Electrolysis of Saline and Low-Grade Water for Hydrogen Production

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

Why Is This Research Important?

This research is crucial for advancing environmental sustainability and energy security. By developing efficient electrolysers that can use saline water, hydrogen production can become more accessible and cost-effective. This contributes to global efforts in reducing carbon emissions and combating climate change, making significant strides towards a sustainable and decarbonized future.

Introduction

This paper discusses the challenge of freshwater scarcity and the potential of utilizing abundant saline and low-grade water for hydrogen production through water electrolysis. The research is significant as it addresses Sustainable Development Goals (SDGs) 6, 7, and 13, which focus on clean water, affordable and clean energy, and climate action.

Hydrogen Production and Its Importance

Hydrogen, as a clean and storable fuel, offers a pathway to carbon neutrality and climate change mitigation. Its versatility allows for widespread applications across industry, households, and transportation sectors. Hydrogen production through water electrolysis, however, is currently dependent on high-purity water, which necessitates costly desalination and purification processes. This limitation underscores the need for innovative approaches to make hydrogen production more economically viable.

Challenges of Saline Water Electrolysis

The electrolysis of saline water presents multiple technical challenges. First, the requirement for ultra-pure water in existing electrolyser systems increases operational costs. Second, the electrochemical reactions at the electrodes, specifically the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), are hindered by impurities present in saline water. Lastly, the presence of these impurities leads to the degradation of membranes and catalysts, reducing the overall efficiency and lifespan of the electrolysers.

Key Advances in Electrode Materials

Recent advancements in electrode materials focus on enhancing the efficiency and selectivity of catalysts that can operate with impure water. The development of catalytic electrode materials that can effectively handle metal ions, chlorides, and bio-organisms is crucial. These materials must exhibit high activity for HER and OER while maintaining stability in the presence of impurities. Reducing the overpotential for OER is essential to improve the energy efficiency of the process, with state-of-the-art catalysts and membranes achieving this under specific conditions.

Reactor Design Considerations

The paper examines various electrolyser technologies and their suitability for saline water electrolysis. Alkaline Water Electrolysers (AWE) use a liquid alkaline electrolyte and a porous diaphragm, offering robustness but allowing unwanted ion migration. Proton Exchange Membrane Water Electrolysers (PEMWE) utilize a solid polymer electrolyte and are highly efficient but sensitive to impurities. Anion Exchange Membrane Water Electrolysers (AEMWE) operate at high pH, minimizing chloride oxidation, though they still face challenges with ion migration. High-Temperature Electrolysers operate with steam at elevated temperatures, potentially reducing water purification needs but are more complex and costly.

Strategies for Improved Selectivity and Stability

To overcome the challenges associated with saline water electrolysis, several strategies are proposed. The alkaline design criterion suggests operating in alkaline conditions to enhance OER selectivity and reduce chlorine evolution. Developing catalysts with selective catalytic sites optimized for OER can improve selectivity over chloride oxidation. Applying Cl- blocking layers on electrodes prevents chloride ions from reaching catalytic sites. Advanced membranes that selectively permit desired ions while blocking impurities are also crucial for improved performance.

Key Data and Examples 

The paper provides several key examples of catalytic performance and stability studies. NiFe layered double hydroxide (LDH) catalysts have demonstrated high current densities (up to 290 mA cm–2) with excellent OER selectivity. Stability studies on NiFeOx and CoOx catalysts show their potential in maintaining performance in saline conditions with minimal chloride interference. Cl- blocking layers, such as MnOx coatings, effectively enhance OER selectivity on IrOx/GC electrodes by preventing chloride ions from reaching the active sites.

Conclusion and Future Directions

The research highlights the potential of using saline and low-grade water for hydrogen production, which can significantly reduce costs and dependency on freshwater resources. Future research should focus on standardizing testing conditions to enable better comparison of catalyst performance across studies. Long-term stability of catalysts and membranes must be addressed to ensure sustained performance. Real-world applications should be explored to validate these technologies in diverse saline water conditions and to scale them up for industrial use.

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