Reduction and oxidation kinetics of NiWO₄ as an oxygen carrier for hydrogen storage by a chemical looping process

NiWO₄ with a volumetric storage density (VSD) of 496 g L^-1 was studied to evaluate its H₂ storage potential as an oxygen carrier under a chemical looping (CL) process scheme. The material was synthesized by precipitation and calcined at 950 °C for 5 hours in air. Characterization consisted in XRD, BET surface area, SEM and EDS analysis. NiWO₄ hydrogen storage reduction-oxidation evaluation was performed by TGA using 5% v H₂/Ar and 2.2% v H₂O/Ar at 800 °C. Global kinetics for the reduction step was studied from 730 to 870 °C using 2 to 5% v of H₂/Ar. While oxidation kinetics was examined from 730 to 870 °C using 0.8 to 2.2% v H₂O/Ar. A hydrogen storage multicycle stability test was performed by exposing NiWO₄ to 17 consecutive redox cycles. XRD results of the synthesized material indicate NiWO₄ as the only crystalline phase. Fully reduced material found only W and Ni species, while reoxidation led back to NiWO₄. BET surface area of synthesized material was 4.25 m² g^-1. SEM results showed fresh NiWO₄ composed of non-porous large particles (1-5 μm). After reduction, the material shown a porous coral-like morphology with particles between 50 to 100 nm. EDS analysis results confirmed the compositions of the reduced (Ni + W) and fully oxidized NiWO₄ species, respectively. Oxygen carrier reaction conversions for both reduction and regeneration steps were 100%. Global kinetic studies indicate a first order reaction for the two reduction steps and during oxidation, with activation energies of 22.1, 48.4 and 53.4 kJ mol^-1 for the two reduction and oxidation steps, respectively. NiWO₄ multicycle stability test shown no loss of VSD and fast reduction and oxidation kinetics under the studied conditions after seventeen consecutive redox cycles, which confirms the potential of this material with respect to current oxygen carriers reported in the literature for hydrogen storage applications.