Electrolysis of Low-Grade and Saline Surface Water (Level 4)

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

Overview

This review article addresses the potential and challenges of using low-grade and saline water for hydrogen production through electrolysis. It emphasizes the development of efficient and selective catalytic electrode materials and the design of electrolysers capable of operating with impure water feeds.

Introduction

Freshwater is becoming increasingly scarce, with over 80% of the worldā€™s population facing high levels of water security risk. This issue is recognized within the Sustainable Development Goals (SDGs), particularly SDG 6 on Clean Water and Sanitation. The paper explores the potential of using low-grade and saline water, which are abundant resources, to address not only SDG 6 but also SDG 7 on Affordable and Clean Energy and SDG 13 on Climate Action. Hydrogen, produced through water electrolysis, presents a promising solution to combat climate change and achieve zero emissions, offering economic and social benefits. However, significant challenges remain in reducing costs and integrating hydrogen into everyday life.

Challenges of Saline Water Electrolysis

The process of splitting water into oxygen and hydrogen requires external energy and is energetically demanding. The presence of impurities, such as non-innocent ions, bacteria, and small particulates in saline water, can poison electrodes and limit their long-term stability. Effective membranes are essential to handle impure water, prevent competing redox reactions, and avoid membrane degradation and biofouling.

Electrolysis Configurations

Several configurations are discussed, each with its own set of challenges:

1. Proton Exchange Membrane Water Electrolysers (PEMWE) use solid acid polymer electrolytes and typically supply water only to the anode. The low pH medium provided by the PEM complicates anode chemistry, making oxygen evolution reaction (OER) selectivity challenging.

2. Alkaline Water Electrolysers (AWE)Ā operate as two-compartment cells separated by porous diaphragms, using liquid alkaline electrolytes. Issues include physical blockages and the migration of ions, which can affect performance.

3. Anion Exchange Membrane Water Electrolysers (AEMWE)Ā utilize anion exchange membranes and can supply water to the cathode, anode, or both. The high operating pH minimizes chloride oxidation, making them interesting for saline water splitting, though competition between OH- and Cl- oxidation remains a concern.

4. High-Temperature ElectrolysersĀ operate at temperatures between 150Ā°C and 1000Ā°C, using solid oxide or ceramic membranes. While this configuration allows for the purification of water before it reaches the catalyst, the high energy demand and limited electrode materials pose significant challenges.

Reactor Design Considerations

Effective catalysts should have low overpotentials to be efficient. Nickel-iron hydroxide catalysts, for instance, show promising performance. Reactor design must also address physical blockages, ion migration, and microbial contamination to maintain long-term stability.

Anode Materials for Electrolysis

Three main strategies for developing anode materials for electrolysis in impure water are presented:

1. Alkaline Design Criterion: Operating in alkaline conditions maximizes OER selectivity over chlorine evolution reaction (ClER).

2. Selective OER Sites: Developing catalysts with active sites that favor OER over ClER is feasible but challenging, as these sites are typically active for both reactions.

3. Cl- Blocking Layers: Using layers that prevent chloride ions from reaching the OER catalyst is another approach. These layers must be permeable to water while blocking chloride ions effectively.

Cathode Materials for H2 Evolution

The primary concerns for cathodes operating in impure water are stability in the presence of impurities and prevention of active site blocking and corrosion. Several strategies are proposed:

1.Membranes: Using membranes to protect the cathode from impurities is one approach.

2. Catalyst Development: Developing catalysts with selective surface chemistry and inherent corrosion resistance is crucial for maintaining long-term stability.

3. Permselective Barrier Layers: Applying these layers to catalyst surfaces can block impurities while allowing the transfer of reagents and products.

Conclusion and Outlook

Several issues need to be addressed to make the electrolysis of impure or saline water viable. The use of appropriate membranes is crucial, as they must handle impurities effectively without significant degradation. Overcoming the competition between chlorine chemistry and water oxidation is essential for successful saline water splitting. Operating in alkaline conditions has shown promise, but transitioning to real seawater presents additional challenges. Standardizing criteria for assessing new materials and understanding membrane blockage by impurities would be beneficial. Islands with abundant renewable energy could serve as ideal test sites for these technologies. Overall, further research and development are needed to advance electrolyser technology and catalyst materials for sustainable hydrogen production using saline and low-grade water.

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