Porphyrin-based porous organic polymers synthesized using the Alder-Longo method: the most traditional synthetic strategy with exceptional capacity

Porphyrin is a typical tetrapyrrole chromophore-based pigment with a special electronic structure and functionalities, which is frequently introduced into various porous organic polymers (POPs). Porphyrin-based POPs are widely used in various fields ranging from environmental and energy to biomedicine-related fields. Currently, most porphyrin-based POPs are prepared via the copolymerization of specific-group-functionalized porphyrins with other building blocks, in which the tedious and inefficient synthesis procedure for the porphyrin greatly hinders the development of such materials. This review aimed to summarize information on porphyrin-based POPs synthesized using the Alder-Longo method, thereby skipping the complex synthesis of porphyrin-bearing monomers, in which the porphyrin macrocycles are formed directly via the cyclic tetramerization of pyrrole with monomers containing multiple aldehyde groups during the polymerization process. The representative applications of porphyrin-based POPs derived using the Alder-Longo method are finally introduced, which pinpoints a clear relationship between the structure and function from the aspect of the building blocks used and porous structures. This review is therefore valuable for the rational design of efficient porphyrin-based porous organic polymer systems that may be utilized in various fields from energy-related conversion/storage technologies to biomedical science.

Fe-Ni porphyrin/mesoporous titania thin film electrodes: a bioinspired nanoarchitecture for photoelectrocatalysis

Porphyrin and porphyrinoid derivatives have been extensively studied in the assembly of catalysts and sensors, seeking biomimetic and bioinspired activity. In particular, Fe and Ni porphyrins can be used for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by immobilization of these molecular catalysts on semiconductor materials. In this study, we designed a hybrid material containing a crystalline mesoporous TiO2 thin film in which the catalytic centres are Ni-porphyrin (NiP), Fe-porphyrin (FeP), and a NiP/FeP bimetallic system to assess whether the coexistence of both metalloporphyrins improves the OER activity. The obtained photoelectrodes were physicochemically and morphologically characterized through high-resolution FE-SEM images, UV-vis and Raman spectroscopies, cyclic voltammetry, and impedance measurements. The results show a differential behavior of the mono- and bimetallic porphyrin systems, where the Fe(iii) centre in FeP may increase the acidity and lower the reduction potential of the Ni2+/3+ couple when co-deposited with NiP leading to an improved photoelectrochemical water-oxidation performance. We have validated the cooperative effect of both metal complexes within this novel system, where the μ-peroxo-bridged interaction between Fe and Ni is integrated into a supramolecular heterometallic structure of porphyrins.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Ni(II)-Incorporated Ethylene Glycol-Linked Tetraphenyl Porphyrin-Based Covalent Organic Polymer as a Catalyst for Methanol Electrooxidation

Methanol oxidation reaction (MOR) is a perfect alternative to the conventional oxygen evolution reaction (OER), generally utilized as the anode reaction for hydrogen generation via the electrochemical water splitting method. Moreover, MOR is also relevant to direct methanol fuel cells (DMFCs). These facts motivate the researchers to develop economical and efficient electrocatalysts for MOR. Herein, we have introduced an ethylene glycol-linked tetraphenyl porphyrin-based (EG-POR) covalent organic polymer (COP). The Ni(II)-incorporated EG-POR material Ni-EG-POR displayed excellent OER and MOR activities in an alkaline medium. The materials were thoroughly characterized using 13C solid-state NMR, Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) surface area analyzer, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric analyzer (TGA), and powder X-ray diffraction (PXRD) techniques. These organic-inorganic hybrid materials showed high chemical and thermal stability. Ni-EG-POR requires an overpotential of 400 mV (vs RHE) in OER and 190 mV (vs RHE) in MOR to achieve a current density of 10 mA cm-2. In addition, the catalyst also showed excellent chronoamperometric and chronopotentiometric stability, indicating that the catalyst can provide stable current over a longer period and its potential as a non-noble metal MOR catalyst.

Recent Advances in Porphyrin-Based Systems for Electrochemical Oxygen Evolution Reaction

Oxygen evolution reaction (OER) plays a pivotal role in the development of renewable energy methods, such as water-splitting devices and the use of Zn–air batteries. First-row transition metal complexes are promising catalyst candidates due to their excellent electrocatalytic performance, rich abundance, and cheap price. Metalloporphyrins are a class of representative high-efficiency complex catalysts owing to their structural and functional characteristics. However, OER based on porphyrin systems previously have been paid little attention in comparison to the well-described oxygen reduction reaction (ORR), hydrogen evolution reaction, and CO2 reduction reaction. Recently, porphyrin-based systems, including both small molecules and porous polymers for electrochemical OER, are emerging. Accordingly, this review summarizes the recent advances of porphyrin-based systems for electrochemical OER. Firstly, the electrochemical OER for water oxidation is discussed, which shows various methodologies to achieve catalysis from homogeneous to heterogeneous processes. Subsequently, the porphyrin-based catalytic systems for bifunctional oxygen electrocatalysis including both OER and ORR are demonstrated. Finally, the future development of porphyrin-based catalytic systems for electrochemical OER is briefly prospected.

Electrocatalytic water oxidation performance in an extended porous organic framework with a covalent alliance of distinct Ru sites

The rational synthesis of durable, earth-abundant efficient electrocatalysts for the oxygen evolution reaction (OER) from water is one of the most important routes for storing renewable energy and minimizing fossil fuel combustion. The prime hurdles for effectively utilizing commercial RuO₂ as (OER) electrocatalysts are its very low stability, catalyst deactivation, and high cost. In this work, we explored a Ru-integrated porous organic polymer (Ru@Bpy-POP) by a facile one-pot Friedel-Crafts alkylation strategy between redox-active (Ru(demob)3Cl2) and a carbazole unit, which is composed of unique features including an extended framework unit, isolated active sites, and tunable electrode kinetics. Ru@Bpy-POP can serve as a bridge between a Metal-Organic Framework (MOF) and POP-based catalytic systems with a balanced combination of covalent bonds (structural stability) and open metal sites (single site catalysis). Ru@Bpy-POP, deposited on a three-dimensional nickel foam electrode support, exhibits a promising electrocatalytic OER activity with an ultra-low ruthenium loading compared to a benchmark RuO₂ catalyst, providing an overpotential of about 270 mV to reach 10 mA cm^-2 in an alkaline medium. Moreover, a high current density of 248 mA cm^-2 was achieved for the Ru@Bpy-POP catalyst at only 1.6 V (vs. RHE), which is much higher than 91 mA cm^-2 for commercial RuO₂. The robust, albeit highly conjugated, POP framework not only triggered facile electro-kinetics but also suppressed aggregation and metallic corrosion during electrolysis. In particular, the benefits of covalent integration of distinct Ru sites into the framework can modulate intermediate adsorption and charge density, which contributes to its exceptional OER activity. All of the critical steps involved in OER are complemented by Density Functional Theory (DFT) calculations, which suggest that electrocatalytic water oxidation proceeds from a closed-shell configuration to open-shell electronic configurations with high-spin states. These open-shell configurations are more stable than their closed-shell counterparts by 1 eV, improving the overall catalytic activity.

Porous organic polymers for electrocatalysis

Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, N₂ reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.