Significant role of Mn(III) sites in eg1 configuration in manganese oxide catalysts for efficient artificial water oxidation

Manganese-Based Metal-Organic Frameworks and Their Derivatives for Electrochemical Water Splitting: Recent Advances and Future Outlook

Metal-organic frameworks (MOFs) and their derivatives have recently attracted significant interest as promising candidates in water splitting due to their well-defined structural and electronic features, three-dimensional architecture, high surface area, abundance of active sites, remarkable stability, and improved capabilities for mass transport and diffusion. Mn-based MOFs and their derivatives have been extensively studied and demonstrated significant potential in water splitting, inspired largely by the natural photosystem-II. Despite the development of numerous Mn-based electrocatalysts, Mn-MOFs stand out due to their strong synergistic interactions, tunable electronic properties, efficient charge and mass transfer, and straightforward synthesis. However, recent reviews on MOFs have largely overlooked the specific advancements in Mn-MOFs and their derivatives for water-splitting applications. By providing an overview of the uses of Mn-MOFs and their materials, this article seeks to close that gap. It looks at their stability, porosity, and structure as well as how they are used in water splitting. This study offers a deeper knowledge of the properties and uses of Mn-MOFs and their related materials by drawing on groundbreaking research. The link between structure, property, and performance is examined, current advancements in the subject are discussed, difficulties faced are addressed, and potential future developments are taken into account.

Structural and Electronic Factors behind the Electrochemical Stability of 3D-Metal Tungstates under Oxygen Evolution Reaction Conditions

Transition metal tungstates (TMTs) possess a wolframite-like lattice structure and preferably form via an electrostatic interaction between a divalent transition metal cation (MII) and an oxyanion of tungsten ([WO4]2-). A unit cell of a TMT is primarily composed of two repeating units, [MO6]oh and [WO6]oh, which are held together via several M-μ2-O-W bridging links. The bond character (ionic or covalent) of this bridging unit determines the stability of the lattice and influences the electronic structure of the bulk TMT materials. Recently, TMTs have been successfully employed as an electrode material for various applications, including electrochemical water splitting. Despite the wide electrocatalytic applications of TMTs, the study of the structure-activity correlation and electronic factors responsible for in situ structural evolution to electroactive species during electrochemical reactions is still in its infancy. Herein, a series of TMTs, MIIWVIO4 (M = Mn/Fe/Co/Ni), have been prepared and employed as electrocatalysts to study the oxygen evolution reaction (OER) under alkaline conditions and to scrutinize the role of transition metals in controlling the energetics of the formation of electroactive species. Since the [WO6]oh unit is common in the TMTs considered, the variation of the central atom of the corresponding [MO6]oh unit plays an intriguing role in controlling the electronic structure and stability of the lattice under anodic potential. Under the OER conditions, a potential-dependent structural transformation of MWO4 is noticed, where MnWO4 appears to be the most labile, whereas NiWO4 is stable up to a high anodic potential of ∼1.68 V (vs RHE). Potential-dependent hydrolytic [WO4]2- dissolution to form MOx active species, traced by in situ Raman and various spectro-/microscopic analyses, can directly be related to the electronic factors of the lattice, viz., crystal field splitting energy (CFSE) of MII in [MO6]oh, formation enthalpy (ΔHf), decomposition enthalpy (ΔHd), and Madelung factor associated with the MWO4 ionic lattice. Additionally, the magnitude of the Löwdin and Bader charges on M of the M-μ2-O-W bond is directly related to the degree of ionicity or covalency in the MWO4 lattice, which indirectly influences the electronic structure and activity. The experimental results substantiated by the computational study explain the electrochemical activity of the TMTs with the help of various structural and electronic factors and bonding interactions in the lattice, which has never been realized. Therefore, the study presented here can be taken as a general guideline to correlate the reactivity to the structure of the inorganic materials.

Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis

Active and nonprecious-metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital components of clean energy conversion devices such as regenerative fuel cells and rechargeable metal-air batteries. Porous manganese oxides (MnO_x) are promising electrocatalyst candidates because of their high surface area and the abundance of Mn. MnO_x catalysts exhibit various oxidation states and crystal structures, which critically affect their electrocatalytic activity. These effects remain elusive mainly because the synthesis of oxidation-state-controlled porous MnO_x with similar structural properties is challenging. In this work, four different mesoporous manganese oxides (m-MnO_x) were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity toward oxygen electrocatalysis. The following activity trends were observed: m-Mn₂O₃ > m-MnO₂ > m-MnO > m-Mn₃O₄ for the ORR and m-MnO₂ > m-Mn₂O₃ > m-MnO ≈ m-Mn₃O₄ for the OER. These activity trends suggest that high-valent Mn species (Mn(III) and Mn(IV)) with disordered atomic arrangements induced by nanostructuring significantly influence electrocatalysis. In situ X-ray absorption spectroscopy was used to analyze the changes in the oxidation states under the ORR and OER conditions, which showed the surface phase transformation and generation of active species during electrocatalysis.

Effect of Potassium Promoter on the Performance of Nickel-Based Catalysts Supported on MnOx in Steam Reforming of Ethanol

The effect of a potassium promoter on the stability of and resistance to a carbon deposit formation on the Ni/MnOx catalyst under SRE conditions was studied at 420 °C for different H2O/EtOH molar ratios in the range from 4/1 to 12/1. The catalysts were prepared by the impregnation method and characterized using several techniques to study their textural, structural, and redox properties before being tested in a SRE reaction. The catalytic tests indicated that the addition of a low amount of potassium (1.6 wt.%) allows a catalyst with high stability to be obtained, which was ascribed to high resistance to carbon formation. The restriction of the amount of carbon deposits originates from the potassium presence on the Ni surface, which leads to (i) a decrease in the number of active sites available for methane decomposition and (ii) an increase in the rate of the steam gasification of carbon formed during SRE reactions.

Investigation of Li-excess Manganese Oxide Spinel Structure for Electrochemical Water Oxidation Catalysis

The rapid development of efficient and cost-effective catalysts is essential for oxygen evolution reaction. Herein, nanostructured spinels LiMn2O4, delithiated λ-MnO2, and Li4Mn5O12 have been synthesized at low temperature and are...

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 MnOx phase with a birnessite-like short-range order and surface-stabilized MnIII 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.

Cobalt-based metal–organic frameworks as functional materials for battery applications

Research progress on cobalt-based metal–organic frameworks as functional materials for battery applications has been presented.

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.

Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage

Exploring new materials with high efficiency and durability is the major requirement in the field of sustainable energy conversion and storage systems. Numerous techniques have been developed in last three decades to enhance the efficiency of the catalyst systems, control over the composition, structure, surface area, pore size, and moreover morphology of the particles. In this respect, metal organic framework (MOF) derived catalysts are emerged as the finest materials with tunable properties and activities for the energy conversion and storage. Recently, several nano- or microstructures of metal oxides, chalcogenides, phosphides, nitrides, carbides, alloys, carbon materials, or their hybrids are explored for the electrochemical energy conversion like oxygen evolution, hydrogen evolution, oxygen reduction, or battery materials. Interest on the efficient energy storage system is also growing looking at the practical applications. Though, several reviews are available on the synthesis and application of MOF and MOF derived materials, their applications for the electrochemical energy conversion and storage is totally a new field of research and developed recently. This review focuses on the systematic design of the materials from MOF and control over their inherent properties to enhance the electrochemical performances.

Alkaline electrochemical water oxidation with multi-shelled cobalt manganese oxide hollow spheres

Multi-shelled hollow spheres of cobalt manganese oxides (CMOs) deposited on Ni foam exhibited superior alkaline electrochemical water oxidation activity and surpassed those of bulk CMO and commercial noble metal-based catalysts. A higher amount of cobalt in the spinel structure resulted in the transformation of the tetragonal to the cubic phase with a decrease in the overpotential of oxygen evolution.

A Single-Source Precursor Approach to Self-Supported Nickel-Manganese-Based Catalysts with Improved Stability for Effective Low-Temperature Dry Reforming of Methane

Self-supported nickel-manganese-based catalysts were synthesized from heterobimetallic nickel manganese oxalate precursors via a versatile reverse micelle approach. The precursors were subjected to thermal degradation (400 °C) in the presence of synthetic air to form respective metal oxides, which were treated under hydrogen (500 °C) to form Ni₂ MnO₄ -O₂ -H₂ , Ni₆ MnO₈ -O₂ -H₂ and NiO-O₂ -H₂ . Similarly, the precursors were also treated directly under hydrogen at the same temperature to form Ni₂ MnO₄ -H₂ and Ni₆ MnO₈ -H₂ . The catalysts were extensively investigated by PXRD, SEM, TEM, XPS and BET analyses. The resulting catalysts were applied for dry reforming of methane (DRM) and exhibit better stability and resistance to coking than coprecipitated catalysts. Further, we show that addition of manganese, which is not an active catalyst for DRM alone, to nickel has a significant promotion effect on both the activity and stability of DRM catalysts, and a Ni/Mn ratio lower than 6:1 enables optimized activity for this system.

Copper-Intercalated Birnessite as a Water Oxidation Catalyst

We report a synthetic method to increase the catalytic activity of birnessite toward water oxidation by intercalating copper in the interlayer region of the layered manganese oxide. Intercalation of copper, verified by XRD, XPS, ICP, and Raman spectroscopy, was accomplished by exposing a suspension of birnessite to a Cu(+)-bearing precursor molecule that underwent disproportionation in solution to yield Cu(0) and Cu(2+). Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to ∼700 mV (at 10 mA/cm(2)) and 240 mV/decade, respectively, for birnessite without copper. Impedance spectroscopy results suggested that the charge transfer resistivity of the Cu-modified sample was significantly lower than Cu-free birnessite, suggesting that Cu in the interlayer increased the conductivity of birnessite leading to an enhancement of water oxidation kinetics. Density functional theory calculations show that the intercalation of Cu(0) into a layered MnO2 model structure led to a change of the electronic properties of the material from a semiconductor to a metallic-like structure. This conclusion from computation is in general agreement with the aforementioned impedance spectroscopy results. X-ray photoelectron spectroscopy (XPS) showed that Cu(0) coexisted with Cu(2+) in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu(2+) showed a reduction in water oxidation kinetics, further emphasizing the importance of Cu(0) for the increased activity of birnessite. The introduction of Cu(0) into the birnessite structure also increased the stability of the electrocatalyst. At a working current of 2 mA, the Cu-modified birnessite took ∼3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.