Light-Triggered CO2 Breathing Foam via Nonsurfactant High Internal

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A Light-triggered CO2 Breathing Foam via Non-Surfactant High Internal Phase Emulsion Shiming Zhang, Dingguan Wang, Qianhao Pan, Qinyuan Gui, Shenglong Liao, and Yapei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11315 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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ACS Applied Materials & Interfaces

A Light-triggered CO2 Breathing Foam via NonSurfactant High Internal Phase Emulsion Shiming Zhang, Dingguan Wang, Qianhao Pan, Qinyuan Gui, Shenglong Liao, Yapei Wang* Department of Chemistry, Renmin University of China, Beijing, 100872, China KEYWORDS: Recyclable CO2 capture, Light-triggered, Gas separation, Surfactant-free high internal phase emulsion, Porous foam

ABSTRACT: Solid materials for CO2 capture and storage have attracted enormous attention for gaseous separation, environmental protection and climate governance. However, their preparation and recovery are meeting the problems of high energy and financial cost. Herein, a controllable CO2 capture and storage process is accomplished in an emulsion-templated polymer foam, in which CO2 is breathed-in at dark and breathed-out under light illumination. Such a process is likely to become a relay of natural CO2 capture by plants which on the contrary breathe out CO2 in nights. Recyclable CO2 capture at room temperature and release under light irradiation guarantee its convenient and cost-effective regeneration in industry. Furthermore, CO2 mixed with CH4 is successfully separated through this reversible breathing in and out system, which offers great promise for CO2 enrichment and practical methane purification.

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INTRODUCTION Natural photosynthesis as powered by sunlight is directing the highly efficient CO2 capture and conversion in nature.1,2 At the same time of CO2 consumption, this vital process yields essential life-needed chemicals such as oxygen and carbohydrate, additionally prevents the rise of air temperature. However, with rapid growth of human industrialization and activities, CO2 exhalation upon using fossil fuels is far beyond the consuming capability by the natural system.3,4 As a result, the composition of atmosphere is worldwide changing which gives rise to tremendous increase of climatic issues.5,6 In addition to improving environmental protection and reducing CO2 release, great efforts are devoted to developing effective strategies and materials to strengthen CO2 capture. This fundamental step is becoming a steadily concerned field that thereby benefits the academic research and industrial production. The past few years have witnessed an explosive development of CO2-adsorbing materials in order to improve the storage capacity and adsorption rate. Examples of zeolites,7,8 zeolitic imidazolate framework,9-12 metal–organic framework,13-17 ionic liquids,18-25 base-containing polymers,26-30 carbon-based materials31-33 have been focused in terms of low cost and easy production. Despite the improvements at the aspects of CO2 capture specificity and absorption rate, the regeneration of these CO2 sorbents upon removing CO2 has to involve endothermic processes, which still relies on the consumption of fossil fuels and induces secondary CO2 pollution. We recently proposed a reversed photosynthesis-like process by which CO2 was captured by a kind of porous particles and released under light illumination.34 Such a process is fossil fuel-derived energy-free as the driving energy only comes from sunlight. Yet the use of CO2 sorbent is at a low level which limits the scalable CO2 capture in a larger space. On the


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other hand, the powder-like products are difficult to reform and fill into reactors with special channels for practical uses. In this study, high internal phase emulsion (HIPE), a type of complex emulsion with less continuous phase to stabilize more disperse phase, was exploited for preparing a porous CO2adsorbing foam with high throughput. Unlike other traditional emulsions, additional surfactants were not included in the entire system to ensure no surfactant contamination within the emulsion-templated products. The CO2 sorbent of polyethyleneimine acted as not only emulsifier but also polymer scaffold of porous foams, which is beneficial for high-performance CO2 storage. To realize its full potential in the industrial areas, the HIPE was modified as a photopolymerizable gel-like product. The fluidic nature of HIPE offers great convenience to fill it into column reactors, followed with polymerization and solidification into a monolithic foam. Moreover, this foam-like CO2 sorbent is regenerable with the combination of photothermal conversion. EXPERIMENTAL SECTION Materials. Polyethyleneimine (PEI, MW = 70000 g/mol, 50 wt.% aqueous solution) and sodium hydroxide were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. 1hydroxycyclohexyl phenyl ketone (HCPK, 99%) was obtained from Energy Chemicals. N, N’Methylenebisacrylamide (97%) and toluene were purchased from Beijing Chemical Works. A tank of mixed gas composed of 70 vol.% methane and 30 vol.% carbon dioxide was received from Beijing Gas Company. Bromothymol Blue was provided by Shanghai Aladdin Bio-Chem Technology Co. Preparation and characterization of HIPE-templated porous Foam. The porous foams were prepared by an oil-in-water (O/W) emulsion-templated method. Typically, the PEI (125 µL, 25


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mg/mL) aqueous solution was added into N, N’ - Methylenebisacrylamide (875 µL, 25 mg/mL), and then added with toluene (4 mL) containing HCPK (1 mg). Two immiscible phases were emulsified into an emulsion by a high shear emulsifier under speed of 16, 000 rpm for 2 min. The as-prepared emulsion was photopolymerized under UV light irradiation for 10 min. Finally, the toluene in the foam was completely removed under low vacuum for 12 h. For preparing foam with the ability of photothermal conversion, the acidified carbon nanotube (1.5 wt.%, 40 µL) was added in the water phase before emulsification. The morphology of PEI emulsion was imaged by an optical microscope (Zeiss Axio Scope A1) and porous foam was characterized by scanning electron microscope (SEM, JEOL-6700). The porosity of the foam was evaluated on a mercury injection apparatus (AutoPore IV 9500). CO2 capture and release. The CO2 capture and release under different temperatures were quantified on a thermo-gravimetric analyzer (TG). Several milligram materials were placed in a platinum pan and heated to 60°C at a rate of 10°C/min under a nitrogen flow (40 mL·min−1) to allow CO2 desorption for 40 min. Then, the nitrogen flow was changed as CO2 flow and the system was cooled down to 25oC for CO2 capture for 40 min. At least 5 cycles were carried out and the capture amount of CO2 was identified according to TGA data. Light-controlled CO2/CH4 gas separation. The separation of mixed gas by the porous foam was tested on a gas chromatography (GC, Agilent Technologies 4820) which can quantify the volume percentage of each gas. Briefly, 0.45 g foam was placed in a 25 mL sealed glass bottle that is fully filled with CH4/CO2 mixed gas. To analyze the gas composition, 1.0 mL of original mixed gas was extracted from the bottle and injected into GC. As for light-controlled gas separation, 0.82 g “dark” foam with ability of photothermal conversion was placed under the continuous illumination of NIR light (light power 3.6 W) for 40 min and then the mixed gas was


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taken out for GC measurement. Then NIR light was removed allowing the foam in the dark for another 40 min and the gas component was measured again on GC following the same method as stated above. Several cycles were carried out for convincing the reversible CO2 capture and release by light modulation. NIR controlled CO2 release. The CO2 light-controlled release is tested via a tandem equipment. Before the test, 0.82 g “dark” foam was kept in open air at room temperature for 2 h to allow the foam fully soaked by CO2. The foam was then filled in a PMMA tube, followed with nitrogen purging. The gas released from the tube was flowed into a bottle of aqueous solution containing pH indicator (Bromothymol Blue) through a Teflon tube. PMMA tube was then placed under a continuous illumination of NIR light (3.8 W) to leach out CO2 from the polymer foam. The color of the pH indicator was recorded by a single-lens reflex camera (Nikon, D7100). RESULTS AND DISCUSSION Polyethyleneimine (PEI) has been investigated as a controllable CO2 sorbent based on the reversible chemical bonding between CO2 and its amine as well as imine groups. To improve the CO2 capture efficiency, PEI was intended to be solely formulated as a porous foam via an oil in water (O/W) emulsion method, which is amenable to scaling up the production with respect to effective cost and convenient preparation35. As illustrated in Figure 1a, an aqueous solution dissolving PEI was mixed with toluene to form an oil-in-water emulsion under vigorous agitation. Without the addition of other surfactants, a high internal phase emulsion (HIPE) was formed, in which the disperse phase of toluene has a volume fraction of 80%. This HIPE emulsion is abnormal as phase inversion routinely occurs in typical emulsions if the disperse phase is over 74.6 vol.% according to the Ostwald packing theory. HIPE only happens in the presence of emulsifiers with appropriate amphiphilicity. In this regard, PEI should have played a


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crucial role in the formation of HIPE emulsions. In practice, PEI is a pH-sensitive polymer which is more hydrophilic at pH below the pKa of amine groups, and on the contrary, more hydrophobic at pH above the pKa. Adjusting pH value offers the possibility to manipulate the amphiphilicity of PEI polymer, which has great effect on their interfacial activities. At pH 8.3, PEI ought to be hydrophilic and prefer to stay in water phase, which is theoretically beneficial for steady O/W emulsion. However, microscopy observations illustrate that the emulsion formed at pH 8.3 is relatively unstable (Figure 1b and 1e). HIPE was formed while its emulsion morphology could be lasted for a long period once changing the pH to 9.3 (Figure 1c and 1f). After the emulsification for 48 hours, the emulsion was partially separated into two isolated phases (Figure S1). Once pH was increased to 11.8, a stable O/W HIPE was readily formed (Figure 1d and 1g). It is believed that the amphiphilic change as a result of deprotonation of PEI at higher pH values accounts for a better interfacial organization.


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Figure 1. (a) Schematic illustration of the preparation process of emulsions. The yellow color represents oil phase of toluene, and the red color represents water phase loaded with PEI at a specific pH. The volume ratio between oil and water is fixed as 4:1 unless otherwise indicated. Nile red was loaded in the oil phase as a fluorescence probe. Optical microscopy images of emulsions prepared at different pH: (b) pH=8.3, (c) pH=9.3, (d) pH=11.8; (e), (f) and (g) are corresponding fluorescence microscopic images of (b), (c), and (d), respectively. In order to identify the distinct interfacial activity of PEI, the interfacial energy between the oil and water phases in the presence of PEI at different pH values was measured by a Pendant drop method. As shown in Figure 2a, a droplet of PEI aqueous solution (10 mg/mL) was suspended in a toluene phase. The interfacial energy as identified by the droplet shape decreases upon increasing the pH value of aqueous solution. In contrast to the interfacial energy at neutral conditions or in the absence of PEI addition, the interfacial energy drops to a low level of 27 mN/m at pH 11.8. It should be noted that solely increasing pH also decreases the interfacial energy, but it is far less than the effect by PEI addition. Assuming all amine groups have the same pKa of 8.5, it is estimated that only 0.08% of amine groups are protonated at pH 11.8. Yet this low degree of protonation provides enough hydrophilic composition to tailor appropriate amphiphilicity of PEI with wonderful interfacial activity, which ensures the high stability of the gel-like HIPE (Figure 2b). Higher degree of protonation at lower pH conditions enables PEI to be more hydrophilic. PEI with 99.90% protonation at pH 5.7 is assumed to be completely dissolved in water phase because it causes negligible change to the interfacial energy. Although the basic environment facilitates the formation of HIPEs (pH 9.3, 10.7, 11.8), only the HIPE prepared at pH 11.8 could be remained without distinct phase separation for almost one week.


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Figure 2. (a) The interfacial energy between toluene and PEI aqueous solution with different pH values. (b) Optical photographs of PEI HIPEs prepared at different pH values. The PEI concentration in the aqueous solution is 10 mg/mL. To discern the exact contribution of PEI to the formation of HIPE at high pH values, detailed self-assembly of PEI in the aqueous solution was studied. Practically, many surfactants even with better emulsifying ability than PEI cannot lead to the formation of HIPE. In this regard, the appropriate amphiphilicity of PEI is not the only factor to account for the formation of HIPE. It is assumed that PEI may be reorganized and become more interfacially active before the assembly at the oil/water interfaces. In terms of partial protonation, PEI is alike an amphiphile at high pH values. Self-assembly of PEI chains at high concentration (25 mg/mL) should occur instead of uniform dissolution in aqueous solution. Micelle-like aggregates are anticipated according to the dynamic light scattering (DLS) and transmission electron microscope (TEM) studies (Figure 3a). As shown in Figure 3b, nano-aggregates with average


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size less than 100 nm were formed when PEI was dissolved in basic aqueous solution. DLS studies also approve that high pH condition is the prerequisite for the formation of PEI nanoaggregates (Figure S2). These aggregates with spherical morphologies were further convinced by TEM observation (Figure 3c). Great insights were also provided into the dynamic self-assembly process of PEI by DPD stimulation. As illustrated in Figure 3d, at the beginning of the base addition, all PEI chains are freely dispersed in a water media and no aggregation is observed. Once the polymers are close to each other, small aggregates containing a low number of polymer chains appear along with the reduction of the systematic energy. Larger aggregates are gradually formed as a result of higher degree of self-assembly (Figure 3e). This dynamic progress of selfassembly was thoroughly monitored in Supplement Movie S1, during which PEI chains are reorganized from freely random status to tangled configuration within the micelle-like aggregates. In comparison with free polymer chains, the nano-aggregates possess better interfacial activity because they can decrease the interfacial energy more efficiently. The appearance of nano-aggregates is assumed to lead to Pickering effect which refers to emulsions stabilized by nanoparticles. As a consequence, the stability of O/W emulsions could be dramatically improved. As presented in previous studies, the abnormal HIPEs are possibly formed in the Pickering emulsions only when the nanoparticle-based emulsifiers possess ideal amphiphilicity and enough concentration. It is thus believed that the particular aggregation of PEI in the basic conditions mainly contributes to the formation of HIPEs. On the other hand, it is convinced that the concentration of PEI is also important to the formation of HIPEs. As compared in Figure 3f and 3g, HIPE could be formed with the use of PEI at low concentration of 5 mg/mL. However, the whole system loses the gel-like performance that it flows downward while turning over the glass vessel (Figure 3g). In principle, such a concentration ought to be


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high enough to provide free PEI molecules to cover the oil-water interface. The relatively poor stability of HIPE is attributed to the decreased number of micellar aggregates at lower PEI concentrations, which becomes more serious if the PEI concentration is further decreased. Unstable O/W or even phase separation occurred when the PEI concentration is below 3 mg/mL. In other words, the instability of HIPE at relative low PEI concentration also suggests the importance of nano-aggregation for stabilizing oil-water interfaces more sufficiently.

Figure 3. (a) Schematic illustration of PEI polymer chain self-assembly in aqueous solution at high pH value. (b) Dynamic lase scattering result of 10 mg/mL PEI aqueous solution at pH = 11.8, the count rate is 150.6 kcps. (c) TEM image of 10 mg/mL PEI aqueous solution. Inset: the


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local amplified TEM image, scale bar is 75 nm (d) DPD simulation of PEI polymer chain movement in pH value of 11.8 and (e) the formation of micelles in same condition. (f) and (g) are optical photographs of PEI-containing O/W emulsions prepared with the use of different PEI concentrations at pH 11.8. The oil-water ratio is fixed at 4:1. The number refers to the exact PEI concentration with unit of mg/mL. The HIPE was extended as a photo-curable complex upon blending it with N, N’methylenebisacrylamide and a photo initiator (HCPK). A rapid free radical polymerization occurs

once

the

HIPE

complex

is

subjected

to

UV

irradiation.

The

N,

N’-

methylenebisacrylamide was polymerized into a cross-linked network which was able to fix the structure of HIPE. A polymeric foam was conveniently produced after the removal of solvents. The fluidic nature offers great convenience to mold the HIPE complex into size- and shapespecific reactors, generating porous foams with high throughput and particular shapes. For example, a cigar shaped foam with 3 cm in height and 1 cm in diameter was obtained when the polymerization of HIPE was taken place in a test tube (Figure 4a). Its specific surface area was estimated as 18.6 m2/g on a mercury-injection apparatus (Figure S3). Principally, massive production of PEI-based foams is expected if photo-polymerization was altered by thermalpolymerization in consideration of limited light penetration. Scanning electron microscope (SEM) observation reveals that the PEI foam has an open cell morphology (Figure 4b), guaranteeing the ease of gas flow for the following CO2 absorption and separation. Other complicated shapes, e.g. heart-like and star-like shapes could be also replicated from specific molds according to the polymerization procedure as stated above (Figure 4c and 4d). With respect to its intrinsic simplicity and generality, this HIPE templated method followed with


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photo-polymerization affords fascinating promise to build monolith foams in enclosed spaces for special requirements.

Figure 4. (a) The optical image of a typical cross-linked porous foam molded from a test tube. (b) Cross-section view of the porous foam under SEM observation. (c) Optical images of a heartshaped porous foam and (d) a star-shaped porous foam. Based on the reversible association between CO2 and the amine and imine groups, PEI endows the porous foam with the ability of absorbing CO2 in a control manner (Figure 5a). Three stages are involved in the process of CO2 absorption, including quick CO2 reaction with external PEI on pore surface, CO2 diffusion within the foam matrix to react with inner PEI, and an equilibrium state once the formation speed and decomposition speed of carbamate are equal. The absorption reaction as the formation of carbamate is temperature dependent. Increasing the temperature is unfavorable to the adsorption reaction, but promotes the decomposition of


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carbamate. As shown in Figure 5b, CO2 adsorption and desorption could be alternatively regulated between room temperature and high temperature at 60 oC. Notably, the porous morphology is beneficial to the highly efficient CO2 adsorption of PEI foam and a weight increase about 2.5 wt.% according to the porous foam is reached (Figure S4). Changing the emulsification conditions may affect the emulsion morphology and thus the CO2 absorption efficiency. As summarized in Figure 5c and Table 1, several conditions, including the PEI concentration, emulsification speed, and water-oil ratio, were attempted to investigate their effect on the foam morphology as well as CO2 absorption. Increasing the fraction of oil phase and shearing rate in the emulsion induces the enrichment of PEI on pore surfaces as a result of the increase of interfacial areas and porosity (Figure S5, Table S1). Thus, the capability of CO2 capture will be improved in these cases (Figure 5c). Additionally, PEI concentration also has great impact on the morphology of PEI foam as well as gas diffusion. At the PEI concentration of 15 mg/mL, PEI nano-aggregates together with free PEI polymers at the O/W interface can not fully cover the wall of foam, leading to porous morphologies with irregular open-cell structures. Increasing PEI concentration up to 25 mg/mL could slightly improve the CO2 capture because more PEI polymers distribute on the pore surface. However, with remarkable increase of PEI concentration, for example at 50 mg/mL, the excessive PEI could cause the formation of porous morphologies with close-cell structures (Figure S6 and Table S2), which are not beneficial to the gas diffusion during CO2 capture and thus the ability of CO2 capture is impaired. Table 1. CO2 capture ability of PEI foam with PEI concentration. The oil to water ratio is 4:1 and emulsifying speed is 16000 rpm. PEI Concentrate (mg·mL-1) CO2 absorbance / wt.%

15 2.510

25

50

100

2.555

1.851

0.345


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Figure 5. (a) The CO2 adsorption by PEI as a result of carbamate between CO2 and amino groups. (b) Several cycles of reversible CO2 absorption and desorption of PEI breathing foam. CO2 absorption temperature is 25 oC and desorption temperature is 60 oC. (c) CO2 absorption ability of porous foams prepared with different emulsification conditions. The PEI foam was extended as a light-responsive CO2 sorbent upon loading “dark materials” with ability of photothermal conversion. Typically, 1.5 wt.% acidified carbon nanotubes (CNT) were doped in the HIPE to generate a “dark” porous foam. In principle, CNT is able to generate thermal heat under light irradiation. As a consequence, the temperature of CNTdoped PEI foam was rapidly increased to 35 oC under light illumination with power of 0.1 W. Higher temperature could be reached if the light power was increased, e.g. 60 oC at light power of 0.3 W (Figure 6a). The foam is quickly cooled down in a few seconds once the light is turned off. This “black” foam possesses satisfactory photostability that its photothermal conversion ability is well retained after on-off light irradiation for more than ten cycles (Figure 6b). Compared with traditional heating ways, the thermal treatment based photothermal conversion


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for regulating CO2 absorption and desorption is fossil fuel-derived energy-free as the driving energy only comes from sunlight.

Figure 6. (a) Temperature changes of the “dark” foam under light irradiation with different light power. The light wavelength is 808 nm. (b) Time-dependent cyclic temperature change of PEI foam under the illumination of NIR light (0.3 W). In terms of highly efficient CO2 absorption, the PEI foam was used to separate the CO2 and CH4 mixture which routinely exists in the natural gases. So far, aqueous alkaline solution is the most readily used system to remove CO2 from the natural gases. Yet such a great deal of alkaline use practically causes severe corrosion to the affiliated equipment and environment. The PEI foam serves as a porous filter to remove CO2 from the natural gases. This separation procedure is more convenient and environment benign. Moreover, the foam doped with photothermal agents can be fully reused with the combination of light treatment. As a proof of concept, a piece of PEI foam (0.45 g) was sealed in a glass reactor full of CO2 and CH4 mixture. As monitored by GCmass spectroscopy, the CO2 component was decreased from 30 vol.% to 20 vol.% at room temperatures under the PEI absorption for 320 min (Figure 7a). If the apparatus was exposed to light illumination, the decrease of the CO2 fraction was slowed down and even the CO2 fraction


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turned to be increased (Figure 7b). This disturbance by light illumination indicates that the lighttriggered CO2 absorption and desorption is available in the complicated atmosphere, ensuring the practical recovery for long term use.

Figure 7. (a) CO2 volume percentage versus time after adding normal porous foam in the bottle filled with CO2/CH4 mixed gas. (b) CO2 volume percentage versus time after adding “dark” foam under cyclic NIR light illumination (3.2 W). To realize its full potential, the PEI foam was loaded into a transparent PMMA tube, serving as a Do-It-Yourself reactor for more practical CO2 absorption (Figure 8a). The reactor was connected with an aqueous pool in which a pH indicator of bromothymol blue was added. Once CO2 is delivered into the aqueous pool, the blue color will be weakened and even faded away because pH is decreased as a result of CO2 dissolution. As shown in Figure 8b, the indicator remains light blue when the reactor is only purged with nitrogen. However, no color change was observed when the nitrogen atmosphere was replaced by a mixture of CO2 and nitrogen (volume ratio, 5:95), indicating CO2 should have been taken up by the PEI foam. Upon exposing the reactor to light illumination for a short time, the aqueous pool became colorless, which is indicative of the CO2 evolution from the reactor (Movie S2). This practical model convinces that this particular class of PEI foam may be readily scaled up and filled in special


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containers for continuous gas separation. Additionally, the economic CO2 removal by light is inherently reliable for recovering and reusing the gas separator in practice.

Figure 8. (a) The schematic illustration of light-controlled CO2 separator by loading lightsensitive PEI foam in a PMMA tube. (b) Optical image of the CO2 separator connected with an aqueous pool containing CO2 indicator; (c) Optical images of the aqueous pool before and after triggering CO2 release by NIR light illumination. The light power is 3.2 W and the irradiation is lasted for 3 min. Conclusions In summary, a surfactant-free emulsion method possessing advantages of high scalability and elegant simplicity was exploited to prepare porous foam with ability of CO2 adsorption. Further combining with photothermal agents, this particular class of CO2 sorbents could be fully


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regenerated after releasing CO2 under light illumination. In comparison with previous CO2 absorbing materials, this renewable system is cost effective on energy consumption, very promising for practical light induced CO2 storage and separation. It is envisioned that this CO2absorbing system would be a perfect supplement to natural CO2 storage. CO2 is expected to be stored at dark when natural plants stop storage and release CO2 into atmosphere. Once it switches to sunlight illumination, CO2 is released and subjected to further conversion. To present the possibility of practical uses in special reactors for industrial purpose, we have attempted to mold the foam into a variety of shapes by taking advantage of the fluidic and polymerizable properties of the particular HIPEs. It is expected that CO2 conversion may be included in this controllable CO2-capturing system if it is loaded with efficient CO2 catalysts. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Supporting information includes: Optical images of PEI emulsions at different pH; The porosity analysis and specific surface area of PEI foam; Time-dependent weight percentage of PEI foam with CO2 absorption; SEM images of PEI foam fabricated at different emulsifying speeds or oil to water ratios; SEM images of PEI foam fabricated at different PEI concentrations. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT


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Light-Triggered CO2 Breathing Foam via Nonsurfactant High Internal

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