Electron Accumulation Induces Efficiency Bottleneck for Hydrogen Production in Carbon Nitride Photocatalysts

This study addresses the light intensity dependence of charge accumulation in a photocatalyst suspension, and its impact on both charge recombination kinetics and steady-state H2 evolution efficiency. Cyanamide surface functionalized melon-type carbon nitride (NCNCNx) has been selected as an example of emerging carbon nitrides photocatalysts because of its excellent charge storage ability. Transient spectroscopic studies (from ps to s) show that the bimolecular recombination of photogenerated electrons and holes in NCNCNx can be well described by a random walk model. Remarkably, the addition of hole scavengers such as 4-methylbenzyl alcohol can lead to ∼400-fold faster recombination kinetics (lifetime shortening to ∼10 ps). We show that this acceleration is not the direct result of ultrafast hole extraction by the scavenger, but is rather caused by long-lived electron accumulation in NCNCNx after hole extraction. The dispersive pseudo-first order recombination kinetics become controlled by the density of accumulated electrons. H2 production and steady-state spectroscopic measurements indicate that the accelerated recombination caused by electron accumulation limits the H2 generation efficiency. The addition of a reversible electron acceptor and mediator, methyl viologen (MV2+), accelerates the extraction of electrons from the NCNCNx and increases the H2 production efficiency under one sun irradiation by more than 30%. These results demonstrate quantitatively that while long-lived electrons are essential to drive photoinduced H2 generation in many photocatalysts, excessive electron accumulation may result in accelerated recombination losses and lower performance, and thus highlight the importance of efficient electron and hole extraction in enabling efficient water splitting photocatalysts.