Boron: Its Role in Energy-Related Processes and Applications

Abstract

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications.

Keywords: OLEDs; boron; electrolytes; hydrogen; small-molecule activation.

Figure 1

Schematic representation of the classes of molecular boron species described in this section.

Figure 2

Selected recent highlights of boron‐based small‐molecule activation. NHC=N‐heterocyclic carbene, CAAC=cyclic (alkyl)(amino)carbene, Dur=2,3,5,6‐tetramethylphenyl, Tip=2,4,6‐triisopropylphenyl.

Figure 3

Hydrogen evolution by thermolysis of ammonia borane and its regeneration.

Figure 4

Partial regeneration of borohydride.

Figure 5

Some C‐, B‐, N‐containing heterocyclic compounds investigated for hydrogen storage.

Figure 6

Examples of boron‐based WCAs.

Figure 7

Examples of borate anions used in battery electrolytes.

Figure 8

Structures of selected (cyano)borate anions and selected properties of the respective EMIm‐ILs: dynamic viscosity η(20 °C), specific conductivity σ(20 °C), melting point (T _m; DSC onset), decomposition temperature (T _dec; DSC onset), and electrochemical window (ΔE=E _ox−E _red).

Figure 9

The most relevant parent boron clusters with respect to metal‐ion batteries (top) and a complex Mg²⁺ salt that contains 1,7‐carboranyl ligands (bottom, thf=tetrahydrofuran).

Figure 10

Representative structures of tri‐ and tetracoordinate boron compounds used as either fluorescent emitters or charge‐transport/blocking materials in OLEDs.

Figure 11

Representative examples of donor–acceptor boron‐based TADF emitters (a) and multiresonance boron‐based TADF emitters (b), as well as a diagram illustrating the difference between TADF and phosphorescence (c).