Layered -Co(OH)2 Nanocones as Electrode Materials for Pseudocapacitors: Understanding the Effect of Interlayer Space on Electrochemical Activity

The effect of space accessible to electrolyte ions on the electrochemical activity is studied for a system of transition-metal hydroxide-based pseudocapacitors. Layered -Co(OH)2 with various intercalated anions is used as a model material. Three types of layered -Co(OH)2 with intercalated anions of dodecyl sulfate, benzoate, or nitrate, are prepared by a simple reflux and an anion-exchange process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations and X-ray diffraction (XRD) data show the formation of layered -Co(OH)2 nanocones with interlayer spacing between adjacent Co(OH)2 single sheets of 1.6, 0.7, and 0.09 nm, corresponding to the anions as listed above. Electrochemical characterization reveals that interlayer space has a great effect on the electrochemical activity of -Co(OH)2 nanocones as an electrode material. For the interlayer spacing of 1.6 nm, in the case of dodecyl sulfate-intercalated -Co(OH)2, the Faradaic reaction takes place more adequately than for benzoate- and nitrate-intercalated -Co(OH)2. As a result, a higher specific capacitance and better cycling stability is obtained for the dodecyl sulfate-intercalated -Co(OH)2. The electrochemical activity obviously reduces when the interlayer space decreases to 0.7 nm. Our results suggest the importance of rational designing the interlayer space of layered transition metal hydroxides for high-performance pseudocapacitors.