Activating Mn Sites by Ni Replacement in α-MnO₂.

Area of Science:

• Materials Science
• Electrochemistry
• Catalysis

Background:

• Transition metal oxides exhibit structure and composition-dependent electrocatalytic activity for oxygen evolution (OER) and oxygen reduction (ORR).
• Manganese oxides are known ORR catalysts, while nickel compounds excel in OER.

Purpose of the Study:

• To investigate the electrocatalytic effects of incorporating nickel into α-MnO₂.
• To understand how Ni incorporation creates distinct active sites with varying electronic configurations and catalytic activities.

Main Methods:

• Synthesis of Ni-modified α-MnO₂ via hydrothermal methods.
• Characterization using X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), transmission electron microscopy with energy-dispersive X-ray spectroscopy (TEM-EDX), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS).
• Electrocatalytic activity assessment via cyclic voltammetry in KOH solution.

Main Results:

• Ni incorporation into the α-MnO₂ lattice (site A) occurs up to ~5% Mn content; higher Ni content forms separate Ni phases (site B).
• Ni incorporation causes subtle changes in electronic configuration, significantly impacting pseudocapacitive responses and revealing Mn 3d orbitals relevant to ORR.
• Site A enhances ORR activity, while site B decreases ORR activity but increases OER rate.

Conclusions:

• Ni-modified α-MnO₂ exhibits tunable electrocatalytic properties for both ORR and OER.
• The electronic structure modifications induced by Ni incorporation are key to enhancing ORR activity.
• Controlling Ni incorporation can selectively tune catalytic performance for specific reactions.