Designing Thiophene-Enriched Fully Conjugated 3D Covalent Organic Framework as Metal-Free Oxygen Reduction Catalyst for Hydrogen Fuel Cells

The application of three-dimensional (3D) covalent organic frameworks (COFs) in renewable energy fields is greatly limited due to their non-conjugated skeletons. Here, we design and successfully synthesize a thiophene-enriched fully conjugated 3D COF (BUCT-COF-11) through an all-thiophene-linked saddle-shaped building block (COThTh-CHO). The BUCT-COF-11 exhibits excellent semiconducting property with intrinsic metal-free oxygen reduction reaction (ORR) activity. Using the COF as cathode catalyst, the assembled anion-exchange membrane fuel cells (AEMFCs) exhibited a high peak power density up to 493 mW cm^-2 . DFT calculations reveal that thiophene introduction in the COF not only improves the conductivity but also optimizes the electronic structure of the sample, which therefore boosts the ORR performance. This is the first report on the application of COFs as metal-free catalysts in fuel cells, demonstrating the great potential of fully conjugated 3D COFs as promising semiconductors in energy fields.

Nitrogen doped carbonaceous materials as platinum free cathode electrocatalysts for oxygen reduction reaction (ORR)

Comparison of physicochemical properties and electrocatalytic behavior of different N-doped carbonaceous materials as potential catalysts for oxygen reduction reaction (ORR) was attended. Ball-milling of graphite with melamine and solvothermal treatment of graphite oxide, graphene nanoplatelets (GNP) with ammonia were used as preparation methods. Elemental analysis and N2 physisorption measurements revealed the synthesis of N-doped materials with strongly different morphological parameters. Contact angle measurements proved that all three samples had good wettability properties. According to analysis of XRD data and Raman spectra a higher nitrogen concentration corresponded to a smaller size of crystallites of the N-doped carbonaceous material. Surface total N content determined by XPS and bulk N content assessed by elemental analysis were close, indicating homogenous inclusion of N in all samples. Rotating disc electrode tests showed that these N-doped materials weremuch less active in acidic medium than in an alkaline environment. Although the presence of in-plane N species is regarded to be advantageous for the ORR activity, no particular correlation was found in these systems with any type of N species. According to Koutecky–Levich analysis, both the N-containing carbonaceous materials and the reference Pt/C catalyst displayed a typical one-step, four-electron ORR route. Both ball-milled sample with high N-content but with low SSA and solvothermally synthesized N-GNP with high SSA but low N content showed significant ORR activity. It could be concluded that beside the total N content other parameters such as SSA, pore structure, structural defects, wettability were also essential for achieving high ORR activity.

Crucial Roles of a Pendant Imidazole Ligand of a Cobalt Porphyrin Complex in the Stoichiometric and Catalytic Reduction of Dioxygen

A cobalt porphyrin complex with a pendant imidazole base ([(L₁ )Co^II ]) is an efficient catalyst for the homogeneous catalytic two-electron reduction of dioxygen by 1,1'-dimethylferrocene (Me₂ Fc) in the presence of triflic acid (HOTf), as compared with a cobalt porphyrin complex without a pendant imidazole base ([(L₂ )Co^II ]). The pendant imidazole ligand plays a crucial role not only to provide an imidazolinium proton for proton-coupled electron transfer (PCET) from [(L₁ )Co^II ] to O₂ in the presence of HOTf but also to facilitate electron transfer (ET) from [(L₁ )Co^II ] to O₂ in the absence of HOTf. The kinetics analysis and the detection of intermediates in the stoichiometric and catalytic reduction of O₂ have provided clues to clarify the crucial roles of the pendant imidazole ligand of [(L₁ )Co^II ] for the first time.

Entropy Enhanced Perovskite Oxide Ceramic for Efficient Electrochemical Reduction of Oxygen to Hydrogen Peroxide

The electrochemical oxygen reduction reaction (ORR) offers a most promising and efficient route to produce hydrogen peroxide (H₂O₂), yet the lack of cost‐effective and high‐performance electrocatalysts have restricted its practical application. Herein, an entropy‐enhancement strategy has been employed to enable the low‐cost perovskite oxide to effectively catalyze the electrosynthesis of H₂O₂. The optimized Pb(NiWMnNbZrTi)_1/6O₃ ceramic is available on a kilogram‐scale and displays commendable ORR activity in alkaline media with high selectivity over 91 % across the wide potential range for H₂O₂ including an outstanding degradation property for organic dyes through the Fenton process. The exceptional performance of this perovskite oxide is attributed to the entropy stabilization‐induced polymorphic transformation assuring the robust structural stability, decreased charge mobility as well as synergistic catalytic effects which we confirm using advanced in situ Raman, transient photovoltage, Rietveld refinement as well as finite elemental analysis.

Strongly Coupled Cobalt Diselenide Monolayers for Selective Electrocatalytic Oxygen Reduction to H2 O2 under Acidic Conditions

Electrosynthesis of hydrogen peroxide (H₂ O₂ ) in the acidic environment could largely prevent its decomposition to water, but efficient catalysts that constitute entirely earth-abundant elements are lacking. Here we report the experimental demonstration of narrowing the interlayer gap of metallic cobalt diselenide (CoSe₂ ), which creates high-performance catalyst to selectively drive two-electron oxygen reduction toward H₂ O₂ in an acidic electrolyte. The enhancement of the interlayer coupling between CoSe₂ atomic layers offers a favorable surface electronic structure that weakens the critical *OOH adsorption, promoting the energetics for H₂ O₂ production. Consequently, on the strongly coupled CoSe₂ catalyst, we achieved Faradaic efficiency of 96.7 %, current density of 50.04 milliamperes per square centimeter, and product rate of 30.60 mg cm^-2  h^-1 . Moreover, this catalyst shows no sign of degradation when operating at -63 milliamperes per square centimeter over 100 hours.

Proton Removal Kinetics That Govern the Hydrogen Peroxide Oxidation Activity of Heterogeneous Bioinorganic Platforms

Precise regulation of proton-coupled electron-transfer (PCET) rates holds the key to simultaneously optimizing the turnover frequency and product selectivity of redox reactions that are central to the realization of renewable energy schemes in a sustainable future. In this work, a self-assembled monolayer (SAM) of a Ru complex electrografted onto a glassy carbon (GC) electrode was prepared as a heterogeneous electrocatalytic interface to facilitate the hydrogen peroxide (H₂O₂) oxidation half-cell reaction of a direct hydrogen peroxide/hydrogen peroxide fuel cell. A functional lipid membrane embedded with catalytic amounts of proton carriers was appended on top of the Ru SAM to construct a hybrid bilayer membrane (HBM) platform that can modulate the thermodynamics and kinetics of proton- and electron-transfer steps independently. The performances of the as-prepared Ru SAMs and HBMs toward H₂O₂ oxidation were investigated using electrochemical means, kinetic isotope effect (KIE) studies, and Tafel analyses. Proton carriers featuring borate, phosphate, and nitrile headgroups were found to dictate the transmembrane proton removal rate, thereby controlling the H₂O₂ oxidation activity. The first significance of this work was the expansion of HBM platforms to GC substrates to overcome the limited redox potential window on gold thiol systems, thereby enabling electrochemical investigations of anodic reactions at the SAM-lipid interface. The second highlight of this work was demonstrating for the first time that deprotonation kinetics can be taken advantage of to enhance the electrocatalytic oxidation performance of a metal complex anchored at the SAM-lipid interface of a HBM platform. When the knowledge gaps regarding how PCET steps govern redox pathways are closed, the advances achieved using our unique bioinorganic platform are envisioned to accelerate the understanding and optimization of electrocatalytic processes involving proton- and electron- transfer steps that are fundamental to the development of high-performance energy devices.

Cu3-Cluster-Doped Monolayer Mo2CO2 (MXene) as an Electron Reservoir for Catalyzing a CO Oxidation Reaction

The catalytic oxidation of CO on Cu₃-cluster-decorated pristine and defective Mo₂CO₂ (MXene) monolayers (Cu₃/p-Mo₂CO₂ and Cu₃/d-Mo₂CO₂) was investigated by first-principles calculations. The stability of the designed catalysts was comprehensively demonstrated via analysis of the energies, geometry distortion, and molecular dynamics simulations at finite temperatures. The difference in the individual adsorption energies, as well as the oxidation and poisoning of Cu₃/p(d)-Mo₂CO₂ under CO and O₂ gas atmospheres, was tested to estimate the catalytic ability. We found that Cu₃/d-Mo₂CO₂ might be a superior catalyst with good stability and reactivity for CO oxidation. The active sites of the Cu₃ cluster acting as an electron reservoir governed its electron-donating and -accepting ability. Different adsorption configurations of O₂ on Cu₃/d-Mo₂CO₂ also gave rise to different reaction activities. The facile rate-limiting energy barrier was attributed to the charge buffer capacity of the Cu₃ cluster that mediates the reaction. Our results may provide clues to fabricate MXene-based materials by depositing small clusters on MXenes and exploring the advanced applications of these materials.