Structure-mediated fluid-involved behaviors promote the performance of a carbon-based supercapacitor

Abstract

Carbon nanotube sponges (CNTSs), a promising electrode material, are limited by inherent hydrophobicity. Herein, various TiO₂ nanostructures were synthesized inside CNTSs, providing pseudocapacitance and regulating the wettability and ion migration of the electrolyte, thereby enhancing the area-specific capacitance by up to 250%. These results offer insights into advanced energy storage systems.

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Supercapacitors have attracted wide attention for their high power density,^1,2 fast charging–discharging rate with high current density,^3,4 long cycle life^5,6 and safety,^7,8 and they are considered to be promising candidates for use in next-generation energy storage systems.^9–12 The performance of a supercapacitor is mainly governed by the characteristics of the electrode material (specific surface area, pore structure, and conductivity) and those of the electrolyte (ionic conductivity and electrochemical performance), and the compatibility between them.^13,14 Notably, the behavior of the electrolyte within the electrode structure plays a non-negligible role.^15,16 From a surface chemistry perspective, sufficient wetting of the electrode surface by the electrolyte is required to ensure an adequate solid–liquid contact area.¹⁷ This is essential for forming the electric double layer¹⁸ and facilitating electrochemical reactions.¹⁹ Furthermore, the migration mode of ions within the electrolyte directly determines the ion transport kinetics, thereby influencing the overall performance of the supercapacitor.^20,21 Insufficient ion migration under high-rate conditions leads to a decline in capacitive performance during rapid charge–discharge cycles.^22,23 Therefore, developing materials possessing high porosity, a large specific surface area, efficient ion transport pathways, and suitable electrolyte wettability is of paramount importance.^14,24,25

Carbon nanotube sponges (CNTSs) are a class of conductive aerogel composed of multi-walled carbon nanotubes, offering multiple advantages.^26–28 The self-supporting conductive network formed by interconnected carbon nanotubes provides an exceptionally high specific surface area and abundant mesopores. These structural features facilitate electrolyte infiltration and rapid ion transport, thereby enhancing power density. Moreover, the network ensures excellent electron conduction pathways and mechanical resilience.^29,30 However, it is important to note that pristine CNTSs, when used as a supercapacitor electrode, possess inherent limitations. These include a relatively monodisperse pore structure, a single charge storage mechanism (EDLC), and poor surface wettability (as, due to the highly non-polar nature of the graphitic carbon surfaces, they exhibit strong inherent hydrophobicity), all of which can restrict their charge storage capability.^31,32 It remains a significant yet challenging task to modify the interfacial properties of CNTSs without compromising their inherent structural and performance benefits.³³

Here in this study, TiO₂ nanostructures with different morphologies were introduced into CNTS via a facile hydrothermal method, forming spherical, urchin-like, and thorn-like TiO₂@CNTS composites. Experimental and simulation results demonstrate that, compared to pristine CNTS, these TiO₂ nanostructures not only enhance electrolyte wettability but also provide additional effective solid–liquid interfaces for energy storage. The area-specific capacitance values of the TiO₂@CNTS composites exhibited significant increases of 196.4%, 204.8%, and 250% for the spherical, urchin-like, and thorn-like TiO₂ nanostructures, respectively, where the thorn-like TiO₂@CNTS composite exhibits the maximal improvement in performance. This work highlights the correlation between electrolyte infiltration behavior and TiO₂ morphology, providing insights into the design of next-generation supercapacitors.

By tuning the conditions during hydrothermal synthesis, spherical, urchin-like, and thorn-like TiO₂ nanostructures were synthesized on carbon nanotube sponges (CNTSs) obtained by the chemical vapor deposition (CVD) method.

In typical synthesis processes, the morphology of TiO₂ nanostructures evolved from spherical to urchin-like as the concentration of HCl increased from 2 mL to 8 mL at low titanium precursor concentrations of 100 to 500 µL. Conversely, TiO₂ is more likely to form bulk and thorn-like structures at a higher titanium precursor concentration of 700 µL, while simultaneous increases of both precursor and HCl concentrations lead to TiO₂ spheres (Fig. 1). The morphological evolution can be ascribed to the fact that the carbon nanotubes serving as a self-supporting framework provide abundant heterogeneous nucleation sites. The precursor concentration determines the initial nucleation density by controlling supersaturation, while hydrochloric acid acts as a kinetic inhibitor, modulating the hydrolysis rate of Ti⁴⁺ and promoting anisotropic growth along the (001) direction through selective adsorption onto specific crystal facets. This synergistic effect between the precursor and acid concentrations leads to a structural transition from sparse spherical shapes to dense urchin-like structures, and finally to a bulk-like architecture, which is consistent with previously reported growth mechanisms.³⁴

Fig. 1 Morphologies of TiO₂@CNTS composites. (a) A schematic illustration of the TiO₂@CNTS composites. (b and c) SEM images of the spherical TiO₂@CNTS composite. (d and e) SEM images of the urchin-like TiO₂@CNTS composite. (f and g) SEM images of the thorn-like TiO₂@CNTS composite. (h) A morphological phase diagram of TiO₂ vs. HCl concentration.

The hydrothermal approach enables the in situ growth of TiO₂ nanostructures on carbon nanotubes. Taking spherical TiO₂@CNTS as an example, the TiO₂ particles grown directly on CNTS form a “string-of-pearls” architecture, demonstrating robust interfacial bonding and sufficient contact between the nanostructures and CNTS, indicating superior electrical conductivity and mechanical stability. Moreover, spherical TiO₂@CNTS exhibited a contact angle of 27° (Fig. S3b), which is much lower than the value of 98° for pristine CNTS. Urchin-like TiO₂@CNTS and thorn-like TiO₂@CNTS also exhibited excellent electrolyte wettability, suggesting a potential increase in the charge storage capability of TiO₂@CNTS-based supercapacitors.

Energy dispersive X-ray spectroscopy (EDS) mapping and X-ray diffraction (XRD) pattern results confirm the TiO₂@CNTS structures (Fig. 2a and b). The characteristic peaks of the hydrothermally synthesized rutile TiO₂ are clearly identified at 26.7° (110), 36.12° (101), 44.08° (111), 56.68° (211), and 69.80° (332).³⁵ The characteristic peak of CNTS appears at 26.7°, corresponding to the (002) plane of graphitic carbon. All three TiO₂ nanostructures possess a rutile crystalline phase. The resistances of TiO₂@CNTS are 0.103 kΩ m⁻¹ (spherical TiO₂@CNTS composite), 0.106 kΩ m⁻¹ (urchin-like TiO₂@CNTS composite), and 0.116 kΩ m⁻¹ (thorn-like TiO₂@CNTS composite). The introduction of TiO₂ nanostructures not only improved the wettability but also mediated ion flow inside the TiO₂@CNTS composites. A laminar flow model was established in COMSOL, with the necessary hydrodynamic and physical parameters such as material type, viscosity coefficient, conductivity, and Young's modulus carefully defined. It can be observed that the ion flow velocity is maximized in the central regions of the pores and decreases near the particle edges due to fluid viscosity effects. Compared to spherical and urchin-like samples, the thorn-like TiO₂@CNTS material possesses a more developed nanoscale pore structure, providing a greater number of transport pathways for ion adsorption and desorption near the electrode surface. Moreover, the interfacial integral length of thorn-like TiO₂@CNTS is 1588.4 µm, which is 4 times that of spherical TiO₂@CNTS and 2 times that of urchin-like TiO₂@CNTS. The larger solid–liquid contact area also ensures the maximum enhancement in the capacitance performance of the thorn-like TiO₂@CNTS sample.

Fig. 2 Characterization of the TiO₂@CNTS composites. (a–d) EDS mapping of the thorn-like TiO₂@CNTS composite. (e) X-ray diffraction (XRD) patterns of TiO₂@CNTS composites with different morphologies. (f) An SEM image of the spherical TiO₂@CNTS composite for mass transfer behavior simulations. (g) The simulation results for mass transfer behavior in the spherical TiO₂@CNTS composite. (h) An SEM image of the thorn-like TiO₂@CNTS composite for mass transfer behavior simulations. (i) The simulation results for mass transfer behavior in the thorn-like TiO₂@CNTS composite.

Subsequently, we evaluated the electrochemical performances of spherical, urchin-like, and thorn-like TiO₂@CNTS composites with scan rates ranging from 10 mV s⁻¹ to 50 mV s⁻¹ (Fig. 3). The thorn-like TiO₂@CNTS composite exhibits specific capacitance of 52.4 mF cm⁻², increasing by 57.8% and 33.3% compared to the spherical and urchin-like samples, respectively. The diffusion-controlled capacitance percentages of the thorn-like TiO₂@CNTS composite were 36%, 29%, 25%, 21% and 19% at 10, 20, 30, 40, and 50 mV s⁻¹, while the spherical and urchin-like TiO₂@CNTS composites exhibit lower diffusion-controlled capacitance percentages, indicating an increase in the pseudocapacitive contribution for the thorn-like sample. The thorn-like TiO₂@CNTS composite also exhibited an increase of up to 100% in specific capacity compared to pristine CNTS.

Fig. 3 Capacitive performances of TiO₂@CNTS composites with various morphologies. (a–c) Cyclic voltammetry (CV) curves of the spherical TiO₂@CNTS composite, urchin-like TiO₂@CNTS composite and thorn-like TiO₂@CNTS composite. (d–f) Pseudo-capacitance calculation results for the spherical TiO₂@CNTS composite, urchin-like TiO₂@CNTS composite and thorn-like TiO₂@CNTS composite. (g–i) The relative pseudo-capacitance and double layer contributions for the spherical TiO₂@CNTS composite, urchin-like TiO₂@CNTS composite and thorn-like TiO₂@CNTS composite at sweep speeds of 1–50 mV s⁻¹.

The electrochemical performances of the TiO₂@CNTS composites exhibit significant morphology dependence (Fig. 4). The cyclic voltammetry (CV) curves (Fig. 4a) show that thorn-like TiO₂@CNTS possesses a smaller potential difference between the reduction and oxidation peaks at a scan rate of 10 mV s⁻¹, indicating faster interfacial charge transfer kinetics and lower polarization. Linear sweep voltammetry (LSV) tests (Fig. 4b) demonstrate that the TiO₂ coating effectively suppresses electrolyte decomposition, thereby broadening the stable potential window at the electrode/electrolyte interface. Galvanostatic charge–discharge (GCD) curves (Fig. 4c–e) confirm that spherical TiO₂@CNTS exhibits superior rate capability (with capacity retention of 90% at a current density of 4.8 mA cm⁻²), and its discharge time is significantly longer than that of pristine CNTS. Furthermore, the self-discharge rate of TiO₂@CNTS is markedly reduced (Fig. 4f); due to improved wettability and optimized ion transport kinetics, its half-voltage time is extended to approximately 2500 seconds. Electrochemical impedance spectroscopy (EIS) analysis (Fig. 4g and h) reveals that the thorn-like TiO₂@CNTS composite electrode possesses significantly lower charge transfer resistance (R_ct) and series resistance (R_s). Finally, thorn-like TiO₂@CNTS maintains capacity retention of 97% after 5000 cycles (Fig. 4i), highlighting the exceptional structural stability conferred by the enhanced integration of thorn-like TiO₂ within the three-dimensional CNTS framework.

Fig. 4 A comparison of the charge and discharge performances of TiO₂@CNTS samples. (a) Cyclic voltammetry (CV) curves of CNTS (black line) and spherical (green line), urchin-like (red line) and thorn-like (blue line) TiO₂@CNTS composite electrodes. (b) Linear sweep voltammetry (LSV) curves of CNTS (black line) and spherical (green line), urchin-like (red line) and thorn-like (blue line) TiO₂@CNTS composite electrodes. (c) Galvanostatic charge–discharge (GCD) curves of thorn-like TiO₂@CNTS at different current densities. (d) GCD curves of CNTS, spherical TiO₂@CNTS and thorn-like TiO₂@CNTS at a sweep speed of 0.6 mA cm⁻². (e) The variation in area-specific capacitance of sphere-like TiO₂@CNTS and thorn-like TiO₂@CNTS as a function of scan rate. (f) A self-discharge comparison between spherical TiO₂@CNTS, thorn-like TiO₂@CNTS and pure CNTS electrodes. (g–i) Electrochemical impedance spectroscopy (EIS, g) analysis, equivalent series resistance (R_s) and charge transfer resistance (R_ct, h) values, and long-term cycling stability (i) of the TiO₂@CNTS composite electrodes.

In this study, TiO₂ nanostructures with different morphologies were successfully incorporated into carbon nanotube sponges (CNTSs) using a simple hydrothermal method. Both experimental and simulation results indicate that the introduction of TiO₂ significantly enhances the hydrophilicity of the electrode and increases the effective solid–liquid contact area, thereby optimizing the fluid-related behaviors (infiltration and ion transport) of the electrolyte. Among them, the thorn-like TiO₂@CNTS composite exhibits superior nanochannel and interfacial characteristics, demonstrating a 250% increase in area-specific capacitance compared to pristine CNTS, along with improved rate performance and cycling stability. This work reveals the intrinsic relationship between electrode nanostructure, electrolyte fluid behavior, and energy storage performance, providing clear insights for the design of next-generation high-performance supercapacitors. Furthermore, these structure-mediated design principles, specifically the transition from isotropic to anisotropic thorn-like geometry, offer a universal strategy to optimize electrolyte fluid behavior and electrochemical performance in other transition-metal-oxide systems, such as MnO₂ and V₂O₅.

Author contributions

Heping Cao: conceptualization, methodology, hydrothermal synthesis, characterization, writing – original draft. Chengqing Tang: methodology, mass transfer behavior simulations, electrochemical performance evaluation, writing – original draft. Jiashuo Duan: data curation, chemical analysis (XRD and EDS mapping). Zhaohui Yang: conceptualization, supervision, validation, writing – review & editing. Yitan Li: conceptualization, project administration, supervision, validation, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: SEM images, FT-IR spectra, water contact angle test data, CMOSOL visualization diagrams, and electrochemical performance data. See DOI: https://doi.org/10.1039/d6na00092d.

Acknowledgements

The National Natural Science Foundation of China (grant no. 22402103), the Program of Taishan Scholar (grant no. tsqn202312040), the Natural Science Foundation of Shandong Province (grant no. 2024HWYQ-011 and 2025KJHZ014), and the Natural Science Foundation of Jiangsu Province (grant no. BK20240418) are acknowledged.

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Footnote

† These authors contributed equally to this work.

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