Extended Data Fig. 1: Photographs of the solar fuel system at ETH Zurich.

a) The solar redox unit, comprising the primary sun-tracking solar paraboloidal concentrator coupled to a secondary planar rotating reflector, and the two solar reactors at the foci. b) The two solar reactors, water-cooled calorimeter, and Lambertian target for solar radiative power measurements (seen via the secondary reflector).