Oxidative Deposition of Manganese Oxide Nanosheets on Nitrogen-Functionalized Carbon Nanotubes Applied in the Alkaline Oxygen Evolution Reaction

Oxygen evolving reactions catalyzed by different manganese oxides: the role of oxidation state and specific surface area

A set of the four manganese oxide powders α-MnO2 (hollandite), δ-MnO2 (birnessite), Mn2O3 (bixbyite), and Mn3O4 (hausmannite) have been synthesized in a phase-pure form and tested as catalysts in three different oxygen evolution reactions (OER): electrochemical OER in KOH (1 mol L−1), chemical OER using aqueous cerium ammonium nitrate, and H2O2 decomposition. The trends in electrochemical (hollandite >> bixbyite > birnessite > hausmannite) and chemical OER (hollandite > birnessite > bixbyite > hausmannite) are different, which can be explained by differences in electric conductivity. H2O2 decomposition and chemical OER, on the other hand, showed the same trend and even a linear correlation of their initial OER rates. A linear correlation between the catalytic performance and the manganese oxidation state of the catalysts was observed. Another trend was observed related to the specific surface area, highlighting the importance of these properties for the OER. Altogether, hollandite was found to be the best performing catalyst in this study due to a combination of the high manganese oxidation state and a large specific surface area. Likely, due to a sufficient electrical conductivity, this intrinsically high OER performance is also found to some extent in electrocatalysis for this specific example.

A mild reduction of Co-doped MnO2 to create abundant oxygen vacancies and active sites for enhanced oxygen evolution reaction

Efficient and non-precious-metal-based catalysts (e.g., manganese-based oxides) for the oxygen evolution reaction (OER) remain a substantial challenge. Creation of oxygen vacancies of manganese-based oxides with the aim to enhance their intrinsic activities is rarely reported, and there is a critical requirement for a mild and facile synthesis strategy to create abundant oxygen vacancies on manganese-based oxides. Herein, Co-doped MnO₂ nanowires were reduced by NaBH₄ solution at room temperature; then, MnCo₂O_4.5 nanosheets with abundant oxygen vacancies and active sites were formed on the surface of Co-doped MnO₂ nanowires. Benefiting from the reduction strategy, the fabricated hierarchical Co-doped-MnO₂@MnCo₂O_4.5 nanowire/nanosheet nanocomposites exhibit higher catalytic activity (an overpotential of 250 mV at a current density of 10 mA cm^-2 in 1.0 M KOH solution) than pristine Co-doped MnO₂ nanowires. The calculated TOF of Co-doped-MnO₂@MnCo₂O_4.5 is 0.034 s^-1 at the overpotential of 300 mV, which is 136-fold higher than that of Co-doped-MnO₂. The excellent OER performance was attributed to the synergistic advantages of abundant oxygen vacancies and active sites over the hierarchical nanowire-nanosheet architectures.

Combination of Highly Efficient Electrocatalytic Water Oxidation with Selective Oxygenation of Organic Substrates using Manganese Borophosphates

One of the key catalytic reactions for life on earth, the oxidation of water to molecular oxygen, occurs in the oxygen-evolving complex of the photosystem II (PSII) mediated by a manganese-containing cluster. Considerable efforts in this research area embrace the development of efficient artificial manganese-based catalysts for the oxygen evolution reaction (OER). Using artificial OER catalysts for selective oxygenation of organic substrates to produce value-added chemicals is a worthwhile objective. However, unsatisfying catalytic performance and poor stability have been a fundamental bottleneck in the field of artificial PSII analogs. Herein, for the first time, a manganese-based anode material is developed and paired up for combining electrocatalytic water oxidation and selective oxygenations of organics delivering the highest efficiency reported to date. This can be achieved by employing helical manganese borophosphates, representing a new class of materials. The uniquely high catalytic activity and durability (over 5 months) of the latter precursors in alkaline media are attributed to its unexpected surface transformation into an amorphous MnO_x phase with a birnessite-like short-range order and surface-stabilized Mn^III sites under extended electrical bias, as unequivocally demonstrated by a combination of in situ Raman and quasi in situ X-ray absorption spectroscopy as well as ex situ methods.

Factors Governing the Activity of α-MnO2 Catalysts in the Oxygen Evolution Reaction: Conductivity versus Exposed Surface Area of Cryptomelane

Cryptomelane (α-(K)MnO2 ) powders were synthesized by different methods leading to only slight differences in their bulk crystal structure and chemical composition, while the BET surface area and the crystallite size differed significantly. Their performance in the oxygen evolution reaction (OER) covered a wide range and their sequence of increasing activity differed when electrocatalysis in alkaline electrolyte and chemical water oxidation using Ce4+ were compared. The decisive factors that explain this difference were identified in the catalysts' microstructure. Chemical water oxidation activity is substantially governed by the exposed surface area, while the electrocatalytic activity is determined largely by the electric conductivity, which was found to correlate with the particle morphology in terms of needle length and aspect ratio in this sample series. This correlation is rather explained by an improved conductivity due to longer needles than by structure sensitivity as was supported by reference experiments using H2 O2 decomposition and carbon black as additive. The most active catalyst R-cryptomelane reached a current density of 10 mA cm-2 at a potential 1.73 V without, and at 1.71 V in the presence of carbon black. The improvement was significantly higher for the catalyst with lower initial activity. However, the materials showed a disappointing catalytic stability during alkaline electrochemical OER, whereas the crystal structure was found to be stable at working conditions.

Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”.

Formation of Ordered Honeycomb-like Structures of Manganese Oxide 2D Nanocrystals with the Birnessite-like Structure and Their Electrocatalytic Properties during Oxygen Evolution Reaction upon Water Splitting in an Alkaline Medium

In this work, a chemical reaction between gaseous ozone and aqueous solution of Mn(CH₃COO)₂ in drops has been researched. It has been shown that the formation of H _x MnO₂·nH₂O nanocrystals with a morphology of nanosheets and a birnessite-like crystal structure with a thickness of 5-8 nm is observed on the surface of drops. These nanocrystals are oriented spontaneously to the solution-gas interface and constitute peculiar ribbons with a width of 1-2 μm, some of which form ordered honeycomb structures (OHS) with a 5-20 μm cell size. To explain the observed effect, the scheme of chemical reactions that take place at the interface between the surface of a drop and ozone has been modeled, and it can be described using a diffusion pattern model taking into account the action of "force fields" on the surface of a drop, which arise due to its curvature. After the drop is dried, these structures practically retain their morphology and form a fractal structure with a geometric area equal to the area of the drop base on the surface of the substrate. The study of the electrocatalytic properties of these structures revealed that they are active electrocatalysts in the oxygen evolution reaction (OER) during water electrolysis in alkaline medium. The most efficient of the obtained electrocatalysts are characterized by an overpotential value of 284 mV at a current of 10 mA/cm² and the Tafel coefficient of 37.7 mV/dec and are currently one of the best among pure manganese oxides. Finally, it has also been assumed that this effect is explained by the morphological features of the structures obtained, which contribute to the removal of oxygen bubbles from the electrode surface during electrolysis.

Verchenko V, Shevelkov AV, Driess M. Boosting Water Oxidation through In Situ Electroconversion of Manganese Gallide: An Intermetallic Precursor Approach

For the first time, the manganese gallide (MnGa4 ) served as an intermetallic precursor, which upon in situ electroconversion in alkaline media produced high-performance and long-term-stable MnOx -based electrocatalysts for water oxidation. Unexpectedly, its electrocorrosion (with the concomitant loss of Ga) leads simultaneously to three crystalline types of MnOx minerals with distinct structures and induced defects: birnessite δ-MnO2 , feitknechtite β-MnOOH, and hausmannite α-Mn3 O4 . The abundance and intrinsic stabilization of MnIII /MnIV active sites in the three MnOx phases explains the superior efficiency and durability of the system for electrocatalytic water oxidation. After electrophoretic deposition of the MnGa4 precursor on conductive nickel foam (NF), a low overpotential of 291 mV, comparable to that of precious-metal-based catalysts, could be achieved at a current density of 10 mA cm-2 with a durability of more than five days.

Water Oxidation Catalysts: The Quest for New Oxide-Based Materials

The expected shortage of fossil fuels as well as the accompanying climate change are among the major challenges of the 21st century. A global shift to a sustainable energy landscape is, therefore, of utmost importance. Over the past few years, solar technologies have entered the energy market and have paved the way to replace fossil-based energy sources, in the long term. In particular, electrochemical solar-to-hydrogen technologies have attracted a lot of interest—not only in academia, but also in industry. Solar water splitting (artificial photosynthesis) is one of the most active areas in contemporary materials and catalysis research. The development of low-cost, efficient, and stable water oxidation catalysts (WOCs) remains crucial for artificial photosynthesis applications, because WOCs still represent a major economical and efficient bottleneck. In the following, we summarize recent advances in water oxidation catalysts development, with selected examples from 2016 onwards. This condensed survey demonstrates that the ongoing quest for new materials and informed catalyst design is a dynamic and rapidly developing research area.