Article pubs.acs.org/JPCC
Ultrahigh-Power Flexible Electrochemical Capacitors Using BinderFree Single-Walled Carbon Nanotube Electrodes and Hydrogel Membranes Jim Kalupson,† Danhao Ma,‡ Clive A. Randall,§,⊥ Ramakrishnan Rajagopalan,*,#,⊥ and Kofi Adu⊥,∥ †
Department of Engineering Science and Mechanics, ‡Department of Energy and Mineral Engineering, §Department of Materials Science and Engineering, ⊥Materials Research Institute, and ∥Department of Physics, Altoona College, The Pennsylvania State University, University Park, Pennsylvania 16802, United States # Department of Engineering, The Pennsylvania State University, Dubois, Pennsylvania 15801, United States ABSTRACT: Binder-free single-walled carbon nanotubes were self-assembled to form a highly dense 20 μm thick carbon nanotube electrode to be used in electrochemical capacitors. Fabrication of symmetric nanotube capacitors using these electrodes and highly ionically conducting poly(vinyl alcohol)-based hydrogel membranes soaked in aqueous sulfuric acid resulted in a capacitor with power density as high as 1040 kW/kg based on the mass of both electrodes. The time constant of the assembled capacitor was ∼15 ms. The capacitors showed no degradation in performance even after 10 000 cycles. Pseudocapacitance was further induced by modifying the surface of nanotube electrodes to almost double the specific capacitance while retaining the high power and cycling performance.
1. INTRODUCTION Electrochemical energy storage devices such as batteries and electrochemical capacitors have attracted considerable attention due to the growing demand for energy storage solutions for automotive, renewable energy systems such as wind, solar, and energy-harvesting technologies.1,2 There are a number of marketing surveys that point to significant growth in the electrochemical capacitor market over the next decade, above S11 billion by 2022.3 There is also an important demonstration emerging which shows that the high power of the capacitors when integrated with secondary batteries aids in improving the cycle life of the battery components, thereby providing major cost saving in a total system in terms of maintenance and down time costs aiding large- and small-scale alternative energy management systems, all providing new investments in the development of supercapacitor.4,5 Charge storage in electrochemical capacitors is mainly due to electrostatic double-layer formation at the electrode/electrolyte interface. Electrochemical capacitors are primarily considered as a high power electrochemical device as compared to battery, which is a high-energy device. Typical time constant for state-ofthe-art electrochemical capacitors are of the order of seconds and are capable of storing energy density of about 5 W h/kg. With the advent of asymmetric electrochemical capacitor design, it is now possible to improve both the cell voltage and energy density.6−8 The improvement in power density is mainly addressed by reducing the equivalent series resistance (ESR) of the device. The origin of ESR in electrochemical double-layer capacitors (EDLCs) can be quite complex as it is influenced by several © 2014 American Chemical Society
sources that include contact resistance from the current collector, electronic and pore resistance of carbon electrode, binder resistance, interfacial resistance of the electrode/electrolyte, and ionic resistance of the separator.9−11 More often, the electrode resistance dominates the power performance of the capacitor, and hence it is critical to design carbon materials with optimum physical properties such as porosity, surface area, and electrical conductivity. In addition to the electrode design, it is also important to fabricate ion-conducting membranes with good electrolyte uptake, ionic conductivity, and minimum interfacial resistance with the electrodes.12−14 Electrochemical impedance spectroscopy can be a powerful tool to measure ESR, as well as providing mechanistic insights into factors that affect rate capability. Porous activated carbons with optimum porosity on the order of 1−3 nm and surface areas greater than 1500 m2/g are primarily used as electrodes in EDLCs. However, due to tortuous and interconnected pore structure, the power capabilities of these electrodes are limited, and typically the relaxation frequency for capacitors made with these materials is less than 1 Hz. More recently, power capability of porous carbons has been improved by a hierarchical pore structure design, but that comes with significant loss in bulk density limiting volumetric capacitance.15−17 Carbon nanotube (CNT), due to its very high electrical conductivity, can be a good electrode material for the Received: October 23, 2013 Revised: January 3, 2014 Published: January 13, 2014 2943
dx.doi.org/10.1021/jp410502s | J. Phys. Chem. C 2014, 118, 2943−2952
The Journal of Physical Chemistry C
Article
washed with copious amount of water and then treated with 1 M H2SO4 for 4 h in order to remove any manganese residues. The electrodes were then washed and dried at 100 °C overnight before being used. 2.3. Synthesis of Poly(vinyl alcohol) Hydrogel Membrane (PVA). Five grams of poly(vinyl alcohol) (Sigma-Aldrich) was dissolved in 45 mL of deionized water by heating the mixture under rigorous stirring at 90 °C until fully dissolved. The polymer solution was cooled down to room temperature, and then 40 μL of cross-linking agent, glutaraldehyde (SigmaAldrich, 50 wt % with water), was added, and the mixture was stirred for 5 min. The resultant solution was then cast onto a glass plate and treated with a doctor blade to obtain an average thickness of 75 μm. The membrane was peeled off and allowed to dry overnight at room temperature under vacuum. The dried membrane was then soaked in aqueous sulfuric acid of different concentrations (1 and 3 M, respectively) for 24 h before being used for electrochemical measurements. The morphology of the membrane was characterized using a Hitachi S-3500N scanning electron microscope. 2.4. Electrochemical Characterization. 2.4.1. Ionic Conductivity Measurements. Ionic conductivity of the synthesized PVA membrane was measured using electrochemical impedance spectroscopy as a function of temperature. The membranes were presoaked in 1 M H2SO4 electrolyte for 24 h before being sandwiched together between two gold foil electrodes in a Teflon cell. The active surface area of the membrane was ∼44 mm2 with a thickness of 100 μm for the PVA, respectively. The assembled cell was placed in a temperature-controlled Delta 9023 furnace. The cell was equilibrated at a defined temperature for 30 min before each measurement. Impedance data were collected using a Solartron 1287 Impedance analyzer at 25, 10, −5, −20, −35, −50, and −65 °C. Ionic conductivity was calculated from the impedance data as follows
design of high-power electrochemical capacitors. CNTs can be either assembled as a buckypaper or directly grown on current collectors.18−27 The high-power density results from the high electrical conductivity, mesoporosity, and high electrolyte accessibility of carbon nanotubes. In this investigation, we report for the first time the design of an aqueous electrochemical capacitor using high surface area ultrathin CNT electrodes with excellent mechanical and electrochemical properties that has a relaxation frequency of 63 Hz (time constant ∼ 15 ms). In order to exploit the high-power capability of CNT electrode, poly(vinyl alcohol) (PVA)-based hydrogel membranes with ionic conductivity on the order of 10−2 S/cm were fabricated to be used as the separator. The designed aqueous capacitors showed excellent cyclability (>10 000 cycles) with good energy densities and coulombic efficiencies. The fabrication methodologies of the individual components are also attractive for a manufacturing scaled process.
2. EXPERIMENTAL SECTION 2.1. Fabrication of SWCNT Electrodes. A wet chemistry postsynthesis self-assembly (PSSA) technique was used to fabricate the single-wall carbon nanotubes (SWCNTs) electrode. In brief, a commercial SWCNT (Thomas Swan Elicarb) of residual noncarbonaceous content of ∼3.5 wt % was used to fabricate the electrode. First, a chlorination technique was used to digest the noncarbonaceous impurities (mostly the growth catalysts) in the CNTs to improve the purity to part per million (ppm) levels. The purified SWCNT was dispersed and suspended in a high-density liquid (Cargille Heavy Liquid) under continuous magnetic stirring for about 4 h. The liquid was thermally evaporated slowly and was trapped cryogenically resulting in the formation of binder-free SWCNT electrode. The electrode was treated at high temperature to 800 °C to remove the liquid remnant. The temperature-programmed oxidation (TPO) on TA Q5000IR thermogravimetric analyzer (TGA), the Renishaw InVia micro-Raman, and the Micrometric ASAP 2020 were then used to characterize the electrode. The TPO of the commercial as-received and purified SWCNTs was performed at 5 °C/min ramp rate from 25 to 1000 °C. The Raman spectra were collected at room temperature in the backscattering configuration using Renishaw inVia spectrometer equipped with a Peltier cooled RenCam ddCCD. A Leica DM LM confocal microscope with 100° ⌀ objective was used to illuminate the sample and collect the scattered light. It was set to operate at
