Nickel-Based Electrocatalysts for Water Electrolysis

Abstract

Currently, hydrogen production is based on the reforming process, leading to the emission of pollutants; therefore, a substitute production method is imminently required. Water electrolysis is an ideal alternative for large-scale hydrogen production, as it does not produce any carbon-based pollutant byproducts. The production of green hydrogen from water electrolysis using intermittent sources (e.g., solar and eolic sources) would facilitate clean energy storage. However, the electrocatalysts currently required for water electrolysis are noble metals, making this potential option expensive and inaccessible for industrial applications. Therefore, there is a need to develop electrocatalysts based on earth-abundant and low-cost metals. Nickel-based electrocatalysts are a fitting alternative because they are economically accessible. Extensive research has focused on developing nickel-based electrocatalysts for hydrogen and oxygen evolution. Theoretical and experimental work have addressed the elucidation of these electrochemical processes and the role of heteroatoms, structure, and morphology. Even though some works tend to be contradictory, they have lit up the path for the development of efficient nickel-based electrocatalysts. For these reasons, a review of recent progress is presented herein.

1. Water Electrolysis

One of the main motivations to produce molecular hydrogen (H₂) is its role as an energy carrier, with an energy density of 140 MJ kg⁻¹ [1], which makes it an ideal alternative for clean energy storage. Water electrolysis, the electrical decomposition of water (H₂O) into H₂ and molecular oxygen (O₂) [2], is considered a clean and efficient H₂ production method since no byproducts are generated, and renewable energies can be used, in contrast to the reforming process, in which carbon dioxide (CO₂) and carbon monoxide (CO) are generated [3]. Although water electrolysis has been known for more than 200 years and provides high-purity H₂ (greater than 99.5%) [4], it represents less than 1% of the total amount of H₂ produced each year globally [5].

The overall reaction for water electrolysis is shown in Equation (1) [6].

$${\mathrm{H}}_2\mathrm{O}{}_{\left(\mathrm{l}\right)}\to {\mathrm{H}}_2{}_{\left(\mathrm{g}\right)}+\frac{1}{2}{\mathrm{O}}_2{}_{\left(\mathrm{g}\right)}$$

(1)

The electrical work needed for water electrolysis according to Equation (1) at standard temperature and pressure is given by the change in free energy, ${\Delta }_{\mathrm{rxn}}{G}^{\mathsf{\theta }}=237.2$ kJ/mol. This marks the thermodynamic lower limit of energy required for producing hydrogen, which in practical terms is equivalent to 33 kWh per 1 kg of hydrogen at an equilibrium voltage of 1.23 V. In the best-case scenario, the electrical energy consumed by an electrolyzer is about 50 kWh, corresponding to an efficiency of 66% [7]. Typical operating voltages range from 1.60 to 2.00 V.

Water electrolysis consists of two electrochemical reactions that occur in parallel at two different electrodes: the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) [8]. A water electrolysis cell consists of an anode, a cathode, a power source, and an electrolyte. When an electrical current is applied to the electrochemical system, electrons flow from the negatively charged source to the cathode to produce H₂. In parallel, to maintain the electrochemical balance, ions travel across the electrolyte to the anode, and they give up electrons to produce O₂ [4].

Understanding the mechanism of water electrolysis is fundamental for the efficient production of H₂. Among the main limitations for its industrial scalability is the efficiency of the OER at the anode. When comparing the OER mechanism with the HER mechanism, the first one is thermodynamically and kinetically more demanding, as it requires the sequential transfer of four electrons and the need to break oxygen-hydrogen bonds to form oxygen–oxygen bonds [9,10].

As stated above, water electrolysis is a process composed of two main reactions: HER and OER. It could be acidic, alkaline, or neutral, depending on the electrolyte composition. The selection of water electrolysis conditions must ensure that the electrolytic system (i) is chemically and electrochemically stable, (ii) dissolves the electrolyte to provide a conductive solution, and (iii) is easy to handle and store. Meanwhile, the supporting electrolyte must be inert and have high-mobility ions to improve the conductivity of the solution [11].

In acidic electrolytes, HER and OER reactions proceed according to Equations (2) and (3):

$$\begin{array}{ccc}4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}& \to & 2{\mathrm{H}}_2\end{array}$$

(2)

$$\begin{array}{ccc}2{\mathrm{H}}_2\mathrm{O}& \to & 4{\mathrm{H}}^{+}+{\mathrm{O}}_2+4{\mathrm{e}}^{-}\hfill \end{array}$$

(3)

whereas in alkaline electrolytes these reactions take place following Equations (4) and (5):

$$\begin{array}{ccc}\hfill 4{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}& \to & 2{\mathrm{H}}_2+4{\mathrm{OH}}^{-}\hfill \end{array}$$

(4)

$$\begin{array}{ccc}\hfill 4{\mathrm{OH}}^{-}& \to & {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\hfill \end{array}$$

(5)

Both HER and OER have multi-step mechanisms, with more than one possible pathway. The cathodic half-reaction, HER, involves three steps: Volmer, Tafel, and Heyrowsky; and it can proceed via two mechanisms: the Volmer–Tafel or the Volmer–Heyrovsky processes [12].

During the Volmer step, a Faradaic reaction generates a hydrogen atom, which is adsorbed on the surface of the catalyst. This can occur either due to the reduction of a proton in acidic media (see Equation (6)), or due to the reduction of a water molecule in alkaline electrolytes, as shown in Equation (9) [12].

The formation of H₂ from the adsorbed hydrogen atoms can proceed via the Tafel or Heyrovsky steps. The formation of H₂ through the Tafel process (Equation (7)) consists of the combination of two hydrogen atoms previously adsorbed on the catalyst to produce a H₂ molecule that will later be desorbed. On the other hand, in the Heyrovsky step, as shown in Equation (8) or Equation (10), H₂ is formed via the cathodic formation of a hydrogen atom that bonds with an ${\mathrm{H}}_{\mathrm{ad}}$ formed through the Volmer step [12,13,14].

Acidic conditions:

Volmer step:

$$2{\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{M}+2{\mathrm{e}}^{-}\to 2{\mathrm{MH}}_{\mathrm{ad}}+2{\mathrm{H}}_2\mathrm{O}$$

(6)

Tafel step:

$$2{\mathrm{MH}}_{\mathrm{ad}}\to {\mathrm{H}}_2+2\mathrm{M}$$

(7)

Heyrovsky step:

$${\mathrm{MH}}_{\mathrm{ad}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+\mathrm{M}$$

(8)

Alkaline conditions:

Volmer step:

$$2{\mathrm{H}}_2\mathrm{O}+\mathrm{M}+2{\mathrm{e}}^{-}\to 2{\mathrm{MH}}_{\mathrm{ad}}+2{\mathrm{OH}}^{-}$$

(9)

Tafel step:

$$2{\mathrm{MH}}_{\mathrm{ad}}\to {\mathrm{H}}_2+2\mathrm{M}$$

Heyrovsky step:

$${\mathrm{MH}}_{\mathrm{ad}}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2+\mathrm{M}+{\mathrm{OH}}^{-}$$

(10)

One of the factors determining the preferred mechanism is the surface coverage of ${\mathrm{H}}_{\mathrm{ad}}$. At high coverage, the Volmer–Tafel mechanism dominates since the surface recombination step becomes more probable, whereas at low coverage, the Volmer–Heyrovsky mechanism is favored [13]. The free energy of hydrogen adsorption ($\Delta G_{\mathrm{H}}$) indicates the strength of the metal–hydrogen bond and must also be taken into account. If this bonding energy is too negative, the Volmer step is favored but the strong MH bond will be difficult to break, thus impeding the other steps. In contrast, a positive $\Delta G_{\mathrm{H}}$ would make the Volmer step less likely to occur, which would slow down the HER. Another factor that must be considered is the stability of the catalyst at different pH values; non-noble metals are unstable in acidic media but can perform well in alkaline media. Combinations of non-noble metals with non-metals can be used to make materials with high catalytic activity in a wide range of pH [13]. In summary, the electrocatalytic material determines the mechanism followed by HER. The choice of electrocatalyst implies a choice of electrolyte pH since the stability of the former limits this choice.

The other half-reaction in water electrolysis, OER, involves the transfer of four electrons. The mechanism followed by OER depends on the electrocatalyst used and the pH of the electrolyte [13]. The mechanisms that have been reported for different catalytic materials present variations in the intermediates involved in the overall reaction, mainly in the reaction that precedes the formation of O₂ [14,15]. There are two main routes for the formation of O₂: the first one is the direct combination of two adsorbed oxygen atoms (MO), Equation (13); the second one is the formation of an oxyhydroxide intermediate (MOOH), Equations (14) and (18), which will then, upon an oxidation step, decompose to O₂ [14,15]. Even though there have been discrepancies in the mechanism reported for OER, and these vary depending on the electrocatalytic conditions, there are three principal intermediates involved: the oxide (MO), the hydroxide (MOH), and the oxyhydroxide (MOOH) [13]. Furthermore, the efficiency of OER depends on the interactions between electrocatalytic material and intermediates [15]. Due to these thermodynamic and kinetic limitations, the OER requires the application of high overpotential values [3,16].

Acidic conditions:

$$\begin{array}{ccc}\hfill \mathrm{M}+{\mathrm{H}}_2\mathrm{O}& \to & \mathrm{MOH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\hfill \end{array}$$

(11)

$$\begin{array}{ccc}\hfill \mathrm{MOH}& \to & \mathrm{MO}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\hfill \end{array}$$

(12)

$$\begin{array}{ccc}\hfill 2\mathrm{MO}& \to & 2\mathrm{M}+{\mathrm{O}}_2\hfill \end{array}$$

(13)

$$\begin{array}{ccc}\hfill \mathrm{MO}+{\mathrm{H}}_2\mathrm{O}& \to & \mathrm{MOOH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\hfill \end{array}$$

(14)

$$\begin{array}{ccc}\hfill \mathrm{MOOH}+{\mathrm{H}}_2\mathrm{O}& \to & \mathrm{M}+{\mathrm{O}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\hfill \end{array}$$

(15)

Alkaline conditions:

$$\begin{array}{ccc}\hfill \mathrm{M}+{\mathrm{OH}}^{-}& \to & \mathrm{MOH}+{\mathrm{e}}^{-}\hfill \end{array}$$

(16)

$$\begin{array}{ccc}\hfill \mathrm{MOH}+{\mathrm{OH}}^{-}& \to & \mathrm{MO}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\hfill \end{array}$$

(17)

$$\begin{array}{ccc}\hfill 2\mathrm{MO}& \to & 2\mathrm{M}+{\mathrm{O}}_2\hfill \end{array}$$

$$\begin{array}{ccc}\hfill \mathrm{MO}+{\mathrm{OH}}^{-}& \to & \mathrm{MOOH}+{\mathrm{e}}^{-}\hfill \end{array}$$

(18)

$$\begin{array}{ccc}\hfill \mathrm{MOOH}+{\mathrm{OH}}^{-}& \to & \mathrm{M}+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\hfill \end{array}$$

(19)

Acid electrolytes are commonly used on acid proton exchange membranes [14]. The main advantage of using an acid electrolyte for water electrolysis is the high concentration of protons available for HER [17,18]. Nevertheless, acidic water electrolysis requires the implementation of noble metals (Pt, Ir, or Ru) or noble metal oxides (IrO₂ or RuO₂) as electrocatalysts for HER or OER, respectively [17,19,20]. In the electrolysis cell’s anode, the potential working range is only suitable for noble metals due to their resistance to aggressive and corrosive environments and their insolubility in acids, giving them a long operating life [19,20,21]. The use of non-noble metals as electrocatalysts in acidic conditions leads to the dissolution of oxides and hydroxides, causing the loss of stability due to changes in the structural and chemical composition of the electrocatalyst [19,20]. This explains the scarcity of cost-effective electrocatalysts for acid water electrolysis [22].

A common alternative to acid electrolytes is the implementation of alkaline electrolytes, such as KOH and NaOH. These are strong bases with high-mobility ions [23]. Due to the low availability of protons in the media, the catalytic activity and exchange current density for HER are low compared to the values reported for acid electrolytes [12]. On the other hand, the main advantage is that alkaline electrolytes provide better stability for non-noble metals since there is no corrosion or risk of dissolution [11,18,24]. The ability to select non-noble metals, such as earth-abundant metals, as electrocatalyst materials makes alkaline electrolytes a cost-effective alternative for industrial applications [14,17,20].

Like alkaline electrolytes, neutral electrolytes enable the use of first-row transition metals as catalysts since the corrosion rate of catalysts under strong acids or bases can also be reduced by using a neutral electrolyte. However, neutral electrolytes have the drawback that HER requires high overpotentials to dissociate water molecules to generate protons, slowing down the kinetics of this reaction [25]. Neutral electrolytes allow the use of bifunctional catalysts, which simplifies the electrochemical systems and can lead to a significant reduction in costs. Neutral media also allow the direct use of seawater as a neutral electrolyte [25,26] and the combination of a metal-based electrocatalyst with biocatalysts for the production of biofuels [26].

2. Electrocatalysis

The dependence of the electron transfer rate on the electrode material suggests that the electrochemical reaction is electrocatalytic, and the electrode acts as an electrocatalyst. An electrocatalyst–electrolyte interface has a heterogeneous distribution, composed of multiple phases, of which one or more can catalyze the elementary steps of the reaction. These phases are anchored to a material with a high surface area that stabilizes the area of the electrocatalyst, acting as a support phase [27]. An electrocatalyst’s key performance parameters are the overpotential, Tafel slope, the exchange current density, the turnover frequency, Faradaic efficiency, electrochemical active surface area, and stability, which provide information on the electrochemical properties of the electrocatalytic system, kinetics parameters, the reaction mechanism, and the rate-determining step [14,15,28].

The overpotential $\eta$ is the difference between the equilibrium potential $E_{\mathrm{eq}}$ and the applied potential $E_{\mathrm{app}}$ (Equation (20)) at which the electrocatalyst provides a given current as a result of the excess potential introduced to overcome the energetic barrier of the reaction [15,29].

$$\eta =E_{\mathrm{app}}-E_{\mathrm{eq}}$$

(20)

The Butler–Volmer (Equation (21)) describes the rate of an electrochemical reaction through the current density (j) in terms of the overpotential. As the scan rate controls the speed at which the potential is swept, j is proportional to the reaction rate [30]. The lower the $\eta$ required to maintain a specific j, the more efficient the electrocatalytic material [31]. In this equation, $j_0$ is the exchange current density, $\alpha$ is the transfer coefficient, f represents the Faraday constant divided by the temperature and the real gas constant (i.e., $f=F/RT$), and n is the number of electrons transferred in the rate-determining step of the reaction:

$$j=j_0\left\{e^{-\alpha nf\eta }-e^{(1-\alpha )nf\eta }\right\}$$

(21)

The Butler–Volmer equation represents the sum of reduction and oxidation reaction currents. These currents are proportional to the rate of reaction. In equilibrium, the total flux of the current is zero because the system has reached a dynamic equilibrium in which the rate of oxidation-reduction (redox) reactions is the same [32]. Otherwise, only one process is dominant at high $\eta$ values. This eliminates one term, leading to Tafel’s equation [9], which can be expressed as the linearized Equation (22).

$$\eta =m_{T}ln{j}_0-m_{T}lnj{m}_T=\frac{RT}{\alpha nF}$$

(22)

The Tafel slope, $m_T$, describes how sensitive the current response is to the overpotential and gives information about the reaction mechanism and the rate-limiting steps [9]. A small Tafel slope is attributed to a rapid increase in the j at lower $\eta$ values [15]. Equally, the exchange current density ($j_0$) can be obtained using the Tafel equation (Equation (22)) that corresponds to the intercept [14]. $j_0$ is proportional to the rate reaction at the equilibrium when the total current is equal to zero, and the anodic and cathodic currents are equal [15,33]. This kinetic parameter depends on the concentration, temperature, electrode catalyst loading, and its specific surface area [33]. It reflects the intrinsic bonding/charge-transfer interactions between the electrocatalyst and the electroactive species [15]. A high value of $j_0$ indicates that the material is a promising electrocatalyst for the target reaction [15,33]. These three kinetic parameters can be determined experimentally by means of voltammetry, specifically linear sweep voltammetry (LSV). The structure of the voltammogram obtained (shape, size, and potential peaks) gives information about the electrochemical processes active on the surface of the working electrode [34,35,36,37].

Based on the voltammogram and using Equation (22), Tafel curves can be plotted to find the Tafel slope. For instance, Yu et al. used LSV to determine the Tafel slopes of three different deposits, namely, Pt, $\alpha$-Ni(OH)₂/Pt, and $\beta$-Ni(OH)₂/Pt, and determined that the $\beta$-Ni(OH)₂/Pt deposit had the highest electrocatalytic activity because this material presented the smallest Tafel slope (39 mV s⁻¹) [38]. Figure 1a,b show the voltammograms and the Tafel plots with their corresponding slopes, respectively.

The turnover frequency (TOF) is a performance parameter that evaluates the rate at which reactants are converted to products per catalyst site per second [39,40]. The TOF values depend on the temperature and pressure [40]. The equation for the determination of TOF in gas evolving reactions is stated in Equation (23), where j is the current density, $N_{Av}$ is the Avogadro number, n is the number of electrons involved, F is Faraday’s constant, and $\Gamma$ is the surface or total concentration of active sites [41].

$$\mathrm{TOF}=\frac{j{N}_{Av}}{nF\Gamma }$$

(23)

TOF values between 10⁻³ and 10⁻² s⁻¹ have been observed [40]. The main limitation for the accurate determination of this parameter is the difficulty of determining the number of active sites. There is an overestimation in the counting of active species in the electrochemical system because it includes chemically inert or buried sites [14,40]. Furthermore, the calculation of $\Gamma$ for multimetallic, non-metallic, and mixed oxide catalysts on a large scale is limited due to its difficulty [40]. An accurate method for the approximation of $\Gamma$ for monometallic films is integrating the peak of a redox reaction characteristic of the material, obtained by means of cyclic voltammetry (CV), that corresponds to the reaction of interest. This method only considers metallic sites that directly participate in the redox reaction [14,30].

Videa et al. reported the determination of $\Gamma$ for Ni(OH)₂ in alkaline media using CV and Equation (24), where ${\Gamma }_{\mathrm{Ni}}{\left(\mathrm{OH}\right)}_2$ is the superficial concentration of Ni(OH)₂, Q is the area under the redox peak involved in the formation of Ni(OH)₂, F is the Faraday constant and n is the number of transferred electrons in the redox process. By integrating the area under the redox peak, I_af (Figure 2), which was involved in the formation of the electroactive species, Ni(OH)₂, the charge is related to ${\Gamma }_{\mathrm{Ni}}{\left(\mathrm{OH}\right)}_2$ according to Equation (24) [42]

$${\Gamma }_{\mathrm{Ni}}{\left(\mathrm{OH}\right)}_2=\frac{Q}{nFA}.$$

(24)

The electrochemical active surface area (ECSA) is a fundamental electrochemical property of an electrified interface. It is the electrode surface area that is accessible for charge transfer or storage [43]. The values of ECSA depend on the electrochemical reaction that takes place in the interface and the materials involved [14,43]. Because every catalytic process takes place on the electroactive surface, extensive quantities obtained in electrochemical experiments must be normalized concerning ECSA. Since changes in the ECSA during an experiment may occur, it is necessary to monitor this property between experiments [44]. Some methods that have been reported for the determination of ECSA are the integration of the redox peak areas, the hydrogen underpotential deposition, carbon monoxide stripping, and the double-layer capacitance ($C_{DL}$). Capacitive characteristics of an interface in an electrochemical system, such as $C_{DL}$, are associated with the charge storage phenomenon. This phenomenon occurs at potential values at which there is no solvent decomposition or electron transfer during the potential application in the interface [45]. $C_{DL}$ can be determined by means of two techniques: CV [23,46] and electrochemical impedance spectroscopy (EIS) [46]. The current measured ($i_C$) in the selected potential window is attributed to the capacitive double-layer charge. When plotted against the scan rate ($\nu$), the slope of the curve gives the value of $C_{DL}$ according to Equation (25).

$$i_C=\nu C_{DL}$$

(25)

Finally, to obtain the ECSA, the value of $C_{DL}$ is divided by the specific capacitance of the sample at the same electrolyte ($C_S$) (Equation (26)) [46,47].

$$\mathrm{ECSA}=\frac{C_{DL}}{C_S}.$$

(26)

The second technique for the determination of $C_{DL}$ is electrochemical impedance spectroscopy (EIS). This technique is useful for the characterization of the electrode–electrolyte interface. It gives an insight into the kinetic properties of an electrocatalytic reaction, using a simple electrochemical approach [48,49]. This technique has also been previously reported for the study of electrocatalytic reactions, such as OER [46,47,48,49,50]. Electrical impedance is the resistance of a system to the flow of current at a specific frequency [51]. An alternating current (ac) potential is applied at a range of frequencies to measure the impedance. Electrochemical processes, such as charge transfer, ion diffusion and capacitance, can be identified on the basis of their frequency-dependent current response [47,49,50,52]. EIS spectra are fitted to an equivalent circuit, in which each component of the circuit is related to an interfacial parameter [47,50,52]. The interfacial parameters obtained via EIS depend on the environment in which the charge transfer occurs, on the electrolyte’s composition, and the electrode [47,52]. The determination of $C_{DL}$ for monometallic films has proven to be a reliable technique, comparable to CV [53,54]. For deposits with multiple components, such as NiFe bimetallic films and high conductivity, EIS is better than CV due to the elimination of the interference of redox and OER currents [46,50]. Batchellor et al. confirmed the accuracy of the ECSA determination using EIS; they obtained an increase of three orders of magnitude in the value of the $C_{DL}$ in the charged state compared to the uncharged state [50]. Finally, based on the estimate provided by ECSA, the roughness factor (RF) can be determined. RF is obtained via the division of the real area by the geometric area [23]. The roughness of a material can be related to the distribution, size, and morphology of the electrocatalyst particles; a rough surface is characterized by irregularity in its structure. Thus, this provides information about the geometry and distribution of active sites [55]. An RF bigger than one indicates a larger and more favorable area for adsorption [56].

There is a need to increase electrocatalyst stability to meet industrial applications without sacrificing its electrochemical activity [14]. The stability of a non-noble metal electrocatalyst is an issue at the industrial level due to the high current densities involved [14,57]. Some catalyst deactivation processes that have been related to low stability are corrosion, material degradation, surface passivation, and catalyst film delamination. Furthermore, in terms of film thickness, a thick film will have more stability compared to a thinner deposit of the same material [58]. A typical test for the evaluation of stability is the accelerated durability test. In this test, the electrocatalyst is subjected to hundreds or thousands of voltammetry cycles at a high scan rate. Before and after this cyclic step, LSV is conducted to evaluate changes in the material activity. Small changes in the $\eta$ indicate the electrocatalyst’s stability.

3. Nickel-Based Electrocatalysts

Even though water electrolysis has the advantage of producing extremely pure hydrogen (>99.9%), it is limited by its energy efficiency, safety, durability, operability, portability, and the high costs of installation and operation [11]. That is why the development of low-cost electrocatalytic materials that efficiently couple electron transfer in HER and OER reactions at low overpotential values are of importance [59,60]. The characteristics of a promising electrocatalyst are a lower overpotential at a current density, lower Tafel slope, and excellent stability [61]. So far, noble metal-based electrocatalysts are the most efficient electrocatalysts for water electrolysis; along with platinum (Pt) for HER and ruthenium (RuO₂) and iridium (IrO₂) oxides for OER [62,63]. However, the implementation of noble metals and their alloys for water electrolysis limits the industrial scalability due to their high costs and scarcity [59,60]. An attractive alternative is the implementation of transition metals, such as Co, Ni, Fe, or Mo, or their alloys as electrocatalysts for water electrolysis [59,64].

Nickel-based electrocatalysts are an economical and efficient alternative for water electrolysis, they have shown good stability, and electrocatalytic activity for HER and OER [61,64,65]. Nickel is a good candidate for electrocatalysts because of its high conductivity, thermal stability, and good electrical properties. Commonly, Ni is a divalent cation, but it can acquire other valences in the range of −1 to +4. This property gives it the facility to undergo various electronic transitions [65]. To improve the electrocatalytic activity of Ni materials, tuning its chemical environment, structure, and morphology is critical [12]. The performance of an electrocatalyst in water electrolysis is modulated by its crystalline structure, chemical composition, hierarchical porosity, size distribution, electrical conductivity, and surface chemistry [65]. Because of this, there has been intensive research into the production of nickel-based electrocatalysts for water electrolysis.

3.1. Classification of Nickel-Based Electrocatalysts by Composition

As mentioned before, by modifying the chemical environment of nickel electrocatalysts, the electrocatalytic activity can be improved. In nickel-based electrocatalysts, a synergistic effect between Ni and neighboring heteroatoms, such as Fe, O, N, and P, improves the surface adsorption properties via the enhancement of the electronic properties and possibly increments in the surface area [65]. The optimization of a nickel electrocatalyst’s properties via the tuning of its composition and crystalline structure can be achieved through cationic and anionic regulation. Cationic regulation involves introducing cations into the structure to regulate the active sites; this is achieved by modifying the free energy of adsorption or desorption. On the other hand, anionic regulation is the incorporation of anions to regulate the interaction properties and the active sites’ reactivity by adjusting its polarization [65,66]. A useful tool to predict the activity of a new electrocatalyst is the volcano plot. The volcano plot is constructed based on Sabatier’s principle, which states that an optimal electrocatalyst should have an adsorption energy that is neither too high nor too low with respect to an intermediate in an electrocatalytic reaction. The Volcano plot is obtained by plotting the reaction rate versus the free energy of adsorption of an intermediate [31,67]. The volcano plots for HER and OER are shown in Figure 3. Although the descriptors of the reaction rate are different, it can be seen that the best materials for HER and OER are noble metals or their oxides.

The production of economical and efficient electrocatalysts for water electrolysis is essential in order to develop new materials with tuned compositions, i.e., materials of which the surfaces offer precise interactions with the reaction intermediates to promote the electron transfer at the lowest overpotential. For the case of the OER (in acid or alkaline media), the key intermediate adsorbed species are OH (Equation (12) or Equation (17)), O (Equation (14) or Equation (18)), and OOH (Equation (15) or Equation (19)). Nørksov et al. showed that the adsorption energies of these intermediates are linearly related [69]. Ideally, the free energies for each of these processes should be 1.23 eV, but these energies correlate according to Equation (27)

$$\Delta G_{\mathrm{O}}+\Delta G_{\mathrm{OOH}}\approx 3.2\mathrm{eV}$$

(27)

which implies a theoretical overpotential of ∼0.37 V. These linear correlations are known as scaling relationships and reveal reactivity trends in elementary reactions, in which related intermediates bind through the same atom or on the same site [70]. These relationships impose limits to the minimum overpotentials required for a process to occur and the maximum catalytic activity that may be obtained [71]. Breaking the scaling, the limitations imposed by the existence of these scaling relationships would require a bifunctional catalyst that was capable of dissociating the pathways involved in the mechanism. Song et al. have shown that nanoclusters of $\gamma$-FeOOH added onto a $\gamma$-NiOOH support act as proton donor/acceptor centers, bypassing the step from O to OOH, shifting the overpotential below the theoretical limit [72].

In

Table 1. Composition of nickel-based alloys reported in the literature.

Composition Classification	Advantages	Disadvantages	Examples	Ref.
Mono- and Polymetallic oxides	Adjustable structure, stable in alkaline electrolyte. Transforms to oxyhydroxides during OER	Low conductivity. Activity depends on the metal type, its oxidation state, and support electrode.	1 LiCo${}_{\frac{1}{3}}$Ni${}_{\frac{1}{3}}$Fe${}_{\frac{1}{3}}$O2 1 LaNiO3	[73,74]
Metallic and bimetallic embedded or decorated in stable host	Transforms into oxyhydroxides during OER.	Unstable in extreme acid or alkaline electrolytes. Unstable against corrosion.	1 Co, 1 NiFe	[75]
Metallic hydroxides Bimetallic Heterostructures	Adjustable structure and composition. Transforms into oxyhydroxides during OER.	Unstable at high positive potentials.	3 Fe Ni hydroxide 3 NiFe LDH-NS@DG10	[76,77]
Metallic and bimetallic oxyhydroxides	Adjustable structure and composition.		3 NiOOH	[78]
Chalcogenides. Sulfur- and selenide-based.	High conductivity. Efficient mass transfer. High density of active sites. Resistance to corrosion and electrolyte poisoning.		1 FeNiS2 1 (NiCo)0.85Se@NiCo LDH	[79,80]
Metallic and bimetallic phosphides	Adjustable structure and composition.	Unstable at high positive potentials.	1 Ni2P, 1 Co3NiP	[81,82]
Phosphochalcogenides	High electronic conductivity. Adjustable electronic structure.		3 Ni0.9Fe0.1PS3	[8]
Bi- and trimetallic oxyphosphides	Direct synthesis. Wide range of compositions and morphology. Intermetallic synergy.	Poor performance in alkaline media.	3 FeCo3Mo3P-O	[83]
Nitrides	High electronic conductivity. Inert in harsh electrolytes.	Unstable in long-term experiments. Difficult morphology and composition control.	1 Ni3FeN	[84]
Boride heterostructures	High conductivity. Corrosion resistance in acidic and alkaline media.		2 Ni3B/MoB	[85]
Bimetallic phosphates	Work in neutral and alkaline electrolytes. Adjustable morphology, facilitating mass transport and electron/proton transfer. Long-term stability.		3 NiMoPOx	[86]
Borates		Work in neutral and alkaline electrolytes. Adjustable morphology. Stability	1 Ni-Bi	[87]

NS = nanosheet, LDH = layered double hydroxides, ¹ For OER, ² For HER, ³ Bifunctional.

some of the most common compositions of nickel-based electrocatalysts are listed with their corresponding general formulae, advantages, disadvantages, and examples.

3.2. Structural Classification of Nickel-Based Electrocatalysts

Another alternative method to improve electrocatalytic activity is modifying the structure and morphology to increase the number of active sites and shorten the paths for charge or mass transport [61]. Several structures of nickel-based electrocatalyst are presented.

3.2.1. Perovskite Structure

Perovskites are a type of oxide with the general formula ABO₃ (Figure 4a), where A represents an alkaline earth element, such as Ca, Ba, and Sr, or a lanthanide element, commonly La or Pr [15,88], and B represents transition metals, such as Ni, Co, or Fe. These usually have oxidation states +3, +4, or a mixture of both, depending on the oxidation state of A. B is at the center of the octahedron, and A fills empty spaces to complete the perovskite structure [15]. Via the partial substitution of atoms of either A or B, it is possible to tune the perovskite structure (A_xA′_(1−x)B_yB′_(1−y)O₃) to enhance the electrocatalytic activity, modify the chemical stability, the ionic conductivity, the cell parameters, or the thermal properties [88,89]. Park and coworkers evaluated the impact of the partial substitution of the transition metal in perovskites in terms of electrocatalytic activity for OER. Via the partial substitution of Mn, Fe, or Co in LaNiO₃, they observed a general improvement in the OER activity, but the best result was obtained via the partial substitution of Fe [89]. Perovskites’ ability to stack A and B cations to obtain ordered-layer structures, such double perovskites (AA′BB′O₆), in which the unit cell is the double of a common unit cell makes it possible to achieve diverse electronic structures and synergistic effects between the cations [14,90].

3.2.2. Spinel

A spinel is a binary oxide with the general formula AB₂O₄ (Figure 4b) [14,93]. Spinels contain two crystallographic sites, octahedral and tetrahedral, which can lead to two different types of structure depending on where the metals are localized: the normal spinel ((A${}_{T_d}^{2+}$)(B${}_{O_h}^{3+}$)₂O₄) and the inverse spinel ((A${}_{O_h}^{2+}$)(B${}_{T_d}^{3+}$) (B${}_{O_h}^{3+}$)₂O₄) [15,94]. The main advantages of this structure are the good electric conductivity due to the mixed-valence system, high charge transfer efficiency, and high stability in harsh alkaline electrolyte and under high anodic potentials. Furthermore, the electrocatalytic activity can be modulated by incorporating secondary metals, such as Ni, Mn, and Zn [14,15,94,95]. Two of the most studied types of spinels are ferrite (MFe₂O₄) and cobaltite (MCo₂O₄) [14,15]. Torres-González et al. reported the application of NiFe₂O₄ hollow nanospheres with high surface area and stability as anode material for water electrolysis in an alkaline microfluidic device [96]. Chien et al. produced a NiCo₂O₄ aerogel for OER with high surface area and a well-connected 3D and through a pore structure that increased the availability of active sites and the easy access of the electrolyte to these sites [97].

3.2.3. Pyrite

Pyrite is a common structure of transition metal dichalcogenides (MX₂), where M could be Fe, Co, or Ni, and X is S or Se (Figure 4c). Cations are localized inside a sulfur or selenium octahedron [98,99]. This structure is characterized by its long-term stability in either acidic or alkaline electrolytes and for the synergistic effect between the binary metals [85]. He et al. reported the synthesis of a bifunctional electrocatalyst for HER and OER. Ni_0.33Co_0.67S₂ nanoneedles displayed a superior electrocatalytic activity for both processes that was attributed to the optimized balance due to the electron transfer capability, the increased exposure of active sites, and the good dissipation of gaseous products [98].

3.2.4. Layered Structures

The layered-structure-type oxides, also known as layered double hydroxides (LDHs), consist of ionic layers that contain interlayer ions of the opposite charge or water molecules that balance the overall charge and increase the spacing between layers (Figure 4d). The layers have a brucite-like structure [15,100]. Transition-metal hydroxides and oxyhydroxides have this type of structure. These compounds show good electric conductivity and are stable in alkaline electrolytes [15]. Ni-Fe LDHs have been used as a protection layer for silicon photocathodes, and electrocatalyst for HER in alkaline solution, displaying outstanding stability [101]. Single monolayers are produced via liquid exfoliation to increase the electrocatalytic activity of the LDH materials. This treatment increases the number of active sites in the structure’s edges, making the inner layer more accessible to the electrolyte, and increasing the electronic conductivity [75]. Liang et al. reported the synthesis and chemical exfoliation of NiCo LDHs and their activity towards OER and an enhancement in their activity due to their increased surface and more exposed active sites [102].

3.2.5. Heterostructures

Heterostructures are hybrid materials comprised of several materials, and in this case, with synergistic effects on the electrocatalysis of water electrolysis (Figure 4e) [79,103]. Each component of the heterostructure accelerates one specific rate-determining step of water electrolysis or enhances a chemical or physical property of the electrocatalyst [78]. The interest in developing heterostructures as electrocatalysts resides in the fact that a single material has limited activity towards water splitting because of either its low electronic conductivity, insufficient active sites, or instability. Instead, heterostructures have a tuned electronic structure that merges the advantages of different materials to produce an ideal or at least a more efficient electrocatalyst [103]. Zheng et al. reported a core-shell hybrid nanostructure, Ni₃(S_0.25Se_0.75)@NiOOH, on nickel foam as a efficient water splitting electrocatalyst. The core was made of nickel chalcogenides, which optimized the electronic structure and the catalytically active nickel oxide/oxyhydroxide shell. This heterostructure harnessed the advantages of both materials and, with a rational design, synthesized an efficient water electrocatalyst [104].

3.2.6. Amorphous Structures

Electrocatalysts with amorphous structures are solids with randomly arranged atoms and can have different compositions, such as oxides, layered double hydroxides (LDHs), binary and ternary oxides, as well as chalcogenides, borides, and phosphates (Figure 4f) [92,105]. The use of amorphous nickel-based electrocatalysts for water electrolysis increases the number of active sites available by increasing the diffusion of electrolytes to inner parts of the material, the ECSA, and the number of defects [92]. He et al. synthesized an amorphous Ni-based bifunctional catalyst that was stable and robust in both processes. The electrocatalyst required an oxidative pretreatment for HER in a near-neutral aqueous buffer. For OER, at anodic potentials, the film was converted to nickel oxide reversibly [106]. Trudel et al. developed a photochemical route to synthesize binary Fe_100−yNi_yO_x and ternary Fe_100−y−zCo_yNi_zO_x amorphous oxides with highly controlled compositions and a homogeneous distribution of metals that showed significant improvement in the thermodynamic and catalytic performance toward OER. Based on the behavior of the Tafel slope, onset overpotential, and the overpotential at 10 mA cm⁻² at different compositions and annealing temperatures, they propose a synergistic effect between Ni and Fe and that Fe and Ni play a specific role in the kinetic and thermodynamic determining steps of the OER mechanism, respectively, suggesting the bifunctional activity of these materials [107,108].

3.3. Structural Considerations for Nickel-Based Electrocatalysts

The electrochemical activity of a material can be tailored by controlling the morphology, crystallinity, size of primary particles, and microstructure [94,109]. The ideal electrocatalyst has a big ECSA and accessible and continuous transport channels for charge carrier transport. This feature enables the electrocatalyst to increment the number of active sites available and facilitates electrolyte access [97]. Definitively knowing the effect that modifications in the structure of a material can have on its electrocatalytic activity is of great importance.

3.3.1. Substrate

Depending on the support used for the deposition or casting of the electrocatalyst, the electrocatalytic activity can be either enhanced or diminished. The correct substrate can increase the loading of the electrocatalyst and favor the electron transfer [61,110]. Conductive substrates and 3D substrates with open pores have been investigated due to their chemical and physical properties, which enhance ionic and electronic conductivity and increase the loading of the electrocatalyst [61,110,111]. Some disadvantages that can come with using substrates are the poor kinetics of electrocatalytic reactions and low durability. These two aspects are strongly related to the weak catalyst–substrate contact, which is responsible for the electronic resistance, vigorous gas evolution, and the consequent peeling off of the catalyst [104,112]. Two substrates that have been widely used to study water electrolysis are carbon-based materials and nickel foam. Carbon-based materials, such as graphene and nanotubes, are conductive substrates with high surface area and stability [61,113,114]. They provide physical support and a conductive path for charge transport [113]. Nickel foam (NF) is a cheap material that is stable in alkaline media and can function as an inert porous support or as a nickel source. The 3D open-pore structure favors the mass transfer and the effective bubble release, and it also has high electrical conductivity and a large surface area [104,111,112]. Finally, a critical factor favoring the use of NF is the direct growth of active material on the surface of the substrate, which enhances the catalyst–surface contact, which improves the charge transport [111].

3.3.2. Defects

The preparation of a perfect crystal without any defects is complex. Many defects can be present in a crystal, such as point defects, line defects, plane defects, and volume defects. All of these defects can modify the electronic and surface properties of an electrocatalyst [115]. The absence of specific atoms, doping with heteroatoms, and the reconstruction of the lattice break the electron-hole symmetry in the structure [115,116]. The defective regions form dangling groups that alter the electronic density and increase the chemical reactivity [116]. Intrinsic defects are beneficial for water electrolysis, as they serve as active sites. For instance, Radinger et al. recently demonstrated an improved electrocatalytic activity and a higher active surface area in relation to the OER performance for a Ni^IIO_x thin film containing Ni^III impurities and additional oxygen vacancies (O_V), which depend on the initial structure and chemical composition of the sample [117]. Moreover, using a defect engineering approach by creating O_V in a spinel nickel ferrite (NiFe₂O₄) structure, the spinel-type catalyst showed an enhanced catalytic OER performance with an OER overpotential as low as 0.35 V at 10 mA cm⁻² and a Tafel slope of 40 mV dec⁻¹. The existence of O_V tunes the electron configuration and creates a significant number of defective donor states in the bandgap to facilitate charge transfer processes [118].

3.3.3. Doping

An electrocatalyst’s surface can be altered by introducing foreign elements, leading to improved electrocatalytic activity for water electrolysis [119]. Doping tunes the native electronic structure of a catalytic surface via the intentional alloying of semiconducting materials with the controlled presence of impurities [65,120]. Swierk et al. evaluated the effect that doping a nickel (oxy)hydroxide structure with iron had on the OER activation energy. The OER activation energy was similar to the activation energy of an IrO₂ catalyst [47]. Ni can also act as a doping agent. It has been reported in the literature that doping electrocatalysts for water electrolysis with Ni increases the surface area and therefore the number of active sites and enhances the conductivity, promoting the full utilization of the electrocatalyst [93,100].

3.3.4. Porosity

Porous structures have proven to enhance the electrocatalytic activity of water electrolysis electrocatalysts. An interconnected porous structure promotes the full utilization of the ECSA and generates a continuous passageway for charge carrier transport, which improves the catalytic process [97,109]. Finally, porosity increases the electrocatalyst’s stability through the efficient liberation of evolving gas bubbles and minimizing the damage of the electrode [109].

3.3.5. Nanostructuring

Since electrocatalytic reactions occur at the electrode–electrolyte interface, they depend on the electrocatalyst’s surface. The implementation of nanostructures increases the ECSA and therefore the number of active sites. The smaller the electrocatalyst particle size, the higher the reaction rate for water electrolysis because nanostructures facilitate the release of smaller bubble gas due to their lower contact with the electrocatalyst. Furthermore, there is an increment in the exposure of active sites to the electrolyte [100,109]. In the literature, the implementation of nickel-based nanostructures as electrocatalysts for water electrolysis is widely reported [121]. Some of the morphologies studied are core-shell nanoparticles [64,122,123,124], hollow structures [109,125], nanowires [100,126], and nanosheets [127,128].

4. Synthesis of Nickel-Based Electrocatalysts

So far, in the literature, many synthesis methods for nickel-based electrocatalysts have been reported, such as hydrothermal synthesis [73,85], solvothermal synthesis [129], chemical vapor deposition [130], and coprecipitation [131]. However, most of these techniques require the use of shape-directing agents and surfactants; they are also time-consuming and complicated [94]. On the other hand, at the industrial scale, the production of nickel-based electrocatalysts is accomplished via the chemical synthesis of powders that are then pasted onto conductive substrates [12]. This is why it is necessary to develop new techniques that allow the production of nickel-based electrocatalyst at an industrial scale and permit the accurate control of composition, structure, and morphology. A comparison of the three most common synthesis techniques for nickel-based electrocatalysts is shown in

Table 2. Common synthesis techniques for nickel-based electrocatalysts.

	Electrodeposition	Hydrothermal Synthesis	Coprecipitation
Description	Reduction and deposition of an ionic species from a conductive electrode	Synthesis of materials in an autoclave with water as a catalyst, temperature above 100 ºC, and high pressure	Simultaneous precipitation of two metallic precursors from a liquid solution
Variants	Galvanostatic or potentiostatic	Surfactant-assisted	Precipitation
Advantages	Direct deposition in substrate. High control of material properties and morphology. Material properties are tuned via changes in bath composition, current density, pulses duration etc. No purification required. Scalable	Direct deposition in substrate. High purity. Controlled size and morphology. High-quality crystals. Lower costs of energy, instrumentation, and precursors. Environmental benign synthesis. Scalable	Provides stoichiometric mixtures with well-defined phases. Crystalline structure is tuned via solution concentration, precursors, temperature, and pH.
Disadvantages	Unwanted blocking of evolved gas. Electrodeposition proceeds via specific reactions, difficult to standardize the technique.	Requires high temperatures and pressures.	Independent precipitation of components can occur. Requires further activation treatment
Examples	NiFe/NF [132], Ni-S-Fe alloy [133]	NiCo2O4 [134], NiFe LDH/NF [135]	NiCoP [136], NiFe [131]

NF = nickel foam, LDH = layered double hydroxides.

.

4.1. Hydrothermal Synthesis of Nickel-Based Electrocatalysts

Hydrothermal synthesis is a straightforward and simple chemical method that occurs in water in a sealed pressure vessel, which enables a reaction to take place at both high temperature and pressure [137]. Water can participate in the reaction as a chemical component or a solvent in a hydrothermal process. Crystalline growth under hydrothermal conditions involves [138]: (i) the dissolution of reactants, producing ions or molecular groups in the hydrothermal medium; (ii) the ions or molecular groups are diffused to the low-temperature region of the vessel, where the seed crystal is grown to form a supersaturated solution; (iii) the ions or molecular groups are adsorbed, decomposed, and desorbed at the growth interface; (iv) the adsorbed material is transported to the interface, prompting the dissolved matter to crystallize to achieve the desired product finally. Several Ni-based electrocatalysts have been synthesis via traditional hydrothermal methods or variants of these. For instance, a mesoporous NiCo₂O₄ material was synthesized through a combination of templating with hydrothermal synthesis [134]. Studies of the electron transfer kinetics in the OER of NiCo₂O₄ showed an improved overpotential of ∼350 mV, and better long-term stability compared to commercially available NiCo₂O₄ [134]. In another work, Liu et. al. synthesized single-crystalline NiFe LDH by means of the fluoride-assisted hydrothermal method [135]. It was demonstrated that the NiFe LDH possessed higher activity for the OER than the well-known RuO₂, exhibiting an ultra-small cell voltage of 1.52 V at a current density of 20 mA cm⁻² at room temperature, as well as good long-term stability [135]. More recently, Ni₂P nanowires were synthesized through a one-step hydrothermal approach, in which water, red phosphorus, nickel acetate, and hexadecyl trimethyl ammonium bromide were used as the solvent, phosphorous and nickel sources, and active agent, respectively [139]. The Ni₂P nanowires were applied as electrocatalysts for HER and OER. The overpotential was 320 mV and the Tafel slope was 73 mV dec⁻¹ for HER. Meanwhile, the Ni₂P nanowires showed an electrocatalytic OER activity with an overpotential of 1.51 V (vs. RHE) and a Tafel slope of 46 mV dec⁻¹. Overall, these examples demonstrated the versatility of the hydrothermal method in the synthesis of Ni-based electrocatalysts for highly efficient water splitting.

4.2. Coprecipitation Synthesis of Nickel-Based Electrocatalysts

Coprecipitation refers to the simultaneous precipitation of two metallic precursors from a common liquid solution. Due to its relative simplicity and its cost-effective and fast process, the coprecipitation method is considered to be one of the most frequently applied strategies in the production of nanomaterials at large scales [140]. The mechanism of homogeneous precipitation involves the separation of the nucleation and growth of the nuclei. In this process, two steps are involved: (i) a short burst of nucleation when the concentration of the precursor species reaches critical supersaturation and (ii) the subsequent slow growth of the nuclei via the diffusion of the solutes to the surface on the crystal [141]. Ni-based electrocatalysts have been synthesized using this strategy. For example, NiFe nanocubes with an average side length of ∼320 nm were synthesized following a precipitation method, in which sodium citrate dihydrate acted as a chelating ligand to control the nucleation rate and crystal growth [131]. The NiFe nanocubes were used as bifunctional electrocatalysts for overall water-splitting, demonstrating overpotentials of 271 and 197 mV for OER and HER, respectively, in 1 M KOH at 10 mA cm⁻² [131]. In another example, NiCoP nanosheet arrays were synthesized via a urea precipitation method, followed by phosphorization [136]. For cathodic HER performance, the NiCoP electrode showed an overpotential of ∼133 mV to reach a current density of 50 mA cm⁻². When used as an OER catalyst, this nanomaterial provides a current density of 50 mA cm⁻² at an overpotential of 308 mV. Moreover, an alkaline electrolyzer requiring a potential of ∼1.77 V to achieve a current density of 50 mA cm⁻² was fabricated by using NiCoP nanosheets as both OER, and HER electrocatalysts [136].

4.3. Electrochemical Synthesis of Nickel-Based Electrocatalysts

Electrolytic deposition or electrodeposition is a technique that consists of the reduction or deposition of an electrically conductive species in a conductive electrode via the application of an electrical current, in which the soluble form of the metal to be deposited is present in the electrolyte. The total amount of metal deposited is proportional to the amount of charge passed during the reaction and thus to the thickness of the layer deposited. The electrodeposition conditions greatly influence the morphology of the product. Some factors that can affect the type of deposit obtained and its quality are the pH, the presence of contaminants, the crystal planes of the substrate side reactions, and the current distribution [142]. The mechanism of electrodeposition consists of four main stages: (i) nucleation, (ii) growth of nuclei, (iii) coalescence, and (iv) layer growth. First, the nucleation process consists of the formation of new sites or nuclei, and it can be outward (2D) or upward (3D), depending on the strength of the interaction with the substrate. For the nuclei to be stable, they must reach a critical dimension that depends on the overpotential applied to the system. Depending on the reaction rate, the nucleation can be instantaneous or progressive. The rate of nucleation determines the morphology of the final deposit. At high overpotentials, fine crystal grains are obtained. In instantaneous nucleation, the nuclei’s growth starts when the nucleation ends. Progressive nucleation takes place when lower overpotential values are applied, producing deposits with larger grains and fewer nuclei; the growth occurs simultaneously [142]. According to the controlled parameter, electrodeposition can be potentiostatic or galvanostatic [143,144]. When comparing galvanostatic deposition with potentiostatic deposition, it has been demonstrated that the first one produces smaller particles with a better dispersion [145]. Salinas-Quezada et al. describe the potential behavior during galvanostatic electrodeposition of nickel in four regimens: (i) nickel nucleation, (ii) nickel growth, (iii) deposition termination due to local changes in pH, and (iv) water reduction (Figure 5) [146]. Additionally, they observed a dependency between the deposition current applied and the electrocatalytic activity of nickel films. When lower deposition current values were applied, 22–33 μA, the electrocatalytic activity favored methanol oxidation, and with higher deposition currents, 38–66 μA, OER was favored, which was attributed to changes in the growth regime because the geometric area defines the diffusion field of metallic ions during this phase. Indeed, the electrochemical activity of a nickel-based electrocatalyst produced via electrodeposition was influenced by the synthesis conditions and the mechanism.

Electrodeposition of Ni-based materials can be carried out in a single step, this is known as co-deposition. In the literature, two types of co-deposition mechanisms have been reported: (i) normal co-deposition and (ii) anomalous co-deposition. Depending on the type of deposition mechanism, the chemical and physical properties of the deposit can change [147]. The general deposition mechanism for metals of the iron group, the fourth period of the periodic table, is shown below Equations (28)–(30). The rate-limiting reaction is expressed as Equation (28) due to the scarcity of adsorption sites for MOH⁺ [147,148].

$$\begin{array}{ccc}\hfill {\mathrm{M}}^{2+}+{\mathrm{OH}}^{-}& \to & {\mathrm{MOH}}^{+}\hfill \end{array}$$

(28)

$$\begin{array}{ccc}\hfill {\mathrm{MOH}}^{+}+{\mathrm{e}}^{-}& \to & {\mathrm{MOH}}_{\mathrm{ad}}\hfill \end{array}$$

(29)

$$\begin{array}{ccc}\hfill {\mathrm{MOH}}_{\mathrm{ad}}+{\mathrm{e}}^{-}& \to & \mathrm{M}+{\mathrm{OH}}^{-}\hfill \end{array}$$

(30)

The main deposition mechanism of bimetallic nickel films is anomalous co-deposition, in which the bath prefers the deposition of the least noble metal [149,150,151]. Two factors that could lead to the preferential deposition of the least noble metal are the suppression of the hydroxide intermediate of the more noble metal by the hydroxide intermediate of the least noble metal and the difference in the dissociation constants between M₁OH⁺ and M₂OH⁺. These factors result in the higher concentration of hydroxide intermediates of the least noble metal and the saturation of the adsorption sites only by the least noble metal, inhibiting the adsorption of the other metallic species [147]. The main factors that alter the deposition process are:

• The pH: high values of pH favor anomalous deposition [147].
• The supporting electrolyte: the supporting electrolyte commonly contains metallic sulfates and chlorides. Chloride salts favor the normal deposition due to the formation of the chloride intermediate (MCl), which has a faster deposition process than MOH⁺ [147,148].
• The buffer capacity of the solution: Sulfate salts can act as a buffer during the deposition process when the electrolyte has a pH around 2, resulting in normal deposition [147].
• The working cycle: during pulsed working cycles, the normal mechanism is favored due to the delay in pH changes [147].
• Electrolyte degradation: the degradation of the electrolyte can cause self-termination of the deposition process altering the structure, morphology, and chemical composition of the deposit [152,153].

Co-deposition of Ni-based materials has been successfully used to obtain Ni-Co powders [154] Ni-Co phosphide [155], and mesoporous Ni-Mo-O composite films [156], all of which have shown high activity for hydrogen evolution. Complex morphologies such as 3D open-cell structures [157] and hierarchical radial nanospheres [158] can also be obtained through this method.

5. Nickel-Based Electrocatalysts for Water Electrolysis

As previously mentioned, H₂ production via water electrolysis is an ideal alternative for energy storage compared to the reforming process as it does not generate byproducts that are harmful to the environment and allows the use of intermittent energies. Nickel-based electrocatalysts are ideal for water electrolysis due to their earth abundance and low cost. These materials have proven to efficiently catalyze OER [12,102,129,159]. HER [8,85,136,160,161] and overall water electrolysis electrocatalysts are being studied [131].

Table 3. Performance parameters of nickel-based electrocatalysts for OER in alkaline media.

Electrocatalyst	Ref.	Substrate	${\mathit{m}}_{\mathit{T}}$	$\mathit{\eta }$ (j/mA cm−2)	$\mathit{\eta }$ (10 mA cm−2)	j ($\mathit{\eta }$/mV)	TOF	Durability
				mV dec−1	mV	mA cm−2	s−1	h
(Ni0.75Fe0.25)Se2	[129]	CFC	47.2	255 (35)	-	52.3 (350)	-	28 (35 mA cm−2)
NiFe	[129]	CFC	66.2	281 (35)	-	-	-	24 (35 mA cm−2)
NiSe2	[129]	CFC	83.6	-	323	-	-	12 (15 mA cm−2)
Co3NiP	[82]	GC	66.5	-	280	225.49 (420)	-	12 (10 mA cm−2)
CoNiP	[82]	GC	77.5	-	290	133.01 (420)	-	12 (10 mA cm−2)
CoNi3P	[82]	GC	89.3	-	300	71.38 (420)	-	12 (10 mA cm−2)
NiP	[82]	GC	92.8	-	330	54.74 (420)	-	12 (10 mA cm−2)
(Ni, Co)0.85Se	[79]	CFC	79	-	255	36 (300)	-	24 (10 mA cm−2)
(Ni, Co)0.85Se@NiCo-LDH	[79]	CFC	77	-	216	97.5 (300)	-	24 (10 mA cm−2)
FeNi-GO	[162]	NF	40	-	220	-	0.38 (300)	-
FeNi-rGO	[162]	NF	39	-	205	-	0.987 (300)	8 (10 mA cm−2)
** Ni2P	[81]	GC	47	-	290	-	-	10 (10 mA cm−2)
$\alpha$-Fe0.4Ni0.6Ox	[163]	FTO	34	250 (1)	-	-	-	24 (1 mA cm−2)
$\alpha$-Fe0.4Co0.4Ni0.2Ox	[163]	FTO	40	280 (1)	-	-	-	24 (1 mA cm−2)
a-NiOx	[108]	FTO	92	100	500	-	-	-
a-Fe0.5Ni0.5Ox	[108]	FTO	36	120	400	–	-	-
a-FeOx	[108]	FTO	72	280	530	-	-	-
Ni3N	[164]	GC	85	-	-	-	-	-
† Ni3N	[164]	GC	45	-	256	52.3 (350)	-	18 (400 mV)
† NiO	[164]	GC	59	-	-	-	-	-
NiOOH	[47]	Au	∼100	-	∼530	-	-	48
Fe0.35Ni0.65OOH	[47]	Au	63	-	∼340	-	-	-
FeNiS2	[80]	CC	46	-	310	-	-	10 (1.7 V)
* NiFe2O4	[94]		243.68	-	-	-	-	-
** NiFe2O4	[94]		98.22	-	-	-	-	-
De-LiCo0.5Ni0.5O2	[73]	CFP	42	290 (5)	-	-	-	-
De-LiCo0.33Ni0.33Fe0.33O2	[73]	CFP	35	240 (5)	-	-	-	-
De-LiCo0.33Ni0.33Mn0.33O2	[73]	CFP	48	310 (5)	-	-	-	-
LiCo0.5Ni0.5O2	[73]	CFP	50	330 (5)	-	-	-	-
LiCo0.33Ni0.33Fe0.33O2	[73]	CFP	45	320 (5)	-	-	-	-
LiCo0.33Ni0.33Mn0.33O2	[73]	CFP	45	340 (5)	-	-	-	-
NiCo	[75]	GC and NF	41	-	334	1.533 (300)	0.1 (300)	13 (10 mA cm−2)
NiFe	[75]	GC and NF	40	-	302	9.351 (300)	0.05 (300)	13 (10 mA cm−2)
Ni-Bi	[87]	ITO	59	425 (1)	-	-	-	-

CFC = carbon fiber composite, * nanofiber, GC = glassy carbon, NF = nickel foam, CFP = carbon fiber paper, ** nanowire, CC = Cabot carbon, a = amorphous, NG = N-doped graphene, † nanosheet, ITO = indium tin oxide, FTO = fluoride doped tin oxide.

compares nickel-based electrocatalysts’ performance parameters for OER and HER in alkaline, acid, and neutral electrolytes. All the electrocatalysts listed in Table 3 and

Table 4. Performance parameters of nickel-based electrocatalysts for OER and HER.

Electrocatalyst	Ref.	Substrate	OER	HER	medium	${\mathit{m}}_{\mathit{T}}$	$\mathit{\eta }$ (10 mA cm−2)	j ($\mathit{\eta }$/ mV)	TOF	Durability
						mV dec−1	mV	mA cm−2	s−1	h
‡ Ni0.9Fe0.1PS3	[8]	GC	X		Al	69	73	-	-	50 (30 mA cm−2)
				X	Al	73	72	-	-	50 (30 mA cm−2)
‡ NiCoP	[136]	NF	X		Al	-	-	50 (300)	-	10 (350 mV)
				X	Al	68.6	-	-	-	10 (160 mV)
‡ NiFe oxide	[131]	CFP	X		Al	48	271	-	0.02 (270)	18 (305 mV)
				X	Al	130	197	-	0.4(401)	18 (238 mV)
Ni3B/MoB	[85]	NF		X	Al	79	119	-	-	15 (10 mA cm−2)
		NF		X	Ac	61	75	-	-	-
Ni3B	[85]	NF		X	Al	86	183	-	-	-
		NF		X	Ac		167	-	-	-
MoB	[85]	NF		X	Al	73	157	-	-	-
		NF		X	Ac		124	-	-	-
NiFeP	[159]	-	X		Al	32	219	-	-	30 (10 mA cm−2)
		-	X		Ac	-	-	-	-	25 (10 mA cm−2)
Ni-NG	[110]	N-G	X		Al	188.6	-	16.3 (400)	-	-
$\alpha$-Ni(OH)2	[10]	GC	X		Al	42	331	0.26 (350)	0.361 (350)	-
$\beta$-Ni(OH)2	[10]	GC	X		Al	111	444	0.03 (350)	0.003	-
NiFeCu	[122]	NF	X		Al	33	180	-	-	20 (20 mA cm−2)
NiFeO4	[96]	GC	X			85	370	-	-	10 (370 mV)
Mn-doped NiS2	[160]	NF		X	Al	57	71	-	1.02 (100)	24 (120 mV)
		NF		X	N	65	84	-	0.63 (100)	24 (180 mV)
NiCo LDH	[102]		X		Al	40	367	-	-	6 (20 mA cm−2)
NiFe oxide	[165]	Au	X		Al	40	280	20(300)	0.008 (300)	2 (10 mA cm−2)
NiFe-LDH/CNT	[166]	CFP	X		Al	31	-	-	0.65 (300)	1 (5 mA cm−2)

Al = alkaline, CFC = carbon fiber composite, GC = glassy carbon; Ac = acid, CFP = carbon fiber paper, CC = Cabot carbon; N = neutral, NG = N-doped graphene, ITO = indium tin oxide; ‡ = bifunctional, NF = nickel foam.

were considered for the comprehensive study of nickel-based electrocatalysts for water electrolysis in the present work, but not all of them are discussed in the following sections.

5.1. Nickel-Based Electrocatalysts for OER

Nickel oxides [94,165] and hydroxides (Ni(OH)₂) [10,167] have been widely investigated for their outstanding performance in OER. Ni(OH)₂ can be found in two phases, $\alpha$-Ni(OH)₂ and $\beta$-Ni(OH)₂, and through a quasi-reversible redox process, they can transform to $\gamma$-NiOOH and $\beta$-NiOOH, respectively (Figure 6) [10]. $\gamma$-NiOOH and $\beta$-NiOOH differ in terms of the interlaying species and the oxidation state of nickel. $\beta$-NiOOH is an ordered structure, with protons between layers, d-spacing of 4.6 Å, and a nickel trivalent oxidation state(Ni^III). $\gamma$-NiOOH is a hydrated structure with interlayer water molecules and alkali ions, a d-spacing of 7 Å and a nickel oxidation state larger than +3 [15]. Despite the extensive research on the development of nickel-based electrocatalyst for water electrolysis, the mechanism of water electrolysis in nickel-based materials remains unclear.

To accurately design a Ni-based electrocatalyst for OER, knowing the active sites and the role of the heteroatoms is critical. The active phase in Ni-based materials has not been clearly identified; $\beta$-NiOOH and $\gamma$-NiOOH have been identified as the active forms. Louie and Bell compared the OER activity of as-deposited and aged Ni films and Ni-Fe oxide films [165]. They concluded that (1) a mixture of ordered and disordered $\beta$-NiOOH were responsible for the high performance, and (2) that the aging process increases the electrocatalytic activity of pure-Ni films ($m_T$ = 42 mV dec⁻¹), making them comparable to NiFe films with 40% Fe ($m_T$ = 35 mV dec⁻¹) (Figure 7).

Later, Bediako et al., through an anodization pre-treatment, evaluated the activity, structural changes, and oxidation states of the different phases of Ni in nickel borate (Ni-B) [168]. After applying a potential of 1 V, the anodized material had an oxidation state of 3.6, associated with Ni^IV in $\gamma$-NiOOH, and the non-anodized material had an oxidation state of 3.1, related to Ni^III in $\beta$-NiOOH. The electrocatalytic activity increases with anodization were attributed to the phase conversion from $\beta$-NiOOH to $\gamma$-NiOOH, and the consequent increase in Ni^IV sites and the interlayer space. Gao et al. compared $\alpha$-Ni(OH)₂ spheres’ and $\beta$-Ni(OH)₂ plates’ electrocatalytic activity for OER. $\alpha$-Ni(OH)₂ spheres were the most active in an alkaline electrolytes (Table 3) [10], attributed to the in situ formation of $\gamma$-NiOOH, and the nickel oxidation state facilitated the formation of NiOOH and the production of O₂. Finally, high stability was displayed by $\alpha$-Ni(OH)₂ spheres. After an increase in activity at 100 cycles, it remained stable until 500 cycles (Figure 8a). In contrast, although $\beta$-Ni(OH)₂ also displayed an increase in activity at 100 cycles, it dramatically dropped at 500 cycles (Figure 8b). To address this discrepancy, Trotochaud et al. studied the effect of electrolyte impurities in OER activity, and specifically, Fe impurities [169]. In the presence of Fe impurities, a 30-fold increase in conductivity and a partial charge transfer effect were observed; these led to an increase in Ni activity. Trotochaud stated that the aging of nickel-based electrodes leads to the incorporation of Fe impurities that enhance the catalytic activity of $\beta$-NiOOH reported in the literature. Thus, it is necessary to accurately determine the active phase of NiOOH in order to discount any possible interference.

The effect that unintentional and intentional addition of Fe has in terms of the electrocatalytic activity of Ni-based materials has been reported [169,170]. Axmann et al., using Mössbauer spectroscopy, described nickel-iron hydroxide as a cluster of Fe^IVO₆ octahedra surrounded by six Ni^IVO₆ octahedra, which altogether formed discrete M⁴⁺ domains in the LDH structure, and which were considered the electroactive sites [170]. Then, Ahn and Bard confirmed the presence of Ni^IV and Fe^IV in NiFeOOH through the implementation of surface interrogation scanning electrochemical microscopy [171]. Through the determination of pseudo-first-order OER rate constants, they observed that the process occurred via two catalytic sites, one with “fast” kinetics ($k$ = 1.70 s⁻¹) and the other with “slow” kinetics ($k$ = 0.04 s⁻¹) for Ni_0.82Fe_0.18OOH, the optimal composition. With an Fe content bigger than 25% uncontrolled segregation was observed, and the rate of reaction decreased ($k$ = 0.34 s⁻¹). The “fast” active site was attributed to Fe^IV sites due to the matching of the fraction of “fast” sites (17.6%) and Fe content (18.19%). The “slow” active site was attributed to Ni due to the similarity of the rate constant with NiOOH (0.04 s⁻¹). Contrastingly, in a theoretical study with density functional theory (DFT) and its variants, Martinez et al. studied the mechanism of OER in Fe-doped NiOOH; $\beta$-NiOOH was considered the active phase [172]. They observed (1) the oxidation of superficial Fe³⁺ to Fe⁴⁺, (2) Fe⁴⁺ stabilization by oxo moieties, and (3) the ability of Ni³⁺ to be reduced to Ni²⁺, which had a synergistic effect that enhanced OER electrocatalysis. Independently of how Fe was incorporated, there is an increment in the electrocatalytic activity of Ni-based materials with the addition of Fe, but the active sites have not been clearly elucidated. As stated above, transition metal doping of Ni-based materials causes atomic distortions and defects, increasing the exposure of active sites, modulating the free energy of adsorption of intermediates, and enhancing water dissociation [160,173]. Via electrodeposition and following dealloying and oxidation treatment, Zhang et al. produced core-shell NiFeCu metal/metal oxide nanoparticles (NPs) with high electrical conductivity, and large surface areas, which were optimal for OER in alkaline media [122]. Cu was related with the increase in ECSA, the optimal ratio was 10:1:21 (Ni:Fe:Cu), with a $C_{DL}$ value of 54.24 mF cm⁻², but the increase in the Cu concentration led to a friable and loose structure. Fe modulated the activity; the optimal concentration of Fe³⁺ ions was 25 mM ($m_T$ = 33 mV dec⁻¹). The trimetallic core-shell NP showed a dependency on the electrolyte pH, indicating that non-concerted proton-electron transfers took place during OER (Figure 9). The dependence of electrocatalytic activity upon pH is correlated to the deprotonation of NiOOH to produce NiOO⁻, which is a key OER intermediate [174].

Doping NiO_xH_y alters the structural evolution of the electrocatalyst during OER. Dette et al. studied the structural evolution of NiO_xH_y nanosheets (NS), CoO_xH_y NS and their alloys and Ni_0.8Fe_0.2O_xH_x NS. The major structural changes were observed with ≥40% of nickel after 200 CV cycles. They cracked or broke apart due to changes in lattice parameters, occasioned by changes in the Ni-Ni distance, from 3.12 Å in Ni(OH)₂ to 2.86 Å in NiOOH (Figure 10d). Nanosheets with ≥80% of Co and Ni_0.8Fe_0.2O_xH_x NS (Figure 10a) did not crack after the 200 cycles and maintained their shape, with a slight increase in roughness. This was attributed to the higher porosity of the initial structure, which allowed the accommodation of strain due to changes in metal–metal distances. Furthermore, the Ni^II/Ni^III redox peak in the voltammogram shifted: a cathodic shift for Co NS and an anodic shift for Ni_0.8Fe_0.2O_xH_x (Figure 10c). Accordingly, doping of Ni-based materials can lead to an improvement in their electrocatalytic activity and the stability of the electrocatalysts towards OER, tuning their electronic and structural properties.

Ni can also serve as a doping agent. Doping Co_0.85Se nanotubes (NT) with Ni and the electrodeposition of mesoporous NiCo-LDH NS in their surface produced a highly electroactive heterostructure for OER [79]. The use of Ni as a doping agent increased the electrical conductivity, the defect concentration, and therefore the number of active sites exposed to the electrolyte due to its non-stochiometric composition. Furthermore, an increase in superficial hydroxy groups (–OH), which enhanced ion permeability, was observed in (Ni,Co)_0.85Se (85.45%) compared to undoped Co_0.85Se (80.54%). Ni and Co were considered the active sites. This heterostructure had excellent electrocatalytic activity and stability (Table 3) due to the porous structure and synergistic relation between the insulating NiCo-LDH and the metallic (Ni, Co)_0.85Se. Introducing an anion to form a salt is an alternative and efficient complementary strategy for the improvement of Ni-based materials for OER electrocatalysis. Wang evaluated the effect of selenylation on the electrocatalytic activity of Ni and NiFe NS synthesized via hydrothermal treatment over carbon fiber cloth [129]. (Ni_0.75Fe_0.25)Se₂ displayed an efficient activity for OER (Table 3). Upon selenylation, the redox peak attributed to Ni^II/Ni^III in NiS₂ voltammogram shifted to 1.39 V (Ni precursor = 1.42 V), and in (Ni_0.75Fe_0.25)Se₂ shifted to 1.45 V (NiFe precursor = 1.47 V). This shift was attributed to the stronger charge transfer after selenylation due to low polarization. Even though both materials shifted to negative values, the (Ni_0.75Fe_0.25)Se₂ redox peak position was more positive, indicating that the oxidation of Ni²⁺ to Ni³⁺ requires more energy, which tends to favor OER. To further enhance the electrocatalytic activity of Ni-based materials, treatments prior to OER electrocatalysis have been implemented, such as anodization [10], dealloying [122], oxidation [122], and exfoliation [75]. The exfoliation of bulk NiFe LDHs, NiCo LDHs, and CoCo LDHs facilitated electron transport on single-layer nanosheets compared to bulk LDHs. Exfoliation reduces the size of the layers, increasing the ECSA and edge length. Edges have open coordination sites that act as catalytic sites. They found that the electrocatalytic activity for exfoliated and bulk LDHs was NiFe > NiCo > CoCo.

5.2. Nickel-Based Electrocatalysts for HER

To date, there is no agreement about the mechanism of HER in Ni-based electrocatalysts [12]. The theoretical study of the HER mechanism helps to identify and study active sites and to develop a path for the efficient synthesis of HER electrocatalysts. An ideal HER electrocatalyst must strongly trap H atoms and desorb molecular H₂ easily. If this is not the case, doping should help to tune the metal-H bond strength to facilitate (1) H⁺ adsorption, (2) H⁺ reduction, and (3) the formation of H₂ and its desorption. Liu et al., using density functional theory (DFT) described the high electrocatalytic performance of Ni₂P by comparing the relative energy change for each step of the HER mechanism of this material with (NiFe) hydrogenase, Pt^III and Ni^III (Figure 11a). Ni₂P exhibited energy changes nearer to zero, than those of Ni^III and Pt^III in the second step; this gave an insight into the Ni₂P-favorable kinetics for HER. Even though a poisoning effect of Ni sites due to the strong bonding of Ni–H was identified, this phenomenon, far from decreasing the HER catalysis, enhanced the electrocatalytic activity of Ni₂P, as it favored the occurrence of HER in the Ni-P bridges (Figure 11b) [176].

In another theoretical study of HER using DFT, Greeley et al., screened pure metals and their alloys to determine highly active compositions for HER. The study was carried out by comparing the free energy of hydrogen adsorption $\Delta G_{\mathrm{H}}$ of each material [177].

Combining theoretical and experimental studies can lead to the design of a highly efficient HER electrocatalyst. Man et al. evaluated the effect that Fe ($d^6$), Co ($d^7$), Mn ($d^8$), and Mo ($d^9$) have in NiP (Ni ($d^{10}$)) via theoretical and experimental methods. The electrocatalysts were synthesized via wet chemistry and the electrode via drop-casting the catalyst ink into glassy carbon [178]. They concluded that the addition of transition metals with d-electron deficiencies enables electron delocalization and therefore the modification of the electronic structure and the hydrogen adsorption energy. However, the efficiency of the electrocatalyst depended on the H coverage. For low and high coverage, Mo and Co were the most efficient doping agents (Figure 12a–c). NiMoP required an $\eta$ of 400 mV at a current density of 10 mA cm⁻², which indicates a high electron mobility because Mo presents the biggest d-electron deficiency (Figure 12c). To further investigate the mechanism of HER, the relationship between doping, structure, and electrocatalytic activity must be evaluated. NiS₂ nanosheets (NS) doped with Mn (Mn-Ni-S) hydrothermally synthesized over nickel foam (NF) were studied by Zeng et al. [160]. Through their theoretical and structural analysis, it was determined that Mn atoms do not alter the crystalline structure of Ni₂S but favor the dissociation of water molecules (the Volmer step), lowering the electronegativity of Ni, leading to a reconfiguration of the Ni electronic structure and the weakening of the Ni-H bond. Mn-Ni-S/nickel foam (NF) outperformed NiS₂/NF in HER electrocatalysis due to (1) the increase in the number of active sites, through an increase in the number of defects and increasing accessibility to the electrolyte due to the nanosheet structure; (2) the acceleration of mass/charge transport because of the high interconnection between sheets and the void spaces generated in the obtained morphology; and (3) the use of nickel foam as a substrate, without the need for binders, promoted intimate contact that produced efficient paths for mass/charge transport. Finally, their activity in an alkaline electrolyte (1 M KOH), a neutral electrolyte (1M PBS, pH 7), and seawater was tested; for alkaline and neutral water electrolysis, the performance parameters are reported in Table 3. The electrocatalytic activity was higher in alkaline electrolytes, but in both electrolytes, Mn-Ni-S/NF displayed robust long-term durability and stability, up to 24 h, with minimal degradation. In seawater, Mn-Ni-S/NF delivered 10 mA cm⁻² at an $\eta$ of 301 mV. Due to impurities, it was stable for 10 h and did not present side-reactions. The stability and high efficiency were attributed to the absence of a binder and the strong interaction between NF and Mn-Ni-S.

Pyrite structures have been demonstrated to be efficient electrocatalysts for HER [129,160] due to their earth abundance, low cost, and stability in alkaline [160], acid [173], and neutral electrolytes [160]. Through thermal conversion, Zhuo et al. evaluated the effect of adding Se to NiP₂ (NiP_1.93Se_0.07) and P to NiSe₂ (NiP_0.09Se_1.91) for acid (0.5 M H₂SO₄) HER electrocatalysis [173]. The highest electrocatalytic activity was achieved by NiP_1.93Se_0.07 (Table 3). To further enhance the electrocatalytical activity of this material, it was grown on carbon fiber paper. The overpotentials required to achieve a current density of 10 mA cm⁻² and the $m_T$ were 84 and 41 mV dec⁻¹.

Finally, heterostructures, as mentioned before, can further enhance the catalytic performance of Ni-based materials for HER. Xu et al. developed a Ni/NiO/CoSe₂ hybrid via solvothermal synthesis and subsequent annealing treatment [179]. This nanocomposite exhibited a smaller $m_T$, of 39 mV dec⁻¹, compared to CoSe₂ nanobelts (48 mV dec⁻¹). The faster reaction kinetic for the nanocomposite was due to the synergistic effect between metallic Ni, which functioned as a conductive path that reduced the internal resistance; NiO, which weakened the O–H bond of the molecule of water adsorbed; and CoSe₂ nanobelts, a substrate that provided active sites.

5.3. Nickel-Based Bifunctional Electrocatalysts for Overall Water Electrolysis

The design of a bifunctional Ni-based electrocatalyst that can efficiently catalyze WE for the industrial production of H₂ in acid, neutral, and alkaline electrolytes is a challenge. The precise control of the composition, morphology, and structure of electrocatalysts can lead to a material that is capable of efficiently catalyzing HER and OER in the same electrolyte. Kumar and Bhattacharyya synthesized NiFe-oxide nanocubes (NiFe-NCs) via coprecipitation and compared their activity for overall alkaline water splitting with NiFe oxide nanoparticles (NPs) [131]. NiFe-NCs delivered a current density of 10 mA cm⁻² at 1.67 V, whereas NiFe-NPs required 1.90 V (Figure 13a). NiFe-NCs outperformed NiFe-NPs because of (1) the mesoporous structure that facilitated mass transport and the efficient liberation of gas-evolved bubbles from the large ECSA, (2) the edges and corners of NCs increased the number of active sites available, and (3) precise tuning of the composition to achieve the optimal content of Fe (40%), which produced the adequate composition of $\alpha$/$\gamma$-Fe₂O₃. The increased electrode conductivity provided efficient paths for electron transfer and suppressed the oxidation of Ni^II to Ni^III. In that work, Ni^II was considered the active species, and Fe^III was the one in charge of facilitating electrode kinetics. NiFe-NC in overall water electrolysis presented minimal degradation after 18 h (Figure 13b). The authors confirmed that efficient and stable Ni-based electrocatalysts could be produced via the precise synthesis techniques that allow the control of structure and composition [131].

Other materials studied for the overall water electrolysis are layered phosphosulfides (MPS₃). Song et al. evaluated Ni_1−xFe_xPS₃ structures for water splitting in an alkaline medium [8]. These types of materials, MPS₃, are stable structures but have low OER and HER electrocatalytic activity. Thus, to enhance their electrocatalytic activity, doping can modulate the hydrogen or oxygen adsorption energies and enhance the material’s conductivity. In this case, Fe doping must be carefully controlled to achieve an optimal mixture of oxidation states between Ni and Fe sites. The optimal doping composition was Ni_0.9Fe_0.1PS₃; it was stable in HER and OER up to 50 h at −30 mA cm⁻² and 30 mA cm⁻², respectively. They observed that the material did not suffer superficial changes during HER, but in OER electrocatalysis, it functioned as a precursor for Ni(Fe)OOH and Ni(Fe)(OH)₂ in alkaline media. The authors concluded that Ni_0.9Fe_0.1PS₃ is the active material for HER, but in OER it works as a precursor. Z. Wu et al. evaluated the effect of adding S to FeP and P to FeS [180]. They concluded that the metal coordination with P produces the most effective sites and that the electrocatalytic activity is insensitive to S substitution. Through structural and compositional analysis, they observed that P atoms tend to remain on the surface and act as a sacrificing agent to partially protect Fe and S atoms from oxidation and consequential inactivation for HER. Finally, they stated that the precise control of the surface and inner layers of the material are essential to effectively modulate the electronic interactions and conductivity, which then modifies the adsorption energy of intermediates. Therefore, the precise addition of cations and anions into nickel materials can enhance the electrocatalytic activity for overall water splitting.

6. Future Perspectives

Extensive theoretical and experimental investigations have been carried out to elucidate the mechanisms of OER and HER and water electrolysis in Ni-based materials, as well as the role of structure and composition. However, there is still a need for the development of low-cost materials with high electrocatalytic activity and high stability [61]. The main focuses in relation to Ni-based catalysts that can be deduced from the vast information available in the literature are as follows [14,61,65]:

• The elucidation of the water electrolysis mechanism in nickel-based electrocatalysts;
• The development of synthesis techniques with high control over morphology, structure, and composition for nickel-based electrocatalysts; and
• The design of nickel-based electrocatalysts with high stability in acid and neutral media

Hydrothermal, solvothermal, and coprecipitation synthesis techniques have been designed to be an accessible and economical route for Ni-based electrocatalysts. They have been paired with theoretical studies, DFT [172,176,177], and in situ experimental techniques, such as in situ Raman spectroscopy [165] or surface interrogation scanning electron microscopy [171], in order to establish or approximate the mechanism of specific Ni-based electrocatalysts for HER and OER. However, if the goal is production at an industrial scale of H₂ through water electrolysis, accessible, precise, and robust techniques for the synthesis and characterization of Ni-based electrocatalysts must be developed. Galvanostatic deposition and elimination voltammetry (EV) are electrochemical techniques for the synthesis and characterization of materials that are accessible, fast to implement, and which offer a high level of control over parameters. Galvanostatic deposition can produce metallic films with electrocatalytic activity [146,153] and is easy to scale for mass production. Martinez compared galvanostatic and potentiostatic deposition of Ni NP in ITO and concluded that the deposits produced through galvanostatic deposition yield smaller particles with less dispersion. They obtained a particle size of 137 ± 43 nm with applied pulses of 5 mA for 1000 ms [145]. Then, Medrano deposited NiPt films on gold and evaluated their electrocatalytic activity towards water electrolysis; these films reached an $\eta$ of 22 mV at 10 mA cm⁻². Later, Salinas-Quezada et al. observed a dependency between the applied deposition current applied and the electrocatalytic activity of nickel films. When lower deposition current values were applied, 22–33 μA, the electrocatalytic activity favored methanol oxidation, and with higher deposition currents, 38–66 μA, OER was favored [181]. This was attributed to changes in the growth regime because, during this stage, the geometric area defines the diffusion field of metallic ions. Thus, electrocatalysts produced via galvanostatic deposition can favor a specific electrocatalytic process. Another advantage of the use of galvanostatic deposition is that it involves direct deposition on the desired substrate. This eliminates the necessity of using a binder or a conductive agent, which only increases the dead volume, decreases the activity of the material, and, if there is not a good substrate-binder-electrocatalyst interaction, decreases the stability of the material. The common synthesis technique for industrial applications is the chemical synthesis of powders. Some drawbacks of this synthesis technique are the need to transfer the electrocatalyst to a conductive substrate [12], and the low density and efficiency due to the fact that this material reaches its maximum performance at very low geometrical loading [79]. Two of the substrates employed in different types of deposition are carbon materials [173,182] and NF [122]. Researchers have reported increases in ECSA and electrical conductivity due to (1) intimate contact between the substrate and catalyst, (2) the efficient liberation of products from active sites, and (3) the generation of efficient pathways for mass/charge transport [160,173,182]. This strategy could not only increase electrocatalytic activity but the stability would also be enhanced by the facile liberation of gas-evolved bubbles from the active surface of the electrode to avoid electrode damage, and the decrease in ECSA due to the accumulation of bubbles [79].

To efficiently synthesize Ni-based electrocatalysts, it is necessary to elucidate the mechanism of action, know the active sites, and to understand the role of transition metals and impurities, as well as the structural transformation that the material undergoes during HER, OER, and water electrolysis. The detailed and precise characterization of composition, morphology, and structure is necessary to develop accessible and economical characterization techniques and to eliminate impurities that may alter electrocatalytic activity. Trotochaud et al. developed a protocol for KOH purification via decantation to address the presence of impurities that may alter the output of the electrocatalyst characterization in alkaline electrolytes [169]. A clear difference in the cyclic voltammograms of purified KOH and electrolytes with intentionally or unintentionally added Fe was observed. The redox peak corresponding to Ni^II/Ni^III shifted to more positive values with the increase in the Fe content (Figure 14 ). Therefore, in order to elucidate the correct mechanism, the development of purification protocols for diverse impurities and electrolytes is required.

Characterization techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), absorption spectroscopy (XAS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS), have been used to study the initial and final conditions of the Ni-based electrocatalyst. However, the study of the process in this interface needs to be further investigated with economical techniques. Elimination voltammetry (EV) is an attractive alternative because it enables the detection of minor processes in the electrode by means of a mathematical treatment of electrochemical signals. The total current is the sum of individual currents—the charging current ($I_c$), kinetic current ($I_k$), and diffusion current ($I_d$). The total current at three different scan rates is combined, and the unwanted currents are eliminated using linear elimination functions. The three resulting currents, $I_c$, $I_k$, and $I_d$, can be studied [183]. An advantage of this technique is that validation can be completed through computational modeling of the desired process [184], and its mathematical treatment can be accomplished via EV, which could also serve as a reference point to compare the deviations of the real process from the ideal or theoretical process. Oriňáková studied the electrodeposition of nickel in a paraffin-impregnated graphite electrode (PIGE) using chloride and sulfate electrolytes. The data recollected from cyclic voltammograms at three different scan rates were studied using EV and a theoretical model to validate the results obtained. This technique allowed the identification of two different processes of nickel electrodeposition, depending on the electrolyte implemented [185].

7. Conclusions

This review has summarized the water electrolysis mechanism and the development of nickel-based electrocatalysts for HER, OER, and overall water electrolysis. Water electrolysis is an ideal alternative for H₂ production, but the lack of efficient and cheap electrocatalysts limits its industrial application. It has been established that the mechanisms of HER and OER must be elucidated in order to develop efficient nickel-based materials for water electrolysis. Furthermore, the role of heteroatoms and structural and morphological features must be considered because they may enhance or diminish the electrocatalytic activity. On the other hand, the accurate determination of performance parameters is critical to correctly assessing the electrocatalytic activity of nickel-based electrocatalysts. By considering these aspects, a cost-effective nickel-based electrocatalyst can be designed for OER, HER, or ultimately for the overall water electrolysis process. The strategies implemented involve modifying composition through doping, modifying structure or morphology through the induction of defects or increasing the electrocatalytic active surface area, and implementing different substrates. However, there is still room for improvement in the synthesis and characterization of these materials. The synthesis of nickel-based electrocatalysts with controlled structures and compositions may provide enhanced electrocatalytic activity by optimizing the electronic conductivity, binding strength to intermediate species, and the lifetime of the material. Additionally, the development of accessible characterization techniques that allow the study of the electrocatalytic process at the interface of electrocatalysts and electrolytes is needed for the accurate study of OER, HER, and overall water electrolysis on nickel-based electrocatalysts. Finally, based on the literature reviewed in this work, it can be concluded that the ideal electrocatalyst is the bifunctional nickel-based electrocatalyst. A nickel-based heterostructure can efficiently catalyze both HER and OER processes, facilitating the industrial scaling of water electrolysis, increasing stability, and reducing material costs. A clear strategy to achieve this is the implementation of a conductive and porous substrate and the precise doping of nickel materials to produce quaternary structures with tuned electrical conductivity, adsorption energy, and structures that can facilitate the evolution of H₂ and O₂. However, the previously mentioned aspects (mechanisms, the role of heteroatoms, structural and morphological features, and performance parameters) must be evaluated in order to optimize the material’s activity.