1. Reading guide:
Redox electrolyte supercapacitors (RE-SC) are a new type of supercapacitors with high energy density and high power density. However, these devices typically exhibit more severe self-discharge than conventional supercapacitors. This review summarizes the latest research progress of RE-SC, focuses on the analysis of redox media on the self-discharge behavior of devices and its influence mechanism, and analyzes the self-discharge inhibition strategies from the aspects of separator modification, electrolyte optimization, electrode material design and device construction, and emphasizes the important role of studying the matching between redox media and electrode materials, especially the interface interaction between redox media and electrode materials, in inhibiting the self-discharge of RE-SC and improving its practicability.
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Shi J, Tian X, Song Y, et al. Redox electrolyte-enhanced carbon-based supercapacitors: recent advances and future perspectives. Energy Mater. Devices, 2023, 1, 9370009.
How to improve the energy density of carbon-based supercapacitors has always been a bottleneck that plagues the development of electric double-layer capacitors. The use of pseudocapacitive electrode materials is an effective way to increase the energy density of the device, but at the expense of power density and cycle life. Studies have shown that the diffusion rate of ions in the liquid phase is about 7 orders of magnitude higher than that in the solid phase. The introduction of soluble redox species into the electrolyte can take advantage of its rapid ion diffusion in the liquid, thereby achieving high energy density while maintaining high power density, showing unique advantages over traditional pseudocapacitors. In conclusion, RE-SC has the following characteristics: (i) the preparation of redox electrolytes is simple and easy to scale, which can avoid the change of production process equipment due to the use of pseudocapacitive electrodes; (ii) the current commercial capacitive carbon electrode materials can be directly used, and the RE-SC devices can be directly fabricated using the traditional SC assembly process; (iii) redox reactions occur at the electrode-electrolyte interface, which facilitates high power performance and long cycle life; (iv) The electrochemical performance can be easily adjusted by adjusting the chemical structure and concentration of the redox medium. Typically, however, the introduction of redox species accelerates the self-discharge of the capacitor, causing severe energy losses. In view of this, in order to further promote the practical application of RE-SC, it is necessary to comprehensively analyze the self-discharge mechanism of the system and the corresponding suppression strategy.
3. Graphic analysis:
Figure 1. Schematic diagram of the energy storage mechanism of porous carbon-based RE-SC, Copyright @ IOP Publishing.
Figure 2. Schematic diagram of the energy storage process and self-discharge suppression strategy of RE-SC.
Figure 3. a) a timeline for the development of carbon-based RE-SCs; b) The number of papers published on carbon-based RE-SC in the last decade. Data from Web of Science, keywords are “redox electrolytes” and “carbon-based supercapacitors”.
Figure 4. a) Surface oxygen functional groups with[Fe(CN)6]3-/[Fe(CN)6]4- Schematic diagram of the synergy between copyright @Elsevier; b) Addition of redox additives to modify porous carbon, copyright @Elsevier; c) Analysis of the synergistic effect of KI and Na2MoO4 using CV curves at a scan rate of 20 mV s-1: (left) Mo-100 and KI-100 samples; (right) theoretical values and 100Mo-KI-1:1 samples, copyright @Elsevier.
Figure 5. a) Open-circuit voltage time diagram of 1 M Na2SO4 (without and with 0.1 M PFC) when using a porous separator (ONP) and an ion exchange membrane (IEM), respectively, copyright @ACS; b) Diagram of the possible orientation of SDS molecules in a mixture and a plot of contact angle and energy density variation in different systems, Copyright @Wiley-VCH.
Figure 6. a) The process by which Cu2+ is converted into insoluble matter during charging and its self-discharge curve, copyright @RSC; b) Mechanism of formation of reversible solid complexes based on violet essence and bromine during charging/discharging, copyright @ACS.
Figure 7. a) Schematic diagram of the electrode-electrolyte interface with 5CB added to the electrolyte. In the discharged state, cations, anions, and 5CB molecules in the electrolyte are evenly distributed; b) When the electrode is electrified, the electric field near the electrode surface aligns the 5CB molecules in the direction of the electric field, thereby increasing the flow viscosity; c) Potential-time plots with and without 5CB, copyright @Elsevier.
Figure 8. a) ECM based on macroporous, mesoporous and microporous structures, copyright @Elsevier; b) Pore structure as a function of specific capacity and open-circuit voltage, copyright @Elsevier; c) Illustration of the charge storage mechanism involved in multiple physical transfer processes in porous carbon electrodes and bulk solutions, iodide is used as a redox medium, self-discharge is denoted by the abbreviation “S-D”, copyright @ACS.
Figure 9. a) Schematic diagram of surface functionalization of porous carbon by dissolving heat and interaction of PPD and HQ redox additives on carbon surface, copyright @Elsevier; b) Schematic diagram of the electrochemical reactions that occur during charging and self-discharge of supercapacitors containing HQ and AQ redox species, as well as the corresponding potential distribution of their electrodes, copyright @RSC; c) Oxygen-containing functional groups anchor redox media[Fe(CN)6]3- Schematic diagram of copyright, copyright @Elsevier; d) Schematic diagram of the CuCl layer before and after the formation of the CuCl layer on the surface near the pores, copyright @Springer Nature.
Figure 10. Schematic diagram of the electrode-electrolyte interface in a supercapacitor and its equivalent circuitry a) without a barrier layer and b) with a barrier layer; c) Open-circuit voltage-time diagram with or without PPO barrier layer, copyright @Elsevier.
4. Summary and outlook:
By constructing long-life carbon-based RE-SCs with both high power density and high energy density, we can effectively fill the gap between conventional SCs and secondary batteries. Although the development of RE-SC has made great progress in the past few years, the research of RE-SC systems is still in its infancy compared to conventional electric double-layer capacitors, and more in-depth research is needed to put them into practical use, especially in the area of self-discharge suppression.
To achieve this goal, the construction of a strongly interacting redox media-electrode interface is a promising solution, and we believe that the following aspects can be focused on in the future:
From the perspective of separator modification, the development of low-cost ion-selective membrane IEM is an effective approach. However, IEM can increase the internal resistance of the system, which reduces the overall electrochemical performance of the device. In contrast, diaphragms based on electrostatic properties are more effective than pore distribution adjustments, because they can capture and fix redox media without clogging the pores, thereby inhibiting their shuttle and reducing self-discharge. We can pay more attention to the directional polarization of the separator and its effect on the overall electrochemical performance.
In terms of electrolyte control, the chemical modification of redox media at the molecular level can realize the universal application of redox media in various types of basic electrolytes, which can reduce the side effects caused by its conductivity and viscosity. In addition, the design and synthesis of new redox active substances with multiple redox centers, so that the redox reaction can occur at the positive and negative electrodes at the same time, and balance the difference in energy storage between the two poles is also worth studying.
In terms of electrode material modification, the trapping of redox medium by electrode materials through pore confinement and functional bonding can be realized, and strong interfacial interaction can be obtained, which is expected to effectively suppress self-discharge. The study of the matching between electrode characteristics (such as pore structure distribution, surface chemical functional groups) and redox media requires the development of advanced characterization methods to comprehensively characterize and evaluate the reaction process at the interface, so as to provide a theoretical basis for accurately regulating the interface interaction between electrode and redox media.
At present, the research on self-discharge is mainly carried out by leakage current, open-circuit voltage, energy retention, etc., and there is a lack of standardized research methods and judgment standards. There is still a long way to go in standardizing the characterization of self-discharge performance. In addition, in addition to ensuring the comprehensive performance improvement of RE-SC itself, it should also have the ability to integrate with other systems to achieve its wider application exploration.
5. About the corresponding author:
Tian Xiaodong is an associate researcher at Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences. He graduated from the University of Chinese Academy of Sciences with a Ph.D. degree and was selected into the Youth Innovation Promotion Association of the Chinese Academy of Sciences and the “Young Outstanding Talents” Support Program of Sanjin Talents. In the past five years, he has presided over 8 projects at all levels such as national, provincial and ministerial level, and enterprise commission, mainly engaged in the structural design of porous carbon/graphite functional materials and their application in adsorption, thermal conductivity, and energy storage.
Yan Song is a researcher and doctoral supervisor at Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, and director of the Key Laboratory of Carbon Materials, Chinese Academy of Sciences. He is mainly engaged in the basic research on the preparation, structure regulation and application of asphalt-based carbon materials and nano-carbon fibers. He is an academic and technical leader in Shanxi Province, and was selected as a “Sanjin Talent” and a Youth Innovation Promotion Association of the Chinese Academy of Sciences. As the person in charge, he has presided over more than 10 projects of the National Natural Science Foundation of China, provincial and ministerial key R&D and funds, the Commission of Science, Technology and Industry for National Defense, and outstanding talents, published more than 200 research papers in mainstream academic journals of materials and chemical engineering, authorized 19 invention patents, and completed 2 scientific and technological achievements appraisal (all of which are internationally advanced). He is a member of the editorial board of journals such as “New Carbon Materials”, “Carbon Letters”, “Carbon”, “Science Journal of Chemistry”, “Materials Research and Development”, a standing committee member of the Energy Materials and Devices Special Committee and the Carbon Materials Special Committee of the Think Tank of Puppet Materials and Device Scientists, and the chairman of the Carbon Materials Special Committee of the Shanxi Chemical Engineering Society. He won the first prize of Shanxi National Defense Industry Science and Technology Innovation Award, the second prize of Shanxi Natural Science Award, and the Lu Jiaxi Young Talent Award of the Chinese Academy of Sciences.
Shengliang Hu is a professor and doctoral supervisor of North University of China, deputy dean of the School of Energy and Power Engineering, and a leader in the discipline direction. He is mainly engaged in the design, preparation and application of new energy materials, specifically involving solar energy conversion and utilization, energy storage technology and application, etc. He has been selected as “Shanxi Province Sanjin Talent”, “Shanxi Province Model Teacher”, “Shanxi Province Model Teacher Morality”, “Shanxi Province University Young and Middle-aged Top-notch Innovative Talents”, “Shanxi Province Youth Sanjin Scholars”, “Shanxi Province Academic and Technical Leader”, “Shanxi Province 131 Young and Middle-aged Top-notch Innovative Talents”, “Shanxi Province University Outstanding Young Academic Leader” and other programs or titles. As the person in charge, he has presided over more than 20 projects of the National Natural Science Foundation of China, provincial and ministerial key R&D and funds, and outstanding talent projects, and 3 teaching reform projects in Shanxi Province. In the internationally renowned journal Angew. Chem. Int. Ed.，Small，Appl. Catal. B，J. Mater. Chem. A/C，Green Chem.，Chem. Eng. J.，Carbon，Solar RRL，ACS Appl. Mater. Interface, Sensor Actuat. More than 140 academic papers have been published in TOP 1 SCI journals such as B, and more than 20 invention patents have been authorized by the state; He has won 4 second prizes of Shanxi Natural Science Award, 1 second prize of Shanxi Provincial Technological Invention Award, 1 first prize of Shanxi Provincial Excellent Paper Award, and 1 second prize of Shanxi Provincial Teaching Achievement Award. He is also the vice chairman and member of the Shanxi Youth Federation of Overseas Chinese, the editorial board member of international journals such as “Current material science”, “Journal of Testing Technology”, “Journal of North University of China”, and an expert in the evaluation of dissertations of the Ministry of Education, the National Natural Science Foundation of China, and the Ministry of Science and Technology. Listed in the “Top 2% of the World’s Top Scientists” list and the “Top 100,000 Global Scientists” list.
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