Facile single-step synthesis of nitrogen-doped reduced graphene oxide-Mn(3)O(4) hybrid functional material for the electrocatalytic reduction of oxygen

The Effect of Sulfur and Nitrogen Doping on the Oxygen Reduction Performance of Graphene/Iron Oxide Electrocatalysts Prepared by Using Microwave-Assisted Synthesis

The synthesis of novel catalysts for the oxygen reduction reaction, by means of a fast one-pot microwave-assisted procedure, is reported herein and deeply explained. In particular, the important role of doping atoms, like sulfur and nitrogen, in Fe2O3-reduced graphene oxide nanocomposites is described to address the modification of catalytic performance. The presence of dopants is confirmed by X-ray Photoelectron Spectroscopy analysis, while the integration of iron oxide nanoparticles, by means of decoration of the graphene structure, is corroborated by electron microscopy, which also confirms that there is no damage to the graphene sheets induced by the synthesis procedure. The electrochemical characterizations put in evidence the synergistic catalysis effects of dopant atoms with Fe2O3 and, in particular, the importance of sulfur introduction into the graphene lattice. Catalytic performance of as-prepared materials toward oxygen reduction shows values close to the Pt/C reference material, commonly used for fuel cell and metal-air battery applications.

Atomic-Level Regulation of Cu-Based Electrocatalyst for Enhancing Oxygen Reduction Reaction: From Single Atoms to Polymetallic Active Sites

The slow kinetics of cathodic oxygen reduction reactions (ORR) in fuel cells and the high cost of commercial Pt-based catalysts limit their large-scale application. Cu-based single-atom catalysts (SACs) have received increasing attention as a promising ORR catalyst due to their high atom utilization, high thermodynamic activity, adjustable electronic structure, and low cost. Herein, the recent research progress of Cu-based catalysts is reviewed from single atom to polymetallic active sites for ORR. First, the design and synthesis method of Cu-based SACs are summarized. Then the atomic-level structure regulation strategy of Cu-based catalyst is proposed to improve the ORR performance. The different ORR catalytic mechanism based on the different Cu active sites is further revealed. Finally, the design principle of high-performance Cu-based SACs is proposed for ORR and the opportunities and challenges are further prospected.

Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

Catalysts for highly water-resistant catalytic decomposition of ozone: Hausmannite Mn3O4 on exposed (101) crystal surface

Recently, ozone pollution has gradually replaced PM2.5 as the main pollutant affecting air pollution. In this study, we synthesized a series of Mn₃O₄ catalysts by hydrothermal method changing the precursors and tested their activities at different relative humidity, gas volume space velocity of 150,000 h^-1 and 5 ppm ozone. Remarkably, Mn₃O₄-SO₄ prepared with MnSO₄ as precursor showed excellent catalytic ozone decomposition activity, almost completely converting 5 ppm of ozone at different relative humidity ranges. Finally, the most active Mn₃O₄-SO₄ catalyst was tested for its usability limit at RH= 90%, after 28 h of testing under high humidity conditions, it had retained successfully the complete decomposition of low concentrations of ozone. The catalysts were characterized by XRD, Raman, HRTEM, XPS, BET, H₂O-TPD and in situ IR NH₃ adsorption. The characterization analysis revealed that the Mn₃O₄-SO₄ surface could exposed a highly active (101) crystalline surface with high specific surface area, excellent hydrophobicity as well as ozone adsorption capacity, which were highly favorable for ozone decomposition under high humidity conditions. In this work, Mn₃O₄ exhibits good catalytic activity, which provides an additional option for future studies of manganese oxides.

Mn3O4 nano-octahedrons embedded in nitrogen-doped graphene oxide as potent anode material for lithium-ion batteries

Mn3O4 nano-octahedrons embedded in N-doped graphene oxide (MNGO) nanosheets were synthesized using a simple, energy-efficient, and rapid microwave-digested hydrothermal route in a single step. The structural and morphological aspects of synthesized materials were evaluated by XRD, IR, Raman, FE-SEM, and HR-TEM techniques. Then, the composite MNGO was tested for its Li-ion storage properties and compared with reduced graphene oxide (rGO) and Mn3O4 materials. The MNGO composite exhibited superior reversible specific capacity, excellent cyclic stability, and outstanding structural integrity throughout the electrochemical studies. The MNGO composite showed a reversible capacity of 898 mA h g-1 after 100 cycles at 100 mA g-1 and Coulombic efficiency of 97.8%. Even at a higher current density of 500 mA g-1, it exhibits a higher specific capacity of 532 mA h g-1 (~1.5 times higher than commercial graphite anode). These results demonstrate that Mn3O4 nano-octahedrons embedded on N-doped GO are a highly durable and potent anode material for LIBs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11581-023-05035-6.

Largely Entangled Diamondoid Framework with High-Density Urea and Divergent Metal Nodes for Selective Scavenging of CO2 and Molecular Dimension-Mediated Size-Exclusive H-Bond Donor Catalysis

Pore environment modulation with high-density polarizing groups in metal-organic frameworks (MOFs) can effectively accomplish selective and multicyclic carbon dioxide (CO₂) adsorption, whereas the incorporation of task-specific organic sites inside these porous vessels promise to evade self-quenching, solubility, and recyclability issues in hydrogen-bond donating (HBD) catalysis. However, concurrent amalgamation of both these attributes over a single platform is rare but extremely demanding in view of sustainable applications. We designed a robust diamondoid framework CSMCRI-17 (CSMCRI = Central Salt and Marine Chemicals Research Institute) from the mixed-ligand assembly of azo group-containing dicarboxylate ligand, urea-functionalized pyridyl linker, and Zn(II) nodes with specific divergent coordination. Seven-fold interpenetration to the microporous structure largely augments N-rich functionality that facilitates high CO₂ uptake in the activated form (17a) with good CO₂ selectivity over N₂ and CH₄ that outperform many reported materials. The framework displays very strong CO₂ affinity and no reduction in adsorption capacity over multiple uptake-release cycles. Benefitting from the pore-wall decoration with urea functionality from the pillaring strut, 17a further demonstrates hydrogen-bond-mediated Friedel-Crafts alkylation of indole with β-nitrostyrene under mild conditions, with multicyclic usability and excellent reactivity toward wide ranges of substituted nucleophiles and electrophiles. Interestingly, interpenetration-generated optimum-sized pores induce poor conversion to sterically encumbered substrate via molecular dimension-mediated size selectivity that is alternatively ascribed from additional control experiments and support the occurrence of HBD reaction within the MOF cavity. The catalytic path is detailed in light of the change of emission intensity of the framework by the electrophile as well as the judicious choice of the substrate, which authenticates the prime role of urea moiety-governed two-point hydrogen bonding.

Brønsted Acid-Functionalized Ionic Co(II) Framework: A Tailored Vessel for Electrocatalytic Oxygen Evolution and Size-Exclusive Optical Speciation of Biothiols

Metal-organic frameworks (MOFs) not only combine globally demanded renewable energy generation and environmental remediation onto a single platform but also rationalize structure-performance synergies to devise smarter materials with remarkable performance. The robust and non-interpenetrated cationic MOF exemplifies a unique bifunctional scaffold for the efficient electrochemical oxygen evolution reaction (OER) and ultrasensitive monitoring of biohazards. The microporous framework containing Brønsted acid-functionalized [Co₂(μ₂-OH)(CO₂)₂] secondary building units (SBUs) exhibits remarkable OER performance in 1 M KOH, requiring 410 mV overpotential to obtain 10 mA cm^-2 anodic current density, and a low Tafel slope of 55 mV/dec with 93.1% Faradaic efficiency. Apart from the high turnover frequency and electrochemically assessable surface area, steady OER performance over 500 cycles under potentiodynamic and potentiostatic conditions result in long-term catalyst durability. The highly emissive attribute from nitrogen-rich fluorescent struts benefits the MOF in recyclable and selective fluoro-detection of three biothiols (l-cysteine, homocysteine, and glutathione) in water with a fast response time. In addition to colorimetric monitoring in the solid and solution phases, control experiments validate size-exclusive biothiol speciation through molecular-dimension-mediated pore diffusion. The role of SBUs in the OER mechanism is detailed from density functional theory-derived free energy analysis, which also validates the importance of accessible N-sites in sensing via portraying framework-analyte supramolecular interactions.

Highly efficient surface sequestration of Pb2+ and Cr3+ from water using a Mn3O4 anchored reduced graphene oxide: Selective removal of Pb2+ from real water

Owing to the ubiquitous existence of detrimental heavy metals in the environment, simple adsorption-oriented approaches are becoming increasingly appealing for the effective removal of Pb²⁺ and Cr³⁺ from water bodies. These techniques use nanocomposites (NC) of reduced graphene oxide (rGO) and Mn₃O₄ (rGO-Mn₃O₄), they employ a hydrothermal technique featuring NaBH₄ and NaOH solutions. Here, spectroscopic and microscopic instrumental techniques were used to evaluate the morphological and physicochemical characteristics of prepared reduced graphene oxide manganese oxide (rGO-Mn₃O₄), revealing that it possessed a well-defined porous structure with a specific surface area of 126 m² g^-1. The prepared rGO-Mn₃O₄ had significant adsorption efficiencies for Pb²⁺ and Cr³⁺, achieving maximum sequestration capacities of 130.28 and 138.51 mg g^-1 for Pb²⁺ and Cr³⁺, respectively, according to the Langmuir model. These adsorption capacities are comparable to or greater than those of previously reported graphene-based materials. The Langmuir isotherm and pseudo-second-order models adequately represented the experimental results. Thermodynamic analysis revealed that adsorption occurred through spontaneous endothermic reactions. Recycling studies showed that the developed r-GO-Mn₃O₄ had excellent recyclability, with <70% removal at the 5th cycle; its feasibility was evaluated using industrial wastewater, suggesting that Pb²⁺ was selectively removed from Pb²⁺ and Cr³⁺ contaminated water. The instrumental analysis and surface phenomena studies presented here revealed that the adsorptive removal processes of both heavy metals involved π electron donor-acceptor interactions, ion exchange, and electrostatic interactions, along with surface complexation. Overall, the developed rGO-Mn₃O₄ has the potential to be a high-value adsorbent for removing heavy metals.

Heteroatom-Doped Carbon-Encapsulated FeP Nanostructure: A Multifunctional Electrocatalyst for Zinc-Air Battery and Water Electrolyzer

The rational design and synthesis of efficient multifunctional electrocatalysts for renewable energy technologies is of significant interest. Herein, we demonstrate a novel approach for the synthesis of a nitrogen and phosphorus dual-doped mesoporous carbon-encapsulated iron phosphide (FeP@NPC) nanostructure and its multifunctional electrocatalytic activity toward an oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction for zinc-air battery (ZAB) and alkaline water-splitting applications. FeP@NPC is obtained by the carbothermal reduction of the precursor complex [Fe(bpy)₃](PF₆)₂ in the presence of melamine without any traditional phosphidating agent. The PF₆^- counteranion is used for the phosphidation of Fe. FeP@NPC obtained at 900 °C (FeP@NPC-900) exhibits excellent bifunctional oxygen electrocatalytic performance with a very low potential gap (ΔE = E_1/2^ORR - E_j10^OER) of 670 mV. The ZAB device delivers a peak power density of 190.15 mW cm^-2 (iR-corrected), specific capacity of 785 mA h g_Zn^-1, and energy density of 706.5 Wh kg_Zn^-1 at 50 mA cm^-2. The ZAB exhibits excellent charge-discharge cycling stability for over 35 h with negligible voltaic efficiency loss (0.9%). Three CR2032 coin-cell-based ZABs made of an FeP@NPC-900 air cathode connected in series power 81 LEDs for 15 min. FeP@NPC-900 also has promising electrocatalytic activity toward water splitting in acidic as well as in alkaline pH. The benchmark current density of 10 mA cm^-2 is achieved with a two-electrode alkaline water electrolyzer at a cell voltage of 1.65 V. ZAB-powered water electrolyzer is made by integrating two rechargeable ZABs connected in series with the two-electrode water electrolyzer. The ZAB powers the electrolyzer for 24 h without a significant loss in the open-circuit voltage. The catalyst retains its initial structural integrity even after continuous water electrolysis for 24 h.

Removal of oxytetracycline promoted by manganese-doped biochar based on density functional theory calculations: Comprehensive evaluation of the effect of transition metal doping

The regulation of surface electrons by non-metal doping of biochar (BC) is environmentally and ecologically significant. However, systematic studies on the regulation of surface electrons by transition metal doping are lacking. The present study is based on the observation that the removal efficiency of oxytetracycline (OTC) by Mn-doped BC is eight times higher than that of undoped BC in 20 min. The effects of Mn doping on the crystal phase formation, persistent free radicals (PFRs), electron density, molecular orbitals, and nucleophilic active sites of BC are investigated, and the intermediate products of OTC are evaluated. Mn doping enhances the signal for sp²-hybridised carbon-carbon double bond, forms more delocalised π-bonds, and promotes the formation of free radicals centred on the carbon atoms. The specific surface area of BC increases, and manganese oxide is formed on the its surface. Density functional theory calculations show that Mn doping accelerates the electron transfer of BC, provides additional electrons for the BC system, and makes this system more ionised. OTC molecules preferentially attack the nucleophilic reaction sites near Mn atoms based on molecular electrostatic potential measurements. Therefore, this study provides new insights into the surface electronic structures regulated by transition metal elements.

A Nd6 molecular butterfly: a unique all-in-one material for SMM, MCE and maiden photosensitized opto-electronic device fabrication

Besides iron, ironically neodymium (Nd) is the most ubiquitously used metal for magnetic purposes, even among the lanthanides, when it comes to the field of molecular magnetism, yet it ranks among the least studied metals. However, strong apathy towards this magnetic lanthanide means that vital information will be missed, which is required for the advancement of the subject. Herein, we have successfully demonstrated the usefulness of a hexanuclear neodymium complex as a magnetic material, and also in electronic device fabrication. A {NdIII6} cage with an aesthetically pleasing butterfly topology was synthesized using a rather non-conventional N-rich pyridyl-pyrazolyl based ligand. The cage shows single molecule magnet (SMM) properties, with an effective energy barrier, U_eff, value of 3.4 K and relaxation time, τ₀, of 3.1 × 10^-4 s, originating from an unusual occurrence of metal centres with different coordination environments. Furthermore, magnetic studies reveal significant cyrogenic magnetic cooling, with a magnetic entropy change of 8.28 J kg^-1 K^-1 at 5 T and 3 K. To the best of our knowledge, the titular compound is the only example of a Nd-complex that exhibits concomitant magnetocaloric effect (MCE) and SMM properties. Complete active space self-consistent field (CASSCF) calculations were carried out to shed light on the origin of the magnetic anisotropy and magnetic relaxation of the compound. The same uniqueness is also true for the first electronic investigation carried out on the Nd complex. The maiden electronic device fabricated using the Nd complex shows an interesting intertwining of electronic and optical features, which contribute towards its improved photosensitized optoelectronic data.

Trioctylphosphine Oxide (TOPO)-Assisted Facile Fabrication of Phosphorus-Incorporated Nanostructured Carbon Nitride Toward Photoelectrochemical Water Splitting with Enhanced Activity

Designing nanostructured arrays of two-dimensional surfaces and interfaces is a versatile approach to increasing their photoelectrochemical activity. Here, phosphorus (P)-incorporated nanostructured carbon nitride (h-PCN) with an enlarged surface area is fabricated by employing trioctylphosphine oxide (TOPO) as a dopant precursor for visible-light-driven photoelectrochemical water splitting to produce hydrogen. The structural, morphological, and electronic properties of the photocatalyst have been characterized through various physicochemical techniques. We show that the incorporation of P into the g-C₃N₄ framework enhances light absorption over broad regimes, charge separation, and migration, as well as the specific surface area, showing excellent photocurrent enhancement (5.4 folds) in the cathodic direction as compared to bulk g-C₃N₄. Moreover, the photocathode shows 3.3-fold enhancement in current at zero biased potential. Without using any cocatalyst, the photoelectrodes produced 27 μmol h^-1 of H₂ and 13 μmol h^-1of O₂ with 95% faradic efficiency. The excellent photoelectrochemical behavior toward water-splitting reactions by the photoelectrode is attributed to the synergistic effect of P incorporation and active sites emerging from the nanostructured architecture of the material. This work demonstrates the facile fabrication of nanostructured P-incorporated g-C₃N₄ toward water-splitting reactions to produce hydrogen without using a cocatalyst in a simple and cost-effective way.

Urea-engineering mediated hydrogen-bond donating Friedel−Crafts alkylation of indoles and nitroalkenes in dual-functionalized and microporous metal-organic framework with high recyclability and pore-fitting-induced size-selectivity

As an effective alternative to Lewis acid activation, hydrogen-bond donating (HBD) organo-catalysis denotes a powerful construction tool to important classes of carbon–carbon bonds, wherein metal-organic frameworks (MOFs) alleviate issues like...

Recent Advances in Flexible Zn-Air Batteries: Materials for Electrodes and Electrolytes

Flexible Zn-air batteries (ZABs) draw much attention due to the merits of high energy density, stability, and safety, and show potential applications for wearable devices. However, the development of flexible ZABs with great energy density, high round-trip efficiency, and long cycle life for practical applications is highly restricted by the lack of highly active oxygen catalysts, high ion-conducting solid-state electrolytes, appropriate Zn anodes, and advanced battery configuration. Promising oxygen catalysts should possess both, superior oxygen reduction reaction and oxygen evolution reaction performance and can be directly used as self-supporting cathodes without loading catalysts on support materials such as carbon cloth. In addition, electrolytes play an important role in ZABs; a good electrolyte should be in all-solid state with high ion conductivity. Moreover, for an excellent Zn anode, it is required to stably contact the electrolyte interface during the bending process. Therefore, in this review, recent advances in ZABs are summarized, including: i) the powder and 3D self-supporting oxygen catalysts, ii) the species of solid-state electrolytes, and iii) the rational design of Zn anodes. Finally, the challenges and opportunities of this promising field are presented.

One-Step Hydrothermal Synthesis of Nitrogen-Doped Reduced Graphene Oxide/Hausmannite Manganese Oxide for Symmetric and Asymmetric Pseudocapacitors

In this paper, the pseudocapacitive performance of nitrogen-doped and undoped reduced graphene oxide/tetragonal hausmannite nanohybrids (N-rGO/Mn₃O₄ and rGO/Mn₃O₄) synthesized using a one-pot hydrothermal method is reported. The nanohybrid electrode materials displayed exceptional electrochemical performance relative to their respective individual precursors (i.e., reduced graphene oxide (rGO), nitrogen-doped reduced graphene oxide (N-rGO), and tetragonal hausmannite (Mn₃O₄)) for symmetric pseudocapacitors. Among the two nanohybrids, N-rGO/Mn₃O₄ displayed greater performance with a high specific capacitance of 345 F g^-1 at a current density of 0.1 A g^-1, excellent specific energy of 12.0 Wh kg^-1 (0.1 A g^-1), and a high power density of 22.5 kW kg^-1 (10.0 A g^-1), while rGO/Mn₃O₄ demonstrated a high specific capacitance of 264 F g^-1 (0.1 A g^-1) with specific energy and power densities of 9.2 Wh kg^-1 (0.1 A g^-1) and 23.6 kW kg^-1 (10.0 A g^-1), respectively. Furthermore, the N-rGO/Mn₃O₄ nanohybrid exhibited an impressive pseudocapacitive performance when fabricated in an asymmetric configuration, having a stable potential window of 2.0 V in 1.0 M Na₂SO₄ electrolyte. The nanohybrid showed excellent specific energy and power densities of 34.6 Wh kg^-1 (0.1 A g^-1) and 14.01 kW kg^-1 (10.0 A g^-1), respectively. These promising results provide a good substance for developing novel carbon-based metal oxide electrode materials in pseudocapacitor applications.

Study on the Synthesis of Mn3o4 Nanooctahedrons and Their Performance for Lithium Ion Batteries

Among the transition metal oxides, the Mn₃O₄ nanostructure possesses high theoretical specific capacity and lower operating voltage. However, the low electrical conductivity of Mn₃O₄ decreases its specific capacity and restricts its application in the energy conversion and energy storage. In this work, well-shaped, octahedron-like Mn₃O₄ nanocrystals were prepared by one-step hydrothermal reduction method. Field emission scanning electron microscope, energy dispersive spectrometer, X-ray diffractometer, X-ray photoelectron spectrometer, high resolution transmission electron microscopy, and Fourier transformation infrared spectrometer were applied to characterize the morphology, the structure, and the composition of formed product. The growth mechanism of Mn₃O₄ nano-octahedron was studied. Cyclic voltammograms, galvanostatic charge-discharge, electrochemical impedance spectroscopy, and rate performance were used to study the electrochemical properties of obtained samples. The experimental results indicate that the component of initial reactants can influence the morphology and composition of the formed manganese oxide. At the current density of 1.0 A g^-1, the discharge specific capacity of as-prepared Mn₃O₄ nano-octahedrons maintains at about 450 mAh g^-1 after 300 cycles. This work proves that the formed Mn₃O₄ nano-octahedrons possess an excellent reversibility and display promising electrochemical properties for the preparation of lithium-ion batteries.

Tuning effect of amorphous Fe2O3 on Mn3O4 for efficient atom-economic synthesis of imines at low temperature: improving [O] transfer cycle of Mn3+/Mn2+ in Mn3O4

A novel Fe₂O₃ modified Mn₃O₄ catalyst (Fe₅Mn₅-100) has been prepared by adopting a simple co-precipitation method following low temperature baking. Fe₅Mn₅-100 showed exceptionally high catalytic activity for the production of imine.

Metal-Free N-Formylation of Amines with CO2 and Hydrosilane by Nitrogen-Doped Graphene Nanosheets

N-Formylation of amines with carbon dioxide (CO₂) as a carbonyl source is emerging as an important way for CO₂ transformation into high-value-added chemicals; however, the developed catalytic systems mainly focused on transition-metal-based homogeneous catalysts. Herein, we reported rationally designed nitrogen-doped graphene nanosheets (NG) as metal-free catalysts for N-formylation of various amines with CO₂ and hydrosilane to formamide under mild conditions. The NG catalyst displayed a wide amine scope with the desired formamide yields up to >99%, demonstrating its comparable catalytic performance to the reported transition-metal-based catalysts. Our experimental research reveals that the N-formylation of aniline involves an initial NG-promoted CO₂ hydrosilylation with PhSiH₃ to silyl formate and a subsequent nucleophilic attack of the aniline to give N-formanilide. Moreover, the key step of CO₂ hydrosilylation can be simplified to a pseudo-first-order reaction under a high CO₂ concentration with an observed reaction rate constant (k_obs) of 226 h^-1 at 40 °C and an apparent activation energy (E_a) of 34 kJ mol^-1. In sharp contrast, a k_obs of 23 h^-1 and E_a of 47 kJ mol^-1 were observed under catalyst-free conditions. Our theoretical investigation indicates that NG-promoted CO₂ hydrosilylation corresponds to an exergonic reaction (ΔG = -0.53 eV), which is much lower in energy state than that of catalyst-free conditions (ΔG = -0.44 eV). Finally, the NG showed outstanding recyclability in the N-formylation reaction with almost unchanged catalytic performance during twelve-time recycling. This research thus represented a breakthrough in metal-free transformation of CO₂ into fine chemicals with low-cost, environment-friendly, and carbon-based catalysts to replace the scarce and expensive transition-metal-based catalysts.

Functional molecule guided evolution of MnO x nanostructure patterns on N-graphene and their oxygen reduction activity

In this work, we systematically followed the growth of MnO_x nanostructures on trimesic acid (TMA)/benzoic acid (BA) functionalised nitrogen doped graphene (NG) and studied their electrocatalytic activity towards oxygen reduction reaction (ORR). In these hybrid materials the MnO_x phase, their morphology and Mn surface valency were guided by the functional molecules, their concentration and the duration of reaction, which in turn significantly affected the ORR activity. During the growth in the presence of TMA, agglomerated nanostructures were formed at 2 h reaction, which transformed to well dispersed 4–7 nm particles at 6 h over a large area of NG. However, in the presence of BA, MnOOH nano-flecks were formed at 2 h and transformed to MnOOH nanowires and oval shaped Mn₃O₄ particles at 8 h of reaction. The valency of surface Mn on the different MnO_x nanostructures was ascertained by X-ray photoelectron spectroscopy (XPS). The ORR activity of samples were studied by cyclic voltammetry (CV) and rotating disc electrode (RDE) in alkaline medium. Among all the studied samples, the highest ORR activity with most efficient 4e⁻ transfer process is observed for TMA modified NG-MnO_X obtained at 6 h of reaction, which is due to its well dispersed nanostructure morphology.

In this work, we systematically followed the growth of MnO_x nanostructures on trimesic acid (TMA)/benzoic acid (BA) functionalised nitrogen doped graphene (NG) and studied their electrocatalytic activity towards oxygen reduction reaction (ORR).

Oxygen Reduction Reaction Activity of Microwave Mediated Solvothermal Synthesized CeO2/g-C3N4 Nanocomposite

Electrocatalytic active species like transition metal oxides have been widely combined with carbon-based nanomaterials for enhanced Oxygen Reduction Reaction (ORR) studies because of the synergistic effect arising between different components. The aim of the present study is to synthesize CeO₂/g-C₃N₄ system and compare the ORR activity with bare CeO₂. Ceria (CeO₂) embedded on g-C₃N₄ nanocomposite was synthesized by a single-step microwave-mediated solvothermal method. This cerium oxide-based nanocomposite displays enhanced ORR activity and electrochemical stability as compared to bare ceria.

Metallosupramolecular Polymer Precursor Design for Multi-Element Co-Doped Carbon Shells with Improved Oxygen Reduction Reaction Catalytic Activity

Heteroatom-doped carbon materials have been extensively studied in the field of electrochemical catalysis to solve the challenges of energy shortage. In particular, there is vigorous research activity in the design of multi-element co-doped carbon materials for the improvement of electrochemical performance. Herein, we developed a supramolecular approach to construct metallosupramolecular polymer hollow spheres, which could be used as precursors for the generation of carbon shells co-doped with B, N, F and Fe elements. The metallosupramolecular polymer hollow spheres were fabricated through a simple route based on the Kirkendall effect. The in situ reaction between the boronate polymer spheres and Fe3+ could easily control the component and shell thickness of the precursors. The as-prepared multi-element co-doped carbon shells showed excellent catalytic activity in an oxygen reduction reaction, with onset potential (Eonset) 0.91 V and half-wave (Ehalf-wave) 0.82 V vs reversible hydrogen electrode (RHE). The fluorine element in the carbon matrix was important for the improvement of oxygen reduction reaction (ORR) activity performance through designing the control experiment. This supramolecular approach may afford a new route to explore good activity and a low-cost catalyst for ORR.

Azadirachta indica leaf-extract-assisted synthesis of CoO–NiO mixed metal oxide for application in a microbial fuel cell as a cathode catalyst

In this study, a highly efficient yet low-cost mixed transition metal oxide of Ni and Co as CoO–NiO was synthesized using Azadirachta indica leaf extract as a bioactive chelating agent.

Boosting CH3OH Production in Electrocatalytic CO2 Reduction over Partially Oxidized 5 nm Cobalt Nanoparticles Dispersed on Single-Layer Nitrogen-Doped Graphene

Herein, we successfully synthesized partially oxidized 5 nm cobalt nanoparticles dispersed on a single-layer nitrogen-doped graphene (SL-NG) (denoted as PO-5 nm Co/SL-NG) catalyst by a unique and simple one-pot synthesis strategy, which was efficiently applied for highly selective electrocatalytic reduction of carbon dioxide to methanol in 0.1 mol dm^-3 aqueous NaHCO₃ medium under mild conditions, reaching the maximum faradaic efficiency (FE) of 71.4% for methanol at -0.90 V versus saturated calomel electrode (SCE), possessing a strong electrocatalytic current density of 4 mA cm^-2 and a high yield of 1.10 mmol dm^-3 h^-1, and the corresponding overpotential is as low as 280 mV. Moreover, at -1.0 V versus SCE, a high current density of 10 mA cm^-2 can be obtained, and the FE for methanol still remains 23.2%. Notably, the proposed catalyst exhibits prominent stability after 10 h electroreduction of CO₂, and the morphology, particle size, structure, and element contents of the catalyst almost remain stable. This work first provides an advanced PO-5 nm Co/SL-NG for selective electroreduction of carbon dioxide into methanol, which simultaneously possesses the merits of high current density, low overpotential, high selectivity, superior FE, and good stability, outperforming most reported electrocatalysts.

Mn3O4 Quantum Dots Supported on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as Oxygen Reduction Electrocatalysts for High-Performance Zn-Air Batteries

Highly efficient and low-cost nonprecious metal electrocatalysts that favor a four-electron pathway for the oxygen reduction reaction (ORR) are essential for high-performance metal-air batteries. Herein, we show an ultrasonication-assisted synthesis method to prepare Mn₃O₄ quantum dots (QDs, ca. 2 nm) anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Mn₃O₄ QDs/N-p-MCNTs) as a high-performance ORR catalyst. The Mn₃O₄ QDs/N-p-MCNTs facilitated the four-electron pathway for the ORR and exhibited sufficient catalytic activity with an onset potential of 0.850 V (vs reversible hydrogen electrode), which is only 38 mV less positive than that of Pt/C (0.888 V). In addition, the Mn₃O₄ QDs/N-p-MCNTs demonstrated superior stability than Pt/C in alkaline solutions. Furthermore, a Zn-air battery using the Mn₃O₄ QDs/N-p-MCNTs cathode catalyst successfully generated a specific capacity of 745 mA h g^-1 at 10 mA cm^-2 without the loss of voltage after continuous discharging for 105 h. The superior ORR activity of Mn₃O₄ QDs/N-p-MCNTs can be ascribed to the homogeneous Mn₃O₄ QDs loaded onto the N-doped carbon skeleton and the synergistic effects of Mn₃O₄ QDs, nitrogen, and carbon nanotubes. The interface binding energy of -3.35 eV calculated by the first-principles density functional theory method illustrated the high stability of the QD-anchored catalyst. The most stable adsorption structure of O₂, at the interface between Mn₃O₄ QDs and the graphene layer, had the binding energy of -1.17 eV, greatly enhancing the ORR activity. In addition to the high ORR activity and stability, the cost of production of Mn₃O₄ QDs/N-p-MCNTs is low, which will broadly facilitate the real application of metal-air batteries.

Synthesis of Large Mesoporous-Macroporous and High Pore Volume, Mixed Crystallographic Phase Manganese Oxide, Mn2O3/Mn3O4 Sponge

The controlled synthesis of mixed crystallographic phase Mn₂O₃/Mn₃O₄ sponge material by varying heating rates and isothermal segments provides valuable information about the morphological and physical properties of the obtained sample. The well-characterized Mn₂O₃/Mn₃O₄ sponge and applicability of difference in reactivity of H₂ and CO₂ desorbed during the synthesis provide new developments in the synthesis of metal oxide materials with unique morphological and surface properties. We report the preparation of a Mn₂O₃/Mn₃O₄ sponge using a metal nitrate salt, water, and Dextran, a biopolymer consisting of glucose monomers. The Mn₂O₃/Mn₃O₄ sponge prepared at 1 °C·min^-1 heating rate to 500 °C and held isothermally for 1 h consisted of large mesopores-macropores (25.5 nm, pore diameter) and a pore volume of 0.413 mL/g. Furthermore, the prepared Mn₂O₃/Mn₃O₄ and 5 mol %-Fe-Mn₂O₃/Mn₃O₄ sponges provide potential avenues in the development of solid-state catalyst materials for alcohol and amine oxidation reactions.

Oxygen-deficient triple perovskites as highly active and durable bifunctional electrocatalysts for oxygen electrode reactions

Highly active and durable bifunctional oxygen electrocatalysts have been of pivotal importance for renewable energy conversion and storage devices, such as unitized regenerative fuel cells and metal-air batteries. Perovskite-based oxygen electrocatalysts have emerged as promising nonprecious metal bifunctional electrocatalysts, yet their catalytic activity and stability still remain to be improved. We report a high-performance oxygen electrocatalyst based on a triple perovskite, Nd_1.5Ba_1.5CoFeMnO_9-δ (NBCFM), which shows superior activity and durability for oxygen electrode reactions to single and double perovskites. When hybridized with nitrogen-doped reduced graphene oxide (N-rGO), the resulting NBCFM/N-rGO catalyst shows further boosted bifunctional oxygen electrode activity (0.698 V), which surpasses that of Pt/C (0.801 V) and Ir/C (0.769 V) catalysts and which, among the perovskite-based electrocatalysts, is the best activity reported to date. The superior catalytic performances of NBCFM could be correlated to its oxygen defect-rich structure, lower charge transfer resistance, and smaller hybridization strength between O 2p and Co 3d orbitals.