Figure 1: Electrolysis of neutral water showing flow of electrons and charged particles to create a circuit.

In order to complete the circuit, charged particles must be able to flow between the solution around the anode and solution around the cathode. A solution of water contains H2O, along with small amounts of OH- and H3O+ from the natural dissociation of water as expressed by the equilibrium equation

H2O ⇌ H3O+ + OH-

Because these ions exist in solution and can flow through a membrane separating solutions, the electrolysis of water is possible without supplementary charged particles. However, due to the limited extent of dissociation, high voltages are required. For this reason, hydrogen extraction uses either acidic or alkaline water, in which case the half equations are different due to the presence of more H+ or OH- in solution, as written below.

In acidic conditions:

H2O → O2 + 2H+ + 2e-

2H+ + 2e- → H2

In basic conditions:

4OH- → O2 + 2H2O + 4e-

2H2O + 2e- → H2

Because there are more charged particles in solution, a lower voltage is required to drive the reactions. Another way to create additional charged particles in solution is to add a salt that will dissociate into charged particles in water [7].

Although there is an opportunity to advance green hydrogen production using proton exchange membrane (PEM) electrolysers1 and anion exchange membrane (AEM) electrolysers,2 the focus of this article is alkaline electrolysers, which use the basic nature of the solution to provide OH- ions. The scarcity of freshwater competes with the scarcity of energy, so developments of seawater use in electrolysis provide a powerful option for the sustainable production of green hydrogen. Three core issues will be addressed by as many novel suggestions found in research released over last year.

Using basic water introduces the first problem:

I. Expense of alkalising water

If KOH, for example, is used to introduce OH- ions, then a strong aqueous KOH solution (20-30%) is required [8].

Two further issues arise when using seawater as the electrolyte in this process:

II. Competition between Cl- and H2O reactions at the anode

Several species can arise from reactions between Cl- ions, the electrodes, and other species in solution. HOCl, OCl-, and Cl2 are all possible products of different reactions. The likelihood of spontaneous occurrence of each depends on the pH (concentration of OH- and H3O+), temperature, induced electrical current, and concentration of Cl-. The predominant oxidation half equations are as follows.

In acidic conditions:

2Cl- → Cl2 + 2e-

In basic conditions:

Cl- + OH- → OCl- + H2O + 2e-

Note the loss of only two electrons compared to the four lost in oxidation of water, creating a more favourable half reaction [9]. The products of Cl- oxidation are corrosive to the anode, posing what is currently considered the biggest compromising factor for the application of seawater electrolysis.

III. Presence of Mg2+ and Ca2+

High concentrations of OH- near the cathode (and therefore higher pH) can cause the precipitation of hydroxides (such as Mg(OH)2 and (Ca(OH)2) with low solubility in water [8]. This solid buildup can surround the cathode and prevent its participation in the reduction reaction forming hydrogen. Research is swift and urgent into the plausible counteractions to these issues and other significant limitations on the functioning of seawater electrolysis. Recent publications propose possible improvements of electrode and catalyst design.

Induced Local Alkaline Environment [10]

Published January 2023. This paper describes a promising approach that could allow seawater to be used as it is sourced, only undergoing a small amount of filtration to remove large solid particles. The publication discusses the addition of a Cr2O3 coating onto CoOx,3 an already well-researched electrode/catalyst design. Chemical reasoning for this design is derived from understanding of Lewis acids and bases. According to the hard and soft acid base theory [11], OH- ions, being hard bases, are more likely to interact with hard acids like Cr3+ thereby increasing their concentration near the electrodes. This creates an alkaline environment that allows water electrolysis to take place at a reasonable voltage. Additionally, as like charges repel, Cl- ions are less likely to approach the cathode and undergo oxidation. The binding of OH- to Cr3+ also reduces the production of OH- at the cathode, thus mitigating pH increase and reducing the precipitation of Mg2+ and Ca2+. The varying concentrations of certain species surrounding each electrode form what is known as the electrical double layer (EDL). This is one of the many studies promising a possible manipulation of this solution that allows critical issues to be resolved. The results not only show significant improvements in the durability of the CoOx catalyst, but also a durability approaching that observed in pre-alkalised seawater. The Cr2O3-CoOx catalyst operated at sufficient current densities (more than 150 mA cm-1) for more than 200 hours. Detailed analysis using a rotating ring-disk electrode confirmed that pH was higher (and therefore OH- concentration was higher) at the surface of the anode coated with Cr2O3 than for the anode without. Additionally, IR spectroscopy indicated the formation of OH- from H2O absorbed onto Cr2O3.

Ag Nanoparticles on Surface [12] Published October 2023.

In a similar approach suggested late last year, the common ion effect was utilised instead of the hard-soft acid-base theory. In this research, silver (Ag) nanoparticles were embedded on the surface of an NiFe-LDH4 catalyst with the aim of forming sparingly soluble AgCl, which would repel Cl- in the solution. This study also sought to create an EDL with a lower Cl- concentration, hence reducing the likelihood of catalyst/electrode erosion due to oxidation. The designed catalyst was able to function at a current density of 400 mA cm-1 for 2500 hours in seawater alkalised with NaOH, significantly longer than the catalyst without Ag nanoparticles and other previously published proposals. Further investigation of the Ni foam skeleton anode after 500 hours of electrolysis demonstrated that the presence of Ag had reduced the extent of erosion. Spectroscopic analysis also indicated the formation of AgCl in the presence of Cl- ions and Ag2O only in the presence of OH-. This successful investigation indicates promising progress, exemplifying longer lasting electrodes than previously thought possible.

Figure 2: Beach near the collection point for seawater used in the study of Ag nanoparticles on catalyst surface [12].

Desirable Effects of Chloride Absorption [13] Published September 2023.

This article suggests that the absorption of Cl- onto Fe can both favour the oxidation of H2O rather than Cl- and reduce the erosion of the Fe catalyst. Again, it utilised a NiFe-LDH anode with different electrolyte solutions to demonstrate the performance with and without the absorption of Cl- onto the catalyst. The results showed that less voltage was required to maintain a current density of 100 mA cm-1 in a solution containing Cl- compared to one without. The anode lasted up to 100 hours at 200 mA cm-1, which was also compared to an anode without Fe: Ni(OH)2. In the latter case, the presence of Cl- required a higher induced voltage to reach a similar level of performance.

Two reasons for improved performance are reported. First is Ni's increased ability to reach higher valence states. The rate determining step for the reaction mechanism of the oxidation of water to O2 relies on the formation of a bond between Ni and OH [14], increasing the valence state of Ni. Ni, with two electrons in its valence shell, is typically most stable when it loses two electrons, resulting in a full valence shell and a total charge of 2+. However, forcing Ni into states of charge more positive than 2+ allows it to hold more OH-, creating more available sites for the desirable oxidation of water to O2. When Cl- is bonded to Fe, Fe tends to hold a less positive charge, allowing Ni to hold more. Research indicated that Ni could reach a valence state of 4+, making it a harder acid than Fe3+ and more attracted to the hard base OH-. This idea also reinforces the likelihood of softer acid Fe3+ absorbing softer base Cl-. Furthermore, the study suggests that Cl- absorption on Fe limits the Fe leaching, as Fe has a lower valence state and is therefore less likely to interact or be eroded by OH- in solution. Finally, the undesirable involvement of oxygen in LDH through an alternate oxidation mechanism (lattice oxidation mechanism or LOM) was shown to decrease. Figure 3 clarifies this, demonstrating an LOM reaction occuring in pure water and not in seawater. This enhanced the desirable adsorbate evolution mechanism (AEM) with the rate determining step referenced above. Raman spectroscopy indicated the conversion of Ni to higher valence states at lower voltages in seawater compared to fresh water, while X-ray absorption spectroscopy confirmed the presence of Fe- Cl bonds. This analysis supports the provided explanation for increased stability of the NiFe LDH catalyst in seawater through both increased favorability of oxidation of H2O (over Cl-) and ‘protection’ of Fe.

Implications for Seawater Electrolysis

The investigation into a hard Lewis acid layer from January is considerably different from those released later in the year, demonstrating changes in catalyst design strategy. Use of the initially perceived undesirable Cl- ion to achieve adequate or even improved performance in seawater compared to freshwater could inspire an attainable method of green hydrogen production. However, these particular studies fail to address the excessive expense of seawater alkalisation and fail to prevent the buildup of precipitate on the cathode. Integrating the Lewis acid layer may result in developments improving the process.

The use of Ag nanoparticles agrees with the idea that an EDL with low Cl- concentration is desirable [Fig. 5], and shows significant improvements on the stability of anodes constructed in such a way. Both this research and the paper published one month earlier utilise the same base catalyst NiFe LDH design, which was first introduced around 2017 [15] and remains one of the most common choices in contemporary research.

While the study investigating the absorption of Cl- into Fe found the electrocatalysts lasted a comparably short 100 hours, its detailed investigation provides valuable insight into the role of Cl- in seawater electrolysis. Parallels can clearly be seen between this and the later release, particularly concerning the induced high concentration of Cl- ions on the surface of the anode. Although not discussed in the earlier publication, the transferable idea of common ion repulsion likely applies, indicating a further minimisation of Cl- oxidation through a concentration gradient [Fig. 6].

There are an abundance of suggestions for modifying the NiFe LDH catalyst similar to the implanting of Ag nanoparticles [4], but the originality and effectiveness of this particular publication justifies its importance in the hunt

Figure 3: Enhancement of desirable AEM oxidation reaction and protection of catalyst with Cl- presence [13].

Figure 4: Concentration of Cl- in electrolyte with Lewis acid coating.