Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes with Significantly Improved Efficiency

Surprising Route to a Monoazaporphyrin and Full Characterization of Its Complexes with Five Different 3d Metals

In the search for mild agents for the oxidative cyclization of tetrapyrromethane to the corresponding corrole, we discovered a route that leads to a monoazaporphyrin with three meso-CF3 groups. Optimization studies that allowed access to appreciable amounts of this new macrocycle paved the way for the preparation of its cobalt, copper, nickel, zinc, and iron complexes. All complexes were fully characterized by various spectroscopic methods and X-ray crystallography. Their photophysical and electrochemical properties were determined and compared to those of analogous porphyrins in order to deduce the effect of the peripheral N atom. Considering the global efforts for designing efficient alternatives to platinum group metal (PGM) catalysts, they were also absorbed onto a porous carbon electrode material and studied as electrocatalysts for the oxygen reduction reaction (ORR). The cobalt complex was found to be operative at a quite positive catalytic onset potential and with good selectivity for the desirable 4-electrons/4-protons pathway.

An 'active site anchoring' strategy for the preparation of PBO fiber derived carbon catalyst towards an efficient oxygen reduction reaction and zinc-air batteries

In order to promote the wide application of clean energy-fuel cells, it is urgent to develop transition metal-based high-efficiency oxygen reduction reaction (ORR) catalytic materials with a low cost and available rich raw material resources to replace the currently used precious metal platinum-based catalytic materials. Herein, a novel 'active-site-anchoring' strategy was developed to synthesize highly-activated carbon-based ORR catalysts. Firstly, poly(p-phenylene benzobisoxazole) (PBO) fiber with a stable chemical structure was selected as the main precursor, and iron was complexed on its surface, and then poly-dopamine (PDA) was coated on the surface of PBO-Fe to form a PBO-Fe-PDA composite structure. Therefore, carbon-based catalyst PBO-Fe-PDA-900 with abundant Fe2O3 active sites was prepared by anchoring iron sites by PDA after pyrolysis. As a result, the PBO-Fe-PDA-900 catalyst displayed a 30 mV higher half-wave potential (0.86 V) than that of a commercial Pt/C electrocatalyst. Finally, PBO-Fe-PDA-900 was used as a cathode material for zinc-air batteries, showing a high peak power density superior to Pt/C. This work offers new prospects for the design of efficient, non-precious metal-based materials in zinc-air batteries.

Panchromatic and Perturbed Absorption Spectral Features and Multiredox Properties of Dicyanovinyl- and Dicyanobutadienyl-Appended Cobalt Corroles

Four new β-functionalized π-extended cobalt corroles with one and two dicyanovinyl (DCV) or dicyanobutadienyl (DCBD) moieties at the 3- and 3,17-positions have been synthesized and characterized by various spectroscopic techniques. Interestingly, the synthesized DCV- and DCBD-appended cobalt corroles displayed panchromatic and near-infrared absorption in the range 300-1100 nm in CH2Cl2 and pyridine solvents. (MN)2-(Cor)Co and A2MN2-(Cor)Co exhibited 8-9 times enhancement in the molar absorptivity of the Q band compared to the parent corrole ((Cor)Co). The unique absorption spectral features of these β-functionalized cobalt corroles are splitting, broadening, and red-shifting in the Soret and Q bands. One DCV unit brings a 30-46 nm red shift, whereas one DCBD unit brings a 40-75 nm red shift in the Q band compared to the corresponding precursors. This is rare that the intensity of the longest Q band is greater than or equal to the Soret-like bands. These corrole derivatives exhibit UV-vis spectral features similar to those of chlorophyll a. A 220 mV positive shift per DCV group and 160 mV positive shift per DCBD group were observed in the first oxidation potentials compared to (Cor)Co in the desired direction for the utility of these cobalt complexes in electrocatalysis. DFT studies revealed that HOMO and LUMO were stabilized after appending DCV and DCBD groups on the corrole macrocycle and exhibited a "push-pull" behavior leading to promising material applications in nonlinear optics (NLO) and catalysis.

Creating Conjugated C-C Bonds between Commercial Carbon Electrode and Molecular Catalyst for Oxygen Reduction to Hydrogen Peroxide

Immobilizing molecular catalysts on electrodes is vital for electrochemical applications. However, creating robust electrode-catalyst interactions while maintaining good catalytic performance and rapid electron transfer is challenging. Here, without introducing any foreign elements, we show a bottom-up synthetic approach of constructing the conjugated C-C bond between the commercial Vulcan carbon electrode and an organometallic catalyst. Characterization results from FTIR, XPS, aberration-corrected TEM and EPR confirmed the successful and uniform heterogenization of the complex. The synthesized Vulcan-LN4 -Co catalyst is highly active and selective in the oxygen reduction reaction in neutral media, showing an 80 % hydrogen peroxide selectivity and a 0.72 V (vs. RHE) onset potential which significantly outperformed the homogenous counterpart. Based on single-crystal XRD and NMR data, we built a model for density functional theory calculations which showed a nearly optimal binding energy for the *OOH intermediate. Our results show that the direct conjugated C-C bonding is an effective approach for heterogenizing molecular catalysts on carbon, opening new opportunities for employing molecular catalysts in electrochemical applications.

Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis

Considering the worldwide efforts for designing catalysts that are not based on platinum group metals while still reserving the many advantages thereof, this study focused on the many variables that dictate the performance of cathodes used for fuel cells, regarding the efficient and selective reduction of oxygen to water. This was done by investigating two kinds of porous carbon electrodes, modified by molecular cobalt(III) complexes chelated by corroles that differ very much in size and electron-withdrawing capability. Examination of the electronic effect uncovered shifts in the CoII/CoIII redox potentials and also large differences in the affinity of the cobalt center to external ligands. Spontaneous absorption of the catalysts was found to depend on the size of the corrole's substituents (C6F5 ≫ CF3 ≫ H) and the metal's axial ligands (PPh3 versus pyridine), as well as on the porosity of the carbon electrodes (BP2000 > Vulcan). The better-performing cobalt-based catalysts were almost as active and selective as 20% platinum on Vulcan in terms of the onset potential and the only 2-10% undesirable formation of hydrogen peroxide. Durability was also addressed by using the best-performing modified cathode in a proper anion-exchange membrane fuel cell setup, revealing very little voltage change during 12 h of operation.

Covalent Metalloporphyrin Polymer Coated on Carbon Nanotubes as Bifunctional Electrocatalysts for Water Splitting

Metalloporphyrins have exhibited excellent electrocatalytic activities for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In order to improve the efficiency and conductivity, these molecular catalysts need to be immobilized on conductive electrode materials. Herein, a facile "one-pot" strategy was developed to coat a covalent metalloporphyrin polymer on a carbon nanotube (CNT) as bifunctional catalysts [denoted as MTIPP@CNTs, H₂TIPP = 5,10,15,20-tetra(4-(imidazole-1-yl)phenyl)porphyrin)] for water splitting in alkaline solution. MTIPP@CNTs have shown excellent electrocatalytic activities for both the HER and OER when metalloporphyrin's central metal is optimized as well as the amount of catalysts that is loaded on the CNT. The overpotential (η₁₀) of NiTIPP@CNT-2 for the OER is only 320 mV at a current density of 10 mA cm^-2 in 1.0 M KOH, and CoTIPP@CNT-1 exhibited an excellent electrocatalytic activity for the HER (η₁₀ = 450 mV for 10 mA cm^-2). Furthermore, the remarkable bifunctional electrocatalytic performance (a cell voltage of 2.04 V with a current density of 10 mA cm^-2) was also explored in the overall water splitting test.

An Easily Prepared Monomeric Cobalt(II) Tetrapyrrole Complex That Efficiently Promotes the 4e-/4H+ Peractivation of O2 to Water

The selective 4e-/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) or Co(III) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. However, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e-/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based on a nonaromatic isocorrole scaffold that is competent for 4e-/4H+ oxygen reduction reaction (ORR). This Co(II) 10,10-dimethyl isocorrole (Co[10-DMIC]) is obtained in just four simple steps and has excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co porphyrin (cobalt 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin, [Co(TPFPP)]) and corrole (cobalt 5,10,15-tris(pentafluorophenyl)corrole triphenylphosphine, Co[TPFPC](PPh3)) homologues demonstrated that the spectroscopic and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, the investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ∼85% selective for the 4e-/4H+ reduction of O2 to H2O over the more kinetically facile 2e-/2H+ process that delivers H2O2. In contrast, the same ORR evaluations conducted for the Co porphyrin and corrole homologues demonstrated that these traditional aromatic systems catalyze the 2e-/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups or ditopic molecular clefts or that impose cofacially oriented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and shows a privileged performance for the 4e-/4H+ peractivation of O2 to water as compared to other simple cobalt containing tetrapyrroles.

Metalloporphyrins as Catalytic Models for Studying Hydrogen and Oxygen Evolution and Oxygen Reduction Reactions

The hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are involved in biological and artificial energy conversions. H-H and O-O bond formation/cleavage are essential steps in these reactions. In nature, intermediates involved in the H-H and O-O bond formation/cleavage are highly reactive and short-lived, making their identification and investigation difficult. In artificial catalysis, the realization of these reactions at considerable rates and close to their thermodynamic reaction equilibria remains a challenge. Therefore, the elucidation of the reaction mechanisms and structure-function relationships is of fundamental significance to understand these reactions and to develop catalysts.This Account describes our recent investigations on catalytic HER, OER, and ORR with metalloporphyrins and derivatives. Metalloporphyrins are used in nature for light harvesting, energy conversion, electron transfer, O₂ activation, and peroxide degradation. Synthetic metal porphyrin complexes are shown to be active for these reactions. We focused on exploring metalloporphyrins to study reaction mechanisms and structure-function relationships because they have stable and tunable structures and characteristic spectroscopic properties.For HER, we identified three H-H bond formation mechanisms and established the correlation between these processes and metal hydride electronic structures. Importantly, we provided direct experimental evidence for the bimetallic homolytic H-H bond formation mechanism by using sterically bulky porphyrins. Homolytic HER has been long proposed but rarely verified because the coupling of active hydride intermediates occurs spontaneously and quickly, making their detection challenging. By blocking the bimolecular mechanism through steric effects, we stabilized and characterized the Ni^III-H intermediate and verified homolytic HER by comparing the reaction behaviors of Ni porphyrins with and without steric effects. We therefore provided an unprecedented example to control homolytic versus heterolytic HER mechanisms through tuning steric effects of molecular catalysts.For the OER, the water nucleophilic attack (WNA) on high-valent terminal Mn-oxo has been proposed for the O-O bond formation in natural and artificial water oxidation. By using Mn tris(pentafluorophenyl)corrole, we identified Mn^V(O) and Mn^IV-peroxo intermediates in chemical and electrochemical OER and provided direct experimental evidence for the Mn-based WNA mechanism. Moreover, we demonstrated several catalyst design strategies to enhance the WNA rate, including the pioneering use of protective axial ligands. By studying Cu porphyrins, we proposed a bimolecular coupling mechanism between two metal-hydroxide radicals to form O-O bonds. Note that late-transition metals do not likely form terminal metal-oxo/oxyl.For the ORR, we presented several strategies to improve activity and selectivity, including providing rapid electron transfer, using electron-donating axial ligands, introducing hydrogen-bonding interactions, constructing dinuclear cooperation, and employing porphyrin-support domino catalysis. Importantly, we used Co porphyrin atropisomers to realize both two-electron and four-electron ORR, representing an unparalleled example to control ORR selectivity by tuning only steric effects without modifying molecular and/or electronic structures.Lastly, we developed several strategies to graft metalloporphyrins on various electrode materials through different covalent bonds. The molecular-engineered materials exhibit boosted electrocatalytic performance, highlighting promising applications of molecular electrocatalysis. Taken together, this Account demonstrates the benefits of exploring metalloporphyrins for the HER, OER, and ORR. The knowledge learned herein is valuable for the development of porphyrin-based catalysts and also other molecular and material catalysts for small molecule activation reactions.

Highly Efficient Photocatalytic Reduction of CO2 to CO by In Situ Formation of a Hybrid Catalytic System Based on Molecular Iron Quaterpyridine Covalently Linked to Carbon Nitride

Efficient and selective photocatalytic CO₂ reduction was obtained within a hybrid system that is formed in situ via a Schiff base condensation between a molecular iron quaterpyridine complex bearing an aldehyde function and carbon nitride. Irradiation (blue LED) of an CH₃ CN solution containing 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), triethylamine (TEA), Feqpy-BA (qpy-BA=4-([2,2':6',2'':6'',2'''-quaterpyridin]-4-yl)benzaldehyde) and C₃ N₄ resulted in CO evolution with a turnover number of 2554 and 95 % selectivity. This hybrid catalytic system unlocks covalent linkage of molecular catalysts with semiconductor photosensitizers via Schiff base reaction for high-efficiency photocatalytic reduction of CO₂ , opening a pathway for diverse photocatalysis.

Corroles at work: a small macrocycle for great applications

Corrole chemistry has witnessed an impressive boost in studies in the last 20 years, thanks to the possibility of preparing corrole derivatives by simple synthetic procedures. The investigation of a large number of corroles has highlighted some peculiar characteristics of these macrocycles, having features different from those of the parent porphyrins. With this progress in the elucidation of corrole properties, attention has been focused on the potential for the exploitation of corrole derivatives in different important application fields. In some areas, the potential of corroles has been studied in certain detail, for example, the use of corrole metal complexes as electrocatalysts for energy conversion. In some other areas, the field is still in its infancy, such as in the exploitation of corroles in solar cells. Herein, we report an overview of the different applications of corroles, focusing on the studies reported in the last five years.

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.

Axial Ligand Coordination Tuning of the Electrocatalytic Activity of Iron Porphyrin Electrografted onto Carbon Nanotubes for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. It is desirable to design transition metal molecular catalysts inspired by enzymatic oxygen activation/reduction processes as an alternative to noble-metal-Pt-based ORR electrocatalysts, especially in view point of fuel cell commercialization. We have fabricated bio-inspired molecular catalysts electrografted onto multiwalled carbon nanotubes (MWCNTs) in which 5,10,15,20-tetra(pentafluorophenyl) iron porphyrin (iron porphyrin FeF₂₀ TPP) is coordinated with covalently electrografted axial ligands varying from thiophene to imidazole on the MWCNTs' surface. The catalysts' electrocatalytic activity varied with the axial coordination environment (i. e., S-thiophene, N-imidazole, and O-carboxylate); the imidazole-coordinated catalyst MWCNTs-Im-FeF₂₀ TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT-Im-FeF₂₀ TPP was loaded onto the cathode of a zinc-air battery, an open-cell voltage (OCV) of 1.35 V and a maximum power density (P_max ) of 110 mW cm^-2 were achieved; this was higher than those of MWCNTs-Thi-FeF₂₀ TPP (OCV=1.30 V, P_max =100 mW cm^-2 ) and MWCNTs-Ox-FeF₂₀ TPP (OCV=1.28 V, P_max =86 mW cm^-2 ) and comparable with a commercial Pt/C catalyst (OCV=1.45 V, P_max =120 mW cm^-2 ) under similar experimental conditions. This study provides a time-saving method to prepare covalently immobilized molecular electrocatalysts on carbon-based materials with structure-performance correlation that is also applicable to the design of other electrografted catalysts for energy conversion.

Identification of the Active Sites of NiCo2O4 and the Support Effect with Carbon Nanotubes for Oxygen Reduction Catalysis

Since the active site of spinel NiCo₂O₄ as an oxygen reduction reaction (ORR) catalyst is under debate, the optimal active sites of NiCo₂O₄ are identified by investigating the catalytic properties of two models of NiCo₂O₄. The results indicate that the ORR activity of five studied active sites of isolated NiCo₂O₄ is primarily limited by excessively binding of *OH, and the Co^III and Ni^III active sites of Co²⁺[Co³⁺Ni³⁺]O₄ are identified as the optimal catalytic sites with overpotentials of 0.69 and 0.76 V, respectively. Moreover, for the investigation of the support effect, the isolated NiCo₂O₄ is supported on pristine and N-doped carbon nanotube (CNT and NCNT). Encouragingly, the ORR activity of Co^III and Ni^III active sites is improved after NiCo₂O₄ is supported on CNT and NCNT due to the weakened binding of *OH. When NiCo₂O₄ is supported on NCNT via the Co^II ion, the rate-determining step of ORR catalyzed by Co^III and Ni^III sites is altered. Moreover, this Co^III site exhibits the highest ORR activity and displays an overpotential of only 0.45 V. Therefore, the support effect can improve the catalytic activity as well as change the mechanism of ORR. This study presents more insights into the active sites and ORR activity of NiCo₂O₄.

Significantly boosted oxygen electrocatalysis with cooperation between cobalt and iron porphyrins

Developing electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of great importance. Herein, Co tetrakis(pentafluorophenyl)porphyrin (Co-P) and Fe chloride tetrakis(pentafluorophenyl)porphyrin (Fe-P) were loaded on carbon nanotubes (CNTs) for combining the electrocatalytic advantages of both Co-P and Fe-P. The resultant (Co-P)0.5(Fe-P)0.5@CNT composite displayed significantly boosted activity for the selective four-electron ORR with a half-wave potential of 0.80 V versus reversible hydrogen electrode (RHE) and for the OER with a potential of 1.65 V versus RHE to obtain 10 mA cm-2 current density in 0.1 M KOH. A Zn-air battery assembled from (Co-P)0.5(Fe-P)0.5@CNT exhibited a small charge-discharge voltage gap of 0.74 V at 2 mA cm-2, a high power density of 174.5 mW cm-2 and a good rechargeable stability (>120 cycles).

Substituents and the induced partial charge effects on cobalt porphyrins catalytic oxygen reduction reactions in acidic medium

Charge states at the catalytic interface can intensely alter the charge transfer mechanism and thus the oxygen reduction performance. Two symmetric cobalt porphyrins with electron deficient 2,1,3-benzothiadiazole (BTD) and electron-donating propeller-like triphenylamine (TPA) derivatives have been designed firstly, to rationally generate intramolecular partial charges, and secondly, to utilize the more exposed molecular orbitals on TPA for enhancing the charge transfer kinetics. The catalytic performance of the two electrocatalysts was examined for oxygen reduction reactions (ORR) in acidic electrolyte. It was found that BCP1/C with two BTD groups showed greater reduction potential but less limiting current density as compared to BCP2/C bearing BTD-TPA units. The reduced potential of BCP2/C was proposed to the introduction of the electron-donating ability of TPA, which may decrease the adsorption affinity of oxygen to the cobalt center. Both dipole-induced partial charge effect and the more exposed cation orbitals of the 3D structural TPA were proposed to contribute to the increased response current of BCP2/C. In addition, BCP2/C attained more than 80% of H₂O₂ generation in acidic solution, which may also relate to the structural effect. These findings may provide new insight into the structural design of organic electrocatalysts and deep understanding on the interfacial charge transfer mechanism for ORR.

Controlling Oxygen Reduction Selectivity through Steric Effects: Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction with Cobalt Porphyrin Atropisomers

Achieving a selective 2 e^- or 4 e^- oxygen reduction reaction (ORR) is critical but challenging. Herein, we report controlling ORR selectivity of Co porphyrins by tuning only steric effects. We designed Co porphyrin 1 with meso-phenyls each bearing a bulky ortho-amido group. Due to the resulted steric hinderance, 1 has four atropisomers with similar electronic structures but dissimilar steric effects. Isomers αβαβ and αααα catalyze ORR with n=2.10 and 3.75 (n is the electron number transferred per O₂ ), respectively, but ααββ and αααβ show poor selectivity with n=2.89-3.10. Isomer αβαβ catalyzes 2 e^- ORR by preventing a bimolecular O₂ activation path, while αααα improves 4 e^- ORR selectivity by improving O₂ binding at its pocket, a feature confirmed by spectroscopy methods, including O K-edge near-edge X-ray absorption fine structure. This work represents an unparalleled example to improve 2 e^- and 4 e^- ORR by tuning only steric effects without changing molecular and electronic structures.

Synthesis, characterization and electrocatalytic properties of bimetallic sulfides CoS/MnS/N-C for oxygen reduction in alkaline media

Synthesis, characterization and oxygen reduction reaction (ORR) catalytic properties of bimetallic sulfides CoS/MnS/N-C catalyst was discussed. The catalyst was derived from a typical Co based zeolitic imidazolate framework (ZIF-67) and manganese aminoporphyrin. 5,15-Bis(4-aminophenyl)-10,20-bis(4-bromophenyl) porphyrin manganese oxoacetate loaded with ZIF-67 forms a porphyrin loaded ZIF-67. This product was then calcined at 800ˆ∘C and vulcanized with thioacetamide to obtain the bimetallic sulfide product CoS/MnS/N-C. The structure of CoS/MnS/N-C was further characterized by XRD, XPS, FESEM and HRTEM spectra which indicated a novel porous and hollow sphere structure. The electrocatalytic properties of the bimetallic material as well as its parent porphyrin and ZIF-67 were also compared in alkaline condition (0.1 M KOH) with a rotating disk electrode. The prepared catalyst CoS/MnS/N-C exhibits a higher catalytic performance than its precursors (PorMnOAc, ZIF-67 and PorMnOAc loaded ZIF-67) with almost four electron transfers under this condition.

Building Tailored Interfaces through Covalent Coupling Reactions at Layers Grafted from Aryldiazonium Salts

Aryldiazonium ions are widely used reagents for surface modification. Attractive aspects of their use include wide substrate compatibility (ranging from plastics to carbons to metals and metal oxides), formation of stable covalent bonding to the substrate, simplicity of modification methods that are compatible with organic and aqueous solvents, and the commercial availability of many aniline precursors with a straightforward conversion to the active reagent. Importantly, the strong bonding of the modifying layer to the surface makes the method ideally suited to further on-surface (postfunctionalization) chemistry. After an initial grafting from a suitable aryldiazonium ion to give an anchor layer, a target species can be coupled to the layer, hugely expanding the range of species that can be immobilized. This strategy has been widely employed to prepare materials for numerous applications including chemical sensors, biosensors, catalysis, optoelectronics, composite materials, and energy conversion and storage. In this Review our goal is first to summarize how a target species with a particular functional group may be covalently coupled to an appropriate anchor layer. We then review applications of the resulting materials.

Metal-Organic-Framework-Supported Molecular Electrocatalysis for the Oxygen Reduction Reaction

Synthesizing molecule@support hybrids is appealing to improve molecular electrocatalysis. We report herein metal-organic framework (MOF)-supported Co porphyrins for the oxygen reduction reaction (ORR) with improved activity and selectivity. Co porphyrins can be grafted on MOF surfaces through ligand exchange. A variety of porphyrin@MOF hybrids were made using this method. Grafted Co porphyrins showed boosted ORR activity with large (>70 mV) anodic shift of the half-wave potential compared to ungrafted porphyrins. By using active MOFs for peroxide reduction, the number of electrons transferred per O₂ increased from 2.65 to 3.70, showing significantly improved selectivity for the 4e ORR. It is demonstrated that H₂ O₂ generated from O₂ reduction at Co porphyrins is further reduced at MOF surfaces, leading to improved 4e ORR. As a practical demonstration, these hybrids were used as air electrode catalysts in Zn-air batteries, which exhibited equal performance to that with Pt-based materials.

Synergistic Modulation of Carbon-Based, Precious-Metal-Free Electrocatalysts for Oxygen Reduction Reaction

Developing alternatives to noble-metal-based catalysts toward the oxygen reduction reaction (ORR) process plays a key role in the application of low-temperature fuel cells. Carbon-based, precious-metal-free electrocatalysts are of great interest due to their low cost, abundant sources, active catalytic performance, and long-term stability. They are also supposed to feature intrinsically high activity and highly dense catalytic sites along with their sufficient exposure, high conductivity, and high chemical stability, as well as effective mass transfer pathways. In this Review, we focus on carbon-based, precious-metal-free nanocatalysts with synergistic modulation of active-site species and their exposure, mass transfer, and charge transport during the electrochemical process. With this knowledge, perspectives on synergistic modulation strategies are proposed to push forward the development of Pt-free ORR catalysts and the wide application of fuel cells.

The cobalt carbide/bimetallic CoFe phosphide dispersed on carbon nanospheres as advanced bifunctional electrocatalysts for the ORR, OER, and rechargeable Zn-air batteries

It is very important, but also challenging to produce high-activity, high durability and affordable non-noble-metal-bifunctional-electrocatalysts for sustainable energy application. Here, one-pot synthesized iron covalent porphyrin polymers (FePor-CPP), with carefully placed Fe, N atoms, a regular porous structure, Co₃[Co(CN)₆]₂ and NaH₂PO₂ precursors were carbonized into N,P-doped carbon nanospheres with the active species of both bimetallic CoFe phosphides and CoC_x nanoparticles (denoted as CoC_x/(Co_0.55Fe_1.945)₂P@C). By employing the CoC_x/(Co_0.55Fe_1.945)₂P@C as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrode catalysts, superior catalytic activity is achieved with E_1/2 of 0.84 V for ORR, and overpotential of 0.39 V at 10 mA cm^-2 for OER in an alkaline medium, respectively. Furthermore, CoC_x/(Co_0.55Fe_1.945)₂P@C as air electrode for rechargeable Zn-air battery shows power density as high as 131 mW cm^-2 and charge-discharge cycle stability, and this suggests the potential application of CoC_x/(Co_0.55Fe_1.945)₂P@C in energy transformation systems. The high electrocatalytic performances are revealed to originate from the change of electronic structure of bimetallic (Co_0.55Fe_1.945)₂P via introducing P into the Co_0.55Fe_1.945 alloy, resulting in a decreased energy gap of CoC_x/(Co_0.55Fe_1.945)₂P@C relative to that of CoC_x/Co_0.55Fe_1.945@C. This work proposes a versatile strategy to develop multifunctional non-precious catalysts for this kind of energy-related electrocatalytic reactions.

Activation of Molecular Oxygen by a Cobalt(II) Tetra-NHC Complex*

The first dicobalt(III) μ₂‐peroxo N‐heterocyclic carbene (NHC) complex is reported. It can be quantitatively generated from a cobalt(II) compound bearing a 16‐membered macrocyclic tetra‐NHC ligand via facile activation of dioxygen from air at ambient conditions. The reaction proceeds via an end‐on superoxo intermediate as demonstrated by EPR studies and DFT. The peroxo moiety can be cleaved upon addition of acetic acid, yielding the corresponding Co^III acetate complex going along with H₂O₂ formation. In contrast, both Co^II and Co^III complexes are also studied as catalysts to utilize air for olefin and alkane oxidation reactions; however, not resulting in product formation. The observations are rationalized by DFT‐calculations, suggesting a nucleophilic nature of the dicobalt(III) μ₂‐peroxo complex. All isolated compounds are characterized by NMR, ESI‐MS, elemental analysis, EPR and SC‐XRD.

In situ synthesis of Co–B-doped porous carbon through laser thermal reduction for an efficient oxygen reduction reaction

High dispersibility Co/B–C catalysts are prepared by laser thermal reduction on boron phenolic resin, exhibiting an efficient ORR electrocatalytic activity.

Nickel hydroxide nanoparticles decorated napthalene sulfonic acid-doped polyaniline nanotubes as efficient catalysts for nitroarene reduction

Nanosize nickel hydroxide decorated 2-napthalene sulfonic acid-doped polyaniline nanotubes nanocomposites (Ni(OH)₂@NSA-PANI NCs) were successfully developed for the catalytic reduction of aromatic nitro compounds. The Ni(OH)₂@NSA-PANI NCs were synthesised by depositing Ni(OH)₂ nanoparticles onto 2-napthalene sulfonic acid doped PANI nanotubes surface. The resulting material was characterized using field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (P-XRD), Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The prepared nanocomposite showed a remarkable ability to catalytically hydrogenate aromatic nitro compounds using sodium borohydride (NaBH₄) as hydrogen source in aqueous medium at room temperature. Kinetic studies were performed using 4-nitrophenol (4-NP) as the model substrate, using the Langmuir-Hinshelwood model. The catalyst showed pseudo-first-order kinetics, with rate constants estimated between 0.08287 and 0.3649 min^-1. Catalyst recyclability without reduced activity was demonstrated over 10 successive cycles. The optimised nanocomposite catalyst demonstrated a low activation energy barrier towards 4-NP reduction.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The recent progress made on porphyrin-based frameworks and their applications in energy-related conversion technologies (e.g., ORR, OER and CO₂RR) and storage technologies (e.g., Zn–air batteries).

Effect of porphyrin metal center on synthesis, structure, morphology and oxygen reduction properties of porphyrin encapsulated metal organic frameworks

A series of different metal centered 5,15-Bis(4-aminophenyl)-10,20-bis(4-bromophenyl)porphyrins (PorM) where M = H2, MnOAc, FeCl, Co or Zn were synthesized, and then encapsulated in zeolite imidazole framework-8 (ZIF-8) by typical template directed strategy to generate PorM@ZIF-8 metal organic frameworks (MOFs). These composites were characterized by UV-vis, FTIR, XRD, FESEM and HRTEM methods. Each prepared PorM@ZIF-8 MOF retained the molecular structure of porphyrin and crystal structure of ZIF-8. It is clearly shown that porphyrin centered metal ions will affect MOFs morphology. Both PorH2@ZIF-8 and PorZn@ZIF-8 gave a rhombic dodecahedron, PorMnOAc@ZIF-8 gave a truncated hexagonal prism-like structure, while no specific structures were obtained for PorFeCl@ZIF-8 and PorCo@ZIF-8 due to aggregation as characterized by FESEM spectrum. Oxygen reduction catalytic ability of ZIF-8, PorM and PorM@ZIF-8 were measured in alkaline condition (0.1 M KOH) with the number of electrons transferred being [Formula: see text] = 2.20–2.60 and generating HO[Formula: see text] as the oxygen reduction product. The catalytic property slightly increased after the porphyrin was encapsulated, due possibly to the capacity limit, inappropriate molecular distance or the direction of encapsulated porphyrin molecular.

Catalytic Four-Electron Reduction of Dioxygen by Ferrocene Derivatives with a Nonheme Iron(III) TAML Complex

A mononuclear nonheme iron(III) complex with a tetraamido macrocyclic ligand (TAML), [(TAML)Fe^III]^- (1), is a selective precatalyst for four-electron reduction of dioxygen by ferrocene derivatives in the presence of acetic acid (CH₃COOH) in acetone. This is the first work to show that a nonheme iron(III) complex catalyzes the four-electron reduction of O₂ by one-electron reductants. An iron(V)-oxo complex, [(TAML)Fe^V(O)]^- (2), was produced by oxygenation of 1 with O₂ via the formation of triacetone triperoxide (TATP), acting as an autocatalyst that shortened the induction time for the generation of 2. Decamethylferrocene (Me₁₀Fc) and octamethylferrocene (Me₈Fc) reduced 2 to 1 by two electrons in the presence of CH₃COOH to produce decamethylferrocenium cation (Me₁₀Fc⁺) and octamethylferrocenium cation (Me₈Fc⁺), respectively. Then, 1 was oxygenated by O₂ to regenerate 2 via the formation of TATP. In the cases of ferrocene (Fc), bromoferrocene (BrFc) and 1,1'-dibromoferrocene (Br₂Fc), initial electron transfer from ferrocene derivatives to 2 occurred; however, neither a second proton-coupled electron transfer from ferrocene derivatives to 2 nor a catalytic four-electron reduction of O₂ occurred.

Tuning of ORR activity through the stabilization of the adsorbates by hydrogen bonding with substituent groups

Metallocorroles and metalloporphyrins (M-N-C) are some of the best alternative molecular catalysts for the replacement of the expensive platinum-group metals (PGM) in oxygen reduction reaction (ORR) catalysis in polymer electrolyte membrane (PEM) fuel cells. To date, Co-based corroles have shown the best performance, but still suffer from the poor stability and the toxicity of the Co metal. Mn-N-C are more stable than Co-N-C, and are also less reactive towards peroxide formation. In this work, using first-principles density functional theory calculations, we study the improvement of the Mn-based corrole ORR activity by exploiting hydrogen bonding with substituent groups to modify the adsorption energies of the ORR intermediates and obtain higher onset potential (Vonset) values. We found that by using phenyl acetic acid as a substituent, Vonset increased from 0.54 V for the unsubstituted corrole to ∼0.9 V which is competitive with the Vonset of the Co-based corroles. Our results suggest that hydrogen bonding with substituent groups should be considered in the analysis and design of the reactivity of active sites in non-PGM ORR catalysts.

First-row transition metal porphyrins for electrocatalytic hydrogen evolution — a SPP/JPP Young Investigator Award paper

A series of first-row transition metal complexes of tetrakis(pentafluorophenyl)porphyrin (1), denoted as 1-M (M [Formula: see text] Mn, Fe, Co, Ni, Cu, and Zn), were synthesized and examined as electrocatalysts for the hydrogen evolution reaction (HER). All these transition metal porphyrins were shown to be active for HER in acetonitrile using trifluoroacetic acid (TFA) as the proton source. The molecular nature and the stability of these metal porphyrins when functioning as HER catalysts were confirmed, and all catalysts gave Faradaic efficiency of >97% for H2 generation during bulk electrolysis. Importantly, by using 1-Cu, a remarkably high turnover frequency (TOF) of 48500 s[Formula: see text] 1-Cu the most efficient among this series of metal porphyrin catalysts. This TOF value also represents one of the highest values reported in the literature. In addition, electrochemical analysis demonstrated that catalytic HER mechanisms with these 1-M complexes are different. These results show that with the same porphyrin ligand, the change of metal ions will have significant impact on both catalytic efficiency and mechanism. This work for the first time provides direct comparison of electrocatalytic HER features of transition metal complexes of tetrakis(pentafluorophenyl)porphyrin under identical conditions, and will be valuable for future design and development of more efficient HER electrocatalysts of this series.

Efficient Nitrate Reduction over Novel Covalent Ag-Salophen Polymer-Derived "Vein-Leaf-Apple"-like Ag@Carbon Structures

Efficient electrocatalysts for nitrate reduction reaction (NO₃^--RR) that could selectively transfer nitrate into harmless nitrogen are required for water-denitrification treatment. The most widely used electrodes for NO₃^--RR including noble metals, transition metals, and their alloys still face many challenges such as lower selectivity and efficiency, high cost, and easy corrosion properties. Metallic Ag with acceptable cost possesses strong corrosion resistance in electrolysis, but its activity is often incompetent for NO₃^--RR. In this work, Ag nanoparticles with a lower loading content (1.99 wt %) on a nitrogen-doped carbon support was successfully used as the robust electrocatalyst for NO₃^--RR in a Cl^--free neutral solution. This Ag@carbon catalyst exhibited an impressive electrochemical performance for NO₃^--RR, with a NO₃^--N conversion yield of 53% and a N₂-N selectivity of 97% at a low electrolysis overpotential (-0.29 V vs RHE). In particular, the prepared Ag@carbon showed better stability and no secondary Ag ion pollution in electrolysis. Its impressive electrocatalytic performance was attributed to the unique "vein-leaf-apple"-like Ag@carbon structures, prepared by thermal conversion of Ag-salophen polymers@CNTs. CNTs served as veins to enhance the electron transportation in electrocatalysts. Salophen polymer-derived mesoporous N-doped carbon plates acted as leaves to concentrate NO₃^- from the electrolyte. Like apples on trees, Ag nanoparticles of about 10-20 nm highly dispersed on carbons selectively converted NO₃^--N into N₂-N. It opens up a cost-acceptable and corrosion-resistant Ag-less electrocatalytic pathway for NO₃^--RR, and the special "vein-leaf-apple"-like Ag@carbon structure could enhance the electrolytic efficiency and N₂-N selectivity for NO₃^--RR.