Surface Hydration of Porous Nickel Hydroxides Facilitates the Reversible Adsorption of CO2 from Ambient Air

Abstract

Direct air capture (DAC) under humid ambient conditions typically requires the use of organic components, with sorbents that are purely inorganic in composition for the most part operating hundreds of degrees above room temperature. In this work, we report porous metal hydroxides as a novel class of water-tolerant, oxidatively and hydrothermally stable low-temperature sorbents that exhibit competitive DAC working capacities of 1.25 mmol/g over 5 consecutive temperature swing adsorption-desorption cycles in the presence of steam and oxygen. Aqueous miscible organic solvent treatments are used to create highly porous structures with surface areas exceeding 700 m²/g that capture CO₂ in the form of bicarbonates under dry conditions, and carbonates under wet conditions. Water exerts a facilitative rather than an inhibiting effect on CO₂ binding, and the presence of hydrating multilayers serves to stabilize carbonate species-akin to moisture swing adsorbents-except for the fact that solvation results in a remarkable (upto 10-fold) increase, not decrease, in DAC capacity. High-valent doping with cerium is used to improve DAC capacities by amplifying surface basicity, evidencing porous nickel hydroxides specifically (and porous metal hydroxides more generally) as a novel class of robust, earth-abundant DAC sorbents.

Scheme 1. (a) Illustration of the Layered Double Hydroxide (LDH) Structure; (b) Depiction of LDH Delamination by Aqueous Miscible Organic Solvent Treatment (AMOST), Adapted with permission from Ref (26), Copyright 2013 Royal Society of Chemistry; and (c) Illustration of α-Ni(OH)₂ and β-Ni(OH)₂ Crystal Structures, Reprinted with Permission from Ref (29), Copyright 2020 Springer Nature.

Figure 1

(a) XRD patterns of pristine samples (dashed lines represent peak positions for α-Ni(OH)₂), (b) N₂ adsorption–desorption isotherms, and (c) pore size distributions derived from BJH analysis of the desorption branches of the PNH sample.

Figure 2

Bright field transmission electron microscopy (TEM) images of α-NiCe_0.1(OH)_x-no AMOST (a) and α-NiCe_0.1(OH)_x (b).

Figure 3

DAC capacities of various porous nickel hydroxides plotted as a function of BET surface area. Adsorption measurements were carried out at 25 °C under 400 ppm of CO₂, 2.1 kPa of H₂O (67% RH), and balance N₂. Dashed line represents a linear regression for the adsorption capacities of the Ce-free sorbents.

Figure 4

(a) Powder X-ray diffraction patterns, (b) S 2p XPS spectra, and (c) FTIR spectra of α-Ni(OH)₂ and α-Ni(OH)₂-SO₄^2– samples obtained at 30 °C under an Ar purge after a 2 h, 200 °C thermal pretreatment under Ar.

Scheme 2. Possible Adsorption Mechanisms Mediating CO₂ Binding onto PNH Materials Including (a) CO₂ Binding in the Form of Interstitial Bicarbonates within Anion Interlayers, (b) CO₂ Adsorption onto Surface Hydroxyls, and (c) CO₂ Binding Enabled by the Molecular Adsorption of Water

Figure 5

(a) DAC adsorption capacity as a function of water pressure for the three α-Ni(OH)₂ materials tested, adsorption under 400 ppm of CO₂ at 25 °C, (b) effect of adsorption temperature on α-NiCe_0.1(OH)_x DAC capacities under dry and humid (2.1 kPa water) conditions, and (c) water adsorption isotherms for α-NiCe_0.1(OH)_x at 25, 50, and 75 °C, and DAC capacities at the corresponding adsorption temperatures and water pressures. The horizontal dashed line represents the estimated water monolayer coverage for α-NiCe_0.1(OH)_x at 3.6 molecules/nm².

Figure 6

CO₂ adsorption energies of PNH materials measured using microcalorimetry at incrementally increased CO₂ pressure at 30 °C. Samples were thermally pretreated for 2 h under vacuum at 200 °C prior to CO₂ adsorption.

Figure 7

FTIR spectra of α-Ni(OH)₂ measured under 400 ppm CO₂ without water vapor (dry), 400 ppm CO₂ and 2.1 kPa H₂O (humid), and 400 ppm CO₂ and 2.1 kPa D₂O (humid D₂O). (b) Evolution in FTIR spectral data over α-Ni(OH)₂ upon switching from a stream containing 400 ppm CO₂ and 2.1 kPa D₂O (humid D₂O) to one containing only 400 ppm CO₂; spectra are reported either 5 or 30 min after switching to a dry stream. Reported spectra are background subtracted from those measured under Ar before exposure to CO₂.

Scheme 3. Depiction of the Proposed Effect of Hydration on the Free Energies of (Bi)Carbonates Formed through CO₂ Binding onto Metal Hydroxide Surfaces

Figure 8

(a) Average adsorption rates for α-NiCe_0.1(OH)_x as a function of the relative humidity of the feed stream. Rates were calculated from breakthrough curves measured at 25 °C under 400 ppm CO₂. (b) Cyclic DAC performance of α-NiCe_0.1(OH)_x over 5 cycles. Adsorption conditions: 25 °C, 400 ppm CO₂, balance N₂. Regeneration conditions: 100 °C for 2 h under 2.1 kPa H₂O, 20 kPa O₂, balance Ar.