Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (<i>h</i> = 70 mmol of O₂/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase

Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction

We investigated the dependence of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to determine the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers positively correlates with the size of the alkali cation: NS/Cs⁺/NS > NS/Rb⁺/NS > NS/K⁺/NS > NS/Na⁺/NS > NS/Li⁺/NS. The thermodynamic origins of these bilayer heights were investigated using molecular dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs⁺/NS has the lowest η (0.45 V), while NS/Li⁺/NS exhibits the highest η (0.68 V) for OER at a current density of 1 mA/cm². Kinetic parameters (η and Tafel slope) associated with NS/Cs⁺/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies

Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS₂ for the Hydrogen Evolution Reaction

We show that intercalation of cations (Na⁺, Ca²⁺, Ni²⁺, and Co²⁺) into the interlayer region of 1T-MoS₂ is an effective strategy to lower the overpotential for the hydrogen evolution reaction (HER). In acidic media the onset potential for 1T-MoS₂ with intercalated ions is lowered by ∼60 mV relative to that for pristine 1T-MoS₂ (onset of ∼180 mV). Density functional theory (DFT) calculations show a lowering in the Gibbs free energy for H-adsorption (Δ<i>G</i>_H) on these intercalated structures relative to intercalant-free 1T-MoS₂. The DFT calculations suggest that Na⁺ intercalation results in a Δ<i>G</i>_H close to zero. Consistent with calculation, experiments show that the intercalation of Na⁺ ions into the interlayer region of 1T-MoS₂ results in the lowest overpotential for the HER