Surface structure and catalytic activity of amorphous metallic alloys Fe-Nb-B-RE (RE = Y,Gd,Tb,Dy) in alkaline solution

Introduction

The Fe-based amorphous alloys possess some outstanding physical and chemical properties (high specific strength, elastic modulus, excellent corrosion resistance and low cost), which are different from the appropriate characteristics of the conventional crystalline alloys (Botta et al., 2013; Koga et al., 2014).

The hydrogen evolution reaction (HER) has been extensively investigated using both wide scale electrode materials and solutions (Navarro-Flores et al., 2005). Different metals and alloys are used as electrode materials for HER. They exhibit high catalytic activity, chemical and corrosion stability and low cost for their production (Safizadeh et al., 2014). HERs have been widely investigated in alkaline solutions on various types of electrodes in order to get electrocatalytic materials that are effective and stable (Sequeira et al., 2011). The characteristics of the amorphous and nanocrystalline metallic electrodes can be modified by changing the elements’ composition and geometric surface area (Domínguez-Crespo et al., 2005).

The aim of this study was to investigate the morphology and electrocatalytic activity of the Fe₈₂Nb₂B₁₄RE₂ (RE = Y, Gd, Tb, Dy) amorphous and nanostructured samples for HER in alkaline solution. The alloy Fe₈₄Nb₂B₁₄ was used as the reference alloy.

Materials and methods

Іnvestigations were carried out for five amorphous alloys in the form of strips (thickness: 20–25 µm and width: 3 mm): Fe₈₄Nb₂B₁₄, Fe₈₂Nb₂B₁₄Y₂, Fe₈₂Nb₂B₁₄Gd₂, Fe₈₂Nb₂B₁₄Tb₂ and Fe₈₂Nb₂B₁₄Dy₂. The alloys were prepared by method melt spinning on a copper wheel with circular velocity 30 m/s at helium atmosphere were received the investigated alloys. The melts were prepared from pure Fe and B and the binary REFe₂ and NbFe₂ compounds. Purity (wt. %) of the initial elements was as follows: Fe – 99.99, Y – 99.96, Gd – 99.96, Tb – 99.96, Dy – 99.96 and B – 99.96.

The thermal stability of the amorphous alloys was studied by the differential scanning calorimetry (NETZSCH DSC 404) method at a scan rate of 10 K/min.

The hydrogen evolution investigations on the as-cast and annealed AMA were carried out in a three-electrode cell using potentiostatic mode method. These measurements were carried out in 1 M KOH solution, using AUTOLAB electrochemical system. The auxiliary electrode was a platinum mesh and the reference electrode was of the type Hg/HgO/1 M KOH. The hydrogen evolution was performed for 1 h. The hydrogen evolution study on the AMA was carried out five times.

The topography and morphology of the as-cast and annealed samples of Fe₈₂Nb₂B₁₄RE₂ (RE = Y, Gd, Tb, Dy) amorphous alloys were analysed using the atomic force microscopy (AFM) and the scanning probe microscope SolverProM.

Results and discussion

Figure 1 shows the DSC curves vs. temperature determined for all the examined alloys at a heating rate of 10 K/min. DSC curves exhibit two peaks indicating the two-stage crystallization process in the alloys. In Chrobak et al. (2011), it was shown that the crystallization of the Fe₈₂Nb₂B₁₄RE₂ (RE = Y, Gd, Tb and Dy) amorphous alloys proceeds in two stages. Table 1 shows the temperatures of the first (T_X1) and the second (T_X2) crystallization peaks for the examined alloys. The substitution of Fe atoms in the amorphous Fe₈₄Nb₂B₁₄ alloy by 2 at.% RE leads to increase of nanocrystallization temperature on 110 K and accelerates nucleation nanocrystals of a-Fe and Fe₂₃B₆ phases with 15–20 nm in size.

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Figure 1.

DSC thermograms of the alloys: 1: Fe₈₄Nb₂B₁₄, 2: Fe₈₂Nb₂B₁₄Y₂, 3: Fe₈₂Nb₂B₁₄Gd₂, 4: Fe₈₂Nb₂B₁₄Tb₂, 5: Fe₈₂Nb₂B₁₄Dy₂ (heating rate 10 K/min).

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Table 1.

Temperatures (±5 K) of the first (T_Х1) and the second (T_X2) stages of crystallization of alloys at heating rate 10 K/min.

Alloys	TХ1	TX2
Fe84Nb2B14	696	811
Fe82Nb2B14Y2	804	1008
Fe82Nb2B14Gd2	798	995
Fe82Nb2B14Tb2	801	1000
Fe82Nb2B14Dy2	801	1000

The surface relief of the initial and annealed alloys for 1 h at TX1 temperature (see Table 1) was studied by AFM. Thermal treatment of the amorphous ribbons changed the characteristics of the surface. All AMAs have amorphous structure (Chrobak et al., 2011). 2D AFM-images of the initial and the annealed Fe₈₂Nb₂B₁₄Dy₂ samples are presented in Figure 2. Figure 3 shows the distribution of the structural units vs. their dimensions of the initial and the annealed samples. The size and counts of the structural units changed on the surface of alloys after their heat treatment. For example, globules of the initial AMA Fe₈₂Nb₂B₁₄Y₂ (maximum counts of the structural units ∼540) were mostly ranging in size from 20 to 23 nm and the globules of the annealed AMA Fe₈₂Nb₂B₁₄Y₂ (maximum counts of the structural units ∼400) were mostly ranging in size from 23 to 25 nm (Figure 3).

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Figure 2.

2D AFM-images of the initial (a) and the annealed (b) Fe₈₂Nb₂B₁₄Dy₂ samples.

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Figure 3.

Distribution of the structural units vs. dimensions of the initial (a) and the annealed samples (b): 1: Fe₈₄Nb₂B₁₄, 2: Fe₈₂Nb₂B₁₄Y₂, 3: Fe₈₂Nb₂B₁₄Gd₂, 4: Fe₈₂Nb₂B₁₄Tb₂, 5: Fe₈₂Nb₂B₁₄Dy₂.

The adhesion work on phase boundaries of Fe₈₄Nb₂B₁₄ alloy/distilled water is 88.5 mJ/m². Alloying of Fe₈₄Nb₂B₁₄ alloy by RE leads to increased the values of the work of adhesion. The highest values of the adhesion work observed in a contact of as-cast Fe₈₄Nb₂B₁₄, Fe₈₂Nb₂B₁₄Tb₂ and Fe₈₂Nb₂B₁₄Dy₂ alloys with 1 M aqueous KOH solution (Figure 4).

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Figure 4.

The work of adhesion on the interface of the initial AMA – distilled water (a); initial AMA – 1 M aqueous KOH (b); annealed AMA – 1 M aqueous KOH (c). AMAs: 1: Fe₈₄Nb₂B₁₄, 2: Fe₈₂Nb₂B₁₄Y₂, 3: Fe₈₂Nb₂B₁₄Gd₂, 4: Fe₈₂Nb₂B₁₄Tb₂, 5: Fe₈₂Nb₂B₁₄Dy₂.

The Fe₈₂Nb₂B₁₄RE₂ amorphous alloys were tested as electrodes for the HER in 1M KOH solution. The intensive evolution of hydrogen begins at – −1.0 V. The volume of the evolved hydrogen for the examined alloys was calculated using Faraday's law (see Table 2). RE alloying for 2 at. % of the basic Fe₈₄Nb₂B₁₄ alloy increases the catalytic activity of the Fe₈₂Nb₂B₁₄RE₂ amorphous alloys for HER in alkaline solution. Thermal treatment has a positive effect on their activity in the HER due to the enhancement of roughness of electrode surface of alloys (Figure 5). Іn particular, heat treatment increased volume of evolved hydrogen to 141 times for Fe₈₄Nb₂B₁₄ alloy and for Fe₈₂Nb₂B₁₄Y₂, Fe₈₂Nb₂B₁₄Gd₂, Fe₈₂Nb₂B₁₄Tb₂ Fe₈₂Nb₂B₁₄Dy₂ alloys to 73, 57, 21, 19 times, respectively.

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Table 2.

Volume of evolved hydrogen for the initial and the annealed amorphous alloys.

State Alloy	VH2, ml/cm2
	Initial	Annealed
Fe84Nb2B14	0.15	21.14
Fe82Nb2B14Y2	0.25	18.36
Fe82Nb2B14Gd2	0.54	30.68
Fe82Nb2B14Tb2	0.72	15.04
Fe82Nb2B14Dy2	0.29	5.42

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Figure 5.

The average surface roughness of the alloys: 1: Fe₈₄Nb₂B₁₄, 2: Fe₈₂Nb₂B₁₄Y₂, 3: Fe₈₂Nb₂B₁₄Gd₂, 4: Fe₈₂Nb₂B₁₄Tb₂, 5: Fe₈₂Nb₂B₁₄Dy₂. The light grey and grey columns are characteristics of the initial and the annealed samples, respectively.

Conclusions

A two-stage crystallization process was observed for Fe₈₂Nb₂B₁₄RE₂ (RE = Y, Gd, Tb, Dy) and Fe₈₄Nb₂B₁₄ amorphous alloys. Alloying of the reference alloy Fe₈₄Nb₂B₁₄ by 2 at. % RE increases the crystallization temperatures of Fe₈₂Nb₂B₁₄RE₂ alloys for ∼110 K and their roughness as well.

Thermal treatment and alloying lead to increasing volume of evolved hydrogen for the Fe₈₂Nb₂B₁₄RE₂ amorphous alloys due to the enhancement of roughness of the electrode surface.