Flame-induced defect-rich in spinel ferrites for efficient solar water splitting

What is it about?

Spinel zinc ferrite (ZnFe2O4, ZFO) is a potential photoanode material for photoelectrochemical (PEC) water splitting because of its ideal bandgap (1.9–2.1 eV) and superior chemical stability in aqueous solutions. However, the low charge collection efficiency significantly hinders the improvement in PEC activity. Herein, we report an ultrafast and effective flame activation route to enhance the charge collection properties of ZFO. First, high-temperature flame (> 1300 ℃) facilitated surface and grain boundary diffusions, increasing the grain size and connectivity of the ZFO nanoparticles. Second, the reducing atmosphere of the flame enabled the formation of surface defects (oxygen vacancy and Fe2+), thereby increasing the charge carrier density and surface adsorption sites. Significantly, these two factors promoted charge transport and transfer kinetics, resulting in a 10-fold increase in the photocurrent density over the unactivated ZFO. Furthermore, we deposited a thin Al2O3 overlayer to passivate the ZFO surface and then the NiFeOx oxygen evolution catalyst (OEC) to expedite hole injection into the electrolyte. This surface passivation and OEC deposition led to a remarkable photocurrent density of ~1 mA/cm2 at 1.23 V versus the reversible hydrogen electrode, which is the highest value among all reported ZFO photoanodes. Notably, the NiFeOx/Al2O3/F-ZFO photoanode achieved excellent photocurrent stability over 55 h (96% retention) and superior faradaic efficiency (FE > 94%). Our flame activation method is also effective in improving the photocurrent densities of other spinel ferrites: CuFe2O4 (93 times), MgFe2O4 (16 times), and NiFe2O4 (12 times).

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Photo by Cullan Smith on Unsplash

Photo by Cullan Smith on Unsplash

Why is it important?

(1) This is the first study that utilizes the flame technique on ZFO photoanode and awakening their PEC performance. We developed and optimized the flame activation conditions, such as flame equivalence ratio, and activation time, to enhance and maximize ZFO’s PEC performance. Furthermore, we studied the impact of flame activation conditions on the physicochemical properties of ZFO in detail. (2) Flame activation on ZFO, which achieved one of the highest photocurrent density values (~1 mA/cm2 at 1.23 V vs. the reversible hydrogen electrode) among all the reported ZFO-based photoanodes by forming a heterostructure with Al2O3 and NiFeOx layers. This combination of three layered heterostructures has never been reported before.

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