Frustrated Lewis Pairs: a New Way to Store Hydrogen

It is the most common element on Earth, can be produced using renewable energy, has the highest energy content per weight unit of any fuel, and its conversion to electricity is very efficient. Hydrogen has all that it takes to be the fuel of the future and the only remaining question seems to be: why aren’t we using it already? The problem is storage. Many methods have been tested and proven insufficient, and storing hydrogen continues to be extremely difficult. Compressing the gas under high pressure is an option, but the range of a car with such a compressed tank will not be more than 500 kilometers. To liquefy hydrogen, temperatures as low as -253 °C are needed, a condition which is not really practical in a car. Porous materials, capable of adsorbing large volumes of gas, have been tried as well. Unfortunately, the release of the gas is in that case often irreversible and/or uncontrollable, making also this option not ready for implementation. And the problems don’t end here. Even if hydrogen could be conveniently stored and controllably released, there are expensive metals needed for the electrodes in fuel cells. These rare metals such as platina are not available on a sufficient scale to provide the whole world with hydrogen-powered cars.

Luckily, there is a new promising alternative, which provides a solution to both problems. These interesting compounds consist of Lewis acids and Lewis bases, which are similar to the regular acids and bases we know. Acids, such as hydrochloric acid (HCl) and acetic acid (the acid in vinegar), want to lose an H⁺ (Figure 1). By losing this H⁺, the acid gains electrons and becomes negatively charged. Bases, on the other hand, want to bind an H⁺ and donate electrons to the H⁺ to form a bond. Acids take up electrons, while bases give them away. Lewis acids and bases work exactly like this, but there is not necessarily an H⁺ involved. It is just about electrons. When you put a Lewis acid and a Lewis base together, the Lewis base will give its electrons to the Lewis acid, and a bond between the two is formed. Now the two compounds are connected and a stable molecule called a Lewis adduct is formed (Figure 1). The base has lost its electrons, the acid has gained them, and everybody is happy.

Figure 1. Acids take up two electrons (represented by a hyphen), while bases donate electrons by forming a bond.

The Lewis adduct in Figure 1 is called ammonia borane and is one of the most promising candidates for hydrogen storage. It is light, solid at room temperature, and hydrogen-rich. Because both boron (B) and nitrogen (N) are light elements, the weight percentage of hydrogen in this molecule is 20%. This means that when you have 100 grams of this compound, you also have 20 grams of hydrogen atoms. To release these as hydrogen gas, a different compound is needed called a frustrated Lewis pair. This is a pair of a Lewis acid and Lewis base which cannot form an adduct because they are too big to come close to each other. When the Lewis acid and Lewis base are too bulky, they will approach each other as closely as possible, but never be able to form a bond (Figure 2). One of the things these frustrated Lewis pairs are really good at is splitting hydrogen gas. Hydrogen gas consists of two hydrogen atoms connected by two electrons (H-H). The Lewis acid of the frustrated pair will take up the electrons and one hydrogen atom, while the Lewis base takes up the remaining H+ (Figure 2). By increasing the temperature or applying a small voltage using cheap carbon electrodes, the hydrogen gas is released and the frustrated Lewis pair returns to its original state.

Figure 2. The frustrated Lewis pair consists of the phosphorus (P) Lewis acid and boron (B) Lewis base. Because the groups attached to P and B are too large, an adduct cannot be formed. When hydrogen (H-H) is added, the Lewis base takes up one H+ and the acid obtains electrons and one H. The reaction can be reversed by applying a voltage or raising the temperature.

Together, the Lewis adduct ammonia borane and a frustrated Lewis pair form a great team. A lot of research is done now into frustrated Lewis pairs to invent cheaper, more stable catalysts and improve the control over the release of hydrogen. If there will be hydrogen-powered cars on the road in the future, then it is quite certain that they have these compounds inside.