Vertically Aligned Carbon Nanotubes Electrodes Research Articles

A comparative study of high density Vertically Aligned Carbon Nanotubes grown onto different grades of aluminum – Application to supercapacitors

Vertically aligned carbon nanotubes (VACNT) have received extensive attention as electrode materials for supercapacitors owing to their superior features: no formatting or binder needed, anisotropy. They can be obtained by aerosol-assisted Catalytic Chemical Vapor Deposition (CCVD), a cost-effective and roll-to-roll compatible process, directly onto aluminum current collectors by lowering their growth temperature. In this paper, a comparative study of high density VACNT grown onto different grades of aluminum collectors is performed. Influence on electrochemical performances of carpet thickness up to 170 μm and VACNT volumetric density ranging from 70 to 350 mg/cm3 is evaluated. VACNT electrodes exhibit high areal capacitance up to 360 mF/cm2 and 25 F/cm3 and average gravimetric capacitance is 45 F/g. At high scan rate volumetric density has low impact while increasing height of VACNT limit performances. Symmetrical supercapacitor based on VACNT/Al electrodes assembled in coin-cell exhibits high energy and power density with high potential difference of 2.7 V and great energy retention capacity (90% at 13 A/g). Such results outline the benefits of the VACNT growth process directly applicable to aluminum current collector meeting industrial requirements (high purity and thin thickness) and exhibit the huge potentialities of such electrode to realize in supercapacitor applications.

Ultrahigh‐Areal Capacitance Flexible Supercapacitors Based on Laser Assisted Construction of Hierarchical Aligned Carbon Nanotubes

AbstractElectrochemical energy storage is a key technology for a clean and sustainable energy supply. In this respect, supercapacitors (SC) have recently received considerable attention due to their excellent performance, including high‐power density and long‐cycle life. However, the poor binding strength between the active materials and substrate, the low active material loading, and small specific capacitance hinder the overall performance improvement of the device. In this study, an ultrahigh‐areal capacitance flexible SC based on the Al micro grid‐based hierarchical vertically aligned carbon nanotubes (VACNTs) is studied. Interestingly, the Al micro grid‐based VACNTs exhibit ultrahigh loading (13 mg cm−2), and the as‐fabricated VACNTs electrode display outstanding electrochemical performance, including an impressive areal capacitance of 1,300 mF cm−2 at the current density of 13 mA cm−2 and excellent stability with a retention ratio of 90% after 20,000 cycles at the current density of 130 mA cm−2. Furthermore, the hierarchical VACNT electrodes show excellent mechanical flexibility when assembled into quasi‐solid‐state SC using Na2SO4‐PVA gel as the electrolyte. The capacitance of this device is hardly changed bending different angles, even 180°. This study demonstrates the tremendous potential of Al micro grid‐based hierarchical VACNTs as electrodes for high‐performance flexible and wearable energy storage devices.

The Roles of an Aluminum Underlayer in the Biocompatibility and Mechanical Integrity of Vertically Aligned Carbon Nanotubes for Interfacing with Retinal Neurons.

Retinal implant devices are becoming an increasingly realizable way to improve the vision of patients blinded by photoreceptor degeneration. As an electrode material that can improve restored visual acuity, carbon nanotubes (CNTs) excel due to their nanoscale topography, flexibility, surface chemistry, and double-layer capacitance. If vertically aligned carbon nanotubes (VACNTs) are biocompatible with retinal neurons and mechanically robust, they can further improve visual acuity—most notably in subretinal implants—because they can be patterned into high-aspect-ratio, micrometer-size electrodes. We investigated the role of an aluminum (Al) underlayer beneath an iron (Fe) catalyst layer used in the growth of VACNTs by chemical vapor deposition (CVD). In particular, we cultured dissociated retinal cells for three days in vitro (DIV) on unfunctionalized and oxygen plasma functionalized VACNTs grown from a Fe catalyst (Fe and Fe+Pl preparations, where Pl signifies the plasma functionalization) and an Fe catalyst with an Al underlayer (Al/Fe and Al/Fe+Pl preparations). The addition of the Al layer increased the mechanical integrity of the VACNT interface and enhanced retinal neurite outgrowth over the Fe preparation. Unexpectedly, the extent of neurite outgrowth was significantly greater in the Al/Fe than in the Al/Fe+Pl preparation, suggesting plasma functionalization can negatively impact biocompatibility for some VACNT preparations. Additionally, we show our VACNT growth process for the Al/Fe preparation can support neurite outgrowth for up to 7 DIV. By demonstrating the retinal neuron biocompatibility, mechanical integrity, and pattern control of our VACNTs, this work offers VACNT electrodes as a solution for improving the restored visual acuity provided by modern retinal implants.

The Study of Porous Geometry Design for Capacitive Deionization Devices through Vertically-Aligned Carbon Nanotube Electrodes

In the face of an increasing water crisis, we are developing tools to optimize capacitive deionization (CDI) for brackish water desalination. CDI can be a competitive technology for brackish water treatment due to its higher energy efficiencies compared to reverse osmosis and its more inherent resistance to fouling [1]. However, CDI is still a developing technology where adsorption capacity and salt removal rates into porous, tortuous carbon electrodes is still low. We have designed vertically-aligned carbon nanotube (VA-CNT) electrodes, with minimal tortuosity, to investigate the role of porous geometry on the performance of flow-by CDI devices, specifically examining changes in diffusion resistance, salt adsorption rate and capacity. Previously, a breadth of carbon materials have been investigated for CDI including activated carbon, carbon aerogels, ordered mesoporous carbons, carbide-derived carbons, and carbon nanotubes, among many others [2]. High salt adsorption capacities have been attributed to the presence of micropores (< 2 nm), [3] and optimal materials seem to have a presence of macroporous pathways with microporous structures. [4, 5] However, many carbon materials have tortuous pores making it challenging to decouple the role of pore diameter on salt adsorption rate. VA-CNTs are an exciting material for investigating this issue due to the ability to manipulate the inter-CNT spacing, to study changes in the ion transport rate as a function of geometry, while maintaining mininimal tortuosity and intrinsic capacitance. In this work, we synthesize VA-CNT electrodes that are sparse (inter-CNT spacings upto 100 nm), and more dense CNTs (inter-CNT spacings upto 25 nm). These CNTs are grown using a standard chemical vapor deposition of ethylene on silicon wafers with iron catalyst. By varying the partial pressure of ethylene and the oxygen content in the furnace, we synthesize CNTs of varying densities. The VA-CNT forests is delaminated from the substrate with a high-temperature oxygen etch, seperating the carbon from the corrosive catalyst. The free-standing films are mounted against Ti metal plates serving as the current collectors, and constrained by a PEEK porous mesh (200 μm window, 40 μm wire diameter). We use a flow-by CDI experimental set up to study the role of varying voltages, electrode thicknesses, CNT densities, and chemical functionalization on desalination performance. We find that in a 1mM NaCl solution, CNT electrodes can adsorb from 5-15 mg salt/g carbon, at rates of 0.2-1 mg/g-min. Through densification, we maintain gravimetric performance, but increase our volumetric salt adsorption capacity from 0.2 to 0.3 g/cm3. We achieve charge efficiencies upto 80% for these systems. We combine this experimental investigation with an electric double layer model for macroporous electrodes to inform the design of carbon electrode materials for optimal ion adsorption and throughput in a flow-by CDI device. [1] T. Humplik, J. Lee, S. O'Hern, B. Fellman, M. Baig, S. Hassan, et al., Nanotechnology, 22, 292001 (2011). [2] S. Porada, R. Zhao, A. v. d. Wal, V. Presser, and P. M. Biesheuvel, Progress in Materials Science, 58, 8 (2013). [3] S. Porada, L. Weinstein, R. Dash, A. v. d. Wal, M. Bryjak, Y. Gogotsi, et al., ACS applied materials & interfaces, 4, 3 (2012). [4] M. Suss, T. Baumann, W. Bourcier, C. Spadaccini, K. Rose, et al., Energy & Environmental Science, 5, 11 (2012). [5] C. Tsouris, J. Mayes, K. Sharma, S. Yiacoumi, et al., Environmental Science & Technology, 45, 23 (2011).

(Invited) Roll-to-Roll Synthesis of Vertically Aligned Carbon Nanotubes with Embedded Redox Polymers

Electrochemical double layer capacitors (EDLCs), often referred to as supercapacitors, have been the focus of considerable research to develop ultra-high surface area materials with high power and energy density. Activated carbon represents the state-of-the-art EDLC electrode material due to its high porosity, high surface area, and chemical stability. More recently, vertically aligned carbon nanotubes (VACNTs) have emerged as an alternative material due to their high conductivity, stability, facile synthesis and the ability to control the ion-accessible area by varying nanotube density. VACNT electrodes have been used to achieve high power EDLCs; however, scalable manufacturing methods remain elusive, and the ability to grow VACNTs directly on current collectors will help overcome many hurdles facing commercial application. While the chemical vapor deposition (CVD) method is relatively versatile in terms of controlling CNT characteristics (e.g., tube diameter, number of walls, and dopant ratio), three primary factors limit continuous manufacturing of nanomaterials: i) substrate size determined by reactor geometry, ii) requirements of complex catalytic substrate preparation, and iii) high operating temperatures that are incompatible with traditional current collectors (e.g., Al foil). In this work, we present a continuous, roll-to-roll process for synthesizing VACNT forests directly on inexpensive Al foil using the ferrocene-xylene liquid injection floating catalyst technique. The Al foil is continuously drawn through a chemical vapor deposition reactor operating at ambient pressure and a relatively low temperature (600 °C). EDLC electrodes comprising VACNT forests synthesized in continuous and stationary CVD processes were directly assembled into supercapacitor cells, which yielded high power densities (1270 W/kg) and energy densities (11.5 Wh/kg). These devices exhibited very low internal series resistance due to their intimate contact with the current collector and excellent cycle stability with no loss in performance over multiple thousands of cycles. The energy density of VACNT electrodes is increased by integrating high charge capacity redox polymers within the VACNT eelctrodes. In the continuous process, electroactive conducting polymers and redox polymers are embedded into VACNTs using electrochemical methods and physical adsorption processes. Current progress on the performance and limitations of incorporating redox polymers with carbon nanomaterials will be discussed along with the advantages and challenges in future material development.

Preparation of vertically aligned carbon nanotubes and their electrochemical performance in supercapacitors

Vertically aligned carbon nanotubes (VACNTs) were grown on highly n-doped silicon substrates by direct current plasma enhanced chemical vapor deposition to be used as supercapacitor electrodes from a gas mixture of acetylene and hydrogen on nickel islands as the catalyst particles. Scanning electron microscopy and transmission electron microscopy have been exploited to characterize the VACNTs. Electrochemical properties of electrodes were studied by cyclic voltammetry, galvanostatic charge–discharge and impedance spectroscopy technique. According to SEM and TEM images growth mechanism was tip-initiated. Capacitances for four different growth durations of 20, 40, 60 and 120min were measured to be 1.5, 5.8, 7.5 and 15mFcm−2, respectively, at 1mA discharge current. VACNT electrodes fabricated by this method contributed to better electrode capacitance as compared with results of similar previous studies. Furthermore the rate capability of electrodes was found to be excellent. The specific capacitances of a fabricated VACNT electrode and an entangled carbon nanotube (ECNT) electrode in 0.5M KCl aqueous solution electrolyte were 70 and 8Fg−1, respectively. Compared with ECNT electrode, VACNT electrode achieved higher specific capacitance. The higher specific capacitance of VACNT was attributed to the high surface area as well as to defects on the VACNTs, which may be formed by the H2-plasma during growth. The maximum specific energy and power of the VACNT electrode were 3.5Whkg−1 and 22kWkg−1, respectively. The results indicate that the VACNTs are promising candidates as electrode materials in supercapacitors.

Highly flexible, all solid-state micro-supercapacitors from vertically aligned carbon nanotubes

We report a highly flexible planar micro-supercapacitor with interdigitated finger electrodes of vertically aligned carbon nanotubes (VACNTs). The planar electrode structures are patterned on a thin polycarbonate substrate with a facile, maskless laser-assisted dry transfer method. Sputtered Ni is used to reduce the in-plane resistance of the VACNT electrodes. An ionogel, an ionic liquid in a semi-solid matrix, is used as an electrolyte to form a fully solid-state device. We measure a specific capacitance of 430 μF cm−2 for a scan rate of 0.1 V s−1 and achieve rectangular cyclic voltammograms at high scan rates of up to 100 V s−1. Minimal change in capacitance is observed under bending. Mechanical fatigue tests with more than 1000 cycles confirm the high flexibility and durability of the novel material combination chosen for this device. Our results indicate that this scalable and facile fabrication technique shows promise for application in integrated energy storage for all solid-state flexible microdevices.

Reduced graphene oxide and vertically aligned carbon nanotubes superhydrophilic films for supercapacitors devices

Reduced graphene oxide (RGO) and vertically aligned carbon nanotubes (VACNT) superhydrophilic films were prepared by chemical vapor deposition techniques for electrical energy storage investigations. These electrodes were characterized in terms of their material and electrochemical properties by scanning electron microscopy (SEM), surface wettability, Fourier transform infrared spectroscopy (FTIR), energy dispersive and Raman spectroscopies, cyclic voltammetry (CV) and galvanostatic charge–discharge. We observed several physical structural and electrochemical similarities between these carbon-based materials with particular attention to very good specific capacitance, ultra-high energy storage and fast power delivery. Our results showed that the main difference between specific capacitance values is attributed to pseudocapacitive contribution and high density of multiwall nanotubes tips. In this work we have tested a supercapacitor device using the VACNT electrodes.

Lithium-ion batteries based on vertically-aligned carbon nanotube electrodes and ionic liquid electrolytes

In conjunction with environmentally benign ionic liquid electrolytes, vertically-aligned carbon nanotubes (VA-CNTs) sheathed with and without a coaxial layer of vanadium oxide (V(2)O(5)) were used as both cathode and anode, respectively, to develop high-performance and high-safety lithium-ion batteries. The VA-CNT anode and V(2)O(5)-VA-CNT cathode showed a high capacity (600 mAh g(-1) and 368 mAh g(-1), respectively) with a high rate capability. This led to potential to achieve a high energy density (297 Wh kg(-1)) and power density (12 kW kg(-1)) for the prototype batteries to significantly outperform the current state-of-the-art Li-ion batteries.

Vertically Aligned Carbon Nanotube Electrodes Directly Grown on a Glassy Carbon Electrode

Three-dimensional microelectrodes were fabricated using glassy carbon electrodes combined with vertically aligned carbon nanotubes (VACNTs). VACNTs were grown on various conducting electrode patterns including a carbon electrode fabricated by pyrolysis of a negative photoresist, with plasma-enhanced chemical vapor deposition using a bilayer Fe/Al catalyst. VACNT electrodes grown on the glassy carbon showed excellent electrochemical behavior, whereas VACNT electrodes grown on Pt showed poor electrochemical performance, presumably due to the poor contact between VACNTs and the Pt electrode. Electron microscopy showed that the VACNT layer was strongly bound to the carbon electrode, while that on Pt tended to peel away. The versatility of the all-carbon microelectrodes was also tested by using them for interfacing stem cells. Their superior mechanical properties and the electrical connectivity between the carbon electrode and the VACNTs, along with the simple fabrication process, suggest that glassy carbon may be a good conducting substrate for VACNT electrodes.

Optimization of MnO 2/vertically aligned carbon nanotube composite for supercapacitor application

The optimization strategy for producing manganese oxide supercapacitors based on vertically aligned carbon nanotubes (VACNTs) deposited on large area electrodes is presented. A single sequential process of sputtering, annealing and plasma enhanced chemical vapour deposition (PECVD) is applied to produce dense and uniform VACNTs electrodes. As dielectric layer of the supercapacitor, manganese oxide is electrodeposited lining the surface of the VACNTs electrodes. The control of the growing parameters such as catalyst thickness layer, temperature and deposition time for tuning the density, length and diameter of the VACNTs and their structure are found to be key points for the optimization of the MnO 2 electrodeposition process in view to improve the efficiency of the supercapacitor devices. The electrochemical properties of the obtained electrodes are characterized using cyclic voltammetry and galvanostatic charge–discharge techniques. A specific capacitance of 642 Fg −1 is obtained for MnO 2/VACNTs nanocomposite electrode at a scan rate of 10 mV s −1.

Vertically aligned carbon nanotube film electrodes for bioelectrocatalytic dioxygen reduction

We have prepared vertically aligned carbon nanotube (VACNT) film electrodes for bioelectrocatalytic dioxygen reduction. The electrodes were prepared by a CNT-transfer technique attaching the as-grown VACNTs to an ITO electrode with a conductive CNT/epoxy glue. The VACNTs greatly enhance the active electrode area as shown by the substantial current increase of the slow electroreduction of hydrogen peroxide as well as the increase of the efficiency of the oxidation and reduction of the water-insoluble redox probe t-butylferrocene. Several methods for immobilisation of the multicopper enzyme laccase, both with and without mediator, were evaluated. Very high non-mediated catalytic dioxygen reduction current was measured using VACNTs functionalised with 1-pyrenesulfonic acid. The VACNT electrodes were successfully applied as cathode in a zinc–oxygen battery, reaching a power density of 275 μW cm −2 at 1.5 V with pyrene-functionalised VACNTs and 115 μW cm −2 at 0.9 V with syringaldazine as a mediator in oxygen saturated buffer.

Electrochemical Charge Storage Properties of Vertically Aligned Carbon Nanotube Films: The Activation-Enhanced Length Effect

Improvement in the safety, efficacy, selectivity, and/or power consumption of devices used for neural stimulation requires a low impedance neural electrode interface that can rapidly store high charge densities without damaging the electrode or neural tissue. In this study, vertically aligned carbon nanotube (VACNT) films were synthesized and investigated in vitro for their potential use as a neural stimulation electrode. Materials and electrochemical (EC) characterization was performed prior and subsequent to EC activation, accomplished by slow potential cycling across the water window, to determine if activation improved the desired properties. EC characterization was performed at a wide range of frequencies using various techniques including cyclic voltammetry, electrochemical impedance spectroscopy, and high-rate potential transient measurements. The results indicated that EC activation, a technique often used to activate and greatly improve the performance of iridium oxide electrodes, was also favorable for VACNT electrodes. The effect of activation was more pronounced as the average length of the VACNTs was increased, especially at lower frequencies. The activation-enhanced length effect is believed to be a result of the VACNT film characteristics that are determined during the deposition process. A phenomenological model was developed to illustrate this concept and has important implications for various charge-storage applications.

Vertically aligned carbon nanotube based electrodes: Fabrication, characterisation and prospects

We present a methodology to fabricate carbon nanotube based electrodes using plasma enhanced chemical vapour deposition. The metal catalyst nanoparticles used to promote nanotube growth are removed using a water plasma treatment in combination with an acid attack. The final integrated microelectrode-based devices present excellent electrocatalytic properties that make them suitable for electrochemical applications. The presented methodology enables the construction of highly regular and dense vertically aligned carbon nanotube (VACNT) forests that can be confined within the patterned bounds of a desired surface. These VACNT electrodes display very low capacitive currents and are amenable to further chemical modifications.