Bioinspired Electrocatalysis of Oxygen Reduction Reaction in Fuel Cells Using Molecular Catalysts

Simultaneous electron and proton conduction in a stable metal organic material with highly selective electrocatalytic oxygen reduction reaction to water

Proton coupled electron transfer (PCET) is considered as the elementary step of several chemical, electrochemical and biological processes and thus the development of dual conducting materials has recently become a major focus in Chemical Science. Herein, we report the highly selective electrocatalytic oxygen reduction to water by the stable dual conducting metal-organic material (MOM) [Cu(INA)2(H2O)4] (INA = isonicotinate). Structural analysis reveals the important role of both, hydrogen bonding and π-interactions, in the formation of a supramolecular 3D network. Theoretical calculations show that hydrogen bonding interactions among the coordinated water molecules and deprotonated carboxylate oxygen atoms induce proton transport (2.26 ± 0.10 × 10-5 S cm-1 at 98% RH) while weak intermolecular π-interactions (π-π and anion-π) provide the pathway for electron transport (1.4 ± 0.1 × 10-7 S cm-1 at 400 K). Such dual proton and electron conductivity leads to a selective oxygen reduction reaction (ORR) to water in an alkaline medium. To the best of our knowledge, this is the first report on electrocatalytic ORR by a dual-conducting metal-organic material.

Iron Porphyrin-Based Composites for Electrocatalytic Oxygen Reduction Reactions

The oxygen reduction reaction (ORR) is one of the most critical reactions in energy conversion systems, and it facilitates the efficient conversion of chemical energy into electrical energy, which is necessary for modern technology. Developing efficient and cost-effective catalysts for ORRs is crucial for advancing and effectively applying renewable energy technologies such as fuel cells, metal-air batteries, and electrochemical sensors. In recent years, iron porphyrin-based composites have emerged as ideal catalysts for facilitating effective ORRs due to their unique structural characteristics, abundance, advances in synthesis, and excellent catalytic properties, which mimic natural enzymatic systems. However, many articles have focused on reviewing porphyrin-based frameworks or metalloporphyrins in general, necessitating research specifically addressing iron porphyrin. This review discusses iron porphyrin as an effective catalyst in ORRs. It provides a comprehensive knowledge of the application of iron porphyrin-based composites for electrocatalytic ORRs, focusing on their properties, synthesis, structural integration with conductive supports, catalytic mechanism, and efficacy. This review also discusses the challenges of applying iron porphyrin-based composites and provides recommendations to address these challenges.

Extrinsically conducting MOFs: guest-promoted enhancement of electrical conductivity, thin film fabrication and applications

Conductive metal-organic frameworks are of current interest in chemical science because of their applications in chemiresistive sensing, electrochemical energy storage, electrocatalysis, etc. Different strategies have been employed to design conductive frameworks. In this review, we discuss the influence of different types of guest species incorporated within the pores or channels of metal-organic frameworks (MOFs) and porous coordination polymers (PCPs) to generate charge transfer pathways and modulate their electrical conductivity. We have classified dopants or guest species into three different categories: (i) metal-based dopants, (ii) molecule and molecular entities and (iii) organic conducting polymers. Different types of metal ions, metal nano-clusters and metal oxides have been used to enhance electrical conductivity in MOFs. Metal ions and metal nano-clusters depend on the hopping process for efficient charge transfer whereas metal-oxides show charge transport through the metal-oxygen pathway. Several types of molecules or molecular entities ranging from neutral TCNQ, I2, and fullerene to ionic methyl viologen, organometallic like nickelcarborane, etc. have been used. In these cases, the charge transfer process varies with the guest species. When organic conducting polymers are the guest, the charge transport occurs through the polymer chains, mostly based on extended π-conjugation. Here we provide a comprehensive and critical review of these strategies to add electrical conductivity to the, in most cases, otherwise insulating MOFs and PCPs. We point out the guest encapsulation process, the geometry and structure of the resulting host-guest complex, the host-guest interactions and the charge transport mechanism for each case. We also present the methods for thin film fabrication of conducting MOFs (both, liquid-phase and gas-phase based methods) and their most relevant applications like electrocatalysis, sensing, charge storage, photoconductivity, photocatalysis,… We end this review with the main obstacles and challenges to be faced and the appealing perspectives of these 21st century materials.

The Excited-State N-H Tautomerization Rate in Free-Base Corroles

Corrole is a tetrapyrrolic dye with a structure that resembles porphyrin, apart from a single missing carbon. The absence of this carbon results in the re-arrangement of the double bonds within the macrocycle, and the presence of three pyrrolic protons in the central cavity in its free-base form. These protons lead to the existence of two distinct tautomeric structures that exist in a dynamic equilibrium. Although the ground-state energies of the tautomers are similar, the excited states show a significant difference in energy which unbalances the equilibrium between the tautomers and results in rapid excited-state tautomerization, favouring one tautomeric species over the other. Although the excited-state tautomerization process has been known for a long time, very few studies have been performed on it, leaving many key aspects of the process poorly understood. Herein we show how ultrafast photoluminescence can be used to experimentally determine the rates of excited-state tautomerization and activation energies of three free-base corrole derivatives thus allowing us to completely describe the excited-state dynamics of the unusual excited state of free-base corrole and opening the door to the development of new materials that can exploit its unique characteristics.

Bioinspired molecular catalysts with a unique tricopper architecture for highly efficient oxygen reduction reaction

In situ growth of intertwined trinuclear copper complexes (nCu3) on a cellulose-derived carbon support (CMC) produced a high-performance electrocatalyst (CMC-nCu3) for the oxygen reduction reaction (ORR), which demonstrated superior performance in zinc-air batteries compared to a commercial Pt/C catalyst. This work highlights the importance of copper-based molecular catalysts with rich and intertwined tricopper structures for boosting both ORR activity and stability.

Morphological and structural design through hard-templating of PGM-free electrocatalysts for AEMFC applications

This study delves into the critical role of customized materials design and synthesis methods in influencing the performance of electrocatalysts for the oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFCs). It introduces a novel approach to obtain platinum-free (PGM-free) electrocatalysts based on the controlled integration of iron active sites onto the surface of silica nanoparticles (NPs) by using nitrogen-based surface ligands. These NPs are used as hard templates to form tailored nanostructured electrocatalysts with an improved iron dispersion into the carbon matrix. By utilizing a wide array of analytical techniques including infrared and X-ray photoelectron spectroscopy techniques, X-ray diffraction and surface area measurements, this work provides insight into the physical parameters that are critical for ORR electrocatalysis with PGM-free electrocatalysts. The new catalysts showed a hierarchical structure containing a large portion of graphitic zones which contribute to the catalyst stability. They also had a high electrochemically active site density reaching 1.47 × 1019 sites g-1 for SAFe_M_P1AP2 and 1.14 × 1019 sites g-1 for SEFe_M_P1AP2, explaining the difference in performance in fuel cell measurements. These findings underscore the potential impact of a controlled materials design for advancing green energy applications.

Atomic-level design of metalloenzyme-like active pockets in metal-organic frameworks for bioinspired catalysis

Natural metalloenzymes with astonishing reaction activity and specificity underpin essential life transformations. Nevertheless, enzymes only operate under mild conditions to keep sophisticated structures active, limiting their potential applications. Artificial metalloenzymes that recapitulate the catalytic activity of enzymes can not only circumvent the enzymatic fragility but also bring versatile functions into practice. Among them, metal-organic frameworks (MOFs) featuring diverse and site-isolated metal sites and supramolecular structures have emerged as promising candidates for metalloenzymes to move toward unparalleled properties and behaviour of enzymes. In this review, we systematically summarize the significant advances in MOF-based metalloenzyme mimics with a special emphasis on active pocket engineering at the atomic level, including primary catalytic sites and secondary coordination spheres. Then, the deep understanding of catalytic mechanisms and their advanced applications are discussed. Finally, a perspective on this emerging frontier research is provided to advance bioinspired catalysis.

Tuning the performance of Fe-porphyrin aerogel-based PGM-free oxygen reduction reaction catalysts in proton exchange membrane fuel cells

Fe-N-C catalysts are currently the leading candidates to replace Pt-based catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells. To maximize their activity, it is necessary to optimize their structure to allow high active site density on one hand, and hierarchical porous structure that will allow good mass transport of reactants and products to and from the active sites on the other hand. Hence, the hierarchical structure of the catalyst plays an important role in the balance between the electrochemical active site density and the mass transport resistance. Aerogels were synthesized in this work to study the interplay between these two parameters. Aerogels are covalent organic frameworks with ultra-low density, high porosity, and large surface area. The relative ease of tuning the composition and pore structure of aerogels make them prominent candidates for catalysis. Herein, we report on a tunable Fe-N-C catalyst based on an Fe porphyrin aerogel, which shows high electrocatalytic oxygen reduction reaction activity with tunable hierarchical pore structure and studied the influence of the porous structure on the overall performance in proton exchange membrane fuel cells.

The synergy between electrochemical substrate oxidation and the oxygen reduction reaction to enable aerobic oxidation

Aerobic substrate oxidation reactions catalyzed by a heterogeneous catalyst can be looked upon as two independent half-cell reactions, viz. anodic substrate oxidation and the cathodic oxygen reduction reaction (ORR). In this context, Fe PANI/C, a well-known catalyst for the ORR, is chosen to validate this hypothesis, wherein the anodic reaction is hydrazine oxidation. Fe PANI/C shows excellent activity in terms of the electrochemical ORR and hydrazine oxidation in both alkaline aqueous and non-aqueous media and taken together the aerobic oxidation efficacy of hydrazine-like small organic molecules is correlated with the electrochemical outcomes.

Molecular Geometry and Electronic Structure of Copper Corroles

Copper corroles are known for their unique multiconfigurational electronic structures in the ground state, which arise from the transfer of electrons from the π orbitals of the corrole to the d-orbital of copper. While density functional theory (DFT) provides reasonably good molecular geometries, the determination of the ground spin state and the associated energetics is heavily influenced by functional choice, particularly the percentage of the Hartree-Fock exchange. Using extended multireference perturbation theory methods (XMS-CASPT2), the functional choice can be assessed. The molecular geometries and electronic structures of both the unsubstituted and the meso-triphenyl copper corroles were investigated. A minimal active space was employed for structural characterization, while larger active spaces are required to examine the electronic structure. The XMS-CASPT2 investigations conclusively identify the ground electronic state as a multiconfigurational singlet (S0) with three dominant electronic configurations in its lowest energy and characteristic saddled structure. In contrast, the planar geometry corresponds to the triplet state (T0), which is approximately 5 kcal/mol higher in energy compared to the S0 state for both the bare and substituted copper corroles. Notably, the planarity of the T0 geometry is reduced in the substituted corrole compared with that in the unsubstituted one. By analyzing the potential energy surface (PES) between the S0 and T0 geometries using XMS-CASPT2, the multiconfigurational electronic structure is shown to transition toward a single electron configuration as the saddling angle decreases (i.e., as one approaches the planar geometry). Despite the ability of the functionals to reproduce the minimum energy structures, only the TPSSh-D3 PES is reasonably close to the XMS-CASPT2 surface. Significant deviations along the PES are observed with other functionals.

Kinetic and mechanistic investigations of dioxygen reduction by a molecular Cu(II) catalyst bearing a pentadentate amidate ligand

A pentadentate Cu(II) complex, [CuII(dpaq)](ClO4) (1), featuring a redox active ligand, H-dpaq (H-dpaq = 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), catalyses four-electron reduction of dioxygen by decamethylferrocene (Fc*) in the presence of trifluoroacetic acid (CF3COOH) in acetone at 298 K. No catalytic oxygen reduction was observed in the presence of stronger Brønsted acids than CF3COOH, such as perchloric acid (HClO4) or trifluoromethanesulphonic acid (HOTf). In contrast, facile catalytic reduction of O2 occurs by Fc* with 1 and HClO4 or HOTf in dimethylformamide (DMF). The use of CF3COOH as the proton source in DMF results in the suppression of O2 reduction under otherwise identical reaction conditions. While the O2 reduction reactions in DMF are linearly dependent on the pKa of Brønsted acids, the acid dependence on catalytic O2-reduction reactivity by 1 in acetone showed complete reversal. Cyclic voltammetry studies using p-chloranil as the probe substrates in the presence of acids in the solvents reveal that the strengths of the protonic acids increase significantly in acetone compared to that in DMF. The amidate-N in [CuII(dpaq)](ClO4) (1) undergoes protonation in the presence of HClO4 or HOTf in DMF to form [CuII(H-dpaq)]2+ (1-H+), but not in the presence of CF3COOH. Enhanced acid strength of CF3COOH in acetone, however, effectively protonates 1 and triggers O2 reduction. Protonation of 1 with HClO4 or HOTf in acetone results in the change of its coordination environment, and this protonated species does not trigger O2 reduction. Detailed kinetic studies indicate that 1-H+ undergoes reduction by two-electrons and the reduced species binds O2 to form a Cu(II)-superoxo intermediate. This is followed by a rate-determining proton-coupled electron-transfer (PCET) reduction to generate the Cu(II)-hydroperoxo intermediate. While catalytic O2 reduction in acetone occurs predominantly via a 4e-/4H+ pathway, product selectivity (H2O vs. H2O2) in DMF depends upon the concentration of the reductant (Fc*). While dioxygen reduction to H2O2 is favoured at low [Fc*], mechanistic studies suggest that O2 reduction with high [Fc*] proceeds via a [2e- + 2e-] mechanism, where the released H2O2 during catalysis is further reduced to water.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Cyclodextrin metal-organic frameworks and derivatives: recent developments and applications

While there is a tremendous amount of scientific research on metal organic frameworks (MOFs) for gas storage/separation, catalysis and energy storage, the development and application of biocompatible MOFs still poses major challenges. In general, they can be synthesised from various biocompatible linkers and metal ions but particularly cyclodextrins (CDs) as cyclic oligosaccharides are an astute choice for the former. Although the field of CD-MOF materials is still in the early stages and their design and fabrication comes with many hurdles, the benefits coming from CDs built in a porous framework are exciting. Versatile host-guest complexation abilities, high encapsulation capacity and hydrophilicity are among the valuable properties inherent to CDs and offer extended and novel applications to MOFs. In this review, we provide an overview of the state-of-the-art synthesis, design, properties and applications of these materials. Initially, a rationale for the preparation of CD-based MOFs is provided, based on the chemical and structural properties of CDs and including their advantages and disadvantages. Further on, the review exhaustively surveys CD-MOF based materials by categorising them into three sub-classes, namely (i) CD-MOFs, (ii) CD-MOF hybrids, obtained via combination with external materials, and (iii) CD-MOF-derived materials prepared under pyrolytic conditions. Subsequently, CD-based MOFs in practical applications, such as drug delivery and cancer therapy, sensors, gas storage, (enantiomer) separations, electrical devices, food industry, and agriculture, are discussed. We conclude by summarizing the state of the art in the field and highlighting some promising future developments of CD-MOFs.

Achieving a highly efficient oxygen reduction reaction via a molecular Fe single atom catalyst

Molecular Fe phthalocyanine (FePc) is successfully anchored on a defective mesoporous carbon framework for the highly efficient oxygen reduction reaction (ORR) with a half-wave potential of 0.86 V (vs. RHE) and a limited current density of 5.40 mA cm^-2. DFT calculations further suggest that the non-planar structure incorporating FePc can promote charge polarization and decrease the energy barrier.

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.

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.

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.

Platinum Group Metal-Free Catalysts for Oxygen Reduction Reaction: Applications in Microbial Fuel Cells

Scientific and technological innovation is increasingly playing a role for promoting the transition towards a circular economy and sustainable development. Thanks to its dual function of harvesting energy from waste and cleaning up waste from organic pollutants, microbial fuel cells (MFCs) provide a revolutionary answer to the global environmental challenges. Yet, one key factor that limits the implementation of larger scale MFCs is the high cost and low durability of current electrode materials, owing to the use of platinum at the cathode side. To address this issue, the scientific community has devoted its research efforts for identifying innovative and low cost materials and components to assemble lab-scale MFC prototypes, fed with wastewaters of different nature. This review work summarizes the state-of the-art of developing platinum group metal-free (PGM-free) catalysts for applications at the cathode side of MFCs. We address how different catalyst families boost oxygen reduction reaction (ORR) in neutral pH, as result of an interplay between surface chemistry and morphology on the efficiency of ORR active sites. We particularly review the properties, performance, and applicability of metal-free carbon-based materials, molecular catalysts based on metal macrocycles supported on carbon nanostructures, M-N-C catalysts activated via pyrolysis, metal oxide-based catalysts, and enzyme catalysts. We finally discuss recent progress on MFC cathode design, providing a guidance for improving cathode activity and stability under MFC operating conditions.

Assessing electron transfer reactions and catalysis in multicopper oxidases with operando X-ray absorption spectroscopy

Here we propose an experimental setup based on operando X-ray absorption spectroscopy (XAS) to understand why copper-containing oxidoreductase enzymes show exceptional performance as catalysts for the oxygen reduction reaction (ORR). An electrode based on carbon nanoparticles organized in mesoporous structures with bilirubin oxidase (BOD) was developed to be used in a home-made operando XAS electrochemical cell, and we probed the electron transfer under ORR regime. In the presence of molecular oxygen, the BOD cofactor containing 4 copper ions require an overpotential about 150 mV to be reduced as compared to that in the absence of oxygen. A second electron transfer step, which occurs faster than the cofactor reduction, suggests that the cooper ions act as a tridimensional redox active electronic bridges for the electron transfer reaction.

Understanding enzyme active sites can elucidate fundamental enzymatic reaction pathways and inform designs for synthetic catalysts. Here, authors employ operando X-ray absorption spectroelectrochemistry to assess copper ions in bilirubin oxidase during oxygen reduction electrocatalysis.

Enhancement of the oxygen reduction reaction electrocatalytic activity of metallo-corroles using contracted cobalt(iii) CF3-corrole incorporated in a high surface area carbon support

Molecular ORR catalysts based on metallo-corrole with the smallest meso-substituent reported to date, Co(iii)CF₃-corrole, was synthesized and compared to the well-studied Co(iii)tpf-corrole when adsorbed on a high surface area carbon support.

Rotaxanation as a sequestering template to preclude incidental metal insertion in complex oligochromophores

Rotaxane as sacrificial template to avoid metal insertion in porphyrinoids during copper catalyzed click reaction.

Nature-inspired electrocatalysts and devices for energy conversion

A NICE approach for the design of nature-inspired electrocatalysts and electrochemical devices for energy conversion.

In situ immobilization of isolated Pd single-atoms on graphene by employing amino-functionalized rigid molecules and their prominent catalytic performance

The isolated Pd single atoms anchored on graphene demonstrate a catalytic activity that is 21.3 times higher than that of Pd/C in the RhB hydrogenation reaction.