Manganese-Oxide Solids as Water-Oxidation Electrocatalysts: The Effect of Intercalating Cations

Abstract

A set of birnessite-type manganese oxides with both monovalent alkaline cations (Li + , Na + , K + , Rb + or Cs +) and divalent alkaline-earth cations (Ca 2+ or Mg 2+) has been synthesized using electrolytic deposition methods. X-ray diffraction gives typical birnessite-type patterns, with diffraction peaks at 2θ = 12.01°and 24.7°assigned to (001) and (002) planes. Electron paramagnetic resonance spectra suggest differences in the stability of Mn(III) species in manganese-oxide layer, which is more stable in birnessite samples with divalent cations than samples with monovalent cations. The activity of samples for an anodic O 2-evolution reaction is considerably affected by electrostatic potential and Lewis acidity of the monovalent cation, following the sequence of Li + > Na + > K + > Rb + > Cs +. While for samples with intercalating divalent alkaline-earth cations, there was slightly lower activity for O 2 evolution, which may be correlated to different stability of Mn(III) species in the birnessite layers. In addition, we observed much higher O 2-evolution activity at an overpotential of ca. 220-230 mV if the samples were heat-treated between 300-500°C once they were electrolytically deposited as LiMnO x films. Clearly dry heating causes changes in both the crystalline structure and electronic structure of manganese-oxide layers, as indicated by associated changes in water-oxidation activity.

Biogenic Manganese Oxide Synthesized by a Marine Bacterial Multicopper Oxidase MnxG Reveals Oxygen Evolution Activity

“…Having synthesized the above sets of biogenic MnO x with diverse morphologies and structures, we employed them as electrocatalysts for water-oxidation reactions as previous reports suggested that synthetic MnO x with varying morphologies show varying OER activities. ,,, Both linear sweep voltammetric (LSV) and chronoamperometric (CA) studies were performed. As shown in Figure , all these biogenic MnO x show activities for anodic O 2 evolution, i.e., employing smaller overpotentials (η < 880 mV at 8.3 A/g·cm 2 , normal hydrogen electrode which is used through this paper) than that of the bare FTO electrode (η > 930 mV at 8.3 A/g·cm 2 vs NHE, Figure A) and showing higher CA current density at 1.6 V (vs NHE) (Figure B).…”

“…The monoclinic birnessite sample was prepared using a protocol that was modified from previously reported procedures. , Electrodeposition was performed on a Princeton Applied Research (Oak Ridge, TN) Model 263 A potentiostat/galvanostat and was analyzed using the PowerStep software package in a three-electrode system. The three-electrode system contains a fluorine-doped tin oxide-coated glass (FTO; Sigma-Aldrich, MO) as the working electrode, a nickel foam (MTI Corp., CA) as the counter electrode, and Ag/AgCl in 3 M NaCl (0.203 V vs NHE; BASi, IN) as the reference electrode.…”

“…Although the oxygen evolution mechanism of the OEC in photosystem II remains elusive, the structural/elemental similarity between the OEC and manganese oxides has inspired a significant amount of research on exploring manganese-oxide catalysts. − For example, one of the commonly studied manganese-oxide OER catalysts is layered birnessite, which consists of an edge-sharing lattice of octahedral manganese-oxide units and intercalating alkaline metal cations between layers to balance the negative charges created by defects (Figure ). ,− …”

“…While most of the manganese oxides are synthesized using strong oxidants, − strong bases, or external electric potentials to oxidize Mn­(II), ,− manganese oxide minerals in nature are usually synthesized through enzymatic pathways under mild conditions. Abiotic oxidation of soluble Mn­(II) to Mn­(III) and Mn­(IV) oxides usually proceeds slowly in seawater (pH 8.16). , However, manganese-oxidase enzymes can expedite this process by 3–5 orders of magnitude .…”