Enhanced activity of Pd/α-MnO2 for electrocatalytic oxygen evolution reaction

This work describes the application of α-MnO₂ and Pd/α-MnO₂ as electrocatalysts in the oxygen evolution reaction (OER). Characterization data revealed that the Pd²⁺ precursor has been oxidized during the synthesis, and the resulting Pd⁴⁺ ions have unprecedently replaced the lattice framework Mn³⁺ ions of α-MnO₂. Furthermore, formation of PdO nanoparticles was also observed. Lower OER overpotential at j = 10 mA cm^?2 (636 mV) was obtained for Pd/α-MnO₂ in relation to α-MnO₂ (700 mV), what is in alignment with the lower charge transfer resistance of Pd/α-MnO₂ (4.9 kΩ cm²) compared to α-MnO₂ (10.4 kΩ cm²). Lower Tafel slope (73 mV dec^?1) and higher TOF (2.87 × 10^?4 s^?1) at overpotential of 350 mV was obtained for Pd/α-MnO₂ in relation to α-MnO₂ (Tafel of 77 mV dec^?1 and TOF of 1.94 × 10^?4 s^?1), indicating a faster electron transfer kinetics promoted by Pd. Pd/α-MnO₂ was stable at j = 14 mA cm^?2 for 6 h.     

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.     

Synthesis of spinel LiMn2O4 nanoparticles through one-step hydrothermal reaction

Phase pure spinel LiMn₂O₄ nanoparticles can be directly synthesized by one-step hydrothermal reaction of γ-MnO₂ with LiOH in an initial Li/Mn ratio of 1 at 200 °C. The reaction might involve a redox reaction between Mn⁴⁺ and OH⁻, and the formation of LiMn₂O₄ at the same time under the proposed hydrothermal conditions. This hydrothermal process is simple since only γ-MnO₂ powders are used as the Mn source, whereas without use of any oxidants, reductants, or low valence Mn source. The electrochemical performance of the as-synthesized LiMn₂O₄ nanoparticles towards Li⁺ insertion/extraction was examined. Rather good capacity and cycle performance, and an especially excellent high rate capability, were observed for the sample that was hydrothermally reacted for 3 days.     

Oxygen vacancy rich α-MnO2 @B/O-g-C3N4 photocatalyst: A thriving 1D-2D surface interaction effective towards photocatalytic O2 and H2 evolution through Z-scheme charge dynamics

Herein, we have prepared a 1D-2D junction based Oxygen Vacancy rich α-MnO₂@B/O-g-C₃N₄ photocatalyst by using 1D α-MnO₂ nanorods over 2D nanosheets of boron, oxygen co-doped exfoliated graphitic carbon nitride (BOCN). The important feature of the above composite material is the availability of oxygen vacancy as well as the presence of dual oxidation state of Mn (Mn⁴⁺, Mn³⁺) which enhances surface activity as well as chemical reaction output of the material. Among the synthesized materials α-MnO₂@B/O-g-C₃N₄ (MBOCN-20) shows best photocatalytic activity by following Z-Scheme charge dynamics towards water oxidation (295.1 μmol h^?1) and reduction (560.1 μmol h^?1) reactions in presence of methanol (hole scavenger) and AgNO₃ (electron scavenger) as sacrificial agent. However, 44.2 and 86.5 μmol h^?1 of O₂ and H₂ evolution was observed in absence of any sacrificial agent. This analysis will confer a valuable blue-print to construct stimulating photocatalysts to achieve the paramount performance towards photocatalytic water redox reaction.     

Catalytic activity and sulfur tolerance for Mn-substituted La0.75Sr0.25CrO3±δ in gas containing H2S

Lanthanum chromites substituted by transition metal are potentially applied as anode catalysts in solid oxide fuel cell fed with fuel gas containing H₂S. In order to understand the effect of composition on catalytic activity and sulfur tolerance of anode catalysts, La_0.75Sr_0.25Cr_1−xMn_xO_3±δ (noted as LSCM, x = 0.2, 0.5, 0.8) series were synthesized and characterized by XRD, XPS and H₂-TPR. The results demonstrate that LSCM55 with moderate Mn-substitution content has the highest activity for H₂S, which is attributed to considerable reducibility of Mn⁴⁺ determined by H₂-TPR. XRD patterns reveal that as-synthesized samples with different Mn-substitution contents have different sulfur tolerance. Sr 3d_5/2, Cr 2p_3/2, Mn 2p_3/2 regions of XPS for samples with different Mn-substitution contents imply that dominance of Mn⁴⁺, responsible for the catalytic activity, is the consequence of the interactions between cations at A-site and B-site. On the other hand, evolution of Mn in H₂S atmosphere indicates Mn³⁺ in Mn⁴⁺-O-Mn³⁺ clusters contributes greatly to the sulfur tolerance of LSCM. By analyzing consistency in the contents of lattice oxygen, S species (mainly sulfate) and Mn³⁺, a hypothetical mechanism of LSCM catalysts involving catalysis and sulfur tolerance was proposed.     

Facial synthesis of p-p heterojunction composites: Evaluation of their electrochemical properties with photovoltaics-electrolyzer water splitting using two-electrode system

Mixed transition metal oxides have garnered widespread interest as alternative electrocatalysts for the oxygen and hydrogen generation reactions; however, they tend to require extended synthetic routes, in addition to possessing limited electrocatalytic activities and stabilities. Herein, we report the observation of a synergistic effect between the non-precious metal oxides Mn₃O₄ and Co₃O₄ with CuO and NiO, wherein the resulting composites exhibit promising properties as catalysts for the alkaline water electrolysis process. The activities of these composites in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) were improved compared to their counterparts, and the dynamic potentials of these processes were reduced. Importantly, low overpotentials of 202 and 380 mV were found for the CuO–Mn₃O₄ composite catalysts for the OER and the HER at 10 mA/cm², respectively. In addition, electrochemical impedance spectroscopy measurements showed a reduced impedance response for the composites, which was dominated by the relaxation of the intermediate frequency associated with the adsorption of the intermediate. Furthermore, the superior catalytic activities of the composites were attributed to their structural properties, high electroactive surface areas, fast electron transport kinetics, and good chemo-electrical bonding between Mn₃O₄ and CuO. Importantly, merging with a marketable silicon-based solar cells, the accumulated PV-EC water splitting device obtains greater hydrogen production under stimulated solar light irradiation. This work offers a typical demonstration and respected strategies for practical large-scale solar H₂ production via an economical PV-EC technology.     

Effects of binary Co–Mn oxides promoters on low-temperature catalytic performance of Pd/Al2O3 for methane combustion

Aiming at fabricating high-activity and stable methane combustion catalysts in the dry/wet conditions, Co–Mn binary oxides were employed as promoters to Pd/Al₂O₃ system herein. The introduction of appropriate amount of manganese made Mn³⁺ maximally enter into the Co₃O₄ spinel structure, conducive to the conversion of Co³⁺ to Co²⁺ by Mn³⁺ and then the enhancement of lattice distortion. Therefore, abundant oxygen vacancies were produced, which enhanced the surface-concentrations of active Pd²⁺ and O_ads species, together with the exchange of oxygen species. The resulting catalyst with a molar Mn/Co ratio of 0.20 performed superior low-temperature activity and durability. Moreover, the synergy of Mn and Co could accelerate the removal process of accumulated OH/H₂O from the active sites, thereby promoting the regeneration of PdO and oxygen vacancies. This endowed the tailored catalyst with remarkable moisture-tolerance and hydrothermal stability, and inspiring enhanced activity (T₉₀ = 350 °C) after removing water vapor.     

NiCo2O4 nanostructures loaded onto pencil graphite rod: An advanced composite material for oxygen evolution reaction

Driving oxygen evolution reaction (OER) at extremely low overpotential and the blockage of oxygen gas inside the catalytic material leads to the deactivation of catalytic activity, therefore it is an essential step in electrochemical energy conversion systems, but still very challenging task. The clay minerals including bentonite and kaolinite are rich with plenty of active centers and favorable chemical composition for the catalysis applications but limited by the insulating properties, thus they cannot be used as an electrode material for the water splitting. The unique presence of clay minerals in the form of pencil graphite rod (PGR) and its attractive architecture enabled us to exploit advantageous features and use them as an in situ electrode for growth of metal oxide nanostructures for the electrolysis applications. The naturally inherent presence of SiO₂ favors the catalytic properties and durability of the electrode whereas the MgO produces the abundant oxygen vacancies and Co³⁺ ions for OER process. Herein, we present a facile approach of using PGR as host substrate and co-catalyst for the loading of Co₃O₄, NiCo₂O₄ and NiO nanostructures and the modified electrode carried high porosity for easily bubbling of oxygen gas, plenty of intrinsic active centers coming from both clay minerals and metal oxides for excellent OER process. The fabricated electrode is physically well-characterized, and it has a natural ability to sustain a long term stability even at higher current densities and industrial electrolyzer conditions. The NiCo₂O₄/PGR, Co₃O₄/PGR, and NiO/PGR electrodes exhibit an overpotential of 234, 242 and 272 mV respectively at a current density of 100 mAcm^?2 in 1.0 M KOH electrolytic solution. The presence of large number of oxygen vacancies through SiO₂ and MgO, high Ni²⁺/Ni³⁺ and Co³⁺/Co²⁺ ratios, multi metal centers, large specific surface area, high pore volume, high electrochemical active surface area and fast charge transport within the NiCo₂O₄/PGR are the main reasons for its superfast OER kinetics. Thus, the proposed method of electrode design will pave a potential way for high performance electrochemical devices like metal air batteries, fuel cell and supercapacitors.     

Electrochemical construction of S-doped MnOx/Mn integrated film on carbon paper in a choline chloride based deep eutectic solvent for enhanced electrochemical water oxidation

The rational design and synthesis of highly efficient, precious metal-free electrocatalysts in the promotion of electrochemical water oxidation is essential for the sustainable use of renewable energy. Herein, an integrated, S-doped MnO_x/Mn film developed on carbon paper (CP) formed by a two-step transformation approach that includes template-free electrodeposition and in-situ electrochemical oxidation is employed as an efficient oxygen evolution reaction (OER) electrocatalyst. The doping of S and in-situ electro-oxidation lead to a tailored electronic structure and phase composition, which endow the composite electrode with a significantly enhanced OER catalytic performance. This S-doped integrated electrode perfectly combines highly active MnO_x outer layers with conductive metallic Mn inner layers to offer abundant catalytic interfaces and electronic paths, leading to favorable reaction kinetics. As a result, the optimal S-doped MnO_x/Mn/CP offers a low overpotential of 435 mV at 10 mA cm^?2 with a Tafel slope of 89.97 mV dec^?1 as well as improved stability. Impressively, a highly intrinsic catalytic activity with a turnover frequency of 0.0112 s^?1 is obtained for S-doped MnO_x/Mn/CP, which is 4.2-fold higher than that of MnO_x/Mn/CP. S doping plays a critical role in stabilizing the Mn^IIIO_x active phase and is ultimately responsible for the enhanced catalytic performance. This work provides an integrated design concept and S doping approach suitable for boosting the OER catalytic performance of MnO_x.     

Reusable water oxidation catalyst with dual active center for enhanced water oxidation: Iridium oxide nanoparticles immobilized on monodisperse-porous Mn5O8 microspheres

A reusable catalyst with dual active center for chemical water oxidation is synthesized for the first time by immobilization of iridium oxide nanoparticles (IrO₂ NPs) on monodisperse-porous manganese oxide microspheres acting both catalytic active center and support. Individual catalytic activity of manganese oxide microspheres is explained by multiple oxidation states of manganese which are capable of forming oxidative oxygen species. Monodisperse-porous microspheres in the form of Mn₅O₈, MnO₂ and Mn₂O₃ are used for synthesis of different catalysts and the highest activity in water oxidation is observed with the catalyst synthesized using Mn₅O₈ microspheres. The catalytic activity is correlated with the total Mn(II) and Mn(III) percentage of manganese oxide type selected for composite catalyst. The oxygen evolution up to 244 μmol is achieved in 30 min with the catalyst synthesized using Mn₅O₈ microspheres. Maximum TON and TOF numbers are obtained as 298 and 557 h^?1 with excellent reusability.     

Improving catalytic activity of layered lithium transition metal oxides for oxygen electrode in metal-air batteries

Lithium transition metal oxide has superior performance for oxygen evolution reaction (OER), while its activity for catalyzing oxygen reduction reaction (ORR) is too low to meet the demand of practical applications. Herein, the NCM-based (NCM, LiNi_1/3Co_1/3Mn_1/3O₂) composite materials are prepared through the two steps method. The NCM-2 (Mn₂O₃/LiNi_1/3Co_1/3Mn_1/3O₂) hybrid material demonstrates excellent ORR catalytic property and high OER catalytic performance, as well as the superior stability. Besides, with NCM-2 hybrid materials as catalysts of air cathode, the Al-air battery and Zn-air battery both exhibit higher power density. Therefore, based on results of Brunauer-Emmett-Teller and O₂ temperature programmed desorption analysis, the improved catalytic performance ascribed to large specific surface area, pore structure and enhanced oxygen adsorption ability. In this work, the catalytic activity of lithium transition metal oxide has been improved, and a new method was provided to synthesize bifunctional catalysts for metal-air batteries.     

Basic concepts of photoelectrochemical solar energy conversion systems

Abstract

The development of novel oxides, nanocomposites and architectures is in demand for the direct conversion of solar energy into other forms of energy, such as chemical and electrical energy. Especially, those oxides and devices, which can be used for photolysis of water (hydrogen and oxygen evolution) and for water purification, are of interest. So far, semiconducting oxides such as TiO₂, Fe₂O₃, WO₃, SrTiO₃, tantalates and niobates are the only class of materials which have shown high stability as photoelectrodes towards corrosion in the rate limiting step in the oxygen evolution reaction (OER) under illumination at the electrode/electrolyte interface during photolysis of water. Oxides have to be developed to be highly conductive and have a bandgap, which can be achieved by tailoring the defect chemistry of the oxides or by formation of a suited mixed oxide phase. Of special interest is the preparation of highly conductive p- and n-type TiO₂ and WO₃ as well as their alloys as corrosion stable and photoelectrocatalytically active electrodes. Bandgap reduction of pure TiO₂ involved the formations of solid solutions of TiO₂–FeO and TiO₂–Fe₂O₃, which were reported to have a bandgap of ～2·2 eV.¹ Besides TiO₂, WO₃ also has a superior stability as a photoelectrode material in the OER. Alloying with FeO also leads to lowering of the bandgap. Alternatively, ternary oxides of the systems Ni–Co–O and Ni–Fe–O are known for their high catalytic activity in the OER. They are considered as potential cocatalysts in the process of water oxidation. The materials can also be used in hybrid photoelectrodes consisting of a photovoltaic structure to absorb the sunlight with a corrosion stable and catalytically active window layer, which is in contact with the electrolyte.     

MnxCo3-xO4 spinel oxides as efficient oxygen evolution reaction catalysts in alkaline media

The design of efficient electrocatalysts for oxygen evolution reaction (OER) is an essential task in developing sustainable water splitting technology for the production of hydrogen. In this work, manganese cobalt spinel oxides with a general formula of Mn_xCo_3-xO₄ (x = 0, 0.5, 1, 1.5, 2) were synthesised via a soft chemistry method. Non-equilibrium mixed powder compositions were produced, resulting in high electrocatalytic activity. The oxygen evolution reaction was evaluated in an alkaline medium (1 M KOH). It was shown that the addition of Mn (up to x ≤ 1) to the cubic Co₃O₄ phase results in an increase of the electrocatalytic performance. The lowest overpotential was obtained for the composition designated as MnCo₂O₄, which exhibited a dual-phase structure (∼30% Co₃O₄ + 70% Mn_1.4Co_1.6O₄): the benchmark current density of 10 mA cm⁻² was achieved at the relatively low overpotential of 327 mV. The corresponding Tafel slope was determined to be ∼79 mV dec⁻¹. Stabilities of the electrodes were tested for 25 h, showing degradation of the MnCo₂O₄ powder, but no degradation, or even a slight activation for other spinels.     

Designed synthesis of hierarchical Mn3O4@SnO2/Co3O4 core-shell nanocomposite for efficient electrocatalytic water splitting

The hierarchical Mn₃O₄@SnO₂/Co₃O₄ core-shell nanocomposite has been successfully synthesized via a facile structural construction strategy. The SnO₂/Co₃O₄ nanosheets (SnO₂/Co₃O₄ NSs) were enclosed on the surface of Mn₃O₄ nanorods (Mn₃O₄ NRs). The prepared materials were investigated as the catalyst toward oxygen evolution reaction (OER) performance in alkaline. By contrast, the design of core-shell hierarchical nanocomposite of Mn₃O₄@SnO₂/Co₃O₄ possesses the obvious electrocatalytic OER performance than others, showing the overpotential of approximately 420 mV at a current density of 10 mA cm⁻² with a low Tafel slope of 70.1 mV dec⁻¹, in which the interesting structure can provide the interfacial and cooperative effect between core (hierarchical SnO₂/Co₃O₄ NSs) and shell (1D Mn₃O₄ NRs) that 1D Mn₃O₄ NRs can act as an electron acceptor and accelerates electron transfer, and that hierarchical SnO₂/Co₃O₄ NSs provide a large specific surface area and multiple exposed surface active sites between electrode material and electrolyte.     

Se-doped cobalt oxide nanoparticle as highly-efficient electrocatalyst for oxygen evolution reaction

Electrolysis of water has been one of the most promising approaches for renewable energy resources while the efficient oxygen evolution reaction (OER) remains challenging. Herein, a series of different ratio of Se doped Co₃O₄ nanoparticles XSe-Co₃O₄ are prepared by hydrothermal method and applied as OER electrocatalysts. Se^2? is doped into the Co₃O₄ crystal lattice by substituting of O^2? and a large number of oxygen vacancies are generated, which provides more available activity sites for OER. Se doping increases the surface ratio of Co²⁺/Co³⁺ and accelerates the electron transport that favors OER activity promotion. The optimized doping ratio of 6%Se–Co₃O₄ presents low overpotential of 281 mV at 10 mA cm^?2, as well as a low Tafel slope of 70 mV dec^?1 in 1 M KOH solution, which has great advantages compared to the recently reported Co₃O₄-based OER electrocatalysts. This work provides new ideas for the development of efficient Co₃O₄-based OER electrocatalysts.     

Comparative analysis of the changes in local Ni/Mn environment in lithium–nickel–manganese oxides with layered and spinel structure during electrochemical extraction and reinsertion of lithium

Electron paramagnetic resonance spectroscopy was used to analyse the changes in local Ni, Mn environment in layered LiNi_1/2Mn_1/2O₂ and spinel LiNi_1/2Mn_3/2O₄ after electrochemical extraction and reinsertion of lithium. For layered LiNi_1/2Mn_1/2O₂, the EPR signal from Mn⁴⁺ is only detected, while residual antiferromagnetic correlations between Ni²⁺ and Mn⁴⁺ ions gives rise to strong resonance absorption for LiNi_1/2Mn_3/2O₄ spinel. The first charge process of layered LiNi_1/2Mn_1/2O₂ leads to oxidation of Ni²⁺ ions located both in the transition metal sites and in the Li sites. The reverse process of reduction of these nickel ions was suggested to proceed between 4.4–3.0 V and 2.5–1.4 V, respectively. Lithium extraction from LiNi_1/2Mn_3/2O₄ spinel leads to oxidation of paramagnetic Ni²⁺ to diamagnetic Ni⁴⁺ without significant changes in the local environment of Mn⁴⁺. For both fully delithiated compositions, Li_1−xNi_1/2Mn_1/2O₂ and Li_1−xNi_1/2Mn_3/2O₄, an EPR spectrum from localized Mn⁴⁺ ions is observed, indicating an exhaustion of paramagnetic Ni²⁺ ions in the vicinity of Mn⁴⁺ ions. Furthermore, it has been found that the Mn⁴⁺ environment including paramagnetic Mn⁴⁺ and Ni²⁺ neighbours is restored after the first cycle of charge/discharge.     

Controlled synthesis of Mn3O4/Co9S8–Ni3S2 on nickel foam as efficient electrocatalyst for oxygen evolution reaction

Advances in electrochemical interfaces have greatly facilitated the development of new energy systems that can replace traditional fossil fuels. Oxygen evolution reaction (OER) is the core reaction in the new energy conversion system to produce hydrogen. Here, nanorods structure of Mn₃O₄/Co₉S₈–Ni₃S₂/NF-4 was designed and assembled. The Mn₃O₄ has served as an appropriate matrix to build a composite structure with Co₉S₈–Ni₃S₂ to enhance the stability of catalyst. And the introduction of Mn regulated the electronic structure of Ni and Co, which increased the OER activity of matericals. Further characterization and electrochemical testing have suggested that between polymetallic can effectively optimize conductivity and enhance reaction kinetics. Mn₃O₄/Co₉S₈–Ni₃S₂/NF-4 can achieve overpotential of 188 mV at the current density of 10 mA cm^?2 in alkaline solution, with small Tafel slope of 43.2 mV dec^?1 and satisfactory stability of 30 h at 10 mA cm^?2. This work may show a feasible reference in the design of high-efficient OER catalysts.     

Role of TiB2 and Bi2O3 additives on the rechargeability of MnO2 in alkaline cells

With an aim to understand the role of recently reported Ti-containing additives like TiB₂ on the rechargeability of manganese oxide cathodes in alkaline cells, a redox reaction involving the chemical oxidation of Mn(OH)₂ with H₂O₂ in KOH solution and a non-redox reaction involving the reaction of Mn(III) acetate with KOH have been carried out in the presence and absence of 1 wt% TiB₂ and 0.5 wt% TiB₂ + 4.5 wt% Bi₂O₃ additives. The solid products formed during the reactions have been analyzed by X-ray diffraction and a redox titration to determine the oxidation state of manganese while the filtrate has been analyzed to determine the amount of dissolved manganese with reaction time. The results suggest that irreversible reactions that follow the disproportionation reaction of dissolved Mn³⁺, which leads to the formation of electrochemically inactive phases like birnessite (δ-MnO₂) and hausmannite (Mn₃O₄) and a consequent decline in capacity retention, are suppressed in the presence of the TiB₂ additive, with the suppression being more effective when Bi₂O₃ is present along with TiB₂.     

Structural characteristics and electrochemical performance of layered Li[Mn0.5−xCr2xNi0.5−x]O2 cathode materials

Li[Mn_0.5−xCr_2xNi_0.5−x]O₂ (0 < 2x <0.2) (Mn/Ni = 1) cathode materials have been synthesized by a solution method. X-ray diffraction patterns of the as-prepared materials were fitted based on a hexagonal unit cell (α-NaFeO₂ layer structure). The extent of Li/Ni intermixing decreased, and layering of the structure increased, with increasing Cr content. Electrochemical cycling of the oxides, at 30 °C in the 3–4.3 V range vs. Li/Li⁺, showed that the first charge capacity increased with increasing Cr content. However, maximum discharge capacity (∼143 mAh g⁻¹) was observed for 2x = 0.05. X-ray absorption near edge spectroscopic (XANES) measurements on the K-edges of transition metals were carried out on pristine and delithiated oxides to elucidate the charge compensation mechanism during electrochemical charging. The XANES data revealed simultaneous oxidation of both Ni and Cr ions, whereas manganese remains as Mn⁴⁺ throughout, and does not participate in charge compensation during oxide delithiation.