Effect of Different Metal Ions between Nanolayers of Manganese Oxide for Water Oxidation Reaction under Acidic Conditions

Abstract

A new approach is used to synthesize various metal ions [Mg(II), Ca(II), Ba(II), Al(III), Zn(II), Cd(II), Mn(II), Fe(III), Co(III), Ni(II), and Cu(II)] between layers of Mn oxide in a pre-synthesized framework of layered Mn−K oxide. These Mn oxides were heated at 60−600 °C, and their electrochemistry and water oxidation reaction (WOR) were investigated at pH = 1. Using this strategy, the framework of the structure of catalysts is the same, and all are synthesized in a layered Mn oxide containing K(I) framework. Thus, comparing different ions in the same framework is easier. After calcination at 300−600 °C, X-ray absorption spectroscopy shows a layered structure with no change in bulk, but XRD shows that a few conversions regarding layered Mn oxide → α-MnO 2 → α-Mn 2 O 3 occur. Indeed, after calcination at 300−600 °C, the formed layered Mn oxide converts into a better WOR catalyst. After calcination at ≥300 °C, small amounts of α-MnO 2 /α-Mn 2 O 3 are formed in the structure, which could be important for WOR. At 600 °C, trace amounts of crystalline α-Mn 2 O 3 are usually formed in the presence of redox-inert ions, which is not an efficient catalyst for WOR. In the presence of redox-active ions, the formation of metal oxides such as Fe, Co, Ni, and Ni oxides is possible at 600 °C. These metal oxides are usually catalysts for WOR. An open structure of layered Mn oxide could increase the WOR activity of these redox-active ions. Thus, Mn ions may not be active sites for WOR and could be a substrate for these redox-active ions. Among the redox-inert ions between the layers of Mn oxides, the presence of Ca(II) at 500 °C results in a significant increase in WOR. Interestingly, a Ca/Mn cluster is present in the biological WOR catalyst.

Innovative Insights into Water-Oxidation Mechanism: Investigating Birnessite’s Reaction with Cerium(IV) Ammonium Nitrate

“…The related patterns for the prepared birnessite are shown in Figure a (Figure S1). − The scanning electron microscopy (SEM) images reveal nanosheets with characteristic thin, flat surfaces that spread extensively in two dimensions, indicating the essential features of birnessite nanosheets (Figure b,c; Figures S2 and S3). At higher resolution, these nanosheets tend to layer or stack in certain areas, hinting at possible aggregation or assembly within the sample.…”

Iron Integration in Nickel Hydroxide Matrix vs Surface for Oxygen-Evolution Reaction: Where the Nernst Equation Does Not Work

“…The SEM image exhibits a certain roughness indicative of a textured topography. This roughened surface could potentially contribute to a larger surface area, which might enhance the material’s catalytic properties or adsorption capabilities. − …”

Composition Tuned Electron Rearrangement of Transition-Metal-Based Compounds Promotes Hydrogen Generation

“…To reduce the energy barriers of the two half reactions of water electrolysis, many advanced catalysts have been well developed. − The transition-metal-based catalysts with rationally designed chemical compositions and microstructures have been found to exhibit excellent catalytic activity that is comparable to or even better than the noble-metal-based materials. ,, In general, the catalysts with multi-transition metals would have better catalytic activity than their single metal counterparts. − The change of anions of the transition-metal-based catalysts has been found to modulate their catalytic characters toward other energy conversion reactions. For example, most of the transition metal oxides (TMOs) − or hydroxides (TM-OH) , are advanced OER catalysts, while the corresponding TM sulfides (TMS) or TM phosphides (TMP) − are usually reported to be active for the hydrogen evolution reaction (HER), even though the catalytic active sites are still assumed to be the transition metal ions. In addition, the modification of chemical composition, even the slight change of the concentration of transition metal ions, would lead to distinct microstructures, which will also affect the catalytic performance of the material. − Therefore, it is critically important to disentangle the roles of chemical composition from the morphological effects, and further determine the influence of both transition metal cations and anions for water splitting electrolysis. − …”