Metalloporphyrins and related metallomacrocycles as electrocatalysts for use in polymer electrolyte fuel cells and water electrolyzers

Efficient electrolytic hydrogen evolution from cobalt porphyrin covalently functionalized with chain-like phosphazene on double-walled carbon nanotubes

The development of efficient electrocatalysts for the hydrogen evolution reaction (HER) is crucial for advancing electrochemical water splitting technology. Herein, we report a novel hybrid electrocatalyst, CoTHPP-PDCP@DWCNTs, synthesized through a covalent functionalization of cobalt-porphyrin (CoTHPP) with chain-like polydichlorophosphazenes (PDCP) and subsequent incorporation onto double-walled carbon nanotubes (DWCNTs). The successful synthesis was confirmed by various spectroscopic techniques, which collectively revealed strong electronic interactions between components, improved charge transfer, and enhanced electrochemical stability. Notably, CoTHPP-PDCP@DWCNTs exhibited excellent HER activity, achieving low overpotentials of 159 mV in 1.0 M KOH and 102 mV in 0.5 M H2SO4 at a current density of 10 mA cm-2, outperforming its precursors and many previously reported non-noble metal electrocatalysts. The enhanced HER performance can be attributed to the synergistic interactions between CoTHPP, PDCP, and DWCNTs, which increased the density of active sites, improved conductivity, and efficient charge transfer. This study highlights the importance of covalent linkage and structural engineering in optimizing HER electrocatalysts, providing insights for designing next-generation high-performance and durable non-noble metal-based electrocatalytic systems.

Construction of hydrophilic-and-cationic metalloporphyrin-based polymers for electrocatalytic small molecule activation

The electrocatalytic activation of small molecules for the production of hydrogen, oxygen, and ammonia represents a novel approach towards achieving environmentally sustainable chemical transformations. Covalent organic polymers have attracted extensive research due to their designable structures and well-defined catalytic sites. Herein, we elaborate on the synthesis, characterization, and electrochemical analyses of a class of hydrophilic and cationic polymers (M-MV) with metalloporphyrins serving as the active central nodes. Owing to the inclusion of cationic viologen units, the as-prepared M-MV exhibited excellent hydrophilicity. When the metal centres vary, three metalloporphyrin-based covalent polymers possess specific catalytic properties for distinct electrocatalytic reactions in aqueous solution. The experimental results show that Co-MV is more favorable for the electrocatalytic HER, Ni-MV is more conducive to the electrocatalytic OER, and Cu-MV exhibits the optimal catalytic activity in the electrocatalytic NO2-RR.

Zeolite Imidazole Framework-Derived Hollow Nickel-Cobalt Layered Double Hydroxide Intercalated with Metalloporphyrin for Enhanced Oxygen Evolution Reaction

The design of highly efficient, stable, and nonprecious-metal-based electrocatalysts for the oxygen evolution reaction (OER) has been a major research topic in the field of hydrogen production from electrolytic water splitting. In this work, a novel hierarchical hollow NiCo layered double hydroxide (NiCo LDH) with Cu-TCPP (TCPP = tetrakis(4-carboxyphenyl)-porphyrin) intercalation (denoted as X-CT/LR, where X is the amount of Cu-TCPP addition) was successfully fabricated by using zeolite imidazole framework-67 (ZIF-67) as a template. Among them, 1-CT/LR has superior OER activity and stability, requiring only a low overpotential of 204 mV to reach a current density of 10 mA/cm2. The experimental results show that the introduced Cu-TCPP, in addition to being an active center itself, also acts as an electron transfer medium in the interlayer to enhance the conductivity of the material. Meanwhile, the hierarchical hollow structure and interlayer domain-limiting effect of NiCo LDH also ensure the dispersion and stabilization of Cu-TCPP, which is favorable to give full play to the synergistic catalytic effect of the two components. This work provides a facile strategy to obtain a nonprecious-metal-based OER electrocatalyst for water splitting.

Tailored Metal-Porphyrin Based Molecular Electrocatalysts for Enhanced Artificial Nitrogen Fixation to Green Ammonia

Electrochemical nitrogen reduction (E-NRR) is one of the most promising approaches to generate green NH3. However, scarce ammonia yields and Faradaic efficiencies (FE) still limit their use on a large scale. Thus, efforts are focusing on different E-NRR catalyst structures and formulations. Among present strategies, molecular electrocatalysts such as metal-porphyrins emerge as an encouraging option due to their planar structures which favor the interaction involving the metal center, responsible for adsorption and activation of nitrogen. Nevertheless, the high hydrophobicity of porphyrins limits the aqueous electrolyte-catalyst interaction lowering yields. This work introduces a new class of metal-porphyrin based catalysts, bearing hydrophilic tris(ethyleneglycol) monomethyl ether chains (metal = Cu(II) and CoII)). Experimental results show that the presence of hydrophilic chains significantly increases ammonia yields and FE, supporting the relevance of fruitful catalyst-electrolyte interactions. This study also investigates the use of hydrophobic branched alkyl chains for comparison, resulting in similar performances with respect to the unsubstituted metal-porphyrin, taken as a reference, further confirming that the appropriate design of electrocatalysts carrying peripheral hydrophilic substituents is able to improve device performances in the generation of green ammonia.

Structural and electronic properties of Li-adsorbed single and bilayer porphyrin sheets as an electrode material for energy storage devices - a DFT analysis

In this study, we adopt density functional theory (DFT) to investigate the structural and electronic properties of monolayer and bilayer 2-D porphyrin sheets (PS) of covalent organic frameworks (COFs) upon interaction with Li atoms as an electrode material for Li-ion batteries. Based on their mechanical properties, our systems exhibit remarkable stability. The adsorption of Li at various sites in the monolayer, including over and between the bilayers of PS, is investigated. Our results indicate that Li at site S3 has the highest adsorption energy, and Li is energetically preferred to intercalate within the bilayer rather than monolayers due to its high adsorption energies. Notably, the charge transfer remains consistent for both systems. The density of state distribution, charge density difference plots, spin density and the band structure results show that the PS has high electrical conductivity. Additionally, the reaction potential was carried out, and the negative reaction potential results demonstrate that the system undergoes a reduction reaction. The resultant theoretical capacity and the open circuit voltage highlight that the PS materials of COFs are an important step for use in the next generation high-performance lithium-ion batteries.

Bifunctional porphyrin-based metal-organic polymers for electrochemical water splitting

Electrochemical water splitting offers the potential for environmentally friendly hydrogen and oxygen gas generation. Here, we present the synthesis, characterization, and electrochemical analyses of four organic polymers where metalloporphyrins are the active center nodes. These materials were obtained from the polymerization reaction of poly(p-phenylene terephtalamide) (PPTA) with the respective amino-functionalized metalloporphyrins, where M = Fe, 1; Co, 2; Ni, 3; Cu, 4. Scanning and transmission electron microscopy images (SEM and TEM) show that these polymers exhibit a layer-type morphology, which is attributed to hydrogen bonding and π-π stacking between the metalloporphyrin nodes. The synthesized materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), UV-Vis spectroscopy, and Fourier-transform infrared spectroscopy (FT-IR). Among the materials studied, the cobalt-based polymer, 2, demonstrates a bifunctional electrocatalytic activity for oxygen (OER) and hydrogen (HER) evolution reactions with overpotentials (η10) of 337 mV and 435 mV, respectively. The Fe, 1, and Ni, 2, polymers are less active for HER with maximum current densities (jmax) of 12.6 and 19.1 mA cm-2 and η10 678 mV, 644 mV. Polymer 2 achieves a jmax of 37.7 mA cm-2 for HER and 133 mA cm-2 for OER. The copper-based material, 4, on the other hand, shows selectivity towards HER with an overpotential (η) of 436 mV and a maximum current density (j) of 45.5 mA cm-2. The bifunctional electrocatalytic performance was tested in the overall water-splitting setup, where polymer 2 requires a cell voltage of 1.64 V at 10 mA cm-2. This work presents a novel approach to heterogenized molecular systems, providing materials with exceptional structural characteristics and enhanced electrocatalytic capabilities.

Boosted hydrogen evolution reaction for a nitrogen-rich azo-bridged metallated porphyrin network

The potential of porous organic polymers (POPs) toward electrocatalytic water splitting have attracted considerable scientific attention, due to their high specific surface areas, superlative porosity and diverse electronic structures; yet it remains challenging. Herein, we report a facile synthesized novel nitrogen-rich azo-bridged metallated porphyrin POP (CoTAPP-CoTNPP) for improving the hydrogen evolution reaction (HER) activity. The incorporation of the cobalt porphyrins and the azo groups endows CoTAPP-CoTNPP with effective charge transfer efficiency and large π-conjugated porous frameworks, thus enhancing the HER performance. Origins of the excellent HER performance of the material are evaluated using a series of structural and electrochemical measurements. Remarkably, CoTAPP-CoTNPP exhibits low overpotentials of 103 and 170 mV to reach 10 mA cm-2 in acidic and alkaline media, respectively, outperforming many previously reported HER electrocatalysts. These results demonstrate the enormous potential of the as-prepared azo-linked porphyrin POP for electrocatalytic water splitting.

Effects of tuning the structural symmetry of cobalt porphyrin on electrocatalytic oxygen reduction reactions

Metalloporphyrins have attracted significant attention as highly promising alternatives to Pt-based electrocatalysts in the realm of oxygen reduction reactions (ORRs). While the structure of porphyrin is widely recognized as a pivotal factor influencing the ORR performance, the impact of molecular symmetry, which is one of the key properties of the molecular structure, has rarely been understood and its effects remain largely unexplored. Herein, we designed and synthesized two triphenylamine (TPA)-substituted cobalt porphyrins, the asymmetric aBz-TCoP and the symmetric Bz-2TCoP, which are doped onto carbon black to construct composite catalysts for ORRs. The electronic structures of both porphyrins are determined through density functional theory (DFT) calculations, and the morphology and electronic states of the composites are examined by spectroscopic techniques. A series of electrochemical measurements demonstrate the superior activity, selectivity and durability of Bz-2TCoP/C to aBz-TCoP/C in ORRs conducted in both acidic and alkaline electrolytes. The improved ORR properties of the symmetric porphyrin may stem from the steric properties rather than the electronic properties of the chemical structure. This work represents a preliminary study on the effects of porphyrin structural symmetry on electrocatalysis and provides a potential strategy for further structural modifications of metalloporphyrins, as non-noble metal electrocatalysts, to enhance the ORR performance.

Development of Electrochemical Sensor Using Iron (III) Phthalocyanine/Gold Nanoparticle/Graphene Hybrid Film for Highly Selective Determination of Nicotine in Human Salivary Samples

Nicotine is the one of the major addictive substances; the overdose of nicotine (NIC) consumption causes increasing heart rate, blood pressure, stroke, lung cancer, and respiratory illnesses. In this study, we have developed a precise and sensitive electrochemical sensor for nicotine detection in saliva samples. It was built on a glassy carbon electrode (GCE) modified with graphene (Gr), iron (III) phthalocyanine-4,4',4″,4'''-tetrasulfonic acid (Fe(III)Pc), and gold nanoparticles (AuNPs/Fe(III)Pc/Gr/GCE). The AuNPs/Fe(III)Pc/Gr nanocomposite was prepared and characterized by using FE-SEM, EDX, and E-mapping techniques to confirm the composite formation as well as the even distribution of elements. Furthermore, the newly prepared AuNPs/Fe(III)Pc/Gr/GCE-nanocomposite-based sensor was used to detect the nicotine in phosphate-buffered solution (0.1 M PBS, pH 7.4). The AuNPs/Fe(III)Pc/Gr/GCE-based sensor offered a linear response against NIC from 0.5 to 27 µM with a limit of detection (LOD) of 17 nM using the amperometry (i-t curve) technique. This electrochemical sensor demonstrated astounding selectivity and sensitivity during NIC detection in the presence of common interfering molecules in 0.1 M PBS. Moreover, the effect of pH on NIC electro-oxidation was studied, which indicated that PBS with pH 7.4 was the best medium for NIC determination. Finally, the AuNPs/Fe(III)Pc/Gr/GCE sensor was used to accurately determine NIC concentration in human saliva samples, and the recovery percentages were also calculated.

Facile Synthesis of β-Tetracyano Vanadyl Porphyrin from Its Tetrabromo Analogue and Its Excellent Catalytic Activity for Bromination and Epoxidation Reactions

Complex 2,3,12,13-tetracyano-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[V^IVOTPP(CN)₄], 2} has been prepared by nucleophilic substitution of β-bromo groups of the corresponding 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[V^IVOTPP(Br)₄], 1} using CuCN in quinoline. Both complexes show biomimetic catalytic activity similar to enzyme haloperoxidases and efficiently brominate various phenol derivatives in the presence of KBr, H₂O₂, and HClO₄ in the aqueous medium. Between these two complexes, 2 exhibits excellent catalytic activity with high turnover frequency (35.5-43.3 s^-1) due to the strong electron-withdrawing nature of the cyano groups attached at β-positions and its moderate nonplanar structure as compared to 1 (TOF = 22.1-27.4 s^-1). Notably, this is the highest turnover frequency value observed for any porphyrin system. The selective epoxidation of various terminal alkenes using complex 2 has also been carried out, and the results are good, specifying the importance of electron-withdrawing cyano groups. Catalysts 1 and 2 are recyclable, and the catalytic activity proceeds through the corresponding [V^VO(OH)TPP(Br)₄] and [V^VO(OH)TPP(CN)₄] intermediates, respectively.

Electrocatalytic oxygen reduction with cobalt corroles bearing cationic substituents

Recent decades have seen increasing interest in developing highly active and selective electrocatalysts for the oxygen reduction reaction (ORR). The active site environment of cytochrome c oxidases (CcOs), including electrostatic and hydrogen-bonding interactions, plays an important role in promoting the selective conversion of dioxygen to water. Herein, we report the synthesis of three Co^III corroles, namely 1 (with a 10-phenyl ortho-trimethylammonium cationic group), 2 (with a 10-phenyl ortho-dimethylamine group) and 3 (with a 10-phenyl para-trimethylammonium cationic group) as well as their electrocatalytic ORR activities in both acidic and neutral solutions. We discovered that 1 is much more active and selective than 2 and 3 for the electrocatalytic four-electron ORR. Importantly, 1 showed ORR activities with half-wave potentials at E_1/2 = 0.75 V versus RHE in 0.5 M H₂SO₄ solutions and at E_1/2 = 0.70 V versus RHE in neutral 0.1 M phosphate buffer solutions. This work is significant for outlining a strategy to increase both the activity and selectivity of metal corroles for the electrocatalytic ORR by introducing cationic units.

V-Porphyrins Encapsulated or Supported on Siliceous Materials: Synthesis, Characterization, and Photoelectrochemical Properties

Metalloporphyrin-containing mesoporous materials, named VTPP@SBA, were prepared via a simple anchoring of vanadyl porphyrin (5,10,15,20-Tetraphenyl-21H,23H-porphine vanadium(IV) oxide) through a SBA-15-type mesoporous material. For comparison, vanadyl porphyrin was also impregnated on SiO2 (VTPP/SiO2). The characterization results of catalysts by XRD, FTIR, DR-UV-vis, and EPR confirm the incorporation of vanadyl porphyrin within the mesoporous SBA-15. These catalysts have also been studied using electrochemical and photoelectrochemical methods. Impedance measurements confirmed that supporting the porphyrin in silica improved the electrical conductivity of samples. In fact, when using mesoporous silica, current densities associated with oxidation/reduction processes appreciably increased, implying an enhancement in charge transfer processes and, therefore, in electrochemical performance. All samples presented n-type semiconductivity and provided an interesting photoelectrocatalytic response upon illumination, especially silica-supported porphyrins. This is the first time that V-porphyrin-derived materials have been tested for photoelectrochemical applications, showing good potential for this use.

A Greener Route to Blue: Solid-State Synthesis of Phthalocyanines

Phthalocyanines are important organic dyes with a broad applicability in optoelectronics, catalysis, sensing and nanomedicine. Currently, phthalocyanines are synthetized in high boiling organic solvents, like dimethylaminoethanol (DMAE), which is a flammable, corrosive, and bioactive substance, miscible with water and harmful to the environment. Here we show a new solid-state approach for the high-yielding synthesis of phthalocyanines, which reduces up to 100-fold the amount of DMAE. Through systematic screening of solid-state reaction parameters, carried out by ball-milling and aging, we reveal the influence of key variables-temperature, presence of a template, and the amount and role of DMAE in the conversion of tBu phthalonitrile to tetra-tBu phthalocyanine. These results set the foundations to synthesize these high-performance dyes through a greener approach, opening the field of solid-state synthesis to a wider family of phthalocyanines.

Promotion of Catalytic Oxygen Reduction Reactions: The Utility of Proton Management Substituents on Cobalt Porphyrins

Three ABAB-type cobalt meso-tetraarylporphyrins with fluorine (F-CoPor), acetic acid (AC-CoPor), and cyanoacetic acid (CN-CoPor) groups at the para-positions of phenyl rings at the 10,20-positions are synthesized and evaluated as catalysts for oxygen reduction reactions (ORRs). In density functional theory calculations, the frontier molecular orbitals of these complexes were found to be stabilized relative to model complexes with electron-withdrawing atoms or moieties on the meso-aryl rings. Electrochemical measurements suggest that electrodes with CN-CoPor (CN-CoPor/C) exhibit the most positive ORR potential values and the highest limiting current density in both acidic and alkali electrolytes, while the F-CoPor/C electrocatalyst exhibits extremely low ORR performance. The electron transfer numbers for the electrocatalysts are more than 3.0, indicating that a mixture of 2- and 4-electron transfer pathways occurs. The results demonstrate that coupling the hydrogen bonding properties and electron-withdrawing abilities through rational design of the substituent at the meso-position is an efficient way to modify the ORR performance.

Metalloporphyrin Metal–Organic Frameworks: Eminent Synthetic Strategies and Recent Practical Exploitations

The emergence of metal–organic frameworks (MOFs) in recent years has stimulated the interest of scientists working in this area as one of the most applicable archetypes of three-dimensional structures that can be used as promising materials in several applications including but not limited to (photo-)catalysis, sensing, separation, adsorption, biological and electrochemical efficiencies and so on. Not only do MOFs have their own specific versatile structures, tunable cavities, and remarkably high surface areas, but they also present many alternative procedures to overcome emerging obstacles. Since the discovery of such highly effective materials, they have been employed for multiple uses; additionally, the efforts towards the synthesis of MOFs with specific properties based on planned (template) synthesis have led to the construction of several promising types of MOFs possessing large biological or bioinspired ligands. Specifically, metalloporphyrin-based MOFs have been created where the porphyrin moieties are either incorporated as struts within the framework to form porphyrinic MOFs or encapsulated inside the cavities to construct porphyrin@MOFs which can combine the peerless properties of porphyrins and porous MOFs simultaneously. In this context, the main aim of this review was to highlight their structure, characteristics, and some of their prominent present-day applications.

Recent Advances Regarding Precious Metal-Based Electrocatalysts for Acidic Water Splitting

Electrochemical water splitting has wide applicability in preparing high-density green energy. The Proton exchange membrane (PEM) water electrolysis system is a promising technique for the generation of hydrogen due to its high electrolytic efficiency, safety and reliability, compactness, and quick response to renewable energy sources. However, the instability of catalysts for electrochemical water splitting under operating conditions limits their practical applications. Until now, only precious metal-based materials have met the requirements for rigorous long-term stability and high catalytic activity under acid conditions. In this review, the recent progress made in this regard is presented and analyzed to clarify the role of precious metals in the promotion of the electrolytic decomposition of water. Reducing precious metal loading, enhancing catalytic activity, and improving catalytic lifetime are crucial directions for developing a new generation of PEM water electrolysis catalysts. A summary of the synthesis of high-performance catalysts based on precious metals and an analysis of the factors affecting catalytic performance were derived from a recent investigation. Finally, we present the remaining challenges and future perspectives as guidelines for practical use.

Synergistic Dual-Atom Molecular Catalyst Derived from Low-Temperature Pyrolyzed Heterobimetallic Macrocycle-N4 Corrole Complex for Oxygen Reduction

A heterobimetallic corrole complex, comprising oxygen reduction reaction (ORR) active non-precious metals Co and Fe with a corrole-N4 center (PhFCC), is successfully synthesized and used to prepare a dual-atom molecular catalyst (DAMC) through subsequent low-temperature pyrolysis. This low-temperature pyrolyzed electrocatalyst exhibited impressive ORR performance, with onset potentials of 0.86 and 0.94 V, and half-wave potentials of 0.75 and 0.85 V, under acidic and basic conditions, respectively. During potential cycling, this DAMC displayed half-wave potential losses of only 25 and 5 mV under acidic and alkaline conditions after 3000 cycles, respectively, demonstrating its excellent stability. Single-cell Nafion-based proton exchange membrane fuel cell performance using this DAMC as the cathode catalyst showed a maximum power density of 225 mW cm^-2 , almost close to that of most metal-N4 macrocycle-based catalysts. The present study showed that preservation of the defined CoN4 structure along with the cocatalytic Fe-Cx site synergistically acted as a dual ORR active center to boost overall ORR performance. The development of DAMC from a heterobimetallic CoN4-macrocyclic system using low-temperature pyrolysis is also advantageous for practical applications.

Catalysts for Oxygen Reduction Reaction in the Polymer Electrolyte Membrane Fuel Cells: A Brief Review

This mini-review presents a short account of materials with exceptional activity towards oxygen reduction reaction. Two main classes of catalytic materials are described, namely platinum group metal (PGM) catalyst and Non-precious metal catalyst. The classes are discussed in terms of possible application in low-temperature hydrogen fuel cells with proton exchange membrane and further commercialization of these devices. A short description of perspective approaches is provided and challenging issues associated with developed catalytic materials are discussed.

Pyrolysis-Free Synthesized Catalyst towards Acidic Oxygen Reduction by Deprotonation

Acidic oxygen reduction is vital for renewable energy devices such as fuel cells. However, many aspects of the catalytic process are still uncertain-especially the large difference in activity in acidic and alkaline media. Thus, the design and synthesis of model catalysts to determine the active centers and the inactivation mechanism are urgently needed. We report a pyrolysis-free synthesis route to fabricate a catalyst (CPF-Fe@NG) for oxygen reduction in acidic conditions. By introducing a deprotonation process, we extended the oxygen reduction reaction (ORR) activity from alkaline to acidic conditions. CPF-Fe@NG demonstrated outstanding performance with a half-wave potential of 853 mV (vs. RHE) and good stability after 10000 cycles in 1 M HClO₄ . The pyrolysis-free route could also be used to assemble fuel cells, with a maximum power density of 126 mW cm^-2 . Our findings offer new insights into the ORR process to optimize catalysts for both mechanistic studies and practical applications.

A Janus-Type Phthalocyanine for the Assembly of Photoactive DNA Origami Coatings

Design and synthesis of novel photosensitizer architectures is a key step toward new multifunctional molecular materials. Photoactive Janus-type molecules provide interesting building blocks for such systems by presenting two well-defined chemical functionalities that can be utilized orthogonally. Herein a multifunctional phthalocyanine is reported, bearing a bulky and positively charged moiety that hinders their aggregation while providing the ability to adhere on DNA origami nanostructures via reversible electrostatic interactions. On the other hand, triethylene glycol moieties render a water-soluble and chemically inert corona that can stabilize the structures. This approach provides insight into the molecular design and synthesis of Janus-type sensitizers that can be combined with biomolecules, rendering optically active biohybrids.

Copolymers of bipyridinium and metal (Zn & Ni) porphyrin derivatives; theoretical insights and electrochemical activity towards CO2

This study reports the electropolymerization of novel keto functionalized octaethyl metal porphyrins (Zn²⁺ and Ni²⁺) in the presence of 4,4′-bipyridine (4,4′-bpy) as a bridging nucleophile. The polymer films were characterized by electrochemical, spectroscopic (UV-Vis, XPS, FT-IR and Raman spectroscopy) and imaging (AFM and SEM) techniques. The absorption and electronic spectra confirm the presence of both porphyrin and 4,4′-bipyridine units in the film. The surface morphology reveals homogeneous film deposition with average roughness values of approx. 8 nm. The theoretical studies performed offered insights into the interplay of different metal centres (Zn²⁺ and Ni²⁺) and the keto functionality of the porphyrin unit in the formation of copolymer films. The electrochemical interaction of polymer films with CO₂ suggests a reversible trap and release of CO₂ with low energy barriers for both the polymers.

Electropolymerization of keto functionalized porphyrins and 4,4′-bipyridine.

Enhanced Oxygen Reduction Reaction Performance Using Intermolecular Forces Coupled with More Exposed Molecular Orbitals of Triphenylamine in Co-porphyrin Electrocatalysts

Triphenylamine (TPA) has often been used as a building block to construct functional organic materials yet is rarely employed in oxygen reduction reaction (ORR) due to its strong electron-donating ability. This versatile segment bears a three-dimensional spatial structure whose effect has not been fully explored in catalytic systems. To this end, five symmetric cobalt porphyrins with carbazole and TPA derivatives have been synthesized and their ORR performance has been evaluated in acid medium. It was found that all compounds produced mainly hydrogen peroxide in oxygen reduction, with CP1 attaching benzyl derivatives and XCP4 possessing TPA-carbazole substituents at the meso-position of porphyrin, showing similar but more positive ORR potential as compared to the other analogues. Importantly, XCP4 achieved the greatest response current and the largest electron transfer numbers and H₂O₂ yields among the investigated molecules. Detailed electrochemical measurements suggested that the dipole-induced partial charges on the porphyrin in tandem with the more exposed molecular orbitals on TPA contributed to this enhancement, with the former attracting more protons to the affinity of reactive sites and the latter increasing the collision frequency between the electrocatalyst and H⁺ in solution. This is the first attempt to integrate the intermolecular forces with more exposed molecular orbitals in altering the electrochemical process.

A cobalt-pyrrole coordination compound as high performance cathode catalyst for direct borohydride fuel cells

Pyrrole and cobalt nitrate were used as nitrogen and metal sources respectively to synthesize a dinitratobis(polypyrrole)cobalt(ii) (Co(polypyrrole)₂(NO₃)₂) adduct as the precursor of a Co–pyrrole/MPC catalyst. Pyrrole has the capability of polymerization and coordination with Co(ii). Taking this advantage, the Co(polypyrrole)₂(NO₃)₂ coordination can form a long-chain structure with abundant and robust Co–N bonds, contributing to significantly increased catalytic sites in the product catalyst. As a result, the obtained Co–pyrrole/MPC (MPC = macroporous carbon) catalyst exhibited high ORR catalytic activity in alkaline media and excellent performance in direct borohydride fuel cell (DBFC). A peak power density up to 325 mW cm⁻² was achieved at ambient condition, outperforming the commercialized Pt/XC-72 benchmark containing 28.6 wt% Pt. The construction of long-chain coordination precursor was verified playing a key role in the electrochemical improvement of Co–pyrrole/MPC catalyst in DBFC.

Co–pyrrole/MPC was synthesized by using pyrrole and cobalt nitrate as nitrogen and metal source, which enabled a higher peak power density than the commercialized 28.6 wt% Pt/XC72 in DBFC.

Investigation of the photoconductive properties of thiophene substituted metallo-phthalocyanines

The synthesis of 4-(thiophen-3-ylethynyl)phthalonitrile was achieved via the Sonogashira cross-coupling reaction. Herein, we report the synthesis of symmetrical metallo-phthalocyanines with cobalt, zinc, and manganese as the metal ions, and tetrakis(ethynylthiophene) groups substituted at the peripheral positions. The new compounds have been characterized by using mass, ¹H NMR, ¹³C NMR, FT-IR and UV-vis spectroscopy. Dark and photoconductivity measurements were carried out on spin coated thiophene substituted metallo-phthalocyanines at various temperatures fixed between 300 and 440 K. Photocurrent generation and charge transport mechanisms are discussed as well. The evaluation of the photoconductive properties of these compounds indicated that the photocurrent generation efficiency and charge transport mechanism exhibited a strong temperature dependence. On the other hand, conductivity measurements revealed that the temperature dependence of the conductivity can be described by the Arrhenius type exponential equation under dark conditions. Furthermore, it was found that the photoconductive behavior of the film can be described successfully by the variable range hopping model for the whole investigated temperature range.

Electrochemical hydrogen and oxygen evolution reactions from a cobalt-porphyrin-based covalent organic polymer

Covalent organic polymers have attracted much attention due to their high specific surface area, superlative porosity, and diversity in electronic structure. Herein, a novel porous cobalt-porphyrin-based covalent organic polymer (CoCOP) is fabricated through the Schiff-base condensation reaction, which is used as a difunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The CoCOP possesses a high surface area and strong synergistic effect between the cobalt-porphyrins and the CN groups, resulting in efficient HER and OER performances. The CoCOP required relatively low overpotentials for both HER (121 mV to reach 1.0 mA cm^-2 and 310 mV to reach 10 mA cm^-2) and OER (166 mV to reach 1.0 mA cm^-2 and 350 mV to reach 10 mA cm^-2) in alkaline media. This work may provide a new idea for the design of non-noble metal-based coordination polymers with excellent structure and high electrocatalytic performance.

Surface-coordinated metal-organic framework thin films (SURMOFs) for electrocatalytic applications

The design and development of highly efficient electrocatalysts are very important in energy storage and conversion. As a kind of inorganic organic hybrid material, metal-organic frameworks (MOFs) have been used as electrocatalysts in electrocatalytic reactions due to their structural diversities and fascinating functionalities. Particularly, MOF thin films are coordinated on substrate surfaces by a liquid phase epitaxial (LPE) layer by layer (LBL) growth method (called surface-coordinated MOF thin films, SURMOFs), and recently have been studied in various applications due to their precisely controlled thickness, preferred growth orientation and homogeneous surface. In this review, we will summarize the preparation and electrocatalysis of SURMOFs and their derived thin films (SURMOF-D). The SURMOF based thin films possess diverse topological structures and flexible properties, providing abundant catalytically active sites and fast charge transfer for efficient electrocatalytic performance in the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CRR), supercapacitors, tandem electrocatalysis and so on. The research challenges and problems of SURMOFs for electrocatalytic applications are also discussed at the end of the review.

Metalloporphyrinic metal-organic frameworks: Controlled synthesis for catalytic applications in environmental and biological media

Recently, as a new sub-family of porous coordination polymers (PCPs), porphyrinic-MOFs (Porph-MOFs) with biomimetic features have been developed using porphyrin macrocycles as ligands and/or pillared linkers. The control over the coordination of the porphyrin ligand and its derivatives however remains a challenge for engineering new tunable Porph-MOF frameworks by self-assembly methods. The key challenges exist in the following respects: (i) collapse of the large open pores of Porph-MOFs during synthesis, (ii) deactivation of unsaturated metal-sites (UMCs) by axial coordination, and (iii) the tendency of both coordinated moieties (at peripheral meso- and beta-carbon sites) and the N₄-pyridine core to coordinate with metal cations. In this respect, this review covers the advances in the design of Porph-MOFs relative to their counterpart covalent organic frameworks (Porph-COFs). The potential utility of custom-designed porphyrin/metalloporphyrins ligands is highlighted. Synthesis strategies of Porph-MOFs are also illustrated with modular design of hybrid guest@host composites (either Porph@MOFs or guest@Porph-MOFs) with exceptional topologies and stability. This review summarizes the synergistic benefits of coordinated porphyrin ligands and functional guest molecules in Porph-MOF composites for enhanced catalytic performance in various redox applications. This review shed lights on the engineering of new tunable hetero-metals open active sites within (metallo)porphyrin-MOFs as out-of-the-box platforms for enhanced catalytic processes in chemical and biological media.

Engineering a solid-state metalloprotein hydrogen evolution catalyst

Hydrogen has the potential to play an important role in decarbonising our energy systems. Crucial to achieving this is the ability to produce clean sources of hydrogen using renewable energy sources. Currently platinum is commonly used as a hydrogen evolution catalyst, however, the scarcity and expense of platinum is driving the need to develop non-platinum-based catalysts. Here we report a protein-based hydrogen evolution catalyst based on a recombinant silk protein from honeybees and a metal macrocycle, cobalt protoporphyrin (CoPPIX). We enhanced the hydrogen evolution activity three fold compared to the unmodified silk protein by varying the coordinating ligands to the metal centre. Finally, to demonstrate the use of our biological catalyst, we built a proton exchange membrane (PEM) water electrolysis cell using CoPPIX-silk as the hydrogen evolution catalyst that is able to produce hydrogen with a 98% Faradaic efficiency. This represents an exciting advance towards allowing protein-based catalysts to be used in electrolysis cells.

Theoretical Study on Electronic Structural Properties of Catalytically Reactive Metalloporphyrin Intermediates

Metalloporphyrins have attracted great attention in the potential application of biomimetic catalysis. Especially, they were widely investigated as green catalysts in the chemical oxidation of various hydrocarbons through the catalytic activation of molecular oxygen. The structural properties of active central metal ions were reported to play a decisive role in catalytic activity. However, those delicate structural changes are difficult to be experimentally captured or elucidated in detail. Herein, we explored the electronic structural properties of metalloporphyrins (metal porphyrin (PMII, PMIIICl)) and their corresponding catalytically active intermediates (metal(III)-peroxo(PMIII-O2), metal(III)-hydroperoxo(PMIII-OH), and metal(IV)-oxo(PMIV=O), (M=Fe, Mn, and Co)) through the density functional theory method. The ground states of these intermediates were determined based on the assessment of relative energy and the corresponding geometric structures of ground states also further confirmed the stability of energy. Furthermore, our analyses of Mulliken charges and frontier molecular orbitals revealed the potential catalytic behavior of reactive metalloporphyrin intermediates.

Enhancement of metallomacrocycle-based oxygen reduction catalysis through immobilization in a tunable silk-protein scaffold

Fuel cells convert chemical energy into electrical current with the use of an oxidant such as oxygen and have the potential to reduce our reliance on fossil fuels. To overcome the slow kinetics of the oxygen reduction reaction (ORR), platinum is often used as the catalyst. However, the scarcity and expense of platinum limits the wide-spread use of fuel cells. In the search for non-platinum oxygen reduction catalysts, metallomacrocycles have attracted significant attention. While progress has been made in understanding how metallomacrocycle-based molecules can catalyze the ORR, their low stability, remains an on-going challenge. Here we report an immobilization strategy whereby hemin (iron protoporphyrin IX, heme b) is converted into an oxygen reduction catalyst which could be operated for over 96 h, with turnover numbers >10⁷. This represents a 3 orders of magnitude improvement over the best reported iron porphyrin ORR catalyst to date. The basis for this improvement in turnover is specific binding of the heme within a recombinant silk protein, which allows for separation of the porphyrin active sites. Use of the silk protein provides a scaffold that can be engineered to improve selectivity and efficiency. Through rational design of the heme binding site, a > 95% selectivity for a four-electron reduction of oxygen to water was obtained, equal to the selectivity obtained using platinum-based catalysts. This work represents an important advance in the field, demonstrating that metallomacrocycle-based ORR catalysts are viable for use in fuel cells.

An unusual donor-acceptor system MnIIPc-TCNQ/F4-TCNQ and the properties of the mixed single crystals of metal phthalocyanines with organic acceptor molecules

The interaction of manganese(ii) phthalocyanine with 7,7,8,8-tetracyanoquinodimethane and its perfluoro derivative proceeds with the oxidation of Mn and the reduction of the acceptor molecules to give the first mixed single crystals of manganese(iii) phthalocyanine with TCNQ/F4-TCNQ radical anions. The crystals have unusual structures with C-Hπ interactions between the ions and their orthogonal arrangement, as well as remarkable redox properties. The charge transfer was proved by spectroscopic and magnetic studies.