The rapidly increasing atmospheric CO 2 concentration has driven research into the development of cost-and energy-efficient materials and processes for the direct air capture of CO 2 (DAC). Solid-supported amine materials can give high CO 2 uptakes and acceptable sorption kinetics, but they are generally prepared in powder forms that are likely not practically deployable in large-scale operations due to significant pressure drops associated with packed-bed gas−solid contactors. To this end, the development of effective gas−solid contactors for CO 2 capture technologies is important to allow processing high flow rates of gas with low-pressure drops and high mass transfer rates. In this study, we demonstrate new laminate-supported amine CO 2 sorbents based on the impregnation of lowmolecular-weight, branched poly(ethyleneimine) (PEI) into an expanded poly(tetrafluoroethylene) (ePTFE) sheet matrix containing embedded silica particles to form free-standing sheets amenable to incorporation into structured gas−solid contactors. The free-standing sheets are functionalized with PEI using a highly scalable wet impregnation method. This method allowed controllable PEI distribution and enough porosity retained inside the sheets to enable practical CO 2 capacities ranging from 0.4 to 1.6 mmol CO 2 /g sorbent under dry conditions. Reversible CO 2 capacities are achieved under both dry and humid temperature swing cycles, indicating promising material stability. The specific thermal energy requirement for the regeneration based on the measured CO 2 and water capacities is 287 kJ/mol CO 2 , where the molar ratio of water to CO 2 of 3.1 is achieved using hydrophobic materials. This is the lowest molar ratio among published DAC sorbents. A larger laminate module is tested under conditions closer to larger-scale operations (linear velocities 0.03, 0.05, and 0.1 m/sec) and demonstrates a stable capacity of 0.80 CO 2 /g sorbent over five cycles of CO 2 adsorption and steam regeneration. The PEI-impregnated ePTFE/silica composite sorbent/contactors demonstrate promising DAC performance derived from the amine-filled silica particles contained in hydrophobic ePTFE domains to reduce water sorption and its concomitant regeneration energy penalty.

We provide a perspective on the development of direct air capture (DAC) as a leading candidate for implementing negative emissions technology (NET). We introduce DAC based on sorption, both liquid and solid, and draw attention to challenges that these technologies will face. We provide an analysis of the limiting mass transfer in the liquid and solid systems and highlight the differences.

The working mechanism of bis-(3-sulfopropyl) disulfide (SPS) during Cu electroless deposition was investigated by means of a real-time observation method that measures both the open-circuit potential and the mass change of an electrode in real time. The real-time observation with the aid of a derivatization method enabled the investigation of a working mechanism of the SPS during a Cu electroless deposition. The main suggested mechanism in this study is the reductive adsorption of SPS: the SPS adsorbs onto the Cu surface in a reduced form, 3-mercapto-1-propane sulfonate (MPS), by the oxidation of formaldehyde. The MPS is then reoxidized to the SPS, reducing the cupric ion to the cuprous ion. This redox process provides an indirect route for the electron transfer from formaldehyde to cupric ions, in addition to a direct electron transfer by an autocatalytic process. When the surface concentration of MPS is low, this process can facilitate a Cu electroless deposition by accelerating the reduction of cupric ions, which is the rate-determining step of the Cu reduction. When the surface concentration is high, however, the MPS blocks the active sites for the formaldehyde oxidation rather than mediates electrons, suppressing the Cu electroless deposition.