Nanostructured manganese oxide on carbon for water oxidation: New findings and challenges

Water-oxidation reaction (WOR) is a sluggish reaction for hydrogen production through water splitting. Herein, Mn oxide/mesoporous carbon (MC) is introduced as an active WOR catalyst under neutral conditions. The composite was synthesized through the reaction of permanganate and MC using a simple and low-cost procedure and characterized by scanning electron microscopy, in situ visible spectroscopy, energy-dispersive spectroscopy, (high-resolution) transmission electron microscopy, and X-ray diffraction. Then, the WOR activity of this composite was investigated under neutral conditions. The experiment shows that nanosized Mn oxide/MC is a promising catalyst for water-splitting systems. The overpotentials for the onset (based on the Tafel plot) and 1.0 mA of WOR are 700 and 940 mV, respectively. Based on the LSV, the Tafel slope is 378.6 mV per decade.     

Large-area manganese oxide nanorod arrays as efficient electrocatalyst for oxygen evolution reaction

Large-area manganese oxide nanorod arrays (MnO₂ NRAs) have been directly grown vertically on Ti foil with a uniform length and diameter by a simple electrochemical method without any templates. The deposition temperature is one of the most important parameters for formation MnO₂ NRAs and at 25 °C no MnO₂ NRAs can be obtained. The results show that MnO₂ has high activity and good stability for oxygen evolution reaction (OER) and the structure of nanorod arrays pronounced enhances MnO₂ activity. The onset potential of MnO₂ NRAs is lower than that of Pt foil and lower 401 mV than that of MnO₂ film, indicating that the structure of MnO₂ NRAs shows an easy OER for water split. The MnO₂ NRAs may be of great potential in electrochemical water split.     

Electrochemical induction of Mn(III) in the structure of Mn(IV) oxide: Toward a new approach for water splitting

Oxygen-evolution reaction (OER) through water-oxidation reaction is an essential reaction for water splitting, which provides electrons for hydrogen production. Mn oxides are among the commonly investigated types of metal oxides as OER catalysts. Recent studies showed that Mn(III) ions in the structure of Mn(IV) oxide have an essential role in OER. Previous works have only focused on adding different materials to induce Mn(III) ions in the structure of Mn(IV). Indeed, Mn(III) ions could be induced in the Mn(IV)-oxide structure in the presence of organic compounds, reductants, fluoride, chloride, and gold nanoparticles. However, a challenging issue in the field is using a stable and perdurable force during OER to induce Mn(III) ions in the structure of Mn(IV). Herein, the effects of inducing Mn(III) ions on OER by different potentials on the surface of the prepared Mn(IV) oxide on fluorine-doped tin oxide were investigated in the phosphate buffer at pH 11, 7, and 3. Mn(IV) oxide under OER/Mn(III) induction shows no decrease in the current density, but for the same sample under OER alone, a decrease (15%) is observed in the current density after only 1000 s. At a lower inducing potential, a decrease for OER is observed, which corresponds to Mn(IV)/(III) reduction to Mn(II), and Mn(II) leaking effect. This investigation sheds new light on OER in the presence of Mn oxide and is a promising, low-cost, and environmentally friendly strategy, which results in an increase in the yield of OER.     

Toward Escherichia coli bacteria-machine for water-oxidation reaction at neutral conditions: Using Ruthenium Red

Water splitting into hydrogen and oxygen is a promising method for storing sustainable but intermittent energies. Ruthenium compounds are promising for the water-oxidation reaction. Herein, an easy method is employed to load a water-oxidation catalyst (Ruthenium Red; ([(NH₃)₅RuORu(NH₃)₄ORu(NH₃)₅]Cl₆)) on the surface of the Escherichia coli bacterial template. After the synthetic procedure, the catalyst is characterized by field emission-scanning electron microscopes, high-resolution transmission electron microscopy, visible spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. Some of the employed methods show that the bacterial template is intact after the synthetic procedure. In the next step, the catalyst is investigated by linear sweep, cyclic, and square wave voltammetry methods. The water-oxidation reaction of the compounds is examined under electrochemical conditions in LiClO₄ (0.25 M) at pH = 6.3. Linear sweep voltammetry shows that the onset overpotential of the water-oxidation reaction of the catalyst is 720 mV (based on extrapolation of the Tafel plot) with a Tafel slope of 226.4 mV per decade. Thus, the presence of Ruthenium Red in the material remarkably increases the activity of the water-oxidation reaction.     

A dinuclear iron complex as a precatalyst for water oxidation under alkaline conditions

Water oxidation is a key reaction for water splitting. The decomposition of Fe-based-molecular structures toward Fe-based oxides is a promising method for oxygen-evolution reaction (OER) through water oxidation. The decomposition of Fe-based-molecular structures method results in a slow decomposition of precatalysts and forms Fe oxide-based catalysts. In this study, the Fe species formed through the decomposition of a dinuclear Fe(III) complex under OER is investigated by X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and the electrochemical method. In addition, using Ni(OH)₂, a new approach is reported for detecting trace Fe species on the electrode surface. The resulting Fe oxide-based catalyst shows a catalytic current with an onset of 621 mV overpotential and the Tafel slope of 113.7 mV/decade at pH 11 in a buffer phosphate.     

Nanostructured manganese oxide electrodes for lithium-ion storage in aqueous lithium sulfate electrolyte

Nanostructured manganese oxide electrodes are fabricated directly by electrochemical deposition. Surface morphology of the electrode deposited at high-current density shows nanowires with diameter 12–16 nm distributed randomly. Nanowires tend to aggregate to clumps when the deposition current density is low. Both annealing temperature and deposition current density affect the electrochemical performance of the deposited manganese oxide electrode in an aqueous lithium sulfate electrolyte. An optimal annealing temperature is found to be 300 °C in terms of the electrode's specific capacity during high-rate charging/discharging. An electrode with thinner nanowires deposited at high-current density has a high-specific capacity because thinner nanowires shorten the diffusion of lithium ions and in favor of high-rate charging/discharging.     

Nickel and cobalt co-doped MnCO3 nanostructures for water oxidation reaction

Nowadays, electrochemical water splitting is a securing alternative for clean-energy production and also an efficient expertise for oxygen and hydrogen production. Compared to single metal-based electrocatalyst, multi metal-based electrocatalyst offers more active sites, high surface area, and distinctive nanostructure for effective water oxidation. In this work, nickel and cobalt co-doped MnCO₃ was successfully synthesized via a facile co-precipitation technique. Rhombohedral crystal phase of MnCO₃ nanostructures and its crystallite sizes were thoroughly analyzed by the XRD spectra. Incorporation of doping element such as Ni and Co in MnCO₃ nanostructures exhibited two different morphologies which enhanced the catalytic performance of the prepared samples. Large surface area and porosity of the nanomaterials improved the stability and activity of the prepared MnCO₃ nanostructures. EDX analysis confirmed Mn, C, O, Ni and Co elements in stoichometric ratio. Moreover, the specific capacitance of (Ni, Co) co-doped MnCO₃ nanostructures attained 581 F/g while the other electrodes attained only 207, 332 and 175 F/g respectively. A small Tafel slope with low overpotential of Ni, Co co-doped MnCO₃ nanostructures was 20.2 mV/dec and 293 mV respectively. Therefore, the prepared electrocatalysts of Ni, Co co-doped MnCO₃ nanostructure is one of the attractive anode material for high performance energy conversion applications.     

Effect of electrodeposition conditions on the electrochemical capacitive behavior of synthesized manganese oxide electrodes

Galvanostatic electrodeposition techniques were applied for the preparation of novel electroactive manganese oxide electrodes. The effects of supersaturation ratio on the morphology and crystal structure of electrodeposited manganese oxide were studied. Manganese oxide electrodes were synthesized by anodic deposition from acetate-containing aqueous solutions on Au coated Si substrates through the control of nucleation and growth processes. By changing deposition parameters, a series of nanocrystalline manganese oxide electrodes with various morphologies (continuous coatings, rod-like structures, aggregated rods and thin sheets) and an antifluorite-type crystal structure was obtained. Detailed chemical and microstructural characterization of as-deposited electrodes was conducted using SEM, TEM and AAS. Manganese oxide thin sheets show instantaneous nucleation and single crystalline growth, rods have a mix of instantaneous/progressive nucleation and polycrystalline growth and continuous coatings form by progressive nucleation and polycrystalline growth.In addition, the electrochemical behavior was investigated by cyclic voltammetry. The experimental results show that manganese oxide electrodes, with rod-like and thin sheet morphology, exhibited enhanced electrochemical performance. The highest specific capacitance (∼230 F g⁻¹) and capacitance retention rates (∼88%) were obtained for manganese oxide thin sheets after 250 cycles in 0.5 M Na₂SO₄ at 20 mV s⁻¹.     

Dysprosium doped copper oxide (Cu1-xDyxO) nanoparticles enabled bifunctional electrode for overall water splitting

The production of hydrogen, a favourable alternative to an unsustainable fossil fuel remains as a significant hurdle with the pertaining challenge in the design of proficient, highly productive and sustainable electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, the dysprosium (Dy) doped copper oxide (Cu_1-xDy_xO) nanoparticles were synthesized via solution combustion technique and utilized as a non-noble metal based bi-functional electrocatalyst for overall water splitting. Due to the improved surface to volume ratio and conductivity, the optimized Cu_1-xDy_xO (x = 0.01, 0.02) electrocatalysts exhibited impressive HER and OER performance respectively in 1 M KOH delivering a current density of 10 mAcm^?2 at a potential of ?0.18 V vs RHE for HER and 1.53 V vs RHE for OER. Moreover, the Dy doped CuO electrocatalyst used as a bi-functional catalyst for overall water splitting achieved a potential of 1.56 V at a current density 10 mAcm^?2 and relatively high current density of 66 mAcm^?2 at a peak potential of 2 V. A long term stability of 24 h was achieved for a cell voltage of 2.2 V at a constant current density of 30 mAcm^?2 with only 10% of the initial current loss. This showcases the accumulative opportunity of dysprosium as a dopant in CuO nanoparticles for fabricating a highly effective and low-cost bi-functional electrocatalyst for overall water splitting.     

Electrolytic splitting of saline water: Durable nickel oxide anode for selective oxygen evolution

Increasing generation of renewable electricity offers a source of green electric power, which can be exploited for the sustainable production of hydrogen through the electrolysis of water. Scarcity of fresh water resources promotes the search for electrode materials, which could be used for splitting of saline water into hydrogen and oxygen without formation of hazardous chlorine compounds, and also withstand highly aggressive chloride medium. In the present study we demonstrate that nickel oxide layer formed by means of simple spray-pyrolysis technique on conductive glass substrate can be used as corrosion resistant and 100% O₂-selective anode for the electrolysis of alkaline (pH 14) chloride solution. Dimensional stability and durability of the anode is secured by the absence of metallic phase, whereas selectivity towards oxygen evolution and absence of chloride oxidation is shown to be controlled thermodynamically. The obtained experimental evidence that hydrogen peroxide is formed as intermediate in oxygen evolution reaction under conditions investigated has led to new mechanistic insights and the mechanism of Ni(IV)-mediated electrocatalytic oxidation of water molecules in alkaline chloride medium has been proposed.     

A facile top-down approach for constructing perovskite oxide nanostructure with abundant oxygen defects as highly efficient water oxidation electrocatalyst

Developing highly efficient electrocatalysts for oxygen evolution reaction (OER) is of significant importance for the application of many energy conversion and storage technologies. Perovskite oxides have attracted great attention as potential OER electro-catalysts. Their performance, however, are strongly limited by large particle size, owing to the high synthesis temperature. Herein, we report a facile top-down strategy for fabricating perovskite oxide nanostructures with large surface area and strongly improved intrinsic OER activity. SrNb_0.1Co_0.7Fe_0.2O_3-δ (SNCF) particles with micro size are treated by (NH₄)₂Fe(SO₄)₂ saturated solution for different time length at room temperature. The obtained catalysts exhibit significantly increased surface area with nanosheet structure in the outer layer. Furthermore, cobalt on the surface are reduced from Co³⁺ to Co²⁺, suggesting oxygen vacancy formation on the surface. The defective SNCF nanostructure exhibits significantly improved OER activity and good stability. The facile methodology reported in this work can be generally applied to other oxide electrocatalysts for energy applications.     

Manganese compounds as water oxidizing catalysts for hydrogen production via water splitting: From manganese complexes to nano-sized manganese oxides

For hydrogen production by water splitting, the water oxidation half reaction is overwhelmingly rate limiting and needs high over-voltage (∼1 V), which results in low conversion efficiencies when working at current densities required. At this high voltage, other chemicals will be also oxidized and this would be environmentally unacceptable for large-scale H₂ production.     

Manganese oxide with different morphology as efficient electrocatalyst for oxygen evolution reaction

Three-dimensional (3D) manganese oxides consisted of tetragonal phase Mn₃O₄ and α-MnO₂ with different morphology have been directly grown vertically on Ti foil by a simple electrochemical method without any template and used as the catalysts for oxygen evolution reaction (OER). The results show that manganese oxides with different morphology show high activity and good stability for OER and the manganese oxide (MnO_x) nanowire arrays obtained at 70 °C show higher activity and better stability than MnO_x with cotton wool structure and MnO_x nanosheet arrays.     

A quinary high entropy metal oxide exhibiting robust and efficient bidirectional O2 reduction and water oxidation

The O₂/H₂O couple-based transformation between renewable energy and electricity has emerged as a key step in implementing a carbon-neutral energy infrastructure. Therefore, an inexpensive and efficient electrocatalyst driving both O₂ reduction and O₂ evolution reaction in water becomes critical that can be directly applied in a unitized regenerative fuel cell in both electrolyzer or fuel cell mode. Here, we have crafted a high entropy metal oxide (HEO) containing readily abundant first-row transition metals (Fe, Cr, Co, Mn, Ni) via a metal-organic framework intermediate followed by regulated annealing at 750 °C. This material exhibited bidirectional ORR and OER activity in alkaline aqueous media (pH 14.0) with excellent energy efficiency on either side, showcasing a difference of 0.79 V (while achieving 10 mA cm⁻² current density) and ∼90% Faradaic efficiency. The in-depth electrochemical and surface analysis pointed out the key formation of the Ni–OOH layer on the HEO particle and the optimal porosity for maximized electrochemical surface area generation as pivotal factors behind its superior reactivity. An alkaline electrolyzer was assembled with this HEO (anode) and Ni-foam (cathode), which demonstrated concurrent production of O₂ and H₂ over 6 h with minimal alterations in the anodic material. Therefore, this robust, inexpensive, and scalable HEO material can boost the progress in developing sustainable electrolyzer/fuel cell assemblies.     

Lessons from metal oxides to find why Nature selected manganese and calcium for water oxidation

The problem of finding a way to store energy from abundant sources such as sunlight, wind or geothermal heat is critical. Water splitting toward hydrogen production is a very promising way for the goal. Although the cathodic reaction is of major interest in hydrogen production, the concurrent anodic water oxidation, which provides cheap electrons for the cathodic reaction, is a limitation for hydrogen formation. The best water-oxidizing catalyst was found by Nature million years ago and used in plants, algae and cyanobacteria. We believe learning from the natural system is very promising, because Nature has been successfully splitting water for millions of years, using an inexpensive and environmentally friendly MnCa oxido cluster. Herein we study the phenol oxidation by some nano-sized metal oxides in the presence of H₂O₂. As metal oxides are functional and structural models for the water-oxidizing complex in Photosystem II, the results can be expanded for the natural site. We suggest that low organic compound oxidation under water oxidation is an important issue to select manganese and calcium ions for water oxidation.     

Rethink about electrolyte: Potassium fluoride as a promising additive to an electrolyte for the water oxidation by a nanolayered Mn oxide

Water oxidation is a bottleneck of the hydrogen production through the water-splitting reaction. Herein, the promising role of fluoride on the water-oxidizing activity of a nanolayered Mn oxide under the electrochemical condition is reported. The experiments show the increase of the water-oxidizing activity of the nanolayered Mn oxide under an electro-water oxidation circumstance in the presence of potassium fluoride as a promising additive to an electrolyte. As a result, the required overpotential is decreased and the yield of oxygen evolution raised in the water-oxidation reaction.     

Benzimidazole Schiff base copper(II) complexes as catalysts for environmental and energy applications: VOC oxidation,oxygen reduction and water splitting reactions

The new copper(II) complexes [Cu(μ-1κO:2κONN′-HL1)(μ-1κO:2κO′-NO₃)]₂.[Cu(μ-1κO:2κONN′-HL1)(CH₃OH)]₂(NO₃)₂ (1) and [Cu(κONN′-HL2)(μ-1κOO’:2κO′-NO₃)]_n (2), derived from the new pro-ligands H₂L1 = 2-(5,6-dihydroindolo[1,2-c]quinazolin-6-yl)-5-methylphenol and H₂L2 = 2-(5,6-dihydroindolo[1,2-c]quinazolin-6-yl)-4-nitrophenol, were synthesized and characterized by elemental analysis, FT-IR, ESI-MS, and their structural features were unveiled by single-crystal X-ray diffraction analysis. This discloses a dimeric structure for 1 and a polymeric infinite 1D metal-organic chain for 2. The complexes were evaluated as catalysts for the oxidation of toluene, a volatile organic compound (VOC), and for oxygen reduction and water splitting reactions. 1 exhibits a higher activity for the peroxidative conversion of toluene to oxygenated products (total yields up to 38%), whereas 2 demonstrates a superior performance for electrochemical energy conversion applications, i.e., for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution (HER) reactions in an alkaline medium in terms of higher ORR current densities, lower Tafel slope (73 mV dec^?1) and higher number of electrons exchanged (3.9), comparable to that of commercial Pt/C. Complex 2 also shows a better performance with lower onset potential and higher current densities for both OER and HER when studied as electrocatalyst for water splitting.     

Manganese oxides treated with organic compounds as catalysts for water oxidation reaction

Manganese oxides of different crystalline structures: α-MnO₂, δ-MnO₂, α,γ-MnO₂ and Mn₂O₃; were treated with the organic compounds picolinic acid, ethylenediamine and pyridine; and were applied as catalysts in the chemical water oxidation reaction using Ce(IV) ammonium nitrate as sacrificial oxidant. The treatment led to modifications in the oxides properties, such as reduction of the particle size, increase of surface area and partial reduction of Mn⁴⁺ to Mn³⁺ for the Mn(IV) oxides, or of Mn³⁺ to Mn²⁺ for Mn₂O₃, because of favored interactions of the organic molecules with the lattice planes with higher d spacing. Oxygen evolution reaction (OER) tests showed the superior catalytic activity of the treated Mn(IV) oxides, for instance α,γ-MnO₂-en presented TOF five times higher than pure α,γ-MnO₂. The increase in surface area as well as the higher Mn³⁺ content caused by the treatment of the Mn(IV) oxides were correlated with the improvement in the OER catalytic activity.     

An aluminum/cobalt/iron/nickel alloy as a precatalyst for water oxidation

Among different strategies, water splitting toward hydrogen production is a promising process to store energy from intermittent sources. However, the anodic water oxidation is a bottleneck for water splitting. In this paper, we report an aluminum/cobalt/iron/nickel alloy as a precatalyst for the electrochemical water oxidation. The alloy electrode contains different metal ions including cobalt, iron, and nickel which all are efficient for water oxidation is tested. We characterized this electrode using scanning electron microscopy, transmission electron microscopy, diffuse reflectance infrared Fourier transform spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and electrochemical methods. After stabilization, the electrode shows an onset overpotential of 200.0 mV and affords a current density of 3.5 mA cm^?2 at an overpotential of 600.0 mV in KOH solution at pH 13.     

Fabrication of MnO2-pillared layered manganese oxide through an exfoliation/reassembling and oxidation process

MnO₂-pillared layered manganese oxide has been first fabricated by a delamination/reassembling process followed by oxidation reaction and then by heat treatment. The structural evolution of MnO₂-pillared layered manganese oxide has been characterized by XRD, SEM, DSC-GTA, IR and N₂ adsorption-desorption. MnO₂-pillared layered manganese oxide shows a relative high thermal stability and mesoporous characteristic. The layered structure with a basal spacing of 0.66 nm could be maintained up to 400 °C. The electrochemical properties of the synthesized MnO₂-pillared layered manganese oxide have been studied using cyclic voltammetry in a mild aqueous electrolyte. Sample MnO₂–BirMO (300 °C) shows good capacitive behavior and cycling stability, and the specific capacitance value is 206 F g⁻¹.