Understanding the
role played by moisture in CO2 sorption
is key for designing the next generation of solid sorbents such as
metal–organic frameworks, which can be used for carbon capture
and conversion as well as for molecular sieving, energy storage, etc. The abundance of water in nature and industrial processes,
including in anthropogenic sources of CO2 has been shown
to significantly affect commercial adsorbent performances, including
their uptake capacity and selectivity. However, less is known about
the role of humidity on CO2 diffusion, even though it is
crucial for economically viable rapid capture processes. In this work,
we have used atomistic simulations and experiments to gain insight
into the effect of humidity on CO2 adsorption, diffusion
and transport properties in UiO-66(Zr), here described as a flexible
structure. We show that depending on the water concentration adsorbed
in the host nanoporosity, the CO2 adsorption can be enhanced
or reduced depending on thermodynamic conditions. At low water loading,
isolated molecules interact with low-energy sites of the sorbent.
At higher loading, nucleation drives water cluster formation, followed
by cluster percolation resulting in a sub-nanoporous adsorbing media
decreasing the overall CO2 diffusion compared to the dry
structures. We finally show that equilibrium parameters such as self-diffusion
coefficients and isotherms can be used to describe the CO2 transport in dry and humid structures through the nano-Darcy equation.