Introduction

The paper discusses the challenges and recent advancements in water electrolysis using low-grade and saline water as feedstock. It aims to explore sustainable hydrogen production through electrolysis while minimizing costs associated with water purification and desalination. Water electrolysis is a promising method for producing hydrogen, which can help achieve zero emissions. However, the scarcity of freshwater resources necessitates the use of low-grade and saline water, which presents additional challenges.

Key Challenges

Water electrolysis typically requires ultra-purified water to avoid complications with catalysts and membranes. Using saline water introduces issues such as pH fluctuations, mineral precipitation, and interference from ions and microbes. Chloride chemistry poses a significant challenge, as chloride ions in saline water can lead to the formation of toxic chlorine gas during electrolysis, competing with the oxygen evolution reaction (OER).

Electrolyser Technologies

The paper reviews various electrolyser technologies, including Alkaline Water Electrolyser (AWE), Proton Exchange Membrane Water Electrolyser (PEMWE), Anion Exchange Membrane Water Electrolyser (AEMWE), and high-temperature electrolysis. Each technology has its advantages and challenges. AWE uses a porous diaphragm and operates with an alkaline electrolyte, while PEMWE utilizes a solid acid polymer electrolyte and requires high-purity water. AEMWE employs an anion exchange membrane suitable for high pH operation but is susceptible to chloride oxidation. High-temperature electrolysis includes proton-conducting ceramics and solid oxide electrolysers, which can purify water via evaporation but have higher operational costs.

Electrode Materials and Catalysts

Electrode materials and catalysts are crucial for the efficiency of the electrolysis process. Anodes must be selective for OER over chlorine evolution reaction (ClER). Strategies include the alkaline design criterion, which favors OER thermodynamically by operating at high pH, designing selective OER sites, and applying chloride blocking layers. Cathodes focus on stability and resistance to impurity deposition. Common materials include Pt and Ni-based alloys. Strategies for improving stability include permselective layers and developing corrosion-resistant alloys.

Reactor Design

Reactor design must address issues related to impurity transport and membrane blockage. Potential solutions include using filtration systems and permselective barriers to improve the efficiency and durability of the electrolysers.

Key Examples and Figures

Key examples and figures include a Pourbaix diagram of aqueous chloride chemistry, different configurations of water electrolysers, the alkaline design criterion for OER selectivity in saline water, and the performance of various oxide catalysts in saline water electrolysis. The paper also discusses the use of MnOx coatings on IrOx electrodes for selective OER and challenges and potential solutions for hydrogen evolution reaction (HER) stability in impure water.

Importance of the Research

This research addresses the critical need for sustainable and cost-effective hydrogen production. By developing electrolysers capable of operating with impure water, reliance on freshwater resources and associated purification costs can be reduced. This aligns with global efforts to achieve clean energy goals and combat climate change.

Implications

The research has far-reaching implications. Economically, reduced costs for hydrogen production can make renewable energy sources more competitive and accessible. Environmentally, utilizing saline water for electrolysis helps conserve freshwater resources and supports sustainable energy practices. Technologically, advances in catalyst and membrane technologies can lead to more efficient and durable electrolysers, accelerating the adoption of hydrogen as a clean fuel.

Conclusion

The paper highlights the importance of developing electrolysers capable of using low-grade and saline water directly. Addressing the challenges associated with impurities and chloride chemistry is crucial for achieving efficient and cost-effective hydrogen production. Future research should focus on optimizing catalyst selectivity, stability, and reactor design to facilitate the practical implementation of these technologies.