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reduction in electrolyzer stack costs over the last few years and is expected to be the dominant technology for sustainable hydrogen production by 2030. 2, 8, 9 Direct Seawater Electrolysis One of the requirements of PEM water electrolysis is the need of highly pure water feeds with a minimum requirement of American Society for Testing and Materials (ASTM) Type II deionized (DI) water (resistivity > 1 MΩ-cm) while ASTM Type I DI water (> 10 MΩ-cm) is preferred.14 ASTM defines Type II water, as required in commercial electrolyzers, as having a resistivity of > 1 MΩ-cm, sodium, and chloride content < 5μg/L and < 50 ppb of total organic carbon (TOC).15 Alkaline electrolyzers are less stringent on water quality as compared to PEM, but still needs high purity water to achieve long-term stability. Such high purity water as required by water electrolysis systems is produced through a combination of either reverse osmosis (RO), multi-stage flash distillation (MSF), electrodialysis, multiple effect distillation (MED) to desalinate water, and commonly an additional technology such as ion exchange or electrodeionization (EDI).16, 17 The additional capital and operating cost associated with water purification has been the common argument that has spurred research activities into direct electrocatalysis of seawater for H2 production, with the rationale that seawater represents ~96.5% of earth’s water resources.18, 19 A technology for direct seawater splitting could potentially be used in coastal arid zones that have limited access to freshwater yet plenty of access to seawater and renewable electricity from solar, wind and geothermal.19-21 Over the last few decades, significant research efforts have gone into direct seawater electrolysis (Figure 1). In the last decade, the field has seen 700+ scientific publications, and 340+ patent applications, which translates into millions of dollars of research funding allocated globally. Seawater electrolysis could be done to either produce chlorine via chloride oxidation or oxygen via water oxidation. Although chlorine is a valuable industrial chemical, the quantities produced for the growing hydrogen market would far exceed global demand for Cl2.22 Therefore, one of the major challenges has been the development of active and stable anode catalysts for selective oxygen evolution over chlorine.18, 19 The competing chlorine evolution reaction (CER) is thermodynamically unfavorable compared to the oxygen evolution reaction (OER) (~480 mV higher in alkaline media), but it is a two-electron reaction, in contrast with OER which involves four electrons. This difference in the numbers of electrons involved makes OER kinetically 3PDF Image | Seawater Electrolysis for Hydrogen Production
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