Substitutional doping of WO₃ for Ca-ion based supercapacitor

Highlights

• Study of Bi, Hf, and Nb doping on the performance of (WO₃) as electrode material.

• Analyze the effect of doping through DFT calculations.

• Corroboration of the DFT calculations by XRD, TEM, XPS, and Raman.

• Testing doped samples in different electrolytes (H⁺, Li⁺, Na⁺, Ca²⁺ and Al³⁺).

• Fabrication of Ca-ion Asymmetric device to prove the practical applicability.

Abstract

In the present work, we analyzed the effect of doping (Bi, Hf, and Nb) on the performance of tungsten oxide (WO₃) based supercapacitor. Through substitutional doping, the properties were altered to enhance its electrochemical charge storage performance. The doped samples were tested in different electrolytes (H⁺, Li⁺, Na⁺, Ca²⁺ and Al³⁺). The cyclic voltammetry, galvanostatic charge–discharge, and EIS studies were performed to identify the best suitable anode material for the fabrication of asymmetric supercapacitor. Nb:WO₃ yielded the highest specific capacitance (C_sp) of 782 Fg⁻¹ in Ca electrolyte. An asymmetric device with Nb:WO₃ as anode and MnO₂ as a cathode using an aqueous Ca ion electrolyte exhibited a high potential window of 2 V and capacitance of 126 Fg⁻¹. The factors responsible for performance of electrode material in different electrolytes were identified through experimental results, theoretical calculations, and relevant simulations. The effect of doping on WO₃ crystal structure and electronic properties along with their binding nature/interaction with different electrolyte ions were first postulated through density functional theory (DFT) calculations.

Introduction

Societal development and advancement are mainly dependent on energy. Our dependency on limited fossil fuels is a major cause for concern along with the environmental damage caused due to the burning of fossil fuels. We have realized the importance of clean and renewable energy in our survival as well as that of the planet. Incessant emission of greenhouse gases, the resulting global warming, and climate change threaten the balance of the planet itself.[1] As the awareness about clean and renewable energy inspires us towards impactful change, the problem of energy storage becomes more critical. Harvesting energy from renewable sources to powering electric vehicles, energy storage is garnering the attention of researchers and governments alike.[2] Li-ion battery has been dominating as an energy storage device for a couple of decades. However, as technology advances the shortcomings of Li-ion batteries become more apparent. Li-ion batteries have relatively lower power density. New technology is more demanding. There are also concerns over the availability of raw material, toxicity, safety, and environmental impact and thus different options are being investigated.[3]

Tungsten oxide (WO₃) is a multi-functional material with good performance in applications such as gas sensing, electrochromism, photo-catalysis, energy storage, etc.[4], [5], [6], [7], [8] WO₃ exhibits pseudocapacitive behavior due to multiple oxidation states, fast surface reactions, and suitable crystal structure for intercalation of electrolyte ions. WO₃ based supercapacitors have been extensively studied and widely reported.[9], [10], [11], [12], [13], [14], [15] Huang et al. [16] reported C_sp of 524F g⁻¹ for tungsten oxide nanosheets arrays with 84% capacitance retention after 4000 cycles in 0.5 M H₂SO₄. He et.al[17] achieved C_sp of 488F g⁻¹ and 84% capacitance retention after 10,000 cycles in 2 M H₂SO₄ electrolyte for 0.6 V potential window. Kim et al. [18] tested the performance of WO₃ in 1 M LiClO₄ + PC between − 0.9 and 0.5 V (vs. SCE) and achieved C_sp of 139 Fg⁻¹ for pristine WO₃. After 2000 cycles 75% capacitance retention was observed. The performance of WO₃ increased after embedding a metallic nanofilament array (NFA). The capacitance and its retention both increased due to the inclusion of conductive NFA. Santhosha et al. [19] demonstrated the synthesis of WO₃ nanoparticles with a high specific capacity of 927 mAh g⁻¹ in the first cycle. However, the capacity retention was very poor and it decreased to 100 mAh g⁻¹ for the second discharge cycle. With successive discharge cycles, the capacity continued to decrease. Garcia et. al[20] also encountered a similar problem. In their report, WO₃ electrode showed a discharge capacity of 120 mAh g⁻¹ for the initial cycles but with poor stability. Capacity retention is a major issue with WO₃ electrodes in sodium-ion electrolytes.

The performance of WO₃ in neutral aqueous electrolytes (Li⁺ and Na⁺ ions) is nowhere near that of H₂SO₄. Major factors affecting the performance is the reaction kinetics of the material, electronic conductivity, movement of electrolyte ion and their interaction with the metal centers. [21] Options other than Li and Na ion containing electrolyte can prove more suitable.[22] Cheaper and abundant elements like calcium and aluminum further bring down the cost while improving the performance of the electrode material.[23], [24]

Utilizing composite electrode has many advantages. The performance of WO₃ was increases significantly with the incorporation of carbon nanotubes, conducting polymers, graphene, and graphene oxide.[21] The choice of materials for the preparation of composite electrodes is dependent on the synergy of materials. Carbon-based materials, in general, have higher electronic conductivity, power density, and surface area than metal oxides. However, the energy density and specific capacitance are lower than metal oxides. [25] Combining two suitable materials brings the best out of them. Another approach to augment and alter the properties is doping of metal oxide with a different metal. Substitutional doping is an effective strategy to tailor the electrochemical, structural, and optical properties of WO₃. Substitutional doping results in modulation of band energies, crystal structure, charge transportation capabilities, and material electrolyte interface. WO₃ has been doped with different metals to improve its performance as a photo-catalyst in photoelectrochemical water splitting and electrochromism.[26], [27], [28], [29], [30], [31], [32] Doping with isovalent elements like molybdenum or chromium will not result in charge mismatching and thus reducing the defects and vacancies. However, doping with non-isovalent elements will create oxygen vacancies and other defects.[33] The size of the dopant element is also important along with its valency. The movement of electrolyte ions in and out of the crystal and the ease with which this happens is determined by the crystal structure of the active material and the electrolyte’s ionic size. Systematic substitutional doping can facilitate crystal structure modification according to application requirements.

In this report, we focus on substitutional doping of different valence metals (Bi³⁺, Hf^4+, and Nb⁵⁺) in WO₃ and its effect on electrochemical charge storage. The electrochemical performance of pristine WO₃ is compared with doped (Bi, Hf, and Nb) WO₃. The results are further analyzed in conjunction with the data from different characterization techniques such as XRD, XPS, Raman, and TEM as well as theoretically with density functional theory (DFT) calculations. The enhancement in electrochemical performance is ascribed to the crystal structure and lattice parameter modifications due to the result of substitutional doping. Further, the doped samples were tested in different aqueous electrolytes (H, Li, Na, Ca, and Al) to analyze their electrochemical performance. Aqueous electrolytes have numerous advantages over organic electrolytes. Aqueous electrolytes have high-ionic conductivity, low cost, non-flammability, non-corrosiveness, safety, and convenient assembly in air, compared to organic electrolytes. To evaluate the performance of doped samples, electrolytes with different ionic sizes and valencies were used. The most commonly used electrolyte for WO₃ is 1 M H₂SO₄. Due to the corrosive nature of the acid electrolyte, mild, neutral aqueous electrolytes are the preferred option. Li-ion is widely studied and the most commonly used electrolyte commercially. The demand for Li and its limited availability is slowly driving up its prices. To further explorer the options Na, Ca, and Al was also selected. These materials are easy to access, abundant and cheap. An asymmetric device was fabricated using the best combination of electrode material and electrolyte.