Influences of Temperature on Band Energetics and Electrochemical Performance of Cerium Oxynitride in a Symmetric Aqua-Based Supercapacitor
Authors: Ghosh Sourav , Priyanga G. Sudha , Kumar U. Naveen , Gangavarapu Ranga Rao , Jeevandoss C.R. , Thomas Tiju 
Published: July 2021

The fast deterioration of fossil fuels motivates the development of eco-friendly, sustainable, and renewable energy resources. Concurrently enhancement of present energy storage technologies is crucial. Supercapacitors (SCs) are known for their quick energy delivery (1-10 kW kg-1) capability and long cycle life (~105 cycles). The advantages of SCs enable high-power applications in several sectors such as industrial, transportation, etc. The increasing demand and specific advantages of SCs have helped to cause a bloom in the market in the last couple of decades. Thus, research on novel materials to improve specific capacitance is increasing with time. Hence, understanding the band energetics of the electrode material and studying the electrochemical performances are pivotal in SC research.

Among several electrode materials, Ce-based materials (oxides and related composites) are often studied in this aspect due to (i) interchangeable oxidation states of cerium (3+/4+), (ii) relative abundance of cerium on the earth’s crust (nearly as rich as copper sources, and indeed most abundant amongst all rare earth metals), and (iii) high-temperature stability. Research on metal oxynitrides is still in its infancy for supercapacitor application. Metal oxynitrides are compositionally in between pure oxides and nitrides. The existence of an oxide layer alongside the nitride phase ensures rapid redox reactions. Rapid redox reactions enhance rate capabilities and enable longer cycle life. Hence, cerium oxynitride is used here as the electrode material for a symmetric supercapacitor device based on an aqueous electrolyte. The influences of temperature on the band energetics of the material and electrochemical performances of the device are studied extensively.

In this work, the lattice constant of CeOxNy is optimized concerning total energy calculations for various temperatures. The configuration of the material is found to be stable when varied against several temperatures (-5 to 60 oC). The electronic structure of CeOxNy for the temperature range from -5 to 60 oC is successfully predicted. The lattice parameter of CeOxNy increases monotonically with increasing temperature. The bandgap variance contributed by lattice thermal expansion is determined using the corresponding lattice constants. As with numerous different semiconductors, fundamental computational estimations in this work on CeOxNy show similar patterns of bandgap narrowing with expanding temperature. The bandgap is reduced by 1.18 eV at 60 oC compared to 0 oC.

The total and partial density of states (DOS) was computed to analyze the electronic properties of CeOxNy at different temperatures. CeOxNy is found to be a wide bandgap semiconductor (Eg= 2.82 eV) at 0 oC. The DOS of CeOxNy shows a significant change in the conduction state because of the expansion in temperature. The Fermi level of CeOxNy also decreases and shifts downwards from 0 oC. As a result of the increased temperature, the width of the conduction state in CeOxNy is reduced, and the height of the peak is increased. The decrease in the bandgap is primarily due to a change in the conduction state and a decrease in the Fermi level value, which predominantly comes from the relatively minor hybridization between Ce-4f and O-2p state electrons.

The observations due to the effect of temperature on the band energetics are reflected in the material’s electrochemical performance. The steady increase in areal capacitance with increasing temperature can be attributed to the narrowing bandgap. The areal capacitance exhibits an upsurge of ~13.4 times (74.73 mF cm-2 from 5.54 mF cm-2 0.1 mA current) at 60 oC than the room temperature (25 oC). However, the time constant increased in both freezing and high temperature. The optimum time constant is 12.12 millisecond at room temperature, which increased to 14.68 millisecond at -5 and 0 oC and 21.54 millisecond at 60 oC. Overall, high aerial capacitance and low time constant indicate CeOxNy as a promising supercapacitor electrode material even at high temperatures.

Figure 1