Hybrid Graphene/WO₃ and Graphene/CeOx Electrodes for High‑Performance Supercapacitors

Abstract

Graphene’s high surface area, conductivity, and mechanical strength make it an attractive scaffold for hybrid supercapacitor electrodes. In this study, we combine CVD‑grown graphene with tungsten oxide (WO₃) and cerium oxide (CeOₓ) deposited by pulsed reactive magnetron sputtering to form layered nanocomposites. Electrochemical testing reveals that increasing the number of graphene layers boosts areal capacitance, reaching 4.55 mF cm⁻² for a three‑layer graphene/WO₃ stack. The performance gain is attributed to the synergistic contribution of the copper oxide interfacial layer that forms beneath the graphene. All electrodes retain 70–90 % of their initial capacitance after 850 charge–discharge cycles, demonstrating robust cycling stability.

Background

Electrochemical capacitors are rapidly emerging as high‑power, long‑life energy storage devices, suitable for portable electronics, electric vehicles, and grid‑level applications. Compared with batteries, supercapacitors store charge through an electrostatic Helmholtz double layer and, in many cases, additional faradaic pseudocapacitance, providing high power density and excellent cycle life while sacrificing energy density. The key to improving performance lies in developing electrode materials that combine large accessible surface area with high electrical conductivity and chemical stability.

Carbon allotropes, especially graphene, are preferred due to their low environmental impact, low cost, and remarkable physical properties. A single graphene sheet offers a theoretical specific capacitance of ≈21 µF cm⁻², while three‑dimensional graphene architectures can deliver energy densities of ~13 Wh kg⁻¹ and power densities of ~8 kW kg⁻¹. However, planar single‑layer graphene still suffers from limited surface area; therefore, hybridization with transition‑metal oxides that provide pseudocapacitance is a proven strategy.

Previous works have demonstrated that combining graphene with metal‑oxide nanoparticles such as Fe₂O₃, MnO₂, or NiO significantly enhances specific capacitance, reaching up to 380 F g⁻¹. In this context, we investigate the synergy between CVD‑grown graphene and two transition‑metal oxides—WO₃ and CeOₓ—using a reproducible, scalable sputtering process. Both oxides are known for their redox activity and high charge‑storage capability, yet their combination with graphene has not been reported to date.

Experimental

Hybrid Electrode Preparation

Continuous graphene films were synthesized by chemical vapor deposition (CVD) on 75 µm thick, 99 % pure polycrystalline copper foils. After a hydrogen plasma etch (100 W, 20 Pa, 10 min), the foils were heated to 1040 °C in a quartz tube while a methane/hydrogen flow (5/20 sccm) was introduced for 20 min to deposit a uniform single‑layer graphene film. The samples were then cooled under high vacuum (3 × 10⁻⁴ Pa).

WO₃ and CeOₓ particles were deposited on the graphene surface by pulsed reactive magnetron sputtering (1 Pa, Ar/O₂ 13/7 sccm, 60 W, 5 s, 10 cm target‑substrate distance). For three‑layer stacks, a polymer‑assisted transfer technique was employed: the graphene/oxide film was coated with PMMA, the copper was etched with FeCl₃, the PMMA/oxide layer was floated onto a fresh graphene/oxide layer, and the PMMA was removed with acetone.

Figure 1a illustrates the complete fabrication workflow. Figure 1b shows the assembled Swagelok cell, consisting of two graphene/oxide electrodes, a glass‑fiber separator soaked in 1 M LiClO₄/EC:DEC (1:1), and an organic electrolyte.

Schematic drawings. a Fabrication of the graphene/MeO stacking. b Cell architecture with separator and electrolyte.

Structural/Morphological Characterization

Raman spectroscopy (Jobin‑Yvon LabRam HR 800) confirmed single‑layer graphene (I₂D/I_G ≈ 2.47, 2D FWHM ≈ 40 cm⁻¹). Scanning electron microscopy (SEM, JEOL JSM7100F) and transmission electron microscopy (TEM, Bioscan Gatan JEOL 1010) revealed uniform dispersion of WO₃ particles (average diameter ≈ 25 nm) on the graphene surface. X‑ray photoelectron spectroscopy (XPS, PHI 5500) was used to quantify the oxygen content of the copper substrate and to confirm the presence of a thin Cu₂O interfacial layer.

Electrochemical Characterization

Electrochemical tests were carried out in a dry N₂ glove box (< 1 ppm O₂, < 1 ppm H₂O) using a Swagelok configuration. Cyclic voltammetry (CV) was performed at scan rates from 10 mV s⁻¹ to 200 mV s⁻¹ over a 1.8 V window. Specific capacitance (C_s) was calculated from the charge balance equation, and interfacial capacitance (C_i) was obtained by dividing C_s by the electrode area.

Results and Discussion

Hybrid Structure

The brief sputtering time was deliberately chosen to preserve the integrity of the graphene lattice. TEM images (Figure 2a,b) show WO₃ nanoparticles uniformly decorating the graphene edge; the larger particles (~25 nm) exhibit a d‑spacing of 0.31 nm, matching the tetragonal WO₃ (101) plane. SEM images (Figure 2c) confirm continuous graphene coverage with minimal multilayer patches.

Morphological and structural characterization. a TEM image of the Gr/WO₃ film and Raman spectrum. b HRTEM of a WO₃ particle and its SAED pattern. c SEM of the as‑grown graphene film. d Raman of transferred graphene on SiO₂.

XPS analysis (Figure 3) indicates that the copper surface underneath graphene develops a thin Cu₂O layer even after hydrogen plasma annealing. The O 1s peak intensity is higher for samples with graphene, implying a thicker oxide interfacial layer that contributes to the total capacitance.

XPS characterization. a O 1s spectrum of Cu with graphene. b O 1s spectrum of bare Cu.

Electrochemical Results

Figure 4a shows the CV curves of a three‑layer graphene/CeOₓ electrode. The interfacial capacitance increases from 0.87 mF cm⁻² for a single graphene layer to 4.55 mF cm⁻² when two additional layers are added (+ 258 %). For the WO₃ system, the capacitance rises from 2.69 to 4.15 mF cm⁻² (+ 54 %) with the same layering.

Electrochemical characterization. a CV curves of three‑layer Gr/CeOₓ at various scan rates. b Interfacial capacitance vs. scan rate. c Capacitance increase with layer number. d Ragone plot of energy and power densities.

The Ragone analysis (Figure 4d) shows that the power density peaks at 1.6 × 10⁻⁴ W cm⁻² for the three‑layer Gr/CeOₓ electrode, comparable to other graphene/metal‑oxide systems. Although the energy density is lower (4.5 × 10⁻⁸ W h cm⁻²), the devices achieve superior power delivery.

Cycling tests (Figure 6a) demonstrate that all electrodes retain 70–90 % of their initial capacitance after 850 cycles. The CeOₓ‑based hybrids exhibit higher charge/discharge efficiency (Figure 6b). Charge‑discharge curves (Figures 6c,d) reveal that the time required for a full cycle increases from ~1.7 s (single layer) to ~4.7 s (three layers) for CeOₓ, and from 1.9 s to 5.5 s for WO₃, reflecting the enhanced energy storage capacity.

Electrode efficiency. a Capacitance retention. b Charge/discharge efficiency. c Charge‑discharge of one vs. three layers (CeOₓ). d Same for WO₃.

Conclusions

Layer‑by‑layer assembly of graphene with WO₃ or CeOₓ nanoparticles yields hybrid electrodes that combine high pseudocapacitance with excellent electrical connectivity. The addition of metal‑oxide particles boosts total capacitance, while further stacking amplifies the effect by increasing the electrode surface area. CeOₓ hybrids provide slightly higher charge/discharge efficiency and maintain > 70 % capacitance after 850 cycles. The observed performance, including a maximum areal capacitance of 4.55 mF cm⁻² and robust cycling, confirms the promise of CVD‑grown graphene/oxide stacks for next‑generation supercapacitors.