The HGM/CNx composites were then applied in vertically irradiated PR conditions using small glass reactors (SA = 4.9 cm 2 , V = 46 mL; Figure , ESI) at different composite loadings with both Ni2P and Pt co-catalysts (chemically deposited) to determine appropriate conditions for scaled-up catalyst recyclability trials (Figure ; Table , ESI). As the HGM/CNx (1:3 HGM:melamine) concentration increased from 1.5 mg mL -1 (1.53 mg cm -2 ) to 48 mg mL -1 (49 mg cm -2 ), the specific activity of the composite declined from a maximum of 19.8 to 4.9 µmolH 2 gCN x -1 h -1 with a Pt co-catalyst and 10.0 to 2.2 µmolH 2 gCN x -1 h -1 with a Ni2P co-catalyst. The areal activity, which reports the time-normalised amount of H2 produced per area, regardless of the amount of composite in the reaction, was found to increase with composite loading from 119 to 1109 µmolH 2 m -2 h -1 at 1.5 and 24 mg mL -1 , respectively with a Pt co-catalyst and from 68 to 490 µmolH 2 m -2 h -1 at 1.5 and 24 mg mL -1 , respectively with a Ni2P co-catalyst. This highlights the trade-off between efficient use of photocatalyst material and maximum hydrogen production.