Stoichiometry-grain size-specific capacitance interrelationships in nickel oxide.

Area of Science:

• Materials Science
• Electrochemistry
• Nanotechnology

Background:

• Nickel oxide (NiO) offers high theoretical specific capacitance (Cs) from double-layer charging and faradaic hydroxide adsorption.
• Practical Cs in NiO is primarily from faradaic processes, influenced by stoichiometry and grain size.
• Understanding the interplay between chemical/structural properties and charge storage is crucial for optimizing NiO performance.

Purpose of the Study:

• To investigate the relationship between stoichiometry, grain size, and specific capacitance in non-stoichiometric nickel oxide.
• To synthesize NiO with varying stoichiometry and grain size using a sol-gel method.
• To correlate synthesis temperature with structural and chemical properties and their impact on charge storage.

Main Methods:

• Sol-gel synthesis of non-stoichiometric NiO at different temperatures (620, 720, 920 °C).
• X-ray diffraction (XRD) for grain size estimation.
• X-ray photoelectron spectroscopy (XPS) for determining Ni2+/Ni3+ stoichiometry.
• Cyclic voltammetry (CV) in 2 M KOH to measure specific capacitance (Cs).

Main Results:

• Synthesis temperature increased grain size from 55 to 194 nm and stoichiometry (Ni2+/Ni3+ fraction) from 70.3 to 99.2 atom%.
• Specific capacitance (Cs) increased from 7.5 to 92.4 F g-1 as grain size decreased and Ni3+ content increased.
• A thermodynamic model suggests nickel vacancy accommodation explains deviations from stoichiometry at smaller grain sizes.

Conclusions:

• Lower grain size and higher Ni3+ content in non-stoichiometric NiO enhance specific capacitance.
• Improved conductivity and efficient NiOOH formation via OH- adsorption contribute to increased Cs.
• The study elucidates the critical interdependencies of stoichiometry, grain size, and electrochemical performance in NiO.