. [32]
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sulfur on the surface or the result of the reduction of some sulfur-containing groups, likely sulfonic acids, with the carbon phase during formation of the composite, and this would lead to the release of sulfur groups as SO2. Treatment at 350 8C further decreased the amount of sulfur on the surface, and as the TA–MS results suggest, the sulfonic groups were removed. Given that GO contains some residual SO42¢ ions,[32] these species could also be removed by heating at 350 8C. The content of oxygen also decreased, which we associate with the removal of oxysulfur species and some carboxylic acids. On the surface of CP treated at 350 8C, the content of sulfur detected by XPS decreased significantly, which is in agreement with the TA–MS analyses, and much more oxygen is attached to the surface, which enhances its surface acidity. Whereas CP-AO and CGO-AO have similar carbon and oxygen contents, the formation of composites definitely stabilizes/preserves the sulfur content, as suggested above on the basis of the TA–MS results. Details on the surface chemistry obtained from deconvolution of the C 1s, O 1s, and S 2p core energy level spectra (Table 3 and Figure 4) indicate an increase in the acidity after the addition of GO, which is demonstrated by an increase in the contribution of all species having C¢O bonds. Interestingly, on the basis of the deconvolution of the C 1s core energy spectrum, the CGO composite has a marked contribution of carbonyl/quinone groups (at a binding energy of 287.4 eV), but the O 1s spectrum shows the predominance of oxygen in O¢C/ O¢S configurations (at a binding energy of 533.3 eV), which we link to the presence of epoxy and sulfonic groups. Their contribution significantly decreased after hot-air treatment of the composites. This treatment also resulted in an increase in the contribution of the species at a binding energy of 531.5 eV (carboxyl/carbonyl groups and/or sulfoxides/sulfones). A marked decrease in the contribution of sulfonic acids is also noticed in the S 2p spectrum (binding energies of 168.6 and 169.6 eV) of CGO-AO. These results fully support the changes in chemistry indicated by the results of other surface characterization methods and discussed above. Speciation of sulfur in the CGO composite also shows that a significant number of sulfonic CP CP-AO CGO CGO-AO acid groups of GO was reduced by the surface of the carbon 80.5 70.3 54.0 72.5 component during formation of 10.8 16.1 12.1 14.4 the composite. The CP-AO 4.4 5.9 24.7 6.25 2.6 5.1 5.8 4.68 sample has its sulfur (small con1.7 2.6 3.5 2.21 tent) mainly in the form of reduced species. The content of surface ele42.4 38.1 13.2 43.6 47.1 58.9 78.9 56.4 ments derived from energy-dis10.5 3.0 7.9 – persive X-ray spectroscopy (EDX) analysis is included in Table S2. Even though in the case of 73.9 76.8 51.3 77.6 – 16.8 – – sulfur the percentage of ele10.6 – 9.7 4.9 ments detected on the surface is 6.4 6.4 32.2 16.6 higher than that obtained from 9.1 – 6.8 0.9 XPS analysis, the trend in the changes is the same. The ele-
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Figure 4. C 1s, O 1s, and S 2p core energy level spectra for the materials studied.
ment maps presented in Figure S2 show a high distribution of sulfur on the surface of the carbon and its composites. ChemSusChem 2015, 8, 1955 – 1965
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Changes in the structure of the carbonaceous material upon formation of the composite are seen in Figure 5, in which the
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Figure 5. X-ray diffraction patterns of the materials studied.
X-ray diffraction patterns are collected. Although for CGO a d002 diffraction peak representing GO (0.82 nm) is clearly visible, it cannot be detected upon heat treatment, which suggests its exfoliation. In the case of CGO-AO, a well-defined peak at 2 q = 25.078 of the graphene component is seen, and it represents an interlayer spacing of 0.35 nm. This indicates that hot-air oxidation of the CGO composite resulted in a very heterogeneous structure, in which two phases of graphene and amorphous carbons are detected. Such a process is, in fact, expected,[37] and it might be associated with the observed shift in the exothermic peaks in the DTA curve assigned to the decomposition of the epoxy groups of GO (Figure 3 b and Figure S3). This process might contribute to the porosity changes (Figure 1 and Table 1). The textural features of the materials studied are presented in Figures 6 and 7 as SEM (SEM images at a high magnification of 300.00 kX also included in Figure S4) and high-resolution (HR)TEM images. The carbon from the polymer shows an interesting texture consisting of an outside thin “skin” covering the porous texture (Figure 6). That “skin” almost resembles wrinkled graphene layers.[38–40] The composite shows mainly aggregated GO layers, which might be attached through dispersive forces to the outer particles of carbon. After thermal treatment, exfoliation of these aggregates is seen. In the HRTEM images, both the organized layered structure of GO and the totally amorphous structure of CP carbon are seen (Figure 7). These GO layers are also visible on the surface of the CGO composite. On the other hand, interesting features are detected in the case of CGO-AO, for which “rings” of a layered structure of GO are detected within the amorphous phase of the carbon component. Visual analysis of the units resembling the graphite structure in CGO-AO indicates an interlayer distance of 0.35 nm, which is consistent with the XRD analysis. On the basis of the results obtained, a mechanism for the formation of the composite is proposed. During polymer carbonization in the presence of ZnCl2, which is a dehydrating agent/pore former,[41, 42] the micro-/mesoporous structure is developed in the carbon phase. Apparently, pores smaller than 10 nm are too small to accommodate dispersed GO layers. ChemSusChem 2015, 8, 1955 – 1965
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Figure 6. SEM images.
Therefore, those GO units were attached through dispersive interactions to the outside surface of the carbon particles. Specific chemical interactions through the involvement of functional groups cannot be ruled out, as the results suggest partial reduction of the GO phase. This process causes an apparent decrease in porosity because of the addition of a 50 % nonporous phase (dilution effect) and because the entrances of some pore at the surface of the granules are blocked by the GO phase. Heat treatment at 350 8C in air causes exfoliation of GO and further, but still partial, reduction of its surface. Moreover, some thermally unstable functional groups from the carbon phase decompose. Interestingly, that process leads to a pore texture that is very similar to that of the CP carbon with the development of an additional volume of small pores as a result of “air activation”. Apparently, chemistry and spatial constrains result in a synergistic effect on the porosity, as the surface area of the composite is higher than the hypothetical one consisting of a physical mixture of both composite components heated at 350 8C. The cyclic voltammetry (CV) curves of our materials measured after equilibration and with broadening of the potential window from ¢0.2 to 0.2–0.8 V are presented in Figure 8 a–d. Only the first cycle for CGO shows a marked reduction in functional groups. This feature is not visible in the CV curve of CGO-AO, owing to apparent removal of these groups during heat treatment. Then, all the CV curves show broad loops with
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Figure 7. HRTEM images.
well-marked oxidation/reduction humps at approximately 0.3 V versus Ag/AgCl, which represent quinone–hydroquinone redox transitions[5] and small humps at approximately 0.08 V versus Ag/AgCl. Given that the latter humps are not visible if the experiments were run in the dark and under visible light with oxygen removed from the electrolyte (Figure 8 e–h), we link them to the oxygen reduction reaction (ORR).[43] Notably, even though the effects are not very pronounced owing to the predominant capacitive behavior, the visible light and oxygen in the systems increase the intensity of the humps, which suggests photoactivity. The effect is most pronounced for the composite (i.e., CGO) with the lowest porosity. The CV curve for the air-oxidized carbon (i.e., CP-AO) shows some chargetransfer limitations, as demonstrated in its deviation from the rectangular shape. Interestingly, no clear response to visible light in the ORR was found. The lack of this feature in the CV curves can be linked to the very small sulfur content on the surface of this carbon. Sulfur species have been implicated in the enhancement of the catalytic activity of carbon for the ORR.[28, 37] The photoactivity is clearly seen in Figure 9 a–d in the current generated under visible-light exposure at a potential of 0.08 V. The photocurrent density is the highest for CGO and ChemSusChem 2015, 8, 1955 – 1965
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the smallest for the CP and CP-AO samples (Figure 9 e). We link this to the presence of GO conducting units in the composites, which help in charge separation, and to the presence of a significant amount of sulfur in sulfonic acid chromophore like moieties, which upon excitation by light can give electrons to oxygen.[9, 44] The highest photocurrent generation was found for the sample with the highest contribution of these species. Not without significance is also the presence of a marked amount of sulfur in the form of bisulfide and thiol. Sulfur incorporated into the carbon matrix has been found to create catalytic sites for oxygen reduction[28, 37, 45, 46] and to help increase the hydrophobicity in small pores, into which oxygen can be adsorbed.[46] A comparison of the measured capacitance is presented in Figure 9 f. Whereas only a small decrease in the capacitance for CGO is found relative to that for the CP sample, CGO-AO exhibits a capacitance that is almost two times higher than of the other two samples tested. The similar capacitance of CP and CGO, in spite of the fact that the volume of the pores is less than 0.7 nm in the latter (most active for EDLC[4, 11–14]), is only 25 % of the capacitance of the CP sample, which suggests that features other than a specific porosity, introduced by GO, determine the capacitive performance. On the basis of earlier studies,[5, 9, 13–19, 25, 26] these features include the conductivity of GO, higher wettability owing to the abundance of oxygen from GO, and the presence of groups participating in redox reactions such as quinones,[5] sulfoxides, and sulfones.[9, 25, 26] Whereas the first two factors increase the degree of utilization of the existing pore space,[13, 14] the latter factor contributes to the pseudocapacitive performance. Indeed, the results collected in Table 1 show that the composite samples have the highest water affinity. The conductivity (Table 1) should also play a role, and the presence of GO in CGO-AO significantly enhances its conductivity relative to that of the CP-AO carbon. The capacitive performance is even more interesting for the CGO-AO sample, especially if its electrochemical capacitance is compared to that of the CP carbon. As described above, these two samples have almost the same pore-size distributions. In this case, the physical mechanism of EDLC is expected to be the same for both materials provided that other factors do not play a role. Apparently, this is not a case, as the specific capacitance of the composite is two times larger than that of the CP carbon. This is a clear indication of the marked effect of surface chemistry, wettability, and conductivity. As seen from the results discussed above, CGO-AO has almost three times more oxygen (atomic concentration %) on the surface and double the amount of groups decomposing between pH 3 and 11 than the CP carbon. The XPS results indicate that CGO-AO also has a higher contribution of quinones, which are known for their pseudocapacitive performance.[5] A much higher sulfonic acid content than that in CP increases the wettability of the composite by providing high hydrophilicity (Table 1). Comparison of the capacitance values for CGO-AO and CP shows that factors other than the texture can increase the capacitive performance by even more than 100 %. The impedance spectroscopy results for all samples exhibit an arc at high frequency and that arc/semicircle is most pro-
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Full Papers Conclusions The results presented in this paper showed for the first time that mixing nanoporous carbon possessing rich surface chemistry with graphite oxide (GO) results in the formation of a new composite with unique properties. This composite shows synergistic effects in both porosity and surface chemistry. Whereas the latter is affected by reactions between both carbonaceous phases, for the former, spatial constrains and chemistry of GO exfoliation also play significant roles. The presence of the reduced GO phase increased the conductivity and also stabilized the presence of sulfur in the composites. Sulfur is important for the generation of photocurrent (enhancing the oxygen reduction reaction and capacitance), and in specific configurations (sulfones), it enhances the surface and thus increases the accessibility of ions to the small pores and the utilization of their species. Given that two of the samples obtained had the same porosity but different chemistry, the direct current conductivity and the wettability, the effect of features other than texture could be evaluated.
Experimental Section Materials
Figure 8. a–d) CV curves recorded in different potential windows and e–h) CV curves measured at a scan rate of 5 mV s¢1 in the dark (D) and under visible-light (VL) exposure.
nounced for the CP sample (Figure S5). On the other hand, it almost does not exist for CGO. The presence of these arcs might be linked to limitations in charge transfer to the small pores, which is affected not only by the sizes of the pores but also by the density of surface groups and the conductivity of the carbon matrix. The internal resistance is similar for all samples (0.67 W for CP, 0.87 W for CP-AO, 0.68 W for CGO, and 0.85 W for CGO-AO). ChemSusChem 2015, 8, 1955 – 1965
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The formula of the organic polymer used as a precursor for the preparation of the nanoporous carbon material is as follows {BES.DM = bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]sulfide, GMA = glycidyl methacrylate}:
The properties of this particular polymer were described in detail by Podkoscielna.[47] Reagent-grade ZnCl2 powder (> 98 %) from Aldrich was used as an activating agent. In the first step, the polymer was added to ZnCl2 dissolved in distilled water (100 mL) in a weight ratio of 1:2. The obtained mixture was dried at 120 8C. Then, the material was carbonized in a horizontal furnace under a flow of nitrogen (100 mL min¢1) at a heating rate of 10 8C min¢1 up to 800 8C with
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in a ratio of 50:50 wt %. GO was obtained by using Hummers method.[48] First, GO was well dispersed in water by sonication, and then the ground carbon powder was added to the GO suspension. The mixture was sonicated for 1 h more and then stirred overnight. Afterward, the carbon/GO suspension was filtered without washing and was dried at 120 8C. The composite obtained in this way is referred to as CGO. Then, the composite (i.e., CGO) was heated in air at 350 8C for 3 h to modify its structure[9, 49, 50] and chemistry.[9, 49, 50] This material is referred to as CGOAO. The same treatment was applied to the CP sample; it is referred to as CP-AO.
Methods Electrochemical measurements: Prior to electrochemical measurements, to avoid polarization of the working electrode the electrodes were wet in sulfuric acid (1 m) for 24 h. Then, the open circuit potential . Cyclic voltammetry (CV) experiments were run in a very narrow potential window (from 0.2 to ¢0.2 V) to confirm a lack of changes in the CV curve with an increase in the number of cycles. Photoelectrochemical measurements were performed in 1 m H2SO4 in a three-electrode cell with a Pt wire counter electrode and a saturated Ag/AgCl (3 m KCl) reference electrode. For the preparaFigure 9. a–d) Examples of current signals in the dark and under solar-light irradiation at + 0.08 V versus Ag/AgCl tion of the electrode, a slurry of for the carbon and composite materials; e) photocurrent response at various bias potentials; f) capacitance values the carbon or carbon/graphene calculated by CV in the dark and under solar-light irradiation (VL). oxide composite, polyvinylidene fluoride, and a carbon black conductive additive (ratio 80:10:10) in N-methyl-2-pyrrolidone was coated on a Ti foil collector with an a holding time of 40 min. The polymer-derived carbon sample was active area of 1 cm2. The electrodes were dried in air at 120 8C. A extensively washed with 1 m HCl to remove Zn from the carbon total mass of the active electrode material was approximately network and was then washed with distilled water in a Soxhlet ap7 mg. A solar simulator (Solar Light Co., INC, XPS-150TM) with paratus to remove all residual soluble impurities until a constant a 420 nm cut-off filter was used as the irradiation source. A VersapH was reached and no chloride ions were detected in the filtrate. STAT MC (Princeton Applied Research) electrochemical workstation The carbon obtained in this manner is referred to as CP. was employed to record the electrochemical behavior. The transiThe new carbonaceous composite was synthesized by mixing polyent photocurrent was obtained under a constant bias potential bemer-derived carbon with graphite oxide (GO) in aqueous solution tween 0.6 and ¢0.2 V versus Ag/AgCl under on/off illumination. ChemSusChem 2015, 8, 1955 – 1965
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Full Papers Dark current equilibrium at the applied potential was allowed before irradiation. Values of the specific capacitance [F g¢1] were estimated by cyclic voltammetry (scan rate of potential 5 mV s¢1). The specific capacitance (C) was calculated according to Equation (1): C¼
Q ðDE mÞ
ð1Þ
in which Q is the charge obtained after integrating the voltammogram, m is the total mass of the electrode, and DE is the potential window. Surface characterization: Sorption of nitrogen at ¢196 8C was performed by using an ASAP 2020 (Micromeritics, Surface Area and Porosity Analyzer) on the active materials/carbons. Before the experiments, samples were out-gassed at 120 8C to constant vacuum (0.13 Pa). The surface area (SBET), total pore volumes (Vt, from the last point of the isotherm at a relative pressure of 0.99), volume of the micropores (Vmic), volume of pores less than 0.7 and 1 nm (V < 0.7 nm and V < 1 nm), mesopore volumes (Vmeso), and pore-size distributions were calculated from the isotherms. The volume of the mesopores represents the difference between the total pore volume and the micropore volume. The surface area (SNLDFT), volume of pores, and pore-size distribution were calculated by using 2 D-NLDFT (www.NLDFT.com) by assuming heterogeneity of the pore sizes.[30] X-ray diffraction (XRD): XRD measurements were conducted on the active carbon components of the electrodes by using standard powder diffraction procedures analyzed by CuKa radiation (40 kV and 40 mA) generated in a Phillips X’Pert X-ray diffractometer. The scan rate used was 2.3 deg min¢1. X-ray photoelectron spectroscopy (XPS): XPS analysis was performed by using a Physical Electronics PHI 5700 spectrometer with non-monochromatic MgKa radiation (300 W, 15 kV, 1253.6 eV) for the analysis of the core level signals of C 1s, O 1s, and S 2p and with a multichannel detector. Spectra of powdered samples were recorded with constant pass energy values at 29.35 eV by using a 720 mm diameter analysis area. Under these conditions, the Au 4f7/2 line was recorded with 1.16 eV full width at half maximum at a binding energy of 84.0 eV. The spectrometer energy scale was calibrated by using Cu 2p3/2, Ag 3d5/2, and Au 4f7/2 photoelectron lines at binding energies of 932.7, 368.3, and 84.0 eV, respectively. The PHI ACCESS ESCA-V6.F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted by using Gauss–Lorentz curves to determine the binding energy of the different element core levels more accurately. The error in the binding energy was estimated to be approximately 0.1 eV. Fourier transform infrared (FTIR) spectroscopy: FTIR spectroscopy was performed on the active carbon components of the electrodes by using a Nicolet Magna-IR 830 spectrometer by using the attenuated total reflectance (ATR) method. The spectrum was generated and collected 64 times and corrected for background noise. The experiments were done on the powdered samples, without the addition of KBr.
were transformed into proton binding curves representing the total amount of protonated sites. From them, the pKa distributions and the numbers of groups represented by certain pKa values were calculated.[51, 52] Thermal analysis–mass spectrometry (TA–MS): Thermogravimetric (TG) curves were obtained by using a TA instrument thermal analyzer (SDT Q 600) that was connected to a gas analysis system (OMNI Star) mass spectrometer. The active carbon components were heated up to 1000 8C (10 8C min¢1) under constant helium flow (100 mL min¢1). From the TG curves, differential TG (DTG) curves were derived. The composition of gases was evaluated by MS, and gas evolution profiles as a function of temperature were obtained. High-resolution transmission electron microscopy (HRTEM): HRTEM was performed on the active carbon components of the electrodes with a JEOL 2100 LaB6 instrument operating at 200 kV. Analyses were performed after the carbon samples were resuspended in ethanol. Scanning electron microscopy (SEM): SEM images were performed at Zeiss Supra 55 VP. The accelerating voltage was 5.00 kV. Scanning was performed in situ on a sample powder without coating. Electron-dispersive X-ray spectroscopy (EDX) analysis was done at magnification 5 KX with an accelerating voltage 15.00 kV, and the contents of the elements on the surface were calculated. DC conductivity measurements: DC conductivity was measured by using the four-probe method on pellets with a composition of 90 wt % carbon-based materials and 10 wt % polytetrafluoroethylene as binder. The prepared composition was pressed by a Carver Press machine by applying 0.02 MPa pressure and disk-shaped well-packed pellets with diameter 8 mm were formed. The pellets were dried in oven for 12 h. The thickness of the pellets was measured by a spring micrometer. Conductivity measurements were performed by using a Keithley 2400 Multimeter. Water affinity evaluation: The changes in surface hydrophilicity were determined by measuring water adsorption affinity. The initial carbon-based samples were dried at 120 8C to constant mass and were then placed in a closed vessel with constant pressure of water vapor at ambient temperature. After 24 h, TA experiments were performed by using a TA instrument thermal analyzer (SDT Q 600). The weigh lost in nitrogen between 30 and 120 8C was assumed as an equivalent to the quantity of water adsorbed on the surface.
Acknowledgements The authors are grateful to Dr. Beata Podkos´cielna of Maria Curie-Sklodowska University (Lublin, Poland) for kindly providing the polymer sample. This research was partially funded by the Junta de Andaluca, Spain (P12-RNM-1565) and the European Fund for Economic and Regional Development (FEDER). Keywords: graphene · photoactivity supercapacitors · surface chemistry
Potentiometric titration: Potentiometric titration measurements were performed on the active carbon components of the electrodes with a 888 Titrando automatic titrator (Metrohm). Details of the experiments are presented in Ref. [22]. The experimental data ChemSusChem 2015, 8, 1955 – 1965
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[1] [2] [3] [4]
·
porosity
·
A. Burke, Electrochim. Acta 2007, 53, 1083. A. G. Pandolfo, A. F. Hollenkamp, J. Power Sources 2006, 157, 11. P. Simon, Y. Gogotsi, Acc. Chem. Res. 2013, 46, 1094. G. Salitra, A. Soffer, L. Eliad, Y. Cohen, D. Aurbach, J. Electrochem. Soc. 2000, 147, 2486.
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Papers [5] H. A. Andreas, B. E. Conway, Electrochim. Acta 2006, 51, 6510. [6] H. Nishihara, T. Kyotani, Adv. Mater. 2012, 24, 4473. [7] M. B. Sassin, C. N. Chervin, D. R. Rolison, J. W. Long, Acc. Chem. Res. 2013, 46, 1062. [8] X. Zhao, B. M. Sanchez, P. J. Dobson, P. S. Grant, Nanoscale 2011, 3, 839. [9] M. Seredych, E. Rodriguez-Castellon, M. J. Biggs, W. Skinner, T. J. Bandosz, Carbon 2014, 78, 540. [10] Y. G. Wang, H. Q. Li, Y. Y. Xia, Adv. Mater. 2006, 18, 2619. [11] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P. L. Taberna, Science 2006, 313, 1760. [12] C. Largeot, C. Portet, J. Chmiola, P. L. Taberna, Y. Gogotsi, P. Simon, J. Am. Chem. Soc. 2008, 130, 2730. [13] M. Seredych, M. Koscinski, M. Sliwinska-Bartkowiak, T. J. Bandosz, ACS Sustainable Chem. Eng. 2013, 1, 1024. [14] M. Seredych, M. Koscinski, M. Sliwinska-Bartkowiak, T. J. Bandosz, J. Power Sources 2012, 220, 243. [15] D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu, T. J. Bandosz, Carbon 2010, 48, 1767. [16] H. Gûmez, M. K. Ram, F. Alvi, P. Villalba, E. Stafanakos, A. Kumar, J. Power Sources 2011, 196, 4102. [17] C. W. Huang, C. M. Chuang, J. M. Ting, H. Teng, J. Power Sources 2008, 183, 406. [18] K.-S. Kim, S.-J. Park, J. Electroanal. Chem. 2012, 673, 58. [19] Z. Tai, X. Yan, J. Lang, Q. Xue, J. Power Sources 2012, 199, 373. [20] K. Singh, M. Seredych, T. J. Bandosz, ChemElectroChem 2014, 1, 565. [21] D.-W. Wang, F. Li, L.-C. Yin, X. Lu, Z.-G. Chen, I. R. Gentle, G. Q. Lu, H.-M. Cheng, Chem. Eur. J. 2012, 18, 5345. [22] D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu, T. J. Bandosz, Adv. Funct. Mater. 2009, 19, 438. [23] H. Itoi, H. Nishihara, T. Kogure, T. Kyotani, J. Am. Chem. Soc. 2011, 133, 1165. [24] M. Seredych, T. J. Bandosz, J. Mater. Chem. A 2013, 1, 11717. [25] X. Zhao, Q. Zhang, C.-M. Chen, B. Zhang, S. Reiche, A. Wang, T. Zhang, R. Schlogl, D. S. Su, Nano Energy 2012, 1, 624. [26] M. Seredych, K. Singh, T. J. Bandosz, Electroanalysis 2014, 26, 109. [27] D. Hulicova-Jurcakova, A. M. Puziy, O. I. Poddubnaya, F. Suarez-Garca, J. M. D. Tascon, G. Q. Lu, J. Am. Chem. Soc. 2009, 131, 5026. [28] J. Liang, Y. Jiao, M. Jaroniec, S. Z. Qiao, Angew. Chem. Int. Ed. 2012, 51, 11496; Angew. Chem. 2012, 124, 11664.
ChemSusChem 2015, 8, 1955 – 1965
www.chemsuschem.org
[29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]
W. Kicinski, M. Szala, M. Bystrzejewski, Carbon 2014, 68, 1. J. Jagiello, J. P. Olivier, Adsorption 2013, 19, 777. M. Seredych, T. J. Bandosz, J. Phys. Chem. C 2007, 111, 15596. C. Petit, M. Seredych, T. J. Bandosz, J. Mater. Chem. 2009, 19, 9176. C. Hontoria-Lucas, A. J. Lûpez-Peinado, J. de D. Lûpez-Gonzlez, M. L. Rojas-Cervantes, R. M. Martn-Aranda, Carbon 1995, 33, 1585. J. Zawadzki, M. Wisniewski, Carbon 2003, 41, 2257. J. L. Figueiredo, M. F. R. Pereira, M. M. A. Freitas, J. J. M. rf¼o, Carbon 1999, 37, 1379. H. C. Chin, Carbon 1981, 19, 175. M. Seredych, J.-C. Idrobo, T. J. Bandosz, J. Mater. Chem. A 2013, 1, 7059. H. P. Boehm, A. Clauss, G. O. Fischer, U. Hofmann, Z. Anorg. Allg. Chem. 1962, 316, 119. M. Seredych, J. A. Rossin, T. J. Bandosz, Carbon 2011, 49, 4392. J. Guo, J. R. Morris, Y. Ihm, C. I. Contescu, N. C. Gallego, G. Duscher, S. J. Pennycook, M. F. Chisholm, Small 2012, 8, 3283. M. Molina-Sabio, F. Rodrguez-Reinoso, Colloids Surf. A 2004, 241, 15. K. Kante, C. Nieto-Delgado, J. R. Rangel-Mendez, T. J. Bandosz, J. Hazard. Mater. 2012, 201 – 202, 141. D. Yu, Q. Zhang, L. Dai, J. Am. Chem. Soc. 2010, 132, 15127. J. E. Beecher, T. Durst, J. M. J. Fr¦chet, A. Godt, A. Pangborn, D. R. Robello, C. S. Willand, D. J. Williams, Adv. Mater. 1993, 5, 632. Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 2012, 6, 205. M. Seredych, T. J. Bandosz, Carbon 2014, 66, 227. B. Podkoscielna, J. Appl. Polym. Sci. 2011, 120, 3020. W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339. T. J. Bandosz, E. Rodriguez-Castellon, J. M. Montenegro, M. Seredych, Carbon 2014, 77, 651. C. O. Ania, M. Seredych, E. Rodriguez-Castellon, T. J. Bandosz, Carbon 2014, 79, 432. J. Jagiello, Langmuir 1994, 10, 2778. J. Jagiello, T. J. Bandosz, J. A. Schwarz, Carbon 1994, 32, 1026.
Received: January 29, 2015 Revised: February 24, 2015 Published online on April 27, 2015
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