Spinel Structural Disorder Influences Solar‐Water‐Splitting Performance of ZnFe₂O₄ Nanorod Photoanodes

Abstract

Zinc spinel ferrite, ZnFe O (ZFO), is an emerging photoanode material for photoelectrochemical (PEC) solar fuel production. However, a lack of fundamental insight into the factors limiting the photocurrent has prevented substantial advance in its performance. Herein, it is found that ZFO nanorod array photoelectrodes with varying crystallinity exhibit vastly different PEC properties. Using a sacrificial hole scavenger (H O ), spatially defined carrier generation, and electrochemical impedance spectroscopy, it is shown that ZFO with a relatively poor crystallinity but a higher spinel inversion degree (due to cation disorder) exhibits superior photogenerated charge separation efficiency and improved majority charge carrier transport compared to ZFO with higher crystallinity and a lower inversion degree. Conversely, the latter condition leads to better charge injection efficiency. Optimization of these factors, and the addition of a nickel-iron oxide cocatalyst overlayer, leads to a new benchmark solar photocurrent for ZFO of 1.0 mA cm at 1.23 V versus reversible hydrogen electrode (RHE) and 1.7 mA cm at 1.6 V versus RHE. Importantly, the observed correlation between the cation disorder and the PEC performance represents a new insight into the factors important to the PEC performance of the spinel ferrites and suggests a path to further improvement.

Effect of the Degree of Inversion on the Electrical Conductivity of Spinel ZnFe2O4

“…Other configurations between normal and inverse are commonly observed and the degree of inversion, x, is the parameter employed to describe them according to T [M 1– x Fe x ] O [M x Fe 2– x ]O 4 , where 0 ≤ x ≤ 1. Among spinel ferrites, zinc ferrite (ZnFe 2 O 4 , ZFO) is widely studied due to its promising application in photoelectrochemistry as a photoanode material for solar‐light‐driven water oxidation ,. The thermodynamically most stable configuration of ZFO at 273.15 K and 100 kPa is the normal spinel structure ( x =0) .…”

“…Zhu et al. have recently reported that ZFO samples with poor crystallinity and a high degree of inversion exhibit a higher efficiency for the photoelectrochemical water oxidation than samples with higher crystallinity and low degree of inversion . Other authors have investigated the influence of the grain size and the synthetic conditions on the electrical properties of ZFO .…”

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting

“…[147] )c )Currentdensityv ersus PE potential curves of the nanocore Mo:BiVO 4 film measured in ap hosphateb uffer solutionc ontaining0 .5 m Na 2 SO 3 (with SO 3 2À as as acrificial oxidant) and the nanocone Mo:BiVO 4 /Fe(Ni)OOH film measured in phosphate buffer solution.R eproduced with permission. [149] ) ChemSusChem 2019, 12,4337 -4352 www.chemsuschem.org [148] )d )Linear scanning photocurrentdensityv ersus electrode potential curveso fZnFe 2 O 4 (zinc spinel ferrite, ZFO)n-PE in 1 m NaOHu nder intermittent simulatedi llumination of one sun (100 mW cm À2 ).…”

“…Lee et al found that thermalt reatment under hydrogen or vacuum couldb oost the photoactivity by up to 20 times. [149] By adjusting the formation temperature, which has as ignificant impact on crystal structure, they obtained a benchmark j ph of 1.0 mA cm À2 at 1.23 Vv ersusR HE with a nickel-iron oxide as an additional catalystt of acilitate the oxygen evolution reaction (Figure 6d). [113] Doping was also achieved by the group of Zou through spray pyrolysis.…”

“…Sivula et al found the link between the degree of cation disorder and PEC performance. [149] By adjusting the formation temperature, which has as ignificant impact on crystal structure, they obtained a benchmark j ph of 1.0 mA cm À2 at 1.23 Vv ersusR HE with a nickel-iron oxide as an additional catalystt of acilitate the oxygen evolution reaction (Figure 6d).…”