Supercapacitors (SCs) have excellent merits of high power density, rapid charge/discharge rates, and long cycle stability [1]. However, their relatively low energy density hampers the practical applications [2]. In general, the energy density of SCs strongly depends on their intrinsic properties of active materials and related inactive indispensable components such as current collector, electrolyte, separator, tabs, and package. Usually, the weight of these inactive components can reach around 50~70% of total mass [3]. In most literature, active materials are evaluated at a relatively low areal mass loading (<5 mg/cm²) to pursue the maximum performance by fully exposing the active sites and shortening the charge and ion transport pathways as much as possible [4]. However, these evaluation methods show limited guidance to design commercial SCs based on such active materials. Low areal mass loading of active materials results in excessive utilization of the current collector. In general, the density of current collector (Al foil: 2.7 g/cm³) is several times larger than that of active materials (compacted density of activated carbons (ACs): 0.4~0.6 g/cm³), hindering the development for high-energy, lightweight and low-cost SCs [5]. ACs are the most promising material for commercial SCs owing to their high surface area, robust electrical conductivity, good chemical stability, and low cost. In commercial SCs, the ACs electrode is fabricated with an optimized mass loading of >10 mg/cm² and a thickness of 100-200 µm [6]. However, the capacitance of SCs is not linear with the mass loading or electrode thickness. Simultaneously, the increased mass loading or electrode thickness results in inferior performances due to the decreased accessible surface area, enlarged electrical resistance, prolonged ion transport channels, and poor electrolyte wetting. Therefore, it is still needed to design a novel active materials-current collector system with high mass loading for the practical applications of SCs.

In this work, we report using carbon nanotubes (CNTs) matrix with high mass loading embedded in Ni foam (NF) to develop high areal capacitance electrodes for SCs. The CNTs matrix was directly grown on the NF (the size of 1×1×0.05 cm) by CVD method at the temperature of 650-800 °C without any other catalysts. Acetylene (C₂H₂) was used as the carbon source at a flow rate of 5 sccm, and the balance gases were H₂ and Ar with flow rates of 100 and 300 sccm, respectively. The mass loading of the CNTs matrix reaches up to 60 mg/cm². Figure 1 (a) and (c) show the top and bottom surface morphologies of CNT-NF electrode. Two electrodes with the mass loading of 60.2 and 59.5 mg/cm² were packaged into a coin cell for evaluating the performances. The glass fiber and 1mol/L MeEt₃NB₄ in Acetonitrile were used as the separator and electrolyte, respectively.

Figure 2 (a) shows the cyclic voltammetry (CV) of SCs at the scan rates of 100 mV/s and 2 mV/s. The calculated capacitance of SCs is 1214 mF. Given that, the corresponding areal, gravimetric, and volumetric capacitance of CNTs matrix active material are calculated as 2.428 F/cm², 40.4 F/g, and 48.6 F/cm³, respectively. Figure b (a) presents the electrochemical impedance spectroscopy (EIS) of SCs in the frequency range from 100 kHz to 100 mHz. The impedance at high frequency is the contact resistance between the active material and current collector, indicating that the contact resistance of the CNT-NF electrode is 0.187 Ω·cm² because the CNTs matrix grows on NF directly without using any binders. After 600 cycles, the capacitance of SCs reaches 1442 mF by an increase of 18.8% and because CNTs matrix has a poor wetting ability in the organic solvents, as shown in Figure 2(c). In figure 2(d), the corresponding contact resistance is 0.205 Ω·cm² after 600 cycles, showing CNT-NF electrode has excellent rate stability.

In conclusion, we have successfully used Ni foam as the scaffold structure and current collector for growing high mass loading CNTs matrix as the active material for high areal capacitance application. Importantly, this 3D CNTs matrix electrode system could offer a promising strategy to simultaneously improve the areal capacitance and energy density (gravimetric or volumetric) of SCs.

References

[1] F.Wei, et al. New Carbon Mater. 34, no. 2 (2019): 132-139. 9.

[2] Z.Li, et al. Nat. Energy 5, no. 2 (2020): 160-168.

[3] F. Zhang, et al. Energy Environ. Sci. 6, no. 5 (2013): 1623-1632.

[4] H.Sun, et al. Nat. Rev. Mater., 2018, 4, 45-60.

[5] H.Nakagawa, et al. J. Electrochem. Soc. 147, no. 1 (2000): 38.

[6] Y.Gogotsi, et al., Science, 2011, 334, 917-918.