Molecular-MN4 vs atomically dispersed M−N4−C electrocatalysts for oxygen reduction reaction

First-principle calculations study of the ORR/OER electrocatalytic activity of ruthenium polyphthalocyanine axially modified with aliphatic thiol groups

In this study, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of ruthenium polyphthalocyanine axially modified with different aliphatic thiol groups, RuPPc-SR (SR = -SCH3, -SC2H5, -SC3H7, -SC4H9, -SC5H11, and -SC6H13), in an acidic medium were simulated using DFT. All -SR groups can effectively enhance the ORR and OER catalytic activities of RuPPc. The ORR and OER overpotentials of RuPPc-SC4H9 are 0.237 V and 0.436 V, respectively, which are far lower than those of RuPPc (0.960 V and 0.903 V). For RuPPc-SC4H9, the four C and S atoms of the -SC4H9 chain and Ru atom are coplanar, and thus, conjugate effects and inductive effects exist between the -SC4H9 chain and Ru atom. This makes the Ru atom exhibit the least positive Bader charge and smallest spin density, and the anti-bonding orbitals of dxz, dyz, and dz2 of the Ru atom shift below the Fermi level (Ef). This makes the adsorption strength of RuPPc-SC4H9 toward ORR and OER intermediates the weakest, which accelerates the reaction process, thus resulting in better ORR and OER catalytic activity. Therefore, the introduction of the aliphatic thiol groups might effectively improve the OER/ORR catalytic activity of RuPPc.

Boron in the Second Coordination Sphere of Fe Single Atom Boosts the Oxygen Reduction Reaction

Metal single atoms coordinated with four nitrogen atoms (M1N4) are regarded as tremendously promising catalysts for the electrocatalytic oxygen reduction reaction (ORR). Nevertheless, the strong bond intensity between the metal center and the O atom in oxygen-containing intermediates significantly limits the ORR activity of M1N4. Herein, the catalytically active B atom is successfully introduced into the second coordination sphere of the Fe single atom (Fe1N4-B-C) to realize the alternative binding of B and O atoms and thus facilitate the ORR activity. Compared with the pristine Fe1N4 catalyst, the synthesized Fe1N4-B-C catalyst exhibits improved ORR catalytic capability with a half-wave potential (E1/2) of 0.80 V and a kinetic current density (JK) of 5.32 mA cm-2 in acid electrolyte. Moreover, in an alkaline electrolyte, the Fe1N4-B-C catalyst displays remarkable ORR activity with E1/2 of 0.87 V and JK of 8.94 mA cm-2 at 0.85 V, outperforming commercial Pt/C. Notably, the mechanistic study has revealed that the active center is the B atom in the second coordination shell of the Fe1N4-B-C catalyst, which avoids the direct bonding of Fe-O. The B center has a moderate binding force to the ORR intermediate, which flattens the ORR energy diagram and thereby improves the ORR performance. Therefore, this study offers a novel strategy for tailoring catalytic performance by tuning the active center of single-atom catalyst.

Oxygen reduction reaction (ORR) in alkaline solution catalysed by an atomically precise catalyst based on a Pd(II) complex supported on multi-walled carbon nanotubes (MWCNTs). Electrochemical and structural considerations

A new atomically precise, single-ion catalyst (MWCNT-LPd) for ORR (oxygen reduction reaction), consisting of a Pd(II) complex of a tetraazacycloalkane anchored on multiwalled carbon nanotubes, has been prepared through a supramolecular approach ensuring a uniform distribution of catalytic centres on the support surface. A tetraazacycloalkane was chosen to saturate the four coordination sites of the typical square planar coordination geometry of Pd(II) with the aim of ascertaining whether the metal ion must have free coordination sites to function effectively in the ORR or whether, as predicted by quantum mechanical calculations, the catalytic effect can be originated from an interaction of O2 in the fifth coordinative position. The results clearly demonstrated that tetracoordination of Pd(II) does not influence its catalytic capacity in the ORR. Electrodes based on this catalyst show ORR performance very close to that of commercial Pt electrodes, despite the low Pd(II) content (1.72% by weight) in the catalyst. The onset potential (Eon) value and the half-wave potential (E1/2) of the catalyst are, respectively, only 53 mV and 24 mV less positive than those observed for the Pt electrode and direct conversion of O2 to H2O reaches 85.0%, compared to 89% of the Pt electrode. Furthermore, a preliminary galvanostatic test (simulating a working fuel cell at a fixed potential) showed that the catalyst maintains its efficiency continuing to produce water throughout the process (the average number of electrons exchanged over time per O2 molecule remains close to 4).

Supercapacitive properties of carbazole-containing cobalt(II) phthalocyanines/reduced graphene oxide composites

This study aims to compare the effect of substituents (position and number) and reduced graphene oxide on the supercapacitive properties of cobalt(II) phthalocyanines. For this purpose, three new tetra- and octa-substituted cobalt(II) phthalocyanines bearing 9H-carbazol-2-yloxy groups on peripheral or non-peripheral positions (1-3) were synthesized. The characterization of the resultant cobalt(II) phthalocyanines was carried out by applying several spectroscopic approaches. The newly synthesized macromolecules were used for the functionalization of reduced graphene oxide (rGO). The obtained nanocomposites (rGO-(1-3)) were utilized for the modification of Ni foam (NiF) electrodes through a facile one-step electrodeposition strategy performed for electrochemical supercapacitor applications. Simultaneous polymerization of the cobalt phthalocyanines and electrochemically reduction of graphene oxide led to the formation of a fabricating layer on the surface of the NiF electrode. The resulting electropolymerized films were characterized by Raman, Fourier-transform infrared (FT-IR), and Field emission scanning electron microscope (FESEM) spectroscopic techniques as well as electrochemical methods. The prepared electrodes possessed superior electrochemical activities owing to the synergistic effect of the cobalt(II) phthalocyanines and rGO. All the modified electrodes displayed high supercapacitaive properties and the highest activity was obtained for the NiF/rGO2-1 electrode. The NiF/rGO2-1 electrode exhibited higher specific capacitance (655.2 F g-1 at 0.5 A g-1) than NiF/1 (338.0 F g-1). Additionally, a specific capacitance of 85.2% was obtained for NiF/rGO2-1 electrode after 3000 charge-discharge cycles. As a result, all the prepared metallophthalocyanines-reduced graphene oxide can be considered alternative agents to develop high performance-next-generation energy storage devices.

Inductive Effect on Single-Atom Sites

Single-atom catalysts exhibit promising electrocatalytic activity, a trait that can be further enhanced through the introduction of heteroatom doping within the carbon skeleton. Nonetheless, the intricate relationship between the doping positions and activity remains incompletely elucidated. This contribution sheds light on an inductive effect of single-atom sites, showcasing that the activity of the oxygen reduction reaction (ORR) can be augmented by reducing the spatial gap between the doped heteroatom and the single-atom sites. Drawing inspiration from this inductive effect, we propose a synthesis strategy involving ligand modification aimed at precisely adjusting the distance between dopants and single-atom sites. This precise synthesis leads to optimized electrocatalytic activity for the ORR. The resultant electrocatalyst, characterized by Fe-N3P1 single-atom sites, demonstrates remarkable ORR activity, thus exhibiting great potential in zinc-air batteries and fuel cells.

Metalation of functionalized benzoquinoline-linked COFs for electrocatalytic oxygen reduction and lithium-sulfur batteries

It is worthwhile to explore and develop multifunctional composites with unique advantages for energy conversion and utilization. Post-synthetic modification (PSM) strategies can endow novel properties to already excellent covalent organic frameworks (COFs). In this study, we prepared a range of COF-based composites via a multi-step PSM strategy. COF-Ph-OH was acquired by demethylation between anhydrous BBr3 and - OMe, and then, M@COF-Ph-OH was further obtained by forming the N - M - O structure. COF-Ph-OH exhibited a 2e--dominated oxygen reduction reaction (ORR) pathway with high H2O2 selectivity, while M@COF-Ph-OH exhibited a 4e--dominated ORR pathway with low H2O2 selectivity, which was due to the introduction of a metal salt with a d electron structure that facilitated the acquisition of electrons and changed the adsorption energy of the reaction intermediate (*OOH). It was proven that the d electron structure was effective at regulating the reaction pathway of the electrocatalytic ORR. Moreover, Co@COF-Ph-OH showed better 4e- ORR properties than Fe@COF-Ph-OH and Ni@COF-Ph-OH. In addition, compared with the other sulfur-impregnated COF-based composites examined in this study, S-Co@COF-Ph-OH had a larger initial capacity, a weaker impedance, and a stronger cycling durability in Li-S batteries, which was attributed to the unique porous structure ensuring high sulfur utilization, the loaded cobalt accelerating LiPS electrostatic adsorption and promoting LiPS catalytic conversion, and the benzoquinoline ring structure being ultra-stable. This work offers not only a rational and feasible strategy for the synthesis of multifunctional COF-based composites, but also promotes their application in electrochemistry.

Insights into dual effect of missing linker-cluster domain defects for photocatalytic 2e- ORR: Radical reaction and electron behavior

Photocatalytic 2e^- ORR paves a promising avenue for hydrogen peroxide (H₂O₂) production. However, the obscure structure-activity relationship between a specific structure and photocatalytic 2e^- ORR restricts the understanding of its intrinsic mechanism. In this work, Metal-Organic Frameworks (MOFs) with missing linker-cluster domain (MLCD) defects were employed as a model to shed new light on the effect of MLCD defect on photocatalytic 2e^- ORR, which mainly focused on the radical reaction and electron behavior. Experiments and theoretical calculations revealed that incorporating MLCD defects significantly lowered the contribution rate of reactive superoxide radical (·O₂^-). Meanwhile, the retarded interfacial charge transfer via "Ti-O″ bridge was caused by the undesirable electron dissipation and augmented orbital-limited resistance induced by electron spin polarization. Therefore, the photocatalytic 2e^- ORR activity was decreased to 30% of its origin by construction MLCD defects. This study provides insights into the internal mechanism of photocatalytic 2e^- ORR for designing and optimizing excellent defective nanomaterials.

Bioinspired and Bioderived Aqueous Electrocatalysis

The development of efficient and sustainable electrochemical systems able to provide clean-energy fuels and chemicals is one of the main current challenges of materials science and engineering. Over the last decades, significant advances have been made in the development of robust electrocatalysts for different reactions, with fundamental insights from both computational and experimental work. Some of the most promising systems in the literature are based on expensive and scarce platinum-group metals; however, natural enzymes show the highest per-site catalytic activities, while their active sites are based exclusively on earth-abundant metals. Additionally, natural biomass provides a valuable feedstock for producing advanced carbonaceous materials with porous hierarchical structures. Utilizing resources and design inspiration from nature can help create more sustainable and cost-effective strategies for manufacturing cost-effective, sustainable, and robust electrochemical materials and devices. This review spans from materials to device engineering; we initially discuss the design of carbon-based materials with bioinspired features (such as enzyme active sites), the utilization of biomass resources to construct tailored carbon materials, and their activity in aqueous electrocatalysis for water splitting, oxygen reduction, and CO₂ reduction. We then delve in the applicability of bioinspired features in electrochemical devices, such as the engineering of bioinspired mass transport and electrode interfaces. Finally, we address remaining challenges, such as the stability of bioinspired active sites or the activity of metal-free carbon materials, and discuss new potential research directions that can open the gates to the implementation of bioinspired sustainable materials in electrochemical devices.

Hierarchical meso-micro porous FeNC derived from tripolycyanamide-based microporous polymer as efficient electrocatalyst for oxygen reduction reaction

Designing porous FeNC nanomaterials with highly efficient active sites is an effective strategy for constructing high-performance oxygen reduction reaction (ORR) electrocatalysts. N-containing porous organic polymers (POPs) have emerged as promising candidates for the preparation of porous FeNC catalysts. Here, N-rich tripolycyanamide-based microporous polymer (TCAMP)-coated SiO₂ nanospheres (SiO₂@TACMP) were prepared as the precursors of an Fe-N doped hierarchical meso-micro porous carbon (Fe-N-HMC) electrocatalyst for the ORR. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations demonstrated that the Fe-N-HMC catalyst possessed a higher content percentage of Fe-N_x active sites and a better distribution of Fe nanoparticles than its Fe-N doped microporous carbon (Fe-N-MC) counterpart. N₂ adsorption-desorption isotherm analysis showed that Fe-N-HMC catalyst exhibited a hierarchical meso-micro porous system, with a Brunauer-Emmett-Teller (BET) surface area (S_BET) of 733 m² g^-1 (∼2 times of Fe-N-MC's S_BET). As a result, Fe-N-HMC catalyst presented a highly efficient ORR performance with a half-wave potential of 0.856 mV, which is similar to the commercial grade 20 wt% Pt/C catalyst and superior to the Fe-N-MC catalyst. Moreover, the as-synthesized Fe-N-HMC catalyst displayed a better durability and methanol tolerance than the commercial Pt/C catalyst. Therefore, Fe-N-HMC shows great promise as an ORR catalyst for fuel batteries and metal-air cells due to its low-cost, high activity, and good stability.

Self-Supporting Metal-Organic Framework-Based Nanoarrays for Electrocatalysis

The replacement of powdery catalysts with self-supporting alternatives for catalyzing various electrochemical reactions is extremely important for the large-scale commercial application of renewable energy storage and conversion technologies. Metal-organic framework (MOF)-based nanoarrays possess tunable compositions, well-defined structure, abundant active sites, effective mass and electron transport, etc., which enable them to exhibit superior electrocatalytic performance in multiple electrochemical reactions. This review presents the latest research progress in developing MOF-based nanoarrays for electrocatalysis. We first highlight the structural features and electrocatalytic advantages of MOF-based nanoarrays, followed by a detailed summary of the design and synthesis strategies of MOF-based nanoarrays, and then describe the recent progress of their application in various electrocatalytic reactions. Finally, the challenges and perspectives are discussed, where further exploration into MOF-based nanoarrays will facilitate the development of electrochemical energy conversion technologies.

Review on Metal-Organic Framework Classification, Synthetic Approaches, and Influencing Factors: Applications in Energy, Drug Delivery, and Wastewater Treatment

Metal ions or clusters that have been bonded with organic linkers to create one- or more-dimensional structures are referred to as metal-organic frameworks (MOFs). Reticular synthesis also forms MOFs with properly designated components that can result in crystals with high porosities and great chemical and thermal stability. Due to the wider surface area, huge pore size, crystalline nature, and tunability, numerous MOFs have been shown to be potential candidates in various fields like gas storage and delivery, energy storage, catalysis, and chemical/biosensing. This study provides a quick overview of the current MOF synthesis techniques in order to familiarize newcomers in the chemical sciences field with the fast-growing MOF research. Beginning with the classification and nomenclature of MOFs, synthesis approaches of MOFs have been demonstrated. We also emphasize the potential applications of MOFs in numerous fields such as gas storage, drug delivery, rechargeable batteries, supercapacitors, and separation membranes. Lastly, the future scope is discussed along with prospective opportunities for the synthesis and application of nano-MOFs, which will help promote their uses in multidisciplinary research.

Fabrication of graphene-oxide and zeolite loaded polyvinylidene fluoride reverse osmosis membrane for saltwater remediation

Incorporation of inorganic and organic materials in polymer has contributed well towards the development of advanced reverse-osmosis membranes; with greater permeation, and salt rejection potential. We are reporting, Zeolite/GO/PVDF based thin-film composite membranes that were successfully synthesized by solution casting process, an eco-friendly, low-cost, and biocompatible technique. PVDF membranes modified with different ratios of GO/Zeo (0.03, 0.05 and 0.07) were characterized by FTIR, SEM, XRD, TGA, and DSC. Membranes were then tested for its potential for water permeation and salt rejection abilities. As prepared membranes owe better pore-distribution, a moderate degree of crystallinity and high absorption capability that is highly needed for micro-filtration phenomena used for desalination of saline water. The modified membranes exhibited enhanced water permeability up to 28.9 L/m²h as compared to pure PVDF membrane having water permeability flux of 15.6 L/m²h. Salt-rejection ability was found increasing for the membranes (up to 98%) modified with different concentration of GO/Zeo, as compare to pure PVDF membrane (82%). During water permeation and salt rejection studies, no deleterious impact was noted for modified PVDF membranes. This development will entail an efficient approach to furnish high-level performance reverse-osmosis membranes, with greater osmotic-pressure bearing capacity and higher stability.

Rational design of M-N4-Gr/V2C heterostructures as highly active ORR catalysts: a density functional theory study

Inspired by the composites of N-doped graphene and transition metal-based materials as well as MXene-based materials, heterostructures (M-N₄-Gr/V₂C) of eight different transition metals (M = Ti, Cr, Mn, Fe, Co, Ni, Cu, and Zn) doped with nitrogen-coordinated graphene and V₂C as potential catalysts for the oxygen reduction reaction (ORR) using density functional theory (DFT) were designed and are described herein. The calculations showed that the heterostructure catalysts (except for Zn-N₄-Gr/V₂C) were thermodynamically stable. Ni-N₄-Gr/V₂C and Co-N₄-Gr/V₂C showed higher activities towards the ORR, with overpotentials as low as 0.32 and 0.45 V, respectively. Excellent catalytic performance results were observed from the change in electronic structure caused by the strong interaction between V₂C and the graphene layers as well as the synergistic effect between the MN₄ groups and the graphene layers. This study further provides insights into the practical application of ORR catalysts for MXene systems through the modulation of the electronic structure of two-dimensional materials.

Metal-Organic Frameworks Derived Electrocatalysts for Oxygen and Carbon Dioxide Reduction Reaction

The increasing demands of energy and environmental concerns have motivated researchers to cultivate renewable energy resources for replacing conventional fossil fuels. The modern energy conversion and storage devices required high efficient and stable electrocatalysts to fulfil the market demands. In previous years, we are witness for considerable developments of scientific attention in Metal-organic Frameworks (MOFs) and their derived nanomaterials in electrocatalysis. In current review article, we have discussed the progress of optimistic strategies and approaches for the manufacturing of MOF-derived functional materials and their presentation as electrocatalysts for significant energy related reactions. MOFs functioning as a self-sacrificing template bid different benefits for the preparation of metal nanostructures, metal oxides and carbon-abundant materials promoting through the porous structure, organic functionalities, abundance of metal sites and large surface area. Thorough study for the recent advancement in the MOF-derived materials, metal-coordinated N-doped carbons with single-atom active sites are emerging candidates for future commercial applications. However, there are some tasks that should be addressed, to attain improved, appreciative and controlled structural parameters for catalytic and chemical behavior.

Bifunctional rare metal-free electrocatalysts synthesized entirely from biomass resources

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are important processes for various energy devices, including polymer electrolyte fuel cells, rechargeable metal-air batteries, and water electrolyzers. We herein report the preparation of a rare metal-free and highly efficient ORR/OER electrocatalyst by calcination of a mixture of blood meal and ascidian-derived cellulose nanofibers. The obtained carbon alloys showed high ORR/OER performances and proved to be promising electrocatalysts. The carbon alloys synthesized entirely from biomass resources not only lead to a new electrocatalyst fabrication process but also contribute to CO2 reduction and the realization of a good life-cycle assessment value in fabrication of a sustainable energy device.

Self-templating synthesis of heteroatom-doped large-scalable carbon anodes for high-performance lithium-ion batteries

We design a simple and cost-effective strategy to construct a large–scalable nitrogen-rich sulfur-doped porous carbon material as a high-performance anode material for lithium-ion batteries.

Enhanced Electrocatalytic Activity of Cobalt-Doped Ceria Embedded on Nitrogen, Sulfur-Doped Reduced Graphene Oxide as an Electrocatalyst for Oxygen Reduction Reaction

N, S-doped rGO was successfully synthesized and embedded Co-doped CeO2 via hydrothermal synthesis. The crystal structure, surface morphology and elemental composition of the prepared catalyst were studied by XRD, Raman spectra, SEM, TEM and XPS analyses. The synthesized electrocatalyst exhibits high onset and halfwave potential during the ORR. This result shows that a combination of N- and S-doped rGO and Co-doped CeO2 leads to a synergistic effect in catalyzing the ORR in alkaline media. Co–CeO2/N, S–rGO displays enhanced ORR performance compared to bare CeO2. The superior stability of the prepared catalyst implies its potential applications beyond fuel cells and metal–air batteries.