Ultra‑Thin TiO₂ Nanomembranes via Atomic Layer Deposition Deliver Record‑High Capacitance for Pseudocapacitor Electrodes

Abstract

TiO₂ is a low‑cost, environmentally benign material with excellent electrochemical potential. Its practical use in supercapacitor electrodes has been limited by high internal ion resistance and poor electrical conductivity. In this study, we employed atomic layer deposition (ALD) to fabricate TiO₂ nanomembranes (NMs) with precisely controlled thicknesses on a three‑dimensional polyurethane sponge template. After calcination, the resulting ultra‑thin, flexible NMs exhibited a mixed anatase/rutile phase that enhanced ion transport. Electrochemical testing revealed that the 100‑cycle NM delivered an outstanding specific capacitance of 2332 F g⁻¹ at 1 A g⁻¹, corresponding to an energy density of 81 Wh kg⁻¹. Two such electrodes assembled in series were able to power a red LED for approximately one minute, demonstrating practical relevance. The findings highlight the promise of ALD‑grown TiO₂ NMs for next‑generation high‑performance pseudocapacitors.

Introduction

Supercapacitors are valued for their high power density, rapid charge–discharge capability, and long cycle life. Pseudocapacitors, which rely on fast Faradaic reactions, can achieve higher capacitance and energy density than purely electrostatic devices. Transition‑metal oxides, especially TiO₂, are attractive as pseudocapacitor electrodes due to their low cost, non‑toxicity, and multiple oxidation states. However, conventional TiO₂ suffers from low electronic conductivity and sluggish ion diffusion, which restrict its performance.

Two‑dimensional (2‑D) nanomembranes offer a high surface‑to‑volume ratio, flexibility, and short ion diffusion pathways—attributes that can overcome these limitations. Precise thickness control is essential for optimizing electrochemical properties, and ALD provides nanometer‑scale deposition accuracy while conformally coating complex 3‑D substrates. Here, we combine ALD with a porous polymer template to fabricate TiO₂ NMs of tunable thickness and investigate their structural and electrochemical characteristics.

Materials and Methods

Fabrication of TiO₂ Nanomembranes

TiO₂ NMs were deposited on commercial polyurethane sponges using ALD with tetrakis‑dimethylamido titanium (TDMAT) and water as precursors under nitrogen purge. Deposition cycles were set to 100, 200, or 400, yielding average thicknesses of ~15, 34, and 71 nm respectively (AFM). The coated sponges were calcined at 500 °C in O₂ to remove the polymer and form a 3‑D porous NM network. The resulting powders were cleaned in ethanol, HCl, and DI water.

Electrode Preparation

TiO₂ NM powders (90 wt %) were mixed with PTFE binder (10 wt %) and milled with a small amount of ethanol to form a homogeneous slurry. The slurry was coated onto cleaned nickel foam, dried at 60 °C, pressed at 10 MPa, and activated in 1 M KOH for 12 h. Active‑material loading was ~1.5 mg cm⁻².

Characterization

Crystalline phases were identified by XRD (Cu Kα) and Raman spectroscopy. Morphology was examined by SEM, and surface topology by AFM. Chemical states were probed by XPS (C 1s as reference). Electrochemical performance was measured in a three‑electrode cell (Ag/AgCl reference, Pt counter) using CV, chronopotentiometry (CP), and EIS on a CHI 660E workstation. Specific capacitance, energy, and power densities were calculated following standard protocols.

Results and Discussion

The ALD process reproduced the sponge’s 3‑D morphology, producing smooth, flexible NMs with uniform thickness. XRD confirmed a mixed anatase/rutile phase; Raman spectra displayed characteristic peaks for both phases, with an additional 610 cm⁻¹ rutile peak appearing only in the 400‑cycle sample, indicating oxygen‑deficient regions due to thicker deposition.

Electrochemical testing revealed pronounced redox peaks in all TiO₂ NM electrodes, absent in bare nickel foam, confirming pseudocapacitive behavior. The 100‑cycle NM exhibited the largest CV area and longest CP discharge time, indicative of superior capacitance. Specific capacitances at 1 A g⁻¹ were 2332 F g⁻¹ (100 cycles), 1780 F g⁻¹ (200 cycles), and 1094 F g⁻¹ (400 cycles). Energy densities followed the same trend: 81 Wh kg⁻¹, 59 Wh kg⁻¹, and 38 Wh kg⁻¹, respectively. Power densities increased from 250 to 1250 W kg⁻¹ as current density rose from 1 to 5 A g⁻¹.

Electrochemical impedance spectroscopy showed the lowest internal resistance for the 100‑cycle NM, reflecting its high surface area and flexible interconnectivity, while the 400‑cycle NM suffered higher resistance due to reduced electrode‑electrolyte interface and increased sheet stiffness.

Cycle stability tests at 5 A g⁻¹ over 40 cycles demonstrated 80.98 % capacitance retention for the 100‑cycle NM, underscoring good durability. Practical viability was illustrated by assembling two such electrodes in series; the device powered a red LED (~1.5 V) for ~1 min after charging at 5 A g⁻¹.

Conclusion

We have demonstrated that ultra‑thin TiO₂ nanomembranes fabricated by ALD on a porous template deliver record‑high pseudocapacitive performance, with a specific capacitance of 2332 F g⁻¹ and an energy density of 81 Wh kg⁻¹ at 1 A g⁻¹. The combination of mixed anatase/rutile phases, high surface area, and excellent flexibility enables rapid ion transport and low internal resistance. These results establish ALD‑grown TiO₂ NMs as a promising, low‑cost platform for high‑performance, wearable energy‑storage devices.

Abbreviations

AFM

Atomic force microscopy

ALD

Atomic layer deposition

CP

Chronopotentiometry

CV

Cyclic voltammetry

DI

De‑ionized water

EIS

Electrochemical impedance spectroscopy

LED

Light‑emitting diode

NMs

Nanomembranes

PTFE

Polytetrafluoroethylene

SEM

Scanning electron microscopy

TDMAT

Tetrakis dimethylamido titanium

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction