Design of Highly Active and Stable Bifunctional Electrocatalysts for Oxygen Reactions

Abstract

A new design strategy for the development of bifunctional electrocatalysts capable of catalyzing both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is proposed. In this strategy, the MnO x lattice is doped with either electropositive (Sr, Ba) or electronegative (Bi, Pb) elements that results in the coincorporation of electron-rich donor (Mn2+) and electron-poor acceptor (Mn4+) defects in the same parent (Mn3+) lattice. These defects effectively catalyze the reduction (ORR) and oxidation (OER) processes on the same electrode surface. This study is based on the results of a previous study on Mn2O3 that showed Mn2+ and Mn4+ as the active sites for ORR and OER processes, respectively. Our results show that BiMnO x is the most promising bifunctional catalyst with OER/ORR activities that are comparable to the individual activities of state-of-the-art commercial Pt or RuO2 catalysts. Stability tests show the catalyst to be stable for more than 3 h of continuous OER or ORR polarization. This work provides a pathway for the individual tuning of defects to control electrocatalytic activities, which opens up new possibilities for the rational design of many perovskite-based oxides.

Unusually High Electron Affinity Enables the High Oxidizing Power of Layered Birnessite

“…We, therefore, used photoluminescence spectroscopy (PL) under ambient air conditions to determine Mn cation concentrations in the birnessite lattice . The characteristic emission of Mn cations occurs at the following spectral energies: 4 T 1g → 6 A 1g of Mn­(II) at 1.78 eV, 3 T 1g → 5 E 1g of Mn­(III) at 1.35 eV, 2 E 1g → 4A 2 of Mn­(IV) at 1.58 eV, and 3 T 2 → 3 A 2 of Mn­(V) at 1.1 eV. , The PL spectrum of birnessite (Figure A) shows three peaks corresponding to Mn­(II), Mn­(III), and Mn­(IV) cations. Deconvolution of the spectrum was performed by fitting with Gaussian peaks of fixed characteristic energies.…”

Formation of the Metastable MnIII Water Oxidation Intermediate in Birnessite is Controlled by a Dissolution‐Deposition Process Involving Labile MnII

“…Figure 1B shows a representative emission spectrum of the birnessite powder measured under ambient air conditions. The characteristic emission of Mn cations occur at the following spectral energies: 4 T 1g → 6 A 1g of Mn II at 1.78 eV, 3 T 1g → 5 E 1g of Mn III at 1.35 eV, and 2 E 1g → 4 A 2 of Mn IV at 1.58 eV [28,29] . Three peaks corresponding to Mn II , Mn III and Mn IV cations can be observed in the PL spectrum of birnessite.…”

“…4 A 2 of Mn IV at 1.58 eV. [28,29] Three peaks corresponding to Mn II , Mn III and Mn IV cations can be observed in the PL spectrum of birnessite. The spectrum was deconvoluted by fitting with Gaussian peaks at fixed characteristic energies.…”

Direct determination of Mn valence states in mixed‐valent manganates by photoluminescence spectroscopy

“…The small concentration of Mn III signal ($8%) is due to the presence of oxygen vacancy defects naturally formed during synthesis, which also renders the lattice n-type, as determined from the Hall Effect measurements. 37 It is of note that the alkali and alkaline earth metals have no emission signal in the spectral range where Mn emission occurs. This is shown for the case of Sr in Figure 4C using the spectrum of Sr(NO 3 ) 2 .…”

“…Perovskite strontium manganite (SrMnO 3-δ ) was synthesized using a sol-gel method reported earlier. 37 Triclinic sodium birnessite was synthesized through the redox reaction between MnO 4…”