Glycerol photoelectrochemical oxidation reaction at carbon nitrides/BiVO₄ materials

The H₂ evolution from water electrolysis can be coupled with co-generation of other added-value products through biomass oxidation. In this study, we investigate the photoelectrochemical oxidation of glycerol using visible-light-responsive carbon nitride /bismuth vanadate (CN/BiVO₄) heterojunction photoelectrocatalysts. Different CN materials were explored, including polymeric carbon nitride (PCN), crystalline poly(heptazine imides) (PHI-Cat, in which Cat = Na, K, or Cs), and poly(triazine imide) (PTI-Li). The CN materials were spin-coated onto fluorine-doped tin oxide (FTO) substrates, followed by the Bi electrodeposition step and conversion to BiVO₄ in the presence of vanadyl acetylacetonate at 500 °C for 2 h. The CN/BiVO₄ heterojunctions presented bandgap energy values, E _g, similar to pure BiVO₄. X-ray diffraction analysis also revealed that the BiVO₄ phase was not altered by the presence of the CN. However, scanning electron microscopy analysis coupled to energy-dispersive X-ray spectroscopy (SEM-EDS) revealed regions rich in Bi and V, and others rich in C and N, suggesting the formation of heterojunctions. Photoelectrochemical studies demonstrated that BiVO₄ is active for both water and glycerol (1.0 mol·L^-1) oxidation, with a fourfold increase in photocurrent at 1.23 V vs reversible hydrogen electrode (RHE) upon glycerol addition. Clearly, the type of nitride employed in the heterojunctions influences the activity of the material for glycerol oxidation, with the photocurrent at 1.23 V vs RHE following the order: PCN/BiVO₄ > BiVO₄ ≈ PHI-Cs/BiVO₄ > PHI-K/BiVO₄ ≈ PHI-Na/BiVO₄ > PTI(Li)/BiVO₄. The SEM-EDS analysis after electrochemical tests revealed that the presence of crystalline CNs induces the segregation of vanadium oxides, contributing to a decrease in activity. On the other hand, the superior performance of PCN/BiVO₄ is attributed to a greater thermal stability of PCN during BiVO₄ synthesis, as indicated by thermogravimetric analysis. These findings highlight the dual importance of electronic compatibility and thermal resilience of CN materials in designing efficient heterojunction photoanodes for biomass-assisted hydrogen production.