Introduction

During the past century with the fast development of modern industries, increase of the environmental pollution and high costs of the fossil fuels have raised the demand of acquiring alternative resources of energy. Extensive efforts have been carried out to obtain new renewable energy resources at lower prices compared to the conventional fuels, and to reduce the damaging effects of climate change by mitigating greenhouse gas emissions (Ref 1, 2).

Hydrogen is considered to be a promising clean energy carrier to replace fossil derivative fuels. One of the most promising techniques for producing high purity hydrogen is alkaline water electrolysis, giving the advantage of a more controllable corrosion rate and cheaper construction materials (Ref 3). However, costs of the electrolysis process have been a great concern since it is highly dependent on the costs of the electrical energy (Ref 2, 4). Therefore, to improve the energy efficiency of the electrolysis process, hydrogen evolution over-potentials that contribute significantly to cell energy losses should be lowered. To achieve this goal, advanced electrolysers with high-performance electrodes need to be developed to obtain hydrogen at diminishing costs with minimum energy consumption (Ref 5). Using high intrinsic active electrode materials with enhanced electrode surface areas is a possible route of increasing the efficiency of the hydrogen evolution reaction (HER) (Ref 6, 7). A good accessibility for diffusive mass transport of the reactants and their products inside the porous structure is another requirement for the electrodes for HER (Ref 8).

Although platinum is the best-known electrocatalyst for HER, its commercial use as cathode electrode material has been limited due to its expensiveness and rarity. In industrial applications, often nickel replaces platinum, since not only it is a catalytically active metal but also it has high stability in alkaline solutions at elevated temperatures (Ref 9).

Among different surface modification techniques, plasma spraying has demonstrated its capability to produce porous electrode coatings in a cost effective process, taking advantage of the surface morphology produced by the spraying process (Ref 7, 9, 10). Possibility of forming coatings at high deposition rates with moderate operating costs has made this method superior compared to other surface coating techniques, such as electrodeposition, sintering, vacuum deposition, etc (Ref 11). Reduction of hydrogen evolution overpotentials when using plasma-sprayed nickel-based cathode electrodes has been addressed in several works (Ref 9, 10, 12).

In recent years, there has been a major interest towards development of plasma-sprayed coatings, utilizing nanosized particles. This interest arouse from the unique features of nanometer scale coatings, such as increased surface areas and superior performances. For Ti-based electrodes, Irissou et al. (Ref 13) showed that the hydrogen evolution overpotentials of nanostructured electrodes were significantly lower compared to the ones prepared by the micron-sized powder. However, nanosized particles cannot be used directly for plasma spraying due to the difficulties with their injection into the core of the plasma jet. Suspension plasma spray (SPS), based on injection of a liquid feedstock, is a novel method developed to overcome the injection problem of sub-micron and nanosized particles. The suspension is basically made by dispersing the feedstock powder particles with the particle sizes ranging from tens of nanometers to a few micrometers in a liquid phase, which is mostly ethanol, water, or a mixture of both. Addition of a dispersant is usually needed to reduce the agglomeration and sedimentation of the particles (Ref 14, 15).

The objective of this work was to develop Ni-based electrode coatings with enhanced surface areas for the hydrogen evolution reaction, using atmospheric plasma spray (APS) and SPS processes. The effects of spraying parameters of each process on the electrochemical active surface areas of the coated samples were studied. The results were compared for the coatings produced by APS and SPS processes.

Material and Methods

Spraying Materials

Two different commercial powders, nickel with the nominal particle size in the range of −75 to +45 μm and nickel oxide with the nominal particle size in the range of −5 μm to +500 nm, were used for APS and SPS processes, respectively.

Nickel powder has a tendency to sediment and agglomerate in the suspension because of its high specific mass and magnetic properties (Ref 16). Due to the difficulties in obtaining a stable nickel suspension, nickel oxide powder was used instead. A series of stable suspensions containing 10 wt.% of NiO powder in ethanol as the solvent were prepared for the SPS process. To increase time-stability of the particle dispersion and to prevent agglomeration of the particles, polyvinylpyrrolidone (PVP) was added as the dispersing agent. 1, 2, 3, 4, and 5 wt.% of PVP were added to the suspension at different test tubes to adjust its mass percentage by sedimentation tests.

Injection Set-Up

The injection system for SPS was composed of two tanks, one containing the suspension and the other one containing ethanol. For the spraying process, the tanks were pressurized with N2 gas. To avoid contamination and clogging of the injection system, the ethanol stored in the tank was used to clean the injection system after each suspension projection. The position of the injector, having a 200-μm diameter nozzle, was adjusted in such a way that the suspension drops penetrate adequately to the plasma jet at the nozzle exit (Ref 17).

Spraying Processes

Plasma spraying experiments were carried out using a Sulzer Metco 3MB atmospheric plasma-spraying gun, mounted on a computer-controlled robotic arm. Coatings were deposited on grit blasted, rectangular low carbon steel coupons of dimensions 25 × 25 × 3 mm3. Argon was used as the primary plasma gas as well as the powder carrier gas, and hydrogen was used as the secondary gas.

To determine optimum operating conditions for producing coatings with a high surface area, while having a minimum number of experimental runs, Taguchi statistical method (Ref 18) was employed in design of the experiments (DOE). Using such an approach greatly reduce the experimental time and cost. DOE is further useful in evaluation of the importance of each selected coating variable on the surface areas of the coatings.

Table 1 shows the L9 orthogonal array designed for APS processes. The table was produced using a combination of standoff distance, current, plasma gas flow rate, and torch traverse speed as spraying variables, each with three selected levels (two levels for the torch traverse speed). The 15 over-layers were deposited at a constant powder feeding rate of 30 g/min to create APS coating. Table 2 shows the L8 orthogonal array used for SPS processes. The array consists of a combination of five variables including standoff distance, suspension flow rate, plasma gas flow rate, current and torch traverse speed, each having two selected levels. Each SPS coating was composed of deposition of 20 over-layers.

Table 1 Plasma spraying parameters used for APS coatings
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Table 2 Plasma spraying parameters used for SPS coatings
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Characterization and Analysis

One of the key parameters to assess the performance of an electrode is its electrochemical active surface area. It is well known that the electrochemical double layer capacitance (ECDLC) of an electrode is proportional to the electrochemical active surface area of the material used (typically 20 μF/cm2) (Ref 19). Therefore, for initial comparison of the electrochemical active surface area of the coatings formed by APS and SPS processes, their ECDLC was measured. A three-electrode configuration was used for all measurements. A Pt wire and an Hg/HgO electrode (saturated 1 M KOH) were used as counter and reference electrodes in a 0.5 M NaOH solution. A geometrical surface area of 0.78 cm2 of each coating was exposed to the electrolyte as the working electrode. The cell was bubbled with nitrogen before and during the measurements to de-oxygenate the electrolyte solution.

Cyclic voltammetry measurements were performed at five scan rates of 0.02, 0.05, 0.1, 0.15, and 0.2 V/s in the potential window from −1.35 to 0.7 V versus Hg/HgO using an in-house potentiostat. The region around −0.1 to 0.4 V versus Hg/HgO is considered to be essentially free of faradic current and was used to evaluate the contribution of the ECDLC to the recorded current (Fig. 1). Three ECDLC measurements were performed on each sample. The capacitance values were calculated by dividing the average current at the center of the positive and negative sweeps of the resulting cyclic voltammograms by the scan rates (Ref 19).

Fig. 1
figure 1

Cyclic voltammetry of S6 sample, and non-faradic region used for ECDLC measurements between the dashed lines (scan rate of 0.02 V/s)

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Signal-to-noise (S/N) ratio, a measure of robustness, was used to analyze the influence of each spraying control factor on the specific surface area of the coatings. Since the goal was to maximize the specific surface area, the S/N ratio of “larger is better” was used. This category is calculated as a logarithmic transformation of loss function:

$$ S/N = \left( { - 10 \times \log \sum {\frac{1}{{Y^{2} }}} /n} \right), $$
(1)

where n is the number of observations, and Y is the observed data.

Morphology of powder particles and coating surfaces were studied by a Hitachi S-3400N VP scanning electron microscope (SEM).

Results and Discussion

Precursor Materials

Figure 2 shows SEM images of the morphology of precursor micron-sized Ni and submicron-sized NiO powders used in this study.

Fig. 2
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Morphology of precursor powders: (a) Ni powder and (b) NiO powder

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Sedimentation results of the suspensions are depicted in Fig. 3. It can be seen that the minimum sedimentation was obtained for the suspension containing 1 wt.% PVP. The viscosity of this suspension was measured by a Cannon-Fenske reverse flow capillary viscometer with an error margin of ±0.38%. Kinematic viscosity of the suspension was measured 1.69 × 10−6 (m2/s) at 22 °C, showing a 15.6% increase with respect to the viscosity of ethanol.

Fig. 3
figure 3

Sedimentation test results for 10 wt.% NiO suspension and 0 to 5 wt.% PVP after 14 days

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ECDLC Measurements

Figure 4 shows the calculated double layer capacitances for all APS and SPS samples. As illustrated in this figure, by coating the sample using either APS or SPS, the ECDLC values are increased significantly compared to the uncoated substrate. Nevertheless, the samples coated with the suspension exhibited a larger ECDLC compared to the APS deposited ones. As indicated above, the double layer capacitance of a sample is in a direct relation with its electrochemical active surface area. Therefore, it can be concluded that the coatings produced by nanosized powder have a larger electrochemical active surface area in comparison with those produced by micron-sized powder. Smaller standard deviation of ECDLC values of APS-coated samples indicate more homogeneous coatings compared to the SPS-coated ones. On the other hand, larger variation of ECDLC between SPS coatings indicate that the SPS process is more sensitive to the spraying parameters, i.e., a small variation of the spraying parameters can greatly affect the surface area of the coatings. This can be mainly related to the much smaller powder particles used in the SPS process.

Fig. 4
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Electric double layer capacitance of APS- and SPS-coated samples

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APS Coatings

ECDLC results, illustrated in Fig. 4, indicate that A7 with 82 μF has the lowest and A1 with approximately 142 μF has the highest capacitance among APS-coated samples. Surface SEM micrographs of these two samples can be seen in Fig. 5.

Fig. 5
figure 5

SEM surface images of APS samples (a) A7 and (b) A1

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The images show that the top surface of A7 coating is composed of larger splats with diameters of >100 μm, while A1 has a finer and rougher surface structure. The presence of semi-molten and re-solidified particles is evident in the latter coating. Plasma sprayed coatings are formed by deposition of flattened splats created by impact and rapid solidification of molten and semimolten particles. The size, morphology, and bonding of the splats along with porosity determine the structural properties of that deposit. The different surface structures can be directly related to the plasma spraying parameters that were used to form the coatings (Ref 20).

Table 3 presents the S/N response data for the electric double layer capacitance. Based on the value differences between the larger and smaller levels, the relative importance of each factor is quantified as factor ranks. The results indicate that standoff distance has the greatest influence on the electro-active surface area, followed by torch traverse speed, current, and hydrogen gas flow. Figure 6 shows the effect of each control factor level on the active surface area.

Table 3 Response table for signal to noise ratios for APS
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Fig. 6
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Main effects plot for S/N ratios for APS

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At 12 cm standoff distance, higher velocity and temperature of the impacting particles provide a better cohesion between the depositing layer and the substrate. Meanwhile, the smaller particles have a tendency to re-solidify because of the more rapid heat loss and deceleration at longer inflight paths. This combination of completely melted, semi-melted and re-solidified particles leads to the formation of deposits with a higher degree of porosity, rougher surfaces, and consequently larger surface areas. Increasing the standoff distance from 12 to 17 cm did not have any significant effect on the electrochemical active surface area of the coatings. By further increase of the standoff distance to 22 cm, the particle velocity and temperature are further reduced at the time of impact to the substrate. Therefore, more inflight particles at longer spray distances are re-solidified, which also causes a weaker adhesion of the particles to the substrate. In this case, only a limited number of particles, which are still molten and semi-molten by the time of impact, would adhere to the substrate and fabricate coatings consisting of larger splats with lower open porosity, and thus, a more limited specific surface area. For a better understanding of the behavior of inflight particles, their temperature and velocity at the point of impact need to be determined using a DPV-2000 on-line diagnostic system.

The second parameter that affected the active surface area is torch traverse speed. The results indicate that by increasing the torch traverse speed from 0.5 to 1 m/s, the active surface area was reduced. There is no clear explanation for this behavior, and further investigations are required. The results in Fig. 6 and Table 3 show that the current and hydrogen gas flow did not have a significant effect on the electrochemical active surface area.

Since the goal here is to maximize the specific surface area, the factor levels that produce the highest mean should be employed. Analysis of the results leads to the conclusion that a factor combination of 17 cm standoff distance, 400 A current, 4.4 NLPM hydrogen flow, and 0.5 m/s gun travel speed generates the maximum surface area. For this combination, an S/N ratio of 43.13 was predicted. However, further experiments are required to confirm its accuracy.

SPS Coatings

Figure 4 shows that S1 with 125 μF and S6 with 348 μF has the smallest and the largest capacitance for SPS coatings, respectively. SEM images of these two surfaces are shown in Fig. 7.

Fig. 7
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SEM surface images of SPS samples: (a) S1 and (b) S6

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As illustrated in Fig. 7, the surface of S1 coating is mainly composed of dispersed cauliflower-like aggregates with diameters larger than 20 μm, and the coating does not cover the whole substrate. For S6 coating, larger amounts of smaller aggregates (~10 μm) are deposited on the surface and form a rougher surface with more specific surface area. Formation of cauliflower-like microstructures using SPS process is widely reported in the literature (Ref 21–23).

Table 4 shows that the significance of the spraying variables for SPS coatings from high to low are current, standoff distance, hydrogen gas flow, torch traverse speed, and suspension flow rate.

Table 4 Response table for signal to noise ratios for SPS
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Figure 8 depicts the effect of various factor levels on the produced surface areas. The influence of spraying variables on the specific surface areas of the SPS coatings can be discussed as follows.

Fig. 8
figure 8

Main effects plot for S/N ratios for SPS

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By increasing the spray distance from 4 to 6 cm, the electrochemical active surface area is increased. This effect can be described by formation of more porous structures due to the deposition of cooler and slower particles at higher spraying distances. The effect of current and hydrogen gas flow can be attributed to their influence on the input power. Higher current and hydrogen gas flow rate result in an elevated input power. For the SPS experiments input power was varied from 22 kW at the hydrogen flow rate of 3 NLPM and current of 450 A to 28 kW at the hydrogen flow rate of 5 NLPM and current of 500 A. It has been shown that higher power levels result in an increasing temperature and velocity of the sprayed particles in addition to the higher temperature of the top surface of the coating (Ref 24), which cause higher expansion of the impinging particles on the surface, densification of coating, and thus, a smaller surface area.

The results of Fig. 8 and Table 4 show that the torch traverse speed had a slightly lower effect on the active surface area compared to the spray distance and input power, and suspension flow rate did not have a significant effect on the surface area. The results indicate that the real surface is maximized at 6 cm standoff distance, 22.8 g/min of suspension flow rate, 3 NLPM hydrogen gas flow, 450 A current, and 1 m/s torch traverse speed. Using a combination of these factor levels, an S/N ratio of 53.76 is predicted.

Summary and Conclusion

APS and SPS coating processes were used to produce nickel-based electrode coatings with enhanced surface areas for hydrogen production. Electrocatalytically active electrodes with large surface areas can significantly reduce the costs of electrolysis and increase the efficiency of the process.

Taguchi method of DOE was used to define the optimum spraying conditions. Electric double layer capacitances of the coatings were determined as measures for their electrochemical active surface areas. The results showed higher surface areas for all SPS deposits compared to the APS ones. This can be attributed to the formation of very fine porous agglomerates with an average diameter of 10 μm on the surface of the SPS coatings. ECDLC was increased by a factor of seven for the APS and by a factor of 17 for the SPS-coated samples with the largest surface areas compared to the sandblasted substrate.

The effects of spraying variables of each process on the specific surface areas of the coatings were studied. By the analysis of the results, the standoff distance in APS and current in SPS were identified as the most significant factors affecting the electrochemical active surface areas of the coatings. Generally, generation of larger surface areas was related to the deposition of semi-molten and re-solidified particles and formation of more porous structures. For further increase of the electrochemical active surface area of APS and SPS deposits, more optimization of coating parameters is required. In addition, both APS- and SPS-coated samples need to be reduced to resume the activity of the electrodes.