Photoreforming of real biomass-derived waste streams using Nb-doped BiVO4 photoanodes for sustainable hydrogen production

Abstract

Solar-induced valorization of agroindustrial residues is a key issue for the so-called sustainable growth. In this study, a novel Nb-doped BiVO₄ photoanode is described and applied for the photoreforming of crude glycerol and flegmass, real byproducts of biodiesel production and sugarcane ethanol refining, respectively. To the best of our knowledge, this is one of the first reports exploring flegmass as a substrate for photoreforming. Nb(V) doping significantly enhances the photoanode performance, with 5% Nb : BiVO₄ exhibiting 80% increase in photocurrent during long-term crude glycerol photoreforming, reaching 2.2 mA cm⁻² at 1.0 V vs. RHE. When flegmass is employed, smaller photocurrents are observed in long-term experiments due to the lower organic content. Nevertheless, the 5% Nb : BiVO₄ photoanode exhibits a 200% increase in photocurrent compared to the pristine oxide. Detailed characterization reveals that Nb⁵⁺ doping occurs dominantly at VO₄ tetrahedral sites, also leading to oxygen vacancies as evidenced by EPR spectroscopy. Electrochemical impedance spectroscopy showed higher charge carrier densities and reduced charge transfer resistance in the Nb-modified photoanodes. Photoreforming of crude glycerol with 5% Nb : BiVO₄ after 6 hours of simulated sunlight irradiation achieved 91.4% substrate conversion, producing green H₂ at a rate of 4.2 μmol cm⁻² h⁻¹, much higher than 2.3 μmol cm⁻² h⁻¹ obtained by undoped BiVO₄. EPR spin trapping experiments indicated that the main reaction mechanism involves direct hole transfer to adsorbed glycerol on the catalyst surface, forming organic radical species. In flegmass photoreforming, 5% Nb : BiVO₄ achieved H₂ evolution rates of 2.4 μmol H₂ per cm² per h, consistently outperforming the bare BiVO₄ photoanode, highlighting its potential for valorizing low-cost, biomass-derived residues into valuable chemicals and renewable fuels.