Since high-purity water is required for electrolysis and seawater is widely available for hydrogen (H₂) production, extensive research has been performed to create direct seawater electrolysis technology. The quest for pure, renewable, and affordable energy has become essential to ensuring worldwide socioeconomic growth in light of the energy shortage and the need to protect the ecosystem [1,2,3,4]. Although freshwater makes up only 3.5% of the world’s water resources, freshwater electrocatalytic splitting is regarded as a safe and effective way to generate adulterated H₂ [5]. Therefore, piezoelectric catalytic wastewater degradation may be achieved by utilizing the energy generated by water flow and low-frequency mechanical energy [6,7]. Several approaches for H₂ production and H₂ chemical materials storage have been established, but discovering an effective and secure method for H₂ production is still required. Asim et al. [8] developed the solid material ammonia borane (NH3BH3) as the most promising H₂ storage material. The same authors developed an amalgamation of gold nanoparticles with metal phosphides and speculated them to be effective catalysts (e.g., Au/Ni₂P and Au/CoP) to improve the H₂ evolution rate [9]. Moreover, in the long-term, the electrolysis of water processes that lead to H₂ generation may increase the likelihood of seawater electrolysis [10]. Simultaneously, seawater electrolysis encounters obstacles from the chlorine evolving reaction (CER) and the kinetically slow evolution reaction of oxygen (OER) when the total salinity is typically 3.5 weight percent and the pH is around 8 [11].

In addition, there are highly promising opportunities for developing inexpensive, effective, and efficient transition metals and their compounds to substitute for catalysts, based on noble metals. In particular, several transition metals, including transition metal phosphides (TMPs), transition metal oxides (TMOs), transition metal dihalides (TMDs), transition metal carbides (TMCs), and transition metal nitrides (TMNs), have shown high activity and stability. TMPs are considered a good alternative to rare metals because of their high electrical conductivity, strong durability against corrosion, as well as substantial catalytic activity. Therefore, due to their distinctive physicochemical properties, TMPs are one of the most intriguing potential electrocatalysts that break through the limits of being suitable. They have been extensively employed in numerous catalytic reactions in the fields of energy transformation and catalysis, such as photocatalytic hydrogen evolution [12]. Because of the availability of natural resources and their excellent conductivity, stability, and metal atom coordination effects, TMPs have received much attention recently [13,14]. Nevertheless, the working efficiency of TMP-based hydrogen evolution reaction (HER) catalysts continues to be impaired by some challenging and unresolved issues. It is still unclear whether active dopants deliver more active sites to enhance the activity of TMPs’ occupant sites [15]. Recent research has revealed that seawater electrolysis has good characteristics and stability [16]. For instance, Wu et al. [11] and Liu et al. [16] constructed catalysts that showed outstanding activity and stability, such as (Ni₂P-Fe₂P). The same authors developed a CoPx@FeOOH catalyst, which was also stable for 80 hours at a high-level current of about 500 mA/cm² in 1.0 M KOH seawater with an overpotential of 283 mV for 100 mA/cm². Chang et al. [17] developed a Fe, P-NiSe₂ NF catalyst for gas phase chemical deposition, which displayed stability over eight days with a significant current density of about 800 mA/cm² at 1.8 V. Moreover, during unrestricted seawater circumstances, the open-circuit voltage for HER at 10 mA/cm² was 290 mV for the CoNiP/CoxP/NF catalyst [18].

Furthermore, TMPs are also potentially useful, non-noble electrochemical catalysts for the evolution reaction of H₂. Due to their excellent HER electrocatalytic efficiency, high conductivity, and durability against corrosion, TMPs have sparked great interest. TMPs are recognized as desirable HER catalyst components compared to other transition metal elements (e.g., metal sulfides) because of their abundant reserves, unique framework, variable composition, and outstanding electrical conductivity [15]. Recent studies have found that transition metal phosphides have exceptional stability and activity in seawater electrolysis. Table 1 provides a quick overview of the characteristics of various recently developed TMP-based electrocatalysts. Although extensive work has been performed, there is still vast room to develop a stable, high-potential catalyst in order to produce sustainable hydrogen from seawater in the long-term.

Some recently published reviews have summarized the progress in water splitting and H₂ evolution reactions. The literature summarized by Shah et al. [36] discussed the structure, mechanism, and potential of transition metal tellurides (TMTs) and phosphides (TMPs) for HER.

Phosphides are the products created when phosphorus is combined with any d- (such as nickel (Ni), molybdenum (Mo), tungsten (W), cobalt (Co), and iron (Fe)) or f-metal. TMPs are resistant metallic substances with acidic as well as metallic sites [39,40]. The metal phosphides react quickly with water and moisture in the air or stored grain to form phosphine gas. Additionally, these materials have complicated structural characteristics and special chemical, physical, and electrical properties because of the crystal lattice interactions between the metal and phosphorus. Previous research on TMP structures can be found in the literature, as reported by [41,42].

TMPs have diverse characteristics that are influenced by several important criteria, including preparation method, P source, capping agent, heating rate, and so on, in addition to their morphology and particle size. TMPs have been used in a wide variety of catalytic reactions because of the versatile nature of their structures [43]. There are numerous methods for synthesizing TMPs outlined in the literature. These methods have been grouped into four different groups: (i) P solvothermal reactions, (ii) solution-phase reactions, (iii) gas–solid reactions, and (iv) other methods [44]. As described by Bhunia et al. [45], different TMP synthesis methods have distinct benefits and drawbacks based on various comparison criteria, including electrocatalyst surface area, TMP conductivity, and other parameters, as shown in Table 2.

Additionally, TMPs have a formula of MxPy due to P’s somewhat stronger electronegativity than metal, which inhibits electron dispersion around the metal atom while enhancing metal-to-P electron transport. The difference between the M–P electronegativity and the M:P ratio determines the properties of the metal phosphide. TMPs, on the other hand, display a mix of covalent and ionic nature bonds due to minor differences in electronegativity. This results in a little positive charge (+) for the metal and a tiny negative charge (−) for the phosphorus. As a result, this unique bond has exceptional thermal and chemical stability as well as strength [46]. On the other hand, TMPs have high activities and good stability due to their earth-abundant resources [47,48,49].

Due to the stoichiometric ratio of metal to phosphorus in their chemical formula (metal-rich phosphides such as M₃P and M₂P, mono-metal phosphides such as MP, and phosphorus-rich metal phosphides such as MP₂ and MP₃), an additional approach to classifying TMPs is to split them into three categories: binary, ternary, and supported types [50], as shown in Table 3. In the open-source literature, several different nanostructures of binary TMPs have been discovered, involving CoP in the form of nanotubes, nanoparticles, nanosheets, and nanorods [51,52]; Co₂P in the form of nanoflowers, nanoparticles, and nanosheets [53,54]; Cu₃P in the form of nanoarrays and nanowires [55,56]; and MoP in the form of nanoflakes and nanoparticles [57,58]. Ternary phosphides are outstanding catalysts for various chemical reactions and exhibit interesting structures. They might be present in the form of nanowires NiCo₂Px [59,60], porous NiCu-P [61,62,63], and core–shell CoMoP [64]. Several studies have addressed the use of supported TMPs and the difficulties associated with their chemistry in their use. These catalysts can behave in diverse forms, for example, alumina, silica, and carbon [65,66,67].

Many catalysts have been investigated in seawater electrolysis up to this point [22,27,39,48].

A complete overview and in-depth knowledge of the reaction process are needed to implement industrial seawater electrolysis and achieve high-efficiency hydrogen production. Notably, the significant water electrolysis reaction is formed by two half-cell reactions, the evolution reaction of hydrogen (HER) and the evolution reaction of oxygen (OER) [71,72], and both depend on the electrolyte’s pH [73]. This means that OER refers to oxidizing water at the anode, while HER refers to reducing water at the cathode to yield H₂. Additionally, water electrolysis, a thermodynamic chemical process, has an overall Gibbs free energy for hydrogen adsorption (ΔG_H*) value of about 237.2 Kj mol⁻¹ [74].

In different pH environments, water decomposes according to Equations (1)–(4).

In acidic pH:

In basic pH:

Seawater is generally rich in salts compared to freshwater, which complicates the electrolytic process [10,75]. The effects of different chemical elements (anions and cations) present in seawater on water electrolysis are discussed in the following paragraphs.

Due to the presence of up to 3.5 wt% salts, seawater has strong ionic conductivity [76]. In seawater, ions of magnesium, sodium, chloride, potassium, calcium, and sulfate account for >99% of the total seawater ion content [77,78]. It is also estimated that artificial seawater has a dissolved solids content totaling approx. 35,000 ppm, of which sodium chloride (NaCl) makes up about 30,000 ppm. The composition of seawater is presented in Table 4 [79,80].

Nevertheless, to simulate real seawater, it has also reportedly been claimed that some Mg²⁺, Ca²⁺, K⁺, and SO₄²⁻ are added. Seawater’s complex ion composition can boost its ionic conductivity, making seawater electrolysis more challenging. As the H⁺ is depleted, for example, the resultant OH combines with both cations (Ca²⁺ and Mg²⁺) to produce insoluble precipitates of calcium oxide and magnesium oxide, respectively. These insoluble precipitates on the electrode surface could obstruct the reaction sites [73,81,82].

Chloride ions can damage both anodes and cathodes in seawater. The active cores of the cathode side’s catalysts are inhibited by chloride ions, slowing down the reaction and hastening the catalysts’ deterioration [83]. However, because the chloride ions may take part in reduction events that are harmful to OER, considerable amounts of chlorine or hypochlorite may develop on the anode side [84]. The reduction processes of Cl⁻ at the anode side are described by Equations (5) and (6) [84,85].

On the other hand, OER is more beneficial kinetically than CER [86]. The volt differential between CER and OER in the basic environment (blue area) is 0.490 V in the pH range of 7.5 to 14, but it decreases under acidic conditions. In these circumstances, OER should have an overpotential in an alkaline medium significantly lower than 0.49 V in an attempt to develop O₂ and prevent CER from producing hypochlorite ions (ClO⁻).