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

Abstract

The effect of space accessible to electrolyte ions on the electrochemical activity is studied for a system of transition-metal hydroxide-based pseudocapacitors. Layered α -Co(OH) 2 with various intercalated anions is used as a model material. Three types of layered α -Co(OH) 2 with intercalated anions of dodecyl sulfate, benzoate, or nitrate, are prepared by a simple refl ux and an anionexchange process. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations and X-ray diffraction (XRD) data show the formation of layered α -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 α -Co(OH) 2 nanocones as an electrode material. For the interlayer spacing of 1.6 nm, in the case of dodecyl sulfate-intercalated α -Co(OH) 2 , the Faradaic reaction takes place more adequately than for benzoate-and nitrate-intercalated α -Co(OH) 2 . As a result, a higher specifi c capacitance and better cycling stability is obtained for the dodecyl sulfate-intercalated α -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. Figure 8 . Schematic drawings of: a) DS-α -Co(OH) 2 , b) NO 3 − -α -Co(OH), and, c) benzoate-α -Co(OH) 2 , where electrolyte ions (OH − ) soak into the interlayer of two adjacent single sheets of Co(OH) 2 .

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“…In contrast, pseudocapacitors can offer far higher specific capacitance and energy density [5]. At present, the pseudocapacitors electrode materials generally involve various metal oxides [9][10][11], metal hydroxides [12,13], metal sulphides [7,14] and conductive polymers [12,14]. Among these materials, transition metal oxides, in Page 3 of 36 A c c e p t e d M a n u s c r i p t 3 terms of achieving high specific capacitance in SCs, are regarded as excellent pseudocapacitors materials [15].…”

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“…For example, the enlargement of interlayer space allows more storage of electrolyte ions as showed in some studies regarding the systems of transition metal hydroxide-based pseudocapacitors. 49 The increase of the specific surface area provides more active sites for electrochemical reactions. The microporous or nanoporous structures could improve the mass transfer during redox reactions.…”

Effects of dodecyl sulfate and nitrate anions on the supercapacitive properties of α-Co(OH) 2

“…The retention of initial capacitance for sample Co(OH) 2 -DS and Co(OH) 2 -NO 3 is 66.7% and 62.5% after 2000 repetitive cycles, respectively. Sample Co(OH) 2 -DS exhibits a slightly better cyclic stability than sample Co(OH) 2 -NO 3 probably due to its larger interlayer space, which may relate to the sufficient electrolyte ions being supplied for redox reaction during the electrochemical processes [18]. The first four and last four galvanostatic charge/discharge cycles for sample Co(OH) 2 -DS and Co(OH) 2 -NO 3 at a current density of 2 A g À1 are presented in Fig.…”

“…b-Co(OH) 2 is a stoichiometric phase of the composition Co(OH) 2 with a brucite-like structure (a = 3.12 Å, c = 4.6 Å) [13,14], while a-form Co(OH) 2 has a similar structure to layered double hydroxides (LDHs) that are composed of hydrotalcite-like layers with a positive residual charge and a series of hydrated anions in the interlayer space to keep charge balance, resulting in a substantial expansion on the interlayer spacing (>7 Å) [15][16][17]. The intercalated anions between the hydrotalcite-like host layers of a-Co(OH) 2 are exchangeable, leading to a possibility for adjusting the interlayer spacing [15,18]. The d-spacing of a-Co(OH) 2 is always determined by the size of interlayer anions, thus a-Co(OH) 2 intercalated with large size anions usually possesses a greater interlayer spacing than that intercalated with small size anions.…”

“…There are many literatures reported on the synthetic method of DS intercalated LDHs because of its large interlamellar space, which make it to be a good candidate for anion-exchange, delamination and restacking [20][21][22]. Three main routes to obtain DS intercalated hydrotalcite-like compounds are anion-exchange [23,24], regeneration from layered double oxide achieved by calcination of a suitable LDH precursor in a solution containing DS anions [25,26] and spontaneous self-assembly which involves one-step co-precipitation of LDH precursors in the presence of DS anions [18,27]. However, anion-exchange can not achieve an efficient intercalation of DS anions under ambient conditions due to the strong affinity between CO 3 2À and the host layers [15,20,28].…”