To test this design principle on a larger scale, we developed modular PEC reactors approaching a m 2 scale for the EIC Horizon Prize "Fuel from the Sun: Artificial Photosynthesis" from the European Commission (Fig. , Supplementary Figs. ). To this end, >100 BiVO4 -PVK|GE|Cu92In8 artificial leaves were assembled over a period of 2 months by a team of 5 postgraduate students and postdoctoral researchers, while a 0.70.5 (0.35) m 2 "artificial tree" reactor was constructed by the Mechanical Workshop (Yusuf Hamied Department of Chemistry, University of Cambridge), to house a 1010 array of 10 cm 2 PEC devices (Fig. , Supplementary Fig. ). A medium-scale test was first performed on the rooftop of the Maxwell Centre in Cambridge, to assess the design of our overall PEC platform (including gas collection and light concentration modules) under real-world operating conditions (Fig. , Supplementary Fig. ). To this end, a 1616 cm 2 reactor was fitted with a custom-made 3Dprinted sample holder accommodating a 32 array of 10 cm 2 artificial leaves. In this setup, the reactor was connected to a 2.5 L external gas tank via tubing, which was fitted with on-off and one-way valves. The gas tank was filled with 1 M KOH solution, and connected to a second empty 2.5 L tank to collect spill-over KOH solution, which was meant to prevent overpressure in the PEC reactor. Accordingly, the excess product and CO2 mixture would flow into the gas tank, where CO2 would be captured in form of carbonate, leaving syngas and O2 to displace the KOH solution (Supplementary Fig. , Movie 2). The synthesis of concentrated syngas was essential to obtain a combustible, non-diluted gaseous fuela key evaluation criterium of the Horizon competition.