Generating hydrogen : Electrolyzing seawater without side-reactions


A team of researchers affiliated with multiple institutions in China has developed a type of electrolysis that works with native seawater and does not have side-reactions or corrosion problems.

In their study, published in the journal Nature, the group tested their process in a real-world location. Heping Xie, with Shenzhen University, has published a Research Briefing in the same journal issue outlining this new effort.

As scientists have become aware of the problems associated with burning fossil fuels, they have turned their attention to alternative sources of fuel for generating power. One possibility is hydrogen, a gas that can be burned without generating harmful greenhouse gases. However, hydrogen has posed problems involved in generating and storing it.

Electrolysis is one method of generating hydrogen, applying electricity to water to split it into its oxygen and hydrogen components. Unfortunately, current electrolysis methods require nearly pure water. In this new effort, the researchers developed an electrolysis process using seawater, a source that is a long way from pure.

The process involves a membrane similar to the type used in waterproof clothing. It has pores that are tiny enough to allow individual molecules through, but prohibits molecules bunched together, as is the case with water. In their device, the outer part of the membrane is exposed to seawater while the inner side is exposed to a small amount of potassium hydroxide (KOH). Inside of the pouch that holds the KOH, the team placed electrodes that generate hydrogen and oxygen on both sides of a separator, which keeps the gas streams clean.

In action, as the device was dipped into seawater, water inside was split, producing hydrogen and oxygen. That reduced the concentration levels of KOH, which pulled more seawater into the device, allowing it to run continuously. In the device, a small amount of seawater was held in a vapor state, which allowed it to pass cleanly through the membrane, where it once again reverted to its water state, providing pure water for electrolysis. Tubes vented the oxygen to collect the hydrogen.

The researchers tested their device in Shenzhen Bay (just north of Hong Kong). Measurements showed it capable of making as much hydrogen as conventional methods, and it was also hardy—it was run for 3,200 hours without signs of degradation.

As hydrogen is a carbon-free alternative energy source with several advantages in- cluding environmental friendliness and high energy density, it can be used in future energy frameworks. There are many methods for producing hydrogen from water electrolysis that offer both high purity and sustainability.

The growing number of scientific reviews on the topic of hydrogen production by the electrochemical splitting of water demonstrates the considerable interest in and financial support for this line of research [1–24]. The hydrogen economy is viewed as a workable solution to the aforementioned issues in light of the rising costs of fossil fuels and increasing environmental degradation. Water electrolysis takes on a special strategic function in this situation [1].

Conventional DC water electrolysis can produce hydrogen. However, the process is not ideal for the environment if the electrical energy for the electrolysis is generated in thermal power plants from fossil fuels due to the release of carbon dioxide. The future of fuel cells is bright, and numerous technologies are being researched globally. Compared with thermal power plants, the amount of carbon dioxide produced during the production

of hydrogen from natural gas for fuel cells can be reduced, although carbon dioxide is still produced. While photo-catalysis is a better method for producing hydrogen, it is still a relatively inefficient process for use in actual applications. Since the cost of energy is declining, by using renewable resources as wind, hydroelectricity, and nuclear power, water electrolysis has recently been considered a method for producing hydrogen [2]. A highly interesting method for producing hydrogen by saltwater electrolysis is the in situ generation of power from waves [25].
A way to lessen environmental pollution caused by power production based on current methods is to produce hydrogen by seawater electrolysis using electricity from local sources and then utilize it in fuel cells.
Two essential components are required for seawater electrolysis to produce hydrogen: cathodes that actively evolve hydrogen during the process and anodes that efficiently develop oxygen rather than chlorine.

The most active noble–metallic material for the hydrogen evolution reaction is platinum, but it cannot be used to produce hydrogen on a large scale. Other cathodic materials, such as nickel and several Ni alloys and composite materials, have shown promise for hydrogen generation over the past ten years [21].

High electrochemical reactivity, high energy density, theoretically infinite availability (as long as water can be split), and the combustion byproduct (water) are all benefits of using hydrogen as a fuel in fuel cells.
The need for hydrogen is expected to treble globally over the next five years, and it should also become a cost-effective and sustainable energy source.

Hydrogen obtained from different methods, e.g., steam methane reforming, methane pyrolysis, and coal gasification have different effects on environment power systems; the transportation, hydrocarbon and ammonia manufacturing, and metalworking industries all use hydrogen.

Most of the actual hydrogen production, which accounts for around 95% of the 60 million tons produced each year in the context of climate change, is not sustainable, requiring the development of cleaner hydrogen production techniques.

The main review papers address subjects related to hydrogen production from water electrolysis: fundamentals of water electrolysis [3–5,26], the technology of electrolysis cells [2,4,19,24], catalytic electrodes for water electrolysis [1,6,16–18,21], hydrogen produc- tion and storage and wastewater valorization [7,13–15,20], renewable energies [6,8], new related technologies [11], and costs and financial considerations [22,24]. The original point of view of this paper is the direct use of seawater as a specific electrolyte, different from pure or alkaline water electrolytes, in order to electrochemically produce hydrogen as a fuel in a sustainable way.

The specific characteristics for seawater electrolysis are usually studied and developed by chemists, but the applications are a popular topic in the energy field. From the analyzed articles, only about 20% of the references dealt with seawater, and they were mainly about electrocatalysts for the hydrogen evolution reaction. The found references on energy subject deal with a different type of electrolyte, pure or alkaline water, so they do not offer an integrated view on sustainable seawater utilization.

Since seawater is the largest naturally occurring free resource of water, the fundamen- tal theoretical concepts of water electrochemistry with a focus on seawater electrochemistry will be presented extensively. The most recent trends in electrocatalysts, emerging technol- ogy, economics, and environmental impact, as well as the direct use of seawater electrol- ysis in combination with renewable energy systems for sustainable development will be also discussed.

Schematic classification of the primary electrocatalyst investigation techniques utilized in the electrolysis of freshwater and seawater to produce hydrogen.
Scheme of the technical and economic analyses of the hydrogen production.
Efficiency of emergent electrochemical technologies suitable for hydrogen production from seawater.
Renewable sources of energy for hydrogen production from seawater electrolysis

reference link : 10.3390/en15228560

More information: Heping Xie et al, A membrane-based seawater electrolyser for hydrogen generation, Nature (2022). DOI: 10.1038/s41586-022-05379-5

A practical method for splitting seawater into hydrogen fuel, Nature (2022). DOI: 10.1038/d41586-022-03601-y


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