Seawater Hydrogen

SCIENTISTS have developed a system that can produce green hydrogen directly from seawater without the need for any pre-treatment processes like desalination. The team behind the development, which involves the introduction of a Lewis acid layer on a transition metal oxide catalyst, say the method shows high potential for commercial application.

Over 97% of the water on Earth's surface is saline water in the oceans, 2% is stored as fresh water in ice caps, glaciers and snow-capped mountain ranges, and just 1% is available for our daily water supply needs.

 
Saline water can be made into potable water through a process called desalination, a technique that some areas around the world rely on to produce fresh water for human consumption and for domestic and industrial use. But desalination is an energy-demanding process, and worse still it is often powered by energy sources which are unsustainable.

Splitting water into its constituent parts is also well understood. The process – known as electrolysis – uses a direct current between two electrodes immersed in an electrolyte to split water into hydrogen and oxygen. Hydrogen is formed at the cathode, or negative electrode, and oxygen at the positive electrode, or anode.

Because a mix of the gases can explode, most electrolysers separate the anode and cathode with a thick, porous plastic sheet, and metal catalysts such as nickel and iron are used to speed up reactions.

Putting both of these processes together, namely desalinating seawater, and then splitting it to create hydrogen has long been hailed as one of the best solutions to provide clean and affordable fuel for energy, that in turn could power everything from a city's electricity, to making steel, producing fertiliser, and even as fuel for airplanes – the list of potential uses is a long one.

However, one of the reasons we're not already using hydrogen fuel to fly around the world, is that saltwater and other impurities corrode electrodes, shortening their life. As those components are typically made of rare metals such as platinum, it costs too much to keep replacing them. Chloride ions in seawater are also a problem and chlorine electro-oxidation reactions (ClOR) compete with oxygen evolution reaction (OER) on the anode during electrolysis. This reaction results in the release of toxic and corrosive chlorine species such as hypochlorite. Hypochlorite is relatively unstable, it can release toxic chlorine gas when mixed with ammonia or acid and it can also eat away at stainless steel.

To get around this, the seawater could be desalinated and purified before processing it, but this is not always financially viable either. Another option is to coat the electrodes with polyanions to suppress corrosion, but this too can be costly.  

Few climate solutions come without downsides. "Green" hydrogen, made by using renewable energy to split water molecules, could power heavy vehicles and decarbonize industries such as steelmaking without spewing a whiff of carbon dioxide. But because the water-splitting machines, or electrolyzers, are designed to work with pure water, scaling up green hydrogen could exacerbate global freshwater shortages. Now, several research teams are reporting advances in producing hydrogen directly from seawater, which could become an inexhaustible source of green hydrogen.

Today, nearly all hydrogen is made by breaking apart methane, burning fossil fuels to generate the needed heat and pressure. Both steps release carbon dioxide. Green hydrogen could replace this dirty hydrogen, but at the moment it costs more than twice as much, roughly $5 per kilogram. That's partly due to the high cost of electrolyzers, which rely on catalysts made from precious metals. The U.S. Department of Energy recently launched a decadelong effort to improve electrolyzers and bring the cost of green hydrogen down to $1 per kilogram.

If they succeed and green hydrogen production skyrockets, pressure could build on the world's freshwater supplies. Generating 1 kilogram of hydrogen using electrolysis takes some 10 kilograms of water. Running trucks and key industries on green hydrogen could require roughly 25 billion cubic meters of fresh water a year, equivalent to the water consumption of a country with 62 million people, according to the International Renewable Energy Agency.

Seawater is nearly limitless, but splitting it comes with its own problems. Electrolyzers are built much like batteries, with a pair of electrodes surrounded by a watery electrolyte. In one design, catalysts at the cathode split water molecules into hydrogen (H+) and hydroxyl (OH-) ions. Excess electrons at the cathode stitch pairs of hydrogen ions into hydrogen gas (H2), which bubbles out of the water. The OH- ions, meanwhile, travel through a membrane between the electrodes to reach the anode, where catalysts knit the oxygen into oxygen gas (O2) that is released.

When seawater is used, however, the same electrical jolt that generates O2 at the anode also converts the chloride ions in saltwater into highly corrosive chlorine gas, which eats away at the electrodes and catalysts. This typically causes electrolyzers to fail in just hours when they can normally operate for years. 

Hydrogen is a versatile chemical used for the production of many products, including fertilizers. Hydrogen is also a key component of fuel cell technology, which harnesses the electricity produced by renewable but intermittent energy sources like solar and wind. Most of the hydrogen produced worldwide derives from a process in which methane is exposed to heat and steam to yield hydrogen.

Hydrogen can also be produced from the electrolysis of water, which uses electricity to split water molecules into hydrogen and oxygen powered by renewable sources such as solar and wind. But there's a catch. Electrolysis requires very clean water that has been deionized, meaning all the impurities, minerals, and electronically charged particles must first be removed. Conventional water purification processes require expensive equipment and can result in energy loss.

Researchers in Johns Hopkins University's Department of Environmental Health and Engineering, in collaboration with Penn State University, have found a way to use seawater as a direct source of hydrogen, with no need for preliminary desalination. Their results appear in Environmental Science & Technology.

"We found that we can use thin-film composite membranes, which are used to purify salt water, in water electrolyzers, splitting the water into hydrogen gas and oxygen, while avoiding producing harmful chlorine gas, which happens with other membrane types."
In their study, Rossi and colleagues tested thin-film composite membranes directly in the electrolyzer-a device that uses electricity to split water into hydrogen and oxygen-accomplishing in a single step both water purification and hydrogen production. They found that the material's porous microstructure allowed only small protons and hydroxide ions to migrate across the membrane, rejecting impurities and other ions that can produce undesirable reactions. The researchers say that this novel approach could replace conventional systems, where expensive ion-exchange membranes are used in combination with ultrapure water feeds.

"Cheap water desalination membranes can be an alternative to more expensive polymer-based membranes and can be used for hydrogen production from low-grade water sources like seawater," said Rossi. "The result is an efficient hydrogen production process from renewable energy sources that eliminates the need for water purification."

He noted that seawater is challenging to use in electrolyzers because of its high salinity. However, it is abundant and available in locations such as coastal areas, where renewable electricity like solar and wind can be generated, but where there is low availability of fresh water. In such locations, other low-grade water sources such as wastewater could potentially be used instead of seawater in this process.  

The U.S. National Science Foundation-funded team integrated water purification technology into a new proof-of-concept design for a seawater electrolyzer, which uses an electric current to split apart the hydrogen and oxygen in water molecules.

This new method for "seawater splitting" could make it easier to turn wind and solar energy into a storable and portable fuel, according to Bruce Logan, an environmental engineer.

"Hydrogen is a great fuel, but you have to make it," Logan said. "The only sustainable way to do that is to use renewable energy and produce it from water. You also need to use water that people do not want to use for other things, and that would be seawater. So, the holy grail of producing hydrogen would be to combine the seawater and the wind and solar energy found in coastal and offshore environments."

Despite the abundance of seawater, it is not commonly used for water splitting. Unless the water is desalinated prior to entering the electrolyzer, an expensive extra step, the chloride ions in seawater turn into toxic chlorine gas, which degrades the equipment and seeps into the environment.

To prevent this, the researchers inserted a thin, semipermeable membrane, originally developed for purifying water in the reverse osmosis treatment process. The reverse osmosis membrane replaced the ion-exchange membrane commonly used in electrolyzers.
"The idea behind reverse osmosis is that you put really high pressure on the water and push it through the membrane and keep the chloride ions behind," Logan said.

Through a series of experiments published in Energy & Environmental Science, the researchers tested two commercially available reverse osmosis membranes and two cation-exchange membranes, a type of ion-exchange membrane that allows the movement of all positively charged ions in the system. 

Clean energy is a top priority for countries worldwide. While conventional power relies on fossil fuels like coal, natural gas, and oil, clean energy comes in various forms such as solar, wind, geothermal, hydroelectric, and biomass.

Hydrogen, too, is a leading energy storage option for renewables and could help reduce the high levels of carbon emissions.
Current research suggests that saltwater electrolysis – the process of splitting water into oxygen and hydrogen – is a viable solution to the common challenges of freshwater electrolysis. Seawater electrolysis could produce sustainable hydrogen without worsening the global freshwater shortage.

According to the United States Department of Energy Alternative Fuel Data Center, pure hydrogen is an abundant element on Earth that shows great promise in supporting the transition to clean, sustainable, and renewable energy.

After hydrogen is produced, it can generate electricity in a fuel cell and only emits water vapor and warm air. Because hydrogen does not release any greenhouse gases, nitrogen oxides, hydrocarbons, or other particulate matter, it does not negatively affect the environment.
Hydrogen has other benefits that will help create a clean energy economy. It's an optimal energy solution in typically challenging areas to decarbonize. It increases the reliability and resilience of the modern power grid. It can also improve public health and the state of the environment.

In addition, it can increase the number of employment opportunities and energy security in global industries. It can help the transportation industry become more sustainable and support the shift to electric vehicles (EVs). And it can contribute to increased revenue and strengthens the world economy.

One challenge driving up costs associated with producing green hydrogen is that electrolyzers require ultrapure water. This makes traditional saltwater electrolysis difficult because many water sources are filled with contaminants.
Although the EPA has strict requirements for water due to the presence of lead, chlorine, and bacteria, it does not necessarily mean all water is free of contaminants.

Seawater electrolysis
Seawater electrolysis research emerged in the early 19th century. Although scientists made advancements in hydrogen production, it never gained traction or became a viable energy solution. In the 20th century, hydrogen was mostly extracted from natural gas and used to power cars, buses, blimps, and rockets.

While using this hydrogen was feasible, its production was energy-intensive and contributed to carbon emissions, one of the main causes of climate change. Additionally, some cities filter municipal solid waste with hydrogen fuel cell technology, which produces hydrogen and prevents waste-derived contamination in local water supplies.

Various researchers and scientists are developing advanced technologies using seawater electrolysis to avoid these challenges. If these technologies work properly, they will produce sustainable hydrogen without using freshwater resources or contributing to carbon emissions.

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