Enhancing CO2 adsorption in Ca-based materials via atomic layer deposition of alumina: Mechanisms and stability

Abstract

Calcium-based thermochemical energy storage materials (Ca(OH)₂/CaCO₃) have emerged as promising candidates for industrial waste heat recovery and solar thermal storage applications, owing to their exceptional energy storage density (3.2 GJ/m³) and cost-effectiveness (<50 $/ton). However, their practical implementation faces significant challenges due to performance degradation caused by sintering and particle agglomeration during cyclic operation. This study demonstrates a synergistic approach combining atomic layer deposition (ALD) of Al₂O₃ coatings and Al³⁺ doping to enhance CO₂ capture in calcium-based materials. Through controlled ALD processing, we achieved nanoscale Al₂O₃ layers (∼3.6 nm) on CaCO₃ substrates. The 20 deposition cycles identified as optimal for balancing Al dispersion and pore accessibility. Materials characterization revealed that Ca(OH)₂-derived samples exhibited superior cyclic stability (after 20 cycles) and thermal storage density (∼2400 kJ/kg). Density functional theory calculations uncovered the atomic-scale mechanisms behind these improvements. Al element doping reduced oxygen vacancy formation energy and shortened Al–O bonds by 21 % (1.89 Å vs. Ca–O 2.40 Å), enhancing structural stability. CO₂ adsorption on Al-doped surfaces adopted a bent configuration (135°). In humid conditions, H₂O co-adsorption further improved performance through a unique [Al–OH₂⋯OCO⋯Ca] bridge structure, promoting bicarbonate formation.