Unveiling the long-term cycling stability mechanism of Na-doped Li4SiO4 for low-concentration CO2 capture: From microstructural evolution to desorption kinetic modeling

Lithium silicate (Li4SiO4) adsorbents garnered considerable interest for CO2 capture due to their high adsorption capacity and stable cycling performance. While Na2CO3 doping has been shown to enhance its the adsorption capacity at low CO2 concentrations, the resulting samples exhibited poor cycling stability. Previous research has predominantly focused on the adsorption process, often overlooking the desorption kinetics. This study addressed this gap by investigating the desorption kinetics and decay mechanisms of Na-doped Li4SiO4 to develop a version with long-term cycling stability from fly ash. The Na/Si ratio and desorption conditions were optimized. Optimal cycling stability was achieved at RNa/Si = 0.25 and a desorption temperature of 700 degrees C, yielding an initial adsorption capacity of 29.45 wt% and retaining 83 % of this capacity after 56 cycles. Phase analysis, desorption kinetics modeling, and examination of kinetic parameter evolution over multiple cycles uncovered the decay mechanisms in Na-doped Li4SiO4. Active phase Li3NaSiO4 ' s poor reversibility contributed to performance degradation. Furthermore, molten LiNaCO3 played a dual role during desorption: it accelerated the diffusion of Li+/Na+/CO32- but hindered CO2 diffusion. When the LiNaCO3 content was low, the diffusion of Li+/ Na+/CO32- became the rate-limiting step; conversely, when the content was high, its reaction with CO2 to form C2O5 2- impeded diffusion, thus becoming the new rate-limiting step.