More importantly, the basic pH solution resulted in a dissolution of the BiVO4 photoanodes, which are only stable under near-neutral pH. This exposed the underlying perovskite light absorbers, indicating degraded devices (metallic Cu reflection) among healthy devices (dark layers) (Supplementary Fig. ). As a result, only 0.64 vol% CO and 0.80 vol% H2 were detected in the large reactor headspace (0.43 vol% CO and 0.58 vol% H2 in the respective gas tank), as certified by the official test report of the competition (Supplementary Data 1). This corresponds to absolute product amounts of 2.39 µmol cm -2 of CO and 1.91 µmol cm -2 of H2 in the reactor headspace, comparable to those of the earlier rooftop test (Fig. ). Despite purging, CO2 content was only 1.61 vol%, whereas N2, O2 and Ar amounted to 97 vol% of the headspace composition (Supplementary Data 1), which indicates air infiltration due to underpressure in the reactor. While these results indicate that further system engineering is necessary, these also provide some general guidelines for PEC reactor design (Text Box 1).