Stoichiometry–grain size-specific capacitance interrelationships in nickel oxide

Abstract

Nickel oxide exhibits almost the highest theoretical specific capacitance (C_s), which includes contributions from non-faradaic double layer charging and faradaic OH⁻ adsorption. However, the realistic and tangible C_s is due to the faradaic process, which can be influenced by chemical (i.e. stoichiometry) and structural (i.e. grain size) changes. Hence, it is necessary to investigate the interrelationships among chemical and structural features and charge storage capacity. Here, a non-stoichiometric nickel oxide (Ni_xO) containing Ni²⁺ and Ni³⁺ was synthesized by a sol–gel method at 620, 720 and 920 °C using Ni(NO₃)₂·6H₂O and citric acid. The grain size as estimated from X-ray diffraction increases from 55 to 194 nm with increase in the synthesis temperature. The stoichiometry measured through Ni²⁺ (or Ni³⁺) fraction from X-ray photoelectron spectroscopy also increases from 70.3 to 99.2 atom% with synthesis temperature. The C_s due to faradaic OH⁻ adsorption was estimated from cyclic voltammetry in 2 M KOH within −0.05 to +0.60 V vs. Hg/Hg₂Cl₂/KCl (sat. in water). This C_s increases from 7.5 to 92.4 F g⁻¹ with a decrease in the grain size and stoichiometry (increase in Ni³⁺) due to possibly the increased conductivity and NiOOH formation through OH⁻ adsorption. The deviation from stoichiometry at lower grain size mainly stems from nickel vacancy accommodation, according to the thermodynamic model proposed here. The interrelationships among stoichiometry, grain size and the specific capacitance of nickel oxide are investigated.