Differentiation of bulk and surface contribution to supercapacitance in amorphous and crystalline NiO.

Supercapacitors have attracted much attention in recent years due to their ability to store energy comparable to batteries such as PbO2/Pb or Ni/MH, and to deliver the stored energy much more rapidly than batteries. Supercapacitors not only store charges between the electrode and electrolyte interfaces as electrochemical double layer capacitor (EDLC), 4] but they also store extra charges with a fast and reversible redox reaction between electrode and the electroactive species in the electrolyte as pseudocapacitors. Such a redox reaction often takes place in oxides where metal ions have multiple valence states. Most studies have focused on increasing the surface area of electrodes in order to increase the capacitance. Although this is a viable approach, it is also critical to determine whether there is significant contribution from the bulk of the materials, and if so, how one can take advantage of the bulk contribution by understanding its characteristics. The existence of the bulk contribution is evident from the fact that the capacitance is much larger in amorphous materials than their crystalline counterparts. However, there is no quantitative understanding concerning the capacitance in electrodes of different structures. With these questions in mind, it is worthwhile to point out that the purpose of the current work is not to demonstrate another supercapacitor with enhanced performance. More importantly, our goal is to establish the quantitative understanding of surface and bulk contributions between amorphous and crystalline electrodes and their electrochemical characteristics, which will benefit future electrode design. We have chosen NiO films as a model system. Our experimental results indicate the surface contribution to supercapacitance is the same for amorphous and crystalline materials where the bulk contribution is three times greater in amorphous materials. The results are believed to also be applicable to other electrode materials. Both crystalline and amorphous NiO films were made by radio requency (RF) magnetron sputtering in a mixture of O2 and Ar gases at a fixed pressure (1.33 Pa). The relative flow rates were varied to control the stoichiometry and crystallinity. At 5% (flow rate) O2, crystalline NiO phase was achieved as shown in Figure 1d. All amorphous films were fabricated at 100% O2, and their representative X-ray diffraction (XRD) patterns are shown in Figure 1c. KOH aqueous solution (1molL ) was chosen as electrolyte in which NiO was electrochemically stable; Figure 1a shows the cyclic voltammograms (CV) of 20 nm an amorphous NiO thin film sample measured at a scanning rate of 100 mVs , within the potential window of 0 V– 0.85 V in reference to the normal hydrogen electrode (NHE). Similar CV curves, but with much smaller current density, were observed in the crystalline NiO films. Peak 1 and peak 2 are a pair of redox processes with insertion/extraction of OH according to: