Regulation crystal phase of Ca2Fe2O5 by synthesis method for efficient hydrogen production in photo-assisted chemical looping process

Abstract

The influence of synthesis methodologies on the photothermal catalytic efficacy of brownmillerite Ca₂Fe₂O₅ in the photo-assisted chemical looping reforming with water splitting (CLRWS) was delineated in this study. Combined with the synthesis methods’ effects on the crystal structure, surface morphology, thermochemical and optical properties of samples, the probable reinforcement mechanism of introducing light into the CLRWS process was elucidated. It was demonstrated that the incorporation of light in CLRWS substantially boosted H₂ production, with a striking enhancement from 0.56 mmol·g_OC^-1 in thermal catalysis to 3.35 mmol·g_OC^-1 under light conditions for mechanochemically synthesized Ca₂Fe₂O₅. Comparative analysis of sol–gel (SG-CFO), coprecipitation (CP-CFO), and mechanochemical (MC-CFO) methods revealed that phase purity and grain size were critical factors of lattice oxygen activity and the bandgap energy and electron-hole recombination rate in photothermal catalysis. SG-CFO exhibited superior phase purity, crystallinity and effective electron-hole pair separation, achieving an 84.2 % fuel conversion and a 6.8 mmol·g_OC^-1 hydrogen yield at 800 ℃ with light, eclipsing the 65.6 % and 5.05 mmol·g_OC^-1 yields recorded at 850 ℃ in the absence of light. Moreover, under light conditions, the H₂ production of MC-CFO was 5.98 times greater than that under dark conditions, primarily due to its small grain size and high charge separation efficiency. The CP-CFO demonstrated the narrowest band gap (1.86 eV) coupled with a wider spectral absorption range relatively, resulting in the most pronounced photothermal effect. This research underscored the imperative of judicious synthesis method selection in the quest for high-performance photothermal catalysts for sustainable H₂ generation.