Enhancing the electrocatalytic activity of Fe phthalocyanines for the oxygen reduction reaction by the presence of axial ligands: Pyridine-functionalized single-walled carbon nanotubes

Same FeN4 Active Site, Different Activity: How Redox Peaks Control Oxygen Reduction on Fe Macrocycles

Macrocycles show high activity for the electrochemical reduction of oxygen in alkaline media. However, even macrocycles with the same metal centers and MN4 active site can vary significantly in activity and selectivity, and to this date, a quantitative insight into the cause of these staggering differences has not been unambiguously reached. These macrocycles form a fundamental platform, similarly to platinum alloys for metal ORR catalyst, to unravel fundamental properties of FeNx catalysts. In this manuscript, we present a systematic study of several macrocycles, with varying active site motif and ligands, using electrochemical techniques, operando spectroscopy, and density functional theory (DFT) simulations. Our study demonstrates the existence of two families of Fe macrocycles for oxygen reduction in alkaline electrolytes: (i) weak *OH binding macrocycles with one peak in the voltammogram and high peroxide selectivity and (ii) macrocycles with close to optimal *OH binding, which exhibit two voltametric peaks and almost no peroxide production. Here, we also propose three mechanisms that would explain our experimental findings. Understanding what differentiates these two families could shed light on how to optimize the activity of pyrolyzed FeNx catalysts.

Bis(formazanate) iron(II) complexes as cathode materials for one-compartment H2O2 fuel cells

In this paper, we describe the synthesis of two six-coordinate, pseudo(octahedral) bis(formazanate) Fe(II) complexes based on newly developed redox-active benzothiazole-substituted formazanate ligands. Complexes [FeII(L1)2], 1, and [FeII(L2)2], 2, were synthesized by reacting 1-(benzothiazol-2-yl)-5-phenyl-3-(pyren-1-yl)formazan (L1H) and 1-(benzothiazol-2-yl)-5-(2-benzoyl-4-chlorophenyl)-3-phenylformazan (L2H), respectively, with appropriate Fe(II) precursors at room temperature. The molecular structures of both bis(formazanate) iron complexes were established using single-crystal XRD, and other characterization methods were utilized to further characterize these complexes, as well as the newly synthesized ligands. Furthermore, the cyclic voltammetry studies of these compounds are documented, revealing that both complexes can undergo electrochemical reductions to create anionic and dianionic species. These complexes were further employed as cathodes in one-compartment membrane-less H2O2 fuel cells, operating in 0.5 M H2O2, with nickel foam serving as the anode. The maximum power densities achieved by the designed H2O2 fuel cells for complexes 1 and 2 were 1.88 mW cm-2 and 3.08 mW cm-2, respectively. This study demonstrates the significant potential of formazanate-based compounds in the development of cathode materials for H2O2 fuel cells.

Molecular Dynamics Simulation of Low-Cycle Fatigue Behavior of Single/Polycrystalline Iron

The fatigue plastic mechanism and dislocation characteristics of engineering materials are the key to studying fatigue damage. In this study, the molecular dynamics (MD) method was employed to investigate the microstructural characteristics and fatigue mechanical properties of both single-crystalline and polycrystalline iron under varying strain amplitudes associated with cyclic hardening, cyclic softening, and cyclic saturation. The occurrence, accumulation, and formation process of the local plastic fatigue damage of monocrystalline/polycrystalline iron under fatigue load are discussed. The local plastic initiation and accumulation of single-crystal iron occur at the intersection of slip planes, which is the dislocation source. The 1/2<111> dislocation plays an important role in the fatigue plastic accumulation of single-crystal iron. Polycrystalline iron undergoes grain rotation and coalescence during cyclic loading. The grain size responsible for plastic deformation gradually increases. The initiation and accumulation of local plasticity occurs at the grain boundary, which eventually leads to fatigue damage at the grain boundary.

Electron Spin-Dependent Electrocatalysis for the Oxygen Reduction Reaction in a Chiro-Self-Assembled Iron Phthalocyanine Device

The chiral-induced spin selectivity effect (CISS) is a breakthrough phenomenon that has revolutionized the field of electrocatalysis. We report the first study on the electron spin-dependent electrocatalysis for the oxygen reduction reaction, ORR, using iron phthalocyanine, FePc, a well-known molecular catalyst for this reaction. The FePc complex belongs to the non-precious catalysts group, whose active site, FeN4, emulates catalytic centers of biocatalysts such as Cytochrome c. This study presents an experimental platform involving FePc self-assembled to a gold electrode surface using chiral peptides (L and D enantiomers), i.e., chiro-self-assembled FePc systems (CSAFePc). The chiral peptides behave as spin filters axial ligands of the FePc. One of the main findings is that the peptides' handedness and length in CSAFePc can optimize the kinetics and thermodynamic factors governing ORR. Moreover, the D-enantiomer promotes the highest electrocatalytic activity of FePc for ORR, shifting the onset potential up to 1.01 V vs. RHE in an alkaline medium, a potential close to the reversible potential of the O2 /H2 O couple. Therefore, this work has exciting implications for developing highly efficient and bioinspired catalysts, considering that, in biological organisms, biocatalysts that promote O2 reduction to water comprise L-enantiomers.

Advances on Axial Coordination Design of Single-Atom Catalysts for Energy Electrocatalysis: A Review

Single-atom catalysts (SACs) have garnered increasingly growing attention in renewable energy scenarios, especially in electrocatalysis due to their unique high efficiency of atom utilization and flexible electronic structure adjustability. The intensive efforts towards the rational design and synthesis of SACs with versatile local configurations have significantly accelerated the development of efficient and sustainable electrocatalysts for a wide range of electrochemical applications. As an emergent coordination avenue, intentionally breaking the planar symmetry of SACs by adding ligands in the axial direction of metal single atoms offers a novel approach for the tuning of both geometric and electronic structures, thereby enhancing electrocatalytic performance at active sites. In this review, we briefly outline the burgeoning research topic of axially coordinated SACs and provide a comprehensive summary of the recent advances in their synthetic strategies and electrocatalytic applications. Besides, the challenges and outlooks in this research field have also been emphasized. The present review provides an in-depth and comprehensive understanding of the axial coordination design of SACs, which could bring new perspectives and solutions for fine regulation of the electronic structures of SACs catering to high-performing energy electrocatalysis.

Substituent Effects in Iron Porphyrin Catalysts for the Hydrogen Evolution Reaction

For a future hydrogen economy, non-precious metal catalysts for the water splitting reactions are needed that can be implemented on a global scale. Metal-nitrogen-carbon (MNC) catalysts with active sites constituting a metal center with fourfold coordination of nitrogen (MN₄ ) show promising performance, but an optimization rooted in structure-property relationships has been hampered by their low structural definition. Porphyrin model complexes are studied to transfer insights from well-defined molecules to MNC systems. This work combines experiment and theory to evaluate the influence of porphyrin substituents on the electronic and electrocatalytic properties of MN₄ centers with respect to the hydrogen evolution reaction (HER) in aqueous electrolyte. We found that the choice of substituent affects their utilization on the carbon support and their electrocatalytic performance. We propose an HER mechanism for supported iron porphyrin complexes involving a [Fe^II (P⋅)]^- radical anion intermediate, in which a porphinic nitrogen atom acts as an internal base. While this work focuses on the HER, the limited influence of a simultaneous interaction with the support and an aqueous electrolyte will likely be transferrable to other catalytic applications.

Electrochemical Sensor Based on Iron(II) Phthalocyanine and Gold Nanoparticles for Nitrite Detection in Meat Products

Nitrites are widely used in the food industry, particularly for the preservation of meat products. Controlling the nitrate content in food is an important task to ensure people's health is not at risk; therefore, the search for, and research of, new materials that will modify the electrodes in the electrochemical sensors that detect and control the nitrate content in food products is an urgent task. In this paper, we describe the electrochemical behavior of a glass carbon electrode (GCE), modified with a Fe(II) tetra-tert-butyl phthalocyanine film (FePc(tBu)₄/GCE), and decorated with gold nanoparticles (Au/FePc(tBu)₄/GCE); this electrode was deposited using gas-phase methods. The composition and morphology of such electrodes were examined using spectroscopy and electron microscopy methods, whereas the main electrochemical characteristics were determined using cyclic voltammetry (CV) and amperometry (CA) methods in the linear ranges of CV 0.25-2.5 mM, CA 2-120 μM in 0.1 M phosphate buffer (pH = 6.8). The results showed that the modification of bare GCEs, with a Au/FePc(tBu)₄ heterostructure, provided a high surface-to-volume ratio, thus ensuring its high sensitivity to nitrite ions of 0.46 μAμM^-1. The sensor based on the Au/FePc(tBu)₄/GCE has a low limit of nitrite detection at 0.35 μM, good repeatability, and stability. The interference study showed that the proposed Au/FePc(tBu)₄/GCE exhibited a selective response in the presence of interfering anions, and the analytical capability of the sensor was demonstrated by determining nitrite ions in real samples of meat products.

Theoretical study of the effect of coordination environment on the activity of metal macrocyclic complexes as electrocatalysts for oxygen reduction

Transition metal macrocyclic complexes are appealing catalysts for electrochemical oxygen reduction reaction (ORR). Here, we perform first-principles calculations to gain a comprehensive understanding on the structure-property relationship of the metal macrocyclic complex systems. Various modifications of the complexes are considered, including centered metal, axial ligand, coordination atom, substituent, and macrocycles. Based on simulation, introduction of appropriate apical ligand can improve the performance of all the three metals, whereas replacement of nitrogen with oxygen or carbon as the coordination atoms may enhance the Ni-centered systems. The antiaromatic ring stabilizes the ∗OOH intermediate, whereas the macrocycle with reduced electron density inhibits the binding with oxygen. By regulating the coordination environment, the overpotential can be significantly reduced. This work may assist the rational design of ORR catalysts and is of great significance for the future development of oxygen reduction catalysts.

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Metal macrocyclic complexes are potential electrocatalysts for ORR

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An understanding on structure-property relationship is gained based on simulation

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Various modifications are considered to improve the performance