A recent paper published in Green Chemistry investigates using bicarbonate of soda as a storage solution for green hydrogen energy.
The demand for efficient and cost-effective energy storage solutions is increasing as the world transitions towards more sustainable energy sources. Renewable energy sources such as wind and solar power are intermittent and seasonal; this requires an effective energy storage method that does not consume fossil fuels.
Using renewable energy to form chemical bonds is an accepted method of energy storage; energy can only be converted from one form to another, never destroyed. Breaking the chemical bonds in molecules releases this energy for further use.
Hydrogen (H2) is hailed as an attractive energy source; it is the most abundant element in the universe and is found in 75% of all matter. Hydrogen gas also has a very high bond dissociation energy of 463 kJ/mol, which releases a large amount of energy when the covalent bond between the two hydrogen atoms is broken.
Green hydrogen production
Hydrogen gas is classed as green when produced without any input of fossil fuels, or the release of greenhouse gases. One method of green hydrogen production is electrolysis — the electrochemical splitting of water into molecular hydrogen and oxygen using renewable energy. When electrolysis is reversed, the hydrogen bonds are broken, and energy is released, allowing hydrogen to act as an energy store that can be turned ‘on and off’.
Electrolysis has an efficiency issue that is partly solved by the use of platinum catalysts that speed up the reaction. However, platinum is a high-value material that is not feasible for large-scale industrial use. This cost issue is one of the reasons why 95% of hydrogen is currently produced using methane as the source, which produces CO2 in the process.
The issue with hydrogen gas as an energy store lies with the danger of transport and physical storage, due to hydrogen’s extreme volatility (explosiveness). Vast amounts of pressure and thick containers are needed to transport hydrogen safely.
While pure hydrogen gas poses dangers for storage and transportation, when locked into a liquid solution, the risks can be mitigated. It is this liquid storage that formed the basis of the recent paper published in Green Chemistry, co-authored by Thomas Autrey Oliver Gutiérrez of the Pacific Northwest National Laboratory. The paper investigates the bicarbonate-formate cycle as a liquid hydrogen carrier.
The bicarbonate-formate cycle
Sodium bicarbonate, commonly known as bicarbonate of soda, is the primary element of the bicarbonate-formate cycle. Aqueous (liquid) formate ions (HCO2−), composed of hydrogen and carbon dioxide, react with water when a catalyst is introduced. The products are molecular hydrogen and bicarbonate (HCO3−) only, which do not have a detrimental impact on the environmental.
This process can also be reversed using pressure changes to release and store hydrogen, acting as a cycle. Once the hydrogen gas is released from the cycle, it can be combined with oxygen in the reverse electrolysis reaction to release the energy stored in the bonds.
Bicarbonate and formate are both non-toxic and non-flammable (unlike molecular hydrogen); this is a neat way of avoiding the issue of hydrogen gas transportation by storing the hydrogen within the formate ions.
The paper emphasises that the bicarbonate-formate cycle is in the early stages of research for hydrogen storage. Currently, the standard amount of hydrogen that can be stored in gas form is 70 kilograms per cubic metre, whereas the cycle stores 20. Moreover, the process relies on electrolysis to release the energy from the hydrogen; further research to decrease the cost of electrolysis is vital for large-scale use.
A benefit of the cycle is that it could be coupled with captured CO2 as part of the formate to maximise sustainability. Carbon capture and utilisation is a promising avenue for decarbonisation of industry and is currently a popular avenue.
While this research is impressive and exciting, improving the bicarbonate-formate cycle’s storage capacity and electrolysis efficiency is the next step to making green hydrogen more economically viable.