Electrochemical supercapacitors(ES) have attracted extensive attention in recent years owing to their ability to supply high power in short-term pulse and long cycling life, which makes them very good energy storage devices for applications such as hybrid power sources for electrical vehicles, portable electronic devices, uninterruptible power supply (UPS), and pulse laser techniques^1-3.

Currently, there are three major types of electrode materials for ES: metal oxides/hydroxides, carbon materials and conducting polymers⁴.Among various materials, Metal oxides/hydroxides are execllent electrode materials for ES due to their rich redox properties involving multiple oxidation states⁵. RuO₂ shows excellent supercapacitive performance, but its high cost and toxic feature impede commercial applications severely. Therefore, various transition metal oxides/hydroxides such as NiO, Co₃O₄, Co(OH)₂, CoOOH and so on with low cost and benign environmental compatibility have been employed as the electrode materials for ES and investigated extensively to substitute RuO₂. However, transition metal oxides/hydroxides are still faced with serious challenge because of the intrinsic poor electrical conductivity. We have recently devoted ourselves to the composite electrode materials based on transition metal hydroxides and Multi-walled carbon nanotubes (MWCNT) .In this paper, we report the facile fabrication of Co (OH)₂/MWCNT composites and demonstrate the excellent capacitive performance.

In a typical synthesis, 10mg MWCNT and certain amount of CoSO₄^.6H₂O were dispersed into 40mL deionized water through ultrasonication, subsequently, 10mL ammonia which was diluted 10-fold was added to above solution gradually under vigorous stirring. Then transferred into a Teflon-lined stainless steel autoclave and maintained at 160 ^oC for 6 h. After cooling down to room temperature, the precipitate was separated by centrifugation, washed with double distilled water and ethanol for several times, and dried at 80 ^oC for 12 h to obtain the final products.

The electrochemical performance of Co(OH)₂/ MWCNT was characterized by cyclic voltammetry (CV) in 1 M KOH aqueous solution at room temperature. The test cell was a three-electrode system in which disk electrode supported Co(OH)₂/MWCNT was assembled as working electrode. A platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively.

Figure 1 shows the CV curves of Co(OH)₂/MWCNT in 1 M KOH solution at a variety of sweep rates varying from 10 to 50 mV/s in the potential range from -0.15 to 0.5 V vs. SCE. Two pairs of redox peaks are observed in each CV curve, the reduction peak and the corresponding oxidation peak is related to the redox reaction:

 Co(OH)₂+ OH^-=CoOOH+ e^-

 While the other is associated with the following redox reaction:

 CoOOH + OH^-=CoO₂ + H₂O + e^-

Figure 2 shows the Galvanostatic charge-discharge curves at various current densities varying from 1 to 10Ag^-1, These charge/discharge curves are approximately symmetric, implying that the electrode has a good electrochemical capacitive characteristic and a superior reversible redox reaction.

References

(1) Xi Wang, Wei Tian, Tianyou Zhai, J. Mater. Chem., 2012, 22, 23310.

(2) Jiewu Cui, Xinyi Zhang, Liang Tong, J. Mater. Chem. A, 2015, 3,10425.

(3) Zhou, H. S.; Zhu, S. M.; Hibino, M.; Honma, I. J. Power Sources2003, 122, 219–223.

(4) Lianbo Ma, Hu Zhou, Xiaoping Shen, RSC Adv., 2014, 4, 53180.

(5) Kian Keat Lee, Wee Shong Chinan,Chorng Haur Sow, J. Mater. Chem. A, 2014, 2,17212 .