by Green hydrogen cannot be seen as environmentally friendly if it drinks large amounts of fresh water or results in the mass production of toxic chlorine, according to researchers at RMIT who say they have created a cheap technique that does neither.
Research into green hydrogen production is advancing rapidly at the moment, as countries around the world struggle to establish themselves in what is expected to become a huge global market for clean fuels. Australia’s vast renewable energy potential and export-oriented economy position it well to compete in high volumes internationally. But as a continent dominated by desert, it is also aware of the scarcity of water and the dangers of sending the life force of its lands abroad. Nine liters (2.4 gallons) of fresh water per kilogram (2.2 lb) of hydrogen bodes ill for mass production.
Creating green hydrogen from seawater is more difficult than using fresh water; there’s corrosion to think about, as well as plenty of impurities and micro-organisms. You need coastal locations close to renewable energy – not an issue for a country as large and relatively empty as Australia, but definitely a factor elsewhere. On some scale, you need to think about what you’re putting back into the ocean after you’re done – whether it’s creating dangerous levels of salinity or pumping high concentrations of toxic chlorine back into the marine environment.
But the benefits are enormous; not only is your water supply free when you use seawater, but if that hydrogen is burned or run through a fuel cell locally, fresh water is emitted that can be filtered through the water table and fed to parched land. The desalinated water is a huge bonus.
Therefore, there are currently many teams working on electrolysis techniques that generate green hydrogen from seawater. In December, we reviewed an efficient Chinese device that uses vapor pressure differentials to spontaneously evaporate pure water from seawater and then electrolyze it. A few weeks ago we highlighted an international team that found a surface treatment for standard electrolysers that converts them to work just as well in seawater. And in 2021, we looked at a very interesting method from Saudi Arabia that captures not just hydrogen, but salable quantities of chlorine and battery lithium phosphate, which would solve an additional global problem and look like a good deal in the market. process.
RMIT
Today, scientists at RMIT Australia announced another approach with great potential for generating highly efficient, low-cost green hydrogen direct from seawater, without generating chlorine.
“The biggest obstacle in using seawater is chlorine, which can be produced as a by-product.” said Dr. Nasir Mahmood, lead researcher on a newly published paper in the peer-reviewed Wiley journal Small. “If we were to meet the world’s needs for hydrogen (using seawater) without solving this problem first, we would produce 240 million tonnes per year of chlorine each year – which is three to four times what the world needs in chlorine. No. it makes sense to replace hydrogen produced by fossil fuels with hydrogen production that could be harming our environment in a different way. Not only does our process omit carbon dioxide, it also produces no chlorine.”
The RMIT device uses a new catalyst made from sheets of nitrogen-doped nickel-molybdenum phosphide (NiMo3P). Along each leaf layer are large pores – well, large at the nanoscale, anyway – designed to accelerate catalytic activity and mass transfer.
RMIT
Nitrogen doping, the team says, performs several functions, including increasing conductivity, optimizing electron density and surface chemistry, and creating new active sites for water catalysis in leaves. The electronegative properties that arise when nitrogen binds to surface metals help prevent unwanted ions and molecules from touching the catalyst surface, and the presence of phosphate, sulfate, nitrate, and hydroxyl ions on the surface serves to block chlorine and prevent the corrosion.
Experimentally, the team found that this catalyst exhibited excellent efficiency and completely suppressed chlorine generation. “The N-NiMo3P leaves exhibit exceptional HER overpotential values (hydrogen evolution reaction) of 23 and 35 mV at 10 mA cm-2 in alkaline electrolytes and seawater, respectively”, says the study. “In addition, for total separation from water, requires only 1.52 and 1.55 V to reach 10 mA cm-2 in alkaline electrolyte and seawater, respectively. These exceptional results demonstrate that low-cost hydrogen can be generated from seawater, regulating the structure and composition of 2D materials.”
“These new catalysts consume very little energy to function and can be used at room temperature,” clarifies Mahmood in a press release. They must also be relatively inexpensive and easy to produce on the large scale that the green hydrogen market must demand.
RMIT
“To be truly sustainable,” says Mahmood, “the hydrogen we use must be 100% carbon-free throughout its production lifecycle and must not cut into the world’s precious freshwater reserves. Our method for producing hydrogen directly from Seawater is simple, scalable and far more cost-effective than any green hydrogen approach currently on the market. With further development, we hope this can promote the establishment of a thriving green hydrogen industry in Australia.”
The team will move to scale up as research continues. The next step is to build a prototype electrolyser system by running a stack of these catalyst sheets to produce large amounts of hydrogen and start optimizing efficiency at the system level at scale. Mahmood believes this technology can help meet the Australian government’s target of producing green hydrogen at AU$2/kg (US$1.40/kg), the level at which it becomes cost-competitive with dirty hydrogen produced with fossil fuels.
Ultimately, that’s the key metric here; For companies to invest heavily in large-scale green hydrogen production, they need to know that they can do so profitably and compete with other hydrogen and fuel sources. We’ll see how it all pans out, but we certainly hope that some of these seawater-splitting innovations work just as well on a swing set as they do in the lab.
Check out a short video below.
RMIT’s efficient and cost-effective catalyst for producing hydrogen from seawater
The article is open access in the journal Small.
Source: RMIT
