Reactive deposition of ultrathin conformal vanadium pentoxide within

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Materials and Interfaces

Reactive deposition of ultrathin conformal vanadium pentoxide within carbon nanotube buckypaper in supercritical fluid CO for electrochemical capacitor 2

Quyet H. Do, and Changchun Zeng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04916 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Industrial & Engineering Chemistry Research

Reactive deposition of ultrathin conformal vanadium pentoxide within carbon nanotube buckypaper in supercritical fluid CO2 for electrochemical capacitor

Quyet H. Do1,2,3, Changchun Zeng1,2,* 1

2

High Performance Materials Institute, Florida State University, Tallahassee, FL 32310, USA Department of Industrial and Manufacturing Engineering, FAMU-FSU College of Engineering,

Tallahassee, FL 32310, USA 3

Saigon Hi-tech Laboratories, Saigon Hi-tech Park, Ho Chi Minh City, Vietnam

* corresponding author, email: [email protected]

Table of contents entry

Uniform thin film of V2O5 was deposited on carbon nanotubes, resulting in outstanding capacitance at high power. Abstract. Vanadium oxide carbon nanotubes sheets (buckypaper) composite electrodes were fabricated by supercritical fluid deposition to produce high performance, binder-free electrochemical capacitor electrodes. The precursor vanadium acetylacetonate (V(acac)3) was deposited using carbon dioxide within the buckypaper followed by in-situ oxidation by oxygen. The deposition process, and the morphology, structure and electrochemical properties of the composite electrodes, were analyzed in detail. Continuous ultrathin layers of V2O5 (up to ~1 nm)

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of high conformity were deposited on the carbon nanotubes and uniformly distributed throughout the buckypaper, which was up to ~200 µm thick. Compared to deposition by physical adsorption, substantially higher loading of the vanadium oxide was achieved, which resulted in excellent electrode performance. For example, a composite with 50 wt% of V2O5 was achieved, which had a total capacitance of 187 F/g in KCl aqueous electrolyte.

1. Introduction Electrochemical capacitors, also commercially called supercapacitors, are energy storage devices with high power capability and long cycle life. The primary technical challenge of current electrochemical capacitors is the energy density, which is substantially lower than that of batteries and fuel cells.1, 2 As the energy storage capacity of a capacitor is proportional to its capacitance, E = 1/ 2CV 2 , an electrode with a higher capacitance would lead to greater energy storage capability. Commercial electrochemical capacitors are primarily based on double-layer capacitance, which is proportional to the surface area of the electrode. The predominant electrode material of this type is activated carbon, which has specific surface area (SSA) of approximately a thousand of m2/g. Even with the exceptionally high SSA, the capacitance that can be realized is still limited to approximately 100 F/g.35 On the other hand, capacitance based on fast redox reactions (pseudocapacitance) of the electrode materials, such as transition metal oxides or conductive polymers, can offer a theoretical capacitance up to thousands of F/g. Ruthenium dioxide has been reported to have excellent capacitance of approximately 600 to 900 F/g, even at high active material loading and high working power because of its high electronic and ionic conductivity. However, ruthenium dioxide is expensive and scarcely available. Therefore, intensive research has focused on the potential of low cost oxides, such as manganese oxide,6, 7 nickel oxide,8 or cobalt oxide,9, 10 for electrochemical electrodes. Vanadium oxide is another low cost material with multiple oxidation valence states that can be varied between II (+2) to V (+5). With a low equivalent molecular weight, its theoretical capacitance could be more than two thousand F/g11. However, the performance of vanadium oxide is severely hindered by its low conductivity. Although the capacitance of about a few hundred to thousands F/g were reported in the literature, the higher values were often measured at very low scan rates and the performance deteriorate rapidly at higher scan rate.12-15 High capacitance at high power can be achieved by overcoming the poor transport properties when an ultrathin, conformal V2O5 layer was deposited on highly conductive materials, such as carbon nanotubes (CNTs)16-18or vanadium nitride (VN).19 Thus conformal thin films of V2O5 deposited on CNTs by atomic layer deposition (ALD) is reported to show excellent pseudocapacitance performance at high scan rates.20 The difficulty of ALD however, is the relatively high cost and low productivity of the process. Supercritical fluid deposition (SFD) is another technique that enables superior conformal deposition of both metal and metal oxide thin films.21-25 This process is akin to chemical vapor deposition (CVD) but uses supercritical fluids (SCFs) to dissolve the precursor instead of relying on evaporation under a


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vacuum. Because of the high precursor concentration in the SFD, and high diffusivity and zero surface tension of the supercritical fluids, conformal deposition can take place at significantly lower temperatures and at higher rates than CVD. Deposition of various metals21, 22, 26, 27 and metal oxides26, 28-30 for electronic devices with excellent uniformity and conformal coverage has been demonstrated. The fabricated films had a thickness from 21 nm to a few hundred nanometers.26 SFD has also been explored for deposition of nanoparticles in porous-materials, for which the unique properties of SCFs are even more advantageous in that they allow for fast and uniform access to the pore structures.29, 31 This method was employed to deposit catalysts on carbon materials for energy conversion and storage. For example, CNTs were used to support noble metals of Pt, Pd and Ru 32-35 particles for fuel cells, and metal oxides of SnO2, RuO2 and Mn3O4 for Li batteries and electrochemical capacitor electrodes.36-38 In these studies, the supporting materials were in powder form. Therefore the deposited materials still required further mixing, dispersion and binding with other ingredients to form the final electrodes. These steps may cause the loss or blockage of access to the active materials, lowering their utilization. Vanadium oxide deposited on binder-free carbon nanotubes networks (CNT buckypapers) as the support by physical adsorption showed excellent pseudocapacitance performance due to the significantly improved transport properties (due to the reduced thickness) and high utility of the oxide.39 However, the process can only achieve uniform deposition of a small amount of active materials, e.g., a few weight percent of the CNT buckypaper substrate. The total performance of the composite electrode was still low. To improve the total electrode performance, in this research, we directly deposited ultra-thin layer (~1 nm) of V2O5 by SFD and in-situ oxidization. The SCF used in the study was supercritical carbon dioxide (scCO2) and oxygen was used as the oxidizing agent. The SFD process, the structures of the deposited materials, and the electrochemical properties of the composite electrode were investigated to establish the deposition-structure-property relationships. 2. Experiment Vanadium oxide deposition Figure 1 shows the schematic of the supercritical fluid deposition system, which includs a high pressure, two-temperature zone reactor (Parr Instrument) with a volume of 250 ml,39 a manifold (75 ml) as the O2 reservoir, and a six-point switching valve for injection. CO2 was supplied by using a high-pressure syringe pump (Teledyne ISCO 100D). The six-point value had two configurations: (I) and (II). In position (I), CO2 was connected to the reactor and the O2 line (99.999%, Airgas South, Inc.) was connected to the manifold. This configuration allowed only CO2 to be delivered to the reactor. When the value was switched to configuration (II), the manifold was connected to the CO2 line on one end and the reactor on the other end, allowing for O2 injection. The configuration was only used when injecting O2 into the reactor.



Figure 1. Schematic of the high-pressure reactive deposition system. A typical deposition process occurred as follows. First, 100 mg V(acac)3 (98%, Strem Chemicals) precursor was placed at the bottom of the reactor and a piece of multi-walled carbon nanotube (MWCNT) buckypaper was place on the deposition stage (Figure 1). The CNT buckypaper was fabricated following the previously reported procedure.39 The reactor was purged with CO2 (99.999%, Airgas South, Inc.), and the temperature was raised to 70 0C. CO2 then slowly fed into the reactor by a syringe pump until the pressure reached 12.55 MPa. The reactor was held at this pressure and temperature while stirring for 30 minutes to dissolve the precursor in CO2. The substrate temperature was then raised to a reaction temperature of 170 °C and system pressure reached 25.51 MPa. Meanwhile, the manifold was filled with 0.69 MPa of O2, then CO2 was pumped into the manifold to reach the same pressure of 25.51 MPa at room temperature. The six-point valve was then switched to configuration II, and the syringe pump was operated at constant flow rate mode (5 ml/min for 4 minutes) to force the O2/CO2 solution from the manifold into the reactor. The pressure in the reactor after the gas injection was around 28.96 MPa. The reactor was kept under constant stirring for another two hours. Thereafter, the pressure was relieved by venting and the reactor allowed to cool. Samples were then retrieved from the reactor. Several reactor operation modes were explored as described in the following: a) Some samples were subjected to a second deposition to increase the oxide materials loading (double deposition); b) Samples were held vertically (vertical deposition, normal mode would parallel, lying on the sample stage) to validate the deposition uniformity; and c) A four-piece buckypaper stack, held together by steel mesh, was used for deposition to explore scale-up feasibility. By varying the weight ratio of precursor/buckypaper, composites with different vanadium oxide weight loadings could be fabricated by using the same reaction conditions. The as-deposited samples were further oxidized by annealing at 300 0C for 1hour.


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Characterization The weight percentage of the as-deposited materials was determined by comparing the weights of the buckypaper before and after the reactive deposition and before annealing. To determine the V2O5 weight percentage in the composite electrode, a base digestion method was used. Samples were immersed in 1M KOH solution for an extended period of time, during which V2O5 was dissolved in the KOH solution. The sample was then washed three times with distilled water to remove any residual V2O5 and dried in an oven at 100 0C for 2 hours. The weight percentage of V2O5 was calculated from the weight of samples before and after the digestion. Morphology and microstructure of the sample was characterized using a scanning electron microscope (SEM) and by transmission electron microscopy (TEM) (JEM–ARM 200 cF). Energy dispersive spectroscopy (EDS) (JEOL JSM–7401F equipped with an EDAX system) was used to detect the vanadium distribution throughout the thickness of the composite electrode. The crystalline structure of the V2O5 was analyzed by x-ray diffraction (Rigaku DMAX 300 Ultima III powder X-ray diffractometer using Cu Kα radiation; data processing: Jade 7.0), while the V–O bond structures were analyzed using an inVia Raman Microscope (Renishaw Inc.). The electrochemical properties of V2O5-buckypaper composites were tested with cyclic voltametry (CV) (VersaSTAT– Princeton Applied Research) in KCl 1M solution, using a three-electrode configuration. The composites, cut into 1x1 cm2 pieces and held by two Pt meshes, were used as the working electrode. Each composite electrode weighed ~3-5 mg. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a Pt wire. The gravimetric specific capacitance was calculated based on the equation, C=i/(m·s), where m, i, and s are the mass of the sample, the current response and the potential scan rate, respectively. Electrochemical impedance measurements were also conducted using the same setup for equivalent circuit modeling of the electrode material. Data fitting of the impedance plot was performed using ZView software. 3. Results and discussion Table1 summarizes the buckypaper/V2O5 composite electrodes fabricated by SFD. Oxide loadings up to fifty weight percent (50 wt%) were achieved, which was significantly higher that than that obtained by adsorption-annealing approach reported in our previous work.39 Moreover, the deposited layer was exceptionally thin and uniform. Table1. Summary of the buckypaper/V2O5 composite electrodes prepared by reactive deposition Sample # Buckypaper configuration V2O5 percentage in the during deposition composite electrodes (%) 1* S N/A



2 S 3.9 3 S 17.2 4 S 27.78 5 S 27.99 6 S 38.23 7 D 50 *: sample used for EDS and SEM study only and was not annealed S stands for single deposition, D stands for double deposition, for which the as-deposited composite was subjected to a second reactive deposition under the same conditions. As an example, Figure 2a shows the SEM micrograph of the composite with 49.32% of as-deposited material. The outer surface of CNT-V2O5 was as smooth as that of neat buckypaper, which indicates a uniform coverage of the oxide on the CNT’s surface.


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Figure 2. Sample with 49.32% deposited material showing uniform deposition: (a) SEM showing the smooth and surface of CNT, and no apparent aggregates, (b) Cross sectional view of the buckypaper, and (c) Vanadium EDS signal across the cross section along the line in (b) (the inset is the EDS spectrum) To examine the uniformity of the deposited vanadium oxide throughout the buckypaper, EDS was conducted across the sample thickness (Figure 2b). The strong vanadium signal (Figure 2c) was relatively uniform throughout the cross section, further confirming the uniform deposition within the relatively thick buckypaper (~ 57 µm). Deposition was also carried out in the same experimental run using two samples placed at different configurations: one oriented horizontally and the other vertically. Comparable percentages of as-deposited materials (37.88% and 38.71%) were observed after deposition, further confirming the deposition uniformity. Uniform oxide deposition within thick electrodes should increase the stored energy. To ascertain the capability of SFD to achieve this, a four-piece buckypaper stack held together by steel mesh underwent the SFD process. Figure 3 shows vanadium oxide loading in each buckypaper. The oxide was reasonably uniformly distributed in buckypaper layers 2-4, as suggested by the similar amount of oxide in those buckypapers. The higher oxide loading in the top layer (layer 1) was caused by the precipitation of the precursors onto the buckypaper during CO2 venting at the end of deposition. Nevertheless, this explorative experiment showed that supercritical fluid deposition had the potential to achieve uniform deposition of oxide materials within thick samples. This is important for scalable fabrication of electrodes with high energy density using SFD.

Figure 3. Distribution of as-deposited materials percentage after deposition of a four-piece stacked buckypaper.



The as-deposited composites were further annealed in air at 300 °C and observed by transmission electron microscopy (TEM). Figure 4 shows the TEM micrographs for the sample with 50% V2O5 (sample 7, Table 1). The oxide layer, which can be distinguished from the CNTs (the parallel lines in the images), is highly conformal and uniform. The layer was about 1 nm thick, which is equal to approximately three crystalline atomic layers.40 The uniformity of the deposited layer by SFD was comparable with that by atomic layer deposition,20 demonstrating the superiority of SFD over solution processes that typically resulted in large grains of metal oxide,17, 36, 41, 42 or non-uniform coating.18

Figure 4.(a) TEM micrographs of composite sample with 50 wt% oxide after annealing and (b) higher magnification view of the marked region in (a). Figure 5a shows the x-ray diffraction results revealing the vanadium oxide crystalline structure. Distinct peaks characteristic of crystalline V2O5 were observed in the annealed V2O5 buckypaper sample. The


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peaks at 2Ɵ of 19.4° and 30.4° corresponded to the (001) and (400) plane of orthorhombic V2O5 (JCPDS: 41–1426).43, 44 The (110) plane peak of V2O5 at about 26° was not resolvable due to the interference with the strong (002) peak of the carbon nanotubes. By comparison, the characteristic diffraction peaks were not observed in the as-deposited samples, suggesting the oxide was likely amorphous. Figure 5b shows the spectra from the Raman spectroscopic analysis to further confirm the presence of vanadium oxide. Despite the relative weak signals due to the minute sample amount (~1nm layer on CNT buckypaper), the presence of vanadium oxide was confirmed by the peaks at 285 cm-1 and 1008 cm-1, which were symmetric stretch of V – O and V = O group, respectively.45 Two additional peaks were also observed around 1150 cm-1 and 1450 cm-1. The exact origin for these peaks is not clear. It is plausible that they may stem from the interaction between the deposited oxide and the underlying carbon nanotubes. Overall the Raman spectroscopy and X-ray diffraction investigations support that the deposited material after SFD contains amorphous vanadium oxide and was converted to crystalline structure after annealing.

Figure 5. (a) X-ray diffraction and (b) Raman spectroscopy of the as-deposited and annealed composites in comparison with pristine buckypaper. The electrochemical properties of the composite electrodes were examined by cyclic voltametry (CV tests). Figure 6a shows CV traces for the composite electrode with V2O5 content of 50% at two different scan rates. The pseudo-peaks were very distinct at a low scan rate of 2 mV/s and became less resolved at a higher scan rate of 100 mV/s. The broadening of the pseudo-peaks and their eventual disappearance at high scan rates was the result of the inability of the intrinsically slow redox reaction to keep pace with the increasingly higher voltage scan rates. At higher scan rates double-layer capacitance became more significant and the CV traces became more rectangular in shape. Figure 6b shows the total capacitance of two composites (50% and 27.18% V2O5) at different scan rates. An excellent capacitance of 187 F/g was achieved in the electrode with 50% V2O5 at the scan rate of 2 mV/s. The capacitance decreased at a higher scan rate. Understandably, the electrodes with higher V2O5 had a higher total capacitance. However, the advantage was more prominent at a low scan rate and diminished at high scan rates (Figure 6b) as the capacitance degradation was higher in samples with higher oxide loading. Figure 6c summarizes the



specific total capacitance and the corresponding specific pseudo-capacitance of V2O5 in the series of composites with different V2O5 content at a high scan rate of 100 mV/s. The pseudo-capacitance component was calculated by subtracting from the total capacitance the double layer capacitance, which is assumed to be proportional to the CNT content of the composite. The total specific capacitance increased as the oxide percentage increased. Note that the total capacitance of the composite was also proportional to its specific surface area, since the charge storage was only located near the surface of the composite and affected by the SSA of the initial buckypaper. Therefore, comparisons of the total capacitance to the double layer capacitance of the pristine buckypaper were more suitable to delineate the performance improvements rendered by the deposited V2O5 in the composites. In this regard, the highest total capacitance value at 100 mV/s (Figure 6c) was ~130 F/g, which was about one order of magnitude higher than that of pristine buckypaper. This remarkable improvement was comparable to that achieved by atomic layer deposition.20 The performance improvement resulted from the combination of high vanadium oxide loading and the high pseudocapacitance of the ultra-thin vanadium oxide layer. For samples with 50% V2O5, the specific pseudocapacitance was 234 F/g at a scan rate of 100 mV/s. This was significantly higher than the 80 F/g reported in the literature17, 41 for electrochemically deposited V2O5CNT composite electrode of similar loading (51.3%) under the same scan rate. An issue that needs to be addressed in the future is the stability of the buckypaper V2O5 composite electrodes, which was rather low in aqueous solution. The capacitance decreased significantly after a few hundred cycles. This was primarily due to the substantial solubility and dissolution loss of the oxide in the aqueous electrolyte solution. The stability may be improved by using an electrolyte system with low vanadium oxide solubility. For example, a recent study showed that the vanadium oxide electrode remained stable after 5000 cycles by using LiCl/polyvinyl alcohol gel electrolyte.49


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Figure 6. Electrochemical properties of the composites: (a) Cyclic voltammetry traces for the composite electrodes with 50% V2O5 at two different scan rates, (b) specific total capacitance at different scan rates for two composite electrodes with different loadings of V2O5, and (c) summary of the specific total capacitance (filled square) of a the series of composite electrodes fabricated in the study, and the V2O5 specific pseudo-capacitance calculated from CV test (open square) and from impedance modeling (filled circle). Scan rate was 100 mV/s. To further understand mechanisms that govern the electrode performance, electrochemical impedance spectroscopy (EIS) tests of electrodes with different oxide contents were carried out in the KCl 1M solution at 0V vs. SCE electrode, from 0.01 Hz to 10000 Hz. Figure 7(a) shows the Nyquist plots for the 50% V2O5 composite electrode. The behavior corresponds to a diffusional impedance, which can be well described by a Randles model of equivalent circuit, as shown in Figure 7b.46 The Nyquist plot can be classified into two frequency regimes: a semicircle part at high and medium frequency range, and a long tail at low frequency range. The semicircle part is caused by a bulk resistance (Rs) in series with a simple double-layer capacitor (C0) and a charge transfer resistance (Rct) of Faradaic reaction between V2O5 with the electrolyte.1 Rs and Rs + Rct can be estimated at two positions where the imaginary impedance Zim approaches to zero (Figure 7a).



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Figure 7.(a) Complex-plane impedance plot of an in-situ deposited CNT-V2O5 composite with 50% oxide, (b) the equivalent circuit model used for the impedance data fitting in (a) The long tail at low frequency is attributed to the ion diffusion inside the electrode, represented by a finite length Warburg element with open circuit terminus (Wo). The values of these parameters for 1 mg of composites with different oxide content were obtained by fitting the model with experimental data, which are listed in Table 2. Due to the convoluted effects of the V2O5 - CNT electrode, the electrolyte solution, and the metal contact, no particular trend was observed for the bulk resistance (Rs). On the other hand, trends were observed for the other model parameters, Rct, C0, and Wo, were only dependent on the structure of the composite electrodes. Table 2 shows that Rct increased and C0 decreased as the oxide content increased. This is the direct result of the accompanied decrease in the CNT content and reduction of the SSA in the composite electrode of the same weight (1mg). For samples with the same SSA (same amount of CNT), the resistance (Rct) was proportional to the charge-transfer resistivity of V2O5, which should be a constant. Indeed, by scaling the resistance of the composite electrode to that of the same amount of CNT weight (1 mg), this same value was obtained for each composite electrode, with an average of 1.09 Ω and a standard deviation of 0.1 Ω. The Warburg element of diffusional impedance consists of the following components: resistance (Rw), time (T) and phase (P), related by following equation (1):46, 47

Zw = R w

Coth ( j ω T ) P ( jω T ) P

(1)

where Zw is the impedance of the Warburg element (Wo). The model fitted resistance RW and time T increases as the thickness of the V2O5 increases, while the phase P is almost constant (~0.5) (Table 2). At a low frequency range (ωT

Reactive deposition of ultrathin conformal vanadium pentoxide within

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