Experimental Investigation of the Cycle Stability of different Iron Oxide Composites for a Redox Hydrogen Storage Process

A promising process to store hydrogen is the thermochemical storage based on the repeated reduction and oxidation (redox) of iron oxide or iron. This storage process is an intermittent, twophase reaction, which takes place under atmospheric pressure in a hydrogen atmosphere during reduction (charging) or in a steam atmosphere during oxidization (discharging). The investigations have been carried out at two constant temperatures, namely 700°C and 800°C. During the storage phase, only iron exists inside the storage reactor a fact that makes the redox system much safer, compared to hydrogen storage under pressure in a tank. This work aims at studying the effect of adding different supporting materials upon producing the iron oxide storage composite samples on their thermochemical cycle stability. Furthermore, the influence of the temperature during the initial sintering process and the redox cycles on the reaction behavior of the iron oxide composites is investigated. It turned out that pure iron oxide pellets have lost about 65% of their redox potential after only three cycles. Applying 10 wt.% of calcium oxide has improved the cycle stability of the iron oxide pellets to over nine cycles. After nine cycles, a loss of redox performance by less than 5% was observed. This has been attributed to the densification of the sample’s outer surface, which is associated with slowing down the gas diffusion rate into/out of the investigated sample. In addition, reducing the temperature during the cycling (800°C to 700°C) and the sintering from 1100°C to 950°C has shown a positive effect on enhancing the cycle stability.