Active Sites For Oxygen Reduction Research Articles

Long-range charge transfer mechanism of the III2IV2 mycobacterial supercomplex

Aerobic life is powered by membrane-bound redox enzymes that shuttle electrons to oxygen and transfer protons across a biological membrane. Structural studies suggest that these energy-transducing enzymes operate as higher-order supercomplexes, but their functional role remains poorly understood and highly debated. Here we resolve the functional dynamics of the 0.7 MDa III2IV2 obligate supercomplex from Mycobacterium smegmatis, a close relative of M. tuberculosis, the causative agent of tuberculosis. By combining computational, biochemical, and high-resolution (2.3 Å) cryo-electron microscopy experiments, we show how the mycobacterial supercomplex catalyses long-range charge transport from its menaquinol oxidation site to the binuclear active site for oxygen reduction. Our data reveal proton and electron pathways responsible for the charge transfer reactions, mechanistic principles of the quinone catalysis, and how unique molecular adaptations, water molecules, and lipid interactions enable the proton-coupled electron transfer (PCET) reactions. Our combined findings provide a mechanistic blueprint of mycobacterial supercomplexes and a basis for developing drugs against pathogenic bacteria.

Membrane potential generation by cytochrome <i>bd</i>

This treatise gives an overview of current thinking on the mechanism of generation of a transmembrane electric potential difference (Δψ) during the catalytic cycle of a bd-type triheme terminal quinol oxidase. It is assumed that the main contribution to the Δψ formation is made by the movement of H+ across the membrane along the intraprotein hydrophilic proton-conducting pathway from the cytoplasm to the active site for oxygen reduction of this bacterial enzyme.

Bifunctional TiCo electrocatalysts based on N-doped graphene cryogels for the oxygen evolution and reduction reactions

Three-dimensional N-doped graphene cryogels (NGC) modified with Ti and Co nanoparticles are investigated as bifunctional oxygen electrocatalysts for potential application in unitized regenerative fuel cells (URFCs). These composites have been successfully produced via a combined solvothermal, freeze-drying, and thermal treatment procedure. Crystallographic structure, surface properties, and morphology are analyzed to study the influence of each metal and the bimetallic combination in forming active sites for oxygen reduction (ORR) and oxygen evolution (OER) reactions. The intrinsic catalytic activity of the composites was determined by rotating disk electrode and further assessed in a gas diffusion electrode to test them in more realistic conditions for URFCs in 6 M KOH. Results revealed the remarkable bifunctional activity of Co/NGC and bimetallic TiCo/NGC composites. The synergistic effect of the metallic cobalt and surface atomic composition of Co3O4 distributed on the N-doped graphene cryogel in Co/NGC is responsible for the high bifunctional performance. Moreover, the partial substitution of cobalt for a less critical raw material, such as titanium, allows the electrocatalyst to maintain great bifunctional activity and stability. The nitrogen doping of the graphene cryogel structure can influence the anatase/rutile titania ratio that forms during pyrolysis. As a result, defects, oxygen vacancies, and Ti3+ species are formed, which, when combined with Co nanoparticles, contribute to the outstanding performance and stability of the TiCo/NGC composite. The excellent performance of N-doped graphene cryogel-based electrocatalysts in GDE conditions indicates their potential use in electrochemical processes, which involve gas-phase reactions.

Generation of Membrane Potential by Cytochrome bd.

An overview of current notions on the mechanism of generation of a transmembrane electric potential difference (Δψ) during the catalytic cycle of a bd-type triheme terminal quinol oxidase is presented in this work. It is suggested that the main contribution to Δψ formation is made by the movement of H+ across the membrane along the intra-protein hydrophilic proton-conducting pathway from the cytoplasm to the active site for oxygen reduction of this bacterial enzyme.

Electrochemical Crosslinking of Alginate-Towards Doped Carbons for Oxygen Reduction.

Electrochemical crosslinking of alginate strands by in situ iron oxidation was explored using a potentiostatic regime. Carbon-based materials co-doped with iron, nitrogen, and/or sulfur were prepared via electrolyte composition variation with a nitrogen-rich compound (rivanol) or through post-treatments with sodium sulfide. Nanometer-sized iron particles were confirmed by transmission and field emission scanning electron microscopy in all samples as a consequence of the homogeneous dispersion of iron in the alginate scaffold and its concomitant growth-limiting effect of alginate chains. Raman spectra confirmed a rise in structural disorder with rivanol/Na2S treatment, which points to more defect sites and edges known to be active sites for oxygen reduction. Fourier transform infrared (FTIR) spectra confirmed the presence of different iron, nitrogen, and sulfur species, with a marked difference between Na2S treated/untreated samples. The most positive onset potential (-0.26 V vs. saturated calomel electrode, SCE) was evidenced for the sample co-doped with N, S, and Fe, surpassing the activity of those with single and/or double doping. The mechanism of oxygen reduction in 0.1 M KOH was dominated by the 2e- reduction pathway at low overpotentials and shifted towards complete 4e- reduction at the most negative explored values. The presented results put forward electrochemically formed alginate gels functionalized by homogeneously dispersed multivalent cations as an excellent starting point in nanomaterial design and engineering.

Keto-enamine-based covalent organic framework with controllable anthraquinone moieties for superior H2O2 photosynthesis from O2 and water

Photocatalytic H2O2 production is a promising strategy for decentralized applications, but to realize efficient H2O2 production in the ambient conditions and absence of organic sacrifice remains challengeable. Herein, anthraquinone functionalized covalent organic frameworks (TpAQ-COFs) were fabricated by β-ketoenamines links of 2,6-diaminoanthraquinone (AQ) and 2, 4, 6-triformylphloroglucinol (Tp), and the structures and compositions were tuned by controlling the condensation time. The increase of time favors the assembly of TpAQ-COFs into more ordered lamellar stacking structure, and induces the increase of enol-imine form. The critical role of anthraquinone moieties as the active sites for oxygen reduction was proved, that is, facilitating charge separation and enhancing the activity and selectivity of H2O2 production. Upon visible light irradiation, the TpAQ-COF synthesized after 12 h reaction has the highest content of anthraquinone moieties and the best performance for H2O2 production (420 μmol h−1 g−1) in the absence of organic sacrificial agents. These findings bring insights to incorporate anthraquinone chemistry into photocatalysts at molecular levels for efficient H2O2 production.

Nitrogen-Doped Carbon-Encapsulated FeCo Alloy Nanostructures with Surface-Dangling Fe(Co)-Nx Active Sites for Oxygen Reduction in Alkaline and Acid Media

Nitrogen-Doped Carbon-Encapsulated FeCo Alloy Nanostructures with Surface-Dangling Fe(Co)-N<i>_x</i> Active Sites for Oxygen Reduction in Alkaline and Acid Media

Modulating Pt Interaction in Au@Pt Nanoparticle/WO3 Photocatalysts for Enhanced Production of Peroxodisulphate

Production of peroxodisulphate (S2O82–) on powdered photocatalysts is a promising way to obtain high-value-added chemicals using solar energy; however, it is still suffering from low efficiency. In the present work, we fabricated bimetallic Au core and Pt shell nanoparticle-loaded WO3 (Au@Pt/WO3) photocatalyst for enhanced production of S2O82–, which is 1.7 times that of Pt nanoparticle-loaded WO3 (Pt/WO3), and was reported to be the best catalyst for S2O82– production. A further study demonstrated that the electronic interaction of Pt was successfully modulated by the core–shell nanoparticle architecture, and the core–shell nanoparticle structure between Au and Pt improved the formation of the surface chemical state of Pt to be metallic Pt, proposed as the active site for oxygen reduction. Besides, the core–shell nanoparticle structure also exposed a large amount of Pt on the surface of the substrate. As a result, Au@Pt/WO3 showed significantly enhanced photocatalytic activity of S2O82– with a quantum efficiency of 5.2% at 420 nm, whereas Pt–Au bimetallic alloy nanoparticle-loaded WO3 with a large amount of surface PtOx and exposed both Au and Pt on the surface showed very low activity. This work provides insights into the bimetallic Pt nanoparticle-loaded WO3 for enhanced photocatalytic activity.

Ultra-dense carbon defects as highly active sites for oxygen reduction catalysis

<h2>Summary</h2> Defective carbons have recently been considered as one of the most promising alternatives to precious metal electrocatalysts. However, atomic structural tailoring of carbon defects poses challenges, especially in regulating defect density to maximize the active sites. Herein, we report an interfacial self-corrosion strategy to control the removal and reconstruction of carbon atoms via a series of thermal redox reactions of ZnO quantum dots and formed CO₂ gas in confined carbon cavity. The ultra-dense carbon defects (HDPC) (2.46 × 10¹³ cm⁻²) in the derived porous carbon served as efficient active sites for oxygen reduction, resulting in an excellent catalyst in both base and acid (half-wave potentials of 0.90 or 0.75 V in 0.1 M KOH or HClO₄). The normalized specific activity and density functional theory calculation reveal a gradient "proximity effect" between carbon defects with different spatial distance, indicating that the quantitative control of carbon defect density is the key to enhancing electrocatalytic activity.

La-doped Ba0.5Sr0.5Co0.8Fe0.2O3- d as Cathode for Protonic-Conducting Solid Oxide Fuel Cells with Enhanced Structure Stability

Introduction As a highly efficient energy conversion device, proton-conducting solid oxide fuel cells (SOFCs) have attracted intensive attention due to its high performance at intermediate temperature.1 Ba0.5Sr0.5Co0.8Fe0.2O3- d (BSCF), as an excellent air electrode material for oxygen-ion conducting SOFCs,2 nevertheless, is not effective when employed in proton-conducting SOFCs. There are several factors limiting the performance of BSCF, including 1) the large amount of Ba2+ in A-site makes the material unstable in an atmosphere containing steam and carbon dioxide acid gases which might occupy active sites for oxygen reduction on the surface of cathode; 2) The large ionic radius of Ba2+ makes the distance between the B-site metal and oxygen far away, suppressing the electron conduction and resulting in low conductivity of BSCF.3 In this work, the modification of BSCF was achieved by partially replacing Ba2+ with La3+. La3+ has a smaller ionic radius and higher affinity with oxygen, which can reduce the unit cell size, increase the electronic conductivity, weaken the interaction between Ba2+ and O2-, and reduce the basicity of the material. This is expected to improve both the catalytic activity of the oxygen reduction reaction (ORR) and the stability of BSCF under SOFC operating condition. Experimental BSCF and La-doped BSCF (0.1La-BSCF) powders were prepared by citric acid-EDTA complex method, and were calcined at 1000 °C for 2 hours to remove the carbon residues and to obtain good crystallization. The button cells with a configuration of Ni-BaZr0.3Ce0.5Y0.2O3 (Ni-BZCY) anode support, thin layer BZCY electrolyte, and the 0.1La-BSCF air electrode were fabricated by the co-pressing method. The electrical conductivity measurements were conducted with the four-probe technique. Results and discussion XRD results indicate that the incorporation of La3+ effectively inhibits the formation of BaCO3 in humidified air, and thus improves the chemical stability of BSCF at 700 °C. Such improvement should be attributed to the improved interactions between La3+ and O2-, which decreases the electron density of Ba2+, leading to the lower basicity of BSCF. In addition, as the ionic radius of La3+ (1.36 Å) is much smaller than that of Ba2+ (1.61 Å), the introduction of La3+ can reduce the unit cell parameters, and lower the distance between B-site cation with oxygen. This effectively improves the electronic conductivity of BSCF at 600 °C from about 35 S cm-1 in air and 55 S cm-1 in O2 to 85 S cm-1 and 120 S cm-1, respectively. Fig. 1 Left: XRD patterns of BSCF and 0.1La-BSCF after synthesis and after heat treatment at 700 oC in humidified air for 100 hours. Right: Conductivity of BSCF and 0.1La-BSCF measured in air. Reference Duan, C.; Huang, J.; Sullivan, N.; O'Hayre, R., Proton-conducting oxides for energy conversion and storage. Applied Physics Reviews 2020, 7 (1), 011314.Shao, Z.; Haile, S. M., A high-performance cathode for the next generation of solid-oxide fuel cells. In Materials for Sustainable Energy: A Collection of Peer-Reviewed Research and Review Articles from Nature Publishing Group, World Scientific: 2011; pp 255-258.Xie, Y.; Shi, N.; Huan, D.; Tan, W.; Zhu, J.; Zheng, X.; Pan, H.; Peng, R.; Xia, C., A Stable and Efficient Cathode for Fluorine-Containing Proton-Conducting Solid Oxide Fuel Cells. ChemSusChem 2018, 11 (19), 3423-3430. Figure 1

Hierarchically Assembled Cobalt Oxynitride Nanorods and N-Doped Carbon Nanofibers for Efficient Bifunctional Oxygen Electrocatalysis with Exceptional Regenerative Efficiency.

Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath-core cobalt oxynitride (CoOx@CoNy) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoOx@CoNy nanorods, composed of metallic Co4N cores and oxidized surfaces, exhibit excellent OER activity (E = 1.69 V at 10 mA cm-2) in an alkaline medium. Although pristine NCNF or CoOx@CoNy alone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E = 0.78 V at -3 mA cm-2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoOx@CoNy nanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoOx@CoNy nanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoOx@CoNy/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn-air batteries.

Duckweed derived nitrogen self-doped porous carbon materials as cost-effective electrocatalysts for oxygen reduction reaction in microbial fuel cells

Cost-effective metal-free electrocatalysts for oxygen reduction reaction were incredible significance of improvement about microbial fuel cells. In this research, a novel nitrogen self-doped porous carbon material is effectively inferred with KOH activation from a natural and renewable biomass, duckweed. Self-doped nitrogen in carbon matrix of nitrogen-doped porous carbon at 800 °C provides abundant active sites for oxygen reduction and improves the oxygen reduction kinetics significantly. Moreover, the porous structure of nitrogen-doped porous carbon at 800 °C encourages the transition of electrolyte and oxygen molecules throughout the oxygen reduction reaction. Oxygen on the three-phase boundary is reduced to water according to a four-electron pathway on nitrogen-doped porous carbon electrocatalyst. The single-chamber microbial fuel cell with nitrogen-doped porous carbon as electrocatalyst achieves comparable power density (625.9 mW m−2) and better stability compared to the commercial Pt/C electrocatalyst. This simple and low-cost approach provides a straightforward strategy to prepare excellent nitrogen-doped electrocatalyst derived from natural and renewable biomass directly as a promising alternate to precious platinum-based catalysts in microbial fuel cells.

Metal-Free Thiophene-Sulfur Covalent Organic Frameworks: Precise and Controllable Synthesis of Catalytic Active Sites for Oxygen Reduction

Defective or heteroatoms-doped metal-free carbon materials (MFCMs) are regarded as efficient oxygen reduction reaction (ORR) catalysts in the past decade. However, the active site structures for ORR in MFCMs are hard to be precisely confirmed and controllably synthesized through the common methods such as high temperature pyrolysis or heteroatom doping process. To verify the precise structure acted as active center for ORR, we firstly report two crystalline metal-free thiophene-sulfur covalent organic frame-works (MFTS-COFs) as ORR catalysts. The MFTS-COFs show more positive catalytic capability than that of the thiophene-free COF, indicating pentacyclic thiophene-sulfur building blocks acted as active centers induce ORR catalytic activity. MFTS-COFs with higher numbers of thiophene-sulfur exhibits better ORR performance. The experimental identification is supported by density functional theory calculations. These results thus demonstrate that rational design and precise synthesis of metal-free crystal-line organic materials can promote the development of new ORR catalysts.

Atom-Coordinated Structure Triggers Selective H2O2 Production

The family of atomic catalysts in defective carbons is now of increasing interest for diverse reactions. In this issue of Chem, Liu and co-workers unveil the nature of M–N–C (M = Mn, Fe, Co, Ni, and Cu) as active sites that can finely tune the oxygen reduction reaction (ORR) pathways spanning 1e–4e transfer processes.

A kinetic Monte Carlo-blueprint for oxygen reduction on oxide-supported PtNi nanoalloys.

To elucidate the effect of the architecture of supported bimetallic nanocatalysts, we developed a new lattice kinetic Monte Carlo based on the classifying and counting adsorption sites with respect to their generalized coordination number. We employed this tool to estimate the activity of MgO-supported PtNi nanoalloys for oxygen reduction. We demonstrated that the presence of Ni atoms in contact with the substrate massively enhances their activity with at least a 7-order of magnitude increase in the turnover of water production with respect to the case where only Pt lay at the interface. We further discussed how the nanoalloy shape affects the activity showing that truncated octahedra are 102 more active than cuboctahedra of similar size. We explained our results in terms of their distinct distribution and occurrence of the most active sites for oxygen reduction leading to the stabilization of different chemical species during the reaction dynamics. Our results suggest that engineering multifaceted and long edge PtNi-nanoalloys with a certain chemical ordering at the support interface would improve their catalytic performance for the oxygen reduction reaction.

Template synthesis of two-dimensional ternary nickel-cobalt-nitrogen co-doped porous carbon film: Promoting the conductivity and more active sites for oxygen reduction

Rational design of a stable, highly active non-precious metal-based electrocatalysts for oxygen reduction reaction (ORR) is vitally important for industrial application of fuel cells technology. As a potential alternative of Pt/C catalyst, two-dimensional (2D) porous carbon materials are widely investigated due to the highly accessible surface area and active sites, wherein carbon films doped with a plurality of metals and non-metal elements are rarely reported due to an uncontrollable synthesis process. Here, a bi-metallic (NiCo alloy nanoparticles) and nonmetallic (N) co-doped porous carbon film (Ni-Co-N@CF) is fabricated by a simple controllable and scalable strategy comprising the synthesis of NiCo alloy nanoparticles, modification of organic molecules, and high-temperature carbonization process. The optimized Ni-Co-N@CF catalyst shows an excellent ORR electrocatalytic activity with a larger electrochemically active surface area (2.31 m2 g−1), a higher half-wave potential (0.86 V) and a lower diffusion limited current density (−4.43 mA cm−2) than all the prepared control catalysts. Moreover, the designated catalyst also exhibits high durability and superior methanol tolerance in alkaline media, significantly better than the commercial Pt/C (20 wt%). The superior ORR performance is attributed to the synergetic interactions of ternary doping of Ni/Co/N in the 2D film skeleton, which not only greatly enhances conductivity but also provides more Co-N active sites.

Facile and simple deposition of cobalt oxide onto oxidized multiwall carbon nanotubes for electrocatalytic oxygen reduction

Cobalt oxide (CoOx) nanoparticles were synthesized onto oxygenated carbon nanotubes (FCNTs) using a simple cyclic voltammetry technique, and used as an electrocatalyst for oxygen reduction reaction (ORR) in alkaline media. The electrodeposition process was optimized by controlling the deposition cycles to achieve the highest catalytic activity. The electrocatalytic activity of the prepared electrocatalyst towards ORR has been studied in 0.1 M KOH. The results showed that coupling of CoOx with FCNTs enhanced the catalytic activity of the obtained catalyst (CoOx/FCNTs) towards ORR in terms of the onset potential (Eonset), the half-wave potential (E1/2) and the kinetic current (IK). Moreover, based on RDE measurements, the catalyst CoOx/FCNTs was able to catalyze the ORR through a 4-electron reduction pathway. The higher activity of CoOx/FCNTs has been explained by the formation of an active interface upon coupling of CoOx with FCNTs leading to creation of more active sites for oxygen reduction and reduction of the reaction intermediates (peroxide). In term of stability, the CoOx/FCNTs catalyst demonstrated a higher stability under ORR condition for a long time (6 h) than the commercial Pt/C catalyst. These findings indicated that CoOx/FCNTs catalyst is a promising non-precious catalyst for ORR electrode in alkaline fuel cells.

Cover Feature: Nanoscopic Combination of Edge and Flat Planes in the Active Site for Oxygen Reduction and Evolution (Eur. J. Inorg. Chem. 42/2019)

Cover Feature: Nanoscopic Combination of Edge and Flat Planes in the Active Site for Oxygen Reduction and Evolution (Eur. J. Inorg. Chem. 42/2019)

Nanoscopic Combination of Edge and Flat Planes in the Active Site for Oxygen Reduction and Evolution

An iron‐containing carbonaceous flat plane was enriched with edges by substrate fine etching and metal oxide loading, resulting in substantial enhancement of the catalytic activity for the oxygen reduction and evolution, which was fundamentally shown using the highly oriented pyrolytic graphite basal plane as the substrate. The evolution close to the standard electrode potential was elucidated by a mechanism involving the nanoscopic edge‐flat plane combination.

Zigzag carbon as efficient and stable oxygen reduction electrocatalyst for proton exchange membrane fuel cells

Non-precious-metal or metal-free catalysts with stability are desirable but challenging for proton exchange membrane fuel cells. Here we partially unzip a multiwall carbon nanotube to synthesize zigzag-edged graphene nanoribbons with a carbon nanotube backbone for electrocatalysis of oxygen reduction in proton exchange membrane fuel cells. Zigzag carbon exhibits a peak areal power density of 0.161 W cm−2 and a peak mass power density of 520 W g−1, superior to most non-precious-metal electrocatalysts. Notably, the stability of zigzag carbon is improved in comparison with a representative iron-nitrogen-carbon catalyst in a fuel cell with hydrogen/oxygen gases at 0.5 V. Density functional theory calculation coupled with experimentation reveal that a zigzag carbon atom is the most active site for oxygen reduction among several types of carbon defects on graphene nanoribbons in acid electrolyte. This work demonstrates that zigzag carbon is a promising electrocatalyst for low-cost and durable proton exchange membrane fuel cells.