Dependence of Interlayer Distance on the Charge Transfer Reaction of Ni-Al Layered Double Hydroxides
Authors: Maki Hideshi , Inoue Masayoshi , Mizuhata Minoru 
Published: February 2020

Ni(OH)2 has been used as electrode material of nickel-metal hydride (Ni-MH) battery and hybrid capacitors. The application of Ni-Al LDH to the cathode material of Ni-MH were attracted owing to its structural stability and high electrochemical properties. The nickel positive active material for the alkali storage battery used widely at present is b-Ni(OH)2. Whereas, if Ni-Al LDH which is a higher-order oxide functions as an active material for a positive electrode, the oxidation reaction in the charging process becomes the multi-electron reaction because Ni-Al LDH has a type crystal structure. Therefore, the capacity per unit weight of the active material will increase, and a drastic improvement of the battery capacity will be achieved. Recently, the reaction products deposit by the interfacial reaction which the thin film continuously forms only on the surface of a solid phase by liquid phase deposition (LPD) method. The active material layers which are extremely adhered to the substrates can be constructed by the LPD method, accordingly, the charge transfer resistance between a current collector and an active material drastically decreased when a current collector is used as a substrate[1,2]. In this research, the Ni-Al LDH was synthesized by the LPD method, and the interlayer distance of the LDH has been changed by the intercalation of the anions which have various ionic radii into the interlayer of the LDH. Furthermore, the influence of the electrode on internal resistance by the interlayer distance of the Ni-Al LDH was discussed by using the layered double hydroxide sample as an active material.
About 15% NH3 aq. was added into 0.4 mol L-1 Ni(NO3)2 aq. until pH 7.5 under stirring, and precipitated b-Ni(OH)2 was filtered off by suction filtration. 0.5 mol L–1 HF was added until the b-Ni(OH)2 dissolved as a nickel fluoride complex. The total F- ion concentration and pH of appropriate amount of the solution were adjusted by hydrofluoric acid and NH3 aq. to 200 mmol L-1 and 8.2, respectively. Then, the total Al3+ concentration was adjusted to 1.2 mmol L-1 by Al(NO3)3 aq. Carbon black powder (CB) or nickel foams (Ni foam) which apply the surface hydrophilization are added as a substrates into the solution, and reacted at 50 °C for 48 h. The prepared Ni-Al LDH(as-dep.LDH) as added the solutions of the various anions, and the anion exchange reaction in the interlayer of the LDH samples for 24 h were carried out. The electrode which made from the anion exchanged Ni-Al LDH as an active material was prepared as our previous report[2]. The Hg/HgO into 6 mol L-1 KOH aq. and the foam nickel plate were employed as the reference electrode and the counter electrode, respectively, while 6 mol L-1 KOH aq. served as an electrolyte solution. The electrochemical impedance spectroscopy (EIS) measurement was performed at 500 mV. The frequency range was 10 kHz - 50 mHz and the voltage amplitude was 5 mV. It was confirmed from the XRD measurement of the anion exchanged LDH that the interlayer distance (d003) of the Ni-Al LDH increased as the anionic radii increased. Neither complex ions (i.e., [Fe(CN)6]4-) nor surface active agent anions (i.e., dodecanesulfonic anion and dodecyl benzenesulfonic anion) did intercalate immediately into the interlayer of the as-dep.LDH because of their large ionic size. It was confirmed that the intercalation of the complex ions and surface active agent anions became possible after the increasing of the interlayer distance by the intercalation of SO4
2- as preliminary intercalation. Fig.1 shows the dependence of the interlayer distance of the Ni-Al LDH on the Ni foam as an active material on the charge transfer resistance (R
ct) concerning the anion exchanged electrodes. It was obvious that the R
ct values decrease as the interlayer distance increases. This is because the ion diffusion in the inter layer becomes easy because of the increase of the interlayer distance, and it can be considered that it is because the Rct value decreases by the improvement of the electrode reaction. Whereas, the interlayer distance dependence of the Ni-Al LDH on the CB substrate hardly appeared on the Rct value concerning the anion exchanged electrodes. It can be concluded that this is because the gross weight of the Ni-Al LDH on CB substrate is less than that on Ni foam, namely, in that case of the CB substrate, the influence on Rct by the increase of the interlayer distance of the active material is relatively slight.
[1] H. Maki, et al., ACS. Appl. Mater. Interfaces, 7, 2015, 17188-17198.
[2] M. Takigawa, et al., Electrochemistry, 83, 2015, 803-806.

Figure 1