(315d) Reduction Kinetics of Hercynite (FeAl2O4) and Hydrogen Production for Solar Thermochemical Water Splitting

Authors: 
Al-Shankiti, I., University of Colorado at Boulder
Idriss, H., Sabic
Weimer, A. W., University of Colorado Boulder
The solar thermochemical water splitting process (STWS) is a promising technology for producing renewable H2. A metal oxide is first reduced with high temperature concentrated sunlight to produce O2. Then, the reduced material is exposed to steam to reoxidize it and produce H2. Un-doped hercynite (FeAl2O4) is shown to be a promising and low-cost iron and aluminum based solar thermochemical water splitting material [1]. The reduction kinetics of FeAl2O4 was studied using a thermogravimetric analysis (TGA) technique. Isoconversional methods and XRD analysis show that hercynite is first formed by reacting aluminum oxide with iron oxide as shown in R1. Then, hercynite is further reduced, forming O-vacancies, while maintaining the spinel structure as shown in R2. Apparent activation energies for both reactions have been determined. This is the first kinetics study to be reported for the total reduction process to form herycnite and then to form the spinel, and to identify H2 production from oxidizing the reduced hercynite with H2O as shown in R3.

Fe2O3 + 2Al2O3 → 2FeAl2O4 + 0.5 O2

R1

FeAl2O4 → FeAl2O4-δ + δ/2 O2

R2

FeAl2O4-δ + δH2O → FeAl2O4 + δH2

R3

1. Muhich, C.L., et al., Predicting the solar thermochemical water splitting ability and reaction mechanism of metal oxides: a case study of the hercynite family of water splitting cycles. Energy & Environmental Science, 2015. 8(12): p. 3687-3699.