## 3 Results and Discussion
### Temporal Dynamics of the Electrical Current Density
These peaks correspond to moments when bubbles detach from the electrode surface, temporarily increasing the measured current density, a phenomenon that is shown in detail in Figure 6a. The comparison highlights how the current density evolves over the 200 s measurement. The peak value was evaluated for potentials from 2.9 to 3.5 V, and a comparison between several measurements is provided for the highest applied potential, showing that the scatter is considerably lower than the difference between measurements at different potentials. The highest ratio represents the smallest difference between the first and the last peak. The low ratio at potentials of 3.2 and 3.3 V indicates that although bubbles merge into a single bubble detaching from the electrode surface, this region is unstable. Figure 6a presents an example of one bubble growth cycle at a potential of 3.4 V, where a single coalesced bubble forms. As the previous coalesced bubble detaches from the microelectrode, small bubbles immediately start growing on the surface. For the presented example, the rapid bubble diameter growth in the first half of a second is well correlated with Equation (2) and observed through the drop of electrical current density. Afterwards, the growth rate decreases as well as the current density variation during the rest of the coalesced bubble's growth. Figure 4: Side-by-side comparison of bubble diameter at different regimes/characteristics of electrical current density curves. The microelectrode surface is denoted with a red line. Figure 5: Comparison of the first and last peak value of measurements at different set values of applied electrical potential (**a**) and comparison between several measurements at the same applied potential of 3.5 V (**b**). Figure 6: (**a**) Example of hydrogen bubble growth at an electric potential of 3.4 V. Each bubble departure is marked by a temporary jump in the electrical current density. (**b**) The difference between the bubble center from the electrode surface and the bubble radius. The detachment of coalesced bubbles from the electrode surface is primarily driven by buoyancy forces [25; 26; 52]. As the bubble grows, the buoyancy force increases until it surpasses the adhesive forces holding the bubble to the electrode, leading to detachment. As the bubble detaches from the surface, the electrode surface is exposed to the electrolyte, enhancing the active area available for the HER, resulting in an increase in the electrical current density. The minimum value of electrical current for the given example is reached approximately at the halfway point of the bubble growth period, indicating that bubble is slowly reaching the final departure diameter and the buoyancy force is starting to prevail, causing the large, coalesced bubble to start lifting from the electrode surface, resulting in enhanced active area of the electrode surface. Figure 6b presents the difference between the vertical distance of the bubble center from the electrode (\(y\)) surface and the bubble radius (\(r\)). At the beginning of the growth cycle, bubbles on the surface have the shape of a truncated sphere, resulting in negative \(y\)-\(r\) values. After the 0.5 s time mark, a bubble starts to form into a complete sphere and starts detaching from the surface, while the emerging microbubbles beneath prevent a full detachment. Based on the provided trend, we postulate that growing current density prior to the bubble detachment, i.e., see Figure 6a between 1.2 and 2.2 s, is due to the slowly and continuously rising bubble that, at some point, creates enough space for enhanced nucleation of microbubbles. This enhancement might be due to improved diffusion of the supersaturated liquid that governs the microbubble growth. This effect, partially observed in previous studies [29; 53], remains to be further analyzed and would require advanced diagnostics of nucleation site distribution where microbubbles form.