<h2>Summary</h2> Sunlight-driven photocatalytic water splitting has been investigated as a potentially scalable and economically feasible means of producing renewable hydrogen. However, reactors suitable for efficient water splitting and prompt collection of gaseous products on a large scale have yet to be demonstrated. Here, we demonstrate a sunlight-powered water-splitting reactor using a fixed Al-doped SrTiO<sub>3</sub> photocatalyst and address the key issues in the reactor design associated with the scale-up. A panel reactor filled with only a 1-mm-deep layer of water was capable of rapid release of product gas bubbles without forced convection. A flat panel reactor with 1 m<sup>2</sup> of light-accepting area retained the intrinsic activity of the photocatalyst and achieved a solar-to-hydrogen energy conversion efficiency of 0.4% by water splitting under natural sunlight irradiation. The concept of a readily extensible water-splitting panel is a viable means for large-scale production of low-cost renewable solar hydrogen.

A Particulate Photocatalyst Water-Splitting Panel for Large-Scale Solar Hydrogen Generation