Why today’s “water” in water splitting is not natural water? Critical up-to-date perspective and future challenges for direct seawater splitting

Hydrogen, the most prominent green energy source, is considered as the next-generation fuel that can replace the current nonrenewable energy sources to realize a sustainable development. However, despite extensive research, a sustainable natural feedstock for water electrolyzers has not yet been found. Moreover, the overuse of high-purity water as feedstock can cause freshwater scarcity in the long run. In addition, the extremely harsh conditions of a highly acidic or basic electrolytic medium in modern electrolyzers restrict the type of electrodes, electrolyzer material, and separator that can be used, thereby increasing the cost of the electrolyzer system. Hence, the use of an abundantly available neutral electrolyte, e.g. "seawater," as a feedstock for electrolyzers has received considerable research attention in recent years. Still, direct splitting of non- manipulated seawater is difficult, owing to the presence of concentrated Cl− ions and the neutral, unbuffered nature of the electrolytic medium (seawater). Different alternative methods have been explored in the literature to achieve progress in seawater splitting. Methods based on developments in the electroactive catalytic material, using additives in the electrolyte, and applying various chlorine-resistant overlayer coatings, presents numerous handicapping maintenance, scalability and applicability challenges. Therefore, new scientific and technological developments are highly required for the direct splitting of seawater to achieve hydrogen sustainably. In this review, we provide a much-needed and relevant perspective on the rapidly expanding research field of electrocatalytic hydrogen production, by analyzing the various obstacles in advancing the uncharted path from the current corrosive half-cell electrolytic systems to the future desired direct seawater splitting technologies. With the advent of the novel electrocatalysts discussed in this review, direct seawater splitting can now be realized, and thus outdates the previous price analysis which questions the scalability of direct seawater splitting. Our study provides an overview of the basics of electrocatalytic water splitting, requirements of an electrocatalyst for performing bifunctional overall water splitting, finally focusing on the dream and need for neutral water, direct seawater splitting, as well as its associated great challenges and current progress in the field.