Electrochemical and Optical Properties of Ni-Al Layered Double Hydroxides Containing Different Intercalated Counter Anions

In this work, nickel aluminum layered double hydroxides (LDHs) containing different charge balancing counter anions (Cl-, CO32-, NO33-, and B033) were prepared. The effect of these counter anions on the stability, and reversibility of the oxidation of the redox active nickel sites in the layers of these LDHs was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), powder X-ray diffraction (XRD) and iodometry.;The LDH containing carbonate counter ions (Ni-Al-CO3) was found to be more stable when subjected to potential scans in basic solution, compared to the LDH containing chloride ions (Ni-Al-Cl). Films of Ni-Al-CO 3 could be subjected to multiple potential cycles in basic electrolyte with no change in their XRD patterns. However, the Ni(II) in the chloride LDH could be oxidized to a much higher average valence (>3.36+) than in the carbonate LDH (>2.59+). The EIS results showed that the onset of oxidation of nickel in the carbonate LDH required a higher direct current (dc) potential than in the chloride LDH. The charge transfer resistance of the partially oxidized carbonate LDH also remained much higher than that of the partially oxidized chloride LDH. The nitrate form of the LDH (Ni-Al-NO3) behaves like the chloride form. Ni(II) in Ni-Al-NO3 could be oxidized to almost 4+. However, the nitrate LDH was unstable when subjected to multiple potential scans in basic solutions. Promising results were obtained for a 1:1 ratio mixed carbonate-borate LDH. The nickel in this mixed carbonate-borate LDH could be oxidized to 3.37+, much higher than in the pure carbonate LDH, and this mixed borate-carbonate LDH was much more stable than the chloride or nitrate LDHs. These results could be of practical importance in the use of nickel LDHs as alternative cathode materials for rechargeable batteries. Commercial NiMH batteries currently use β-Ni(OH)2. The nickel in the powder is oxidized to 3+ on charging and reduced back to 2+ on discharge. However, overcharging can cause the conversion of β-Ni(OH)2 to γ-NiOOH resulting in a large increase in volume and failure of the cell. Because nickel in α-Ni(OH)2 can be oxidized above 3+ with little change in volume, α-Ni(OH)2 has a higher energy storage density than β-Ni(OH)2. However, α-Ni(OH) 2 is not stable in the basic electrolytes used in these batteries. The mixed carbonate-borate LDH has very similar structure to α-Ni(OH) 2, and appears to possesses a good combination of high stability in base and ease of nickel oxidation to a high average valence.;The electrochromic properties of the Ni-Al LDH films were also investigated. A reversible transmittance change of up to about 20% with a coloring response time of 9.7 s and a bleaching response time of 38.3 s was obtained for 10 μg Ni-Al-C1 films in 0.1M NaOH, when subjected to a potential steps between 0.5 V and 1.2 V. The effects of film thickness, electrolyte, film deposition method, scan rate and potential range were also investigated. In pH 10 sodium borate buffer, the transmittance change was dramatically improved from 12% to 35% by the addition of the 0.1 mM [Co(bpy)3]2+ to the electrolyte. It is proposed that the cobalt cations mediated the reduction of electrochemically oxidized nickel in the films, making the colour change associated with the oxidation-reduction of the nickel sites more reversible.