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摘要: 碳基材料(如碳纳米管、石墨烯和介孔碳)是典型的电化学双电层超级电容器电极材料。虽然碳基材料表现出优异的电化学稳定性能,但其比电容较低。因此,常用赝电容材料与其复合。赝电容材料中,二氧化锰(MnO2)因理论比电容高、价格低、储量丰富和环境友好等特点,被广泛应用于超级电容器中。然而,MnO2导电性能差、在循环充放电过程中相转变严重和体积变化大等问题,导致其在实际应用中常表现出较低的比电容。为了研发高性能MnO2/碳基超级电容器,必须深入研究其储能机理。因此,本文分析和总结了4种MnO2材料的电荷储能机理:电解液阳离子的表面吸附机理、电解液阳离子的嵌入-脱出机理、隧道储能机理和电荷补偿机理。虽然电荷补偿机理是涉及阳离子预先插入的MnO2 (AxMnO2)材料,但4种机理的本质都是Mn3+和Mn4+之间的相互转化,且由于储能过程复杂,MnO2基超级电容器储能过程常是几种机理共同作用的结果。最后,对高性能MnO2/碳基超级电容器的前景进行了展望,对其面临的主要挑战和发展策略进行了总结。Abstract: Carbon-based materials, such as carbon nanotubes, graphene and mesoporous carbons, are typical electrochemical double-layer capacitive electrodes of supercapacitors (SCs). Although these carbon electrode materials have excellent electrochemical stability, they usually have a low capacitance. Therefore, pseudocapacitive materials are often combined with them to increase capacitance. Among these pseudocapacitive materials, manganese dioxide (MnO2) has been widely used because of its high theoretical specific capacitance, low-cost, abundance, and environmentally friendly nature. However, the use of MnO2 often produces rather low actual specific capacitances due to its poor electrical conductivity, phase transformation and large volumetric changes during repeated charge and discharge. To explore high-performance MnO2/carbon composite electrode materials, it is necessary to understand the charge storage mechanisms of MnO2. These are analyzed and classified into four types: surface chemisorption of cations, intercalation-deintercalation of cations, a tunnel storage mechanism and a charge compensation mechanism. Although the fourth involves pre-interaction of the cations in MnO2, the essence of all these mechanisms is the valence transition of manganese atoms between +3 and +4, and many mechanisms are usually involved in MnO2-based SCs because of the complicated charge storage process. Critical challenges and possible strategies for achieving high-performance MnO2/carbon-based SCs are discussed and prospective solutions are presented.
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Key words:
- Carbon-based materials /
- Manganese dioxide /
- Charge storage mechanism /
- Supercapacitors
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Figure 1. Ragone plot of several EES devices[4]. Reprinted with permission.
Figure 3. In-situ Raman spectra of the MnO2 electrode at different charge-discharge states[34]. Reprinted with permission.
Figure 5. Insertion process of monovalent and bivalent cations[47]. Reprinted with permission.
Figure 6. (a) To obtain Na0.5MnO2 via electrochemical oxidation at different CV cycle numbers. (b) Specific capacitance of the electrode as a function of cycle number at 10 mV s−1 during electrochemical oxidation. (c) The charge-storage mechanism of the Na0.5MnO2[50]. Reprinted with permission.
Figure 7. (a) Schematic showing different reaction processes of MnO2 nanosheets and ZnxMnO2 nanowires. (b) Schematic illustration of the designed aqueous ZnxMnO2//activated CC zinc-ion HSCs[53]. Reprinted with permission.
Table 1. Dwelling time or structure type, SSA and specific capacitance of MnO2.
Samples SSA (m2 g−1) Capacitance (F g−1) Refs. Dwelling time (1,2,3,6 h) MnO2-1H 16.736 387.1 [23] MnO2-2H 2.841 230.6 MnO2-4H 2.163 242.5 MnO2-6H 1.087 193.1 Structure MnO2-N 18 270 [31] MnO2-HU 71 215 MnO2-SB 15 242 Structure R-MnO2 24.13 144 [32] S-MnO2 68.30 133 F-MnO2 83.17 171 L-MnO2 8.78 117 *Note: In reference[31], MnO2-N, MnO2-H, and MnO2-S represent MnO2 nanorods, MnO2 hollow urchins, and MnO2 smooth balls, respectively. Table 2. Specific capacitance dependence on MnO2 phase structures and SSA.
MnO2 Tunnel Size(nm) SSA(m2 g−1) Specific capacitance (F g−1) Electrolyte Refs. α 2×2(1D) 0.46×0.46 19.29 241 Na2SO4 [33] α(m) 2×2(1D) 0.46×0.46 123.39 297 Na2SO4 [33] α 2×2(1D) 0.46×0.46 29 125 K2SO4 [40] α(m) 2×2(1D) 0.46×0.46 200 150 K2SO4 [41] α(m)(H2SO4) 208 150 K2SO4 [41] α(m)(H2O) 8 125 K2SO4 [41] δ 1×∞(2D) 0.7 20.93 236 Na2SO4 [33] δ 1×∞(2D) 0.7 45 225 K2SO4 [40] δ(H2O) 1×∞(2D) 0.7 17 110 K2SO4 [41] δ(H2SO4) 89 105 K2SO4 [41] δ 3 80 K2SO4 [41] γ 1×2(1D) 0.23×0.46 31.56 107 Na2SO4 [33] γ 1×2(1D) 0.23×0.46 85 87 K2SO4 [40] γ 1×2(1D) 0.23×0.46 41 30 K2SO4 [41] λ(spinel) 3D - 5.21 21 Na2SO4 [33] λ(spinel) 3D - 156 241 K2SO4 [40] λ(spinel) 3D - 35 70 K2SO4 [41] β 1×1(1D) 0.189×0.189 - 9 Na2SO4 [33] β 1×1(1D) 0.189×0.189 35 28 K2SO4 [40] β 1×1(1D) 0.189×0.189 1 5 K2SO4 [41] -
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