Direct Electrolytic Splitting of Seawater: Significance and Challenges

The rapid development of the world economy and human society has accelerated the consumption of fossil fuels and presents a series of critical problems, such as environmental pollution, energy shortages and global warming. As a clean, efficient and sustainable renewable energy, hydrogen energy is regarded as the energy source with the second-greatest development potential in the 21st century. However, hydrogen production mainly relies on natural gas reforming and coal gasification technologies. Current hydrogen production technology consumes a significant amount of fossil fuels and emits a high level of carbon dioxide, aggravating the greenhouse effect, environmental pollution and global warming. To further leverage the advantages of hydrogen as an efficient and clean energy source and achieve zero-carbon emissions, it is imperative that environmentally friendly hydrogen production technologies are developed.

Hydrogen production from water electrolysis has advantages including low pollution, a fast yield, simple process, high purity and the use of renewable energy (solar energy, wind energy, tidal energy, etc.). Such technologies require processes to be carried out in alkaline and acidic environments, which necessitates an adequate corrosion resistance of devices and diaphragms. More importantly, in specific regions, freshwater resources are scarce. If large-scale industrialized water electrolysis for hydrogen production is to be carried out, the availability of freshwater resources must be considered. Seawater is one of the most abundant natural resources on Earth, accounting for approximately 96.5% of the world’s total water storage; thus, the development of direct seawater electrolysis technology for hydrogen production is of great social significance.

Direct seawater electrolysis is an effective solution to the shortage of freshwater resources for large-scale hydrogen production. Additionally, this process can utilize electricity generated by renewable energy sources, such as tidal energy, wind energy and solar energy. Compared with traditional freshwater electrolysis technology for generating hydrogen under acidic or alkaline conditions, this process has three obvious advantages: first, it does not depend on freshwater resources, and work efficiently in coastal areas and other regions; second, it is less corrosive to the device and diaphragm, prolonging their lifespan; and third, it can reduce the cost of large-scale hydrogen production. Therefore, direct seawater electrolysis, as an economical and efficient hydrogen production method, has broad development prospects for coastal areas and other areas with abundant seawater resources.

The seawater electrolysis reaction includes two half-reactions, the cathode hydrogen evolution reaction (HER) and the anode oxygen evolution reaction (OER). The theoretical minimum voltage needed to simultaneously drive HER and OER is 1.23 V, but additional potential for deactivation and overcoming the energy barrier of the original reaction is required during practical electrolysis. Therefore, reducing the overpotential of water electrolysis and energy consumption is the key to advancing hydrogen production through water electrolysis. Electrocatalysts can greatly reduce the overpotential and improve the reaction rate. At present, freshwater electrolysis constitutes the basis of direct seawater electrolysis for hydrogen production. From the perspective of the current state of technological development, utilizing direct seawater electrolysis technology for hydrogen production is completely feasible. However, existing hydrogen production technologies operating under freshwater conditions cannot be directly used for seawater electrolysis. Since the concentration of chloride ions in seawater is as high as 0.5 M, anodes under seawater electrolysis undergo a chloride ion oxidation reaction to generate strong corrosive substances such as hypochlorite, accelerating the corrosion and poisoning of electrocatalysts. This obstacle presents a technical bottleneck in this process. To realize large-scale industrial hydrogen production via seawater electrolysis, it is thus necessary to develop and deliver feasible solutions to the presented challenges. Since electrocatalysts are a key material in seawater electrolysis, the development of electrocatalytic materials with a high activity and strong stability is of great practical significance. At the same time, basic scientific issues, such as the elucidation of the catalytic mechanism and structural evolution of direct seawater electrolysis for hydrogen production, also require attention.

At present, it is well-known that seawater electrolysis is an economical and efficient way to obtain hydrogen energy. Researchers are committed to the development of catalysts for this procedure in neutral solutions. The electrocatalysts used can be divided into three categories: (1) precious metals; (2) non-precious metals and alloys; and (3) transition-metal compounds. The first and second types of electrocatalysts have a high catalytic activity, but they experience high corrosion during seawater electrolysis, resulting in poor stability [1]. In addition, the scarcity and high cost of precious metals also greatly hinder their large-scale applications in seawater hydrogen production. The third class of transition-metal compound catalysts mainly includes nitrides (TMNs) [2,3], carbides (TMCs) [4], chalcogenides (TMSs) [5,6] and phosphides (TMPs) [7,8,9]. For example, our group reported that the CoNiP/Co_xP heterojunction catalyst exhibits excellent activity and stability in direct seawater electrolysis for hydrogen evolution [7], operating stably for 500 h under a current density of 10 mA cm⁻². Sun et al. reported an electrode material (NiFe-LDH/NiS_x/Ni) composed of a nickel–iron hydroxide layer coated on a nickel sulfide layer, which demonstrated excellent catalytic performance [10]. As the inner layer of nickel sulfide evolves into a negatively charged sulfate protective layer during the electrolysis of alkaline seawater, it can operate stably for 1000 h at a high current density of 400 mA cm⁻². However, the catalytic performance of these electrocatalysts in seawater conditions is significantly lower than that in alkaline environments.

To date, the performance and stability of catalysts for seawater electrolysis are still unable to meet the requirements of industrial applications. This is due to four main reasons. First, few studies on the theoretical design and experimental development of catalysts have been carried out. Second, the pH value of electrode surfaces fluctuates to a large extent during seawater electrolysis, leading to the potential deactivation and instability of the catalyst. Third, the pH value may increase during the cathodic hydrogen evolution reaction, causing cations such as Mg²⁺ and Ca²⁺ in seawater to precipitate on the surface of the cathode, hindering the active sites on the surface of the electrode and deactivating the catalyst. Finally, chlorine evolution may exhibit a side reaction during the oxygen evolution reaction of the anode; thus, catalyst is deactivated and destabilized, corroding the water electrolysis device. To solve the abovementioned problems, novel seawater electrolysis catalyst designs have been proposed based on the following. First, catalysts should be designed with a high selectivity under neutrality, so their surface-active sites are in the middle of the oxygen evolution reaction (OER), preventing the interference of other anions (Cl⁻, SO₄²⁻, etc.). Then, a protective layer can be deposited on the surface of the catalyst under neutral conditions to inhibit the diffusion of anions and cations from seawater to the catalyst surface. Finally, electrocatalysts exhibiting excellent performance under neutral conditions may be designed so that the potential difference of the OER reaction is within 480 mV and the chlorine evolution reaction (ClER) thus does not occur during the OER process. Therefore, it is feasible to develop electrocatalysts with a high activity, low cost and resistance to anions and cations in seawater solutions; however, challenges in direct seawater electrolysis remain to be overcome.