"My hometown is in Ningxiang, Hunan. Hunan is in a good development trend in the fields of advanced energy storage materials, new energy systems, and other areas, and I also hope to quickly integrate into the development of my hometown by combining my professional advantages," said Gao Ping, a doctoral graduate from the University of Ulm and a professor at the School of Chemistry of Xiangtan University.

Not long ago, he and his team completed such a feat: using the porphyrin structure molecules originally present in chlorophyll and hemoglobin, they created an electrochemical polymer.

Porphyrin is a class of large molecular heterocycles formed by four pyrrole subunits connected through methylene bridges (=CH-) on the α-carbon atoms, which have good biocompatibility.

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He and his research group have designed a new type of porphyrin conjugated polymer - poly[5,15,10,20-tetrathiafulvalene porphyrin] copper (COP-CuT2TP) through the means of electrochemical polymerization.

This porphyrin conjugated polymer has the characteristic of 16 electron transfer, and its highly conjugated molecular structure can effectively inhibit the dissolution of organic cathode materials, thereby achieving the performance of organic lithium batteries with high capacity and long cycle life.The research team also found that there is an auto-stripping phenomenon of active molecules during the charging and discharging process, which can further improve the utilization rate of organic active molecules.

When using porphyrin-conjugated polymers as cathode materials, they exhibit a high discharge specific capacity of 420mAh/g in lithium batteries, a discharge specific energy of 900W/Kg, and can achieve 8000 cycles of stability.

In general, this new type of porphyrin-based conjugated polymer constructed through electrochemical polymerization strategy is not only excellent in performance but also stable in structure. Compared with the reports, the cathode made with porphyrin-based conjugated polymers has higher energy density and better cycle stability.

This not only provides strong support for the development of the next generation of organic cathodes with both high capacity and long cycle life, but also enriches the types of high-performance organic cathodes. At the same time, the auto-stripping behavior found in the research is also conducive to a better understanding of the energy storage mechanism of organic batteries.

Undoubtedly, this is a potential high-performance organic cathode material, which has brought a positive effect to the development of the next generation of high-performance organic lithium batteries.Leveraging the inherent flexibility of porphyrin-based conjugated polymers, they can be utilized for the fabrication of flexible energy storage electrodes, which can then be integrated into flexible wearable devices.

Furthermore, by employing porphyrin-based conjugated polymers, it is possible to create organic electrode materials that possess both high energy density and long cycle life, thereby playing a role in scenarios such as smart grids and electric vehicles.

It is reported that currently commercialized lithium-ion batteries use inorganic compounds as cathode active materials, and their discharge specific capacity has approached the theoretical value, which limits the further improvement of energy density. In addition, the uneven distribution of raw materials worldwide may lead to instability in the price of lithium batteries.

At the same time, the recycling technology for lithium batteries using inorganic salt systems is not only complex but also very costly. Therefore, it is very necessary to develop sustainable organic materials and apply them to the next generation of lithium batteries.

Organic molecules contain abundant elements such as carbon, hydrogen, and oxygen. They are not only widely available but can also be directly extracted from nature or designed and synthesized through experimental means. Through molecular regulation, the discharge specific capacity can be accurately controlled, and the recycling and treatment costs of organic materials are also relatively low.However, organic molecular electrode materials can be dissolved in organic electrolytes, and their intrinsic electronic conductivity is relatively low, which greatly limits their commercialization process.

Due to the highly conjugated structure of porphyrins and their characteristic of multi-electron transfer, these molecules exhibit good biocompatibility and electrochemical activity, and have been extensively studied in fields such as solar cells, catalysis, and biomedicine.

However, no one has attempted to study them as electrochemical energy storage materials before.

In 2014, during his doctoral studies in Germany, Gao Ping discovered the excellent charge storage performance of porphyrin-based molecules in lithium batteries during a casual conversation and attempt with a friend. Later, they also published a research paper [1].

After returning to China in 2017, Gao Ping's team began to focus on the design and development of porphyrin-based electrochemical energy storage materials. Through molecular design methods, they regulate the active functional groups of porphyrin molecules, metal ion coordination structures, and the optimization of composite electrodes, aiming to develop organic metal secondary batteries that balance sustainability, high capacity, long cycle life, and high power.In order to further explore and understand the application and behavior of porphyrin-based compounds in electrochemical energy storage, he and his students have conducted over six years of basic research since returning to the country.

Focusing on the design optimization of porphyrin molecules, he and his team have discovered some interesting experimental results. For instance, they found that by in-situ polymerization behavior, stable organic cathodes can be constructed, and have been supported by two National Natural Science Foundation grants.

The study of modifying porphyrin molecules with thiophene groups began in 2020. Preliminary research indicated that although the porphyrin molecular framework has a 4-electron transfer, the relatively large inherent molecular weight of the porphyrin structure limits the play of theoretical specific capacity.

This study aims to further improve the discharge specific capacity and cycle stability of porphyrin electrodes. Thiophene groups have good electrochemical activity and possess bipolar characteristics, meaning they can be oxidized or reduced themselves, thus theoretically capable of charge storage.

At the beginning of the study, they designed porphyrin molecules modified with thiophene, bithiophene, and terthiophene groups, synthesizing three types of porphyrin functional materials with different thiophene modifications.As a cathode material, its discharge specific capacity has been significantly improved, with a gravimetric capacity that can reach 300mAh/g. However, the comprehensive electrochemical performance of the material is not ideal, especially the poor cycle stability, which may be due to the molecules being soluble in organic solvents.

In 2021, the research team reorganized their thoughts and started from the construction of porphyrin polymers, hoping to solve the problem of molecular dissolution.

Due to the high activity of the thiophene group at the α-position, they utilized the polymerizable characteristic of the thiophene active site and attempted the strategy of electrochemical oxidation polymerization, rebuilding the three-electrode system, and optimizing the type of auxiliary electrolyte, the concentration of active molecules, the polymerization time, and other parameters.

Ultimately, they obtained a thienothiophene-based porphyrin polymer—it has a regular hexagonal cylindrical appearance, a very stable structure, and is completely insoluble in conventional organic solvents.

At the same time, they found that using a mono-thienyl porphyrin monomer could not yield the corresponding porphyrin polymer, which may be related to the chain length of the thiophene functional group.Through electrochemical performance testing, the research team found that compared to individual molecules, the thienyl-based porphyrin polymer has the same high discharge specific capacity, and its cycle stability can also be significantly improved, with almost no capacity decay after 8000 long cycles.

In the mechanism research phase, they also discovered the self-peeling phenomenon of the porphyrin polymer molecules, that is, during the cycling process, the originally regular hexagonal cylindrical porphyrin polymer will gradually self-peel into a flake-like morphology.

For the strange behavior of gradually increasing capacity during the long cycle process and the reason for having high power performance, the team also found the answer from the above phenomena.

Through this, they confirmed that porphyrin-based materials can play a role in organic lithium batteries, taking into account the performance of high capacity, long cycle, and high power.

It is also reported that most of the synthetic experiments of this work were completed by a master's student. At the beginning, after the research team proposed this idea, the experimental progress was not smooth, especially the synthesis yield of porphyrin molecules was not high, and multiple syntheses were often required to obtain the target product that meets the quality requirements.Moreover, after they synthesized the target monomer molecules, they found that the cycle was not stable. Subsequently, by adjusting the experimental plan and using electrochemical polymerization strategies to construct porphyrin-based polymers, the above problem was finally solved.

In the characterization of the charge storage mechanism, they originally hoped to use in-situ infrared spectroscopy and in-situ Raman spectroscopy for research. However, at that time, the group did not have in-situ characterization equipment. Later, with the help of Professor Liu Jilei from Hunan University, Gao and others filled the equipment gap.

However, the research on in-situ spectral characterization was not smooth. The team did not have much experience in optimizing the sample preparation in the in-situ battery for different materials.

Later, after optimizing the detailed parameters, they obtained normal in-situ spectral data, which well explained the reversible storage behavior of anions and cations in the porphyrin polymer.

Finally, the related paper was published in Angewandte Chemie International Edition (IF 16.6) with the title "Porphyrin-Thiophene Based Conjugated Polymer Cathode with High Capacity for Lithium-Organic Batteries."Xing Wu is the first author, with Professor Liu Jilei from Hunan University and Professor Gao Ping from Xiangtan University serving as corresponding authors.

Subsequently, the research team will continue to optimize the structure of porphyrin molecules, the electrolyte system, and the interface modification.

They also plan to attempt to in-situ load this polymer onto flexible current collectors with high electrical conductivity or related materials, hoping to solve the problem of the material's inherent low electrical conductivity and construct composite electrodes with high electronic conductivity.

They will also try to use it for the research of large batteries, that is, pouch batteries, aiming to develop flexible electrochemical energy storage devices with low cost, long cycle life, and high capacity.

At present, the team is simultaneously advancing several topics, has obtained good data, and expects to achieve results in device applications in the future.