PV-driven green hydrogen production is instrumental for achieving sustainable development goals globally. As a synergetic effort to enhance electrocatalytic water splitting performance, structuring electrodes is equally important as increasing the intrinsic activity of catalysts for practical applications. Hierarchically porous electrodes offer abundant cavities for gas nucleation and reduced contact line for fast departure of hydrogen/oxygen bubbles in order to minimize the bubble coverage on catalyst and the associated transport overpotential. With microscopic high-speed imaging, we capture the dynamics of bubbles generated on porous electrode surface and reveal the correlations among the bubble population, bubble number density, bubble departure frequency with bubble overpotential and catalytic performance. Our characterization results elucidate the fundamental roles of electrode structure and interfacial mass transport in reducing overpotential of water splitting, including bubble ohmic overpotential, bubble overpotential and concentration overpotential. By tailoring the electrode surface structures with 2D materials, we have observed various oxygen bubble behaviors at the electrolyte-electrode interface, which result in different transport energy losses. In addition, based on the spin-polarized density functional theory, first-principles simulation is carried out to probe more insights into pure/doped catalyst-substrate interface, where the charge transfer is influenced by interfacial bonding and reconstruction.