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High-valent metal-oxo moieties have been implicated as key intermediates preceding various oxidation processes. The critical O–O bond formation step in the Kok cycle that is presumed to generate molecular oxygen occurs through the high-valent Mn-oxo species of the water oxidation complex, i.e., the Mn4Ca cluster in photosystem II. Here, we report the spectroscopic characterization of new intermediates during the water oxidation reaction of manganese-based heterogeneous catalysts and assign them as low-spin Mn(IV)-oxo species. Recently, the effects of the spin state in transition metal catalysts on catalytic reactivity have been intensely studied; however, no detailed characterization of a low-spin Mn(IV)-oxo intermediate species currently exists. We demonstrate that a low-spin configuration of Mn(IV), S = 1/2, is stably present in a heterogeneous electrocatalyst of Ni-doped monodisperse 10-nm Mn3O4 nanoparticles via oxo-ligand field engineering. An unprecedented signal (g = 1.83) is found to evolve in the electron paramagnetic resonance spectrum during the stepwise transition from the Jahn–Teller-distorted Mn(III). In-situ Raman analysis directly provides the evidence for Mn(IV)-oxo species as the active intermediate species. Computational analysis confirmed that the substituted nickel species induces the formation of a z-axis-compressed octahedral C4v crystal field that stabilizes the low-spin Mn(IV)-oxo intermediates. Understanding reaction intermediates provides a foundation for active electrocatalysts' design, but it remains elusive for heterogeneous electrocatalysts. Here, the authors report the spectroscopic characterization of low-spin Mn(IV)-oxo as the active intermediates during electrochemical water oxidation. [ABSTRACT FROM AUTHOR]

As the demand for energy has dramatically increased in the past decade, electrochemical water splitting has been regarded as an attractive approach to produce renewable hydrogen energy. However, large overpotentials of oxygen‐evolving reaction (OER) is a key bottleneck for practical application. Thus, water‐oxidizing electrocatalysts with low cost and high efficiency should be developed. Here, 5 nm‐sized Mn3O4 nanoparticles (NPs) are synthesized by a hydrothermal method, which is appropriate for large‐scale production. To further improve their performance, various 3d transition metal elements are successfully doped in Mn3O4 NPs. Ni‐doped Mn3O4 NPs exhibit the highest efficiency among the Mn3O4 NPs doped with various elements. Based on structural analysis, the Ni‐doping process leads to the lattice distortion of their tetragonal spinel structure and it strongly correlates with the enhancement of OER activity. The overpotential at the current density of 10 mA cm–2 is 524 and 458 mV for pristine and 5 at% doped Mn3O4 NPs under neutral condition. The heteroatom‐doping process in sub‐10 nm‐sized nanocatalysts is expected to be a promising methodology to induce distorted structure related to active species. Thus, it can be effective to improve catalytic performance of various heterogeneous nano‐catalysts. [ABSTRACT FROM AUTHOR]

Electrochemical water splitting is one of the ways to produce environmentally‐friendly hydrogen energy. Transition‐metal (TM)‐based catalysts have been attracting attention due to their low cost and abundance, but their insufficient activity still remains a challenge. Here, 4 nm Mn3O4 nanoparticles (NPs) are successfully synthesized and their electrochemical behavior is investigated. Using electrokinetic analyses, an identical water oxidizing mechanism is demonstrated between the 4 and 8 nm Mn3O4 NPs. In addition, it is confirmed that the overall increase in the active surface area is strongly correlated with the superb catalytic activity of the 4 nm Mn3O4 NPs. To further enhance the oxygen evolution reaction (OER) performance, Ni foam substrate is introduced to maximize the entire number of the NPs participating in OER. The 4 nm Mn3O4/Ni foam electrode exhibits outstanding electrocatalytic activity for OER with overpotential of 395 mV at a current density of 10 mA cm−2 under neutral conditions (0.5 m PBS, pH 7). [ABSTRACT FROM AUTHOR]

Electrochemical water splitting is one of the most important ways to produce hydrogen fuel for an environmentally friendly energy system. Low‐cost and efficient electrocatalysts for oxygen evolution reaction (OER) have been studied to overcome the sluggish kinetics of OER, which is considered to be a bottleneck that affects the overall efficiency of water splitting. However, the atomic‐level understanding and control of the catalyst surface still remains elusive. Here, we show that methylamine treatment of manganese oxide nanoparticles can effectively control their surface charge and enhance their OER catalytic activity under neutral condition. Methylamine treatment can reduce the overpotential for OER by nearly 100 mV at the current density 5 mA cm−2. Through spectroscopic studies, potentiometric titration, and zeta potential analysis, we discovered that methylamine does not exist on the surface after methylamine treatment but deprotonates the surface of nanoparticles and induce additional negative charge on the surface. Surprisingly, treatment with not only methylamine but other amines can also improve the OER performance and the catalytic activity of the amine treated nanoparticles is influenced by the pKa values of amines. Underwater stars: After methylamine treatment of OH− ligand exchanged Mn3O4 nanoparticles, oxygen evolving reaction performance is significantly enhanced. Detailed mechanistic study reveals that methylamine deprotonates the surface and generates additional negative charges on the surface. The catalytic performance could also be enhanced with different amines and the decrease in overpotential is associated with pKa of amines. [ABSTRACT FROM AUTHOR]