Analysis of stresses generated during hydrogen extraction from and injection into $Ni(OH)_2$/NiOOH film electrode

Abstract

Stresses generated during the hydrogen extraction from and injection into the $Ni(OH)_2$/NiOOH film electrode in 0.1 M KOH solution have been analysed by using laser beam deflection technique combined with potentiostatic current transient and quartz crystal microbalance (QCM) techniques. From the values of the film thickness measured, the time to complete phase transformation observed and of the potential step given between the hydrogen extraction/injection potential and plateau potential, the velocity and the mobility of phase boundary movement (PBM) were determined to be $2.34 X 10^{-6}$ to $5.85 X 10^{-6} cm s^{-1}$ and $1.56 X 10^{-5}$ to $1.17 X 10^{-4} cm s^{-1} V{-1}$, respectively. The larger mobility for PBM during the hydrogen injection than that during the hydrogen extraction indicates that the PBM is more facile during the hydrogen injection than that during the hydrogen extraction. From the mass change measured from QCM, it is suggested that, during the hydrogen extraction, the PBM continues until the insertion of $K^{+}$ ions begins to occur simultaneously with the desertion of $H_2O$ molecules. During the hydrogen injection, the PBM is accompanied by the desertion of $K^{+}$ ions, followed by the insertion of $H_2O$ molecules at the electrode/electrolyte interface. The tensile and compressive deflections measured during the hydrogen extraction from and injection into the $Ni(OH)_2$/NiOOH electrode, respectively, rose sharply and then were relaxed to a small extent. From current transient, mass change transient and deflection transient simultaneously measured, it is suggested that the occurrence of sharp tensile and compressive movements in the initial stage is traced back to the PBM and the following relaxation of tensile and compressive deflections is attributed to the desertion/insertion of $H_{2}O$ molecules. Most of stresses developed during the hydrogen extraction and injection originate mainly from the PBM, not the desertion/insertion of $K...