ARTICLE pubs.acs.org/JPCC
Electrochemical Capacitors Based on Graphene Oxide Sheets Using Different Aqueous Electrolytes Chien-Te Hsieh,†,* Shu-Min Hsu,† Jia-Yi Lin,† and Hsisheng Teng‡ † ‡
Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan ABSTRACT: Graphene oxide (GO)-based electrochemical capacitors have been fabricated and investigated in 1 M Li2SO4 and Na2SO4 aqueous electrolytes. The GO sheets were derived from natural graphite powders and subsequently coated over carbon paper forming a composite electrode. The GO sheets have highly oxidized planes and edges, occupied by oxygen functionalities including carboxyl, carbonyl, and ether groups. The GO-based capacitor displays specific capacitances of 238.0 and 98.8 F/g at 0.5 mA/cm2 in Li2SO4 and Na2SO4 electrolytes, respectively. The electrochemically active areas for Li and Na ions are calculated to be 452.8 and 219.3 m2/g at the first discharge cycle, respectively. The staking layer of the hydrated Li molecules forms dual layers, whereas the hydrated Na molecules tend toward a monolayer adsorption on the oxidized sheets. On the basis of the Randles plot, the apparent diffusion coefficient of Liþ is calculated to be 3.1 1015 cm2/s, which is about three times higher than that of Naþ in the GO-based electrodes. Compared with the Na2SO4 electrolyte, the GO-based capacitor in Li2SO4 exhibits high stable capacitance, low inner resistance, and high diffusivity. This originates from the smaller ionic size and the lower hydration number, thus facilitating the performance of the capacitor.
1. INTRODUCTION An increasing demand and growing concerns for energy requirement and global warming have stimulated intense research on the development of energy storage.1,2 Electrochemical capacitors (ECs or so-called supercapacitors) are considered a promising candidate for alternative energy storage devices owing to their high rate capability, long cycle life, and low maintenance cost. For the double-layer formation in an electrochemical capacitor, it is generally recognized that, the larger the surface area that can provide for the adsorption of electrolytes on the electrodes, the more the energy that can be stored in the capacitor.35 The utilization of various carbonaceous materials, such as activated carbon granules, activated carbon fabric, and carbon nanotubes, as electrode materials for ECs has been extensively investigated.68 However, a major concern with these materials is that the entire surface area is not electrochemically accessible by an electrolyte.9,10 This has been attributed to the following facts: (i) a small micropore size (pore size Cx O þ Mþ T > Cx O==Mþ
ð2Þ
where Mþ is a metallic ion (e.g., Liþ and Naþ), and >CxO//Mþ is a cation adsorbed by a carbonyl or a quinonetype site, basically derived from an ion-dipole attraction. This specific adsorption process is different from the formation of >Cx//Mþ on nonspecific sites through the dispersion interactions.10 However, this effectively offers an excess specific double layer capacitance due to the local changes of electronic charge density. To explore the surface occupancy on the GO sheets in an aqueous electrolyte, an approach was adopted, which analyzed 12371
dx.doi.org/10.1021/jp2032687 |J. Phys. Chem. C 2011, 115, 12367–12374
The Journal of Physical Chemistry C
ARTICLE
Table 2. Components and Related Parameters of the Equivalent Circuit (eq 5) for the GO-Based Capacitors in 1 M Aqueous Electrolytes (Deviation for Each Component
