Boron-oxy-carbide sheets: A wide voltage symmetric supercapacitor electrode with high temperature tolerance

https://doi.org/10.1016/j.cej.2022.136983Get rights and content

Highlights

  • Boron -oxy- carbide is prepared by hydrothermal assisted carbonization technique.

  • BOC electrode operates in both negative and positive potential window over 1.0 V.

  • BOC SC operates over 2 V and breaks thermodynamic potential in aqueous electrolyte.

  • Achieves higher energy and power density value of 38.75 Wh kg−1 and 18,750 W kg−1.

Abstract

In view of exploring the boron-based nanostructures for the application of energy storage, we have synthesized boron-oxy-carbide material with hydrothermal assisted annealing process and analyzed its performance in aqueous and organic electrolyte. The ability of BOC material to operate both in positive and negative region in Li2SO4 electrolyte encourage us to construct a symmetric device. Splendidly, the BOC material breaks the restriction of low thermodynamic potential window of water (1.23 V) equipped with high cell voltage of 2 V and shown excellent energy density of 16.15 Wh kg−1 at 500 W kg−1 of power density. To extend its operating voltage, a symmetric coin cell was fabricated and tested in TEABF4 electrolyte. As expected, the cell operates upto 3 V and shows maximum energy density of 38.75 Wh kg−1 at 1125 W kg−1 with tremendous rate capability. Moreover, the temperature dependent electrochemical behavior was tested at different temperature from −15 °C to 80 °C. The device shows considerable increment in the specific capacitance with 15.8 F g−1 at −15 °C and 26.24 F g−1 at 80 °C.

Introduction

The clean and renewable energy resources (wind, geothermal, bio-mechanical, solar energy) along with the related energy conversion and storage technologies becomes indispensable to meet the uncontrollable energy demand [1]. In order to maintain the balanced state between power supply and demand, various kinds of electrical energy storage technologies have been invented, such as flywheels (mechanical energy storage), fuel cells (chemical energy storage and conversion), and batteries & supercapacitors (SCs) devices (electrochemical energy storage), respectively [2], [3]. Among them, SCs or electrochemical capacitors probing the energy demanding sectors through its ultra-fast storing ability, simple operation mode, excellent reversibility with huge power density [4], [5]. These unique features enable the SCs to fill the energy/ power void between the high-power capacitors and high energy fuel cells/ batteries [6], [7]. Into the bargain, the SCs liberates minimal thermochemical heat owing to their simplest form of charge storage mechanism [8] which promotes them to have a wider space in industrial power, electric vehicles (EVs), and consumer electronics. More importantly, the high-power density of SCs make them as an ideal primary energy system for capturing regenerative braking energy in EVs and as secondary energy system (in combination with batteries) to drive EVs [9], [10]. Not only limited to EVs, SCs also find applications in uninterrupted power supply (UPS), wind energy, ac line filtering system in the recent years [9], [11]. Herein, it is worth to mention that the performance metrics of the SC device determines their practical applicability[12]. In general, the performance metrics (energy & power density) of a SC is determined by their capacitance, and operating voltage window which in turn relies on the nature of electrode material, configuration (symmetric, asymmetric, hybrid) and the nature of electrolyte [5]. For instance, aqueous electrolyte possesses higher ionic conductivity (>100 mS/cm) and safer to operate [13], [14]. It is well known that the water splitting reactions in acid and basic electrolytes hinders the extension of voltage window above 1.23 V in most of the aqueous SCs [15] and there the emergence of neutral electrolytes (Li2SO4, Na2SO4 etc.) to be the best candidate for higher working voltages begins. The enhanced adsorption of cations (Li+ and Na+) at the electrode–electrolyte interface inhibits the adsorption of H+ ions from the water part of the electrolyte onto the electrode surface and restricts the hydrogen evolution reaction [16]. In case of the configuration, the construction of supercapacitors with asymmetric electrodes (ASC) is the widely used methodology to extend the operating voltage window. However, the different energy storage mechanism of capacitor type and battery type electrodes results in sluggish dynamics and leads to serious damage in the stability of ASCs [17]. Thus, the pioneering works stimulated us to focus on the construction of symmetric SCs with an electrode material that shadows the benefits of asymmetric electrodes. Recently, much light was shed on the structure modification of electrode materials such as carbon based materials (graphene and their derivatives or hybrids), transition metal oxides (TMOs), transition metal dichalcogenides (TMDCs), conducting polymers (CPs), metal organic frameworks (MOF), and MXenes respectively [18], [19]. This area of research brings in the materials with high specific surface area, rich pores in different size, thereby inducing the electron transport and diffusion kinetics of ions [20].Therefore, the development of new or advanced electrode materials with high capacitance, energy/power ratio, rate capability are being considered as prime factors by researchers [21]. But to extend the practical utility of supercapacitor technology, their operation in harsh environment conditions is being neglected in commonly studied supercapacitor materials.

In this scenario, ceramic materials with their corrosion resistant, and high temperature resistant properties still makes them charismatic for the development of supercapacitor electrode[22], [23], [24]. However, their poor surface activity has been overlooked compare to other superior properties. So, it is necessary to focus on improving the surface activity of ceramic materials to make them a viable candidate for SC electrode. Recently, researchers developed hard or glass ceramic materials as electrodes for batteries and SCs via engineering the material properties. For instance, silicon-oxy carbide (SiOC) materials incorporated graphene composite papers are developed and used as anode for lithium-ion batteries and electrodes for SCs [25], [26]. In our recent study, carbothermally converted siloxene sheets into SiOC lamellas are examined as high-performance electrode for SCs that can store charges via electric double layer and intercalation/de-intercalation capacitance [27]. As an alternative to SiOC, boron-based ceramics are of great interest due to the high theoretical capacitance of boron (400F g−1) that is four-fold higher than graphene, the wonder material [28]. Therefore, studies on the supercapacitive properties of elemental boron in different forms such as sheets, wires, were explored in this decade. However, the current limitation in the use of boron electrodes for supercapacitors is their limited operating voltage window (less than1.0 V) due to the occurrence of hydrogen evolution reaction and oxygen evolution reaction [29], [30]. Further, very recent works demonstrated that the inclusion of oxygen defects in boron sheets or hybridization with carbon results in enhanced capacitive performances [31], [32]. Therefore, developing a multicomponent material made of boron, oxygen, and carbon, i.e., the boron-oxy-carbide might produce significant electrochemical performances via synergic effect from the individual components. Till date, there are few works reported the boron based multicomponent electrodes for SC and batteries. Chang et al. recently reported the application of B4C@C core–shell ceramics as an electrode for solid state supercapacitors [33]. In our recent work, we demonstrated the use of BOC nanostructures for supercapacitors applications, however, their energy density is very moderate compared to the state of art devices [34]. This is mainly due to the poor dispersion of boron and their suboxides in the carbon matrix that might result in poor electrical conductivity which restricts the material unable to operate in aqueous configuration (neutral, acidic and basic electrolytes).

Therefore, in this work, we have prepared the BOC nanostructures in a facile two-step synthetic route (hydrothermal followed by annealing process) to modify the morphology into sheets and explored their charge-storage properties in aqueous and ionic electrolytes. Interestingly, the homogeneous dispersion of boron oxide inside the carbon matrix with more free carbon resulted from hydrothermal activation throughout the 2D sheet structure of BOC enables the aqueous electrolyte ions to access the active sites and overcomes the evolution reaction at the both the potentials to work as negative and positive electrode [0.0; 1.0 V] and [-1.0; 0.0 V] vs Ag/AgCl reference electrode in aqueous electrolyte. Accordingly, we designed the symmetric supercapacitor (SSC) with BOC electrodes in Li2SO4 electrolyte that doubles the single component and sweeps over stable voltage window of 2.0 V thereby producing higher energy and power density.

Section snippets

Materials

All the chemical reagents were research grade and used as received without any further purification. Deionized water was used in all the experiments. Borax (Na₂[B₄O₅(OH)₄] ·8H₂O), carbon black, acetonitrile, and N-Methyl-2-pyrrolidone (NMP) were procured from Dae Jung Chemicals, South Korea. Cellulose and Polyvinylidene fluoride (PVDF) were acquired from Sigma Aldrich Ltd., South Korea. Tetraethylammonium tetrafluoroborate (TEABF4) was purchased from Alfa Aesar Chemicals, South Korea.

Preparation of boron-oxy-carbide (BOC) nanostructures

The BOC

Synthesis and structural characterization of BOC nanostructures

Fig. 1 schemes the preparation of BOC nanostructures using hydrothermal reaction followed by high temperature annealing process using cellulose and borax as starting materials. It is worth to mention that the thermal treatment of (i) cellulose (>150 °C) will result in the formation of furfural products [36], and (ii) borax (>130 °C) will decompose into boron oxide (B2O3) powders [37], respectively. Therefore, the hydrothermal reaction between these two materials might result in the formation of

Conclusion

This work highlights the credibility of boron-oxy-carbide nanostructures in energy storage applications. The ability of BOC electrode to operate in both positive and negative regions upto 1 V overcomes the thermodynamic potential and shows excellent energy storage performance operating upto high voltage of 2 V in aqueous Li2SO4 electrolyte. Then the cell voltage was extended upto 3 V when employing organic TEABF4 electrolyte. An excellent device capacitance of 43.36 F g−1 with maximum energy

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2019R1A2C3009747, 2021R1A4A2000934))

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