On May 23, 2022, Professor Pang Huan of Yangzhou University and Professor Yusuke Yamauchi of the University of Queensland, an academic journal founded by Tsinghua University, published a review paper entitled “Porphyrin-based framework materials for energy conversion” at Nano Research Energy. The latest advances in the application of porphyrin-based skeleton materials in electrocatalysis and photocatalysis are introduced in detail.
Due to the demands of modern agriculture and industry, global energy consumption has increased exponentially over the past few decades. Energy conversion technologies based on electrocatalysis and photocatalysis have been studied more widely than ever before, and they have great potential to alleviate energy shortages and solve environmental pollution, providing a guarantee for the sustainable development of social development. Although electrocatalysis and photocatalytic techniques have been extensively studied and achieved some results in recent years, there are still some shortcomings. Porphyrin-based skeleton material has rich metal active sites, adjustable crystal structure, high specific surface area and other characteristics, porphyrin-based skeleton material combines the advantages of homogeneous and heterogeneous catalysts, overcomes the shortcomings of traditional electrocatalysis and photocatalytic technology, and has broad application prospects in the field of electrocatalysis and photocatalysis.
In this article, the Pang Huan Group & Yusuke Yamauchi Group briefly introduces the types of porphyrin structural blocks. They can be used as a linking agent for frame structures or can be assembled directly with the skeleton material into a high-performance catalyst. The latest research progress of porphyrin-based frame material catalysts in electrocatalysis and photocatalysis is summarized and introduced.
1) The author first gave a brief introduction to metal-organic frameworks (porous coordination polymers), covalent organic frameworks, porphyrins, metalloporphyrins, and porphyrin-based frame materials.
2) The authors then summarize and discuss the latest advances in the application of porphyrin-based skeleton materials in electrocatalysis and photocatalysis. Porphyrin-based skeleton materials mainly summarize the original porphyrin fund organic framework, porphyrin fund organic framework complex, porphyrin fund organic framework derivatives, and original porphyrin-based covalent organic framework porphyrin-based covalent organic framework complex. Electrocatalysis mainly summarizes electrocatalytic water decomposition, electrocatalytic carbon dioxide reduction, and electrocatalytic oxygen reduction reactions. Photocatalysis mainly summarizes photocatalytic water decomposition and photocatalytic carbon dioxide reduction.
3) The author finally put forward the key difficulties and challenges that porphyrin-based skeleton materials still face in electrocatalysis and photocatalysis, and gives theoretically feasible solutions for each difficulty and challenge, while the author also proposes the future development opportunities of porphyrin-based skeleton materials, and is full of high expectations for the future vigorous development of porphyrin-based skeleton materials.
Brief introduction of porphyrin frame material
Porphyrin is a macromolecular heterocyclic material that is replaced by various functional groups by porphyrins (C20H14N4) in the middle or β position. Porphyrin can be combined with many metal ions or secondary building units (SBUs) in the center of porphyrin to form metal composites, also known as metal porphyrins. The structure of the porphyrin nucleus, porphyrin, and metalloporphyrin is shown in Figure 1.
Fig. 1 Structural diagram of the porphyrin nucleus, porphyrin and metalloporphyrin
MOFs and COFs, which are directly coordinated with metal ions or SBUs as connectomes, are known as metal porphyrin frames (MPFs) and covalent porphyrin frames (CPFs). On this basis, it can be assembled with other functional materials into related composites, and related derivatives can be synthesized by methods such as calcination. In addition to being applied to skeleton materials as organic ligands, porphyrins or metal porphyrins can also be integrated into the skeleton material as guest molecules by microporous encapsulation and surface adsorption or grafting, called porphyrin @ frame composites. After decades of development, porphyrin-based skeleton materials have been extensively studied and generally exhibit multiple functions. Porphyrin-based skeleton materials are widely used in catalytic reactions because of their simple synthesis and good chemical stability. Most of the reported examples of porphyrin-based organic binders for frame materials are shown in Figure 2.
Fig. 2 Common porphyrin-based frame material organic ligators.
Recent advances in the application of porphyrin-based skeleton materials in electrocatalysis and photocatalysis
The authors summarize the latest advances in the application of porphyrin-based skeleton materials in electrocatalysis and photocatalysis (Figure 3).
Fig. 3 Porphyrin-based skeleton materials are applied in electrocatalysis and photocatalysis
1) Electrocatalytic water decomposition
Electrocatalytic water decomposition is a promising high-efficiency high-purity H2 and O2 renewable energy conversion method. Electrocatalytic water decomposition consists of two basic reactions: electrochemical hydrogen evolution (HER) and oxygen evolution (OER). Currently, platinum-based materials are the most effective HER electrocatalysts. At the same time, the current OER process uses an efficient precious metal material to reduce the high overpotential caused by the slow transfer of four electrons. However, the high price of precious metal catalysts limits the development of electrocatalytic water decomposition. Low-cost MPFs/CPFs and their composites/derivatives have broad application prospects in electrocatalytic energy conversion due to their large specific surface area and uniformly distributed active metal potentials (Figure 4). Professor Qiu Fengxian and Professor Karl M. Kadish collaborated to report a HER electrocatalyst prepared by in situ growth of MoS2-Pt nanosheets on rose-shaped Cu-TCPP MOF-derived sulfur-nitrogen-doped carbon-based substrates. There is a heterogeneous interface and tight interaction between MoS2 (MoS2-Pt) and the sulfur-nitrogen-doped carbon-based substrate, which facilitates rapid electron transfer and conductivity. In the Heyrovsky step, good hydrophilicity and well-designed porous structures also facilitate the transport of the substance. They increase the probability of collisions between the active site and the intermediate (H*), resulting in fast HER efficiency. CuSNC@MoS2-Pt electrocatalyst has good HER electrocatalytic activity in alkaline environments, and the Tafel slope is 55.7 mV dec-1.
Fig. 4 Application of porphyrin-based skeleton material in electrocatalytic water decomposition
2) Electrocatalytic carbon dioxide reduction
Large amounts of carbon dioxide emissions cause global warming and cause serious damage to the ecological environment. Hence the emergence of strategies to reduce greenhouse gases. In existing strategies, electrocatalytic reduction of carbon dioxide due to its operating ambient temperature and pressure, as well as effective control of the target product by adjusting the application potential, is a promising approach. Electrocatalytic carbon dioxide has a variety of pathways to produce different products, such as C1 (CO, CH4, etc.), C2 (C2H4, C2H5OH, etc.), C3 (CH3CH2CH2OH, etc.). Among these pathways, reduction of CO2 to CO is currently one of the most promising methods due to its high selectivity and high current density, as well as the ease of separating gaseous products from liquid water. Molecular catalysts centered on non-precious metals porphyrin and metalloporphyrin have been extensively studied due to their inherent macrocyclic ligand structure and adjustable metal centers of oxidation state, thereby reducing the overpotential and promoting the conversion of CO2 to CO. Porphyrin skeleton materials based on porphyrins and metal porphyrins have attracted interest for their excellent electrocatalytic properties for electrocatalytic carbon dioxide reduction (Figure 5). Professors Liu Honglai, Zhuang Xiaodong and Xu Qiang reported a coronary ether cobalt porphyrin COF (TAPP(Co)-B18C6-COF)) for electrocatalytic carbon dioxide reduction. The coronal ether units incorporated in COFs not only enhance the hydrophilicity of the framework, but also promote the electron transfer of the co-porphyrin nucleus. In addition, the crown ether unit enhances the binding capacity of carbon dioxide. This work provides new insights into COFs and electrocatalysis.
Fig. 5 Application of porphyrin-based skeleton material in electrocatalytic carbon dioxide reduction
3) Electrocatalytic oxygen reduction
Fuel cells and metal-air batteries are among the most promising devices in the new generation of energy conversion technologies to meet our growing energy needs. However, the overall efficiency of these two excellent energy conversion devices is severely limited by the oxygen reduction reaction (ORR). In ORR, molecular oxygen is electrochemically reduced to H2O in an acidic solution or OH- in an alkaline solution by the four-electron pathway, or to H2O2 in an acidic solution or HO2- in an alkaline solution by the two-electron pathway as an intermediate component. The four-electron pathway has higher kinetics and efficiency than the two-electron pathway. In addition, platinum-based materials are ideal for four-electron orr electrocatalysts in practical applications, but due to the scarcity, expensiveness, and low durability of platinum catalysts, the widespread use of ORR devices is limited. Given these constraints, the development of alternative energy sources rich in the earth’s resources, highly reactive and durable is a vital and challenging task. Porphyrin-based MOFs and COFs have proven to be highly competitive electrocatalysts for ORR due to their high specific surface area, adjustable chemical composition, numerous active sites, and adjustable pore size and topology (Figure 6). Prof. Cao reported that MOF-loaded cobalt porphyrin was used for ORR, improving activity and selectivity. Cobalt porphyrins can be grafted to the MOF surface by ligand exchange. Porphyrin @MOF complexes were prepared by this method. Compared with ungranched porphyrins, grafted Coporphyrin has a large half-wave potential offset (>70 mV), enhancing ORR activity. Peroxide reduction was performed using active MOFs, and the number of electrons transferred per O2 was increased from 2.65 to 3.70, and the selectivity of the four electron ORR was significantly improved.
Fig. 6 Application of porphyrin-based skeleton material in electrocatalytic oxygen reduction
4) Photocatalytic water decomposition
The discovery of water-decomposing photocatalysts dates back to 1972, Fujishima and Honda. The semi-conductive properties of TiO2 were discovered, which can degrade water to H2 and O2 under ultraviolet light. Based on this discovery, the application of semiconductor materials in photochemical water decomposition has been widely studied, including metal oxides, metal nitrides and metal sulfides. The principle of photocatalytic separation of water is shown in Figure 7. However, such semiconductor materials still have great shortcomings, such as poor stability, low yield, low quantum efficiency, and most importantly, slow photocatalytic activity, which cannot effectively promote the decomposition of water. The development of a new type of photocatalyst with high photocatalytic activity under visible light illumination is one of the hot spots in the study of photocatalytic hydrolysis. The central role of porphyrin groups in natural photosynthesis inspired this research, proposing their application as molecular and supramolecular light collection systems. On this basis, the application of porphyrin-based skeleton materials in photocatalytic water decomposition has been widely studied. However, research on porphyrin-based frame materials in photocatalytic water decomposition has focused on MPFs and their associated composites as potential next-generation semiconductors. In addition, the Z type is a system that uses two photocatalysts to split water into hydrogen. Since the electron transfer during this reaction is A-type, it is called a Z-type reaction (Figure 7). Z-type photocatalytic system is an effective photolyzysed water hydrogen production system developed in recent years. It can transfer carriers under the impetus of the interfacial electric field, achieving an increase in quantum efficiency.
Fig. 7 Application of porphyrin-based skeleton materials in photocatalytic water decomposition
5) Photocatalytic carbon dioxide reduction
In nature, plants or microbes capture sunlight as an energy supply and use it to convert carbon dioxide into energy carriers and oxygen. Inspired by natural photosynthetic systems, artificial photosynthesis is considered a promising way to enable the conversion from solar to chemical energy, both for energy regeneration and for mitigating the impact of carbon dioxide on climate change. The inorganic semiconductor materials described above also contribute to photocatalytic carbon dioxide conversion. For photocatalytic carbon dioxide reduction, these materials have a large band gap under visible light conditions and a limited light absorption range, which is not conducive to practical applications. In recent years, the application of porphyrin-based frame materials in photocatalytic carbon dioxide reduction has been widely studied due to its inherent large surface area, controllable pore size and high density of catalytic active sites (Figure 8). Professor Jianrong Li reports on a strategy to enhance the stability and functionality of unstable Zr(IV)-MOFs by in situ porphyrin substitution. The addition of metalloporphyrin gives BUT-110 MOFs a high photoretrivation activity of catalytic carbon dioxide without photosensitizers.
Fig. 8 Application of porphyrin-based skeleton material in photocatalytic carbon dioxide reduction
Difficult challenges and solution strategies faced by porphyrin-based skeleton materials
Porphyrin-based skeleton material has a large pore size, open pore channels and high porosity, which improves the mass transfer performance. Although a great deal of research has been done on porphyrin-based frame materials as highly efficient electrical/photocatalysts, most studies are still in the early experimental stages. For practical applications, there are still problems and bottlenecks that need to be solved.
1) Developing efficient non-precious metal porphyrin skeleton material catalysts to replace precious metal catalysts remains a significant challenge and requires significant effort to modify the precursors of frame materials to achieve specific applications of electrical/photocatalysis. In addition, the electrical/photocatalytic properties of porphyrin-based skeleton materials are far from reaching the actual industrial level.
2) The design and construction of porphyrin blocks relies primarily on highly symmetrical guided designs, which limits the diversity of the porphyrin framework family and affects their potential catalytic applications. Understanding the structure-activity relationship of porphyrin skeleton materials is of great significance for the design and synthesis of porphyrin-based skeleton materials.
3) In practical applications, due to the harsh catalytic conditions, it is particularly important to improve the structure and long-term mechanical stability of the porphyrin-based skeleton material. In addition, partially decomposed porphyrin MOFs should be further studied to understand their reaction mechanism and true active sites. It is difficult to obtain an accurate crystal structure for porphyrin COFs prepared by the one-pot method, so the influence of amorphous porphyrin COFs on their catalytic activity and reaction mechanism is still a big challenge.
4) Due to the high cost of preparation of porphyrin skeleton materials, there is an urgent need to develop new synthetic methods. Since the cost of porphyrin-based skeleton materials is an important factor to consider in large-scale industrial applications, reducing synthetic steps is also an important research direction in the future. However, it is extremely difficult to develop new synthetic paths and reduce synthetic steps.
Although the above challenges currently exist in porphyrin-based frame materials, as research progresses, a variety of strategies can be employed to address these challenges.
1) The design and development of novel porphyrin-based backbone materials can significantly enhance their catalytic activity by increasing the exposure of catalytically active sites. First, by adjusting the porosity of the contact active site, the mass transfer process of the porphyrin-based backbone material can be enhanced. Secondly, the growth of porphyrin-based frame materials on a conductive substrate can enhance the electron transfer process by constructing a conductive porphyrin-based frame material and introducing electron collection and donor nodes. Third, in porphyrin-based skeleton materials, its intrinsic catalytic activity can be directly increased by manufacturing fewer defects and adjusting the coordination of the metal. Fourth, the large-plane structure of the porphyrin block guarantees a strong π interaction with the object molecule. The coupling of porphyrin-based frame materials to another object allows them to have synergistic properties in a limited porous environment, thereby improving their catalytic performance.
2) New structures can be constructed by introducing porphyrin elements of asymmetric substituents and nodes with an abnormal number of connections. In addition, the combination of hybrid porphyrin units with flexible organic ligators can prepare novel porphyrin-based skeleton materials with multiple porosity and topologies.
3) The use of highly oxidized metal center nodes (such as Zr4+) can enhance the thermal and chemical stability of porphyrin MOFs. Zr-based porphyrin-MOFs, especially Zr chain nodes, are generally very stable in strongly acidic and alkaline solutions. Adding -CF3 groups to the holes can improve the stability of MOFs in weak acids and weak bases. In addition, porphyrin-based COFs usually have high chemical stability due to covalent organic ligands, and further research in this regard needs to be further investigated.
4) Using a simple and low-cost imine exchange strategy, COF-367-Co nanosheets can be prepared on a large scale and with high yield. This strategy can be used for the construction of other bonded porphyrin-based COFs, which has good application prospects.
Related paper information:
Jiawei Gu, Yi Peng, Ting Zhou, Jiao Ma, Huan Pang, and Yusuke Yamauchi. Porphyrin-based framework materials for energy conversion. Nano Res. Energy 2022, 1: e9120009. https://doi.org/10.26599/NRE.2022.9120009
As a sister journal of Nano Research, Nano Research Energy (ISSN: 2791-0091; e-ISSN: 2790-8119; Official website: https://www.sciopen.com/journal/2790-8119It was launched in March 2022 and is co-edited by Professor Qu Liangti of Tsinghua University and Professor Chunyi Zhi of the City University of Hong Kong. Nano Research Energy is an international multidisciplinary, all-English open access journal, focusing on the cutting-edge research and application of nanomaterials and nanoscience technology in new energy-related fields, benchmarking against the top international energy journals, and committed to publishing high-level original research and review papers. Before 2023, the APC fee will be waived, and all teachers are welcome to submit articles.
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