A study by a team of researchers from Technische Universität Berlin (TUB) and Fritz-Haber-Institut der Max-Planck-Gesellschaft has found that direct seawater splitting for hydrogen production has substantial drawbacks compared to conventional water splitting and offers almost no advantage.

In an open-access paper in the RSC journal Energy & Environmental Science, the researchers report that their analysis shows that direct seawater purification is less promising than a two-step scenario for splitting seawater—first purification by reverse osmosis and then splitting in a conventional water electrolyzer—as the capital and operating costs of water purification are insignificant compared to those of electrolysis of pure water.

For the implementation of a sustainable energy economy, the greatest challenge is the weather-depending, fluctuating electricity production of wind and solar power plants. To meet this challenge and to satisfy the constant energy demand of society, electricity must be stored in times of overproduction to provide energy when little sunshine and wind is available. To store the green electricity in a highly scalable way, it must be converted into chemical energy. The central process for this conversion is electrocatalytic water splitting in which hydrogen and oxygen are formed.

Hydrogen can directly be stored, transported, and reconverted into electricity in a fuel cell. Further, hydrogen is the starting point for the formation of other fuels such as methanol, ammonia, or liquid organic hydrogen carriers. Due to the central role of water splitting in a sustainable energy economy, the cost efficiency of this process is crucial and even one percent could save billions of dollars.

More than half of the costs of electrolytic hydrogen production are caused by the required electricity, and, besides that, the capital cost of the electrolyser is another major part. Additionally, H₂O is needed for water splitting. However, so far, this aspect has seldom been considered. As 96.5% of the global water is seawater and less than 1% is nonfrozen freshwater, direct seawater splitting (DSS, in this report DSS includes the usage of non-purified seawater with or without the addition of additives such as bases or buffers) seems desirable.

… Herein, we answer the question: Can DSS be competitive to a two-step scenario where seawater is first purified and subsequently split?

—Driess et al.

Two ways to make hydrogen from seawater and green electricity. The pathway on the left shows DSS where only one device is required. In the pathway on the right, the seawater is first desalinated by reverse osmosis and then the water splitting is performed. For this pathway, electricity and a device are required for both, the desalination and the actual water splitting. Diess et al.

The analysis found that the energy requirements, the capital, and the operating costs of seawater desalination are marginal compared to those of water splitting. This leads to the conclusion that the benefits of direct seawater splitting are marginal.

However, the analysis also found that the disadvantages of DSS are considerably large. These include electrolyzer lifetime (challenged by unavoidable impurities); an always changing feed as seawater changes seasonally and topologically; corrosive Cl^- oxidation species; precipitation of solids; blocking of the ion exchange membranes; and biofouling.

Further, the authors noted, with current technology, a direct seawater electrolyzer must be operated at a high flow velocity; it must be designed to wash deposited species away; and regular acid cleanings might be required. All these aspects lead to increased capital and operating costs.

The use of additives such as acids, bases or buffers in the feed of a direct seawater electrolyzer might be “magnitudes more expensive”, or the conductivity and ion transport properties of the electrolyte will be low leading to significant efficiency losses.

Resources

• Matthias Driess, Jan Niklas Hausmann, Robert Schlögl and Prashanth W. Menezes (2021) “Is Direct Seawater Splitting Economically Meaningful?” Energy Environ. Sci. doi: 10.1039/D0EE03659E