**1. Introduction**

With the increasing utilization of solar and wind energy worldwide, the intermittent nature of these resources restricts the sustainability of the energy supply. Due to environmental concerns regarding air quality, reducing the impact of fossil fuel use has required the development of new energy technologies for alternative energy options. In addition, minimizing the effect of fossil fuel consumption owing to environmental concerns regarding air quality issues mandates the development of new technologies for alternative energy solutions. There is great potential for discovering new energetic materials and employing them in applications in all of these energy technology development initiatives. BBILs are a novel class of energetic materials with a wide variety of characteristics that may be tailored to specific applications in the energy area [1].

Paul Walden first discovered ILs in 1914 and determined that the ethylammonium nitrate salt had a low melting point of 13°C [2–4]. In 1992, Wilkes and Zaworotko created air and water-stable ILs, known as the second generation, utilizing 1ethyl-3-methylimidazolium cation ([EMIM]+ ) and tetrafluoroborate ([BF4] − ) or hexafluorophosphate anion ([PF6] − ). They discovered that whereas these ILs are normally water insensitive, prolonged exposure to moisture produces alterations in their

characteristics [4]. The continuous fascination with ILs arises from their excellent features in future research and potential applications [1, 5, 7].

ILs are a novel family of solvents with melting temperatures less than 100°C, consisting of a combination of a bulky cation and an inorganic or organic anion [1, 2, 4–10]. An advantage of low melting point solvents is that they have negligible vapor pressures.

As a result, unlike liquids, ILs do not evaporate under standard conditions. ILs are salts with high thermal stability (most ILs are stable up to about 300–400°C) [9], chemical stability, nonflammability, a broad electrochemical window (2–6 V) [10, 11], and high electrical conductivity (1.3–8.5 mS cm−1) [12]. ILs are also less hazardous and effective solvents for both organic and inorganic compounds.

ILs are prominent solvents in electrochemistry due to the constituent anions being oxidized at sufficiently large potentials and organic cations being reduced at low potentials. ILs are generally aprotic, so problems with hydrogen ions that occur in protic solvents can be avoided. In recent years, ILs have great attention as an environmentally friendly ("greenness") liquid that is a candidate to replace commonly used, solvent-based, volatile, and flammable electrolytes that tend to be corrosive [6]. When creating ILs for applications, commonly at least one ion is weakly coordinated; either the cation or the anion is weakly coordinated; and in some ILs, both the ions are weakly coordinated. Solvent properties of ILs vary depending on the nature of the ions in their structure; anions with high charge density and organic cations with short alkyl chains stabilize more polar molecules [13, 14]. ILs have advantages as well as disadvantages. **Table 1** shows the summary of the physicochemical properties of ILs. The main disadvantages are high density, high viscosity, and low conductivity.

ILs are liquids showing high ion density. The density of ILs typically ranges between 1.2 and 1.5 g cm−3, while certain ILs, such as those based on the dicyanamide anion, has a density of less than 1 g cm−3. Because viscosity is important in conductivity and diffusion, conductivity decreases as viscosity increases. ILs have a significantly higher viscosity (30–50 cP) than water (H2O = 0.89 cP at 25°C) [15].

(30–50 cP) than water (H2O = 0.89 cP at 25°C) [15]. The conductivities at room temperature for ILs are in the wide range of from 0.1 to 18 mS cm−1, but even at the highest conductivity, they are much lower compared to conventional aqueous


#### **Table 1.**

*Summary of the physicochemical properties of ILs (adapted from Refs [13–16]).*

electrolyte solutions. Dilution of pure ILs with molecular fluids provides an increase in the conductivity of the medium. For example, pure [EMIM][BF4 − ] has a specific conductivity of 14 mS cm−1, while 2 moles of dm−3 solution in acetonitrile shows a conductivity of 47 mS cm−1. In this context, it can be said that diluting pure ILs with a molecular diluent also reduces the viscosity of the mixture [16].

At room temperature or below, RTILs are typically composed of organic or inorganic anions with weak basic properties and organic cations with low molecular symmetry [11, 13, 16]. It is a liquid group that offers more advantages over organic electrolyte solutions, including low vapor pressure, nonflammability, electrochemical and chemical stability, and high ionic conductivity. RTILs are currently of great interest in both academia and industry. To date, thousands of RTILs with unique physical features and functionalities have been created for use in energy applications. Some special boron-based RTILs have been investigated in EAs such as electrolyte materials in lithium batteries, fuel cells, and solar cells, as they exhibit ionic conductivity values of more than 10–2 S cm−1 at room temperature [11, 13, 16–22].

This review emphasizes on the highlights of boron's ability to facilitate the development of BBILs in EAs such as an atom and small molecule activation toward hyperbolic fuel additive, dye synthesized solar cells (DSSCs), advanced secondary batteries, hydrogen production and storage, electrolyte materials for electroreduction, and CO2 capture.

The first chapter emphasizes the role of boron cations and anions in the production of BBILs designed for usage in various EAs. Following that, examples of boron cations and anions found in the literature are given. Then, BBILs utilized in EA were thoroughly explored. Before detailing the selected examples of each area, a brief introduction was made and then closed with a brief perspective. Ultimately, this review highlights BBILs with less toxic and less expensive starting materials for future energy demands, as well as the possibility for ILs to play an essential part in meeting some of the future difficulties.

## **1.1 Boron's role in ILs**

With the increasing demand for renewable energy and green chemistry, boron has been playing a key role in energy-related research, from synthesizing energy-rich molecules to energy storage to converting electrical energy to light. In this regard, specifically constructed ILs have attracted the attention of researchers by including a variety of tribologically active elements such as sulfur, nitrogen, phosphorus, nitrogen, zinc, molybdenum, halogens, boron, and so on. Among these elements, boron's versatile chemistry makes it prominent in its use in EAs. Given the growing interest in boron chemistry, it is critical to understand why and how boron may become a favored element in IL functionalization [22–31].

Boron is one of the few elements known to show excellent harmony with ILs energy applications [22, 31]. **Table 2** lists the physical, atomic, and other characteristics of boron. Boron, symbol B, atomic number 5, and group 3A of the periodic table, is not found in elemental form in nature but may be produced in pure form by several processes [24, 25]. This position of boron between metals and nonmetals allows it to be employed in a wide range of research areas. Boron has an empty p orbital, so it has an electron deficiency. It has a sensitivity to undergo chemical reactions to saturate the coordination sphere and valence shell. Owing to the chemical properties and orbital nature of boron allow the formation of many useful ILs, including neutral, anionic, and cationic species.


#### **Table 2.**

*Physical, atomic and other properties of boron.*

While forming a compound, the empty p-orbital of the boron can lead to significant delocalization and is attacked readily by nucleophiles such as water or halides [22–38]. As a result, by bond cleavage of the neutral tri-coordinated borate, boron readily forms trivalent compounds with electrophile molecules such as oxides, sulfides, nitrides, and halides. Moreover, fluoride (F− ) and boron trifluoride [BF3] − may combine to create [BF4] − the anionic tetracoordinated borate species [31].

Because boron has an intrinsic electron deficiency, it is primarily defined by its Lewis acidity since it easily forms adducts by seizing electron pairs from Lewis bases [22, 28–30]. However, when certain conditions are met, the boron atom can become negatively charged or polarized and therefore, act as a nucleophile or Lewis base [27–30]. For example, boron and hydrogen combine to form many borane anions. The high hydrogen capacity of these borane anions makes them a suitable material for hydrogen storage. They are also possible candidates for ILs employed in electrochemical devices due to their unsymmetrical borate anions and bulky and anionic boron aggregates.

Many boron-atom/molecule-based ILs can be developed by tailoring cation-anion ion couples for specific purposes. Hitherto, elemental boron, carboranes, and organoboron compounds (cationic borinium (R2B1L2), borenium (R2B1L1), and boronium ions (R2B1), tetrahedral boron anions ([BH4 − ], [B12H12<sup>−</sup> ]), orthoborate anion ([BO3 − ]) were often employed in the synthesis of BBILs [22, 23, 28, 31]. Because of its electron-deficient nature, boron forms a series of highly chemically, electrochemically, and thermally stable anions and negatively charged boron clusters. For instance, organoboron anion compounds from the boron ion family have been frequently used in the formation of stable ILs in recent years, as they can dissolve in solvents with a low dielectric constant with reasonable solubility [31]. It has been indicated that carborane-based ILs with relatively high boron content, such as 1-carba-closododecaborate [CB11H12] −1, are exceptionally stable toward oxidation and coordination reactions due to their unique molecular structures [31, 32, 39–46].
