New insight leads to record performance with photoanode material

Zinc spinel ferrite, ZnFe₂O₄ (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 the performance of ZFO. In a report published today in Advanced Materials, LIMNO finds that ZFO nanorod array photoelectrodes with varying crystallinity (prepared by altering the synthesis temperature) exhibit vastly different PEC properties. Lead author Dr. Xiaodi Zhu and her collaborators show that ZFO with a relatively poor crystallinity but a higher spinel inversion degree (due to disorder in the positions of Fe³⁺ and Zn²⁺ ions) 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 co-catalyst overlayer, leads to a new benchmark solar photocurrent for ZFO of 1.0 mA cm^–2 at 1.23 V vs. RHE and 1.7 mA cm^–2 at 1.6 V vs. 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.