Applications of nanocarbons in redox flow batteries
摘要: 氧化还原液流电池(RFB)被认为是最高效的电网级大规模电化学储能技术，随着能源危机和环境污染的加剧，其引起广泛的关注。电荷传输性质是与储能装置的电化学性能有关的关键因素。通常通过调节材料形态和尺寸有效地减小离子的扩散距离，进而提高离子的扩散系数和电子的传递效率。纳米炭具有特殊的微结构和电子结构，并能呈现出众多奇异的物化特性，例如高离子电导、优异的导热性和出色的机械性能，其在电化学储能中起着不可或缺的作用。调控碳的微观结构是改善其电子和离子传输行为的有效策略。本文回顾了纳米炭在RFB中的功能，特别是着眼于电极（悬浮电极）和双极板（集电极）中使用的纳米炭材料的改性和设计，其可提高能量效率、功率密度和流动池的稳定性。希望对纳米炭材料在氧化还原液流电池中的应用进行更全面系统的介绍，可为高性能氧化还原液流电池的设计提供新的视角。Abstract: Redox flow batteries (RFBs), regarded as the most effective grid-scale electrochemical energy storage technology, are attracting wide attention because of the problems of the energy crisis and environmental pollution. Charge transport properties are critical factors related to the electrochemical performance of energy storage devices. Nanocarbons, which have special morphologies and many physicochemical properties, such as high ionic conductivity, high thermal conductivity and excellent mechanical properties, can play an indispensable role in electrochemical energy storage. Adjusting the microstructure of carbon materials is an effective strategy to improve their electron and ion transport behavior. In this work, the functions of nanocarbons in RFBs are reviewed, especially focusing on the modification and design of nanocarbons used in the electrodes, suspended electrodes in semi-solid RFBs, and bipolar plates (collectors) used to improve the energy efficiency, power density and the stability of high-performance RFBs. A more systematic and comprehensive understanding of the role that nanocarbons play in RFBs could provide a new perspective for the design of high-performance RFB electrodes.
Figure 1. Classification of common carbon materials: according to bonding (hybridization of carbon atom orbitals) and dimension. (i.e., the number of dimensions not limited to the nanometer scale). Reprinted with permission.
Figure 3. Schematic diagram of carbon electrode optimization improvement in aqueous flow battery. Reprinted with permission.
Figure 6. Schematic diagram of (a) the chemical reaction part in a typical VRFBs system cell and (b) the microstructure of the synthesized composite BPs. Reprinted with permission.
Table 1. Several carbon fiber electrodes reported.
Electrode Treatment of carbon fiber electrode Performance Current density(mA cm−2) Ref. New double diameter carbon fiber electrode It combines the advantages of high permeability of large fiber electrode and large specific surface area of small fiber electrode The energy efficiency is 79.43% 150  Electrospun carbon fiber Electrospun carbon fibers with different structural properties (including pore size and pore distribution) were prepared by changing the concentration of the precursor The energy efficiency is 79%, electrolyte utilization rate is 74% 300  Nanopore engineered
When copper oxide etching is used to form a nano-catalysis layer on the carbon fiber surface, an ultra-thin nano-porous catalytic layer is formed on the carbon fiber surface The energy efficiency is 85.1%, and the 2000 cycle test remains stable 320 
Table 2. Biochar used as flow electrodes for flow batteries previously reported.
Biomass source Batteries system Performance Current density(mA cm−2) Ref. Mycobacterium cell wall VRFBs It shows high stability under more than 1000 continuous cycles 200  The twin cocoon VRFBs The average discharge capacity is 83% and the energy efficiency is 20% higher 100  Pomelo peel Zinc bromide flow batteries Energy efficiency is up to 81.2%, and no degradation was observed in the 100 cycles 100  Biomass kiwi fruit VRFBs The average energy efficiency is 80%, showing excellent electrochemical activity and stability in charge and discharge tests 150  Shoulder blade of grass VRFBs The energy efficiency reaches 72.4%, and the corresponding discharge capacity has also increased by 11.1% 50 
Table 3. Some researches on suspended electrodes in SSFBs reported.
Suspension system Research factors Ref. LiNi1/3Co1/3Mn1/3O2-based suspensions Conductive carbon black content  Li4Ti5O12-based organic suspensions Amount of active substance in the suspension  Na2SO4-based suspensions Dispersion time and mixing methodology  LiPF6-based suspensions Rheological properties of carbon black suspension  Li4Ti5O12-based suspensions The type of carbon black, its concentration range and the flow rate range 
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