Si/GaN photocathode has been demonstrated with long-term stability for more than 3,000 hours with high efficiency (ABPE ~11.9%) and high photocurrent density (~35 mA/cm2). However, the fundamental understanding of the mechanism that leads to such extraordinary performance still remains elusive. In this project, we demonstrate that Si/GaN photocathodes can generate sustained hydrogen production with a ~100% Faradaic efficiency (FE) without the use of any electrocatalyst. We also perform accelerated testing and demonstrate that the Si/GaN photocathode can sustain high current density under intensified 3.5 suns illumination for more than 150 hours. We observe that under operating conditions, the photocurrent density and onset potential of Si/GaN photocathodes self-improve. Specifically, we utilize a suite of advanced characterization techniques and first-principles calculations to elucidate the origins of this self-improving behavior. Under operating conditions, Si/GaN photocathodes are subjected to a chemical transformation at the GaN surface that provides protection and enhances the PEC performance by adding active catalytic sites for hydrogen evolution reactions. To the best of our knowledge, this is the first time that protected Si photocathodes show such a stability and FE without the use of any electrocatalysts, as the result of a beneficial chemical transformation. This study reveals a self-improving behavior of Si/GaN photocathode active for hydrogen production, and sets an investigation paradigm for fundamental understanding of the pivotal role of chemical transformation in PEC water splitting. The gained knowledge from this work can provide feedback to the further optimization of the PEC devices.