Photocatalytic Activity and Stability of Carbon Nitride-Pyrite Composites

Julian Kaulbersch, Roland Marschall

University of Bayreuth, 95440 Bayreuth, Germany

One of the primary challenges of global warming is the urgent need to change the fossil fuels reliance to sustainable alternatives. The major problem occurring right now is the storability of the renewable energy sources. For our current applications electricity alone cannot meet all the different demands. Therefore an alternative should be explored. An valuable alternative are high-density energy molecules. Hydrogen emerges as a promising option, due to its properties and absence of greenhouse gas. Currently, the main method of hydrogen production is steam reforming of natural gas. However, the global goal is to develop a sustainable process for green hydrogen to truly stop the global warming.[1]

The main method for renewable hydrogen production is electrolysis, this leads to the production of green hydrogen. To diversify the hydrogen production different methods should be explored and optimized. One of the promising methods for the hydrogen production is photocatalysis. This can involve the overall water slitting (OWS) or with the use of a sacrificial reagent hydrogen evolution (HER) to directly use the solar energy for the energy conversion. [2] One of the most discussed newer photo absorbers in the community is carbon nitride, especially graphitic carbon nitride. Its yellow color with the absorption in the visible solar spectrum, relatively high stability, cheap production and chemical resilience as organic photo absorber are promising aspects.[3]

Nonetheless, a notable limitation arises from our recent work,[4] which has illustrated the phenomenon of photo self-degradation within defect-rich graphitic carbon nitride containing heptazine units. To preserve the advantages while augmenting stability, a crystalline carbon nitride containing lithium cations and chloride anions, primarily composed of triazine units, has been synthesized by the research group of Maggard.[5]

We will present the photostability and catalytic performance of crystalline carbon nitrides and compare it with conventional graphitic carbon nitrides. Ion chromatography results show an improved stability in the crystalline carbon nitride structure. To mitigate recombination reactions, we introduced a co-catalyst in the form of pyrite (FeS₂) at 1, 5, and 10 wt% loading. The co-catalyst addition has enhanced the activity for both carbon nitride types. While the graphitic carbon nitride continues to face issues of self-degradation, the crystalline phase improve its stability when the co-catalyst is present. [6]