Bi-Functional Paraffin@Polyaniline/TiO2/PCN-222(Fe) Microcapsules for Solar Thermal Energy Storage and CO2 Photoreduction

A novel type of bi-functional microencapsulated phase change material (MEPCM) microcapsules with thermal energy storage (TES) and carbon dioxide (CO2) photoreduction was designed and fabricated. The polyaniline (PANI)/titanium dioxide (TiO2)/PCN-222(Fe) hybrid shell encloses phase change material (PCM) paraffin by the facile and environment-friendly Pickering emulsion polymerization, in which TiO2 and PCN-222(Fe) nanoparticles (NPs) were used as Pickering stabilizer. Furthermore, a ternary heterojunction of PANI/(TiO2)/PCN-222(Fe) was constructed due to the tight contact of the three components on the hybrid shell. The results indicate that the maximum enthalpy of MEPCMs is 174.7 J·g−1 with encapsulation efficiency of 77.2%, and the thermal properties, chemical composition, and morphological structure were well maintained after 500 high–low temperature cycles test. Besides, the MEPCM was employed to reduce CO2 into carbon monoxide (CO) and methane (CH4) under natural light irradiation. The CO evolution rate reached up to 45.16 μmol g−1 h−1 because of the suitable band gap and efficient charge migration efficiency, which is 5.4, 11, and 62 times higher than pure PCN-222(Fe), PANI, and TiO2, respectively. Moreover, the CO evolution rate decayed inapparently after five CO2 photoreduction cycles. The as-prepared bi-functional MEPCM as the temperature regulating building materials and air purification medium will stimulate a potential application.


Introduction
Energy and environmental issues have become two crucial obstacles restricting the construction of low-carbon energy systems [1]. Thermal energy storage (TES) is a clean and effective technology to solve the environmental crisis and reduce consumption of fossil fuels [2]. Phase change materials (PCMs), such as organics (paraffin, fatty acids, esters, alcohols, and glycols), inorganics (salt hydrates or metallic), and eutectics (mixtures of inorganic and organic), act a pivotal part in the sustainable progress of economic development under the background of increasing energy consumption due to the valuable heat storage capability, physical-chemistry durability, and small volume change [3,4]. PCMs can store heat energy at high temperatures and then release heat energy at low-temperature conditions, which shows excellent application potential in thermal insulation materials, thermo-regulated textiles, photothermal conversion, etc. [5][6][7]. However, the leakage and poor thermal conductivity of PCMs during the high-low temperature change limits the direct application under practical conditions [8]. Therefore, many encapsulated technologies (microcapsules, porous materials adsorption, blending, etc.) for confining PCMs have been developed to overcome the leakage [9][10][11].
Microencapsulation is one of the main methods of fabricating core-shell structures, where the PCMs as the core materials are confined by the shell material [12]. Generally, also achieve efficient separation of hole-electron pairs to improve the photocatalysis of CO 2 into CO and CH 4 . The current work aims to develop a novel manufacturing technique for the MEPCMs and investigate their solar thermal storage capacity and photocatalytic CO 2 reduction properties.  Figure 1a,b exhibit the SEM images of KH-570 modified TiO 2 and PCN-222(Fe) NPs, respectively. The particle size of KH-570 modified TiO 2 was about 30 nm. The particle size of PCN-222(Fe) NPs was around 50 nm, and their morphology was regular and generally in a fusiform shape. Figure 1c-g exhibit SEM images of PPTP-1 to PPTP-5, in which a series of MEPCMs with apparent spherical structure can be observed, where the diameter of MEPCMs increased with the decrease of the addition of TiO 2 and PCN-222(Fe) NPs, and the MEPCMs size distribution is between 1 and 8 µm. Furthermore, the amount of TiO 2 and PCN-222(Fe) NPs embedded on the surface of the MEPCM decreased with the continuous increase of the amount of PANI, resulting in the poor dispersion and aggregation of MEPCMs. Moreover, with the decrease of PCN-222(Fe), the size of MEPCMs accordingly decreased. Among them, PPTP-3 MEPCM has an ideal dispersion (Figure 1e), where more NPs are accumulated on the surface. The core-shell structure of PPTP-3 can be observed in the TEM image (Figure 1h), where the TiO 2 and PCN-222(Fe) NPs embedded on the surface can also be observed. Figure 1i is the SEM image of PPTP-3 at the acceleration voltage of 10 kV, and the EDX elemental mappings of PPTP-3 show that C, N, Ti, O, Fe, and Zr elements are dispersed on the surface of MEPCMs, indicating that TiO 2 and PCN-222(Fe) NPs are uniformly embedded on the PANI shell. The above results show that the PPTP MEPCMs with paraffin as the core and PANI/TiO 2 /PCN-222(Fe) as the hybrid shell have been successfully prepared.

Chemical Composition Characterization
FT-IR spectra of TiO 2 , KH-570 modified TiO 2 , paraffin, PANI, and PPTP MEPCMs are shown in Figure 2a. For the TiO 2 , the broad peaks at around 560 and 3400 cm −1 are attributed to the Ti-O and hydroxyl groups stretching vibrations, respectively. The peaks of modified TiO 2 at 1633 and 1714 cm −1 are attributed to C=C and C=O vibrations of KH-570, indicating the successful modification of TiO 2 by KH-570. The peaks of paraffin at 2917 and 2852 cm −1 correspond to the asymmetric stretching vibration and symmetric stretching vibration of -CH 2 . The peak at 1471 cm −1 is attributed to -CH 2 bending vibration, and the peak at 718 cm −1 is assigned to the in-plane rocking vibration of the alkyl chain. In the spectrum of PANI, the peaks at 1564 and 1471 cm −1 correspond to C=N and C=C stretching mode of quinone and benzene units, respectively, 1294 cm −1 is indexed with C-N stretching mode for benzene ring. For the spectrum of PCN-222(Fe), the symmetric Fe-N stretching at 998 cm −1 appears, indicating the successful complexation of Fe with porphyrin ring. In the spectrum of the PPTP-3, the characteristic peaks of the different components can be detected, suggesting that the as-prepared MEPCM consists of paraffin, TiO 2 , PANI, and PCN-222(Fe).  [43]. The wide diffraction angle 2θ of PANI is around 18.4 • , which is a unique characteristic diffraction peak of polymer materials [44]. Paraffin diffraction peaks at 2θ are 6.1, 21. The peak at 402.4 eV is the protonated amine of PANI molecular chains [47]. The highresolution Fe 2p spectrum was fitted into two characteristic Fe species at 710.7 (Fe 2p 3/2 ) and 724.5 eV (Fe 2p 1/2 ) (Figure 1f), indicating the introduction of the Fe component into the MOF nanocrystal [48]. In the high-resolution Ti 2p spectrum, signals allocated to Ti 2p 1/2 and Ti 2p 3/2 are observed at 463.5 and 457.9 eV (Figure 1g), which indicates the existence of Ti in a tetravalent form [49]. The XPS analysis shows the stable compound states of C, N, Fe, and Ti in the PPTP-3. In addition, the EDX spectrum of PPTP-3 is shown in Figure 2h, the contents of C, N, O, Fe, Ti, and Zr are 56, 18, 6, 4, 13, and 3 atom%, respectively, which also indicates the PPTP MEPCMs with paraffin as core and PANI/TiO 2 /PCN-222(Fe) as hybrid shell are successfully fabricated. The above results show that the MEPCMs with paraffin as the core and PANI/TiO 2 /PPTP-222(Fe) as the hybrid shell have been successfully prepared.

Pore Structure and Specific Surface Area Analysis
The pore structure and specific surface area of photocatalysts have an essential impact on the performance, and these parameters of materials were analyzed by the BET analyzer. The pore size of PPTP-3 was calculated according to the BET method, which was mainly concentrated at approximately 1.3, 3.2, and 3.8 nm, respectively (illustration in Figure 2i). These pore sizes match the pore sizes in PANI and PCN-222(Fe) ( Figure S1), and the other larger mesopores may result from the piled pores among the MEPCM [50]. According to IUPAC classification, the N 2 adsorption-desorption isotherm belongs to type IV, and a small amount of N 2 adsorption appears under low relative pressure, indicating the existence of micropores. A wide H1 type hysteresis ring appears in the medium pressure region, which proves the existence of mesopores. The BET specific surface area and pore volume of PPTP-3 were 583 m 2 /g and 0.628 cm 3 /g, respectively, which well inherits the pore structure of PCN-222(Fe) and PANI.

Phase Change and Thermal Properties Analysis
The phase change characteristics and thermal properties of pure paraffin and PPTP MEPCMs were analyzed by DSC (Figure 3a). The curves include three solid-liquid phase change peaks in the endothermic stage and three corresponding liquid-solid phase change peaks in the exothermic stage. All the PPTP MEPCMs have similar phase change behavior compared with paraffin. Still, the melting peak temperature T m decreases slightly, and the crystallization peak temperature T c increases marginally due to the crystallization confinement of paraffin microencapsulated in the inner space of the hybrid shell [51]. The T c and T m of high melting point paraffin used in this work are 50.3 and 45.1 • C, the corresponding crystallization enthalpy ∆H c and melting enthalpy ∆H m are 226.3 J·g −1 and 228.5 J·g −1 , respectively. A series of PPTP MEPCMs showed excellent encapsulation efficiency E en except for PPTP-1, the E en of PPTP-2 to PPTP-5 MEPCMs is close to 80% and had good heat storage performance ( Table 1). The values of encapsulation efficiency (E en ), energy storage efficiency (E es ), and thermal storage capability (C es ) were calculated according to Equations (S1)-(S3) in the Supporting Information. The thermal stability of PPTP-3 was verified by 500 heating-cooling cycles experiments. In Figure 3b, the enthalpy of PPTP-3 hardly decreases after 500 cycles, indicating that as-fabricated MEPCMs have superior thermal stability. Furthermore, the thermal stability of the materials was tested by TGA (Figure 3c,d). The results show that the PANI/TiO 2 /PCN-222(Fe) hybrid shell has a good protective effect on paraffin and improves its thermal stability. Significantly, the weight loss of modified P25 was attributed to the removal of KH-570 grafted on the surface of P25 during the temperature of 200~450 • C. PPTP-3 was tested for 500 high-low temperature cycles in the temperature range of 10~60 • C. As shown in Figure 3e, after the high-low temperature cycles test, no liquid paraffin remained on the filter paper, proving that PANI/TiO 2 /PCN-222(Fe) hybrid shell can prevent the internal melting core PCM from leaking.

Photothermal Conversion Performance
To investigate the photothermal absorption-release properties of the materials, a 300 W Xenon lamp (λ > 300 nm) was used to irradiate paraffin, PCN-222(Fe)/PANI/TiO 2 mixture (the composition ratio is the same as the shell of PPTP-3), and PPTP-3 at the same time and the surface temperature was recorded every 30 s by an infrared thermal imager (Figure 4). In the process of light irradiation and removal, it can be observed that there is an apparent difference in the surface temperature. In Figure 4a, in the course of the light irradiation, the temperature of the PCN-222(Fe)/PANI/TiO 2 mixture continues to rise rapidly within 150 s, and the maximum temperature reaches about 85 • C. In contrast, the temperature rise speed of PPTP-3 is slightly slower than that of the mixture due to the heat absorption of internal paraffin; the maximum temperature within 150 s reaches about 49 • C. Meanwhile, the ascending speed of paraffin surface temperature is the slowest, and there is a distinct temperature hysteresis due to the poor photothermal conversion capability. In the cooling process after removing the light source (Figure 4b), in contrast to the fastest decrease of the temperature of the PCN-222(Fe)/PANI/TiO 2 mixture, the temperature of PPTP-3 dropped slower but faster than that of paraffin. Furthermore, to observe the temperature variation, the temperature response curves of samples over time were collected (Figure 4c). During the light radiation stage, the temperature of the PCN-222(Fe)/PANI/TiO 2 mixture almost linearly and rapidly increases to 78.5 • C in 75 s because of the ultra-high thermal conductivity of the mixture without paraffin. Nevertheless, for paraffin and PPTP-3 MEPCM, the temperatures rise rate are slow and increased to 24.8 and 44.6 • C after 160 s, respectively. After removing the irradiation light source, the temperature of the PCN-222(Fe)/PANI/TiO 2 mixture quickly decreases from 78.5 to 12 • C within 100 s because of the high thermal conductivity of the hybrid shell. Meanwhile, the surface temperature of PPTP-3 and paraffin drops to 12 • C after cooling for respectively 120 s and 180 s due to the thermal insulation of the inner paraffin. The above results show that PCN-222(Fe)/PANI/TiO 2 shell has superior photothermal conversion capacity than paraffin, and PPTP-3 integrates the photothermal conversion and heat energy storage, which significantly improves the thermal response and thermal regulation performance of PPTP-3.

Photoelectric Performance
To investigate the photoelectrochemical characteristics of PPTP MEPCMs, TiO 2 , PCN-222(Fe), PANI, TiO 2 /PCN-222(Fe)/PANI ternary mixture, and PPTP MEPCMs were characterized by employing electrochemical workstation. The transferability of photogenerated electrons was analyzed by observing the photocurrent-time (I-t) and the EIS Nyquist plot changes. The evolution of I-t was inversely proportional to that of the EIS Nyquist plot ( Figure 5). The I-t curves are shown in Figure 5a, the photocurrent intensity of the PPTP-1 to PPTP-5 MEPCMs is better than that of TiO 2 , PCN-222(Fe), PANI, and the ternary mixture. Furthermore, with the mass ratio of PCN-222(Fe)/PANI in the MEPCMs increasing first and then decreasing, the photocurrent intensity of PPTP-1 to PPTP-5 MEPCMs also increases first and then decreases, and in which the photocurrent intensity of PPTP-3 reaches the highest due to the maximum mass fraction of PCN-222(Fe) in the MEPCM. It can be observed that the EIS curves are similar to arc shape (Figure 5b), where the radian size represents the impedance value. The EIS curves show that the impedance of PPTP-3 is the smallest, indicating the optimal photoelectron-hole pair separation efficiency because of the suitable heterojunction composition ratio. In brief, the PPTP-3 ternary hybrid shell can effectively enhance the separation efficiency of electron-hole pairs in the catalyst and then improve the photocatalytic reduction capacity.  (Figure 6b), where TiO 2 is an indirect semiconductor, n = 4; PCN-222(Fe) and PANI are direct semiconductors, n = 1 [52]. Thus, the E g of TiO 2 , PCN-222(Fe), and PANI are 3.1, 1.63, and 2.1 eV, respectively. Moreover, the E g of the ternary mixture PCN-222(Fe)/PANI/TiO 2 and PPTP-3 are 1.91 and 1.82 eV, respectively, indicating the integration of ternary heterojunction has more influence than the simple mixture. To obtain the bandgap structure of each material, the flat band potentials (E FB ) of TiO 2 and PCN-222(Fe) were calculated by Mott-Schottky plots. The tangent slopes of Mott Schottky curves on TiO 2 and PCN-222(Fe) are positive, indicating that they are n-type semiconductors (Figure 6c,d). It can be seen from the Mott-Schottky plots that the E FB of TiO 2 and PCN-222(Fe) are −0.06 and −0.46 V vs. Ag/AgCl, respectively. Based on Yuan's research [53], the E CB potential of n-type semiconductors is 0.2 V higher than E FB . Therefore, the E CB values of TiO 2 and PCN-222(Fe) are −0.26 V and −0.66 V, according to the formula E g = E VB − E CB ; thus, the valence band potential (E VB ) of TiO 2 and PCN-222(Fe) is 2.84 V and 0.97 V, respectively. The above data provide theoretical support for the subsequent catalytic mechanism.

Photocatalytic Performance of Catalysts for CO 2 Photoreduction
The results of CO 2 photoreduction catalyzed by various photocatalysts are shown in Figure 7a, where the main product is CO with a small amount of CH 4 . The reduction rate of CO 2 into CO catalyzed by PANI, TiO 2 , and PCN-222(Fe) are 4.12, 0.73, and 8.4 µmol g −1 h −1 , respectively. The reduction rate of CO 2 into CO over PPTP-1 to PPTP-5 first increases and then decreases. The PPTP-3 reaches the optimal value of 45.16 µmol g −1 h −1 , which is mainly due to the highest mass fraction of PCN-222(Fe) in PPTP-3 causing the optimal photocurrent density, hole-electron separation efficiency, and suitable energy band. As the optimal photocatalyst, PPTP-3 has the highest CO yield, which is 11, 5.4, and 62 times higher than that of PANI, PCN-222(Fe), and TiO 2 , respectively. Moreover, the selectivity of CO catalyzed by PPTP MEPCMs is relatively high, where the selectivity of CO over PPTP MEPCMs is almost higher than 96%. To verify the photocatalytic specificity of heterojunction of PPTP-3, the ternary mixture (PCN-222(Fe)/PANI/TiO 2 ) was also adopted to carry out the CO 2 photoreduction, where the reduction rate of CO 2 into CO is 20.23 µmol g −1 h −1 with the CH 4 reduction rate of 3.25 µmol g −1 h −1 , indicating the heterojunction of PPTP-3 shell has better catalytic performance because of the more favorable light absorption of the heterojunction catalyst and the weaker recombination of charges and holes between the three components. The results of total CO evolution yield by PCN-222(Fe), TiO 2 , PANI, and PPTP-3 with Xe lamp within 8 h are shown in Figure 7b. In the process of photoreduction within 8 h, although the reduction rate of CO 2 catalyzed by PPTP-3 decreased slightly after 3 h, the evolution rate of CO was still maintained at a comparatively high level. The evolution amount of CO catalyzed by PPTP-3 is much higher than that of the other three single components, indicating that the ideal activity of PPTP-3 remains during the long-term catalytic process. In addition, the catalytic stability of PPTP-3 was further verified by cycle experiments (Figure 7c). After five cycles of 1 h, the CO produced rate did not decay significantly, and there was still a 90% CO produced rate compared with the first cycle. The structural stability of PPTP-3 after five cycles was verified by FTIR spectra. It can be seen from Figure 7d that the functional groups in the FTIR spectra of PPTP-3 before and after the cycles remained well.

Photocatalysis Mechanism of PPTP-3 MEPCM for CO 2 Photoreduction
The mechanism of CO 2 photoreduction catalyzed by PPTP-3 MEPCM is shown in Figure 8. In the hybrid shell of PPTP-3, PCN-222(Fe) and TiO 2 NPs are dispersedly embedded on the conductive polymer PANI. PANI, as a p-type conductive polymer, can absorb natural light and further induce π-π* transition. Then, the excited state electrons transfer from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The potentials of the excited state electrons on the LUMO and HOMO orbitals are 0.5 and −1.6 eV, respectively, and E g is 2.1 eV [54], which matches the bandgap parameters of PANI in this work. Based on the above analysis of the bandgap structures of PCN-222(Fe), TiO 2 , and PANI, a ternary heterojunction of PCN-222(Fe)/TiO 2 /PANI was proposed. From the potential values of the three components, it can be seen that PCN-222(Fe) and TiO 2 have an excellent potential match with PANI, and then an internal electric field was established. Under the irradiation of natural light, quantities of photogenerated electrons accumulated on the LUMO of PANI, and then the electrons were quickly transferred to the conduction band (CB) PCN-222(Fe). The standard potential (−0.33 V vs. NHE) of O 2 / • O 2 is more negative than the CB of TiO 2 (−0.26 eV), in which the •O 2 − cannot generate in thermodynamics. Therefore, the electrons on the CB of TiO 2 will continue to migrate to the HOMO of PANI. As such, VB of PCN-222(Fe) (0.97 eV) and HOMO potential of PANI (0.5 eV) are more negative than H 2 O/ • OH (1.99 V vs. NHE) or OH − / • OH (2.34 V vs. NHE), which also prohibits the generation of • OH. Therefore, in the process of illumination, the CB values of reduction potentials E 0 (CO 2 /CO, 0.53 V vs. NHE) and E 0 (CO 2 /CH 4 , 0.24 V vs. NHE) are more positive than the CB value of PCN-222(Fe) (−0.66 V vs. NHE). Most of the electrons of CB on PCN-222(Fe) were used to reduce CO 2 , to maintain the strong reducibility of CB belonging to PCN-222(Fe). MOFs tend to produce H 2 O 2 during the photoreduction reaction. Accordingly, excessive H 2 O 2 will consume photoinduced electrons and further weaken the reduction ability of the substrate. By compounding with other materials, such as TiO 2 or PANI, the generation of H 2 O 2 can be effectively reduced, thus improving the photoreduction performance. When exposed to light, photogenerated holes (H + ) of PPTP-3 MEPCM can be produced simultaneously. Since the HOMO potential (0.5 V vs. NHE) of PANI is more negative than the VB of PCN-222(Fe), the photogenerated H + on the VB of PCN-222(Fe) will transfer to the HOMO of PANI. The oxidation potential E 0 (O 2 /H 2 O, 0.82 V vs. NHE) is more VB potential of TiO 2 is more negative than the HOMO potential of PANI. Hence, H 2 O will be oxidized to O 2 and • OH on the VB of TiO 2 . In PCN-222(Fe)/TiO 2 /PANI ternary heterojunction, PANI can not only increase the concentration of photogenerated electrons on CB of PCN-222(Fe) but also absorb the photogenerated H + of PCN-222(Fe) and TiO 2 , reducing the recombination of electrons and H + in PCN-222(Fe) and TiO 2 . Thus, according to Equations (1)-(3) [55], the reduction of CO 2 and oxidation of H 2 O will proceed successfully.

Performance Comparison of MEPCMs with Organic−Inorganic Hybrid Shells in the Literature
Preparation conditions and key parameters are important references to evaluate the comprehensive performance of MEPCMs microcapsules with organic−inorganic hybrid shells. First, the emulsion process requires additional co-emulsifiers in most literature (Table S2). In this work, only the Pickering stabilizers were added without co-emulsifiers in the preparation of microcapsules, making the preparation process green and environmentally friendly. The ∆H m and E en of as-prepared microcapsules in this work are ideal under the condition that no co-emulsifier was added, especially ∆H m basically achieves the highest value. The thermal stability of microcapsules was verified by 500 heating-cooling cycles, in which the enthalpy of microcapsules hardly decreases after 500 cycles. Few MEPCMs microcapsules with organic−inorganic hybrid shells have additional functions other than TES, but the integration of TES and CO 2 photoreduction has not been seen before in the literature. Therefore, performance comparison of PPTP-3 with ternary heterojunction photocatalysts in the literature is listed in Table S3. The evolution rate of CO catalyzed by PPTP-3 exceeds most of the recently reported ternary heterojunction photocatalysts, further indicating the ternary heterojunction of the PPTP-3 shell has the superior structural advantage and light utilization efficiency.

Conclusions
In summary, we have fabricated a novel type of bi-functional paraffin@PANI/ TiO 2 /PCN-222(Fe) MEPCMs with thermal energy storage and CO 2 photoreduction ability by Pickering emulsion polymerization. The PPTP MEPCMs are composed of paraffin as PCM core and PANI/TiO 2 /PCN-222(Fe) as hybrid shell, in which the ternary heterojunction was also constructed on the shell owing to the tight contact of the three components. The MEPCMs realized the optimal phase change enthalpy of 174.7 J·g −1 with an encapsulation efficiency of 77.2%. The thermal properties, chemical composition, and morphological structure were retained well after 500 high-low temperature cycles test. Under natural light irradiation, the photoreduction of CO 2 into CO and CH 4 by the MEPCMs showed excellent activity because of the suitable band gap and enhanced separation of the photogenerated electron-hole pair. The results CO evolution rate was 45.16 µmol g −1 h −1 , which is 5.4, 11, and 62 times higher than pure PCN-222(Fe), PANI, and TiO 2 , respectively. The CO evolution rate could maintain basically after five cycles of CO 2 photoreduction within 1 h, and there was still 90% of CO yield compared with the initial test. The as-fabricated bi-functional MEPCMs will show potential applications in the energy-saving building materials and construction of photocatalytic systems due to the solar energy storage and CO 2 photoreduction.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no competing financial interest.