Performance of hydrogen evolution reaction was conducted using 8-types of Ni woven mesh and 5-types of Ni expanded mesh with different wire diameter and pore size. Woven mesh was purchased at NILACO corp (Japan) and expanded mesh was supplied from DEXMET (USA). Woven and expanded mesh was treated in acid (0.1 M HCl solution for 10 min at room temperature) and base solution (1 M NaOH solution for 10 min at room temperature) before hydrogen evolution reaction to remove residual metal powder and organic impurities. The detailed structure parameters of woven mesh and expanded mesh samples was summarized in Tables 1, 2, respectively. Expanded metal sample names in Table 2 were expressed by XNiY-Z, where the X meant nominal thickness (inch), Y represented strand width (inch), and Z indicated the pore width (inch) along the long axis.

The electrochemical active surface area (ECSA) of the woven mesh and expanded mesh was caculated by cyclic voltammetry (CV) in KOH 30 wt% solution. A Pt plate was used as the counter electrode with Hg/HgO reference electrode. CV is measured at scan rates of 10, 20, 50, and 100 mV/s in the range from −1.0 to −0.1 V. The electrical double layer is calculated in the range from −0.4 to −0.2 V and is the point at which a constant potential occurs due to H-adsorption. The electrochemical active surface area was calculated following equation. E C S A ( c m 2 ) = S   ( V ⋅ A ) / v   ( V / s ) Q ( C / c m 2 ) (1) where S is area of electrical double layer form cyclic voltammetry (V∙A), v means scan rate in cycle voltammetry measurements (V/s), and Q indicates electrical charge with monolayer adsorption of hydrogen on Ni plate (C/cm²).

Hydrogen evolution reaction was conducted in a half-cell to investigate overpotential in KOH 30 wt% solution as electrolyte. The general three-eclectrode measurement was performed in electrolytic cell. Complexed-structure materials and Pt plate was used as working and counter electrode, respectively, with Hg/HgO reference electrode. The potential required for the hydrogen evolution reaction was supplied by an SP-240 potentiostat (Biologic, France) with EC-lab software. Current was measured from 0 to 300 mA depending on applied potential.

The overpotential is a major factor that can confirm the performance of the electrolytic cell in the hydrogen evolution reacion. Since Hg/HgO/1M KOH electrode is used as the reference electrode, correction is required to calculate the overpotential. The method of calculating the overpotential is as follows. O v e r p o t e n t i a l ( V ) = | A p p l i e d   p o t e n t i a l ( V H g / H g O ) + 0.097 + 0.059 × pH | (2)

The potential for obtaining a current of 150 mA was compared in this study. The overpotential at 150 mA current of each complex-structured material was calculated and shown in Table 3.

Eight types of meshes and five types of expanded meshes were prepared to determine the correlation between the performance of hydrogen evolution reaction and the structural properties of the mesh. Figure 1 is a light microscopic image of nickel woven and expanded mesh taken with a light microscope.

Detailed structural parameters of the woven mesh are shown in Table 1. The woven mesh with the largest strand width and pore width is 20 woven mesh, and the woven mesh with the smallest strand width and pore width is 200 woven mesh. The 30, 40, and 50 woven meshes have the same strand width, but the pore widths are progressively smaller, meaning that they become denser. 60, 100 woven mesh and 150, 200 woven mesh have the same strand width at 70 and 50 μm, respectively.

Table 2 shows the structure parameters for the expanded mesh (Figures 1H,I), which shows the irregular strand width and random pore width unlike woven mesh. Thus, it is hardly possible to analyze the structure of the mesh.

The new parameter to determine the structural surface area of the material was derived in order to check the relationship between the structural properties of complex structural materials and the overpotential in alkaline water electrolysis. First, the structure of woven mesh was calculated using the method reported in the paper of Zhang et al. (2010).

The woven mesh is composed of a junction segments and an independent segments as shown in Scheme 2A. The independent segment is the shape of a cylinder, and the structure surface area can be calculated as follows; A r e a   o f   i n d e p e n d e n t   s e g m e n t     ( c m 2 )   = π × 1 2 × s   ( c m ) × w   ( c m ) (3) where s and w is strand and pore width (cm), respectively. And the area of the junction can be calculated by the following formula. A r e a   o f   j u n t i o n   s e g m e n t   ( c m 2 )   = 3 2 × π × 1 4 × s 2   ( c m ) (4)

The number of independent and junction segment (Scheme 2B) can be calculated using the number of pores (n). The number of independent segments is 2n(n+1), and (n+1) ² is the number of junction segments. The structural surface area of the woven mesh can be calculated by calculating the area of the independent and junction segments and multiplying each number. S t r u c t u r a l   s u r f a c e   a r e a   o f   w o v e n   m e s h   ( − ) = { 2 n ( c m − 1 ) [ n ( c m − 1 ) + 1 ] } × [ π × 1 2 × s   ( c m ) × w   ( c m ) ] + [ ( n + 1 ) 2 ] × [ 3 2 × π × 1 4 × s 2   ( c m 2 ) ] (5)

Next, the structural surface area of the expanded mesh was calculated. The expanded mesh is divided into two parts: an independent segment and an intersection segment as shown in Scheme 2C. The shape of the independent part is a cuboid, and the area can be calculated as follows. A r e a   o f   i n d e p e n d e n t   seg m e n t ( c m 2 )   = 2 × [ s     ( c m ) + t     ( c m ) ]   × w   ( c m ) (6) where s represents strand width (cm), t means thickness (cm), and w is pore width (cm). The shape of the intersection segment can be calculated using the following formula as a rhombus.   A r e a   o f   i n t e r s e c t i o n   seg m e n t   ( c m 2 )   = 2 × 1 2 × a     ( c m ) × b     ( c m ) (7) where a and b is vertical and horizontal width of rhombus (cm), respectively. The number of intersection and independent segment can be calculated using the number of pores (LWD: n, SWD: m) in the direction of long way of design (LWD) and short way of design (SWD) as shown in Scheme 2D. The number of non-overlapping pores on each side is equal to the number of pores in the LWD multiplied by the number of pores in the SWD. Structural surface area can be calculated using the following formula. S t r u c t u r a l   s u r f a c e   a r e a   o f   e x p a n d e d   m e s h   ( − ) = n ( c m − 1 ) × m ( c m − 1 ) × [ 8 × s   ( c m ) × w   ( c m ) + 8 × t   ( c m ) × w   ( c m ) + 2 × a   ( c m ) × b   ( c m ) ] (8)

The coverage area, which is a 2-dimentional factor, is shown in Scheme 2E and it was calculated using the simple image conversion method, which calculates the 2-dimentional area of a structure via an optical image without considering a special measurement or calculation formula. The area occupied by strand among images of complex electrode photographed through the simple image conversion method was converted into a single color (white). The area ratio of the single color part occupied by the strand in the total area of the converted image is defined as the coverage area. C o v e r a g e   a r e a   ( % ) = A r e a   o f   w h i t e   c o l o r   ( c m 2 ) T o t a l   a r e a   ( c m ) × 100 ( % ) (9)

The calculated structural surface area and coverage area of woven mesh and expanded mesh are shown in Tables 4, 5, respectively. The structural surface area was calculated according to the use of a circular electrode with a diameter of 2 cm.

The manufacturing parameters are strand width and pore width in the manufacturing method of woven mesh. Thus, the ratio of pore width to strand width can also be considered as a structural property. The ratio of pore width to strand width was calculated using the following formula. R a t i o   o f   P o r e   t o   s t r a n d   w i d t h   ( % ) = P o r e   w i d t h   ( c m ) S t r a n d   w i d t h   ( c m ) × 100   ( % ) (10) The ratio of pore width to strand width can be considered as a 1-dimensional structural parameter because it is a comparison between a line (pore width) and a line (strand width). The calculated result of the ratio of pore width to strand width is shown in Table 4.

Hydrogen evolution reaction was performed using eight types of Ni woven mesh as cathode (Figure 2A). Figure 2B is the overpotential at 150 mA current according to the structural surface area, which is a 3-dimentional factor. There was little relationship between the overpotential and the structural surface area.

The coverage area, 2-dimentional factor and overpotential at 150 mA current were correlated in Figure 2C. The coverage area of the woven mesh lower the overpotential in the HER. The effect of the ratio of pore width to strand width was investigated in Figure 2D. The overpotential decrease as the ratio of pore width to strand width converges to 1. It was found that the ratio of pore width to strand width, the 1-dimensional factor has a higher tendency to overpotential than the structural surface area, 3-dimensional factor.

The increase in the coverage area, 2-dimensional area of the working electrode facing the counter electrode, is sufficient for HER. In addition, the ratio of pore width to strand width of woven mesh exhibited low overpotential when mesh strands and pores were uniformly arranged.

Figure 3A shows the measured current according to the overpotential in the hydrogen evolution reaction of the expanded mesh. The overpotential of the four expanded meshes increased as the coverage area increased. The structural surface area, which is a 3-dimentional factor, showed low overpotential. As the structural surface area increased from 0.41 to 2.40 cm², the overpotential decreased from 376 to 322 mV. From the relationship between overpotential and structural properties in the expanded mesh, it seemed that the structural surface area, which is a 3-dimentional factor, has a greater effect than the coverage area, which is a 2-dimentional factor. The ratio of pore width to strand width, a 1-dimentional factor (Figure 2D) considered in the woven mesh, was excluded due to the characteristics of the expanded mesh having an irregular pore structure.

The higher value of electrochemically active surface area (ECSA) shows high performance in an electrochemical reaction. ECSA of woven mesh and expanded mesh were measured and the structural characteristics were compared. The ratio of pore width to strand width of the woven mesh was compared with the electrochemically active surface area (Figure 4A). As the ratio of pore width to strand width converges to 1, it showed a high electrochemically active surface area. And the relationship between the electrochemically active surface area of the expanded mesh and the structural surface area was tested (Figure 4B). The electrochemically active surface area increased as the structural area increased for the expanded mesh, which is in line with the previous study results (Figure 3C).