Non-selective charge transfer to metal co-catalysts limits oxygen evolution photocatalysis in cobalt-doped carbon nitrides

Metal functionalisation of organic photocatalysts for solar fuels production is a common design strategy. However, herein we demonstrate that recombination on non-charge-selective metal co-catalysts can limit the performance of metal-doped organic photocatalysts by incorporating varying amounts of cobalt into a potassium poly(heptazine imide) framework. The optimal loading of 0.67 wt.% achieves an oxygen evolution rate of 31.2 µmol h-1 under visible illumination in the presence of the sacrificial electron scavenger AgNO3. Contrastingly, dark electrocatalytic oxygen evolution rates are maximised at 2.1 wt.% cobalt loading. We use transient absorption spectroscopy to rationalise this difference in optimal metal loading. Photocatalytic activity is shown to correlate with the steady accumulation of photo-oxidised cobalt, with both being maximal at a cobalt loading of 0.67 wt.%. Ultrafast transient absorption demonstrates that picosecond reduction of accumulated oxidised cobalt is a major light-driven loss pathway which competes with the desired nanosecond photoelectron transfer to AgNO3.The optimal 0.67 wt.% cobalt loading for photocatalysis is rationalised as resulting from a compromise between maximising the density of cobalt catalytic sites for water oxidation whilst avoiding excessive losses resulting from oxidised cobalt sites functioning as recombination centres for photogenerated electrons. We conclude that the efficiencies of many organic photocatalysts could be improved by incorporating catalytic sites which are more selective to the desired electron transfer direction.