Fundamental Insight into Humid CO 2 Uptake in Direct Air Capture Nanocomposites Using Fluorescence and Portable NMR Relaxometry

. Direct air capture (DAC) technology is being explored as a pathway for reducing greenhouse gas emissions through the efficient removal of CO 2 from the atmosphere. However, there remains a knowledge gap regarding structure-property-performance factors that impact the behavior of these systems in diverse, real-world environments. In aminopolymer-based DAC systems, gas diffusion is tightly coupled with polymer mobility, which is in turn affected by a large matrix of variables, including interactions with the pore wall of the support, nanoconfinement, the presence of co-adsorbates (moisture), and electrostatic crosslinks that develop as a function of CO 2 chemisorption. Higher throughput, benchtop techniques for studying and understanding mobility in these systems would lead to more rapid advances in the field. Here, we demonstrate the value of a fluorescence technique for monitoring polymer mobility within nanocomposite capture materials as a function of CO 2 and water adsorption in a series of humidified polyethylenimine-Al 2 O 3 composite materials. The approach allows us to correlate changes in mobility with CO 2 adsorption kinetics as a function of relative humidity. We further couple this information with NMR relaxometry data attained using a portable single-sided magnetic resonance device, and we employ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to correlate the formation of different relative amounts of carbamates and carbonates with the environmental conditions. These results provide a blueprint for using benchtop techniques to promote fundamental understanding in DAC systems that can in turn enable more efficient operation in real-world conditions.


Introduction
While the global clean energy transition continues to gain momentum, 1 certain industries will be intrinsically difficult to fully decarbonize. 2Achieving net zero carbon emissions in those instances will ultimately require the use of robust carbon capture, utilization, and storage technologies. 35][6] Amongst the direct air capture (DAC) technologies currently being investigated for pilot and commercial scale deployment, solid adsorbents based on supported amines are garnering much enthusiasm. 7][10][11][12][13][14] However, certain fundamental knowledge gaps remain that will influence how these and similar materials are employed over long cycle times, [15][16][17] in different environments, 18,19 and within different operational design constraints in real-world DAC systems. 20e such knowledge gap involves structure-property-performance relationships associated with polymer segmental mobility in confinement. 21The mobility of these aminopolymer sorbents directly influences gas diffusion through these materials. 22As CO 2 reacts with available amines within the polymer matrix, the formation of electrostatic crosslinks in the form of carbamate species can significantly rigidify the polymer, 23 effectively converting it from a liquid or rubbery sorbent into a glassy encapsulant.The physical state of the polymer adsorbent not only affects optimal cycle times for CO 2 sorption; it also governs subsequent O 2 diffusion in and out of the material.As residual oxygen in the sorbent chamber during the CO 2 desorption step can lead to fast sorbent degradation, 24 selective O 2 removal prior to CO 2 desorption plays a key role in optimizing amine-based sorbent lifetimes.This optimization requires fundamental understanding of gas diffusion in these materials, and hence polymer mobility.Aminopolymer mobility is governed by a matrix of complex and often counterbalancing factors.These include the architecture and molecular weight of the polymeric sorbent; 25 surface and interface effects in thin films; [26][27][28] the presence of moisture and other additives that can serve as plasticizers 23 and even change the mechanism of adsorption; 29 partial degradation of the sorbent 30 that can lead to rigid moieties 31 in the polymer; and relative CO 2 uptake that results in various degrees of ionic cross-linking. 133][34] We recently outlined the merits and drawbacks of an array of techniques for studying polymer mobility in confinement, 35 which included differential scanning calorimetry, 36 dielectric spectroscopy, 37 NMR, 21 and neutron scattering, 38,39 among others. 40,41 owever, given the large matrix of variables that influence polymer mobility in confinement that can change as a function of lifetime and cycling, higher throughput techniques for quantifying mobility could lead to more rapid advances in the understanding and deployment of these materials in DAC systems.
3][44][45] We recently detailed the development of a powerful fluorescent probe designed specifically to study aminopolymer mobility, and we demonstrated its usefulness in studying DAC sorbents. 35Here, we build on that foundational study by examining poly(ethylenimine) (PEI) mobility as a function of CO 2 uptake across a wide range of temperatures and relative humidity (RH) in a mesoporous γ-Al 2 O 3 composite.We further couple this information with NMR relaxometry data attained using a versatile and portable magnetic resonance device known as NMR Mobile Universal Surface Explorer (MOUSE). 46We then employ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to correlate the formation of different relative amounts of carbamates and carbonates with the environmental conditions.This approach provides a blueprint for using benchtop analytical techniques to promote fundamental understanding of structure-property-performance relationships that can in turn enable more efficient operation of DAC materials in a variety of real-world climates and environments.

Moisture and Fluorescence.
We previously detailed the merits of employing fluorescent probes based on tetraphenylethylene (TPE) in a proof-of-concept study for qualitatively comparing aminopolymer mobility across confined mesoporous DAC composites. 35Here, we develop these probes further to study branched PEI mobility in model DAC systems as a function of real-world operating conditions, beginning with moisture uptake as a function of RH exposure.PEI composites with γ-Al 2 O 3 are quite hygroscopic; 24 a composite that is 40 wt.%PEI can uptake as much 1.7 equivalents in weight of water relative to PEI when allowed to equilibrate in a stream of humid N 2 (vide infra).This amount of moisture can significantly alter the mobility of the PEI matrix.However, disentangling the effects of moisture on the fluorescence response of these probes requires a thorough understanding of the quenching mechanisms at play.TPE-based molecules behave as aggregation induced emission (AIE) probes; they tend to be non-emissive when well solvated in low or non-viscous solvents, where unrestricted intramolecular rotations provide non-radiative pathways for excited state decay.However, in viscous or glassy media, or in an aggregated state, these rotations become restricted to varying degrees, and a strong wavelength dependent emission is then observed from TPE that is a function of the mobility of its matrix.More details of the mechanism responsible for this behavior have been thoroughly reviewed elsewhere. 47,48 n contrast to aggregation caused quenching (ACQ) probes such as perylene, 49 where water can induce aggregation and p-stacking interactions that open non-radiative decay pathways, the addition of water to a non-emissive AIE probe solution will often induce aggregation and turn on fluorescence.While high energy vibrations in water molecules can themselves quench fluorescence, 50 the restriction of intramolecular rotation appears to dominate the fluorescence response of most AIE probes in aqueous solutions.
In this work, we employ tetrakis(4-hydroxyphenyl)ethylene (THPE) as our AIE probe.The hydroxyl groups dramatically enhance its solubility in PEI relative to the parent TPE compound.
We recently demonstrated that THPE emits near 460 nm and appears blue in a frozen/glassy/aggregated state, while it appears green and emits closer to 530 nm in a viscous PEI solution. 35We further demonstrated that the ratio of emission at these two wavelengths can serve as a measure of the relative mobility of the polymer matrix.The fluorescence of THPE in mixed alcohol/water solutions was also recently investigated. 51Between 0 and 82 vol.% water in ethanol, no THPE fluorescence could be detected as the probe was still well solvated.Only when the water fraction was at or above 84 vol.% was blue emission observed, as THPE aggregated/precipitated under these conditions.
Here, we prepared a similar series of THPE solutions in PEI and water to determine the relative volume fractions we could attribute change in the THPE fluorescence response to change in polymer mobility, and at what volume fractions of water we would need to account for probe aggregation.In all cases, the concentration of the probe was 0.1 wt.% relative to the total mass of the solution.Between 0 and 90 vol.% water, we only observed green fluorescence.As can be seen in Figure 1a, the intensity of the fluorescence systematically decreases as the water content increases and sample viscosity decreases.However, between 90 and 95 vol.% water, THPE began to aggregate and stick to the walls of the cuvette, where its blue emission could be observed.Bright blue emission was pronounced in 100% water, where THPE formed a heterogeneous suspension.
The results suggest that changes in the photoluminescence quantum yield (PLQY) of THPE as a function of water concentration in aqueous PEI solutions ≤ 90 vol.% water are not caused by probe aggregation.We acknowledge results in bulk solutions are not perfectly transferable to confined systems.Even though the overall fraction of water present in composites equilibrated at 100% RH (vide infra) is considerably lower than what we measured here in the bulk, it is known that confinement can, in some cases, change the solubility of small molecules relative to bulk solutions. 52However, the results are an important first step in quantifying AIE response of THPE in humidified PEI.Turning to confined systems, we then prepared a series of composite samples equilibrated for 72 h at different values of RH according to a procedure described in the Methods section.The amount of water adsorbed by a 40 wt.% PEI composite (12 mg of PEI in 18 mg of Al 2 O 3 ) at each RH is recorded in Table 1.A systematic increase in moisture uptake between ~2 and 20 mg is observed in this series.The composite also contained ~0.4 wt.% THPE relative to PEI.The emission spectrum of each humidified composite is illustrated in Figure 1b.The emission l max are recorded in Table 1.Several observations are worth noting about the data in Figure 1b.First, if we compare the dry bulk PEI spectrum with that of the dry composite, there is a dramatic blue shift in the emission from l max of 533 nm to near 468 nm in confinement, consistent with a sharp reduction in mobility of the polymer matrix.These results are also consistent with our previous work in mesoporous silica composites using both tethered and dispersed TPE derivatives. 35If we then consider the samples equilibrated to different values of RH, we observe a systematic red shift in l max as a function of increasing RH.This result is consistent with the literature which has hypothesized that moisture acts as a plasticizer for PEI chains that can weaken the inter-and intramolecular hydrogen bonding and dipole-dipole interactions in PEI, 23 all which will enhance its mobility.Interestingly, the l max of the emission spectrum of the composite sample equilibrated to 100% RH is nearly identical to that of the bulk dry sample, suggesting that the effects of confinement and humidity on mobility may counterbalance each other.While care should be exercised in not overinterpreting the fluorescence response, since (as mentioned) the high energy vibrations in water molecules can themselves also contribute to fluorescence quenching, the implications of these fluorescence measurements on PEI mobility are fully consistent with the NMR relaxometry data that will be presented in a subsequent section.
Finally, we monitored the fluorescence response of a dry composite by flowing 100 sccm of 100% RH N 2 through the sample over the course of 1 h (Figure 1c).The emission l max immediately begins red shifting and a new peak grows in near 530 nm, suggesting mobility increases as the sample is becoming humidified.However, the results also indicate the composite has not yet reached equilibrium after just 1 h.Going forward, all samples were allowed to equilibrate at a given RH for 72 h prior to further measurements or manipulation.
Two samples of a 40 wt.%PEI:Al 2 O 3 composite (0.4 wt.% THPE) were prepared.The first was kept under a dry, inert (N 2 ) environment, and the second was pseudo-saturated with CO 2 in a stream of simulated flue gas (45 sccm N 2 , 5 sccm CO 2 ) for 12 h at 30 °C.The fluorescence spectrum of each sample was then recorded at 30, 0, and -30 °C.In previous work, 35 we demonstrated how quantifying the emission spectra via ratiometric analysis at two wavelengths as a function of temperature can give accurate indication of relative polymer mobilities across different samples.Here, we perform this analysis at these three temperatures as we flow CO 2 over the pristine composite.We then compare how much the mobility of the polymer changes at a given temperature relative to its theoretical 'limit' when saturated with CO 2 .The normalized data is illustrated in Figure 2.  If we first consider the data at 30 °C, we observe that 75% of the total mobility change occurs within the first 1 h of simulated flue gas exposure (black trace, Fig. 2).The remaining changes occur very slowly over the next 11 h.In contrast, after 1 h at 0 °C, mobility only changes by 30% of its pseudo-theoretical limit.Less than 10% change in mobility is seen at -30 °C in this time.The samples at 0 and -30 °C effectively level off after 1 h and do not approach their theoretical limit, even after 12 h of CO 2 exposure.The results can be rationalized by considering that PEI becomes glassier much faster or with less overall CO 2 uptake at colder temperatures, which in turn inhibits further gas diffusion and uptake.The activation barrier for CO 2 adsorption and/or proton transfer may also play a more significant role at these lower temperatures.We contend this technique will be particularly useful for studying an array of materials and additives for understanding and optimizing sub-ambient capture processes, 18,53 but we turn now to studying changes in polymer mobility under a variety of simulated DAC conditions, namely 100 sccm of 400 ppm CO 2 in N 2 at different RH.
After equilibrating for 72 h at a given RH, a 40 wt.%PEI:Al 2 O 3 composite was loaded into a flow through quartz cuvette, where the exhaust gas was routed through a LiCOR 850 quantitative CO 2 /H 2 O infrared analyzer.This allowed us to simultaneously monitor CO 2 sorption, RH of the sweep gas, and any changes in the emission spectra of the sample.Six different samples were equilibrated to the same RH values employed in Table 1.Changes in the emission spectra of a representative sample (at 53% RH) as a function of time and CO 2 uptake is illustrated in Figure 3a, with the emission spectra of the other samples shown in Figure S1 in the Supporting Information (SI).Changes in all 6 samples are quantified in Figure 3b, plotted as ratiometric intensities recorded as a function of time.As can be seen in Fig. 3a for the data at 53% RH, flowing humid CO 2 causes a blue shift in the emission spectra and an overall increase in the PLQY.The data is consistent with a decrease in polymer mobility with CO 2 uptake, and it stands in contrast to the data in Fig. 1c, where flowing CO 2 -free moisture over an otherwise dry system causes a redshift in the emission spectra, a decrease in the PLQY, and increased polymer mobility.When comparing the change in the ratiometric intensity of all six composites in Fig. 3b, a few initial observations are worth noting.
First, the drier the sample, the more rapid the polymer mobility changes and becomes glassier as it begins uptaking CO 2 .Also noteworthy, the data suggests that samples between 0 and 33% RH all reach approximately the same mobility after 90 min under these conditions.Above 33% RH, mobility after 90 min of CO 2 capture trends higher with increased RH.
Full interpretation of the data in Fig. 3b requires that CO 2 sorption data for these six samples be considered.We illustrate in Figure S2 of the Supporting Information how we can monitor both adsorption and desorption in a continuous stream of 400 ppm CO 2 .Integration of the area between the curve and the 400 ppm baseline can be used to quantify CO 2 uptake in these experiments, with the adsorption and desorption cycles in full agreement.In Figure 4, we plot the CO 2 adsorption data for all six samples collected concurrently with the fluorescence measurements above.When individual data traces in this figure approach 400 ppm, it is an indication they have stopped uptaking significant amounts of CO 2 from the 400 ppm stream.Closer inspection of the data reveals that at 90 min, the driest sample is near saturation, while the sample under 100% RH has uptaken twice as much CO 2 and is still adsorbing it.Indeed, there is a strong correlation of CO 2 uptake with RH.When we consider both fluorescence and CO 2 sorption data, we observe that the dry samples become glassier much faster, despite having uptaken the same or less CO 2 than the wet samples.5][56][57][58] We and others hypothesize that the relative formation of bicarbonates and certain carbamate species in the presence of moisture are responsible for different degrees of electrostatic crosslinking in these samples that in turn impact polymer mobility.We probe this hypothesis further in the next two sections by correlating the polymer mobility data inferred by fluorescence with NMR relaxometry data on these same samples.We further study the formation of different species in these reactions using DRIFTS to disentangle specific molecular contributions to polymer mobility.

NMR Mobile Universal Surface Explorer (MOUSE).
The NMR MOUSE was originally developed as a compact and mobile tool for noninvasive clinical diagnostics and the investigation of materials properties of arbitrarily large objects. 46The technique has since been used extensively for non-destructive and in field evaluation of rubber and elastomers, cultural heritage items, and food products. 59Unlike traditional NMR, chemical shift information cannot be attained with this technique; however, through the detection of differences in the relaxation times of hydrogen protons, NMR MOUSE data can be used, for example, to differentiate clay-bound water vs. movable water, gas, light oil, and viscous oils present in porous media. 60Here, we use the technique to evaluate the mobility of PEI in Al 2 O 3 as a function of moisture and CO 2 uptake, both to corroborate the fluorescence mobility data, and also to demonstrate the value of the technique as a stand-alone diagnostic in operando tool for evaluating the health and performance of DAC composites in the field.
We selected twelve samples for this study, namely, the six composites listed in those same values of RH but exposed for 90 min to a stream of 400 ppm CO 2 .The mobilities of these twelve samples were measured with fluorescence (see the first and last data points of each trace in Fig. 3b).Here, we record and discuss both the transverse, spin-spin relaxation time (T 2 ) as well as the longitudinal, spin-lattice relaxation time (T 1 ) for each sample.T 2 relaxation is a complex phenomenon influenced by a number of mechanisms, a major one being dipolar coupling between spins. 61T 2 values for these twelve samples were determined by collecting an echo train and fitting the amplitude decay curve (Figures S3 and S4 in the SI).
Ultimately, this relaxation parameter is a measure of the coherence time of the spins.Strong dipolar coupling in rigid samples induces faster decoherence, while in samples with high molecular mobility, T 2 can be longer as the process is less efficient.In polymeric systems, greater entanglements and/or crosslinking density results in a decrease in mobility that manifests as shorter T 2 values. 62,63 n Figure 5, we illustrate the T 2 values collected for these twelve samples as a function of RH and CO 2 exposure.Performing an inverse Laplace transform on the T 2 echo train decay curves produces relaxation spectra (plotted in Fig. S3 and S4) showing most spectra are bimodal with two distinct T 2 values per sample, referred to here as the 'a' and 'b' component of the spectrum.Thus, a biexponential function was used to constrain the fitting of the T 2 echo train decay curve to facilitate comparison between samples.This bimodality is observed even in the dry composite that has not seen CO 2 and is possibly explained by protons near the branched PEI chain ends relaxing at a significantly different rate than protons in or near the backbone, a phenomenon not uncommon for side-chain bearing polymers. 64,65  few general trends are worth noting from the data in Figure 6.
First, for both the pristine set of samples and the set exposed to CO 2 , T 2 increases with increasing RH, suggesting moisture increases the polymer mobility.Second, PEI composites that have been exposed to CO 2 generally display shorter T 2 values than the pristine composites.This trend is particularly evident in the drier samples, indicating CO 2 exposure results in a much more rigid matrix in drier conditions.As RH increases, the difference in T 2 between pristine and CO 2 exposed samples begins to converge until there is no statistical difference at 100% RH.Finally, we note that T 2 measured for bulk anhydrous PEI (dashed lines in Figure 5) is quite similar to that of the PEI:Al 2 O 3 composite at 100% RH.Those results are fully consistent with the mobility data implied by fluorescence (compare solid and dashed black traces in Figure 1b).
T 1 relaxation is a measure of the rate of energy transfer from the nuclear spin system to neighboring lattice molecules.The frequency of this measurement is dictated by temperature and the magnetic field strength.T 1 relaxation is also affected by dipolar spin coupling, as well as mobility of the lattice molecules, quadrupolar coupling, anisotropy, paramagnetic effects, spin diffusion, etc. 61 Overall, T 1 relaxation is a longer process than that of T 2 .Because processes like polymeric mobility can serve as an energy sink from spins to the lattice, higher mobility in the lattice will lead to shorter T 1 relaxation times (in contrast to T 2 ).In Figure S5, we plot T 1 values for all twelve samples.Similar trends and conclusions can be inferred here regarding polymer mobility as can be inferred from the T 2 and fluorescence experiments, at least for the samples ≤ 75% RH.The 100% RH samples are clear outliers, being much higher in magnitude than all the other T 1 values.We believe this phenomenon is likely due to phase separation of large amounts of water in the 100% RH samples, which could give rise to an altogether different T 1 signal, and was recently invoked to explain other observations in humidified PEI composites. 24That hypothesis is further supported by the DRIFTS measurements in the next section which illustrate a dramatic increase in the stretching frequency associated with hydroxyl groups near 3650 cm -1 for the 100% RH sample.Regardless, the T 1 and T 2 data as a whole serve to corroborate the fluorescence mobility data and the interpretation of structure-property-performance relationships.

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) study of product formation.
In dry conditions, CO 2 reacts with two amines (primary or secondary) to form ammonium carbamate ion pairs (RNH 3 + COO -/R 2 NH 2 + COO -).The stabilization of these ions over two amines not only keeps the capacity low at 0.5 mol bound CO 2 per mole of amine, 56,66,67 but also contributes to crosslinking within the polymer or between polymer chains that significantly impedes subsequent CO 2 diffusion. 38,68,69 Hwever, unlike in dry conditions, the exact molecular interactions in humid conditions are still debated.[56][57][58] Despite all these variables, humid conditions at low CO 2 concentrations have been mainly claimed to promote CO 2 diffusion and can even double the CO 2 capacity relative to the anhydrous state. 54- 58 hile the general proposed mechanism in anhydrous conditions involves ammonium carbamate ion formation and crosslinking, when moisture is present, water-stabilized ammonium bicarbonate or hydronium carbamate ions are thought to bind through one ethylamine (Figure 6). 66Note, ammonium carbamates can be present in both dry and humid conditions; they are also the only species illustrated in Figure 6 that result in inter-chain ionic crosslinking.Ammonium Carbamate Ammonium Bicarbonate Hydronium Carbamate 400 ppm CO 2 at 0%, 11%, 33%, 53%, 75% and 100% RH for 90 min.The background for each sample was collected just before exposing the sample to the humidified CO 2 stream.From the spectra shown in Figure 7, we can ascertain that the surface reactions between CO 2 and PEI/Al 2 O 3 vary with RH.Under dry conditions, the band associated with primary amines in PEI at 3305 -3602 cm -1 is depleted as they are the main binding sites for CO 2 .In contrast, hydroxyl groups (-OH at 3656 cm -1 ) play an increasing role in the mechanism for 75%-100% RH CO 2 streams.
Furthermore, as a function of increasing humidity, characteristic bands associated with -CH 2 and -CH 3 in PEI (at 2792 -2966 cm -1 ) are less depleted, and the peaks associated with ammonium carbamates (-NH 3 + /-NH 2 + ) spanning from 1800 -2790 cm -1 are less intense.This is especially visible in the time-dependent spectra shown in Figure S6 in the SI, where these features were almost absent after 5 min at 100% RH.Many species of interest have absorption peaks in the region between 1800 -1200 cm -1 , which makes it challenging to assign peaks properly, sometimes leading to disputes in the community. 70Furthermore, PEI/Al 2 O 3 is a highly heterogeneous system, with primary, secondary and tertiary amines present in PEI and a large pore size distribution in Al 2 O 3 .As such, this composite system is rarely chosen for systematic studies of the role of humidity using DRIFTS.
Based on the combined literature from multiple aminopolymers, small molecule amines, and mostly silica-based supports, we propose the tentative peak assignments in Table 2, where the peaks increasing with humidity are listed in bold.IR signatures from ammonium carbamate can be observed for all levels of humidity, while other peaks grew or appeared only as humid CO 2 was introduced to the system.Such peaks have been assigned to carbamic acid at 1699 cm -1 and 1512 cm -1 , some weak contributions of bicarbonate at 1345 and 1616 cm -1 were observed, and the features at 1293 cm -1 and 1582 cm -1 best fit the absorption of hydronium carbamate, which has been mainly reported for tetraethylenepentamine (TEPA) thin films by Miller and co-workers. 67The authors also computationally validated the rapid diffusion of CO 2 through hydronium carbamate species in TEPA during humid conditions.While the formation of bicarbonates in humid conditions is sometimes put forward as the major mechanism, 56 we measure only subtle contributions from these species, which is in line with the measurement of bicarbonate formation predominantly with tertiary amines 54 or with secondary amines at low PEI loadings. 55Peaks associated with carbamic acid and hydronium carbamate experience the highest increase in intensity in humid conditions with our materials.As the latter species capture CO 2 without promoting inter-chain ionic crosslinking, these results provide support at the molecular level for understanding humiditypromoted polymer mobility observed with both fluorescence and NMR relaxometry measurements.
Finally, we note that Potter and co-workers demonstrated the acidity of Al 2 O 3 supports can impact the reaction mechanism of CO 2 with tethered aminopropylsilyl groups in dry conditions. 57pecially at low amine-loading, CO 2 was shown to interact directly with the Al 2 O 3 support, while these interactions were suppressed at higher amine-loadings.We did perform both small and wideangle X-ray scattering (SAXS, WAXS) measurements to probe the role of Al 2 O 3 in our system (Figure S7).While we observed that the impact of CO 2 on the Al 2 O 3 crystal structure is negligible at 100% RH conditions, likely due to the high levels of hydroxylation on the surface of Al 2 O 3 , in dry conditions, the presence of CO 2 dramatically degrades the Al 2 O 3 crystal structure (see brief discussion in SI).The result serves to highlight the non-innocent role that the changing support may have on influencing PEI mobility at the polymer-support interface as the composite uptakes CO 2 .However, a more detailed study of the nature of the degradation of the Al 2 O 3 crystal structure as a function of CO 2 capture is outside the scope of this current manuscript and will be reported elsewhere.

Conclusions.
This work demonstrated the value of two benchtop techniques for studying polymer mobility within nanocomposite DAC materials as a function of CO 2 and water adsorption.For experiments that required both flowing gas and the maintenance of a given RH, a set of mass flow controllers was employed.Dry 400 ppm CO 2 passed through one controller, and a second stream of 400 ppm CO 2 passed through a separate controller and then a water bubbler.These lines were then combined into one stream for tuning RH, passed through a given sample, and then connected as exhaust gas to a LiCOR 850 quantitative CO 2 /H 2 O analyzer.
Thermogravimetric analysis (TGA).PEI content in the Al 2 O 3 composite was estimated using a TA Instrument Q600 TGA apparatus according to a literature procedure. 74Weight loss from 120 to 900 °C under a 100 mL/min flow of N 2 diluted air was recorded at 10 °C/min and normalized by the residual mass at 900 °C.
Photoluminescence (PL) spectroscopy.PL experiments were performed on a custom-built Princeton Instruments spectrometer using a liquid N 2 -cooled Si CCD (PyLoN) array for collecting visible-NIR spectra (400-900 nm).Intensity calibration was preformed daily using an IntelliCal USB-LSVN (9000-410) calibration lamp.Samples were placed in a 2 mm quartz cuvette and excited with a 365 nm LED (7.5 nm FWHM) in an oxygen-free environment.A 400 nm longpass filter was employed between the sample and collection fiber.Emission spectra were collected from 200 -800 nm using a 150 g/mm grating with 800 nm blaze and 3 mm slit; 20 spectra were averaged

S10
A reaction with CO2 severely degrades the crystal structure of g-Al2O3 in the anhydrous material (green trace).This loss in crystallinity indicates that CO2 directly interacts with the reactive surface of g-Al2O3 leading to increased disorder that propagates through the g-Al2O3 matrix.When the CO2 stream is hydrated (orange trace), the g-Al2O3 surface is hydroxylated and appears less reactive to the CO2, causing a lesser degree of disorder.We do not observe the same trend with PEI/SBA-15 samples.SiO2 is less reactive than g-Al2O3, and the CO2 capacity of PEI infiltrated in SBA-15 can be up to 2 orders of magnitude less than that infiltrated in g-Al2O3. 7 VI. References.

Figure 1 .
Figure 1.(a) Series of aqueous-PEI solutions (0 -100 vol.% water) containing 0.1 wt.% THPE irradiated at 365 nm.The intensity of the green fluorescence systematically decreases as the aqueous-PEI solutions becomes less viscous until above 90 vol.% water, when THPE begins to aggregate and emit blue light.(b) Fluorescence spectra of 0.4 wt.% THPE in a 40 wt.% PEI:Al 2 O 3 composite after equilibrating to a given RH for 72 h.(c) Change in fluorescence of a dry composite of 0.4 wt.% THPE in 40 wt.% PEI:Al 2 O 3 as a function of time upon exposure to 100 sccm of a humidified stream of N 2 (100% RH).

Figure 3 .
Figure 3. 40 wt.%PEI:Al 2 O 3 composite, 0.4 wt% THPE, pre-equilibrated to a given RH for 72 h, 100 sccm of 400 ppm humid CO 2 in N 2 .(a) Change in the emission spectra with time at 53% RH.(b) Summary of ratiometric fluorescence intensity (460 nm/530 nm) of six samples at different RH as a function of time with CO 2 flow.

Figure 6 .
Figure 6.Possible molecular structures resulting from CO 2 capture by PEI in dry (red) and humid (red, blue, and green) conditions.

Figure 7 .
Figure 7. DRIFT spectra of PEI:Al 2 O 3 after 90 min of exposure to 400 ppm CO 2 at 0, 11, 33, 53, 75, and 100% RH.The spectra are offset for clarity.A new sample was used for each RH condition.(a) Significant bands and peaks are assigned in the range of 4000 -1800 cm -1 .(b) Peaks of interest are assigned between 1800-1200 cm -1 , where the peaks increasing as a function of increasing humidity are depicted in dashed blue lines, and the other features are highlighted with solid grey lines.The peak assignment of (a) is recorded inTable S1 and (b) in Table 2. CA refers to carbamic First, we assessed the efficacy of employing a fluorescent probe technique for understanding polymer mobility in a series of humidified PEI:Al 2 O 3 composites, correlating measurements of moisture uptake with changes in the emission spectra.We then did the same for CO 2 sorption in these materials across a range of temperatures as well as values of RH.We correlated the fluorescence data and implied changes in mobility with data from a second benchtop technique, namely a portable magnetic resonance sensor known as the NMR MOUSE.Changes in spin-spin and spinlattice relaxation measurements with this technique helped corroborate conclusions drawn from fluorescence regarding polymer mobility.We then coupled all this information with DRIFTS data, which provided molecular level support for understanding the fundamental underpinnings of humidity-promoted polymer mobility in these materials observed with both fluorescence and NMR relaxometry measurements.Given the large matrix of variables that can influence polymer mobility in confinement, which continue to change as a function of sorbent lifetime and cycling, these high throughput techniques for quantifying in operando changes in mobility position the field for rapid advancement in understanding and deployment of these materials in real-world DAC systems.Methods.Materials.Tetrakis(4-hydroxyphenyl)ethylene (THPE) was purchased from TCI.Branched polyethyleneimine (PEI, M n = 600 g/mol, M w = 800 g/mol) was purchased from Sigma; prior to use, PEI was stirred and heated at 100 °C under 25 mtorr vacuum for 72 h to remove any residual volatile organics.Mesoporous γ-Al 2 O 3 was purchased from Sasol and dried at 100 °C under 25 mtorr vacuum overnight prior to any use.All other reagents and chemicals were purchased from Aldrich and used without purification, unless otherwise noted.Composite Preparation.Sorbents were prepared by first doping bulk PEI (1 g) with 1 wt.%THPE (10 mg) and stirring under N 2 in the dark at 50 °C for 1 h.Mesoporous γ-Al 2 O 3 (1.5 g) was then impregnated with this mixture, targeting a ~70% pore fill, by first stirring doped PEI and Al 2 O 3 in separate methanol solutions (~10 mg/mL) for 1 h, and then combining and stirring overnight.Methanol was removed from this combined mixture with a rotary evaporator.The composite was then dried at 25 mTorr vacuum overnight at 100 °C in the dark.Thermogravimetric analysis (TGA) was employed to verify the weight fraction of PEI in the composite was ~40 wt.%.Relative Humidity.Samples were pre-equilibrated to a given RH with the following procedure.150 mg of 40 wt.%PEI:Al 2 O 3 were evenly dispersed into five quartz cuvettes (30 mg in each).Five different aqueous saturated salt solutions were prepared to create chambers with five different RH values.Nitrogen was bubbled through each saturated solution overnight to remove oxygen.Using an N 2 purged glove bag, the cuvettes were transferred under N 2 into the chambers, which were then sealed.The samples equilibrated at a given RH for 72 h before the chambers were opened and the cuvettes were sealed with a Teflon lined cap.The PEI/Al 2 O 3 composites were weighed before and after the 72 h equilibration time to calculate the mass of water adsorbed.

Table 1 .
Moisture Uptake in PEI/Al 2 O 3 Composites and Corresponding Influence on Fluorescence

Table 1
that were equilibrated to different values of RH, and a set of six additional composites equilibrated to

Table 2 .
Assignment of IR peaks/bands observed in humidified PEI/Al 2 O 3 composites.