(186d) Improving Cycle Performance of Copper Oxide Thermochemical Heat Storage Materials By Doping Cheap Metal Oxides

1. Introduction

Thermochemical redox couples can satisfy the demand of high-temperature energy storage in the next-generation concentrated solar power plants. Copper oxide owns the advantage of high reaction temperature and high energy storage density, but suffers from poor reversibility and cycling stability due to the sintering problem. The purpose of our research is to inhibit the sintering of copper oxide, so as to improve its reversibility, prolong its cycle life, and increase its application value.

2. Methods

2.1 Synthesis of samples

Mixed oxides were synthesized using high-temperature solid-phase synthesis method. They were mixed by ball milling for 30 min via a planetary ball mill. The mixed powders were then calcined at 900 °C for 4 h with a heating rate of 10 °C/min to obtain fresh samples.

• Redox reactivity of samples

The redox reactivity tests of the powders were monitored using a Netzsch STA 449 Jupiter F3 simultaneous thermal analyzer in a typical run, a certain amount (8–10 mg) of the mixed powders was placed into an alumina crucible and heated up to 1100 °C and subsequent cooled down to 700 °C. Heating and cooling rates was 20 °C/min to better evaluate the thermal reaction performance of the powders and a constant air (80 vol % N2 + 20 vol % O2) flow of 100 mL/min was used. The redox cycles were carried out in a tubular muffle furnace under a dynamic air flow of 100 mL/min. In the first cycle, the sample was heated from room temperature to 1100 °C and cooled to 700 °C at 20 °C /min. In the following cycles, the sample was heated from 700 °C to 1100 °C and cooled to 700 °C at 20 °C /min. We took about 1 g of sample out every 100 cycles to test the thermogravimetric (TG) and other characterization analyses.

• Characterization of samples

Powder X-ray diffraction (XRD) analyses were carried out employing a PANalytical B.V. (Netherlands) X-pert powder diffractometer (Cu Kα radiation) in a diffraction angle (2θ) range of 10−80° with a step of 0.02° and 30 s counting time per angle. The microstructure and morphology of the materials before and after redox cycles were observed by scanning electron microscope (SEM) via a SU8010 field emission scanning electron microscope. Transmission electron microscope combined with energy dispersive spectrometer (TEM-EDS) was identify the elemental distribution of samples after different processes.

3. Results

3.1 Reaction characteristics of samples

Inexpensive metal oxides MgO and Al₂O₃ are used to be doped in copper oxide through the high-temperature solid-phase synthesis method. The molar ratio of MgO and Al₂O₃ is 1:1, in order to generate stable MgAl₂O₄ spinel. In the TG result, the reoxidation rate of CuO doped with 10 wt% MgAl₂O₄ (CuO-10%MgAl) can reach 99.9%. DSC test result shows that the enthalpy change of reduction reaction of CuO-10%MgAl is 507.8 kJ/kg, and the enthalpy change of oxidation reaction is 507.4 kJ/kg. The reversibility of CuO-10%MgAl is excellent up to 250 cycles, remaining above 96%.

3.2 Microstructural characterization of samples

SEM and SEM-EDS images of CuO-10%MgAl at different stages (fresh sample, after reduction and after oxidation) show that MgAl₂O₄ is loaded on the surface of CuO as small particles. SEM images of CuO-10%MgAl after different cycles show that the particle size of CuO and MgAl₂O₄ particles did not change much compared with that at the first cycle, and MgAl₂O₄ particles were still evenly distributed on the surface of CuO. It indicates that MgAl₂O₄ prevents the further sintering and growth of CuO particles. Besides, Cu₂O has enough area exposed to air to better absorb oxygen for oxidation reactions.

Conclusion

In summary, we have prepared mixed oxides through solid-phase synthesis, and found an optimal doping ratio of 10wt% of MgAl₂O₄ dioxide (CuO-10%MgAl) through thermogravimetric analysis, where the reoxidation rate reaches 99.9%. While keeping a high reduction reaction enthalpy of 507.8 kJ/kg, its oxidation reaction enthalpy reaches 507.4 kJ/kg. The material also shows an excellent performance during the long-term operation, where the oxidation reaction degree maintains 96% after 250 cycles. Through characterizations, we find MgAl₂O₄ attaches the surface of CuO particles to prevent sintering and agglomeration of CuO.