Porphyrin-based frameworks, as specific kinds of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in energy-related conversion processes, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and CO2 reduction reaction (CO2RR), and also in energy-related storage technologies such as rechargeable Zn-air batteries. This review starts by summarizing typical crystal structures, molecular building blocks, and common synthetic procedures of various porphyrin-based frameworks used in energy-related technologies. Then, a brief introduction is provided and representative applications of porphyrin-based frameworks in ORR, OER, Zn-air batteries, and CO2RR are discussed. The performance comparison of these porphyrin-based frameworks in each field is also summarized and discussed, which pinpoints a clear structure-activity relationship. In addition to utilizing highly active porphyrin units for catalytic conversions, regulating the porous structures of porphyrin-based frameworks will enhance mass transfer and growing porphyrin-based frameworks on conductive supports will accelerate electron transfer, which will result in the improvement of the electrocatalytic performance. This review is therefore valuable for the rational design of more efficient porphyrin-based framework catalytic systems in energy-related conversion and storage technologies.

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

Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO2 electrochemical and visible-light-driven reduction. Since the first reports in the 1970s, an enormous body of work has been accumulated regarding the two-electron two-proton reduction of the gas, along with mechanistic and spectroscopic efforts to rationalize the reactivity and establish guidelines for structure-reactivity relationships. The ability to fine tune the ligand structure and the almost unlimited possibilities of designing new complexes have led to highly selective and efficient catalysts. Recent efforts toward developing hybrid systems upon combining molecular catalysts with conductive or semi-conductive materials have converged to high catalytic performances in water solutions, to the inclusion of these catalysts into CO2 electrolyzers and photo-electrochemical devices, and to the discovery of catalytic pathways beyond two electrons. Combined with the continuous mechanistic efforts and new developments for in situ and in operando spectroscopic studies, molecular catalysis of CO2 reduction remains a highly creative approach.

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Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes

Conversion of CO2 into valuable molecules is a field of intensive investigation with the aim of developing scalable technologies for making fuels using renewable energy sources. While electrochemical reduction into CO and formate are approaching industrial maturity, a current challenge is obtaining more reduced products like methanol. However, literature on the matter is scarce, and even more for the use of molecular catalysts. Here, we demonstrate that cobalt phthalocyanine, a well-known catalyst for the electrochemical conversion of CO2 to CO, can also catalyze the reaction from CO2 or CO to methanol in aqueous electrolytes at ambient conditions of temperature and pressure. The studies identify formaldehyde as a key intermediate and an unexpected pH effect on selectivity. This paves the way for establishing a sequential process where CO2 is first converted to CO which is subsequently used as a reactant to produce methanol. Under ideal conditions, the reaction shows a global Faradaic efficiency of 19.5 % and chemical selectivity of 7.5 %.

Aqueous Electrochemical Reduction of Carbon Dioxide and Carbon Monoxide into Methanol with Cobalt Phthalocyanine

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Transition Metal Complexes as Catalysts for the Electroconversion of CO 2 : An Organometallic Perspective

ConspectusThe electrocatalytic CO2 reduction reaction (CO2RR) to generate fixed forms of carbons that have commercial value is a lucrative avenue to ameliorate the growing concerns about the detrimental effect of CO2 emissions as well as to generate carbon-based feed chemicals, which are generally obtained from the petrochemical industry. The area of electrochemical CO2RR has seen substantial activity in the past decade, and several good catalysts have been reported. While the focus was initially on the rate and overpotential of electrocatalysis, it is gradually shifting toward the more chemically challenging issue of selectivity. CO2 can be partially reduced to produce several C1 products like CO, HCOOH, CH3OH, etc. before its complete 8e-/8H+ reduction to CH4. In addition to that, the low-valent electron-rich metal centers deployed to activate CO2, a Lewis acid, are prone to reduce protons, which are a substrate for CO2RR, leading to competing hydrogen evolution reaction (HER). Similarly, the low-valent metal is prone to oxidation by atmospheric O2 (i.e., it can catalyze the oxygen reduction reaction, ORR), necessitating strictly anaerobic conditions for CO2RR. Not only is the requirement of O2-free reaction conditions impractical, but it also leads to the release of partially reduced O2 species such as O2-, H2O2, etc., which are reactive and result in oxidative degradation of the catalyst.In this Account, mechanistic investigations of CO2RR by detecting and, often, chemically trapping and characterizing reaction intermediates are used to understand the factors that determine the selectivity in CO2RR. The spectroscopic data obtained from different intermediates have been identified in different CO2RR catalysts to develop an electronic structure selectivity relationship that is deemed to be important for deciding the selectivity of 2e-/2H+ CO2RR. The roles played by the spin state, hydrogen bonding, and heterogenization in determining the rate and selectivity of CO2RR (producing only CO, only HCOOH, or only CH4) are discussed using examples of both iron porphyrin and non-heme bioinspired artificial mimics. In addition, strategies are demonstrated where the competition between CO2RR and HER as well as CO2RR and ORR could be skewed overwhelmingly in favor of CO2RR in both cases.

Selectivity in Electrochemical CO2 Reduction.

Metallo-porphyrin complexes such as cobalt and iron porphyrins (CoP and FeP) have shown potential as electrocatalysts for CO2 reduction. Here we report that introducing amino substituents enhances ...

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Enhanced Electrochemical Reduction of CO2 Catalyzed by Cobalt and Iron Amino Porphyrin Complexes

Enhanced electrocatalytic activity of iron amino porphyrins using a flow cell for reduction of CO2 to CO

Electrochemical reduction of carbon dioxide (CO₂) is a sustainable solution to conversion of CO₂ into value-added products such as hydrocarbons and carbon monoxide (CO). However, designing high-efficiency molecule-based electrocatalysts is challenging. In this work, we designed and synthesized iron-porphyrin-pyridine (Fe-TPPy) catalysts in a strategy that combined molecular design and a nanoscale approach. The catalytic activity of these compounds toward CO₂ reduction was evaluated under both homogeneous and heterogeneous conditions. Tuning of Fe-TPPy with anisole electron-donating substituents improved the catalytic efficiency up to 76% with a current density of −1.3 mA/cm² and a turnover frequency (TOF) of 1 s–¹. The faradaic efficiency was further enhanced to 92% with a current density of −30 mA/cm² and a TOF of 5 s–¹ after immobilization of the porphyrins onto carbon nanotubes. Density functional theory calculations confirmed that the push–pull pyridine–anisole interaction facilitates CO₂ binding, resulting in an enhancement of the overall catalytic efficiency. This work provides an effective strategy for improvement of electrocatalytic performance that could inspire the design of future molecular catalysts.

Electrocatalytic Reduction of CO2 to CH4 and CO in Aqueous Solution Using Pyridine-Porphyrins Immobilized onto Carbon Nanotubes

Enhanced Electrochemical Reduction of CO2 to CO upon Immobilization onto Carbon Nanotubes Using an Iron-Porphyrin Dimer

The ever-growing level of carbon dioxide (CO2) in our atmosphere, is at once a threat and an opportunity. The development of sustainable and cost-effective pathways to convert CO2 to value-added chemicals is central to reducing its atmospheric presence. Electrochemical CO2 reduction reactions (CO2RRs) driven by renewable electricity are among the most promising techniques to utilize this abundant resource; however, in order to reach a system viable for industrial implementation, continued improvements to the design of electrocatalysts is essential to improve the economic prospects of the technology. This review summarizes recent developments in heterogeneous porphyrin-based electrocatalysts for CO2 capture and conversion. We specifically discuss the various chemical modifications necessary for different immobilization strategies, and how these choices influence catalytic properties. Although a variety of molecular catalysts have been proposed for CO2RRs, the stability and tunability of porphyrin-based catalysts make their use particularly promising in this field. We discuss the current challenges facing CO2RRs using these catalysts and our own solutions that have been pursued to address these hurdles.

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Caitlin Dao

Papers

Enhanced Electrochemical Reduction of CO2 Catalyzed by Cobalt and Iron Amino Porphyrin Complexes

Enhanced electrocatalytic activity of iron amino porphyrins using a flow cell for reduction of CO2 to CO

Electrocatalytic Reduction of CO2 to CH4 and CO in Aqueous Solution Using Pyridine-Porphyrins Immobilized onto Carbon Nanotubes

Enhanced Electrochemical Reduction of CO2 to CO upon Immobilization onto Carbon Nanotubes Using an Iron-Porphyrin Dimer

Immobilization strategies for porphyrin-based molecular catalysts for the electroreduction of CO2