α-Co(OH)2/carbon nanofoam composite as electrochemical capacitor electrode operating at 2 V in aqueous medium
Introduction
In the recent decades, interest in developing new technologies to address energy challenges is substantially increasing. Considerable effort has been focused on research and development of more efficient energy storage devices and systems. An example of such device is the electrochemical capacitor (EC), also called supercapacitor, ultracapacitor, or power capacitor [1], [2]. Primary advantages of ECs are: high power capability (60–120 s), excellent reversibility (90–95% or higher), and long cycle life (>105). In general, they exhibit 20–200 times larger capacitance per unit volume or mass than conventional capacitors [3], [4]. One of the most important components of an EC is the electrode material. In this context, ECs electrodes materials have been produced by different methods such as chemical precipitation [5], [6], [7], sol–gel [8], chemical vapor deposition [9], electrostatic spray deposition [10], and electrodeposition [11], [12], [13], [14]. Among these techniques, electrodeposition has stood out due to its cost-effectiveness, application at room temperature (no need of high vacuum) and scale-up ability. Also, it allows the deposition of the active material directly on the current collector, eliminating the need for binders and extra-process steps. Besides, the structural and morphological properties of the deposits can be tailored by relatively simple parameters such as potential, current density, bath concentration, pH, and temperature.
Despite the recent efforts on the development of new ECs, they still present some drawbacks, in particular low energy densities (<10 Wh kg−1 in most of commercial ECs) [15] which is much lower than batteries (>100 Wh kg−1). Thus, there is an interest in increasing energy density in ECs to values close to or even beyond those of batteries, without sacrificing their high power capability. For instance, taking into account that the energy density in ECs is obtained by E = CV2/2, where C is the total capacitance and V is the operating voltage, increasing V is a promising strategy to improve their energy density. This can be achieved for example by employing optimized combinations of active electrodes and non-aqueous electrolytes such as organic solvents and ionic liquids (ILs). Although these electrolytes can operate in a very broad potential range, they still present some drawbacks i.e. poor conductivity and high viscosity when compared to aqueous medium. Besides, most of organic electrolytes and ILs are expensive and normally environment-unfriendly. In this context, aqueous electrolytes are still more advantageous from a cost and ecological viewpoint.
Among the various ECs materials, Co(OH)2 has been reported as a promising pseudocapacitive material owing to its layered structure, low cost, environmental friendliness, and high theoretical specific capacitance (3460 F g−1) [15]. Three polymorphic phases of Co(OH)2 have been theoretically proposed in the literature, labeled as α-Co(OH)2, β-Co(OH)2, and γ-Co(OH)2. The existence of these phases can be based on the polymorphism of the naturally occurring mineral CoO(OH) known as heterogenite [16]. Also, it can be compared by analogy to FeO(OH) where three forms occur specifically goethite (α-Fe(OH)2), lepidocrocite (β-Fe(OH)2), and feroxyhyte (γ-Fe(OH)2). However, as far as we know, only α- and β-Co(OH)2 have been synthetized [17], [18], [19], [20]. The α-cobalt hydroxides have the same structure as hydrotalcite-like compounds which contain positively charged Co(OH)2−x layers and charge balancing anions such as , , Cl−, etc. in the interlayer gallery [21]. On the other hand, the β-form is a stoichiometric phase of the composition Co(OH)2 with brucite-like structure consisting of a hexagonal packing of hydroxyl ions with Co(II) filling alternate rows of octahedral sites. Hence, α-hydroxides have a larger interlayer spacing (generally >7 Å depending on the intercalated anions) than that of the β-form (4.6 Å) resulting in a higher electrochemical activity.
As ECs are energy storage devices with high power densities, their resistance must be kept as low as possible [22]. In order to have low resistance materials, several metals have been used as current collectors in ECs. Nevertheless, metal substrates have some limitations mainly the weight and capacitive inactivity which results in an increase in the “dead” mass loading for the EC assembling leading to a poor performance of the final product. To overcome this drawback, several groups have implemented the use of carbon-based (paper, fabric, foam, etc.) materials as substrates for ECs [23], [24], [25]. For example, carbon foam is a synthetic thin, lightweight, fire resistant, with good electrical conductivity, and high surface area material.
Therefore, we report on the design of a α-Co(OH)2/carbon nanofoam composite as electrochemical capacitor electrode. A detailed electrochemical investigation revealed that this composite can operate in a potential range as high as 2 V in aqueous medium. By combining the formation of Co(OH)2 clusters deposited onto functionalized carbon nanofoams, high specific capacitance of approximately 300 F g−1 (1 A g−1), considering the total mass loading of the composite, was achieved. In addition, in order to evaluate the energy and power density, the composite electrode was coupled with carbon nanofoam forming an asymmetric EC with high energy density.
Section snippets
CNF functionalization
As-received carbon nanofoams from Marketech® (0.2 mm thick, 400 m2/g specific surface area) were functionalized by applying a positive potential of +1.5 V/SCE (5 min) in 1 M HNO3. For comparison, non-functionalized and functionalized CNF were analyzed and designated herein as NF-CNF and F-CNF, respectively.
Co(OH)2 electrodeposition
Co(OH)2 was electrodeposited onto NF-CNF and F-CNF which served as current collector to form a composite electrode. The deposits were obtained from a bath composed of 0.05 M Co(NO3)2 and
CNF functionalization
Fig. 1a and b displays the CVs for NF-CNF and F-CNF in 1 M KOH obtained at different scan rates. NF-CNF (Fig. 1a) presents rectangular-like voltammogram, typical of carbon-based materials. In addition, the working potential window is considerable high (approximately 2.4 V) because hydrogen and oxygen evolution only take place at high potential values. Béguin et al. [26] proposed that such behavior occurs in activated carbon materials due to the following hydrogen storage mechanism:
Conclusions
In conclusion, we have demonstrated the production of a α-Co(OH)2/carbon nanofoam composite electrode operating at 2 V in aqueous medium. By combining the functionalization of the carbon nanofoam substrate, use of pulse electrodeposition and enhancement of the potential window, a composite electrode with specific capacitance close to 300 F g−1 at current density of 1 A g−1, considering its total mass loading, was obtained. The best performance composite electrode (PC/F-CNF) was connected to
Acknowledgments
The authors would like to acknowledge Fundação para a Ciência e a Tecnologia (FCT) under the projects M-ERA.NET/0002/2012 and UID/QUI/00100/2013, and COST Action MP 1004 “Hybrid Energy Storage Devices and Systems for Mobile and Stationary Applications”.
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