Tapping hydrogen fuel from the ocean: A review on photocatalytic, photoelectrochemical and electrolytic splitting of seawater

Direct splitting of earth-abundant seawater provides an eco-friendly route for the production of clean H-2, but is hampered by selectivity and stability issues. Direct seawater electrolysis is the most established technology, attaining high current densities in the order of 1-2 A cm(-2). Alternatively, light-driven processes such as photocatalytic and photoelectrochemical seawater splitting are particularly promising as well, as they rely on renewable solar power. Solar-to-Hydrogen efficiencies have increased over the past decade from negligible values to about 2%. Especially the absence of large local pH changes (in the order of several tenths of a pH unit compared to up to 9 pH units for electrolysis) is a strong asset for pure photocatalysis. This may lead to less adverse side-reactions such as Cl-2 and ClO- formation, (acid or base induced) corrosion and scaling. Besides, additional requirements for electrolytic cells, e.g. membranes and electricity input, are not needed in pure photocatalysis systems. In this review, the state-of-the-art technologies in light-driven seawater splitting are compared to electrochemical approaches with a focus on sustainability and stability. Promising advances are identified at the level of the catalyst as well as the process, and insight is provided in solutions crossing different fields.

Automated summary

Direct splitting of earth-abundant seawater for hydrogen production faces challenges in selectivity and stability. While direct seawater electrolysis is a well-established technology, light-driven processes like photocatalytic and photoelectrochemical seawater splitting show promise in utilizing renewable solar power. Advances in solar-to-hydrogen efficiencies have been made, reaching around 2% over the past decade.