Optimization of $Ni(OH)_2$ electrodes

Nickel hydroxide (Ni(OH)2) is a layered hydroxide material that possesses a characteristic two-dimensional layered structure, in which nickel cations are sandwiched between hydroxide ion sheets. This structural arrangement facilitates ion diffusion and reversible redox reactions, making Ni (OH)2 a widely studied pseudocapacitive material due to its high theoretical capacitance. The electrochemical performance of Ni(OH)2 is strongly influenced by structural features formed during synthesis, making control of synthesis conditions critical for optimizing structure and charge-storage capacity. The influence of different precipitating agents, urea and hexamethylenetetramine (HMT), on the structure and electrochemical behavior of hydrothermally synthesized Ni(OH)2 was systematically examined. At elevated temperatures, carbonate forms and becomes incorporated into the structure, particularly in urea-derived samples, producing a more compact and crystalline phase with reduced interlayer spacing. This limits access to redox-active sites and reduces capacitance. In contrast, low-temperature synthesis with HMT yields a turbostratic α-phase with expanded interlayer and a more open structure, enhancing ion transport and cycling stability. In situ Ni K-edge X-ray absorption spectroscopy (XAS) was employed to directly observe the redox behavior of nickel valence state during charging and discharging processes. The measurements confirmed enhanced redox activity and reversibility of the Ni2+/Ni3+ redox couple in carbonate free structures with expanded interlayer.