Influences of morphology and structure on Mn-based multi-step thermochemical H2O splitting cycle

Abstract

The multi-step thermochemical water splitting cycle based on MnFe2O4–Na2CO3 has emerged as an attractive process due to its relatively low reaction temperature. The main challenges are how to enhance its H2/O2 release rates and capacities. Herein, a series of MnFe2O4 with different microstructures synthesized by hydrothermal method were tested in H2O splitting reaction. It was found that MnFe2O4 samples with smaller particle size and fine crystallinity exhibited higher H2 production which was benefit from enhanced intimate contact between homogeneous particles and better ionic transport within fine crystals. In order to enhance the reaction kinetics of the O2 release reaction, a hydrolysis treatment was introduced to the multi-step cycle. As a result, the lamellar structure of Na+ extracted Na1−x(Mn1/3Fe2/3)O2 oxide became unstable and collapsed into cubic MnFe2O4 spinel structure more easily under heating, a structure characteristic suitable for O2 release reaction. Compared to direct O2 release reaction between layered Na(Mn1/3Fe2/3)O2 oxide and CO2, the hydrolysis treatment lead to much faster reaction rate.