Mesoporous 2‑D VO₂ Microarrays Deliver 265 F/g Capacitance, 66 % Retention at 10 A/g, and 3000‑Cycle Stability for High‑Performance Supercapacitors

Abstract

We report a facile, liquid‑interface‑derived synthesis of two‑dimensional (2D) mesoporous VO₂ microarrays. The arrays consist of needle‑like VO₂ particles that are uniformly porous, with ~2 nm crack‑like pores extending 20–100 nm deep across the particle surface. The organic–inorganic interface serves as a soft template, guiding the formation of millimetre‑scale sheets (~1 µm thick). This architecture yields a high specific capacitance of 265 F g⁻¹ at 1 A g⁻¹, retains 182 F g⁻¹ at 10 A g⁻¹, and shows excellent cycling stability (239 F g⁻¹ after 3000 cycles at 2 A g⁻¹). The superior performance stems from the unique mesoporous structure and intrinsic conductivity of the VO₂ microarrays.

Background

Supercapacitors are promising energy‑storage devices that offer an order‑of‑magnitude higher energy density and longer cycling life than conventional batteries, thanks to fast surface‑charge mechanisms. They are generally classified into electrical double‑layer capacitors (EDLCs) based on mesoporous carbons, and pseudocapacitors that rely on Faradaic redox reactions of metal oxides or conducting polymers. Although pseudocapacitance can reach ten‑fold higher values than EDLCs, it is often limited by low surface area and poor electrical conductivity, which compromise power density and cycle life.

Transition‑metal oxides (TMOs) such as RuO₂, MnO₂, Fe₂O₃, NiO, and SnO₂ have been extensively explored as electrode materials. Vanadium oxides—V₂O₅, VO₂, and V₆O₁₃—are especially attractive because of their multiple oxidation states, low cost, abundant supply, and good structural stability. VO₂ stands out due to its mixed‑valence (V³⁺/V⁵⁺) electronic structure, which confers higher conductivity compared to other vanadium oxides. Prior works have demonstrated impressive pseudocapacitance for VO₂ composites: VO₂/rGO (426 F g⁻¹ at 1 A g⁻¹), VO₂/CNT (485 F g⁻¹ at 2 A g⁻¹), and VO₂/CNT (1550 F g⁻¹ via ALD). Pure VO₂ nanocrystals, however, have typically shown modest capacitance (<100 F g⁻¹) due to uncontrolled morphology and insufficient surface area.

We previously introduced a toluene–water hydrothermal system that enables the controlled growth of metal‑oxide nanocrystals at an organic–aqueous liquid interface. In that system, nucleation occurs in the aqueous phase, surfactant adsorption drives the crystals into the organic phase, and the liquid interface acts as a template for anisotropic growth. Here, we extend this approach to synthesize 2D VO₂ microarrays with a unique mesoporous architecture that combines high conductivity, large surface area, and short ion diffusion pathways.

Methods

Materials

V₂O₅, 30 % H₂O₂, toluene, oleic acid, and tert‑butylamine were purchased from Sigma Aldrich and used without further purification. Deionised water (Milli‑Q) was employed throughout.

Preparation of 2D VO₂ Microarrays

In a typical run, 7.5 mL 30 % H₂O₂ was mixed with 150 mL DI water, followed by 0.534 g V₂O₅. After stirring at room temperature until a dark golden‑yellow solution formed, the mixture served as the aqueous phase. The organic phase comprised 30 mL toluene, 12 mL oleic acid, and 1.5 mL tert‑butylamine. Both phases were combined in a 200 mL autoclave and heated to 200 °C for 48 h. The VO₂ microarrays nucleated at the liquid interface and floated onto the aqueous layer. After centrifugation, the product was washed and dried at 200 °C for 2 h under vacuum.

Material Characterization

XRD was performed on a D5005HR diffractometer (Cu Kα, 40 kV, 40 mA). TEM (JEM‑2100F) and FESEM (SU‑70) with EDS provided morphological and compositional data. XPS (ESCALAB 250) assessed surface chemistry. BET surface area and pore distribution were measured by nitrogen adsorption at 77 K (Micrometritics ASAP 2020).

Electrochemical Characterization

Electrodes were prepared by mixing 80 wt % active material, 10 wt % acetylene black (AB), and 10 wt % PVDF in N‑methyl‑2‑pyrrolidone (NMP), then coating the slurry onto Ni foil and drying at 80 °C overnight. The working electrodes were tested in a three‑electrode cell with 1 M Na₂SO₄ as electrolyte. Cyclic voltammetry (CV) was recorded with a PARSTAT 2273 at various scan rates. Electrochemical impedance spectroscopy (EIS) was measured from 10 kHz to 0.01 Hz (10 mV ac). Conductivity was determined by a four‑point probe (ST‑2258A) after compressing the powders into a 0.2 mm thick disc under 30 MPa.

Results and Discussion

The liquid‑interface synthesis proceeds in two stages: (1) nucleation of VO₂ nanosheets at the interface; (2) growth of needle‑like VO₂ units on the nanosheet surface in the aqueous phase. As the reaction proceeds, the nanosheets transform into aggregates of irregular particles, culminating in millimetre‑scale, 1 µm‑thick 2D microarrays (Fig. 1). The needle‑like units are ~350 nm wide and ~1 µm long, each exhibiting a uniform mesoporous network of ~2 nm pores with depths of 20–100 nm (Fig. 1c). BET analysis reveals a surface area of 80 m² g⁻¹ and a narrow pore size distribution centred at 2.85 nm (Fig. 2a), ideal for fast ion transport.

XRD patterns confirm a mixture of VO₂ (B) and VO₂ (R) phases, with the B phase dominating (Fig. 2b). This phase is known for its superior electronic conductivity, supporting the observed high capacitance.

Surface chemistry assessed by XPS shows a V 2p binding energy of 516.7 eV, characteristic of V⁴⁺, confirming the stoichiometry of VO₂ (Fig. 3).

Electrochemical tests demonstrate excellent performance. CV curves remain rectangular even at 50 mV s⁻¹, indicating highly reversible redox behaviour (Fig. 4a). EIS reveals a near‑vertical low‑frequency line and a minimal semicircle, giving an equivalent series resistance (ESR) of 1.07 Ω (Fig. 4b).

Galvanostatic charge–discharge curves at 0.5–10 A g⁻¹ yield a specific capacitance of 275 F g⁻¹ at 0.5 A g⁻¹, 265 F g⁻¹ at 1 A g⁻¹, and 182 F g⁻¹ at 10 A g⁻¹, retaining 66 % of the low‑rate value (Fig. 5b). After 3000 cycles at 2 A g⁻¹, the capacitance remains at 239 F g⁻¹, a 100 % retention (Fig. 5c). In contrast, microarrays lacking the mesoporous architecture (VO₂‑S and VO₂‑F) show markedly lower capacitance and rapid fading.

These results underscore the critical role of the mesoporous, needle‑like structure and the high intrinsic conductivity of VO₂ (B) in achieving superior pseudocapacitance, rapid rate capability, and durable cycling stability.

Conclusions

We have developed a simple, scalable liquid‑interface method to fabricate 2D VO₂ microarrays with a uniform, mesoporous needle‑like architecture. The resulting electrodes exhibit a high specific capacitance of 265 F g⁻¹ at 1 A g⁻¹, retain 66 % of this value at 10 A g⁻¹, and maintain 239 F g⁻¹ after 3000 cycles at 2 A g⁻¹. The combination of large surface area, short ion diffusion pathways, and inherent conductivity of VO₂ (B) accounts for the outstanding performance, positioning these microarrays as promising candidates for next‑generation supercapacitors.