Life cycle assessment of earth-abundant photocatalysts for enhanced photocatalytic hydrogen production

Abstract

Hydrogen (H2) can play a critical role in global greenhouse gas (GHG) mitigation. Photocatalytic water splitting using solar radiation is a promising H2 technology. Titanium dioxide (TiO2) and carbon nitride (g–C3N4)–based photocatalysts are the most widely used photocatalytic materials because of their activity and abundance. Several attempts have been made to improve the photocatalytic performance of these materials in terms of their activity level, life span, response to visible radiation, and stability. However, the environmental impacts of these modifications are often not included in existing studies. This research, therefore, develops a cradle-to-grave life cycle assessment (LCA) framework to evaluate and compare the GHG footprints of four alternative pathways: TiO2 nanorods and fluorine-doped carbon nitride quantum dots embedded with TiO2 (CNF: TNR/TiO2), g-C3N4, and g-C3N4/BiOI composite. Unlike most studies that focus only on certain stages such as laboratory-scale photocatalytic fabrication, this study includes utility-scale cell production, assembly, operation, and end of life to give a more accurate and precise environmental performance estimation. The results show that g-C3N4/BiOI has the lowest GHG footprint (0.38 kg CO2 eq per kg of H2) and CNF: TNR/TiO2 has the lowest energy payback time (0.4 years). In every pathway, energy use in material extraction processes makes up the largest GHG contribution, between 83% and 89%. Sensitivity and uncertainty analyses were conducted under the impact of various input parameters on the life cycle GHG emissions of hydrogen production. Photocatalytic water splitting is highly feasible for adaptation as a mainstream hydrogen production pathway in the future.