Oxygen Reduction on Transition-Metal Porphyrins in Acid Electrolyte I. Activity

Anodic Commodity Polymer Recycling: The Merger of Iron-Electrocatalysis with Scalable Hydrogen Evolution Reaction

Plastics are omnipresent in our everyday life, and accumulation of post-consumer plastic waste in our environment represents a major societal challenge. Hence, methods for plastic waste recycling are in high demand for a future circular economy. Specifically, the degradation of post-consumer polymers towards value-added small molecules constitutes a sustainable strategy for a carbon circular economy. Despite of recent advances, chemical polymer degradation continues to be largely limited to chemical redox agents or low energy efficiency in photochemical processes. We herein report a powerful iron-catalyzed degradation of high molecular weight polystyrenes through electrochemistry to efficiently deliver monomeric benzoyl products. The robustness of the ferraelectrocatalysis was mirrored by the degradation of various real-life post-consumer plastics, also on gram scale. The cathodic half reaction was largely represented by the hydrogen evolution reaction (HER). The scalable electro-polymer degradation could be solely fueled by solar energy through a commercially available solar panel, indicating an outstanding potential for a decentralized green hydrogen economy.

A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N₄ ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen₂N₂)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen₂N₂)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen₂N₂)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen₂N₂)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen₂N₂)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

Recent advances in porphyrin-based nanocomposites for effective targeted imaging and therapy

Porphyrins are organic compounds that continue to attract much theoretical interest, and have been called the "pigments of life". They have a wide role in photodynamic and sonodynamic therapy, along with uses in magnetic resonance, fluorescence and photoacoustic imaging. There is a vast range of porphyrins that have been isolated or designed, but few of them have real clinical applications. Due to the hydrophobic properties of porphyrins, and their tendency to aggregate by stacking of the planar molecules they are difficult to work with in aqueous media. Therefore encapsulating them in nanoparticles (NPs) or attachment to various delivery vehicles have been used to improve delivery characteristics. Porphyrins can be used in a composite designed material with properties that allow specific targeting, immune tolerance, extended tissue lifetime and improved hydrophilicity. Drug delivery, healing and repairing of damaged organs, and cancer theranostics are some of the medical uses of porphyrin-based nanocomposites covered in this review.

Engineering Energy Level of Metal Center: Ru Single-Atom Site for Efficient and Durable Oxygen Reduction Catalysis

Emerging as a new frontier in heterogeneous catalysis, single-atom site catalysts (SSCs) have sparked enormous attention and bring about new opportunities to oxygen reduction electrocatalysis. Despite considerable progress achieved recently, most of the reported SSCs suffer from either insufficient activity or unsatisfactory stability, which severely retards their practical application. Here, we demonstrate a novel Ru-SSC with appropriate adsorption free energy of OH* (ΔG_OH*) to confer excellent activity and low Fenton reactivity to maintain long-term stability. The as-developed Ru-SSC exhibits encouraging oxygen reduction reaction turnover frequency of 4.99 e^- s^-1 sites^-1, far exceeding the state-of-the-art Fe-SSC counterpart (0.816 e^- s^-1 sites^-1), as a result of Ru energy level regulation via spontaneous OH binding. Furthermore, Ru-SSC exhibits greatly suppressed Fenton reactivity, with restrained generation of reactive oxygen species directly observed, thus endowing the Ru-SSC with much more superior stability (only 17 mV negative shift after 20 000 cycles) than the Fe-SSC counterpart (31 mV). The practical application of Ru-SSC is further validated by its excellent activity and stability in a real fuel cell device.

Influence of Iron Salt Anions on Formation and Oxygen Reduction Activity of Fe/N-Doped Mesoporous Carbon Fuel Cell Catalysts

Doping carbon materials with transition metal ions can greatly expand their utility, given these metal ions' unique catalytic activity, for example, in oxygen reduction in proton exchange membrane fuel cells. Unlike main group dopants, a counter anion to the metal cation must be selected and this choice has hitherto received little attention for this synthesis method. Herein, we describe the profound effects that the anion has on the resultant iron/nitrogen-doped ordered mesoporous carbons (Fe-OMC). To increase the iron loading and the number of iron-centered catalytically active sites, we selected three iron salts Fe(OAc)₂, Fe(OTf)₂, and Fe(BF₄)₂·6H₂O, which show greatly enhanced solubility in the liquid carbon precursor (furfurylamine) compared to FeCl₃·6H₂O. The increased solubility leads to a significantly higher iron loading in the Fe-OMC prepared with Fe(OTf)₂, but the increase in performance as cathode catalysts in fuel cells is only marginal. The Fe-OMCs prepared with Fe(OAc)₂ and Fe(BF₄)₂·6H₂O exhibited similar or lower iron loadings compared to the Fe-OMC prepared with FeCl₃·6H₂O despite their much higher solubilities. Most importantly, the different iron salts affect not only the final iron loading, but also which type of iron species forms in the Fe-OMC with different types showing different catalytic activity.

Single-Atom Cr-N4 Sites Designed for Durable Oxygen Reduction Catalysis in Acid Media

Single-atom catalysts (SACs) are attracting widespread interest for the catalytic oxygen reduction reaction (ORR), with Fe-N_x SACs exhibiting the most promising activity. However, Fe-based catalysts suffer serious stability issues as a result of oxidative corrosion through the Fenton reaction. Herein, using a metal-organic framework as an anchoring matrix, we for the first time obtained pyrolyzed Cr/N/C SACs for the ORR, where the atomically dispersed Cr is confirmed to have a Cr-N₄ coordination structure. The Cr/N/C catalyst exhibits excellent ORR activity with an optimal half-wave potential of 0.773 V versus RHE. More excitingly, the Fenton reaction is substantially reduced and, thus, the final catalysts show superb stability. The innovative and robust active site for the ORR opens a new possibility to circumvent the stability issue of the non-noble metal ORR catalysts.

Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction

Similarities are established between well-known reactivity descriptors of metal electrodes for their activity in the oxygen reduction reaction (ORR) and the reactivity of molecular catalysts, in particular macrocyclic MN4 metal complexes confined to electrode surfaces. We show that there is a correlation between the M^III /M^II redox potential of MN4 chelates and the M-O₂ binding energies. Specifically, the binding energy of O₂ (and other O species) follows the M^III -OH/M^II redox transition for MnN4 and FeN4 chelates. The ORR volcano plot for MN4 catalysts is similar to that for metal catalysts: catalysts on the weak binding side (mostly CoN4 chelates) yield mainly H₂ O₂ as the product, with an ORR onset potential independent of the pH value on the NHE scale (and therefore pH-dependent on the RHE scale); catalysts on the stronger binding side yield H₂ O as the product with the expected pH-dependence on the NHE scale. The suggested descriptors also apply to heat-treated pyrolyzed MN4 catalysts.

Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal-Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems

Detailed understanding of the nature of the active centers in non-precious-metal-based electrocatalyst, and their role in oxygen reduction reaction (ORR) mechanistic pathways will have a profound effect on successful commercialization of emission-free energy devices such as fuel cells. Recently, using pyrolyzed model structures of iron porphyrins, we have demonstrated that a covalent integration of the Fe-N _x sites into π-conjugated carbon basal plane modifies electron donating/withdrawing capability of the carbonaceous ligand, consequently improving ORR activity. Here, we employ a combination of in situ X-ray spectroscopy and electrochemical methods to identify the various structural and functional forms of the active centers in non-heme Fe/N/C catalysts. Both methods corroboratively confirm the single site 2e^- × 2e^- mechanism in alkaline media on the primary Fe²⁺-N₄ centers and the dual-site 2e^- × 2e^- mechanism in acid media with the significant role of the surface bound coexisting Fe/Fe _x O _y nanoparticles (NPs) as the secondary active sites.

Systematic selection of metalloporphyrin-based catalysts for oxygen reduction by modulation of the donor-acceptor intermolecular hardness

Incisive modulation of the intermolecular hardness between metalloporphyrins and O2 can lead to the identification of promising catalysts for oxygen reduction. The dependency of the electrocatalytic reduction of O2 by metalloporphyrins on the nature of the central metal yields a volcano-type curve, which is rationalized to be in accordance with the Sabatier principle by using an approximation of the electrophilicity of the complexes. By using electrochemical and UV/Vis data, the influence of a selection of meso-substituents on the change in the energy for the π→π* excitation of manganese porphyrins was evaluated allowing one to quantitatively correlate the influence of the various ligands on the electrocatalysis of O2 reduction by the complexes. A manganese porphyrin was identified that electrocatalyzes the reduction of oxygen at low overpotentials without generating hydrogen peroxide. The activity of the complex became remarkably enhanced upon its pyrolysis at 650 °C.

Oxygen reduction reaction usingN4-metallomacrocyclic catalysts: fundamentals on rational catalyst design

In this review, we describe and discuss the developments in the use of metalloporphyrins and metallophthalocyanines as catalysts for oxygen reduction in aqueous electrolytes. The main goal of most researchers in this field has been to design catalysts which can achieve facile reduction of oxygen by the four-electron transfer pathway at the lowest overpotential possible. With this in mind, the primary objective of this review was to bring to light the research frontiers uncovering important milestones towards the synthesis and design of promising N4-metallomacrocyclic catalysts which accomplish the four-electron reduction of oxygen, and, based on literature, to draw attention to the fundamental requirements for synthesis of improved catalysts operating at low overpotentials. Our emphasis was not to make parallel comparisons between individual classes of N4-metallomacrocyclic complexes with respect to their activity, but rather to focus on the commonalities of the fundamental properties that govern their reactivities and how these may be aptly manipulated to develop better catalysts. Therefore, besides discussion of the progress attained with regard to synthesis and design of catalysts with high selectivity towards four-electron reduction of O2, a major part of the review highlights quantitative structure-activity relationships (QSAR) which govern the activity and stability of these complexes, which when well understood, refined and carefully implemented should constitute a fundamental gateway for rational design of better catalysts.

Monomeric and Polymeric Tetra-aminophthalocyanatocobalt(II) Modified Electrodes: Electrocatalytic Reduction of Oxygen

The monomeric and polymeric tetra-aminophthalocyane to, cobalt(II) species adsorbed onto graphite electrodes are active in electrocatalytic oxygen reduction. While the monomeric species is unstable, the polymerized species is an effective and stable reduction catalyst over a wide pH range. Both the two-electron reduction of oxygen to hydrogen peroxide and the four-electron reduction of oxygen to water are characterized by cyclic voltammetry, rotating disc and rotating ring-disc studies with appropriate theoretical analysis. Some mechanistic information is obtained. This is the first cobalt phthalocyanine species to provide a four-electron reduction pathway which exists over a wide pH range and is stable. The stability is associated with the polymerization since the monomeric species is not stable.