A reversible photo-electrochemical device operating under concentrated irradiation could offer a stand-alone solution for producing solar fuel (in photo-driven electrolysis mode) and power (in fuel cell mode). This strategy would present the advantage of high mass-specific power density. Herein, we demonstrate such a reversible device in a fully automated and controlled experiment with a high-flux solar simulator, with concentrated solar irradiation ranging from 101 to 518 suns (i.e., 101 to 518 kW/m2). In electrolysis mode, a solar-to-hydrogen efficiency of 18.11% and a current density of 292 mA/cm2 are achieved at 518 suns with a device mass-specific power density of 6.76 W/kg. In fuel cell mode, a stable power density output of 35.7 mW/cm2 at a 50 mA/cm2 load and 48.2% voltage efficiency is achieved in constant gas configuration. Utilizing more permeable and hydrophobic gas diffusion layers on the cathode side of the fuel cell improves water removal and leads to an increase in power output by a factor of more than 2.5.