Mechanistic Investigation of Water Oxidation Catalyzed by Uniform, Assembled MnO Nanoparticles

Abstract

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.

Nickel‐Doping Effect on Mn3O4 Nanoparticles for Electrochemical Water Oxidation under Neutral Condition

“…The Mn(III) species were also generalized on the surface of NPs during catalysis. Notably, we investigated that the exceptional efficiency on the nano‐catalysts was attributed to their unique reaction mechanism that is totally different from the bulk counterparts, combined with electrokinetic study and in situ spectroscopic analysis . Furthermore, the kinetic parameters including reaction rate constant ( k ) were extracted via electrochemical impedance analysis…”

“…At high phosphate concentration over 0.1 M, the zeroth‐order dependence was observed for both pristine and 5 at% Ni‐doped Mn 3 O 4 NPs. Based on the electrokinetic analyses, the overall electrochemical rate law for OER on the nanocatalysts was identical to previously reported Mn‐oxide NPs for OER electrocatalysts …”

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.…”

“…Moreover, high cost‐effectiveness (Mn is the 10th most abundant element in the earth's crust), low toxicity, and robust stability in neutral condition are other attractive properties of a manganese‐based catalyst as OECs 31, 32, 33, 34, 35, 36. In addition, the multioxidation states of manganese‐based catalyst would facilitate the local hole transport around Mn centers via a low barrier of O—Mn—O pathway, which is likely to boost the water oxidation activity 33.…”

Solar‐Driven Production of Hydrogen Peroxide from Water and Dioxygen

“…Manganese oxides (MnO x )h ave attracted special attention as water oxidation catalysts due to their naturala bundance, low toxicity,a nd compositional similarity to nature's water splitting inorganic cluster,t he oxygen-evolving complex (OEC) of photosystem II (PSII). [73][74][75][76][77][78][79][80] Although MnO x usually acts as a water oxidationc atalyst for four-electron/four-proton oxidation of H 2 Ot oe volve O 2 , [1,[91][92][93][94][95][96][97][98][99] electrocatalytic H 2 Oo xidation with MnO x in the presenceo fabutylammonium sulfate (BAS)/base mixture (1.0 m)a tp H10a taconstant appliedp otential resulted in production of H 2 O 2 rather than O 2 . [100] Faradaic efficiency for the H 2 O 2 production during the controlled potential electrolysis for 10 min over an MnO x electrocatalyst in BAS electrolyte (1.0 m)a ta na ppliedp otentialo f0 .8 Vv ersusA g/AgCl reached1 00 %i nt he pH region between1 0.0 and 10.4 ( Figure 2).…”