Enhancing the Effectiveness of Oxygen Evolution Reaction by Electrodeposition of Transition Metal Nanoparticles on Nickel Foam Material

Abstract

Electrochemical oxygen evolution reaction (OER) activity was studied on nickel foam-based electrodes. The OER was investigated in 0.1 M NaOH solution at room temperature on as-received and Co- or Mo-modified Ni foam anodes. Corresponding values of charge-transfer resistance, exchange current-density for the OER and other electrochemical parameters for the examined Ni foam composites were recorded. The electrodeposition of Co or Mo on Ni foam base-materials resulted in a significant enhancement of the OER electrocatalytic activity. The quality and extent of Co, and Mo electrodeposition on Ni foam were characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis.

1. Introduction

In recent years, the development of renewable energy sources has dramatically increased [1,2], resulting from both a gradual depletion of crude oil sources, as well as progressive degradation of the natural environment [3,4]. Unfortunately, currently-used green energy technologies are strongly dependent on weather conditions and topography. For instance, the energy production of solar panels reaches its peak value at noon; however, it does not meet the demands of energy consumption [5]. This phenomenon is well-known under the name of solar energy’s duck curve. Appropriate storage of this energy excess could be the only solution to the above problem [6]. Currently, most of the produced renewable energy is stored in lithium-ion battery systems, because of their high-efficiency and convenience in exploitation. However, all batteries have their limitations, such as slow charge and discharge time, which is strongly dependent on ambient temperature, as well as limited operational lifetime. Moreover, used batteries could become environmentally important hazardous waste. One of the most promising new energy storage technologies is based on hydrogen generation by means of alkaline water electrolysis (AWE) [7,8]. Such produced hydrogen can be considered an entirely environmentally friendly energy carrier, since the only by-products generated during the hydrogen combustion are heat and water (see Equation (1)) [9].

$$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}+Q$$

(1)

While many catalysts are known to reduce the overpotential of hydrogen evolution reaction (HER), the more complex oxygen evolution (OER) process still requires a large amount of energy to proceed. Contrary to a comparatively fast single-electron transfer cathodic process, the OER is related to a slow and complex, four-electron transfer reaction (see Equation (2)) [10,11].

$$4{\mathrm{OH}}^{-}\to {\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$

(2)

Hence, in order to minimize the overpotential of the OER, the researchers have made significant efforts in developing new types of anodes. As a result, electrodes based on ruthenium [12] and iridium [13] oxides were found to exhibit outstanding electrochemical properties towards the OER; however, because of their high costs, the use of these materials on the industrial scale is not economically practical. Therefore, in recent years, the researchers’ attention has been shifted to more abundant and inexpensive transitional metals (Ni, Co, Mn, Mo, Fe) [14,15,16] and their alloys [17], oxides and carbides [18,19] to replace platinum group metals.

Thus, one of the most commonly used base materials to make electrodes, in the hydrogen production industry, are nickel and its derivatives. The employment of nickel and nickel-coated electrodes results from their unique properties, such as high catalytic activity in both hydrogen and oxygen evolution reactions. In addition, nickel exhibits superior anticorrosion properties in an alkaline environment. Generally, it is well-known that the OER on metals proceeds on the surface covered by their oxides, where nickel oxide/hydroxide species are especially catalytic in this reaction [20,21,22,23,24,25,26].

However, in order to compete with the electrocatalytic properties of electrodes based on noble metals, newly developed electrodes have to be characterized by large and highly active surface areas (HASA). Large values of HASA could be easily achieved by employing various materials, including foams, fibres, nanowires or nanocones. Such a base material could then be modified with a trace amount of transition metal(s) by one of the deposition methods (electrochemical, electroless, CVD: chemical vapour deposition or PVD: physical vapour deposition), which could further enhance the electrocatalytic properties of these innovative materials [20,21].

Nickel foam could be considered one of the most promising candidates for fabricating such a highly reactive anode in the process of the OER, as in addition to its unique electrocatalytic properties, Ni foam also provides large values of HASA. Nickel is also relatively inexpensive, compared to most noble metals [22].

In this work, electrochemically modified Ni foam materials were assessed as potential anodes towards the OER in alkaline 0.1 M NaOH environment through the employment of major electrochemical techniques. In addition, the structure and quality of electrodeposited Co and Mo on the surface of nickel foam were evaluated by means of a combined scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) methodology.

2. Results and Discussion

2.1. SEM/EDX Characterization of Ni Foam and Co-, and Mo-Modified Nickel Foam Electrodes

Figure 1a,b below illustrate SEM micrograph pictures of the electrodeposition effect of Co and Mo at small and trace amounts, respectively, on the Ni foam surface, recorded at the same magnification level. For the Co-modified Ni foam (at ca. 1 wt.% Co), the surface of the base material is densely covered with a granular structure of small Co nuclei (see Figure 1a). On the other hand, a sample of Mo-modified Ni foam (at ca. 0.2 wt.% Mo) is comparatively shown in a SEM micrograph picture of Figure 1b. The electrodeposited sample is covered with quite an irregular, amorphous and discontinuous structure through the Ni foam surface. In addition, the more detailed information on the composition of the prepared samples is shown in

Table 1. EDX-derived (for an acceleration voltage of 15 kV) chemical composition of surface elements for freshly prepared: Co- and Mo-modified Ni foam samples.

	Element/%	Spectrum 1	Spectrum 2
Co-modified Ni foam	Ni	95.83	92.84
	Co	1.06	1.25
	O	3.11	5.91
	Total:	100.00	100.00
Mo-modified Ni foam	Ni	92.08	86.42
	Mo	0.25	0.33
	O	2.61	5.06
	C	5.06	8.19
	Total:	100.00	100.00

. Moreover, Figure 1c,d demonstrate the EDX pattern of Co- and Mo-modified Ni foam electrodes, respectively.

In addition, the SEM-approximated Co and Mo grain size value came to 50.0 ± 3.5 and 512 ± 9.8 nm, respectively. The above was performed through utilization of the Image Analysis Program (NIS-Elements Basic Research on Nikon).

2.2. OER Characterizations of Ni Foam, Co- and Mo-Modified Ni Foam Electrodes

Cyclic voltammetry (CV) characterization of the surface oxidation process on fresh, Co- and Mo-modified Ni foam electrodes in 0.1 M NaOH solution during 50 CV sweeps, carried-out over the potential span 1.2–1.9 V/RHE (reversible hydrogen electrode) with a scan rate of 50 mV s⁻¹ (only three last sweeps are presented) is shown in Figure 2 below. Thus, for pure and Co-modified nickel foam samples, the CV profiles displayed an oxidation peak (centred at ca. 1.49 and 1.45 V, correspondingly) along with a cathodic reduction peak (starting around 1.3 V). The Mo modification caused a shift of the oxidation peak to more positive potentials than that for pure Ni foam. In contrast, the reduction peak was no longer visible, as it was most likely moved outside the measured potential range. These features refer to the electrochemical formation of the surface β-NiOOH phase and its reduction to β-Ni(OH)₂ layer, correspondingly (Equation (3)).

β-Ni(OH)₂ + OH¯ ↔ β-NiOOH + H₂O + e⁻

(3)

Tafel polarization plots recorded at room temperature for electrochemically oxidized, pure Ni foam, Mo- and Co-modified Ni foam electrodes are shown in Figure 3. Hence, for the nickel foam electrode, the recorded anodic Tafel slope (parameter b_a in Figure 3) and the Tafel-derived exchange current-density (j₀) value for the OER came to 54 mV dec⁻¹ and 7.1 × 10⁻¹¹ A cm⁻² over the low overpotential range. The modification utilizing surface Co or Mo deposition caused the values of b_a and j₀ parameters to rise to 83 and 74 mV dec⁻¹, and 7.0 × 10⁻⁸ and 9.6 × 10⁻⁸ A cm⁻², correspondingly, for the Co- and Mo-modified electrodes. Simultaneously, for larger overpotential values, the reaction system (not presented in Figure 3) moved into the mass transport limitations (see e.g., Ref. [27]). Pre-electrooxidation of Ni foam surfaces proved to be necessary to increase an initial value of the j₀ parameter for low overpotential range, as compared to an analogous Ni foam- based electrode by 3.6× [23]. Additionally, such-recorded exchange current-density was higher than that for other Ni foam [24] and similar 3D-structured [25] Ni electrodes by 7.0× and 1.6×, respectively. Moreover, as compared to similar catalysts [25], Mo-modified Ni foam electrode exhibited superior OER catalytic activity (with over 17× higher value of the j₀ parameter) to that of Ni-Mo composite coating.

Nowadays, there is a trend in literature to show the Tafel polarization results for a set overpotential, typically at η = 0.300 V or a set current-density, usually at 10 mA cm⁻² [26,27]. Hence, the current densities at η = 0.3 V for the Co-modified Ni foam electrode came to 9.2 × 10⁻⁵ A cm⁻², which was superior to those recorded in literature for various cobalt-modified anodes (see e.g., the recorded values of 1.4 × 10⁻⁶–1.0 × 10⁻⁵ A cm⁻² for cobalt oxide electrodes by Lyons and Brandon in Ref. [28]. On the other hand, the recorded b_a parameter for the examined electrodes was similar to those derived on Co and Mo modified 3D-structured electrodes (39 to 80 mV dec⁻¹) by Kubisztal and Budniok, Badruzzaman et al. and Lyons, and Brandon in References [25,26,27].

In order to further characterize the electrocatalytic properties of Ni foam-based electrodes towards the OER, the electrochemical impedance spectroscopy (EIS) technique was applied. The complex-plane EIS spectra are shown in Figure 4. It should be mentioned that all electrodes have exhibited two somewhat depressed semicircles, where the high-frequency arc could be associated with the kinetics of the oxygen evolution reaction (interfacial charge-transfer reaction step). In contrast, the intermediate/low-frequency semicircle corresponds to the surface adsorption of the reaction intermediates [29,30,31,32,33,34]. The electrochemical parameters [Faradic reaction resistance (R_ct), the adsorption resistance for reaction intermediates (R_Ads), double-layer capacitance (C_dl) and pseudo-capacitance (C_Ads)] were obtained by means of two constant phase element (CPE)-modified Randles equivalent circuit model (Figure 5), and their values are presented in

Table 2. Electrochemical parameters for the OER, obtained at as received Ni foam, Co- and Mo-modified Ni foam electrodes in contact with 0.1 M NaOH supporting solution. The results obtained here were recorded by fitting the CPE-modified Randles equivalent circuit (see Figure 5) to the experimentally obtained impedance data (reproducibility usually below 10%, χ² = 4.03 × 10⁻⁵ to 1.07 × 10⁻³.

η/mV	Rct/Ω g	Cdl/µF g−1 s φ1−1	RAds/Ω g	CAds/µF g−1 s φ2−1
Ni Foam
270	0.187 ± 0.033	120,933 ± 12,554	41.756 ± 4.524	144,293 ± 11,985
320	0.129 ± 0.025	127,147 ± 12,430	9.047 ± 0.206	103,520 ± 11,383
370	0.073 ± 0.008	114,453 ± 6488	1.844 ± 0.105	81,360 ± 5257
420	0.086 ± 0.008	89,627 ± 3612	0.616 ± 0.008	67,867 ± 2587
470	0.075 ± 0.007	81,493 ± 3352	0.289 ± 0.007	73,840 ± 3282
520	0.061 ± 0.005	67,600 ± 3119	0.176 ± 0.005	90,933 ± 3627
570	0.064 ± 0.005	76,773 ± 2646	0.114 ± 0.005	78,717 ± 5113
620	0.047 ± 0.002	8720 ± 414	0.123 ± 0.003	129,227 ± 9950
Co-modified Ni foam
270	0.052 ± 0.006	541,047 ± 15,371	5.265 ± 0.417	670,980 ± 28,526
320	0.035 ± 0.004	224,899 ± 26,583	1.239 ± 0.009	690,541 ± 6229
370	0.030 ± 0.004	218,716 ± 10,076	0.217 ± 0.004	412,736 ± 7235
420	0.027 ± 0.003	188,682 ± 8730	0.214 ± 0.004	444,831 ± 6886
470	0.028 ± 0.003	141,993 ± 5113	0.128 ± 0.003	475,169 ± 10,406
520	0.024 ± 0.002	121,588 ± 3725	0.089 ± 0.002	488,446 ± 13,374
570	0.024 ± 0.002	161,622 ± 5287	0.052 ± 0.002	576,419 ± 31,772
620	0.022 ± 0.004	105,473 ± 9599	0.041 ± 0.005	557,230 ± 91,804
Mo-modified Ni foam
270	0.081 ± 0.011	822,987 ± 85,015	2.274 ± 0.052	1,743,525 ± 143,562
320	0.069 ± 0.010	540,724 ± 66,936	0.667 ± 0.018	1,916,698 ± 61,430
370	0.066 ± 0.009	599,202 ± 157,063	0.296 ± 0.007	1,680,668 ± 168,453
420	0.068 ± 0.017	1,024,082 ± 152,342	0.181 ± 0.014	1,045,659 ± 157,476
470	0.052 ± 0.004	200,204 ± 20,927	0.150 ± 0.006	1,512,226 ± 94,060
520	0.035 ± 0.008	783,766 ± 74,144	0.111 ± 0.008	1,337,607 ± 118,659
570	0.030 ± 0.011	1,008,571 ± 140,070	0.091 ± 0.011	916,327 ± 126,682
620	0.023 ± 0.003	1,271,429 ± 181,585	0.087 ± 0.003	1,524,620 ± 324,842

. The CPE element was used within the circuit in order to account for the capacitance dispersion effect, represented by distorted semicircles in the Nyquist impedance plots.

Thus, for the oxidized Ni foam electrode, the recorded R_ct parameter diminished from 0.187 Ω g to 0.047 Ω g at η = 270 and 620 mV, respectively. Similarly, the C_dl parameter decreased from the value of 120,933 to reach 8720 μF g⁻¹ s^φ1−1 for the same overpotential span (see Table 2 for details). The above effect could be associated with substantial blocking of the electrode surface by freshly formed O₂ bubbles, which could easily be visually noticed, especially at increased anodic overpotentials. Simultaneously, the value of the R_Ads parameter reduced from 41.756 to 0.123 Ω g for the corresponding potential range, while the C_Ads parameter oscillated between an initial value of 144,293 (at 270 mV) and a minimum value of 67,867 μF g⁻¹ s^φ2−1 recorded at 420 mV. The latter understandably implies that the kinetics of the OER become facilitated upon the rise of the dc electrode potential. However, the R_Ads parameter values shown in Table 2 are essentially larger than those recorded for the R_ct parameter. The above suggests that the OER process is limited by the reaction intermediates’ adsorption kinetics (also see References [23,34]).

Then, the R_ct parameter for the Co and Mo-modified Ni foam electrodes ranged from 0.052 to 0.022 Ω g and from 0.081 to 0.023 Ω g for the overpotential range: 270–620 mV, respectively. The former implies a considerable reduction of the R_ct parameter (as compared to those R_ct values recorded for the oxidized Ni foam surface); namely, by 3.6× and 2.3× at 270 mV, and by 2.1× at 620 mV for the Co and Mo-modified catalyst materials, respectively (see again Table 2 for details). Similarly, changing radius of the low-frequency semicircle (adsorption of reaction intermediates response) also resulted in a relatively constant decrease of the R_Ads parameter value by ca. 8× and 3× for the cobalt-based, and by 18× and 1.4× for the Mo-modified Ni foam electrodes for the overpotentials of 270 and 620 mV.

Furthermore, deposition of catalytic nanostructures (see Figure 1a,b) resulted in a considerable enhancement of the electrochemically accessible surface area. The latter could be easily observed in the cyclic voltammetry plot presented for the baseline and the Co and Mo-modified Ni foam materials in Figure 2. Consequently, significant surface modification for the Co- and Mo-modified Ni foams is demonstrated by considerably increased current-density within the corresponding CV profiles.

The reported values of the C_dl parameter at η = 270 mV came to 541,047 μF g⁻¹ s ^φ1−1 (Co-modified Ni foam) and 822,987 μF g⁻¹ s ^φ1−1 for the Mo-based Ni foam catalyst material, which translates to ca. 4.5× and 6.8× augmentation of the HASA values for both modified electrodes, in contrast to the oxidized Ni foam anode. Once these surface coefficients are referred to the previously calculated ratios of the R_ct parameter, it becomes evident that the cobalt-activated nickel foam anode exhibited superior OER performance. In addition, dimensionless φ₁ and φ₂ parameters (where φ determines the constant phase angle in the complex-plane plot and 0 ≤ φ ≤ 1) of the CPE circuits (see Figure 5) oscillated between: 0.66–0.97 and 0.49–0.97, respectively.

The impedance results obtained for the examined electrodes in this work were generally in line with those OER impedance studies presented by Si et al. [35] and by Lyons, and Brandon [28] on cobalt oxide electrodes, and by Choi et al. [36] and by Wu, and He [37] on Mo-based composites. Moreover,

Table 3. Electrochemical parameters for the OER performed at the surfaces of different catalyst materials, under alkaline conditions.

η/mV	Solution	Catalysts	Rct/Ω cm2	j(η= 0.3V)/A cm−2	Ref.
270	0.1 M NaOH	Co-modified Ni foam	31.9	9.2 × 10−5	This work
420	0.1 M NaOH	Co-modified Ni foam	16.6	-	This work
430	0.1 M KOH	Co(OH)2/C	8.5	-	35
420	0.1 M NaOH	Co oxide/Fe	17.5	1.15 × 10−5	28
270	0.1 M NaOH	Mo-modified Ni foam	27.1	1.6 × 10−4	This work
420	0.1 M NaOH	Mo-modified Ni foam	22.6	-	This work
290	1.0 M KOH	NiMoO4 nanowire/Ni foam	981 *	-	36
210	30% KOH	Ni-Fe-C-Mo/Ni mesh	364 *	-	37
438	1.0 M NaOH	Pt	80	~4 × 10−5 **	40
278	0.1 M KOH	RuO2/GC ***	142 *	~5 × 10−4 **	41
250	0.1 M KOH	Pd/FTO ****	42 *	~12 × 10−3 **	42

* surface-normalized values (see information below); ** values estimated from the Tafel plots; *** glassy carbon; **** fluorine-doped tin oxide.

presents a comparison of the impedance-recorded R_ct and j (polarization plot-estimated for η = 0.3 V) parameter values in this work (the charge-transfer resistance was derived through the C_dl-approximated surface area of the nickel foam electrodes [23,38,39]) with the results recorded on similar electrodes in References [35,36,37]. It should be noted that the R_ct results presented for Ni-Mo based electrodes in Refs. [36,37] were given as raw data, without providing any specific information on the electrodes’ surface area. Thus, in order to make a meaningful comparison, one could estimate the HASA values through dividing the recorded interfacial capacitance by a commonly used value of 20 mF cm⁻² in literature for smooth and homogeneous surfaces (see Refs. [38,39]). Having done so, the surface-normalized R_ct parameter would come to 981 and 364 Ω cm² for Ni-Fe-C-Mo and NiMoO₄ electrode surfaces, correspondingly. These estimated resistance values are radically greater than those recorded in this work. In addition, the plot-derived value of the j parameter (1.15 × 10⁻⁵ A cm⁻²) for Co oxide/Fe electrode came very close to the current-densities recorded in this work.

Furthermore, in order to fully access the suitability (and possibly also superiority) of the developed electrodes for their possible commercial use, Table 3 also presents the R_ct and j (η = 0.3 V) parameter values for the OER, obtained on selected noble metals/oxide (Pt, RuO₂ and Pd), under similar experimental conditions [40,41,42]. Hence, the recorded (or estimated) charge-transfer resistance parameter values for bulk Pt, RuO₂/GC and Pd/FTO electrodes were significantly greater than those recorded for the Co (or Mo)-modified nickel foam specimens. On the other hand, only fluorine-doped tin oxide palladium electrode exhibited radically higher values (ca. 12 × 10⁻³ A cm⁻²) of Tafel plot-derived current-density parameter than those recorded in the present work for the catalyst (Co, Mo)-modified nickel foam materials.

In addition, the exchange current-density values (j₀) for the OER recorded for Co- and Mo-modified Ni foam samples were calculated based on the linear dependence of -log R_ct vs. overpotential/ƞ, fulfilled by kinetically controlled reactions through employing the Butler-Volmer equation and the relation between the j₀ and the R_ct parameter for overpotential approaching 0 [43,44]. Hence, the impedance-based exchange current-density value for the studied overpotential range (ƞ = 270–620 mV with 50 mV potential increments) came to 6.06 × 10⁻⁵ A cm⁻² for the as-received Ni foam sample. However, significantly higher values of the j₀ parameter were recorded for both Co- and the Mo-modified Ni foam samples, i.e., 2.89 × 10⁻⁴ A cm⁻² and 1.04 × 10⁻⁴ A cm⁻², correspondingly (see Figure 6 below). In addition, the R_ct-derived exchange-current densities are somewhat higher in comparison to those of other OER works performed on electrodes modified by Co and Mo elements in alkaline solutions [25,45]. There, the exchange current-density values recorded for Ni + Mo composite coating came to 9.7 × 10⁻⁵ A cm⁻² [25] and 5.2 × 10⁻⁵ A cm⁻² [45]. Similarly, values of the j₀ parameter, recorded for high overpotential range on Co-based compsite electrodes came to 3.5 × 10⁻⁵ A cm⁻² [46] and 1.1 × 10⁻⁵ A cm⁻² [28].

3. Materials and Methods

3.1. Solutions and Chemical Reagents

In this work, all supporting solutions were prepared by dissolving chemical reagents in ultra-pure water (18.2 MΩ cm) from the Hydrolab (Spring 30 s) purification system. 0.1 M NaOH working electrolyte was prepared from sodium hydroxide pellets (99.996%, AESAR, Ward Hill, NY, USA). Before each series of experiments, atmospheric air was removed from central and both side cell’s compartments by bubbling with high purity argon (Ar 6.0 grade, Linde, Puławy, Poland) for 15 and 5 min, respectively. Furthermore, argon gas flow was kept above the solutions throughout the experiments. 0.5 M H₂SO₄ (stock H₂SO₄: 96%, Merck, Darmstadt, Germany) solution was used for periodic charging of a Pd reversible hydrogen electrode (RHE).

3.2. Preparation of Electrochemical Cell and Electrodes

The OER activity of pure Ni foam, Co- and Mo-modified Ni foam electrodes was examined in an electrochemical cell made of Pyrex glass. The cell comprised three separate compartments. In the central compartment, a Ni foam-based working electrode (WE) was placed, when both Pd (1.0 mm diameter wire, 99.98+% purity, AlfaAesar, Ward Hill, NY, USA) reversible hydrogen electrode and a Pt (1.0 mm diameter wire, 99.9998% purity, Johnson Matthey, Inc., London, UK) counter electrode (CE) were located in separate side compartments.

Nickel foam base electrode material was delivered by MTI Corporation (Richmond, CA, USA) (purity: >99.99% Ni; thickness: 1.6 mm; surface density: 346 g m⁻²; porosity: ≥95%; estimated electrochemically active surface 14.9 cm²) [38]. All examined electrodes were 1 cm × 1 cm (ca. 35 mg). Before electrodeposition of Co and Mo elements on Ni foam base material, freshly cut foam samples were degreased in acetone bath for 15 min under ultrasonication. Then, they were air dried and etched in 2 M HCl for 15 min at 60 °C, also under ultrasonication. Electrodeposition of Co and Mo catalysts on Ni foam was performed according to the conditions and bath compositions shown in

Table 4. Operating parameters and bath compositions employed for electrodeposition of Co and Mo onto Ni foam substrate.

Bath Constituents	Concentration (M)	Operating Parameters
Electrodeposition of Co
CoCl2 × 6 H2O (99%, Sigma Aldrich, Saint Louis, MO, USA)	0.10	Anode: Pt foil
NaCl (99.9%, POCH, Gliwice, Poland)	0.02	Cathode: Ni foam
		Temperature: 303 K
		Deposition time: 300 s
		Current-density: 0.52 mA cm−2
		Solution pH: 4.5
Electrodeposition of Mo
(NH4)6Mo7O24·4H2O (≥99.3%, Merck, Darmstadt, Germany) CH3CO2NH4 (≥98.0%, Merck, Darmstadt, Germany) CH3COOK (≥99.0%, Sigma Aldrich, Saint Louis, USA)	1.95 × 10−4 0.26 0.26	Anode: Pt foil
		Cathode: Ni foam
		Temperature: 303 K
		Deposition time: 1800 s
		Current density: 5.2 mA cm−2
		Solution pH: 6.8

. The bath’s pH for Co electrodeposition was readjusted with 0.1 M NaOH or 0.1 M HCl solutions prior to the experiment. Similarly, before electrodeposition of Mo, the solution’s pH was modified with 0.1 M NaOH, accordingly. No significant pH change was observed during both metal deposition trials.

Such-modified Ni foam materials were left in a desiccator for an extended period of time before being weighed. Before conducting a series of electrochemical experiments, as received, Co- and Mo-modified Ni foam samples underwent extensive electrooxidation by means of 50 cyclic voltammetry sweeps, carried-out for the potential range 1.2–1.9 V/RHE, at a sweep rate of 50 mV s⁻¹.

The palladium RHE was sealed in soft glass. Before its use, the reference electrode was cleaned in hot, concentrated sulphuric acid, followed by cathodic (galvanostatic) charging with hydrogen in 0.5 M H₂SO₄ until H₂ bubbles in the electrolyte were clearly visible. The stability of such prepared RHE was occasionally checked by recording its potential shift in time (it should then be noted that all potentials throughout this work are given on the RHE scale). The CE electrode, prior to its use, was flame-annealed. Similarly, before each series of electrochemical experiments, the electrochemical cell was taken apart and soaked in hot sulphuric acid for at least 4 h. After having been cooled to about 30 °C, the cell was thoroughly rinsed with Hydrolab ultra-pure water.

3.3. Experimental Methodology

Electrochemical performance of the OER electrocatalysts (pure Ni foam, Co-, and Mo-modified Ni foam electrodes) was conducted by a.c. impedance spectroscopy and quasi steady-state Tafel polarization techniques by means of an AUTM204 + FRA32M Multi-Autolab potentiostat/galvanostat system, at room temperature (293 K). For the a.c. impedance measurements, the generator provided an output signal of 5 mV rms and the frequency was swept between 1.0 × 10⁵ and 2 × 10⁻² Hz. The instruments were controlled by Nova 2.1 software for Windows (Metrohm Autolab B.V., Opacz-Kolonia, Poland). Also, all data analysis was performed with the above-mentioned software package. Typically, three impedance measurements were performed at each potential value, independently at two foam electrodes, where reproducibility of such obtained results was usually below 10%. The obtained impedance spectra were fitted by means of a complex, nonlinear, least-squares immittance fitting program. Furthermore, quasi-potentiostatic anodic polarization experiments (recorded at a scan-rate of 0.5 mV s⁻¹) for the OER were carried-out at all Ni foam-based samples.

Additionally, SEM/EDX surface spectroscopy characterization of all examined (Ni foam, Co- and Mo-modified Ni foam) samples was carried out by means of Merlin FE-SEM microscope (Zeiss, Jena, Germany), equipped with Bruker XFlash 5010 EDX instrumentation (with 125 eV resolution) (Bruker, Billerica, MA, USA).

4. Conclusions

Electrochemically deposited small amounts of transition metals in the form of cobalt (at ~1 wt.%) and molybdenum (at ~0.28 wt.%) nanoparticle deposits on the surface of Ni foam material significantly enhanced catalytic activity of the base foam material towards anodic evolution of oxygen in 0.1 M NaOH solution. The above is primarily related to superior OER catalytic properties of Co and Mo nano-deposits, in addition to the considerable modification of electrochemically-active surface for these catalyst materials. The results of this work showed a real possibility for replacing expensive and scarcely found in the natural environment noble metals with significantly cheaper transition metals for the mass production of commercial alkaline water electrolysers.