Spinel ferrites

Sunlight-assisted water splitting to produce oxygen, and more importantly, hydrogen using photoelectrochemical cells has been aggressively investigated for decades as a potentially inexpensive, renewable route to hydrogen fuel.

The best photoelectrochemical (PEC) schemes to date perform this reaction with impressive overall energy efficiency (> 12%), but rely on prohibitively expensive electrode designs comprising epitaxially grown semiconductor layer structures.

Alternative designs featuring cheaper simple oxide materials such as titanium dioxide (TiO₂) or iron oxide (Fe₂O₃) suffer from drawbacks. These include poor solar light harvesting in the visible part of the solar spectrum (TiO₂); rapid electron-hole pair recombination kinetics (Fe₂O₃); or poor match of the energy levels of the valence and conduction bands with the respective energies required to drive the oxidation and reduction reactions.

In the first 2017 issue of the Journal of Photonics for Energy, Dereje H. Taffa and coauthors describe recent research on spinel ferrites as the photoactive materials in PEC solar cells.

In their open-access article, “Photoelectrochemical and theoretical investigations of spinel type ferrites (M_xFe_3–xO₄) for water splitting: a mini-review,” the authors note that spinel ferrites are widely investigated in other scientific spheres, for example as magnetic materials; as spin filters in spintronics; and as active materials in batteries and electrochemical capacitors.

Crystal structures of spinel ferrites: (a) normal spinel, (b) inverse spinel, and (c) orthorhombic spinel.

What makes spinel ferrites attractive as PEC materials (in addition to their low cost) is their ability to absorb light in the visible portion of the solar spectrum and the possibility that their electronic properties and electronic bandgap can be tailored by the choice of the substituted metal ion “M” in the M_xFe_3–xO₄ structure.

Taffa et al. review classes of spinel ferrites suitable for PEC water splitting in terms of their syntheses, performance, and stability. To date, the quantum efficiencies for water splitting at unbiased spinel ferrite-derived photoelectrodes is poor (often < 0.1%), largely due to poor mobility of photogenerated charge carriers.

They review strategies to improve PEC water splitting at spinel ferrites, such as cation doping to improve charge mobility, incorporation of catalysts at the semiconductor surface to improve reaction kinetics, and schemes involving integration of spinel ferrites with more traditional PEC electrode semiconductors, such as TiO₂.

Additionally, theoretical investigations of the effects of the distribution of the “M” cations in the M_xFe_3–xO₄ structures of spinel ferrites on the electronic structure of the resulting semiconductor are reviewed. Different density functional theory (DFT) approaches have been combined with a semi-empirical on-site correction (U) to yield substantial improvements over DFT alone in predicting spinel ferrite bandgaps, as well as their magnetic properties as based on composition.

While the efficiency for PEC water splitting at spinel ferrite-derived photoelectrodes remains modest, the topic continues to be compelling, as the variability of spinel ferrite structure and composition suggest substantial room for improvement. They envision substantial breakthroughs as methods for synthesizing spinel ferrites — and thus control of composition and structure — improve.

The mini-review by Taffa et al. is a terrific window into this scientifically rich playing field.

Coauthors are Ralf Dillert, Anna C. Ulpe, Katharina C. L. Bauerfeind, Thomas Bredow, Detlef W. Bahnemann, and Michael Wark.

Source: dx.doi.org/10.1117/1.JPE.7.012009

–Jeremy Pietron is a staff scientist at the US Naval Research Lab, a member of the editorial board of the Journal of Photonics for Energy, and a guest editor, along with Roland Marschall, of the journal’s special section on solar fuels photocatalysis.

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