High capture capacity of CO₂ in a nickel intercalated Ti₃C₂T_x MXene–fluorohectorite clay heterostructure

Abstract

Carbon dioxide (CO₂) capture under elevated pressure conditions is of particular relevance for pre-combustion capture and syngas purification processes. Here, we report CO₂ adsorption in a nickel-intercalated titanium carbide Ni–Ti₃C₂T_x MXene–fluorohectorite clay heterostructure, designed to modify the high-pressure adsorption behavior characteristic of pristine MXenes. The heterostructure exhibits a CO₂ adsorption capacity of 1.909 mmol g⁻¹ at 50 bar and retains measurable uptake upon pressure release, with 0.602 mmol g⁻¹ remaining at 1 bar after desorption. These results indicate that MXene–clay heterostructures are promising candidates for high-pressure CO₂ separation, while also providing a platform for future exploration of CO₂ conversion strategies beyond the scope of the present study.

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Introduction

Overproduction of greenhouse gases such as CO₂, and lack of ‘green’ fuels, can be targeted simultaneously in one “package” – using nanoporous materials that both can capture CO₂ and subsequently convert it into environmentally friendly fuels by nanoconfinement and catalysis.^1–3 Among materials suitable for CO₂ capture and separation, layered silicates are particularly promising,^4–12 offering high CO₂ volumetric uptake at low pressures with fast kinetics, and full desorption at ambient pressures.¹³ The Ni-cation-exchanged synthetic fluorohectorite clay mineral (Ni-Fht) can capture up to 6.45 mmol g⁻¹ of CO₂ at 55 bar.¹⁰ CO₂ is naturally released within minutes when the pressure is lowered to ambient or slightly below.^5,6

Among available materials, two-dimensional (2D) transition metal carbides, nitrides or carbonitrides known as MXenes stand out as one of the most promising catalysts for CO₂ conversion to ‘green’ value-added chemicals such as hydrocarbon fuels.^13–17

Experimental studies show that dimethyl sulfoxide (DMSO) intercalated titanium carbide (Ti₃C₂T_x) MXene has an adsorption capacity of 5.79 mmol g⁻¹ at 40 bar and slow CO₂ release from the interlayers in a range of months, which makes the storage in this system very interesting for potential real applications.¹⁸ For pristine Ti₃C₂T_x MXene, the gravimetric adsorption capacity of CO₂ into the interlayer is 1.3 mmol g⁻¹ (ref. 18) at 40 bar. Li–Mo₂C has reported adsorption capacity 3.66 mmol g⁻¹ at 40 bar.¹⁹ The largest CO₂ adsorption capacity reported was reached for surface adsorption on termination-depleted Ti₃C₂ flakes, not the powder or compact sample, reaching 12 mmol g⁻¹.²⁰

Comparing clays and MXenes directly, they both provide high surface area and tuneable interlayer spacing that allows CO₂ to enter, if intercalated with proper cation/species in the interlayer. Clays are typically low-cost, and are especially efficient in reversible and selective capture of CO₂ at lower pressures and easy desorption, whereas MXenes reported in experimental works are efficient at high pressures, provide strong CO₂ binding and slow release and have high catalytic activity for CO₂ reduction.^17,21,22

This work was motivated by the question of whether chemical heterogeneity introduced by clay–MXene stacking could modify the pressure dependence and reversibility of CO₂ intercalation in MXenes. A key motivation for introducing the Fht clay component was to shift the CO₂ adsorption onset of MXenes toward lower pressures, as pristine MXenes typically starts to show uptake only above ∼20–25 bar. Moreover, heterostructures of 2D materials often exhibit emergent properties that cannot be predicted from their individual components. Exploring such systems is therefore essential for establishing design and synthesis rules rather than optimizing a single performance parameter.

We also wanted to understand how chemically heterogeneous 2D interfaces modify gas intercalation, retention, and reversibility in layered solids. We aimed to use advantages of two layered materials – clays for efficient CO₂ capture at low pressures and MXenes with their large CO₂ binding capacity as mentioned above. Also, as our previous studies on clays show, presence of Ni species in the interlayer is the key in CO₂ capture,^6,8,23–25 so we incorporated it into the final heterostructure. As the last point, dilution of MXenes with clay will lower the cost of the final material, compared to pure MXene, hence it minimizes ecological footprint.

Materials and methods

As our model system, we have assembled a MXene–clay heterostructure with the Ni species in the interlayers as described further, using single nanosheets of clay and Ti₃C₂T_x MXene (Fig. 1). Initially, Ti₃C₂T_x MXene (Fig. 2a) was synthesized via an optimized top-down wet-chemical etching of Al-layers from Ti₃AlC₂ MAX phase.²⁶ Then, the synthetic fluorohectorite clay (Fht, Fig. 2b) was prepared using a protocol described by Breu et al.²⁷ Individual nanosheets of Fht clay and Ti₃C₂T_x MXene were analyzed using AFM and SEM (Fig. 3, Fig. S1 and S2 in the SI).

Fig. 1 Schematics of the synthesis protocol.

Fig. 2 Crystal structure of (a) Ti₃C₂–OH MXene, (b) fluorohectorite clay, (c) illustration of MXene stack and clay single sheet, (d) heterostructure of MXene and clay, (e) process of drying and CO₂ intercalation into the MXene stack with contraction and subsequent expansion of interlayer.

Fig. 3 XRD pattern of the wet and dry Ni-Fht (a) and Ni–Ti₃C₂T_x MXene (c). As the physical structure of Ni–Ti₃C₂T_x MXene was the thin film, drying was not sufficient and interlayer remained partially wet. In the insets SEM images of (b) Fht stacks and (d) Ti₃C₂T_x MXene single sheets.

The Ni intercalated Ti₃C₂T_x MXene–Fht clay heterostructure was prepared in two main steps (Fig. 1) by first mixing 100 ml of 0.5 wt% of precursor clay Na-Fht²⁷ in water with 50 ml of 1 wt% Ti₃C₂T_x MXene and stirring under N₂ flow for 1 hour. In step 2, Ni interlayer intercalation was done in a 1 M water solution of NiCl₂ (stock solution of 5 g of NiCl₂ in 100 ml of water). Next, we quickly added 100 ml of 1 M NiCl₂ to the mixture of Fht clay and Ti₃C₂T_x MXene nanosheets and stirred under N₂ bubbling for 18 hours. The sample was centrifuged and washed with deionized water 3 times at 30 000 RCF for 30 min. The supernatant was decanted after each centrifugation cycle and the sample was vortexed for 10 min after adding deionized water into the sediment of the Ni–Ti₃C₂T_x MXene–Fht clay sample on the bottom of the Falcon tube. After the last wash, the sample was freeze-dried for a week. An illustration of the resulting structure is shown in Fig. 2c and d. The Ni–Ti₃C₂T_x MXene reference sample was prepared as described above in step 2, just skipping addition of Na-Fht clay in the beginning (step 1 in the preparation route protocol).

For the AFM measurements, 0.1 wt% water solution of delaminated nanosheets was drop-casted on plasma functionalized Si@SiO₂ (100 nm) substrate. The sample was scanned using MultiMode AFM operating with the Nanoscope III controller. Images were captured in tapping mode, using TAP190Al-G AFM probes purchased from Budget Sensors. Images were processed using Gwyddion software²⁸ by aligning rows, flattening and scar corrections.

In situ CO₂ intercalation experiments were done at the BM01 beamline, ESRF, France in transmission mode using PILATUS3 X_2M detector, wavelength of X-ray beam was 1.042 Å. The samples were loaded into closed quartz capillaries, with glass wool filled above the powder sample. Samples were mounted on the goniometer head connected to a CO₂ handling system. First, prior to CO₂ exposure, the sample was heated under continuous pumping of vacuum at 120 °C, using an Oxford cryostream700+, for 2 hours, until we did not observe any shift of basal peak corresponding to distance between individual clay and MXene nanosheets due to the removal of water from the interlayers. After drying, studied sample was cooled to 20 °C under vacuum. In the next step, the studied samples were exposed to CO₂ at 10 and 35 bar, respectively. After exposing the sample to 35 bar, the pressure was reduced to ambient. During the whole in situ CO₂ intercalation experiment, scans were recorded continuously at 1 s intervals. A schematic of the drying and CO₂ intercalation is displayed in Fig. 2(e).

Gravimetric CO₂ adsorption measurements were conducted using an in-house commercial apparatus including a Rubotherm high-pressure magnetic suspension balance.²⁹

Before the measurement, the sample was outgassed at 120 °C for 3 hours under secondary vacuum. The pressure is measured with Tecsis-Series P3382. The temperature was recorded with Pt-100 sensor placed close to the sample crucible. The complete system is placed into a heating chamber to maintain constant temperature within 25 ± 0.3 °C. The suspension balance has a resolution of 10 μg. The sample mass variation m_meas (g) is measured as well as pressure and temperature when equilibrium is reached (with our criterion, when four of the five last mass measurements (noticed each 5 min) are included in an interval of 50 µg). The measured quantity is the excess adsorbed amount, which is obtained by correcting for the buoyancy of the skeletal volume of the sample material and the suspended metal parts. This volume is evaluated by a direct helium buoyancy effect measurement, according to helium not adsorbed at high pressure (10 bar up to 100 bar). The gas phase densities were determined using the equation of state for CO₂ ³⁰ and Helium.³¹

After the first adsorption–desorption cycle, we performed another CO₂ adsorption–desorption cycle after pumping the sample in vacuum for 2 hours. Moreover, adsorption–desorption was measured again after 10 months, when the sample from the first gravimetric experiments, after exposure to CO₂, was left at an ambient atmosphere.

Fourier transform infrared (FTIR) spectra of the sample were obtained using Thermo Scientific Nicolet iS50 FTIR Spectrometer (resolution 2 cm⁻¹, DTGS detector, KBr beamsplitter, Happ-Ganzel apodization, KBr windows) in region 400–4000 cm⁻¹ with transmission technique. The samples were suspended in nujol mull.

Results and discussion

Following the heterostructure sample preparation and mass ratio of clay and MXene in the resulting structure, total surface area of clay, a^clay_svs. surface area of Ti₃C₂, a^MXene_s is a^clay_s/a^MXene_s = 1.621. If we assume that a typical Fht clay sheet has dimension from 2 × 2 µm² up to 70 × 70 µm², with median surface area of 8.3 × 6 µm² (Fig. 3a and S1) and Ti₃C₂T_x MXene nanosheet has median size 1.5 × 2.6 µm², as is demonstrated by AFM and SEM (Fig. 3b and S2) and in accordance with the literature reports,³² the ratio of Fht clay to Ti₃C₂T_x MXene nanosheets is approximately 1 : 8.

There are three types of interfaces that we assume in the heterostructure here: MXene–MXene, MXene–clay and clay–clay, as well as wedge-like pores of the clay–MXene aggregate. Considering the clay–clay interface, if it is present at all, the amount of this is negligible considering the amount of clay nanosheets and their size vs. Ti₃C₂T_x MXene sheets, assuming homogeneous distribution of clay nanosheets within the whole sample (Fig. 2d).

Inspecting the reference samples of pure Ni-Fht clay and Ni–Ti₃C₂T_x MXene, there are sharp peaks before and after drying the samples, corresponding to a Ni-intercalated superstructure interlayers (see Fig. 3a and c)-(002) for the wet Ni-Fht clay with d-spacing of 14.8 Å and (002) for the Ni–Ti₃C₂T_x MXene with d-spacing 15.0 Å, both in the wet state. However, for the Ni–Ti₃C₂T_x MXene–Fht clay heterostructure, we observe two broad peaks after drying, that can be attributed to two different interfaces, of MXene–MXene and MXene–clay. The clay nanosheets are distributed and diluted in the heterostructure and thus clay–clay layers do not contribute significantly to the diffraction pattern with the out-of plane diffraction peaks.

Our previous publication²⁵ demonstrates formation both of Ni²⁺ and Ni-hydroxide in the clay interlayer, where that latter interlayer species (exact formula^23,25 [Ni(OH)_0.83(H₂O)_1.17]_0.37^1.17+) is responsible for the capture of CO₂ in the clay interlayer. We suggest that a similar mechanism might also be present in the Ni–Ti₃C₂T_x MXene–clay heterostructure, as our FTIR spectra clearly proves presence of α-Ni(OH)₂ (see the SI and Fig. S4).

Following the position of basal diffraction peak, we observed that the Ni–MXene–clay heterostructure d-spacing, d_basal, shrinks from 14.4 Å in one water layer hydrated state to 12 Å in the dried state. After cooling the sample down to 20 °C, partial intercalation of evaporated water is observed, expanding the interlayer to 13.9 Å, probably because of water trapped during evaporation in the glass wool in the capillary above the powder sample. The initial d-spacing of the partially hydrated sample before exposure to CO₂ is thus 13.9 Å. After CO₂ exposure, the interlayers expand within a few seconds already at 10 bar (Fig. 4a and b), followed by further expansion at 35 bar with d(10 bar) = 14.3 Å and d(35 bar) = 14.5 Å. The total expansion of interlayer is 0.6 Å. Comparing this with the pure Ni–Ti₃C₂T_x MXene sample, its interlayer expansion at 35 bar is much smaller: 0.3 Å (Fig. S3).

Fig. 4 (a) basal stacking X-ray Bragg peak for the Ni–Ti₃C₂T_x MXene–Fht clay heterostructure, corresponding to d-spacing of stacks for dry sample, sample cooled after drying and sample after exposure to 10 and 35 bar. (b) d_basal of heterostructure at different pressures of CO₂. (c) Gravimetric CO₂ adsorption isotherm for the Ni–Ti₃C₂T_x MXene–Fht clay heterostructure at 25 °C in the first and second cycle and after leaving the sample to recover after CO₂ adsorption for 10 months.

Gravimetric adsorption data were taken in two repeated cycles and after 10 months (Fig. 4c). Original adsorption data show that the adsorption capacity of CO₂ in the Ni–MXene–clay heterostructure is 1.909 mmol g⁻¹ at 50 bar. The desorption branch shows substantial hysteresis, a large amount of CO₂ stays adsorbed during the pressure decreases, especially at CO₂ low pressure range. After releasing the pressure to 1 bar, CO₂ is not completely released from the interlayer such as is the case for clays,^4–8,33 an amount corresponding to 0.602 mmol g⁻¹ of CO₂ stays trapped in the interlayers. The amount of CO₂ adsorbed increases gradually with pressure. The shape of isotherm is due to multilayer adsorption due to the increase of the interlayer distance with the CO₂ adsorption. The observed CO₂ capacity is comparable with the highest adsorbing Ni-intercalated Fht with surface charge 0.3 per unit cell, 2.14 mmol g⁻¹.³⁴ The values observed for pristine Ti₃C₂T_x MXene intercalated with Na is 1.3 mmol g⁻¹ (ref. 18) after intercalation into the interlayers between individual nanosheets and 12 mmol g⁻¹ for surface adherence of CO₂ in termination-depleted Ti₃C₂.²⁰

Gravimetry data in the 2^nd cycle taken after the first adsorption cycle demonstrates that the capacity of the heterostructure is not recovered immediately, as part of CO₂ stays adsorbed. However, data taken 10 months after the original experiments demonstrate full recovery of the system, as the original adsorption capacity is reached again (Fig. 4c).

Compared to pristine MXenes, which exhibit negligible CO₂ adsorption below ∼20 bar, the heterostructure shows measurable uptake already at lower pressures (see Fig. 4), indicating a widening of the effective adsorption window reported for pure MXenes so far. This effect is particularly relevant for high-pressure gas separation scenarios, where even moderate reductions in the required operating pressure can be technologically meaningful.

Compared to state-of-the-art MOFs^35,36 such as the CALF-20,³⁷ with adsorption capacity of 2.55 mmol g⁻¹ at 0.2 bar,³⁸ MOF-74 with 10.4 mmol g⁻¹ at 35 bar and 4.9 mmol g⁻¹ at 1 bar,³⁹ MOF-177 with 33.5 mmol g⁻¹ at 35 bar,³⁹ MOF-2 with 3.2 mmol g⁻¹ at 35 bar and 0.7 mmol g⁻¹ at 1 bar,³⁹ MOF-505 with 10.2 mmol g⁻¹ at 35 bar and 3.3 mmol g⁻¹ at 1 bar,³⁹ MOF-5 with 2.1 mmol g⁻¹ at 1 bar (ref. 40) and 15.22 mmol g⁻¹ at 14 bar;⁴¹ zeolites^35,42,43 such as ZIF-8 with 5.11 mmol g⁻¹ at 30 bar (ref. 44) and 0.8 mmol g⁻¹ at 1 bar,⁴⁵ SBA-15 with 0.4 mmol g⁻¹ at 1 bar,⁴⁶ zeolite 4A with 6.217 mmol g⁻¹ at 1 bar;⁴⁷ our sample's absolute capacity is moderate, the unique pressure window and partial retention distinguish this system (see summary table for comparison of materials for CO₂ capture in the SI).

Our study also demonstrated that Ni intercalation substantially enhances the adsorption of CO₂ compared to Na-intercalated Ti₃C₂T_x MXene. Unlike in clays that release CO₂ after pressure is decreased back to atmospheric pressure, Ni–Ti₃C₂T_x MXene–Fht clay heterostructure keeps temporarily ∼30% of CO₂, with respect to the amount of CO₂ adsorbed at 50 bar (Fig. 4c). The reason for this is likely due to the strong reactivity of CO₂ with –OH and surface terminations^48–51 of Ti₃C₂T_x MXene as well as to presence of lone pairs of electrons.⁵² Clays do not have surface terminations that react with CO₂, thus CO₂ adheres to the interlayer species and is only partly released after the pressure decrease. Assuming that CO₂ in the Ti₃C₂T_x MXene–MXene and MXene–clay interlayers binds both to the Ni hydroxide islands and to the MXene surface, we suggest that CO₂ attached to the Ti₃C₂T_x MXene surfaces remains in the interlayers and that CO₂ adsorbed by Ni hydroxide inclusions is released at ambient pressure similar to the case of Ni-intercalated clays. This idea is supported by the strong binding energies of CO₂, E_ads reported for the Ti₂C and Nb₂C MXene sheets in a range −1.89 to −2.77 eV,⁵³ corresponding to chemisorption, compared with the E_ads of CO₂ binding on pure Ni(OH)₂: −1.56 eV (ref. 54) and E_ads of CO₂ on Ni intercalated fluorohectorite clay: −0.72 eV.⁸ Unfortunately, both FTIR (see the SI) file and XPS⁵⁵ do not allow to determine at which sites CO₂ binds, so we do not have direct experimental proof of binding sites of CO₂ in our heterostructure.

The present work supports the idea that opening the interlayer space between nanosheets and exposing a larger surface area of individual nanosheets, allows intercalating more CO₂. This implies that the intercalation of DMSO⁵⁴ or other large molecule into the interlayers can substantially increase CO₂ non-reversible uptake by the heterostructure, and we plan such studies as a next step. Also, adding more clay nanosheets into the heterostructure might enhance reversible CO₂ adsorption.

Conclusions

The aim of the study focused on tailoring CO₂ adsorption behaviour toward pressure regimes that are relevant for pre-combustion CO₂ capture, operating with the elevated CO₂ partial pressures. The combination of MXene with a layered clay component was designed to modify the adsorption characteristics of pristine MXenes, which typically show negligible CO₂ uptake at low pressures and significant adsorption only above ∼30–40 bar.

We demonstrated that Ni intercalated-Ti₃C₂T_x MXene–fluorohectorite clay heterostructure with the Ti₃C₂T_x MXene to synthetic fluorohectorite clay nanosheet ratio 8 : 1 adsorbs large amount of CO₂, reaching 1.909 mmol g⁻¹ at 50 bars and residual 0.602 mmol g⁻¹ after release of pressure to atmospheric pressure. This means that 30% of CO₂ captured at 50 bars stays in the structure, intercalated between Ti₃C₂T_x MXene and clay nanosheets. We postulate that upon CO₂ intercalation, CO₂ binds both to the Ti₃C₂T_x MXene surface termination groups and Ni hydroxide inclusions in the interlayer, and part of CO₂ which is immobilized on surface remains in the structure.

The results also demonstrate that the heterostructure retains the high-pressure adsorption capability characteristic of MXenes while enabling measurable CO₂ uptake at lower pressures compared to the pristine material, effectively widening the operational pressure window. This shift in adsorption onset is relevant for high-pressure gas separation scenarios, where even moderate reductions in the required operating pressure can be technologically desired.

While the material also exhibits CO₂ adsorption at near-atmospheric pressure, the present performance indicates that the heterostructure is not primarily optimized for post-combustion CO₂ capture, which operates under low CO₂ partial pressures and requires high selectivity and capacity in the low-pressure regime. Instead, the current results position the MXene–clay heterostructure as a promising candidate for high-pressure CO₂ separation, with potential for further optimization through compositional and structural tuning.

The partial retention of CO₂ upon pressure release to atmospheric conditions suggests that the MXene–clay heterostructure may provide a platform for further optimization of adsorption performance and for future exploration of CO₂ conversion strategies, which are beyond the scope of the present study.

Author contributions

BP and JOF conceptualized research. BP prepared Ni–MXene–clay heterostructures, did AFM, synchrotron experiments, processed and analysed data and wrote original and revised versions of manuscript. AT and NCBS synthesized and characterized Ti₃C₂T_x MXene. Group of J. Breu from Bayreuth University synthesized clay. NH performed gravimetric adsorption measurements. HD and AHS participated at synchrotron experiments. IM performed FTIR experiment and interpreted data. KWBH provided data on pure Ni-Fht. SR measured XPS. GW, BA and JOF supervised research. All authors reviewed the manuscript. We acknowledge Daniel Hohenberge for synthesis of clay.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data are available by request from Barbara Pacakova, Jon Otto Fossum and Babak Anasori.

SI contains details about AFM, gravimetry, CO₂ capture in Ni-Ti₃C₂T_x, FTIR, and summary table of adsorption capacity of different materials with references. See DOI: https://doi.org/10.1039/d5nr03026a.

Acknowledgements

Work was supported by the funding from the Research Council of Norway, project numbers 250619 and 315135.

We acknowledge the assistance provided by the Advanced Multiscale Materials for Key Enabling Technologies project, supported by the Ministry of Education, Youth, and Sports of the Czech Republic. Project No. CZ.02.01.01/00/22_008/0004558, Co-funded by the European Union. The UMON's authors acknowledge the financial support form the European Regional Development Fund (ERFD/FEDER) CRUCIAL-SORBINOV co-financed by the Walloon Region.

We also acknowledge funding support from the U.S. National Science Foundation, award number CMMI-2134607, and from Dutch Polymer Institute (DPI), project number 854.

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