Effects of Ni Electrode Surface Microstructure on Hydrogen Bubble Generation Behavior in Alkaline Water Electrolysis

Abstract

Introduction Alkaline water electrolysis (AWE) has attracted considerable attention for hydrogen production; however, the generated bubbles can cover the electrode surface at high current densities, significantly increasing the overpotential 1) . Further research is required to understand the effects of the geometric surface structure on the bubble generation behavior and electrolysis performance. This study investigated the relationship between the Ni electrode surface structure and the hydrogen bubble generation behavior using electrochemical measurements and a microscopic video camera. Experimental Polycrystalline Ni rods (⌀3.0 mm) and hemispherical single-crystal Ni 2) (⌀3.0 mm) were used as the working electrodes. The polycrystalline Ni rods had different surface roughness, achieved by polishing the bottom surfaces with papers of different grades ( G p = 240, 1000, 4000, and 8000). Single-crystal Ni electrodes were prepared with surface orientations of Ni(111), Ni(100), and Ni(110) using a controlled-atmosphere flame fusion method 2) . The Ni(110) sample with the smoothest surface was used as the electrode in the experiments. The skewness of the probability density of the surface height ( S sk / -) was measured using confocal laser scanning microscopy (VK-X160, Keyence Co., Ltd.). Electrochemical measurements were performed using three-electrode electrochemical cells with 0.10, 0.50, and 1.0 M KOH solutions as electrolytes. A reversible hydrogen electrode (RHE) and Ni ring (⌀60 mm) were used as reference and counter electrodes, respectively. The current density ( i /A cm −2 ) was normalized to the projected area of the working electrode surface. After pretreatment, linear sweep voltammetry (LSV) was conducted within a potential window ( E ) of 0.00 to −0.40 V at a scan rate of 0.10 mV s −1 . The bubble formation behavior on the electrode surface was monitored using an inverted microscope video camera. To detect and track the time evolution of bubble generation and determine the number of bubbles generated, the bubble-generation images were analyzed using an original program 3) . The i value at the onset point of bubble formation, i b , was determined as the point at which the number of generated bubbles began to increase. Results and Discussion Fig. 1 shows the polarization curve and bubble behavior at different i values ((i)–(iv)) in 1.0 M KOH ( G p = 1000). Two linear regions were observed, with the transition point marked by an orange circle. The slope in the higher negative potential range was comparable to the Tafel slope of the Volmer–Heyrovsky reaction (120 mV dec −1 ) 2) . The red circle in Fig. 1 indicates the onset point of bubble formation, which was situated near the transition point. Hydrogen bubbles were not observed in the lower negative potential range (i) and their generation began at i b (ii). Thereafter, the number and size of bubbles increased (iii). In the higher negative potential range, the bubbles covered most of the electrode surface (iv). The potential at i b is defined as E b . Figure 2 shows the relationship between E b and G p . E b increased with increasing G p , indicating that a rougher electrode surface enhanced bubble nucleation. S sk , which represents the deviation in height distribution from the mean plane of the surface, was introduced to clarify the factors that influenced this result. Fig. 3 illustrates the relationship between E b and S sk , and Fig. 4 shows images of the electrode surface for S sk ≈ 0 (rough) and S sk < 0 (smooth). Upon polishing, large convex asperities are removed from the electrode surface, resulting in a smoother surface ( S sk < 0). Fig. 3 shows that E b is smaller for electrodes with S sk ≈ 0 than for electrodes with S sk < 0. Comparing the surface characteristics of electrodes with S sk ≈ 0 and S sk < 0, those with S sk ≈ 0 had deeper concave features. Deeper concaves result in slower diffusion of the dissolved hydrogen produced by the electrolytic reaction, leading to a higher degree of supersaturation in the concave regions. Because a higher supersaturation enhances bubble nucleation, E b is lower for electrodes with S sk ≈ 0. Conclusion The effects of the electrode surface characteristics on bubble generation during the hydrogen evolution reaction were investigated using a Ni electrode. The polarization curve exhibited two linear regions, and the potential at the onset of bubble formation ( E b ) coincided with the point at which the linear slope changed. E b increased as the convexity of the electrode surface decreased, whereas concave regions enhanced the degree of supersaturation and bubble nucleation. References 1) D. Kitajima, et al. , Electrochim. Acta , 502 , 144772, (2024). 2) K. L. Varvaris, et al ., J. Phys. Chem. C , 127 , 14711-14722 (2023). 3) K. Toyama, et al ., Electrochemistry , 93(2) , 027011 (2025). Figure 1