Insights into the Electrochemical Behavior of Manganese Oxides as Catalysts for the Oxygen Reduction and Evolution Reactions: Monometallic Core-Shell Mn/Mn3 O4

Room-temperature synthesis of NiFe-hexamethylenetetramine as lattice oxygen involved electrocatalyst for efficient oxygen evolution reaction

The design and synthesis of an oxygen evolution reaction (OER) electrocatalyst following lattice oxygen mechanism (LOM) through a straightforward strategy is crucial for achieving efficient electrocatalytic hydrogen production; however, it remains a formidable challenge. Herein, a novel and highly efficient LOM-based OER electrocatalyst, NiFe-hexamethylenetetramine (NiFe-HMT) coordination compound, is fabricated through a straightforward co-precipitation strategy at room temperature within 30 min. The obtained NiFe-HMT exhibits remarkable OER activity with low overpotentials of 269 and 352 mV to achieve 100 and 1000 mA cm-2, respectively. Experimental results and theoretical calculations reveal that the incorporation of Fe can effectively activate the lattice oxygen in the reconstructed oxyhydroxides, thereby shifting the OER pathway from adsorbate evolution mechanism to LOM. Additionally, compared with NiFe-LDH, NiFe-HMT is more favorable for forming highly active oxyhydroxides and exhibits more significant lattice oxygen activity. Furthermore, NiFe-HMT can be scaled up to more than 10 g in a single batch and stored stability for over 142 days without any significant decline in activity, thereby indicating its potential for large-scale implementation. This study provides valuable insights into developing high-performance OER electrocatalysts following the LOM pathway.

Electronic and surface engineering of Mn active sites by femtosecond lasers: enhancing catalytic performance for seawater electrolysis through Mn4+-OH- layers

Laser-induced modifications of La0.51Sr0.49MnO3 (LSMO) perovskite electrocatalysts are explored for enhanced seawater oxidation under alkaline conditions. Femtosecond (FS) laser treatment stabilizes Mn in the high oxidation state (Mn4+), significantly altering the electronic structure and surface morphology of the catalyst. These changes lead to increased covalency between the Mn d-band and O 2p orbitals, facilitating efficient charge transfer and lowering activation barriers for oxygen evolution reaction (OER) intermediates. Laser treatment also induces a porous, roughened surface, enhancing active site density, hydrophilicity, and ion exchange, while minimizing Jahn-Teller distortions to further stabilize the catalyst during the OER. Additionally, the formation of a robust hydroxide layer protects against corrosive species in seawater, ensuring long-term durability. These combined effects result in significantly improved OER kinetics, selectivity, and stability, positioning laser-treated LSMO (LT-LSMO) as a promising candidate for direct seawater electrolysis applications.

Breaking the Stability-Activity Trade-off of Oxygen Electrocatalyst by Gallium Bilateral-Regulation for High-Performance Zinc-Air Batteries

The rational design of metal oxide catalysts with enhanced oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance is crucial for the practical application of aqueous rechargeable zinc-air batteries (a-r-ZABs). Precisely regulating the electronic environment of metal-oxygen (M-O) active species is critical yet challenging for improving their activity and stability toward OER and ORR. Herein, we propose an atomic-level bilateral regulation strategy by introducing atomically dispersed Ga for continuously tuning the electronic environment of Ru-O and Mn-O in the Ga/MnRuO2 catalyst. The Ga/MnRuO2 catalyst breaks the stability-activity restriction, showing remarkable bifunctional performance with a low potential gap (ΔE) of 0.605 V and super durability with negligible performance degradation (300,000 ORR cycles or 30,000 OER cycles). The theoretical calculations revealed that the strong coupling electron interactions between Ga and Ru-O/Mn-O tuned the valence state distribution of the metal center, effectively modulating the adsorption behavior of *O/*OH, thus optimizing the reaction pathways and reducing the reaction barriers. The a-r-ZABs based on Ga/MnRuO2 catalysts exhibited excellent performance with a wide working temperature range of -20-60 °C and a long lifetime of 2308 hours (i.e., 13,848 cycles) under a current density of 5 mA cm-2 at -20 °C.

Morphology-Driven Bifunctional Activity of Layered Birnessite-Based Materials toward Oxygen Electrocatalysis

The chemical, structural, and morphological diversity of birnessite, a 2D layered MnO2, has opened avenues for its application as an electrocatalyst toward both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Among pristine birnessites prepared by different methods, the freestanding flakes (primary structure) obtained from molten salt (MS-KMnO) showed remarkable bifunctional activity as compared to samples with thicker plates or a hierarchical honeycomb-like (type-I secondary structure) morphology. While the ORR onset potential (E onset) and halfwave potential (E 1/2) for MS-KMnO were recorded at 0.89 and 0.81 V vs RHE, respectively, the OER overpotential (η) was found to be 300 mV. We demonstrated heat-induced secondary structure evolution by modification of the molten salt method, which led to a decrease in activity. In contrast to previous studies, the Co-doped birnessite (Co-KMnO) prepared in molten salt showed lower bifunctional activity (ORR, E 1/2 = 0.72 V; OER, η= 460 mV) as compared to MS-KMnO. Co-KMnO showed an interwoven wrinkled sheet-like (type-II secondary structure) morphology, with Co3+ present in both the in-layer and the interlayer. However, in Co-KMnO/360 prepared at a lower reaction temperature, the areal coverage of the type-II structure reduces, leading to an increase in ORR (E 1/2 = 0.76 V) and OER (η = 440 mV) activity. The chronopotentiometry for 100 h at a constant OER current of 50 mA cm-2 showed an increase in potential from 1.62 to 1.89 V and the characterization of the sample post-treatment showed degradation of the layered structure in MS-KMnO. The samples obtained after 1000 CV cycles in both the ORR and the OER regions showed the formation of secondary structures with a substantial decrease in the Mn3+/Mn4+ ratio. This study demonstrates that morphology tuning within the 2D birnessite system has a marked effect on its bifunctional activity.

Enhanced immobilization of trace nickel by nanoplastic-Fe-Mn oxide complexes in sedimentary systems

Fe/Mn oxides are widely distributed mineral components in marine sediments and act as significant scavengers of trace metals. The emergence of plastic-rock complexes has led to an increasing recognition that plastics may influence the environmental behavior of minerals. Plastics, especially nanoplastics, can affect the formation of Fe/Mn oxides and their ability to immobilize heavy metals. In this study, the role of polystyrene nanoplastics (PS NPs) in the mineralization of FeMn oxides and their effects on the immobilization of heavy metals (using Ni(II) as an example) at the trace concentrations in the environment were investigated. Characterization analysis indicated that PS NPs not only adsorb Fe and Mn ions from the environment through electrostatic attraction (the force that draws together objects with opposite electrical charges) but also serve as a substrate for the heterogeneous nucleation and growth of FeMn oxides. The large specific surface area of the PS NPs provides a site for the growth of FeMn. This results in smaller particle sizes and larger specific surface areas for the generated FeMn oxides. Consequently, Fe-PS-Mn@SiO2 exhibits significantly greater adsorption efficiency for Ni(II) under various environmental conditions (such as different pH and salinity) compared to Fe-Mn@SiO2. Additionally, Fe-PS-Mn@SiO2 remained stable under sunlight at 60 °C over 1.5 years. These findings presented new insights into the impact of NPs on mineral formation and environmental behavior, expanding our understanding of the actual fate of NPs in sediment environments.

Electrocatalysis of Oxygen Evolution Reaction Promoted by CoNiMn Films Synthesized by Electrodeposition

Recently, efforts have been made to address the environmental damage caused by fossil-fuel-based primary energy sources. Interest in efficient technologies for converting and storing energy using renewable sources, especially sunlight, has increased, with the aim of replicating the natural photosynthesis process. However, artificial photosynthesis faces challenges with unfavorable kinetics and thermodynamics, requiring the use of stable catalysts for the hydrogen evolution (HER) and oxygen evolution (OER) reactions to generate H2 and O2, respectively. OER is the most prohibitive of the half-reactions by the highly sluggish kinetics. Mixed oxides, particularly those based on first-row transition metals, have shown promising results as catalysts for the OER. This work reports the synthesis of CoNiMn oxide via electrodeposition on fluoride tin oxide followed by electrochemical activation. This approach seeks to explore the synergistic effect between the elements and to produce a catalyst with superior efficiency and stability for the electrocatalysis of the OER compared to the monometallic and bimetallic oxides. The CoNiMn film was structurally and electrochemically characterized. The electrodeposited CoNiMn hybrid films demonstrated low overpotentials compared with standard OER electrocatalysts, with CoNiMn films outperforming all single and bimetallic oxide films. The activated CoNiMn film required an overpotential of 100 mV at 10 mA cm-2 (430 mV at 25 mA cm-2) and Tafel slope of 58 mV dec-1. The film was active for 15 h at 100 mA cm-2 and showed no significant change in morphology and structure after the chronopotentiometry, indicating that it is a promising and cost-effective alternative to enhance the OER activity using abundant elements.

Bimetal Oxides Anchored on Carbon Nanotubes/Nanosheets as High-Efficiency and Durable Bifunctional Oxygen Catalyst for Advanced Zn-Air Battery: Experiments and DFT Calculations

To meet increasing requirement for innovative energy storage and conversion technology, it is urgent to prepare effective, affordable, and long-term stable oxygen electrocatalysts to replace precious metal-based counterparts. Herein, a two-step pyrolysis strategy is developed for controlled synthesis of Fe2O3 and Mn3O4 anchored on carbon nanotubes/nanosheets (Fe2O3-Mn3O4-CNTs/NSs). The typical catalyst has a high half-wave potential (E1/2 = 0.87 V) for oxygen reduction reaction (ORR), accompanied with a smaller overpotential (η10 = 290 mV) for oxygen evolution reaction (OER), showing substantial improvement in the ORR and OER performances. As well, density functional theory calculations are performed to illustrate the catalytic mechanism, where the in situ generated Fe2O3 directly correlates to the reduced energy barrier, rather than Mn3O4. The Fe2O3-Mn3O4-CNTs/NSs-based Zn-air battery exhibits a high-power density (153 mW cm-2) and satisfyingly long durability (1650 charge/discharge cycles/550 h). This work provides a new reference for preparation of highly reversible oxygen conversion catalysts.

Enhancing oxygen reduction activity of dinuclear copper complexes loaded on an N-doped carbon support via a low-temperature pyrolysis strategy

Bioinspired by the active sites of multicopper oxidases (MCOs), bi/multinuclear copper complexes have attracted great attention in promoting catalytic activity for the oxygen reduction reaction (ORR). Herein, we report the preparation of a Cu-N-C electrocatalyst Cu-BPOZ@CNB-400 for efficient ORR, which was obtained by low temperature pyrolysis of a dinuclear 2,5-bis(2-pyridyl)-1,3,4-oxadiazole (BPOZ) copper complex loaded on a N-doped carbon support at 400 °C. Cu-BPOZ@CNB-400 exhibited a half-wave potential (E1/2) of 0.86 V vs. RHE for the ORR in 0.1 M KOH solution, which was significantly higher than that of the Cu-BPOZ@CNB-800 (E1/2 = 0.83 V) catalyst treated under high temperature (at 800 °C) and the control catalyst Cu-Phen@CNB-400 (E1/2 = 0.82 V) derived from low-temperature-treatment (at 400 °C) of a mononuclear phenanthroline-coordinated-Cu complex loaded on a N-doped carbon support. When Cu-BPOZ@CNB-400 was applied as the cathode catalyst in zinc-air batteries a maximum power density (Pmax) of 127 mW cm-2 could be achieved, demonstrating comparable catalyst performance to the commercial 20 wt% Pt/C (Pmax = 122 mW cm-2) and the control Cu-Phen@CNB-400 catalyst (Pmax = 105 mW cm-2) under similar experimental conditions. Low-temperature pyrolysis of dinuclear copper complexes on a carbon support improved the charge transfer efficiency, inhibited metal aggregation, and could produce highly dispersed Cu-N-C catalysts with dinuclear copper sites for promoting the 4e--reduction selectivity of the ORR. It thus provides a cost-effective approach for the controllable fabrication of efficient ORR catalysts to be applied for energy conversion devices.

Heterointerface MnO2/RuO2 with rich oxygen vacancies for enhanced oxygen evolution in acidic media

The design and synthesis of oxygen evolution reaction (OER) electrocatalysts that operate efficiently and stably under acidic conditions are important for the preparation of green hydrogen energy. The low intrinsic catalytic activity and poor acid resistance of commercial RuO2 limit its further development, and the construction of heterointerface structures is the most promising strategy to break through the intrinsic activity limitation of electrocatalysts. Herein, we synthesized spherical and oxygen vacancy-rich heterointerface MnO2/RuO2 using morphology control, which promoted the kinetics of the oxygen evolution reaction with the interaction between oxygen vacancies and the oxide heterointerface. MnO2/RuO2 was reported to be an acidic OER catalyst with excellent performance and stability, requiring only an ultra-low overpotential of 181 mV in 0.5 M H2SO4 to achieve a current density of 10 mA cm-2. The catalyst activity remained essentially unchanged in a 140 h stability test with an ultra-high mass activity (858.9 A g-1@ 1.5 V), which was far superior to commercial RuO2 and most previously reported noble metal-based acidic OER catalysts. The experimental results showed that the effect of more oxygen vacancies and the heterointerfaces of manganese ruthenium oxides broke the intrinsic activity limitation, provided more active sites for the OER, accelerated reaction kinetics, and improved the stability of the catalyst. The excellent performance of the catalyst suggests that MnO2/RuO2 provides a new idea for the design and study of heterointerfaces in metal oxide nanomaterials.

Geometric and Electronic Engineering in Co/VN Nanoparticles to Boost Bifunctional Oxygen Electrocatalysis for Aqueous/Flexible Zn-Air Batteries

Modulating metal-metal and metal-support interactions is one of the potent tools for augmenting catalytic performance. Herein, highly active Co/VN nanoparticles are well dispersed on three-dimensional porous carbon nanofoam (Co/VN@NC) with the assistance of dicyandiamide. Studies certify that the consequential disordered carbon substrate reinforces the confinement of electrons, while the coupling of diverse components optimizes charge redistribution among species. Besides, theoretical analyses confirm that the regulated electron configuration can significantly tune the binding strength between the active sites and intermediates, thus optimizing reaction energy barriers. Therefore, Co/VN@NC exhibits a competitive potential difference (ΔE, 0.65 V) between the half-wave potential of ORR and OER potential at 10 mA cm-2, outperforming Pt/C+RuO2 (0.67 V). Further, catalyst-based aqueous/flexible ZABs present superior performances with peak power densities of 156 and 85 mW cm-2, superior to Pt/C-based counterparts (128 and 73 mW cm-2). This research provides a pivotal foundation for the evolution of bifunctional catalysts in the energy sector.

In Situ Assembly of a Superaerophobic CoMn/CuNiP Heterostructure as a Trifunctional Electrocatalyst for Ampere-Level Current Density Urea-Assisted Hydrogen Production

Urea electrolysis is a promising energy-efficient hydrogen production process with environmental benefits, but the lack of efficient and sustainable ampere-level current density electrocatalysts fabricated through simple methods is a major challenge for commercialization. Herein, we present an efficient and stable heterostructure electrocatalyst for full urea and water electrolysis in a convenient and time-efficient preparation manner. Overall, superhydrophilic/superaerophobic CoMn/CuNiP/NF exhibits exceptional performance for the hydrogen evolution reaction (HER) (-33.8, -184.4, and -234.8 mV at -10, -500, and -1000 mA cm-2, respectively), urea electro-oxidation reaction (UOR) [1.28, 1.43, and 1.51 V (vs RHE) at 10, 500, and 1000 mA cm-2, respectively], and oxygen evolution reaction (OER) [1.45, 1.67, and 1.74 V (vs RHE) at 10, 500, and 1000 mA cm-2, respectively]. Moreover, the superaerophobic CoMn/CuNiP/NF demonstrates promising potential in full urea (1.33, 1.57, and 1.60 V at 10, 500, and 1000 mA cm-2, respectively) and water (1.46 V, 1.78, and 1.86 at 10, 500, and 1000 mA cm-2, respectively) electrolysis. Based on X-ray photoelectron spectroscopy results, it was determined that the surface of the CoMn/CuNiP electrode was rich in redox pairs such as Ni2+/Ni3+, Cu+/Cu2+, Co2+/Co3+, and Mn2+/Mn3+, which are crucial for the formation of active sites for the OER and UOR, such as NiOOH, MnOOH, and CoOOH, thereby enhancing the catalytic activity. Besides, the in situ assembled CoMn/CuNiP/NF displayed highly stable performance for HER, OER, and UOR with high Faradaic efficiency for over 500 h. This research offers a simple and efficient method for manufacturing a high-efficiency and stable trifunctional electrocatalyst capable of delivering ampere-level current density in urea-assisted hydrogen production. Our density functional theory calculations reveal the potential of CoMn/CuNiP as an effective catalyst, enhancing the electronic properties and catalytic performance. The near-zero Gibbs free-energy change for HER underscores its promise, while reduced CO2 desorption energies and charge redistribution support efficient UOR. These findings signify CoMn/CuNiP's potential for electrochemical applications.