Fe plays a critical, but not yet understood, role in enhancing the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts. We report electrochemical, in situ electrical, photoelectron spectroscopy, and X-ray diffraction measurements on Ni1–xFex(OH)2/Ni1–xFexOOH thin films to investigate the changes in electronic properties, OER activity, and structure as a result of Fe inclusion. We developed a simple method for purification of KOH electrolyte that uses precipitated bulk Ni(OH)2 to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni(OH)2/NiOOH reveals new Ni redox features and no significant OER current until >400 mV overpotential, different from previous reports which were likely affected by Fe impurities. We show through controlled crystallization that β-NiOOH is less active for OER than the disordered γ-NiOOH starting material and that previous reports of increased activity for β-NiOOH are due to incorporation of Fe-impurities during the crystallization process. Through...

Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation

Review of the structure and the electrochemistry of nickel hydroxides and oxy-hydroxides

This review article summarizes the last few decades of research on nickel hydroxide, an important material in physics and chemistry, that has many applications in engineering including, significantly, batteries. First, the structures of the two known polymorphs, denoted as α-Ni(OH)2 and β-Ni(OH)2, are described. The various types of disorder, which are frequently present in nickel hydroxide materials, are discussed including hydration, stacking fault disorder, mechanical stresses and the incorporation of ionic impurities. Several related materials are discussed, including intercalated α-derivatives and basic nickel salts. Next, a number of methods to prepare, or synthesize, nickel hydroxides are summarized, including chemical precipitation, electrochemical precipitation, sol-gel synthesis, chemical ageing, hydrothermal and solvothermal synthesis, electrochemical oxidation, microwave-assisted synthesis, and sonochemical methods. Finally, the known physical properties of the nickel hydroxides are reviewed, including their magnetic, vibrational, optical, electrical and mechanical properties. The last section in this paper is intended to serve as a summary of both the potentially useful properties of these materials and the methods for the identification and characterization of 'unknown' nickel hydroxide-based samples.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.2014.0792

Nickel hydroxides and related materials: a review of their structures, synthesis and properties.

Powders of single-crystalline β-nickel hydroxide (β-Ni(OH)2) nanosheets with the hexagonal structure have been successfully synthesized by the hydrothermal method at 200 °C using nickel acetate as the nickel source and aqueous ammonia as both an alkaline and complexing reagent The yields of β-Ni(OH)2 nanosheet powders were higher than 924% This method is simple and low-cost for large-scale production of powders of single-crystalline β-Ni(OH)2 nanosheets Single-crystalline nickel oxide (NiO) nanosheets have been synthesized by thermal decomposition method at 400 °C for 2 h using single-crystalline β-Ni(OH)2 nanosheets as the precursor The sheet shape of β-Ni(OH)2 was sustained after thermal decomposition to NiO The as-prepared products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetric analysis (DSC), and thermogravimetric analysis (TG)

R. S. McEwen

Papers

Crystallographic studies on nickel hydroxide and the higher nickel oxides