Hydrous ruthenium oxide-tantalum pentoxide thin film electrodes prepared by thermal decomposition for electrochemical capacitors

Abstract

In this work, we have fabricated high-performance thin-film electrodes for electrochemical capacitors (ECs) via thermal decomposition syntheses of RuO₂–Ta₂O₅ coating layers on Ti substrates. The influences of decomposition temperature as well as the Ru/Ta molar ratio on material and electrochemical properties of the EC electrodes are systematically investigated. The thermal decomposition of 300 °C preserves a large fraction of hydrous RuO₂·xH₂O within the hybrid oxide and consequently improves the electrode capacitance. The amorphous Ta₂O₅ incorporation can manipulate the RuO₂ crystallinity and thus its specific capacitance. An optimal Ru/Ta molar ratio of 7:3 is determined for the RuO₂–Ta₂O₅ electrode, which can deliver an energy density of 4.8 Wh kg⁻¹ at a power density of 8720 W kg⁻¹ (or 8.3 Wh L⁻¹ at 15,000 W L⁻¹). In addition, an excellent durability of 97.6% capacitance retention after 3000 charge-discharge cycles is found for this electrode. The proposed RuO₂–Ta₂O₅ thin-film electrode has paved the way for next-generation ECs with superior capacitances, energy/power densities, and cyclability.

Introduction

Electrochemical capacitors (ECs) are widely used as the primary energy storage devices when high power along with ultra-fast response time is needed. ECs have a long cycle life with a moderate energy storage capacity [1,2]. Successful implementation of ECs as an energy storage medium has been demonstrated for camera flashes, back-up power units, regenerative braking systems, and stop-start systems of electric vehicles [3]. Basically, ECs can be classified into two major categories: electrical double-layer capacitors (EDLCs) and pseudo-capacitors. EDLCs store electrostatic charge at the electrode/electrolyte interface. Having a high surface area as well as a proper porosity is the major requirement for an EDLC electrode since the amount of charge that can be stored is a strong function of the accessible electrode surface [[3], [4], [5], [6]]. In contrast, the capacitive performance of pseudo-capacitor materials relies on the fast and reversible faradaic reactions that take place near the surface/sub-surface of the electrodes [7]. Developing high-capacitance electrode materials and the cost-effective synthesis routes are of significant importance for ECs. In addition, understanding the relationship between material properties and electrochemical performance is vital for better design of electrode materials for various EC applications.

Recently, the implementation of metal oxides (e.g. MnO₂ [8,9], SnO₂ [10], ZnO [11], Co₃O₄ [12], and RuO₂ [[13], [14], [15]]) in the electrodes has been shown to provide promising EC performance. In particular, low-crystalline RuO₂ was confirmed to be one of the most ideal electrode materials due to its high specific capacitance, superior electrochemical reversibility, excellent electrical conductivity (~10⁵ S cm⁻¹), and high chemical stability in acidic or alkaline electrolyte [7,[16], [17], [18]]. Despite having superb electrochemical properties, the long-term stability of RuO₂-based ECs still has room to improve.

Several approaches including electro-deposition [[19], [20], [21], [22]], sol-gel method combined with hydrothermal synthesis [23], and thermal decomposition [[24], [25], [26]] have been successfully developed to prepare RuO₂ electrodes. Among these methods, thermal decomposition is a highly efficient route to synthesize high-quality RuO₂ materials through oxidation of a precursor (e.g., RuCl₃·xH₂O) at elevated temperatures [27]. This method is reliable, scalable, cost-effective, simple, facile, and precursor saving. Although it is practically important, systematic investigations, which are crucially needed, are relatively insufficient in the literature. Some pioneering studies have reported that the charge-discharge performance and cycling stability of the RuO₂ prepared by thermal decomposition can be improved via doping other species (e.g., Ta, Ti, and Sn oxides) [24,25,[28], [29], [30]]. Accordingly, researchers are committed not only to exploring to reduce the amount of RuO₂ required to improve its efficiency, but also improve the energy density and power. Metal oxides are a type of substance that researchers often choose because metal oxide can increase the dispersion of particles in composites [31,32]. Incorporation of the metal oxides can enhance the anti-corrosion capability of the electrodes along with reducing the overall cost. The optimal temperature for thermal decomposition and the effects of the foreign dopants and their concentration levels on EC properties deserve more detailed study.

In this work, we have adopted a one-step thermal decomposition technique to fabricate RuO₂–Ta₂O₅ thin film onto Ti foil. The resulting hybrid oxides have been served as high-performance electrodes for ECs. The thermal decomposition temperature is found to be a crucial factor affecting the hydrous states as well as the capacitances of the electrodes. In addition, various Ru/Ta molar ratios of the precursors have been implemented for studying the chemical, crystallinity, and film topography effects on the electrochemical properties of the oxide electrodes. This topic (various Ru/Ta molar ratios) is for the first time systematically investigated. By knowing the role of Ta₂O₅ incorporation, an appropriate Ru/Ta ratio is proposed to optimize the capacitive performance and cycling stability of the hybrid EC electrode.