## 1 Introduction
### Bubble Growth Dynamics
Key processes affected include mass transport, with migration of H+ ions toward the cathode, their reduction into H2 gas at the electrocatalytic surface, and the subsequent transport of products away from the electrode via diffusion and convection [31]. The presence of bubbles reduces the effective active surface area of the electrode, leading to a non-uniform current density distribution. This localized increase in current density accelerates bubble growth and the depletion of reactants at active sites. Bubbles continue growing via diffusion-driven gas transfer and induce moderate convective flows, which can enhance mass transport. Once detached, bubbles generate wake flows that locally mix the electrolyte, reducing concentration gradients and improving reactant supply. However, bubbles also obstruct ion conduction pathways, increasing the effective resistance of the electrolyte and contributing to ohmic losses [31]. The relationship between current density (\(j\)) and fractional bubble coverage (\(\sigma\)) can be described as [47]  
\[j=\frac{I}{A\left(1-\sigma\right)}, \tag{6}\]  
where \(I\) presents electrical current and \(A\) area of the electrode surface, therefore \(I/A\) presents a superficial current density, which must be strictly distinguished from the actual current density. At low current densities, kinetic overpotentials dominate, and bubble coverage (\(\sigma\)) primarily determines the total overpotential. In contrast, at higher current densities, ohmic losses become the dominant factor due to dynamic changes in resistance as bubbles nucleate, grow, and detach. Periodic bubble behavior influences the overall resistance of the electrolyte, while associated convective flows mitigate concentration overpotentials by enhancing mass transport and reducing local concentration gradients. The turbulence induced by bubble detachment further improves mixing efficiency, amplifying the positive effects on mass transport. However, this periodic bubble activity also adds complexity to the dynamic electrochemical environment. The formation of multiple bubbles in practical electrochemical systems is typically uncontrolled. [PERSON] et al. [48] suggested some guidelines and provided simulations, showing that engineering cavity sizes and modifying the gas diffusion layer thickness can tune the process of bubble nucleation. Their numerical simulation was later used for theoretical model validation by [PERSON] et al. [49], where good agreement of the lower and the upper bound for the size of the effective nucleation cavity was confirmed, as the results proved consistent with experimental findings. For in-depth research on hydrogen bubble evolution, isolated bubble events, and the study of their impact on gas-evolving electrodes is the main objective, leading researchers to develop methods that enable the observation of single bubbles. This is often achieved using nano- or micro-sized electrodes that facilitate desired phenomena and allow for detailed investigations of individual bubble behavior associated with electrochemical phenomena [6, 31, 36]. From the applicative point of view, research on microelectrodes is fundamental for understanding and optimizing processes on porous electrodes, their modeling, and a better understanding of the forces that determine bubble detachment, and further provides guidelines for porous catalyst research and development [29, 50, 51]. The present study aims to provide further information on the bubble growth regimes and related parameters, such as the departure frequency and diameter. Specifically, the effect of the electrical potential on the aforementioned parameters is focused on. A series of experiments is performed with 100 \(\upmu\)m platinum microelectrodes operating in a 0.5 M aqueous solution of H2 SO4. High-speed visualization is coupled with data acquisition to analyze the behavior of the hydrogen bubbles during their growth, coalescence with adjacent bubbles, and departure from the microelectrode.