Intrinsic Direct Air Capture

TL;DR

This paper introduces new thermodynamic metrics to evaluate solid sorbents for Direct Air Capture, enabling optimization of materials and cycle conditions based solely on intrinsic properties, independent of system design.

Contribution

It develops universal thermodynamic metrics for DAC sorbents, applicable to any cycle, and demonstrates their use with a large MOF database to optimize material and cycle selection.

Findings

Performance depends on thermodynamic path taken during the cycle.

Optimal sorbents should be evaluated based on relative change in uptake, not static selectivity.

Lower temperatures enhance CO2 uptake divergence from N2, improving selectivity.

Abstract

We present new metrics to evaluate solid sorbent materials for Direct Air Capture (DAC). These new metrics provide a theoretical upper bound on CO2 captured per energy as well as a theoretical upper limit on the purity of the captured CO2. These new metrics are based entirely on intrinsic material properties and are therefore agnostic to the design of the DAC system. These metrics apply to any adsorption-refresh cycle design. In this work we demonstrate the use of these metrics with the example of temperature-pressure swing refresh cycles. The main requirement for applying these metrics is to describe the equilibrium uptake (along with a few other materials properties) of each species in terms of the thermodynamic variables (e.g. temperature, pressure). We derive these metrics from thermodynamic energy balances. To apply these metrics on a set of examples, we first generated approximations of the necessary materials properties for 11 660 metal-organic framework materials (MOFs). We find that the performance of the sorbents is highly dependent on the path through thermodynamic parameter space. These metrics allow for: 1) finding the optimum materials given a particular refresh cycle, and 2) finding the optimum refresh cycles given a particular sorbent. Applying these metrics to the database of MOFs lead to the following insights: 1) start cold - the equilibrium uptake of CO2 diverges from that of N2 at lower temperatures, and 2) selectivity of CO2 vs other gases at any one point in the cycle does not matter - what matters is the relative change in uptake along the cycle.