Storing H2 with Si2BN

By Thomas HornigoldMar 26 2018

Table of Contents

The Hydrogen Economy
Transporting Hydrogen
Si₂BN as a Solution

The Hydrogen Economy

A recurring dream that could one day become a reality involves the so-called “hydrogen economy.” Hydrogen, which can be produced by electrolysis (passing an electrical current through water), can also be burned as a clean fuel. The only by-product of burning hydrogen is water.

The idea is that hydrogen fuels and fuel-cell cars could replace existing vehicles. Hydrogen could be created by electrolysis that is powered by solar power plants and wind farms at times when supply is highest. It will then act as a store for this energy, getting around the intermittency problem.

However, several practical factors hinder this ideal. One of them is the inevitable energy losses involved in hydrogen fuel production. Burning fuel to generate electricity, to then produce a fuel to burn again, is a process with too many steps to be efficient. Additionally, until hydrogen produced by renewables becomes price-competitive, the financial incentives also won’t be in its favor. There is also the issue of transporting and storing hydrogen - to this day, the perfect way to store hydrogen is still being explored.

Transporting Hydrogen

Hydrogen has a very high energy per unit mass, but it’s not particularly dense at normal temperatures. Transporting gaseous hydrogen around requires large tanks and regular refueling. Compressed gas or liquid hydrogen can be used, but the high pressures and low temperatures needed for this kind of storage present their own challenges.

For this reason, there has been considerable materials research effort dedicated to finding materials with suitable chemical properties - the ability to store large amounts of hydrogen, at high densities, in a reversible way, allowing for the hydrogen to be accessed and burned. Adsorbing hydrogen onto the surface of a suitable material would be far denser and more compact, with a lower explosion potential, so this could be a possibility.

Si₂BN as a Solution

Naturally, due to its famously useful properties as a semiconductor, silicon came in for considerable analysis and earlier this decade, scientists began making theoretical predictions about a material called Si₂BN – a 2D material with a hexagonal lattice of silicon, boron, and nitrogen atoms. The material was predicted back in 2016, and it's already been thoroughly characterized using theoretical calculations.

Amongst these, a paper in Nano Energy in 2017 described that as Si₂BN is predicted to have the properties associated with many 2D materials (strength, flexibility, tunable band gap, high thermal conductivity and high electron mobility), it might be useful as a high-capacity anode for lithium-ion batteries. The paper’s calculations suggest that the key to this is the Si-Si-bond, which gives the structure its electronically favorable properties, allowing lithium ions to be adsorbed easily. In fact, the ions are stored directly over the center of the lattice hexagon.

The theoretical capacity of this material to adsorb lithium ions could be as much as 5x greater than existing anode materials, and it compares favorably to other 2D materials that have already been synthesized, such as Silicene, borophene, and 2D black phosphorous. As lithium/sodium ions are intercalated into the material, they cause its structure to buckle. This both allows it to exceed the higher capacities of other 2D materials and allows for fast diffusion of ions – an important factor affecting battery recharging.

In 2D materials, a key point to understand is that the properties of such materials can be highly influenced by doping, adding additional molecules, or stacking more than one 2D material together. Depositing a layer of atoms into a bulk material is unlikely to change its properties too much, but on the surface of a new material, you can shape the electrostatic potentials to your will.

The theoretical use of Si₂BN as an efficient store of hydrogen has so far focused on a version “doped” by adding palladium ad-atoms onto the lattice. This then makes the material an efficient store of H₂ molecules, with a high binding energy of up to 2eV. According to these theoretical calculations, first published in the Journal of Hydrogen Energy in 2017, the H₂ gravimetric storage capacities (roughly speaking, the fraction of the storage unit’s weight that is hydrogen at maximum capacity) was between 6.95-10.21% by weight; each Pd adatom can store up to six H₂ molecules.

This potentially puts Si₂BN in an unusual position. The US Department of Energy set targets for its fuel cells regarding their gravimetric capacity. They hoped for 6.5% as the ultimate target for gravimetric storage density, but the maths suggests that Si₂BN can exceed that. It now remains to ensure that it meets other criteria – affordability, stability, safety and so on – as well as demonstrating that it can be mass-manufactured, to become a serious contender.

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