User:Gary thelake/sandbox seawater WE

Seawater
Ambient seawater presents challenges because of the presence of salt and other impurities. Approaches may or may not involve desalination before electrolysis. Traditional electrolysis produces toxic and corrosive chlorine ions (e.g., and ). Multiple methods have been advanced for electrolysing unprocessed seawater. Typical proton exchange membrane (PEM) electrolysers require desalination.

Indirect seawater electrolysis involves two steps: desalting seawater using a pre-treatment device and then producing hydrogen through traditional water electrolysis. This method improves efficiency, reduces corrosion, and extends catalyst lifespan. Some argue that the costs of seawater desalination are relatively small compared to water splitting, suggesting that research should focus on developing more efficient two-step desalination-coupled water splitting processes.

However, indirect seawater electrolysis plants require more space, energy, and maintenance, and some believe that the water purity achieved through seawater reverse osmosis (SWRO) may not be sufficient, necessitating additional equipment and cost. In contrast, direct seawater electrolysis skips the pre-treatment step and introduces seawater directly into the electrolyzer to produce hydrogen. This approach is seen as more promising due to limited freshwater resources, the need to prioritize basic human needs, and the potential to reduce energy consumption and costs. Membranes are critical for the efficiency of electrolysis, but they can be negatively affected by foreign ions in seawater, shortening their lifespan and reducing the efficiency of the electrolysis process.

One approach involves combining forward osmosis membranes with water splitting to produce hydrogen continuously from impure water sources. Water splitting generates a concentration gradient balanced by water influx via forward osmosis, allowing for continual extraction of pure water. However, this configuration has challenges such as the potential for Cl ions to pass through the membrane and cause damage, as well as the risk of hydrogen and oxygen mixing without a separator.

To address these issues, a low-cost semipermeable membrane was introduced between the electrodes to separate the generated gases, reducing membrane costs and minimizing Cl oxidation. Additionally, research shows that using transition metal-based materials can support water electrolysis efficiently. Some studies have explored the use of low-cost reverse osmosis membranes  (<10$/m2) to replace expensive ion exchange membranes (500-1000$/m2). The use of reverse osmosis membranes becomes economically attractive in water electrolyzer systems as opposed to ion exchange membranes due to their cost-effectiveness and the high proton selectivity they offer for cation salts, especially when high-concentration electrolytes are employed.

An alternative method involves employing a hydrophobic membrane to prevent ions from entering the cell stack. This method combines a hydrophobic porous polytetrafluoroethylene (PTFE) waterproof breathable membrane with a self-dampening electrolyte, utilizing a hygroscopic sulfuric acid solution with a commercial alkaline electrolyzer to generate hydrogen gas from seawater. At a larger scale, this seawater electrolysis system can consistently produce 386 L of H2 per hour for over 3200 hours without experiencing significant catalyst corrosion or membrane wetting. The process capitalizes on the disparity in water vapor pressure between seawater and the self-dampening electrolyte to drive seawater evaporation and water vapor diffusion, followed by the liquefaction of the adsorbed water vapor on the self-dampening electrolyte.