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

: 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. Contributions: Conceptualization, B.P.; methodology, B.P., M.Ł. and T.M.; investigation, M.Ł., M.K; data curation, T.M., M.L. and M.K.; writing—original draft preparation, M.Ł., T.M.; editing, B.P. and T.M.; supervision, I.M.K. All authors have read and agreed to the published version of the manuscript.


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

SEM/EDX Characterization of Ni Foam and Co-, and Mo-Modified Nickel Foam Electrodes
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). shown in Table 1. Moreover, Figure 1c,d demonstrate the EDX pattern of Co-and Momodified Ni foam electrodes, respectively.   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)). 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).

OER Characterizations of Ni Foam, Co-and Mo-Modified Ni Foam Electrodes
Cyclic voltammetry (CV) characterization of the surface oxidation process on fresh, Coand Mo-modified Ni foam electrodes in 0.1 M NaOH solution during 50 CV sweeps, carriedout 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 (j 0 ) 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 j 0 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 Momodified 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 j 0 parameter for low overpotential range, as compared to an analogous Ni foambased 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 j 0 parameter) to that of Ni-Mo composite coating. 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 ba in Figure 3) and the Tafelderived 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 ba 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 3Dstructured [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 ba 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- 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 cobaltmodified 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. 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.
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 13 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 (Rct), the adsorption resistance for reaction intermediates (RAds), double-layer capacitance (Cdl) and pseudo-capacitance (CAds)] 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. 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.    Figure 5 below.
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 13 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 (Rct), the adsorption resistance for reaction intermediates (RAds), double-layer capacitance (Cdl) and pseudo-capacitance (CAds)] 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. 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 O 2 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]). 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%, χ 2 = 4.03 × 10 −5 to 1.07 × 10 −3 .

η/mV
R ct /Ω g C dl /µF g −1 s ϕ1−1 R Ads /Ω g C Ads /µF g −1 s ϕ2−1 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 ϕ 1 and ϕ 2 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 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 cm 2 for Ni-Fe-C-Mo and NiMoO 4 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, RuO 2 and Pd), under similar experimental conditions [40][41][42]. Hence, the recorded (or estimated) charge-transfer resistance parameter values for bulk Pt, RuO 2 /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 (j 0 ) 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/η, fulfilled by kinetically controlled reactions through employing the Butler-Volmer equation and the relation between the j 0 and the R ct parameter for overpotential approaching 0 [43,44]. Hence, the impedance-based exchange current-density value for the studied overpotential range (η = 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 j 0 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].
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 Rct 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 Rct vs. overpotential/ƞ, fulfilled by kinetically controlled reactions through employing the Butler-Volmer equation and the relation between the j0 and the Rct 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 Rct-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].

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:

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 H 2 SO 4 (stock H 2 SO 4 : 96%, Merck, Darmstadt, Germany) solution was used for periodic charging of a Pd reversible hydrogen electrode (RHE).

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 cm 2 ) [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. 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 H 2 SO 4 until H 2 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.

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 × 10 5 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, leastsquares 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).

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.