A US-based research team has found a way to separate hydrogen from a mix of other gases in a new, cost-effective way using pumps.
Purified hydrogen is used for refining metals, manufacturing fertilisers and powering fuel cells for heavy vehicles, but the purification process can involve several difficult steps.
A research team led by Penn State associate professor of chemical engineering, Chris Arges, demonstrated that the process can be simplified using a pump outfitted with newly developed membrane materials.
The researchers used an electrochemical hydrogen pump to both separate and compress hydrogen with an 85 per cent recovery rate from fuel gas mixtures known as syngas, and 98.8 per cent recovery rate from conventional water gas shift reactor exit stream — the highest value recorded. The team detailed their approach in ACS Energy Letters.
Traditional methods for hydrogen separations employ a water gas shift reactor, which involves an extra step, according to Mr Arges. The water gas shift reactor first converts carbon monoxide into carbon dioxide, which is then sent through an absorption process to separate the hydrogen from it. Then the purified hydrogen is pressurised using a compressor for immediate use or for storage.
The key, Mr Arges said, is to use high-temperature, proton-selective polymer electrolyte membranes (PEMs), which can separate hydrogen from carbon dioxide and carbon monoxide and other gas molecules quickly and cost-effectively.
About the pump
The electrochemical pump, equipped with the PEM and other new materials Arges developed, is more efficient than conventional methods, because it simultaneously separates and compresses hydrogen from gas mixtures. It also can operate at temperatures of 200 to 250 degrees Celsius – 20 to 70 degrees higher than other high-temperature, PEM-type electromechanical pumps – which improves its ability to separate hydrogen from unwanted gases.
“This is an effective and potentially cost-saving way to purify hydrogen, especially when there is a large carbon monoxide content,” Mr Arges said.
“No one has ever purified hydrogen to this extent with a gas feed that contained more than three per cent of carbon monoxide using an electrochemical hydrogen pump, and we achieved it with mixtures that consist of up to 40 per cent carbon monoxide by using a relatively new class of high-temperature PEM and electrode ionomer binder materials.”
How it works
To carry out the separation, the team created an “electrode sandwich” where electrodes with opposing charges form the “bread” and the membrane is the “deli meat”. The electrode ionomer binder materials are designed to keep the electrodes together, like the gluten of the bread.
In the pump, the positively charged electrode, or bread slice, breaks down the hydrogen into two protons and two electrons. The protons pass through the membrane, or deli meat, while the electrons travel externally through the pump using a wire that touches the positively charged electrode. The protons then travel through the membrane to the negatively charged electrode and recombine with the electrons to reform the hydrogen.
The PEM works by permitting the passage of protons but preventing the larger molecules of carbon monoxide, carbon dioxide, methane and nitrogen from coming through, according to Mr Arges. For the electrode to work effectively in the hydrogen pump, Mr Arges and his team synthesised a special phosphonic acid ionomer binder that acts as an adhesive to keep the electrode particles together.
“The binder is effective for making a mechanically robust, porous electrode that permits gas transport so hydrogen can react on the electrocatalyst surface while also shuttling protons to and from the membrane,” Mr Arges said.
The researchers plan to investigate how their approach and tools will aid in purifying hydrogen when stored in existing natural gas pipelines. Distributing and storing hydrogen in this manner has never been accomplished, but holds great interest, according to Mr Arges. He explained that hydrogen could aid in generating electric power via a fuel cell or turbine generator to support solar or wind energy-based systems and a variety of more sustainable applications.
“The challenge is that hydrogen has to be stored at low concentrations in the pipeline – less than five per cent – because it can degrade the pipeline, but end-use applications require more than 99 per cent pure hydrogen,” Mr Arges said.
Mr Arges filed two US patent applications on components used in this research while he was on faculty at Louisiana State University. One is on high-temperature PEMs, and the other is on the electrochemical hydrogen pump using the high-temperature PEMs and phosphonic acid ionomer electrode binder.
He is currently licensing the technology for a startup company he co-founded with his wife, Hiral Arges, called Ionomer Solutions LLC.
Deepra Bhattacharya, Penn State doctoral student in chemical engineering, co-authored the paper. Other contributors include Gokul Venugopalan, postdoctoral researcher in the Chemistry and Nanoscience Research Center at the National Renewable Energy Laboratory in Golden, Colorado, and former doctoral student of Mr Arges; and Evan Andrews, Luis Briceno-Mena, José Romagnoli and John Flake, chemical engineering researchers from Louisiana State University.
The US Department of Energy’s Office of Energy Efficiency and Renewable Energy funded this work.