Palladium is one of the most intensely studied model materials for hydrogen in metals. Nanoporous Pd (np Pd) forms a macroscopic sample with a nanoscale network structure. It can efficiently absorb hydrogen due to its large specific surface area and short diffusion paths. Notably, np Pd has been reported to undergo more than 1000 loading/unloading cycles through the $\alpha / \alpha ’$ phase transformation without degradation. This study investigates hydrogen charging kinetics, with a focus on identifying rate-limiting factors and on understanding the limiting subprocess from the perspective of kinetic rate theory. Nanoporous Pd was synthesized via electrochemical dealloying, followed by thermal coarsening. This yields a tunable nanoscale structure with characteristic microstructural length scales ranging from 20 to >200 nm, allowing for precise control over the specific surface area and diffusion length. The hydrogen fraction was controlled electrochemically with the sample wetted by an acidic electrolyte. Hydrogen solubility and interaction dynamics were analyzed based on chronoamperometry and electrochemical impedance spectroscopy. The interfacial hydrogen injection process is identified as the rate-limiting factor for charging/discharging. The ligament size controls interfacial area per volume and, thus, emerges as a critical structural parameter for kinetics. The injection process is further examined in the context of the Pd-H miscibility gap, which is a central characteristic of this and other metal hydride systems. We discuss how the Butler-Volmer equation can be adapted to model the injection rate consistent with the equation of state for the composition-dependent chemical potential at equilibrium in an interacting solid solution. The proposed kinetic model explains observations on how the characteristic charging time in np Pd-H varies with the composition. This provides the basis for further studies of the fundamental mechanisms of hydrogen absorption in metals.