Ultra‑High Capacity Supercapacitor Electrodes from One‑Step Hydrothermal Synthesis of Chlorine‑Doped Carbonated Cobalt Hydroxide Nanowires on Nickel Foam

Abstract

We present a binder‑free electrode comprising self‑stabilized chlorine‑doped carbonated cobalt hydroxide nanowires (Co‑ClNWs) grown directly on nickel foam via a single‑step hydrothermal procedure. The needle‑shaped nanowires, 3–10 nm in diameter and highly mesoporous, furnish a large active surface and unobstructed ion pathways. In a three‑electrode setup, the Co‑ClNWs(NiE) electrode delivers a specific capacitance above 2150 F g⁻¹ at 1 A g⁻¹, retains 94.3 % of its capacitance after 500 cycles, and attains an energy density of 41.8 W h kg⁻¹ at a power density of 1280 W kg⁻¹ when employed as the positive electrode of an asymmetric supercapacitor. Compared with conventional electrodes, the one‑step method provides superior electrochemical performance while simplifying fabrication, offering a promising route for next‑generation energy‑storage devices.

Introduction

Supercapacitors are pivotal for fast‑charging, high‑power, and long‑cycle energy storage, making them attractive for military, automotive, and portable electronics applications [1–7]. They are classified as electrical double‑layer capacitors (EDLCs) or pseudocapacitors, the latter storing charge through Faradaic redox reactions [8–11]. While ruthenium oxide offers excellent performance, its cost, low porosity, and toxicity hinder commercial use [12]. Thus, earth‑abundant metal oxides/hydroxides such as NiO, Co₃O₄, Fe₃O₄, Fe₂O₃, V₂O₅, Co(OH)₂, and Ni(OH)₂ are being explored [13–14].

Co(OH)₂, with reversible redox reactions and a high theoretical capacity, stands out as a promising candidate [14]. Morphology control—especially high surface area and mesoporosity—has proven critical for achieving high capacitance [6,15–18]. Mahmood et al. demonstrated chlorine‑doped carbonated cobalt hydroxide nanowires (Co(CO₃)₀.₃₅Cl₀.₂₀(OH)₁.₁₀) with exceptional capacitance and energy density, attributing the performance to hydrophilic surfaces and anionic stabilizers that mitigate polarization [19].

Structural innovation, such as core–shell or hierarchical architectures, can further enhance ion transport and electrode–electrolyte contact. For instance, a MnCo‑LDH@Ni(OH)₂ core–shell on nickel foam achieved 2320 F g⁻¹ at 3 A g⁻¹, yet the synthesis involved complex steps [24]. A straightforward, scalable route to uniform, high‑performance electrodes remains desirable.

In this work, we synthesize mesoporous chlorine‑doped carbonated cobalt hydroxide nanowires (Co‑ClNWs) directly on nickel foam via a facile one‑step hydrothermal method, yielding the Co‑ClNWs(NiE) electrode. The electrode’s intrinsic design maximizes active site utilization and ion accessibility, offering a benchmark for cobalt‑based supercapacitor materials and a blueprint for industrial scaling.

Methods

Synthesis of Co‑ClNWs on Ni Foam

Commercial nickel foam (Canrd Co., Ltd.) was cleaned in 0.5 M HCl for 30 min, rinsed with deionized water and ethanol, and dried at 80 °C for 12 h. A solution of 3.5 g CoCl₂·6H₂O and 0.9 g urea in 100 mL deionized water was stirred for 30 min until fully dissolved. Nickel foams were submerged in the solution inside a stainless steel autoclave and heated at 120 °C for 20 h. After cooling, the foams were washed, dried under vacuum, and stored for use.

Material Characterizations

Morphology was examined by FE‑SEM (MIRA3 TESCA) and TEM (FEI Tecnai). XRD patterns were recorded on a SIEMENS D500 diffractometer (Cu Kα, λ = 0.15056 nm). XPS was performed on an ESCALAB 250 (Al Kα). Nitrogen adsorption–desorption isotherms (ASAP 2020) provided BET surface areas and pore size distributions via QSDFT.

Electrochemical Measurements

The Co‑ClNWs(NiE) electrode (geometric area = 1 cm², mass = 3 mg, thickness = 0.25 mm) was tested in a three‑electrode cell with Pt counter and Hg/HgO reference in 6 M KOH. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) (5 mV, 0.01 Hz–100 kHz) were conducted on a CHI660E workstation. Specific capacitance was calculated from Eq. (1). For the asymmetric supercapacitor (ASC), Co‑ClNWs(NiE) served as the positive electrode and activated carbon (AC) as the negative electrode; the mass ratio was determined by Eq. (2). Energy and power densities were derived from Eqs. (3)–(5).

Results and Discussion

Characterization of the Co‑ClNWs(NiE)

SEM images (Fig. 1) show that the nanowires densely coat the layered nickel foam, forming a 3‑D interconnected network. The high‑magnification view (Fig. 1b) reveals needle‑shaped Co‑ClNWs (3–10 nm diameter) aligned in a staggered fashion, preserving open pathways for electrolyte ingress. TEM (Fig. 3) confirms the single‑crystalline nature of the nanowires, with lattice fringes spacing of 0.508 nm (17.4°), matching the (100) plane of Co(CO₃)₀.₃₅Cl₀.₂₀(OH)₁.₁₀ (JCPDS 38‑0547). The mesoporous surface (Fig. 3d–e) offers pore sizes > 2 nm, enhancing ion diffusion. XRD (Fig. 4a) and XPS (Fig. 4b–d) verify phase purity and the presence of Co²⁺/Co³⁺, Cl⁻, and carbonate groups. Raman spectra (Fig. 4e) display characteristic peaks at 95, 813, 1045, and 1554 cm⁻¹, confirming the chemical composition. N₂ adsorption (Fig. 4f) shows a type IV isotherm with H3 hysteresis, indicating abundant meso‑ and macropores; the BET surface area is ~5 m² g⁻¹, modest but compensated by high crystallinity and active sites.

Electrochemical Performance of the Co‑ClNWs(E) Electrode

CV curves (Fig. 5a) of the Co‑ClNWs(E) electrode exhibit symmetric redox peaks at scan rates 2–20 mV s⁻¹, confirming pseudocapacitive behavior. GCD curves (Fig. 5b) yield specific capacitances of 975, 950, 900, 825, and 640 F g⁻¹ at 1–8 A g⁻¹, respectively. EIS (Fig. 5d) shows a semicircle diameter of ~2 Ω, indicating moderate charge transfer resistance. After 500 cycles (Fig. 5e), the CV shape remains unchanged, indicating robust cycling stability.

Electrochemical Performance of the Co‑ClNWs(NiE) Electrode

Co‑ClNWs(NiE) delivers superior performance: CV curves (Fig. 6a) maintain clear redox peaks up to 20 mV s⁻¹. GCD at 1 A g⁻¹ (Fig. 6b) shows a plateau confirming Faradaic reactions, and a specific capacitance of 2150 F g⁻¹—over twice that of Co‑ClNWs(E). At higher rates, capacitances of 1985, 1872, 1599, and 944 F g⁻¹ are observed at 2, 3, 5, and 8 A g⁻¹, respectively, demonstrating excellent rate capability. Cycling stability (Fig. 6d) retains 94.3 % after 500 cycles. EIS (Fig. 6f) reveals reduced series resistance and steeper low‑frequency slope after cycling, indicating improved ion diffusion.

The ASC assembled with Co‑ClNWs(NiE) as the positive electrode and AC as the negative electrode operates at 1.6 V. CV (Fig. 6g) and GCD (Fig. 6i) measurements show a specific capacitance of 117.5 F g⁻¹ at 1 A g⁻¹, energy density of 41.8 W h kg⁻¹ at 1280 W kg⁻¹, and 21.2 W h kg⁻¹ at 6397 W kg⁻¹ (8 A g⁻¹), surpassing many recent reports [45–46].

Conclusion

We have developed a binder‑free Co‑ClNWs(NiE) electrode via a single‑step hydrothermal method. The electrode achieves a specific capacitance of 2150 F g⁻¹ at 1 A g⁻¹, retains 94.3 % after 500 cycles, and delivers an energy density of 41.8 W h kg⁻¹ at 1280 W kg⁻¹. These results highlight Co(CO₃)₀.₃₅Cl₀.₂₀(OH)₁.₁₀ nanowires as a promising candidate for high‑energy supercapacitors, and demonstrate that direct growth on nickel foam maximizes active‑site utilization and electrode conductivity.

Abbreviations

ASC:

Asymmetric supercapacitor

Co-ClNWs:

Chlorine‑doped carbonated cobalt hydroxide nanowires

Co-ClNWs(E):

Co‑ClNWs adhered to nickel foam via PTFE

Co-ClNWs(NiE):

Co‑ClNWs grown directly on nickel foam

CV:

Cyclic voltammetry

EDLCs:

Electrical double‑layer capacitors

GCD:

Galvanostatic charge and discharge

MBT and MAT:

Electrochemical impedance spectra before (MBT) and after (MAT) cycling