Hydrogen is an efficient energy carrier. In many countries it can replace natural gas for heating and cooking in households with minor modifications to the existing infrastructure. It can also power cars and it can be used to store energy. The gas is relatively safe to use because its low density will in the event of a leak make it rise quickly and disperse. On the other hand it can enbrittle metals and it ignites easily, compression and liquefaction is expensive. For the Tokyo 2020 Olympics hydrogen will be shipped in from Australia to provide energy for the entire event. Even the Olympic torch will be powered by hydrogen. Too bad this particular hydrogen is produced from low-grade coal making it not exactly environment-friendly.
Ideally hydrogen is produced from solar or wind energy and the conversion then takes place via electrolytic water splitting. Everything is settled then? Not so easy. Water splitting requires a lot of water and there is not enough clean fresh water around if hydrogen is to be used on a massive scale. Seawater then? There is a catch. Whereas fresh water electrolysis gets you oxygen on the anode and hydrogen on the cathode. With seawater chlorine gas also evolves on account of the dissolved sodium chloride. In fact all industrially produced chlorine is made from brine electrolysis. Then what to do with all that chlorine? You can convert all of it to household bleach and distribute it for free making Earth the cleanest planet in the universe or .... find a way to suppress chlorine formation in electrolysis or try something else altogether with seawater. With that in mind, what has been happening in research labs around the world? Time for a quick update.
In 2018 Vos et al. (DOI) tackled the problem of chlorine evolution head-on. The oxygen half-reaction is thermodynamically more favorable but is kinetically slower (involving 4 electrons and not 2). A coating of manganese dioxide on a iridium oxide electrode was found to impart selectivity in the oxygen reaction versus the chlorine reaction. The manganese layer is passive and porous and inhabitable for chlorine ions. In this work as a stand-in for seawater a sodium chloride solution was used but less concentrated than the real thing. The article mentions scientist John Bockris who already in 1970 worked on water electrolysis and coined the term "hydrogen economy". Readers be warned, his alleged secret catalysts never materialized, his later work on cold fusion and forays into alchemy met with controversy.
In another 2018 effort (Hsu et al. DOI) the iridium oxide electrode was coated with a cyanometalate layer. The water was actual seawater and was sourced from West Coast Park in Singapore, the more than thorough supplemental information even has a photo of the collection site. An efficiency of 20% was reported. Amikam et sal. (2018, DOI) have investigated in another way the suppression of the chlorine formation. They found out that conducting the electrolysis (artificial seawater, nickel electrodes) with added sodium hydroxide not only eliminated this side reaction but also salted out sodium chloride through the common ion effect. Solutions can be surprisingly simple.
Artificial seawater ((1 M KOH with 0.5 M NaCl) was again used in a 2019 study (Kuang et al. DOI) with this time an electrode based on a nickel-iron hydroxide coating on a nickel sulfide coating on a porous nickel foam.
Another contender for hydrogen production from seawater is photocatalytic water splitting, no need for electrodes here, just add a catalyst, aim a light source and hydrogen again should evolve. Research has come up with a bunch of catalyst systems, a recent example is metal-doped strontium titanate perovskite (Sahrma et al. 2019 DOI). In this particular effort sea water (Tamsui River in Taiwan) was also tried but it behaved badly compared to fresh water.
One adventurous 2019 research article (Kim et al. 2019 DOI) has the promise of simultaneous carbon dioxide consumption, electricity generation and hydrogen evolution in seawater (sea of Ulsan). To good to be true? In the proposal a carbon dioxide feed continuously acidifies a seawater platinum cathode compartment, the other compartment is an alkaline solution with a zinc electrode. This constitutes a working galvanic cell with accumulation of potassium bicarbonate , a spent zinc electrode and hydrogen gas production. Too good to be true.
Aluminum metal is a known redox partner for water but the oxide outer layer poses a problem and aluminum salts have to be regenerated. In a 2017 effort (Lu et al. DOI) the liquid gallium-indium alloy galistan (think thermometer) is proposed as a remedy for use of Al with artificial seawater. The alloy destroys the oxide layer and can be reused.
Hydrogen can also be replaced by hydrogen peroxide as fuel carrier. The production of this compound from seawater was demonstrated by Mase et al (2016 , DOI) with a tungsten trioxide catalyst. This seawater was made from commercially available red sea salt and with respect to experimental reproducibility preferable over real seawater.
Someone is yelling methanol. If it is not going to be the hydrogen economy or the hydrogen peroxide economy there is always the methanol economy and Hogerwaard et al. have been viewing a bucket of seawater as a bucket of future methanol. A 2019 analysis (DOI) envisions artificial islands stacked to the brim with advanced solar technology and electrodialysis units. In this technique seawater is desalinated using an electrode to force the sodium ions past an ion-exchange membrane. Hydrogen gas production is a side effect in commercial desalination plants based on electrodialysis but in a combined dialysis / electrolysis process the main aim becomes hydrogen production and in addition because hydrogen ions replace the sodium ions the formation of carbon dioxide is also accelerated. In a final step hydrogen and carbon dioxide combine to form the methanol.