Development and optoelectronic characterization of photoactive materials for photo(electro)reforming of industrial waste streams and low carbon hydrogen evolution

Abstract

This work focuses on the development and characterization of novel photocatalytic materials and photoanodes capable of harvesting visible light to promote the photo(electro)reforming of organic substrates, especially real industrial biomass derived waste streams, aiming at the production of low carbon hydrogen and value added oxidation products. Hydrothermal and solvothermal synthetic strategies were employed to introduce oxygen vacancies and dopant species into wide bandgap semiconductor oxides such as Bi2WO6, Nb2O5, and TiO2, effectively narrowing their band gaps and enhancing visible light absorption. Additionally, oxynitride materials (NbTiON) and covalent organic frameworks (COFs) were synthesized as alternative photocatalysts with tunable optoelectronic properties. The obtained materials were comprehensively characterized by different techniques to elucidate their morphology, optoelectronic properties, and charge carrier dynamics. The most promising photocatalysts were immobilized as thin films to fabricate photoanodes, which were evaluated under simulated solar (AM 1.5G, 100 mW cm 2) and visible irradiation (> 420 nm, 100 mW cm 2) in photoelectrochemical cells (PECs) containing glycerol or flegmass as sacrificial agents. PECs with BiVO4 photoanodes doped with 5 wt.% Nb have converted 91.4% of glycerol with 88.9% selectivity toward formic acid formation after 5 hours of simulated sunlight illumination, achieving a H2 evolution rate (HER) of 4.2 μmol H2 cm 2 h 1. This value is 84% higher than that of unmodified BiVO4. Additionally, oxygen vacancies engineering was employed to improve the photoelectrocatalytic performance of Bi2WO6 based photoanodes towards crude glycerol conversion, from 17.0% for the pristine oxide to 24.5%. Concomitantly, HER was enhanced from 32 to 63 μmol h 1 cm 2. Looking at new materials for photoreforming, the properties of thiophene based COFs were modulated through linkage conversion from imine to amide resulting in a 300% increase in the photocatalytic HER compared to its imine form. These findings highlight the synergistic benefits of defect engineering, doping, and alternative synthetic pathways in tailoring the optoelectronic and catalytic properties of semiconductors. Overall, this work advances the design of efficient, stable, and abundant photoelectrodes for solar driven biomass valorization and hydrogen production.