Electrodeposition of Electroactive Co–B and Co–B–C Alloys for Water Splitting Process in 8 M NaOH Solutions
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The aim of this work was to determine parameters of obtaining Co–B and Co–B–C alloys characterized by low overpotential in hydrogen evolution reactions. There were tests performed to examine the influence of borax and arginine concentration changes as well as the value of cathodic current intensity on the composition, morphology, synthesis current efficiency, and effectiveness of hydrogen evolution. Carbon was added to Co–B alloys to improve its properties of water decomposition in 8 M NaOH solution at 90 °C. The results of the experiments confirmed a decrease in overpotential values from 48 mV/dec for Co–8.5B alloy to 33 mV/dec for Co–8.2B–9.6C alloy obtained at the highest concentration (0.1 M) of arginine in the electrolyte. It was observed that alloys of good catalytic properties featured an amorphous structure. The coatings were analyzed with the use of a scanning electron microscope (SEM) and X-ray diffraction (XRD). The content of boron was assessed based on the glow discharge spectroscopy (GDS) method, whereas carbon was analyzed with the method of adsorption spectroscopy of infrared radiation.
KeywordsElectrodeposition Hydrogen evolution Cobalt alloys Non-noble metals catalyst Metal–carbon alloys
Recently, there has been more and more focus on the development and implementation of environmentally friendly technologies. One of the concepts elaborated by Japanese scientists is the application of sustainable energy in the form of methane through electrolytic production of hydrogen [1, 2, 3]. It relies on utilization of CO2 generated by mankind to methane. The source of the hydrogen essential in this reaction will be gaseous H2 obtained in the process of seawater electrolysis. The electrolysis will be powered by current generated by solar panels. The obtained methane can be easily transported, stored, or converted into other energy carriers. One of the key materials required in this technological process are cathodes for hydrogen production.
One of the currently applied electrode materials in industrial production of hydrogen is metallic nickel and Ni-based alloys [4, 5, 6]. It mainly results from their good catalytic properties and relatively low cost in comparison to highly active but expensive platinum metals. Improvement of catalytic efficiency can be done through creation of alloys characterized by the presence of phases of low hydrogen evolution overpotential. One of the possible solutions is the creation of platinum metal alloys with nickel or cobalt—Ni–Pd [7, 8], Co–Pd , Co–Rh , and Co–Ru [11, 12]—which enables to lower the content of expensive metals in the electrode while maintaining very good properties. Addition of molybdenum or tungsten allows to lower both the value of overpotential and corrosive resistance in comparison to pure Ni or Co [13, 14, 15, 16, 17]. Moreover, in many cases, the presence of a magnetic field during synthesis can also decrease the overpotential [18, 19, 20]. It was also found that addition of carbon in alloys of cobalt with molybdenum significantly lowered the energy of electrons bonds Co 2p3/2 and Mo 3d5/2.
It means transfer of the charge from carbon to metal in alloy and causes a faster transfer of the charge from metal to the hydrogen atom and thus acceleration of the hydrogen evolution process on such an alloy. This phenomenon also takes place in the case of alloys Ni–Mo–C, Co–W–C, and Ni–Fe–C [21, 22, 23, 24]. Combining nickel, molybdenum or cobalt, and non-metals into semiconducting compounds Ni–S , Co–Se , Mo–Se [26, 27], and Mo–S  has been intensely investigated recently. It results from a low price of the metals and their wide accessibility while maintaining catalytic properties close to the platinum group of metals.
The work presents test results concerning electrochemical synthesis of Co–B alloys. They featured good electrocatalytic properties. Moreover, addition of carbon in the form of arginine to the bath allowed to obtain Co–B–C alloys which presented higher electrochemical activity at increased corrosive resistance.
The base electrolyte was made by dissolving cobalt sulfate (CoSO4∙7H2O), ascorbic acid (C6H8O6), ammonium chloride (NH4Cl), sodium lauryl sulfate (SLC), and saccharine in distilled water. In order to obtain Co–B alloys, 0–0.15 M of borax (Na2B4O7∙10H2O) was added to the base electrolyte. The source of carbon was from 0.001 to 0.1 M of arginine added to the electrolyte during electrodeposition of Co–B–C alloys. Arginine can dissociate in water and create electroactive ions, which can be a precursor for carbon electrodeposition.
Composition of the base electrolyte
The electrochemical processes were realized on the potentiostat Bio-Logic SP-200 equipped with EC-LAB software. Electrodeposition was performed in a measuring cell where the anode was a platinum sheet of 6 cm2 area, and the deposition process was conducted on previously etched copper plates of 2.8 cm2. The distance between electrodes is approximately 7 cm. Preparation of the working electrode relied on chemical polishing in a mixture of concentrated acids H3PO4, HNO3, and CH3COOH in a volume relation of 1:1:1 at temperature 65 °C for 45 s.
The obtained alloys were analyzed with regard to the content of cobalt, boron, and carbon. The cobalt content was determined with the EDS method while for boron, the glow discharge spectroscopy (GDS) method (Jobin Yvon, 10,000 RF) was applied, whereas the content of carbon was assessed by burning and then analyzing with absorption spectroscopy of infrared radiation.
Morphology of the deposited alloy surface was examined with the scanning microscope Philips XL30. The phase composition of the cathodic deposition was assessed with the use of X-ray diffraction analysis applying filtered X-rays Cu Kα. The size of the cathodic deposit grains was estimated from the half-width of the peak of the intensity of Cobalt-fcc using the Scherrer equation.
Catalytic activity of the obtained alloys was tested during cathodic evolution of hydrogen in 8 M NaOH at temperature of 90 °C. The reference electrode was a calomel electrode, whereas the anode was a platinum sheet of 6 cm2 area. The process of hydrogen evolution was carried out on a specially designed plastic container. The material of the cell has good resistance against strong alkaline environment. Directional coefficients of the lines obtained in galvanostatic polarization measurements were determined.
Results and Discussion
Obtaining Co–B Alloys
The Co–B alloys were from the base electrolyte to which borax was added (0–0.15 M). Moreover, sodium lauryl sulfate (SLC) and saccharine were added to the electrolyte. For the prepared solutions, the process of electrolysis was conducted for 2 h for three different current densities (250, 500, and 1000 A/m2).
On the basis of the obtained experimental data, it can be observed that the increase in the current density is accompanied by a decrease in cathodic current efficiency.
- φ –
cathodic efficiency [%]
mass of deposited coating [g]
theoretical mass, based on Faraday’s law [g]
current intensity [A]
time of electrodeposition [s]
Faraday’s constant [C]
- m Co , m C , m B
mass of cobalt, boron, and carbon in coating [g]
- M Co , M C , M B
molar mass of cobalt, boron, and carbon [g/mol]
Structure of Co–B Alloys
For all the obtained alloys, the diffraction X-ray analysis was conducted applying filtered X-ray radiation Cu Kα. The figures present diffractograms of cathodic deposits of Co–B alloys deposited from solutions of borax concentrations from 0 to 0.15 M for three different cathodic current densities (250, 500, and 1000 A/m2).
Grain sizes for individual samples were estimated from diffractograms of cathodic deposits of Co–B alloys through an analysis of the peak half-width of Miller indicators  for a regular face-centered cubic structure. For alloys of amorphous structure, the crystalline size was assessed from the half-width of the blurred peak present on the diffractograms.
For samples obtained in the electrodeposition process for a current density of 1000 A/m2 (Fig. 3c), occurrence of peaks originating from the fcc structure was noticed. Also, a transition from a hexagonal structure into an amorphous one was observed. The crystal size for deposited pure cobalt was decreased from 6.4 to 2.3 nm. Addition of borax to the electrolyte caused an increase in their size to 4.9 nm for the Co–8.5B alloy, and then the size fell to 2.3 nm which is connected with a change of the structure from hexagonal to amorphous.
SEM micrographs of Co–B alloys electrodeposited for 2 h in base electrolyte (0.2 M CoSO4∙7H2O) with different concentrations of Na2B4O7
The analysis of the SEM images indicates a high correlation of the alloy grain shape and the applied current density in the electrodeposition process, whereas the grain size is determined by concentration of borax in the electrolyte used in the deposition process. The presented images were taken at ×500 magnification. The cathodic deposit is compact and homogeneous; there are no cracks on it. For current density of 250 A/m2, the grain size is growing with the increase of borax concentration in the electrolyte used to obtain the Co–B alloy until reaching a size of 20 μm for a solution of the highest concentration of borax (0.15 M).
For coatings obtained at a current density of 500 A/m2, the grains are very small for 0.05 M of borax in the electrolyte. For 0.1 M of borax, creation of bigger clusters of deposited alloy is observed. An increase in borax content makes the structure more irregular with a grain size around 30–40 μm which is visible for a concentration of 0.15 M.
For the highest applied current density of 1000 A/m2, the grain shape is irregular. An increase of borax concentration in the electrolyte is accompanied by a more and more expanded structure, and grain borders are clearly visible. The grain size changes sharply with an increase of borax concentration in the electrolyte which results from a change of the structure and the intensity of hydrogen evolution, which have a strong impact for surface development.
Hydrogen Evolution of Co–B Alloys
The change in boron content in the Co–B alloy as well as the structure change of the obtained alloy probably significantly influence the alloy’s electrocatalytic properties. In order to follow the changes, tests were made conducted on hydrogen evolution on Co–B alloys at 90 °C in 8 M NaOH.
Polarization curves comprise two sections of different directional coefficients of the Tafel equation. The first area of the polarization curve is the area of the examined inclination of the Tafel slope in the process of hydrogen evolution at low current densities. The second area of the polarization curve is the area in which the basic conditions of the Tafel slope occurrence are not met. Hydrogen evolution on this area takes place violently.
An analysis of curves for Co–B alloys deposited on galvanostatic mode from solutions of different borax concentrations at a density 250 A/m2 (Fig. 4a) suggests that an increase in boron content in the alloy changes the inclination of the Tafel slope from 123 mV/dec for Co–7.4B alloy to 69 mV/dec for the alloy of the highest borax concentration in the electrolyte. It indicates differences in alloy activity for different borax contents. An increase in boron content positively influenced catalytic properties. The mechanism of hydrogen evolution process is not defined and depends on the surface morphology and occurrence of domains of nanocrystalline and amorphous structures. For alloys obtained at a current density of 500 A/m2 (Fig. 4b), it is visible that an increase in borax content in the electrolyte used for obtaining Co–B alloys does not influence the alloy catalytic activity. Inclination of the Tafel slope is constant and amounts at 160 mV/dec. For the highest current density of 1000A/m2 (Fig. 4c), the galvanostatic curves show that the best catalytic properties are found in the Co–8.5B alloy as the Tafel slope inclination is the lowest and amounts at 48 mV/dec. It was obtained from a solution of 0.075 M borax concentration in the electrolyte. Further increase in borax concentration in the electrolyte caused an increase of the Tafel slope inclination and subsequent worsening of catalytic properties. In the discussed case, the Tafel slope inclination is close to the value of 35 mV/dec; therefore, it can be claimed that the slowest phase is the recombination of two adsorbed hydrogen atoms with creation of hydrogen molecules. The increase of the catalytic properties is due the mixture of amorphous Co and CoB phases with lover overpotential of hydrogen evolution present. We have to take into consideration that the irregular agglomerates shown on SEM images for 0.15 M of borax in electrolyte obtained in the highest current density can be connected with improvement of the catalytic properties. Their shape is very well-developed, and it is increasing the active center on the surface of the electrode. Moreover, the hydrogen evolution reaction takes place during the synthesis which can affect the surface evolution process.
Obtaining of Co–B–C Alloys
The Co–B–C alloys were obtained in the process of electrodeposition from the electrolyte containing aqueous solutions of cobalt sulfate (CoSO4∙7H2O, 0.2 M), ascorbic acid (C6H8O6, 0.2 M), ammonium chloride (NH4Cl, 0.2 M), borax (Na2B4O7·10H2O, 0.1 M), and arginine concentrated from 0 to 0.1 M. Moreover, sodium lauryl sulfate (SLC) and saccharine were added to the electrolyte. For the prepared solutions, the process of electrolysis was conducted for 2 h for three applied current densities (250, 500, and 1000 A/m2).
Structure of Co–B–C Alloys
In Fig. 8a, the increase in arginine concentration in the electrolyte for alloys deposited at a current density of 250 A/m2 caused an increase in crystalline size from 4.5 to 5.6 nm for a concentration of 0.005 M. Then, a sharp fall of the crystal size to 2.8 nm for the Co–5.6B–6.6C alloy was observed. It is connected with the increase in boron content in the alloy. Next, a gradual increase in Co crystal size is observed at 4.9 nm for an alloy deposited from the electrolyte of the highest concentration of arginine 0.1 M. For the Co–1.7B–5.3C alloy, an amorphous structure was observed. An increase of carbon in the alloy makes the structure to become a mixture of amorphous and hexagonal structures. Further increase in carbon content in the alloy causes a transition of amorphous structure into a hexagonal one for an alloy deposited from the solution of the highest arginine concentration.
In the case of current density of 500 A/m2 (Fig. 8b), the increase of arginine in the electrolyte causes an increase in crystal size from 2.0 nm for the Co–9.7B alloy to 3.5 nm for the Co–5.0B–10.2C alloy. It results from the creation of an amorphous structure for the Co–6.8B–7.0C alloy. Further increase of arginine concentration in the electrolyte for obtaining Co–B–C alloys caused a gradual increase of the grain size. It is connected with occurrence of a hexagonal structure on the basis of the amorphous one. An increase in carbon content in the alloy is accompanied by an increase of the hexagonal structure participation in the alloy structure to finally transit into it completely for Co–5.0B–10.2C.
Co–B–C alloys deposited from solutions of arginine concentration from 0 to 0.1 M for current density 1000A/m2 are characterized by an amorphous structure which is clearly seen for Co–9.5B–4.3C and Co–8.6B–6.3C alloys deposited from solutions of arginine concentration from 0.001 to 0.005 M. Further increase of arginine concentration in the electrolyte causes an increase of crystalline size up to 4.6 nm for the alloy of the highest content of carbon. It is caused by occurrence of a hexagonal structure on the basis of the amorphous one for the Co–5.7B–6.7C alloy. Significant changes occurring in the estimated cobalt crystal size for a current density of 500 and 1000 A/m2 are likely to result from application of high current density and differentiation of composition of the obtained cathodic deposits. Changes in the crystalline size of alloys obtained during electrodeposition in three applied current densities are visible in SEM photographs obtained with the use of a scanning microscope. But they are creating agglomerates of crystals, and the size of particles is much bigger.
SEM micrographs of Co–B–C alloys electrodeposited for 2 h in the electrolyte (0.2 M CoSO4∙7H2O, 0.1 M Na2B4O7) with different concentrations of arginine
Hydrogen Evolution on Co–B–C Alloys
The increase in carbon content in the alloy made the points on the galvanostatic curve in the process of hydrogen evolution to move away. It means differences in activities of alloys obtained from electrolytes of different arginine contents. Mechanisms of hydrogen evolution on Co–B–C alloys are the same as on Co–B alloys. For the Co–8.2B–9.6C alloy deposited at a current density of 250 A/m2, the Tafel slope inclination amounts at 33 mV/dec which means that the slowest phase is the reaction of recombination of two adsorbed adjacent atoms with creation of a hydrogen molecule. For Co–B–C alloys deposited at a current density of 500 A/m2, the Tafel slope inclination is about 66 mV/dec for all alloys with arginine. The mechanism of the hydrogen evolution process is not defined, and it depends on morphology and occurrence of domains of amorphous and hexagonal structure in the structure. For alloys deposited at a density of 1000 A/m2, the Tafel slope inclination varies between 42 and 46 mV/dec for all Co–B–C alloys. It suggests that the slowest phase is the recombination of two adsorbed atoms with creation of a hydrogen molecule. Although Co–B–C alloys were usually of hexagonal structure, addition of carbon to Co–B alloy caused that catalytic properties of the alloys were better than catalytic properties of Co–B amorphous alloys.
The hydrogen evolution electroactivity of electrodeposited Co and Co-based alloys in alkaline solutions
The best conditions for electrodeposition of Co–B–C alloys were obtained in the electrolyte of 0.1 M borax concentration and 0.005–0.05 M of arginine; the obtained alloys were characterized by a mixture of structures: nanocrystalline and amorphous.
The highest cathodic current efficiencies in the process of Co–B and Co–B–C alloy deposition were observed for current density of 250 A/m2, at high borax concentrations in the electrolyte.
- Creation of the amorphous structure was observed only for high content of boron in Co–B alloy.
The process of hydrogen evolution on Co–B–C alloys significantly relies on the current density applied to obtain the alloy; the most favorable was applying alloys obtained during deposition at the highest current density; alloys obtained this way featured effective hydrogen evolution, and the phase determining the reaction velocity was recombination of two adsorbed atoms with creation of a hydrogen molecule.
Addition of carbon and creation of a three-component alloy Co–B–C improved the alloy activity in the process of hydrogen evolution.
Addition of carbon decreased the Tafel slope inclination to the value of 33 mV/dec for the Co–8.2B–9.6C alloy and improved efficiency of hydrogen evolution.
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