**1. Introduction**

Hydrogen is a clean fuel and an energy carrier that can be used for energy conversion and storage and is considered as a possible substitute for fossil fuels [1]. Electrocatalytic water splitting offers an ideal approach for highly pure hydrogen production. However, despite the multitude of on-going research, the development of an optimized, cost-effective and sustainable catalyst, which possesses a high catalytic activity for hydrogen evolution reaction (HER) is still rather appealing. Commonly, the ability of a given metal to catalyze the HER is estimated based on the exchange current density (ECD), i.e., the current density in the absence of net electrolysis at zero overpotential (at formal equilibrium potential for hydrogen evolution reaction in the particular solution). It is known, that the higher the ECD, the lower the overvoltage that must be applied to create a significant current flow. Hence, elaborated electrocatalysts should manifest exchange current densities equivalent or analogous to the ECD of polycrystalline platinum (~1 × 10−<sup>3</sup> A/cm<sup>2</sup> in alkaline electrolytes) [2].

A considerable part of research on the design of effective cathode materials for HER has been focused on Mo alloys with iron group metals (Ni, Co, Fe) due to their superior catalytic performance

in alkaline media [3–5], stability at elevated temperatures [6] and reasonable corrosion and oxidation resistance [7–10]. These characteristics, combined with good electrical conductivity, easy usage and reasonable price, are attractive parameters for selecting the cathode material for water electrolysis. It was revealed that the activity of the Ni–*x* at.% Mo (12 ≤ *x* ≤ 29) alloy for HER is much higher than that of separate metallic nickel and molybdenum electrodes [11–15]. This phenomenon is attributed to the synergistic effect of Mo dispersed in the Ni matrix, which increases the real surface area of the electrode [12,16]. In addition, the enhancement of the catalytic activity for the HER of the Ni–15 at.% Mo alloy was ascribed to the modification of electron density in d-orbitals upon alloying nickel with molybdenum [17]. In other words, this model implies that some of the electrons of the iron group metal (Ni, Fe, Co) with more filled d-bands are shared with Mo having less-filled d-orbitals. This leads to maximal bond strength and stability of the intermetallic alloy phases [18,19].

Commonly, Mo alloys with iron group metals (Ni, Co, Fe) can be synthesized by applying mechanical alloying [3,20–22], powder metallurgy [11,23,24], spraying [25] and laser cladding techniques [26]. However, fabrication processes in aqueous media are often considered as simpler, cheaper and more environmentally friendly fabrication methods than those requiring sophisticated apparatus, volatile and corrosive chemicals and extra energy that must be incurred to keep the system in a liquid state. Thus, molybdenum can be successfully co-electrodeposited in the presence of iron group metal ions (Ni(II), Co(II), Fe(II)) and appropriate complexing agents from an aqueous electrolyte. It is assumed that the molybdate ions are reduced to molybdenum oxide or hydroxide, which in the presence of iron group metal (Ni, Fe or Co) species allows the formation of the corresponding binary alloy deposits. The effective Mo alloys electrodeposition with iron group metals were carried out from citrate [3,5,7,27], citrate–ammonia [10,12], citrate-gluconate [28], ammonia [29] and pyrophosphate [30,31] aqueous electrolytes.

Moreover, it was claimed that Ni–Mo electrodes show higher electrocatalytic activity than other Ni-based binary alloys such as Ni–Co, Ni–Fe, Ni–Zn and Ni–Cr [5,29]. For a given reason, the fabrication of Ni–Mo alloys possessing the highest activity for effective hydrogen production was the target for the vast research in the last decades. There are numerous reports certifying that the catalytic activity for hydrogen evolution is qualitatively proportional to the Mo content in Mo-based alloys [11,13,17]. Therefore, researchers' efforts were directed to optimize the plating bath vs. deposition conditions in order to obtain Mo-rich alloys as effective catalysts for the HER. It was shown that electrodeposition from ammonia based aqueous solutions produces Mo alloys with up to ~41 at.% of Mo [32–36]. Coatings containing more than 40 at.% Mo have been electrodeposited from ammonium-citrate solution in the presence of imidazolium-based ionic liquids as an additive [37]. Ammonia is frequently added to improve the cathode current efficiency, however, there is also some data about its effect on decreasing the Mo content in bimetallic Mo system with iron group metal (Ni, Co, Fe) [34]. Thus, despite the reduced current efficiency, typically ammonia-free electrolytes are used for the preparation of Mo-rich coatings, e.g., the Fe–Mo electrodes containing up to 59 at.% of Mo have been prepared from a pyrophosphate bath [38,39]. Binary Fe–Mo alloys with 49 at.% of Mo have been electrochemically formed from an aqueous trisodium nitrilotriacetate bath [40]. A considerable increase in Mo content, i.e., up to 70 at.%, can be caused by the addition of Mo powder to the electrolyte [41]. In addition, it has been noted that Mo content in alloys composition can be increased by carrying out the electrodeposition under the pulse current mode [42]. The highest Mo content, 74 at.%, achieved so far by induced electrodeposition in aqueous citrate electrolyte was reported for a Ni–Mo alloy [43].

Therefore, based on the mentioned above, the given research was focused on the electrodeposition of Ni-, Co- and Fe- Mo-rich alloys from a highly saturated ammonium acetate bath. The electrolyte's composition given in Reference [44] was adapted for the electrodeposition of binary Mo-containing alloys. In order to determine the influence of the nature of the iron group metal on the catalytic activity of target coatings (Ni-, Co- and Fe- Mo-rich alloys) the electrochemical conditions were tuned in such way to ensure deposition of alloys with similar content of Mo. The catalytic activity of the electrochemically fabricated Mo-based alloy electrodes for the HER was explored in a 30 wt.% NaOH

solution. In order to compare the electrochemical activity with other typical electrode materials, experiments were also performed using bare platinum electrode (same geometrical area) under the same conditions.

#### **2. Materials and Methods**

#### *2.1. Mo-Rich Alloys Electrodeposition*

Mo-rich alloys, namely Ni–Mo, Co–Mo and Fe–Mo, were prepared from highly saturated ammonium acetate electrolytes (Table 1) based on bath composition proposed in Reference [44] for Mo films deposition. All solutions were prepared from chemicals of analytical grade (A.R.) dissolved in demineralized water. The electrodeposition of coatings was carried out at 30 ◦C in order to lower the viscosity of the concentrated solutions and to avoid salt precipitation. Cu rod (surface area of 1 cm2), platinum sheet (3 × 7 cm2) and a saturated Ag/AgCl electrode were used as a working, counter and reference electrodes, respectively. Prior to the electrodeposition, Cu rods were washed and cleaned in an ultrasonic bath for 6–7 min and etched in an HNO3:CH3COOH:H3PO4 (1:1:1) solution at 60 ◦C. The thickness of the prepared Mo alloy coatings with iron group metals was calculated from gravimetric and elemental analysis data. Further, the electrocatalytic activity for the HER in 30 wt.% NaOH of fabricated cathodes has been investigated.

**Table 1.** Composition of electrolytes for Ni–Mo (Baths No. 1–3), Co–Mo (Bath No. 4) and Fe–Mo (Bath No. 5) coatings electrodeposition.


#### *2.2. Morphological and Structural Study*

The surface morphology and chemical composition of the prepared Mo-based deposits were examined with the scanning electron microscope (SEM, Hitachi TM3000, Tokyo, Japan) equipped with an INCA energy dispersive X-ray spectroscopy detector (EDS, Oxford Instruments, Buckinghamshire, UK) at an accelerating voltage of 20 kV, respectively. Based on the chemical composition of the obtained alloys, the current efficiency (CE) was calculated according to the Faradays' law:

$$\text{CE } \left( \% \right) = \frac{Fm}{It} \left[ \frac{\text{x}\_i \text{n}\_i}{M\_i} + \frac{\text{x}\_{\text{Mo}} \cdot \text{n}\_{\text{Mo}}}{M\_{\text{Mo}}} \right] \times 100\% \tag{1}$$

where *F* is Faradays constant (96485 C); *m* is the weight of the electrodeposit (g); *I* is an applied current (A); *t* is the time of electrodeposition (s); *xi, ni, Mi* is the content (wt.%), electrons transferred per ion, and molecular weight (g/mol) of Ni, Co or Fe, respectively; *x*Mo*, n*Mo*, M*Mo is the content, wt.%, electrons transferred per particular ion, respectively; molecular weight of Mo.

The structure of the electrodeposited alloys was investigated by X-ray diffraction (XRD) methods (Rigaku MiniFlex II, Tokyo, Japan). XRD patterns were produced with Cu Kα radiation (1.5406 Å) in 2θ scanning mode from 20 to 100◦ with a step of 0.01◦.

#### *2.3. Electrochemical Measurements*

The voltammetric measurements for the evaluation of the kinetic parameters of Mo-rich alloys for the HER were performed in a 30 wt.% NaOH solution at several temperatures (25–65 ◦C, with the increment of 10 ◦C) in a thermostatic cell. A platinum wire was used as an auxiliary electrode and a saturated Ag/AgCl electrode was used as the reference electrode. All potentials are given with respect to the Ag/AgCl reference electrode. Potentiodynamic polarization hydrogen evolution curves were recorded at the sweep rate of 2 mV/s. The cathode potential was scanned from its open circuit potential (OCP) up to −1 V. Voltammetric curves were recorded using a potentiostat/galvanostat AUTOLAB equipped with GPES software (version 4.9). Extrapolation of the polarization curves obtained at different temperatures, in the coordinates lg*i* − η to value η = 0 give the possibility to determine the ECD (*i*0). The overvoltage, η, was calculated from the following equation:

$$
\eta = E - E\_{\rm r} \tag{2}
$$

$$E\_{\rm r} = -\left(\frac{2.3RT}{F}\right) \text{pH} \tag{3}$$

where η is an overpotential of the HER (V), *E* is an experimental potential value at which the reaction takes place (V); *E*r is the reversible potential value calculated from the Nernst equation (V), *R* is the universal gas constant (8.314472 J/K mol); *T* is the temperature (K); *F* is the Faraday constant (96,485 J/mol).

For the calculation of overpotentials at temperatures other than 25 ◦C, the tabulated data [45] of the temperature dependence of the potential of the saturated Ag/AgCl electrode vs. the hydrogen electrode, were used.

#### **3. Results and Discussion**

#### *3.1. Design of Mo-Rich Alloys Coatings*

The high percentage of molybdenum in Mo-based alloy electrodeposits leads commonly to the growth of the ECD value but also has a positive influence on their corrosion resistance and microhardness [10]. On the other hand, the electrodeposition of coatings having a very high molybdenum content (>38 at.% of Mo) is more sensitive to side reactions, namely the evolution of hydrogen, which can lead to the appearance of a dense net of cracks, bumps and small pits that diminish practical application of such coatings for the HER. Thus, the first step of the given research was dedicated to the selection of the optimum electrochemical conditions (bath chemistry, applied current density) in order to obtain high-quality Mo-rich alloys with a reasonable deposition rate. The first investigated system was Ni–Mo (Table 1, Baths 1–3) since a high amount of publications have reported [15,46,47] that the Ni–Mo alloy is the most promising non-noble catalysts for the HER among other refractory metal-based electrodes.

Previously, it was shown that if the ratio [Ni(II)]:[Mo(VI)] is approaching 10, the amount of Mo in the Ni–Mo deposit decreases dramatically from 65 to 20 at.% [48]. Therefore, in order to obtain Mo-rich alloys, the ratio was kept at 0.25, 0.5, 1.0. Another parameter, which influences the refractory metal content in the alloys is the applied current density. Based on a preliminary study, two cathodic current densities, namely 30 and 100 mA/cm2, have been chosen for electrodeposition of Mo-rich alloys.

Taking these parameters into account, the dependence of Mo content on the [Ni(II)]:[Mo(VI)] ratio and the cathodic current density was evaluated (Table 2). Namely, as it was anticipated, the amount of Mo in the alloys decreases from ~85 to 36 at.% as the Ni(II) increased in the bath. Here it should be mentioned, that only the content of the metallic phase was taken into account for the evaluation regardless of the ambiguous values of oxygen and other light elements detected by the EDS analysis. The highest content of molybdenum in the Ni–Mo deposits, around 85 at.%, was achieved at a [Ni(II)]/[Mo(VI)] ratio equal to 0.25 in the plating bath. This Mo amount is significantly higher in comparison with previous works reported for Ni–Mo alloys obtained from aqueous electrolytes and is close to that found for Ni–Mo alloys prepared by metallurgical [11] or mechanical alloying techniques [49]. In order to reveal the interdependencies between bath chemistry and applied current densities, partial current densities (PSD) for NiPCD, MoPCD reduction and hydrogen evolution were also evaluated based on Faraday's law (Table 2).

**Table 2.** Dependence of composition, morphology and partial current densities of electrodeposited Ni–Mo alloys on the [Ni(II)]:[Mo(VI)] ratio and applied cathodic current density. Molybdenum at.% content is specified on the SEM images.


Namely, the increase of the [Ni(II)]:[Mo(VI)] ratio increases the NiPCD and consequently Ni content in the deposit the higher applied current density (overpotential) accelerates the reduction of Ni(II) rather than Mo(VI) compounds. The side reaction is accelerated by a higher Mo content in the alloy that leads to the propagation of large micro-cracks, especially for Ni–Mo alloys deposited at a [Ni(II)]/[Mo(VI)] ratio ≤ 0.5 and having more than 50 at.% of Mo. Our results are in a good agreemen<sup>t</sup> with Reference [50], where it was shown that the cracks in the Ni–Mo alloys deposited from a citrate solution have been tracked at a Mo content higher than ~30 at.% but from an ammonium-citrate electrolyte [3] they appear even at lower Mo content (~21 at.%).

Hence, in order to obtain Mo-rich alloys without visible defects, the applied current density of 30 mA/cm<sup>2</sup> and a [Ni(II)]:[Mo(VI)] ratio of 0.5 should be viewed as the optimum conditions. This ratio allows for a four times increase in the MoPCD in comparison with the ratio of 0.25. It suggests that the electroactive complex should contain both molybdenum and nickel species. At the higher ratio (higher Ni(II) concentration), the MoPCD does not change significantly but the Mo content in the alloy decreases (from 54 to 40 at.%). Notably, at a current density of 100 mA/cm2, the MoPCD is practically the same for all investigated [Ni(II)]:[Mo(VI)] ratios, suggesting that electroactive Mo-containing species under such conditions reaches saturation and has no significant effect on alloy electrodeposition. Furthermore, at a higher applied current density, an additional roughening due to the pronounced nodular structure of the coatings is obtained, that can be interconnected with abundant hydrogen evolution, which leads to cracks and holes on the surface (Table 2). On the one hand, in Reference [51], it was shown that the

cracked surfaces are characterized by higher HER activity and ascribed to an increased surface area of the active centers due to the microcracks but on the other hand, a certain amount of the hydrogen enters the open pores (cracks) of the deposit and it starts peeling off around the pores. Hence, it is obvious that the application of such coatings in the industrial processes is not recommendable [30]. Moreover, due to high PCDs for hydrogen reduction, the current efficiency in all investigated cases is rather low (<10%). This is a common characteristic for Ni–Mo co-deposition [52] that is associated with the formation of a mixed Mo oxides layer in the presence of an excess of Mo(VI) ions in the bath, which hinders the further reduction.

Accordingly, based on the experimental results obtained for Ni–Mo alloys, the following optimal conditions were adapted for electrodeposition of Co–Mo and Fe–Mo alloys: cathodic current density 30 mA/cm<sup>2</sup> and [Me(II)/Mo(VI)] = 0.5. This allowed for the electrodeposition of Mo-rich coatings (Co–52 at.% Mo and Fe–54 at.% Mo) coupled with suitable morphology (crack-free coatings with a less rough globular surface) and to evaluate the influence of iron group metal on the catalytic activity for HER in alkaline media. The obtained morphology of Mo-rich alloys was quite similar regardless of the iron group metal (Figure 1).

**Figure 1.** SEM images of electrodeposited at 30 mA/cm<sup>2</sup> and [Me(II)]/[MoO4 <sup>2</sup>−] = 0.5 coatings: Ni–Mo (**a**), Co–Mo (**b**) and Fe–Mo (**c**). The time of electrolysis was 1 h and the thickness of all deposits was ~10 μm.

The structure and crystallite size was evaluated by XRD analysis. A characteristic broad peak at 2θ = 43◦–44◦ was obtained for Ni–54 at.% Mo and is depicted in Figure 2a. According to the thermodynamic equilibrium data, the solubility limit of Mo in the fcc Ni structure at room temperature is ~17 at.%. When the Mo content exceeds this limit, an amorphous microstructure can be noticed and the formation of intermetallic Ni4Mo, Ni3Mo, NiMo compounds becomes possible for Ni–Mo alloys having >25 at.% of Mo [31,53]; a line with a solid solution of Ni in Mo. It is also known that the broadening of the XRD peak is related to the refinement of crystallite size that typically occurs with an increasing Mo content [31]. According to the literature, the mean crystallite size of Ni–Mo coatings can decrease from 50 to 2 nm by increasing the Mo content from 1 to 38 at.%, respectively [35,37,54]. This corresponds to a crystallite size of the investigated Ni–Mo coatings as small as ~2 nm. Notably, Ni–Mo deposits consisting of such small crystallites can have a lower overpotential for hydrogen evolution due to the larger concentration of crystal lattice defects and dislocations, which are considered as active centers for HER [12,19].

 **Figure 2.** XRD pattern for: (**a**) Ni–54 Mo, (**b**) Fe–54 Mo and (**c**) Co–52 Mo electrodeposits (composition is given in at.%).

The XRD patterns for Fe–54 at.% Mo and Co–52 at.% Mo alloys showed the analogous crystalline structure to the Ni–54 at.% Mo coating (Figure 2b,c). Since the presence of one broad peak in the X-ray diffraction patterns makes it difficult to interpret the results, it can be only proposed that a mixture of Mo solid solution in the iron group metal and corresponding intermetallic compounds were formed. For the Fe–Mo (also Fe–W) system, the Mössbauer spectroscopy supports this presumption and suggests that deposits having more than 17 at.% of refractory metal consists of a mixture of molybdenum solid solution in α-Fe and intermetallic phases, e.g., Fe3Mo, Fe2Mo [55,56]. In the case of the Co–Mo alloy, some of the studies reported that a Mo solid solution in cobalt and intermetallic Co3Mo is formed [36,57].

The different intermetallic phases should have an impact on their activity for the HER. It was emphasized that the maximum electrocatalytic activity could be achieved for intermetallic phases of highest symmetry and minimal entropy, such as Laves phases or A3B types (Co3Mo, Ni3Mo, Fe3Mo) and the Brewer theory for intermetallic bonding predicts as the most stable systems [58]. Thus, it was reported that films consisting of a Co3Mo phase (for Co–*x* at.% Mo, 18 ≤ *x* ≤ 28) have the best electrocatalytic properties among other Co–Mo alloys having lower molybdenum contents [59]. Furthermore, in Reference [60], it was concluded that Co3Mo intermetallic compounds are more stable in a hot alkaline solution than other Co–Mo phases. Similarly, in the case of the Fe–Mo alloy system, the lowest overvoltage for hydrogen evolution at a current density of 200 mA/cm<sup>2</sup> has been observed for the Fe–47 at.% Mo sample with a predominant Fe3Mo intermetallic compound phase [38].

## *3.2. Catalytic Behavior*

The electrocatalytic activity for hydrogen evolution in 30 wt.% NaOH of Ni–Mo, Co–Mo and Fe–Mo samples containing ~52–54 at.% of Mo in their composition was evaluated using a linear scan voltammetry method that allows for the determination of the apparent exchange current densities. Polarization curves and semi-logarithmic coordinates of all chosen systems obtained at 25 ◦C are presented in Figure 3. In order to compare the catalytic behavior of Mo-rich Ni-, Co-, Fe-Mo coatings, the cast Mo and Pt electrodes were used. Table 3 summarizes the calculated apparent exchange current densities (*i*0), the overpotentials at a selected current density of 200 mA/cm<sup>2</sup> (η0.2) and the current densities obtained at an overpotential of 0.3 V (*i*0.3); calculated Tafel slopes (*b*c).

**Figure 3.** Cathodic polarization curves of (**a**) Mo-rich electrodeposits in 30 wt.% KOH at 25◦C and (**b**) plots in semi-logarithmic coordinates. The scan rate was 2 mV/s (composition is given in at.%).

**Table 3.** Calculated apparent exchange current densities (*i*0), Tafel slopes (*b*c), overpotentials (η*i*) at *i* = 0.3 A/cm<sup>2</sup> and current densities (*i*η) at η = −0.3 V for hydrogen evolution different electrodes (composition is given in at.%).


Binary Mo alloy cathodes possess 10<sup>5</sup> times higher apparent exchange current densities than cast Mo, thus are more active for the HER. However, all alloys demonstrated lower apparent exchange current densities for the HER at 25 ◦C in comparison with a bare Pt electrode. However, the overpotentials required to obtain current densities of 300 mA/cm<sup>2</sup> and current densities at −0.3 V for active bimetallic Mo alloys ware similar to those determined for Pt, making them competitive electrodes for hydrogen production. Notably, as it was mentioned above, the nature of the iron group metal affects the catalytic activity for the HER and the cathodic current density for the Co–52 at.% Mo coating is higher in comparison to that of Ni–54 at.% and Fe–54 at.% Mo, thus indicating the best catalytic performance among the synthesized electrodes, which is consistent with the lowest η0.2 and *i*0.3 values.

It is well known that the lower Tafel slope implies a lower electrochemical electrode polarization during the HER process, particularly at a high current density. The values of the Tafel slope for Ni–54 at.% Mo and Co–52 at.% Mo deposits under high polarization conditions are 128 mV/s and 132 mV/dec, respectively. Meanwhile, the Tafel slope for Fe–54 at.% Mo under these conditions shifted to more positive values, i.e., increased up to 152 mV/dec and it can be related to the higher iron affinity to the air and the presence of a thin oxide film on the surface that is characterized by a lower conductivity that impedes the electron transfer rate [15].

An improvement of catalytic activity for the HER with the operation temperature, as is desired for practical industrial alkaline electrolysis has been reported [36,42]. Accordingly, in the present study, the electrodeposited alloy electrodes were tested at temperatures ranging from 25 to 65 ◦C by applying 10 ◦C increments. A general comparison of the performance of electrodeposited Ni–54 at.% Mo, Fe–54 at.% Mo, Co–52 at.% Mo and cast separate metals in the temperature range 25–65 ◦C is given in Figure 4. The apparent exchange current densities were calculated from the linear region at low overpotential values and are presented in Table 4. As it was expected, the electrocatalytic activity of Ni–54 at.% Mo, Fe–54 at.% Mo and Co–52 at.% Mo is significantly higher than cast Mo in the whole tested temperature range. Moreover, the results sugges<sup>t</sup> that at an elevated temperature (>35 ◦C) Mo-rich alloys have a more prominent HER outperformance than Pt investigated in our laboratory. Although the HER activity, namely i0, was found to increase with temperature, the Tafel slopes for the Mo-based alloys remained almost constant and varied in the range of 120–150 mV/dec. This phenomenon has been discussed in terms of the entropic contribution towards free energy of activation [61]. Among all investigated systems, the Co–52 at.% Mo electrode demonstrates the best performance towards the HER, particularly at temperatures higher than 45 ◦C. These results correspond well with findings published in Reference [62] where it was confirmed that Co–Mo co-deposits are characterized by a higher catalytic activity and stability in alkaline water electrolysis than Ni–Mo, Co–W and Ni–W alloy electrodes. Moreover, in Reference [63], it was shown that catalytic activity depends on the metal-hydrogen bond strength and absorption sites in the alloy available to hydrogen and thus the electrochemically charged H content decreases in the series of Co–Mo > Co–W > Ni–Mo. Furthermore, comparing Co–Mo, Co–W and Ni–Mo electrodeposits, the thermal desorption of hydrogen occurs at the lowest temperature on Co–Mo showing a faster recombination step of H atoms possible on this alloy, thus improving its electrocatalytic performance [63].

**Figure 4.** Effect of temperature on cathodic polarization curves at Ni–54 Mo (**a**), Fe–54 Mo (**c**), Co–52 Mo (**e**) electrodeposits in 30 wt.% KOH at different temperatures and plots in semi-logarithmic coordinates for Ni–54 Mo (**b**), Fe–54 Mo (**d**), Co–52 Mo (**f**) electrodes. The scan rate was 2 mV/s (composition is given in at.%).


**Table 4.** Experimental values of the apparent exchange current (*i*0, in mA/cm2) and Tafel slopes (*b*c, in mV/dec) for Ni–54 Mo, Co–52 Mo and Fe–54 Mo alloy electrodes at different temperatures (composition is given in at.%).

In many previous publications the Ni–Mo alloy coatings are characterized by a lower overpotential value, as compared to the Co–Mo [14]. Though, there is also some information suggesting that Co–Mo coatings have a better catalytic activity for the HER compared to Ni–Mo deposits [62,64]. At the first glance, these controversial results could be attributed to the different alloy preparation techniques that are capable of yielding an uneven composition, morphology and structure of the prepared samples and thus, directly influence the catalytic properties of the samples. However, the clear tendency between the nature of the iron group metal effect during alloying with Mo and catalytic behavior also cannot be easily defined even for the Ni–Mo, Co–Mo and Fe–Mo cathodes fabricated using the same electrodeposition technique (Table 5). As it can be seen, the exchange current densities, even for alloys having a similar chemical composition, vary depending on the selected alkaline media and temperature.

In general, the present study shows the catalytic activity for the Ni–Mo alloy is comparable to previously reported samples under similar experimental conditions (Figure 5). Moreover, in the case of the Co–Mo and Fe–Mo system, the apparent exchange current density calculated in our work is significantly higher than it was expected from other authors observations (the corresponding columns are not given in the figure due to a significantly lower value). This could be attributed to the more concentrated alkaline media and higher temperature used in the present study.

**Figure 5.** The comparison of apparent exchange current densities (*i*0) towards the HER on Mo alloys with iron group metals determined in this study (\*) with the published data. The experiments were performed in NaOH at 60–65 ◦C. The composition of alloys is given in at.%.

Notably, the Fe–Mo coating demonstrates lower exchange current densities for the HER in an alkaline environment among other electrodes investigated in this study. This may be related to the higher iron affinity to the air by forming an oxide, hydroxide, or mixed film that physically separates the metal surface from the electrolyte.


**Table 5.** Comparison of the HER apparent exchange current density values, *i*0, extracted from the published data for electrodeposited Ni–Mo, Co–Mo and Fe–Mo alloy electrodes.

In order to obtain a more complete picture of electrocatalytic behavior for the HER, the corresponding *E*a values for all tested systems have been calculated considering the linear dependence lg(*i*0) = *f*(1/ *T*) using Arrhenius equation:

$$E\_{\mathbf{a}} = -2.303R \frac{\partial (\text{lg}\,\dot{\mathbf{a}})}{\partial (1/T)}\tag{4}$$

where *E*a is the activation energy (J/mol), *i*0—apparen<sup>t</sup> exchange current density, *T*—temperature (K).

Figure 6 shows the Arrhenius plots for the as-deposited Ni–54 at.% Mo, Co–52 at.% Mo and Fe–54 at.% Mo electrodes. From the slope of these plots *E*a values of 36.6, 32.5 and 27.9 kJ/mol for Fe–Mo, Ni–Mo and Co–Mo electrodes were determined, respectively. It is known that the lower the *E*a value is, the lower the energy requirements for hydrogen production. Thus, it is obvious that the charge transfer rate is favored by Co–52 at.% Mo alloys electrodeposition, since this electrode showed slightly lower activation energy than that of the Ni–54 at.% Mo and Fe–54 at.% Mo coatings.

**Figure 6.** Arrhenius plots for Ni–54 Mo, Co–52 Mo and Fe–54 Mo electrocatalysts (composition is given in at.%).
