Hydrogen from Saline Water Electrolysis

Figure 1: The Pourbaix diagram of an aqueous saline electrolyte.


The paper discusses the challenges and potential solutions for producing hydrogen fuel using water electrolysis, specifically focusing on the use of low-grade and saline water instead of highly purified water. This is significant because freshwater is becoming scarce, and using abundant saline water can address both clean energy and climate action goals.

The Importance of Hydrogen

Hydrogen is a storable and clean fuel that can help achieve carbon neutrality. It can be used in various sectors, including industry, households, and transport. The hydrogen market is expected to grow significantly, creating economic and social benefits, including job opportunities.

Water Electrolysis and Use of Saline Water

Current electrolysers require very pure water, which involves extensive purification processes that are costly and energy-intensive. Desalination and water purification add significant costs, especially when using renewable energy sources.

Using saline or low-grade water directly can reduce costs and simplify the engineering of electrolysers, particularly for large-scale offshore hydrogen production. However, ideal catalytic electrode materials and membranes are needed to handle the impurities and competing reactions in saline water.

Electrolysis Process and Reactions

Water electrolysis involves splitting water into hydrogen and oxygen. This requires external energy and specific conditions, depending on whether the environment is acidic or alkaline.

Challenges and Advances

Saline water contains impurities that can degrade catalysts and block membranes. Local pH changes during electrolysis can cause precipitations that block the cathode and lead to catalyst degradation. Additionally, competing chloride reactions, especially in saline water, can lead to the production of harmful chlorine gas.

Recent research focuses on developing catalysts that can selectively drive the desired reactions even in the presence of impurities. Catalysts that preferentially facilitate the oxygen evolution reaction (OER) over chloride oxidation are essential.

The paper reviews various configurations like Alkaline Water Electrolyser (AWE), Proton Exchange Membrane Water Electrolyser (PEMWE), Anion Exchange Membrane Water Electrolyser (AEMWE), and high-temperature electrolysis systems. The choice of membrane impacts the efficiency and stability of the electrolyser, especially in handling impurities.

Operating in alkaline conditions can help maximize the selectivity for OER, reducing unwanted chloride reactions. Using manganese oxide coatings can block chloride ions and protect the catalysts.

Long-term stability of cathodes in saline water is a concern due to the deposition of impurities and corrosion. Research is focused on developing cost-effective catalysts using abundant elements like Ni, Mn, and Fe.

Coating catalysts with layers that allow selective passage of ions can prevent degradation and improve stability.

Conclusion and Future Direction

The paper emphasizes the need for standardized testing criteria for new materials in saline water applications. For saline water electrolysers to be viable, they must compete with traditional systems in terms of cost. Islands with abundant renewable energy sources are ideal for testing new technologies due to their high import costs for fossil fuels.

The hydrogen market is projected to grow by 33% to US$155 billion by 2022. Examples of technological innovations include NiFe layered double hydroxides (LDH) and MnOx coatings for improved performance in saline water electrolysis.

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