Inhibition of Charge Disproportionation of MnO₂ Electrocatalysts for Efficient Water Oxidation under Neutral Conditions

Abstract

The development of Mn-oxide electrocatalysts for the oxidation of H(2)O to O(2) has been the subject of intensive researches not only for their importance as components of artificial photosynthetic systems, but also as O(2)-evolving centers in photosystem II. However, limited knowledge of the mechanisms underlying this oxidation reaction hampers the ability to rationally design effective catalysts. Herein, using in situ spectroelectrochemical techniques, we demonstrate that the stabilization of surface-associated intermediate Mn(3+) species relative to charge disproportionation is an effective strategy to lower the overpotential for water oxidation by MnO(2). The formation of N-Mn bonds via the coordination of amine groups of poly(allylamine hydrochloride) to the surface Mn sites of MnO(2) electrodes effectively stabilized the Mn(3+) species, resulting in an ~500-mV negative shift of the onset potential for the O(2) evolution reaction at neutral pH.

Efficient Water Splitting Cascade Photoanodes with Ligand‐Engineered MnO Cocatalysts

“…Therefore, most of the previously reported results related to BiVO 4 ‐based photoanodes13, 14, 16, 19, 47, 51, 52, 53 were measured in sulfite oxidation condition to show photo‐electrochemical properties of BiVO 4 ‐based electrodes independently of its poor water oxidation kinetics, as shown in Table S2 (Supporting Information). The photo‐electrochemical current densities of the BiVO 4 ‐based photoanodes4, 13, 14, 16, 17, 19, 20, 21, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 64, 65, 66, 67, 68, 69, 77 were plotted as a function of potential versus RHE. Thus, we measured PEC properties of BiVO 4 ‐based anodes under sulfite oxidation to figure out the effect of ligand engineering.…”

“…Specifically, under neutral condition, manganese oxide nanoparticles exhibited superior performance to previously reported transition metal–based catalysts 32, 33. Several researches show that Mn‐based oxides have superior catalytic activity at neutral pH and are highly active in water oxidation compared to low activity oxides, as shown in Table S1 (Supporting Information) 31, 32, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49. As compared to other OECs, Mn‐based oxides showed good catalytic properties (Tafel slope and overpotential) in the neutral pH condition for water oxidation.…”

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

“…However, although the Ca 2+ is slowly leached out of the material in acid (8), manganese is conserved by repeated reductive dissolution, followed by reoxidation at the solid surface. Takashima et al (38,39) argue that the strong pH dependence of catalytic activity can be explained by the effect of pH on the disproportionation of Mn 3+ to Mn 2+ and Mn 4+ , with the divalent manganese dissolving in the aqueous solution. For such equilibrium to be maintained during catalysis, adsorption of manganese species on the surface and their transport into, or out of, the structure must be rapid and reversible, as is likely to be the case here.…”

Redox properties of birnessite from a defect perspective

“…Currently, one of the biggest challenges is to develop a cheap and efficient catalyst for the former reaction with a low overpotential, which is still needed to overcome the kinetic barriers (already lowered by the catalyst) along the path from reactants to products. Birnessite, similar to the oxygen-evolving complex in Photosystem II for photosynthesis in regard to the coexistence of Mn(III) (nominal Mn 3+ ) and Mn(IV) (nominal Mn 4+ ) within its structure (1)(2)(3), has already shown a moderate catalytic performance with the overpotentials reported from 0.6 V to 0.8 V (4)(5)(6)(7)(8).…”