Theoretical investigation of the post-combustion recovery process in cobalt-based zero-carbon fuels

The application of Co-based recyclable zero-carbon fuels, utilizing oxygen both as a heat transfer fluid and reactant, has gained significant attention as a potential solution to address the intermittency challenges associated with zero-carbon renewable sources. However, the widespread adoption of these fuels faces obstacles, particularly when industrial waste heat deviates from the optimal operational temperature range of 900-1000 degrees C. Despite efforts to modify operation temperatures, the underlying reaction mechanism remains unclear. To elucidate the crucial factors influencing the recovery process of Co-based recyclable zero-carbon fuels, the density functional theory (DFT) method was employed to analyze the deoxygenation process of the spinel structure. This is essential as the recovery and combustion process involve the dissociation and reconstruction of crystal structures. In this study, the desorption steps of oxygen were studied separately. After the complete desorption of the first layer atoms from the slab surface, the second layer is exposed. To ensure structure stability, the surface undergoes reconstruction. The analysis revealed that the concentration of oxygen vacancy of the same type on the slab surface significantly impacts the energy barrier of deoxygenation, particularly in the initial two stages. This finding is further supported by examining the deoxygenation process in Co2MO4 (M=Mn, Mg, Ni, and Cu) fuels, as reported in previous studies. Notably, the surface energy emerged as a key parameter for these Co-based spinel structure fuels. Modifying the surface energy, by lowering it, was identified to enhance the structural stability to increase the recovery temperature. Conversely, improving the surface energy will decrease the stability and lower the recovery temperature. Understanding and manipulating these factors, as proposed in this study, offer avenues for tailoring Co-based recyclable zero-carbon fuels for various industrial processes. This work serves as a crucial guide for further research and development in this field, potentially unlocking new possibilities to broaden the operation temperature range and enhance the performance of these fuels.