Thousands of tonnes of hydrogen isotope mixtures are processed annually to produce heavy-water and ensure the safe decontamination of tritium isotopes. Whilst there are currently technologies in use in industry, they consume an enormous amount of energy and require large capital investments.
A team of researchers from the Physics Department and the National Graphene Institute at Manchester University, in the UK, have developed a graphene-based electrochemical pump, with the potential of it being used commercially to reduce the industry’s footprint.
The separation of hydrogen isotopes is a monumental task which costs the nuclear industry a lot of money and energy. The production of heavy water (D2O), is on the scale of thousands of tonnes per year. Currently, the production of 1 kg of heavy water requires 10 Mwh of energy, the same amount that an average household in the United States uses annually.
There are two main reasons why the production is so expensive. The first, is that there is a low abundance of deuterium isotopes which currently requires a huge amount of water to be processed to produce a sufficient amount of enriched heavy water. The second, is that current technology is only equipped to separate these isotopes after hundreds of stages.
Aside from the production of heavy water, the separation, and removal, of tritium contaminants is another challenging area. Nuclear reactors often suffer at the hands of tritium contaminants, which often leads to a decrease in their operational efficiency. Many nuclear facilitates rely on tritium-based fuel, and when accidents occur at these facilities they leave behind thousands of tonnes of diluted tritiated water.
There is currently a demand for new technologies that produce an enhanced separation efficiency for deuterated and tritiated water, by reducing the number of stages and reducing the overall energy consumption.
The team of Manchester-based researchers have created a graphene-based electrochemical pump using a roll-to-roll fabrication method which uses a large-area membrane (30 m2) composed of CVD-grown graphene supported on a nafion (a commercially available polymer) substrate.
The researchers decorated the graphene sheet with palladium nanoparticles, through electron-beam evaporation methods, and covered with a carbon cloth to electrically contact the graphene sheet across the whole area of the device.
The researchers analysed the isotopes selectivity and hydrogen throughput using a custom fuel-cell system that employed the use of a mass spectrometer (MS).
The researchers detail that it is not the holes in the graphene sheet that provide a permeation path, as the device is electrochemically pumped. It is the covering of the electrically conducting graphene sheet, combined with the carbon cloth, which helps with ion selectivity.
One of the other characteristics of electrochemical pumping is the throughput. The throughput dependence revolves around Faradays constant, and resultantly the Faradaic efficiency. The researchers managed to achieve a 100% Faradaic efficiency, which allowed for the conversion of the electric current into a hydrogen flux.
Despite cracks and defects in the graphene sheet, the researchers managed to achieve a separation factor of 8, which lies just below current technologies used in industry. However, once scaled up the researchers believe that the energy consumption will be magnitudes lower than the separation processes exhibited today.
The separation efficiency for tritium uses a proton-deuteron sieving mechanism to achieve a separation factor of around 30. In comparison, current methodologies involve a cryogenic distillation process and only achieve a separation factor of 1.8 for hydrogen isotope mixtures.
The researchers enriched, as per current technology, the deuterium from its naturally occurring concentration of 0.015% up to 20% and only used 3,800 moles of hydrogen to create 1 mole of deuterium. By just using this small membrane of 30 m2, the researchers estimate that up to 50,000 m3 of gas mixtures could be processed in a year. The energy consumption was found to be roughly 20 GJ per kg of enriched product, which is 10 GJ per kg less than current technologies use.
In addition, the commercialisation potential of graphene in today’s world allows for there to be an abundance of graphene for use in these pumps. With respect to the scale-up process, the pump could be easily scaled and the researchers believe that there will be no foreseeable problems associated with the scale-up process.
Upon optimisation and scale-up, it is thought that the pump will be comparable in terms of separation efficiency of heavy water compared to technology nowadays, and even greater for tritium separations.
There have also been no environmental issues found with the pump as it uses no toxic or corrosive chemicals through the fabrication stage.
As such, the simplicity, efficiency and green-nature of the graphene-based pump will most likely be a consideration for the nuclear and other related industries over the next few years, with the researchers stating that “the graphene-based separation seems significant enough to justify rapid introduction of this disruptive technology even within the highly conservative nuclear industry”.
Source: AzoNano, Liam Critchley
Publication Journal: “Scalable and efficient separation of hydrogen isotopes using graphene-based electrochemical pumping”- Lozada-Hilgado M, et al, Nature Communications, 2017, DOI: 10.1038/ncomms15215