Bench Talk

As the world’s dependence on fossil fuels and non-renewable energy sources is shrinking in today’s greener society, we are seeing a shift towards a higher usage of renewable energy in all forms. However, one of the limiting factors of renewable energy has been the ability to store this energy, and this is the reason why it has taken electric vehicles a long time to reach the market. The same can be said for portable technologies, such as phones and laptops, where one of the major selling points of these technologies are their battery life. Battery life, when it comes down to it, is the battery’s ability to efficiently store energy until its usage is required.

The underpinning principles of energy storage, and therefore any advances that have arisen in recent years, are due to various chemical interactions and principles. It should be noted that these chemical reactions are not like the reactions most would associate with chemistry—i.e., taking one molecule and reacting it with another to create a different molecule. Rather, the principles are electrochemically based, which involves the change in the charge of an ion to facilitate electron movements and includes mechanisms such as ion and electron migration mechanisms, the utilization of holes in a solid-state lattice, and electrochemical half reactions, amongst others. Here, we detail how some common energy storage devices rely on chemistry to function efficiently.

Capacitors are one of the oldest devices to store energy and do so by storing potential energy in an electric field. Capacitors work using a phenomenon known as capacitance, which is the change in electrical charge with respect to a change in the electrical potential. There are many different types of capacitors, all of which have slightly different mechanisms, but standard capacitors possess two conductive regions separated by a non-conductive dielectric plate or a vacuum.

In a capacitor, when an electrical charge from another component, device, or other electrical source causes a charge to manifest in one of the conductors, a force will be exerted on the charge carriers within the other conductor. This causes the charge carriers to attract oppositely charged species while repelling like charges to the other conductor, which leads to a separation of charges between the surfaces of the two conductors. Because both conductors have an equal amount of charges, the dielectric region separating the two conductors develops an electric field.

Energy can be stored within a capacitor because the conductor regions are close together. This short distance causes the charges to be attracted to each other in the presence of the electric field. The capacitor is then able to store a large amount of electrical energy for a defined voltage by doing a large amount of work to move the plates together. The amount of work done exponentially increases as the distance between the plates decreases.

There are many different batteries in use today—from aluminum-based batteries to various types of lithium batteries (Li-air, Li-metal, etc.) and redox flow batteries. Given that the focus here is on energy storage, the focus is on rechargeable Li-ion batteries because they are so widely used and redox-flow batteries as they are used for larger scale energy storage applications.

Li-ion

Li-ion batteries store energy when they are charged and slowly release this energy upon discharge—i.e., when the device holding the battery is turned on. For storing energy, these batteries undergo charging, which is facilitated by a reduction reaction at the cathode. This reduction reaction causes electrons from an external source—such as mains electricity—to combine with the lithium ions. This then causes the ions to migrate and intercalate within the anode; whereupon, the energy is essentially stored in the form of bound electrons within the lithium ions. When a user switches a device on, the battery will undergo a discharge process via an oxidation reaction. The discharge process moves the lithium ions to the cathode and releases electrons, which generates the usable current. If a battery is not performing either of these mechanisms, then the ions desorb into the electrolyte between the two electrodes.

Redox-Flow

Redox-flow batteries are designed more for bulk energy storage. Though redox-flow batteries can be used with various transition metal ions, they are most commonly used with iron, vanadium, and zinc. Redox-flow batteries are very different than Li-ion systems. One of the main differences is that two solvent systems are involved in the electrochemical process. Each of these solvent systems has active components—often a metallic salt—dissolved within the solvent. This dissolution can take the form of a partial or full dissolution of the active component.

The electrolyte solutions are often kept in separate external tanks. The solutions are pumped around the system while passing an electrode, yet kept separate from each other by an ion separator membrane. The ion-exchange mechanism between the two electrodes creates the electrical potential and generates electricity. The reason for their use in larger-scale energy storage applications is that the capacity to store energy within these tanks is much greater than conventional batteries.

In these systems, the electrochemical reactions and energy storage occur within the electrolyte rather than the electrode and rely upon electrochemical half reactions. Upon discharge, the anodic side undergoes an oxidation reaction. This reaction releases an electron, which then travels to the cathodic side of the cell where it is accepted via a reduction reaction. The charging mechanism is the same principle, only that the direction of the current and the electrochemical reactions are reversed. The whole system is balanced through the exchange of positively charged hydrogen ions to maintain charge neutrality.

Hydrogen storage has been more recently realized than the other methods detailed here, yet it is an important area for the development of electric vehicles (EV). Hydrogen storage is very versatile, and the hydrogen ions, which are the energy source in this instance, can be stored in a number of gaseous, liquid, and solid forms. Physical storage of hydrogen is basic to understand, and takes the form of compressed hydrogen gas, liquid hydrogen stored in tanks, and hydrogen cooled to cryogenic conditions then compressed.

However, there are many different types of hydrogen storage in solid forms which use the principles of organic and materials chemistry to store the hydrogen safely. In each of these storage mechanisms, the hydrogen will be stored until a stimulus—such as heat—causes the chemical nature of the storage medium to change and release the hydrogen.

There are two common methods of storing hydrogen within solid materials:

• The first method is through chemical absorption in the surface of a solid—such as a metal organic framework (MOF).
• The second method is through interstitial migration into a solid lattice. This method also starts with hydrogen molecules being adsorbed in the surface of the solid. The hydrogen molecules are then dissociated into hydrogen atoms and stored within the holes of the solid-state lattice.

The final methods in which hydrogen can be stored—again relies heavily on chemistry—are through chemical reactions.

• The first method is through the reaction with metals to form a metal hydride. In this type of chemical reaction, the hydrogen becomes an interstitial ion between the metal ions.
• The second method is through liquid organic reactions with chemical compounds that possess a high affinity for hydrogen absorption—such as carbazole derivatives and toluene.
• The third method is through the reaction of hydrogen-containing materials with water or other oxygen-rich solvents. In this type of chemical reaction, the hydrogen is stored in both the material and the solvent.

Conclusion

Overall, there are many ways to store energy, but in many cases the type of energy storage device used is dependent upon the intended application. However, regardless of the application every energy storage device relies on efficient electrochemical mechanisms to not only migrate any relevant ions and electrons but to also store the energy potential for when it is required. Therefore, chemistry is at the heart of any energy storage application.