Activating Mn Sites by Ni Replacement in α-MnO2

Sami M Alharbi; Mohammed A Alkhalifah; Benjamin Howchen; Athi N A Rahmah; Veronica Celorrio; David J Fermin

doi:10.1021/acsmaterialsau.3c00051

Activating Mn Sites by Ni Replacement in α-MnO₂

ACS Mater Au. 2023 Nov 20;4(1):74-81. doi: 10.1021/acsmaterialsau.3c00051. eCollection 2024 Jan 10.

Authors

Sami M Alharbi^{1

2}, Mohammed A Alkhalifah^{1

3}, Benjamin Howchen¹, Athi N A Rahmah¹, Veronica Celorrio⁴, David J Fermin¹

Affiliations

¹ School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, U.K.
² Department of Chemistry, College of Science, Qassim University, Buraydah 52571, Saudi Arabia.
³ Department of Chemistry, College of Science, King Faisal University, P.O. Box 380, Al-Ahsa, 31982, Saudi Arabia.
⁴ Diamond Light Source Ltd., Diamond House, Harwell Campus, Didcot OX11 0DE, U.K.

Abstract

Transition metal oxides are characterized by an acute structure and composition dependent electrocatalytic activity toward the oxygen evolution (OER) and oxygen reduction (ORR) reactions. For instance, Mn containing oxides are among the most active ORR catalysts, while Ni based compounds tend to show high activity toward the OER in alkaline solutions. In this study, we show that incorporation of Ni into α-MnO₂, by adding Ni precursor into the Mn-containing hydrothermal solution, can generate distinctive sites with different electronic configurations and contrasting electrocatalytic activity. The structure and composition of the Ni modified hollandite α-MnO₂ phase were investigated by X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy (TEM-EDX), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and X-ray photoelectron spectroscopy (XPS). Our analysis suggests that Mn replacement by Ni into the α-MnO₂ lattice (site A) occurs up to approximately 5% of the total Mn content, while further increasing Ni content promotes the nucleation of separate Ni phases (site B). XAS and XRD show that the introduction of sites A and B have a negligible effect on the overall Mn oxidation state and bonding characteristics, while very subtle changes in the XPS spectra appear to suggest changes in the electronic configuration upon Ni incorporation into the α-MnO₂ lattice. On the other hand, changes in the electronic structure promoted by site A have a significant impact in the pseudocapacitive responses obtained by cyclic voltammetry in KOH solution at pH 13, revealing the appearance of Mn 3d orbitals at the energy (potential) range relevant to the ORR. The evolution of Mn 3d upon Ni replacement significantly increases the catalytic activity of α-MnO₂ toward the ORR. Interestingly, the formation of segregated Ni phases (site B) leads to a decrease in the ORR activity while increasing the OER rate.