# Humidity-Enhanced Direct Air Capture of Carbon Dioxide Using Amine-Grafted Covalent Organic Frameworks Under Ambient and Sub-ambient Temperatures

## TL;DR

A new material captures more carbon dioxide from air when it's cold and humid, which could help reduce atmospheric CO2 in most real-world conditions.

## Contribution

A novel amine-grafted COF shows enhanced CO2 capture under cold and humid conditions, which is critical for DAC in diverse climates.

## Key findings

- ImCOF-TAEA achieves a 137% increase in CO2 adsorption capacity under 70% humidity at 25°C.
- CO2 uptake further increases by 205% at 15°C under 70% humidity.
- The material shows excellent recyclability and low regeneration temperature requirements.

## Abstract

The significant rise
in atmospheric CO2 and its impact
on accelerating climate change have triggered intense efforts to develop
porous sorbents for direct air capture (DAC), a route toward carbon-neutrality.
Amine-functionalized covalent organic frameworks (COFs), an emerging
class of crystalline porous materials, have recently shown promising
potential for DAC at ambient, indoor temperatures (25 °C). However,
most of Earth’s land area has annual mean temperatures below
25 °C, accompanied by nonzero and variable relative humidity
(RH). The performance of amine-grafted COFs under cold, humid conditions
remains largely unexplored, even though such climates represent the
majority of potential DAC deployment sites. Herein, we report a systematic
investigation of a tetrahydroquinoline-linked COF covalently functionalized
with diverse amines, evaluating its CO2 adsorption behavior
across a broad range of ambient to sub-ambient temperatures (25 °C
to −20 °C) and relative humidities (0%–70%). A
unique tris­(2-aminoethyl)­amine-functionalized COF (ImCOF-TAEA) achieved
a pseudoequilibrium capacity of 0.46 ± 0.02 mmol g–1 under dry conditions, rising ∼137% to 1.09 ± 0.09 mmol
g–1 under 70% RH using 400 ppm of CO2 at 25 °C. Upon cooling to 15 °C under 70% RH, the uptake
further increased to 1.25 ± 0.02 mmol g–1,
showing a 205% enhancement relative to dry conditions. In
situ spectroscopic analysis supports the mechanism behind
the unusually high enhancement in CO2 adsorption under
humid conditions. ImCOF-TAEA also demonstrates excellent recyclability
under ambient/sub-ambient conditions and has modest (45 °C-65
°C) requirements for regeneration.