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2019 Volume 35 Issue 4
2019, 35(4): 345-346
doi: 10.3866/PKU.WHXB201803142
Abstract:
2019, 35(4): 347-348
doi: 10.3866/PKU.WHXB201804021
Abstract:
2019, 35(4): 349-350
doi: 10.3866/PKU.WHXB201805023
Abstract:
2019, 35(4): 351-352
doi: 10.3866/PKU.WHXB201806041
Abstract:
2019, 35(4): 353-354
doi: 10.3866/PKU.WHXB201806061
Abstract:
2019, 35(4): 355-360
doi: 10.3866/PKU.WHXB201805161
Abstract:
Recently, non-fullerene polymer solar cells (NPSCs) have been developed rapidly because of the flexible energy-level variability and excellent optical absorption properties of non-fullerene electron acceptors. Among them, fused-ring electron acceptors (FREAs) with acceptor-donor- acceptor (A-D-A) structures have been extensively exploited in high-performance NPSCs. These FREAs often employ central aromatic fused rings attached to several rigid side-chains and flanked by two electron-deficient terminals. Many efforts have focused on the modification of the central flat conjugated backbone in order to gain broad and strong absorption and dense stacking. However, the preparation of such FREAs is relatively complex, especially for large fused-ring structures. In a previous work, we provided a simple and useful method to extend the effective conjugation length and broaden the absorption spectrum of the acceptor by noncovalent intramolecular interactions. On this basis, in this work, we have designed and synthesized a new A-D-A-type FREA (ITOIC-2Cl) that uses 4, 9-dihydro-s-indaceno[1, 2-b:5, 6-b']dithiophene (IDT) as a central donor unit, bis(alkoxy)-substituted thiophene rings as conformational locking π-bridges between the donor and acceptor units, and cyanoindanones modified with two high-electron-affinity chlorine atoms as end-capping acceptor units. On one hand, we can attain good backbone planarity in the solid state via the noncovalent conformational locking induced by sulfur−oxygen (S···O) and oxygen−hydrogen (CH···O) interactions, which are not strong enough to lock the coplanar conformation in solution, thus simultaneously endowing ITOIC-2Cl with good solubility. On the other hand, we can enhance the intramolecular charge transfer by enhancing the electron deficiency of the terminal groups. The optical and electrochemical properties of ITOIC-2Cl were systematically explored. Moreover, in combination with the donor polymer of [(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1, 2-b:4, 5-b']dithiophene))-alt-(5, 5-(1', 3'-di-2-thienyl-5', 7'-bis(2-ethylhexyl)benzo[1', 2'-c:4', 5'-c']dithiophene-4, 8-dione))] (PBDB-T), the photovoltaic performances of the devices and the corresponding blend morphologies were studied. ITOIC-2Cl exhibited a broad absorption spectrum up to 900 nm, which is beneficial for broad harvesting of photons across the visible and NIR region. The PBDB-T:ITOIC-2Cl-based blend films exhibited favorable fibrous nanostructures with appropriate nanoscale phase separation, verified by atomic force microscopy and transmission electron microscopy characterizations. This morphology is beneficial for charge transport. Through the space-charge-limited current measurement, the PBDB-T:ITOIC-2Cl-based device exhibited the high hole/electron mobility of 1.85 × 10−4/1.19 × 10−4 cm2∙V−1∙s−1. The PBDB-T:ITOIC-2Cl-based devices obtained a high power conversion efficiency of 9.37%, with an open-circuit voltage (Voc) of 0.886 V, short-circuit current (Jsc) of 17.09 mA cm−2, and a fill factor (FF) of 61.8%. These results thus demonstrate the efficacy of the proposed strategy for designing high-performance non-fullerene FREAs. ![]()
Recently, non-fullerene polymer solar cells (NPSCs) have been developed rapidly because of the flexible energy-level variability and excellent optical absorption properties of non-fullerene electron acceptors. Among them, fused-ring electron acceptors (FREAs) with acceptor-donor- acceptor (A-D-A) structures have been extensively exploited in high-performance NPSCs. These FREAs often employ central aromatic fused rings attached to several rigid side-chains and flanked by two electron-deficient terminals. Many efforts have focused on the modification of the central flat conjugated backbone in order to gain broad and strong absorption and dense stacking. However, the preparation of such FREAs is relatively complex, especially for large fused-ring structures. In a previous work, we provided a simple and useful method to extend the effective conjugation length and broaden the absorption spectrum of the acceptor by noncovalent intramolecular interactions. On this basis, in this work, we have designed and synthesized a new A-D-A-type FREA (ITOIC-2Cl) that uses 4, 9-dihydro-s-indaceno[1, 2-b:5, 6-b']dithiophene (IDT) as a central donor unit, bis(alkoxy)-substituted thiophene rings as conformational locking π-bridges between the donor and acceptor units, and cyanoindanones modified with two high-electron-affinity chlorine atoms as end-capping acceptor units. On one hand, we can attain good backbone planarity in the solid state via the noncovalent conformational locking induced by sulfur−oxygen (S···O) and oxygen−hydrogen (CH···O) interactions, which are not strong enough to lock the coplanar conformation in solution, thus simultaneously endowing ITOIC-2Cl with good solubility. On the other hand, we can enhance the intramolecular charge transfer by enhancing the electron deficiency of the terminal groups. The optical and electrochemical properties of ITOIC-2Cl were systematically explored. Moreover, in combination with the donor polymer of [(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1, 2-b:4, 5-b']dithiophene))-alt-(5, 5-(1', 3'-di-2-thienyl-5', 7'-bis(2-ethylhexyl)benzo[1', 2'-c:4', 5'-c']dithiophene-4, 8-dione))] (PBDB-T), the photovoltaic performances of the devices and the corresponding blend morphologies were studied. ITOIC-2Cl exhibited a broad absorption spectrum up to 900 nm, which is beneficial for broad harvesting of photons across the visible and NIR region. The PBDB-T:ITOIC-2Cl-based blend films exhibited favorable fibrous nanostructures with appropriate nanoscale phase separation, verified by atomic force microscopy and transmission electron microscopy characterizations. This morphology is beneficial for charge transport. Through the space-charge-limited current measurement, the PBDB-T:ITOIC-2Cl-based device exhibited the high hole/electron mobility of 1.85 × 10−4/1.19 × 10−4 cm2∙V−1∙s−1. The PBDB-T:ITOIC-2Cl-based devices obtained a high power conversion efficiency of 9.37%, with an open-circuit voltage (Voc) of 0.886 V, short-circuit current (Jsc) of 17.09 mA cm−2, and a fill factor (FF) of 61.8%. These results thus demonstrate the efficacy of the proposed strategy for designing high-performance non-fullerene FREAs.
2019, 35(4): 361-370
doi: 10.3866/PKU.WHXB201805102
Abstract:
Lithium-ion batteries (LIBs) possess many virtues, such as low weight, a high energy density, and a long service life, and are regarded as an essential component of a low-carbon economy. Nowadays, LIBs are widely used in consumer electronics, as well as military and aviation products, and are the focus of significant research in the emerging field of energy materials. The cathode material is one of the most important parts of the LIB; its electrochemical performance plays an important role in the battery voltage, power/energy density, cycle life, and safety. LiFePO4 is a superior cathode material compared to spinel manganite (LiMn2O4) and layered lithium nickel-cobalt-manganese oxide (LiMO2 (M = Mn, Co, Ni)), and LiFePO4 has many advantages, such as excellent thermal stability, cycling performance, economic viability, and environmental friendliness. The theoretical diffusion coefficient of LiFePO4 is 10−8 cm2∙s−1, which is sufficient for Li+ de-intercalation in nanoparticles. However, the one-dimensional transport channels are easily blocked by structural defects, resulting in a lower diffusion coefficient and poor rate performance. The electronic conductivity of LiFePO4 is about 10−8 S∙cm−1, and this also limits the rate performance. Moreover, the low-temperature performance, low yield, and patent problems are also significant problems facing LiFePO4. In contrast, the stability and cost are not significant limitations to more extensive applications; rather, it is the energy density and power density that must be improved. To meet the above demands, in-depth research on the factors affecting the electrochemical performance of LiFePO4 is required. Many factors affect the electrochemical performance of LiFePO4, such as the synthetic method, particle size, electrolyte environment, electrode structure, and temperature. Based on the current state of research into LiFePO4, we have focused our review on the following three aspects: the characteristics of the nanoparticles, interface environment of the material, and the electrode structure. Finally, we summarize the relationship between the structure and electrochemical performance of LiFePO4 cathode materials: (1) the bulk phase characteristics of the material (phase structure, doping, nanocrystallization, defects, and lithium-ion transport mechanism), (2) interface structure and interface reconstruction under different electrolyte environments, and (3) the electrode structure. Our conclusions have great significance for future research. ![]()
Lithium-ion batteries (LIBs) possess many virtues, such as low weight, a high energy density, and a long service life, and are regarded as an essential component of a low-carbon economy. Nowadays, LIBs are widely used in consumer electronics, as well as military and aviation products, and are the focus of significant research in the emerging field of energy materials. The cathode material is one of the most important parts of the LIB; its electrochemical performance plays an important role in the battery voltage, power/energy density, cycle life, and safety. LiFePO4 is a superior cathode material compared to spinel manganite (LiMn2O4) and layered lithium nickel-cobalt-manganese oxide (LiMO2 (M = Mn, Co, Ni)), and LiFePO4 has many advantages, such as excellent thermal stability, cycling performance, economic viability, and environmental friendliness. The theoretical diffusion coefficient of LiFePO4 is 10−8 cm2∙s−1, which is sufficient for Li+ de-intercalation in nanoparticles. However, the one-dimensional transport channels are easily blocked by structural defects, resulting in a lower diffusion coefficient and poor rate performance. The electronic conductivity of LiFePO4 is about 10−8 S∙cm−1, and this also limits the rate performance. Moreover, the low-temperature performance, low yield, and patent problems are also significant problems facing LiFePO4. In contrast, the stability and cost are not significant limitations to more extensive applications; rather, it is the energy density and power density that must be improved. To meet the above demands, in-depth research on the factors affecting the electrochemical performance of LiFePO4 is required. Many factors affect the electrochemical performance of LiFePO4, such as the synthetic method, particle size, electrolyte environment, electrode structure, and temperature. Based on the current state of research into LiFePO4, we have focused our review on the following three aspects: the characteristics of the nanoparticles, interface environment of the material, and the electrode structure. Finally, we summarize the relationship between the structure and electrochemical performance of LiFePO4 cathode materials: (1) the bulk phase characteristics of the material (phase structure, doping, nanocrystallization, defects, and lithium-ion transport mechanism), (2) interface structure and interface reconstruction under different electrolyte environments, and (3) the electrode structure. Our conclusions have great significance for future research.
2019, 35(4): 371-377
doi: 10.3866/PKU.WHXB201805022
Abstract:
In the last few decades, noble metal nanoparticles (MNP) have been widely used as imaging probes, in the field of bio-imaging, due to their localized surface plasmon resonance (LSPR) phenomenon. Compared to fluorescent probes, MNP imaging exhibits high sensitivity and outstanding signal-to-noise ratio, while the particle itself has good photostability; this makes the MNP probe the perfect candidate for long-term imaging. Currently the most popular MNP imaging and analysis method employs a dark-field microscope with a spectroscope. Since most dark-field microscopes use halogen lamp or mercury lamp as their illumination source, the illumination intensity and wavelength spectrum are limited. Both camera and spectroscopy require longer exposure time to collect sufficient scattering signal to generate a reasonable quality image and scattering spectrum. The narrow illumination spectrum also limits the size of the MNP that can be used (larger-diameter MNP tend to scatter in the near-infrared region). Therefore, a high-intensity and wide-spectrum illumination source is urgently needed in MNP imaging. In this study, we custom-designed a multi-mode dark field microscope by using a supercontinuum laser, comprising of a lightsheet illumination mode for wide-field imaging and a back focus mode for live spectrum analysis, as its illumination source. The total output of the supercontinuum laser was 2 W. Since it was a coherent illumination source it could be focused by the microscope objective to a near diffraction limit area for sufficient intensity. Moreover, since its wavelength spectrum was between 450 nm and 2200 nm, which covered most of the visible and near infrared region, it made the detection of the large-diameter MNP single particle possible. In the back-focus mode, the supercontinuum laser first passed through an annular filter and then entered the objective from the microscope back port. In the lightsheet illumination mode, the laser was focused by a 400-mm cylindrical concave mirror to create a "sheet" and illuminate the sample from its side. In both the illumination modes, the illumination radiation was blocked from the camera to obtain the dark field illumination effect. By using a multi-mode dark field microscope, we could observe a 30-nm-diameter MNP single particle with a color CCD camera in its lightsheet illumination mode and a spectrum time resolution of 1 ms in its back-focus illumination mode. This custom-designed microscope could not only be used to study the MNP single particle in living cells, but more importantly, its application could also be potentially extended to all the MNP-probe-based cell imaging. ![]()
In the last few decades, noble metal nanoparticles (MNP) have been widely used as imaging probes, in the field of bio-imaging, due to their localized surface plasmon resonance (LSPR) phenomenon. Compared to fluorescent probes, MNP imaging exhibits high sensitivity and outstanding signal-to-noise ratio, while the particle itself has good photostability; this makes the MNP probe the perfect candidate for long-term imaging. Currently the most popular MNP imaging and analysis method employs a dark-field microscope with a spectroscope. Since most dark-field microscopes use halogen lamp or mercury lamp as their illumination source, the illumination intensity and wavelength spectrum are limited. Both camera and spectroscopy require longer exposure time to collect sufficient scattering signal to generate a reasonable quality image and scattering spectrum. The narrow illumination spectrum also limits the size of the MNP that can be used (larger-diameter MNP tend to scatter in the near-infrared region). Therefore, a high-intensity and wide-spectrum illumination source is urgently needed in MNP imaging. In this study, we custom-designed a multi-mode dark field microscope by using a supercontinuum laser, comprising of a lightsheet illumination mode for wide-field imaging and a back focus mode for live spectrum analysis, as its illumination source. The total output of the supercontinuum laser was 2 W. Since it was a coherent illumination source it could be focused by the microscope objective to a near diffraction limit area for sufficient intensity. Moreover, since its wavelength spectrum was between 450 nm and 2200 nm, which covered most of the visible and near infrared region, it made the detection of the large-diameter MNP single particle possible. In the back-focus mode, the supercontinuum laser first passed through an annular filter and then entered the objective from the microscope back port. In the lightsheet illumination mode, the laser was focused by a 400-mm cylindrical concave mirror to create a "sheet" and illuminate the sample from its side. In both the illumination modes, the illumination radiation was blocked from the camera to obtain the dark field illumination effect. By using a multi-mode dark field microscope, we could observe a 30-nm-diameter MNP single particle with a color CCD camera in its lightsheet illumination mode and a spectrum time resolution of 1 ms in its back-focus illumination mode. This custom-designed microscope could not only be used to study the MNP single particle in living cells, but more importantly, its application could also be potentially extended to all the MNP-probe-based cell imaging.
2019, 35(4): 385-393
doi: 10.3866/PKU.WHXB201805291
Abstract:
Two-dimensional transition metal dichalcogenides (TMDs) possess the potential to be widely applied in optoelectronic devices, sensors, photocatalysis, and many other fields because of their intrinsic physical, chemical, and mechanical properties. Generally, the van der Waals (vdW) heterostructures fabricated from these TMDs exhibit excellent electronic properties. However, the spectral responses of most vdW heterostructures are limited by the inherent band gaps; it is thus essential to tune the band gaps for specific applications. In this paper, we performed a first-principles theoretical study on the structures and properties of WX2 (X = S, Se, Te), as well as the vdW heterostructures WS2/WSe2, WS2/WTe2, and WSe2/WTe2. The impacts of the number of layers on the properties of WX2 and the strain on the band gaps of vdW heterostructures were demonstrated. We found that every monolayer WX2 (X = S, Se, Te) is a direct gap semiconductor, and as the number of layers increases, their band gaps decrease and they become indirect bandgap semiconductors. The spin-orbit coupling (SOC) effect on their band structures is significant and can decrease the band gap by approximately 300 meV compared with those that do no incorporate SOC effects. The properties of WX2 can be accurately described by the HSE06 + SOC approach. WS2/WSe2, WS2/WTe2, and WSe2/WTe2 heterostructures are direct gap semiconductors with band gaps of 1.10, 0.32, and 0.61 eV, respectively. These three heterostructures exhibit type-II band alignments, which facilitate photo-induced electron-hole separation. In addition, they have quite small electron and hole effective masses, indicating that the separated electrons and holes can move very quickly to reduce the recombination rate of electrons and holes. There is an explicit red-shift of the optical absorption spectra of the three heterostructures compared with those of the monolayer components, and the most obvious redshift occurs in WSe2/WTe2. Both uniaxial and biaxial strains can alter the band gaps of these vdW heterostructures. Once the strain exceeds 4%, a transition from semiconductor to metal characteristics occurs. This work provides a way to tune the electronic properties and band gaps of vdW heterostructures for incorporation in high-performance optoelectronic devices. ![]()
Two-dimensional transition metal dichalcogenides (TMDs) possess the potential to be widely applied in optoelectronic devices, sensors, photocatalysis, and many other fields because of their intrinsic physical, chemical, and mechanical properties. Generally, the van der Waals (vdW) heterostructures fabricated from these TMDs exhibit excellent electronic properties. However, the spectral responses of most vdW heterostructures are limited by the inherent band gaps; it is thus essential to tune the band gaps for specific applications. In this paper, we performed a first-principles theoretical study on the structures and properties of WX2 (X = S, Se, Te), as well as the vdW heterostructures WS2/WSe2, WS2/WTe2, and WSe2/WTe2. The impacts of the number of layers on the properties of WX2 and the strain on the band gaps of vdW heterostructures were demonstrated. We found that every monolayer WX2 (X = S, Se, Te) is a direct gap semiconductor, and as the number of layers increases, their band gaps decrease and they become indirect bandgap semiconductors. The spin-orbit coupling (SOC) effect on their band structures is significant and can decrease the band gap by approximately 300 meV compared with those that do no incorporate SOC effects. The properties of WX2 can be accurately described by the HSE06 + SOC approach. WS2/WSe2, WS2/WTe2, and WSe2/WTe2 heterostructures are direct gap semiconductors with band gaps of 1.10, 0.32, and 0.61 eV, respectively. These three heterostructures exhibit type-II band alignments, which facilitate photo-induced electron-hole separation. In addition, they have quite small electron and hole effective masses, indicating that the separated electrons and holes can move very quickly to reduce the recombination rate of electrons and holes. There is an explicit red-shift of the optical absorption spectra of the three heterostructures compared with those of the monolayer components, and the most obvious redshift occurs in WSe2/WTe2. Both uniaxial and biaxial strains can alter the band gaps of these vdW heterostructures. Once the strain exceeds 4%, a transition from semiconductor to metal characteristics occurs. This work provides a way to tune the electronic properties and band gaps of vdW heterostructures for incorporation in high-performance optoelectronic devices.
2019, 35(4): 408-414
doi: 10.3866/PKU.WHXB201803051
Abstract:
A polymer-surfactant complex is significant in understanding the interactions between amphiphilic molecules and has great potential for use in a vast number of industries. In addition, the stimuli-responsive polymer-surfactant complex represents a hot research topic for the colloid community. However, the use of CO2 gas to tune their interaction and the corresponding morphological change in the polymer-surfactant complex has been less documented. In this work, the commercially available triblock copolymer Pluronic F127 was used as a starting material and the macromolecular initiator Br-F127-Br was synthesized via esterification. Then, the pentablock copolymer poly(2-(diethylamino)ethyl methacrylate))-block-F127-block-poly(2-(diethylamino)ethyl methacrylate)) (PDEAEAM-b-F127-b-PDEAEMA) was prepared via atom transfer radical polymerization (ATRP) of Br-F127-Br and the monomer 2-(diethylamino)ethyl methacrylate. Both Br-F127-Br and PDEAEAM-b-F127-b-PDEAEMA were characterized by FT-IR and 1H NMR spectroscopies as well as gel permeation chromatography (GPC). The results indicated that both Br-F127-Br and PDEAEAM-b-F127-b-PDEAEMA were synthesized successfully. The CO2-responsive behavior of the pentablock copolymer was examined by tracking the changes in pH and electrical conductivity of the polymer solution after alternatingly bubbling CO2 and N2. It was found that cyclic streaming of CO2/N2 could alter the pH of the polymer solution between 7.2 and 5.3, leading to the protonation degree of PDEAEAM-b-F127-b-PDEAEMA varying between 0.26 and 0.96; this in turn varied the electrical conductivity of the polymer solution between 19.4 μS∙cm−1 and 70.6 μS∙cm−1. The reversible changes in pH and electrical conductivity of the polymer solution indicate the good CO2-stimuli responsiveness of PDEAEAM-b-F127-b-PDEAEMA. The interaction of PDEAEAM-b-F127-b-PDEAEMA with an anionic fluorocarbon surfactant potassium nonafluoro-1-butanesulfonate (C4F9SO3K) with and without CO2 was studied by ultraviolet-visible absorption spectrometry (UV-Vis), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The transmittance of the mixed solution of PDEAEAM-b-F127-b-PDEAEMA and C4F9SO3K could be varied between 84% and 52% in the absence and presence of CO2, indicating the formation of aggregates with different sizes. The DLS results showed that the size of aggregates could be modified reversibly between tens of nanometers and several micrometers by bubbling CO2 and replacing CO2 by N2. The TEM image revealed the reversible morphological transition of the aggregates from spherical to wormlike micelles after bubbling CO2. The carbonic acid formed from CO2 and water can protonate the PDEAEMA in the pentablock copolymer to form PDEAEMA·H+, and thus the interaction between the pentablock copolymer and C4F9SO3K becomes strong. When CO2 is replaced by N2, PDEAEMA·H+ reverts to PDEAEMA, and the interaction becomes weak once again. It can therefore be concluded that the protonation/deprotonation process of the pentablock copolymer can be controlled by bubbling CO2/N2. The protonation/deprotonation process can "switch" the electrostatic attraction of PDEAEAM-b-F127-b-PDEAEMA to C4F9SO3K, thereby tuning the hydrophilic-lipophilic balance (HLB) of the polymer-surfactant complex reversibly, leading to the reversible morphological transition of the aggregates. The strategy of CO2-controllable morphological alteration of a polymer–surfactant complex opens a new avenue for preparing gas-sensitive soft materials. ![]()
A polymer-surfactant complex is significant in understanding the interactions between amphiphilic molecules and has great potential for use in a vast number of industries. In addition, the stimuli-responsive polymer-surfactant complex represents a hot research topic for the colloid community. However, the use of CO2 gas to tune their interaction and the corresponding morphological change in the polymer-surfactant complex has been less documented. In this work, the commercially available triblock copolymer Pluronic F127 was used as a starting material and the macromolecular initiator Br-F127-Br was synthesized via esterification. Then, the pentablock copolymer poly(2-(diethylamino)ethyl methacrylate))-block-F127-block-poly(2-(diethylamino)ethyl methacrylate)) (PDEAEAM-b-F127-b-PDEAEMA) was prepared via atom transfer radical polymerization (ATRP) of Br-F127-Br and the monomer 2-(diethylamino)ethyl methacrylate. Both Br-F127-Br and PDEAEAM-b-F127-b-PDEAEMA were characterized by FT-IR and 1H NMR spectroscopies as well as gel permeation chromatography (GPC). The results indicated that both Br-F127-Br and PDEAEAM-b-F127-b-PDEAEMA were synthesized successfully. The CO2-responsive behavior of the pentablock copolymer was examined by tracking the changes in pH and electrical conductivity of the polymer solution after alternatingly bubbling CO2 and N2. It was found that cyclic streaming of CO2/N2 could alter the pH of the polymer solution between 7.2 and 5.3, leading to the protonation degree of PDEAEAM-b-F127-b-PDEAEMA varying between 0.26 and 0.96; this in turn varied the electrical conductivity of the polymer solution between 19.4 μS∙cm−1 and 70.6 μS∙cm−1. The reversible changes in pH and electrical conductivity of the polymer solution indicate the good CO2-stimuli responsiveness of PDEAEAM-b-F127-b-PDEAEMA. The interaction of PDEAEAM-b-F127-b-PDEAEMA with an anionic fluorocarbon surfactant potassium nonafluoro-1-butanesulfonate (C4F9SO3K) with and without CO2 was studied by ultraviolet-visible absorption spectrometry (UV-Vis), dynamic light scattering (DLS), and transmission electron microscopy (TEM). The transmittance of the mixed solution of PDEAEAM-b-F127-b-PDEAEMA and C4F9SO3K could be varied between 84% and 52% in the absence and presence of CO2, indicating the formation of aggregates with different sizes. The DLS results showed that the size of aggregates could be modified reversibly between tens of nanometers and several micrometers by bubbling CO2 and replacing CO2 by N2. The TEM image revealed the reversible morphological transition of the aggregates from spherical to wormlike micelles after bubbling CO2. The carbonic acid formed from CO2 and water can protonate the PDEAEMA in the pentablock copolymer to form PDEAEMA·H+, and thus the interaction between the pentablock copolymer and C4F9SO3K becomes strong. When CO2 is replaced by N2, PDEAEMA·H+ reverts to PDEAEMA, and the interaction becomes weak once again. It can therefore be concluded that the protonation/deprotonation process of the pentablock copolymer can be controlled by bubbling CO2/N2. The protonation/deprotonation process can "switch" the electrostatic attraction of PDEAEAM-b-F127-b-PDEAEMA to C4F9SO3K, thereby tuning the hydrophilic-lipophilic balance (HLB) of the polymer-surfactant complex reversibly, leading to the reversible morphological transition of the aggregates. The strategy of CO2-controllable morphological alteration of a polymer–surfactant complex opens a new avenue for preparing gas-sensitive soft materials.
2019, 35(4): 415-421
doi: 10.3866/PKU.WHXB201803141
Abstract:
The average diameter and size distribution of dispersed-phase droplets are important factors affecting the properties of emulsions, and the changes in these parameters with time and environment can be used to evaluate the emulsion stability. Traditional size characterization methods such as dynamic light scattering (DLS) are not applicable to highly concentrated emulsions. Herein, we report an imaging-based method to measure the droplet size in highly concentrated emulsions. This method comprises three steps: 1) emulsions are labeled with a fluorescent dye, 2) three optical slices with a certain distance between two adjacent focal plans are measured sequentially via confocal laser scanning microscopy, 3) the sizes of dispersed-phase droplets are determined from the apparent diameters of droplets in the optical slices. When the apparent diameter of a droplet in the three optical slices increases or decreases monotonically, droplet diameter is calculated according to the following equations: DC1–2 = {D22 +[(D12 − D22)/4δz + δz]2}1/2 or DC2–3 = {D32 + [(D22 – D32)/4δz + δz]2}1/2, where D1, D2, D3 is the apparent diameter of the droplet measured from the consecutively-obtained optical slices 1−3, respectively; DC1–2 represents the calculated diameter of the droplet from the slices 1 and 2, and DC2–3 is that from the slices 2 and 3, and δz is the distance between two focal planes of the adjacent optical slices. To avoid an obvious interference from the droplet movement, we use the equation 2|DC1–2− DC2–3|/(DC1–2 + DC2–3) = X, where a smaller X value indicates a less extent of movement during measurement, and that the calculated average diameter (DC1–2 + DC2–3)/2 is closer to the measured size of the droplet. The experimental results showed that when X was 15%, the difference between the calculated and measured diameters was about 10%. When X was less than 15%, the calculated average droplet diameter was adopted as an effective diameter. However, when the condition D1= D2 ≥ D3 (or D3 = D2≥ D1) was met, D2 was used as the effective droplet diameter. The present method combines the advantages of fluorescent labeling, double optical slices analysis, and a strategy for eliminating the error caused by droplet movement. The stability of highly concentrated emulsions (oil volume percentage: 60%−80%), prepared by mixing a crude oil model mixture containing n-decane, naphthaline, decalin, and tetraphenylporphyrin (92.3%, 4.1%, 3.6%, and 0.1‰ by mass, respectively) with aqueous solutions containing surfactants, was studied with the proposed method. The experimental results indicated that the present method allowed for the effective and accurate measurement of the anti-coalescence stability of emulsion dispersed-phase droplets. In contrast, the widely adopted "bottle test" method could not provide accurate information on the anti-coalescence stability of the dispersed phase droplets. ![]()
The average diameter and size distribution of dispersed-phase droplets are important factors affecting the properties of emulsions, and the changes in these parameters with time and environment can be used to evaluate the emulsion stability. Traditional size characterization methods such as dynamic light scattering (DLS) are not applicable to highly concentrated emulsions. Herein, we report an imaging-based method to measure the droplet size in highly concentrated emulsions. This method comprises three steps: 1) emulsions are labeled with a fluorescent dye, 2) three optical slices with a certain distance between two adjacent focal plans are measured sequentially via confocal laser scanning microscopy, 3) the sizes of dispersed-phase droplets are determined from the apparent diameters of droplets in the optical slices. When the apparent diameter of a droplet in the three optical slices increases or decreases monotonically, droplet diameter is calculated according to the following equations: DC1–2 = {D22 +[(D12 − D22)/4δz + δz]2}1/2 or DC2–3 = {D32 + [(D22 – D32)/4δz + δz]2}1/2, where D1, D2, D3 is the apparent diameter of the droplet measured from the consecutively-obtained optical slices 1−3, respectively; DC1–2 represents the calculated diameter of the droplet from the slices 1 and 2, and DC2–3 is that from the slices 2 and 3, and δz is the distance between two focal planes of the adjacent optical slices. To avoid an obvious interference from the droplet movement, we use the equation 2|DC1–2− DC2–3|/(DC1–2 + DC2–3) = X, where a smaller X value indicates a less extent of movement during measurement, and that the calculated average diameter (DC1–2 + DC2–3)/2 is closer to the measured size of the droplet. The experimental results showed that when X was 15%, the difference between the calculated and measured diameters was about 10%. When X was less than 15%, the calculated average droplet diameter was adopted as an effective diameter. However, when the condition D1= D2 ≥ D3 (or D3 = D2≥ D1) was met, D2 was used as the effective droplet diameter. The present method combines the advantages of fluorescent labeling, double optical slices analysis, and a strategy for eliminating the error caused by droplet movement. The stability of highly concentrated emulsions (oil volume percentage: 60%−80%), prepared by mixing a crude oil model mixture containing n-decane, naphthaline, decalin, and tetraphenylporphyrin (92.3%, 4.1%, 3.6%, and 0.1‰ by mass, respectively) with aqueous solutions containing surfactants, was studied with the proposed method. The experimental results indicated that the present method allowed for the effective and accurate measurement of the anti-coalescence stability of emulsion dispersed-phase droplets. In contrast, the widely adopted "bottle test" method could not provide accurate information on the anti-coalescence stability of the dispersed phase droplets.
2019, 35(4): 422-430
doi: 10.3866/PKU.WHXB201805301
Abstract:
Perovskite is widely used as catalyst supports because of its flexible composition, good redox performance, and excellent thermal stability. However, the use of perovskite oxides as catalyst supports has two disadvantages: low surface area due to synthesizing the perovskite structure at high temperatures, and native perovskite surfaces preferentially have A-sites instead of catalytically active sites. On the other hand, interaction between the support and metal affects the size and valence state of noble metals. Therefore, perovskite oxides with different structures were prepared and were used to support Au catalysts, in order to obtain excellent catalytic activity and high stability. Specifically, stoichiometric LaMnO3 and nonstoichiometric LaMn1.2O3 perovskites were prepared by the ethylene glycol sol-gel method, and then the LaMnO3-AE oxide was prepared by treating LaMnO3 perovskite with dilute nitric acid. The perovskite-supported Au catalyst was prepared by the deposition precipitation method and its catalytic activity for CO oxidation was evaluated. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR), it was found that LaMnO3 and LaMn1.2O3 perovskite carriers were beneficial for the dispersion of Au; however, the Au nanoparticle size significantly increased with increasing calcination temperature, indicating poor Au thermal stability. In contrast, LaMnO3 perovskite (LaMnO3-AE) etched by nitric acid is not conducive to dispersion of Au, but it is beneficial for improving the thermal stability of Au. Au was always maintained in the zero-valence state after calcination at different temperature. H2-TPR results revealed that the reducibility of the catalysts changed largely after thermal treatment at high temperatures, and was mainly influenced by the agglomeration of Au nanoparticles. Although the reducibility of the Au/LaMnO3-AE catalyst calcined at 250 ℃ is lower than that of Au/LaMn1.2O3 and Au/LaMnO3 catalysts calcined at the same temperature, the former exhibited higher reducibility when the catalyst was calcined at high temperatures (500 and 900 ℃). In the CO oxidation reaction, the catalytic activity of all the prepared catalysts decreased when the calcination temperature was increased from 250 to 500, 700, and 900 ℃. However, the catalytic activity of the Au/LaMn1.2O3 catalyst was significantly higher than those of LaMnO3- and LaMnO3-AE-supported Au catalyst, when calcination temperature was below 500 ℃, while the activity of the Au/LaMnO3-AE catalyst was significantly higher than those of the Au/LaMnO3 and Au/LaMn1.2O3 catalysts when the calcination temperature was more than 700 ℃. As shown in characterization results, after the catalyst was calcined at high temperatures (700 and 900 ℃), the Au nanoparticle size on the Au/LaMnO3-AE catalyst was lower than those on Au/LaMnO3 and Au/LaMn1.2O3 catalysts, leading to high reducibility and catalytic activity of the Au/LaMnO3-AE catalyst. The Au/LaMnO3-AE catalyst also exhibited high stability in CO oxidation. The catalytic activity of the Au/LaMnO3-AE catalyst can be maintained for 20 h at 130 ℃. ![]()
Perovskite is widely used as catalyst supports because of its flexible composition, good redox performance, and excellent thermal stability. However, the use of perovskite oxides as catalyst supports has two disadvantages: low surface area due to synthesizing the perovskite structure at high temperatures, and native perovskite surfaces preferentially have A-sites instead of catalytically active sites. On the other hand, interaction between the support and metal affects the size and valence state of noble metals. Therefore, perovskite oxides with different structures were prepared and were used to support Au catalysts, in order to obtain excellent catalytic activity and high stability. Specifically, stoichiometric LaMnO3 and nonstoichiometric LaMn1.2O3 perovskites were prepared by the ethylene glycol sol-gel method, and then the LaMnO3-AE oxide was prepared by treating LaMnO3 perovskite with dilute nitric acid. The perovskite-supported Au catalyst was prepared by the deposition precipitation method and its catalytic activity for CO oxidation was evaluated. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR), it was found that LaMnO3 and LaMn1.2O3 perovskite carriers were beneficial for the dispersion of Au; however, the Au nanoparticle size significantly increased with increasing calcination temperature, indicating poor Au thermal stability. In contrast, LaMnO3 perovskite (LaMnO3-AE) etched by nitric acid is not conducive to dispersion of Au, but it is beneficial for improving the thermal stability of Au. Au was always maintained in the zero-valence state after calcination at different temperature. H2-TPR results revealed that the reducibility of the catalysts changed largely after thermal treatment at high temperatures, and was mainly influenced by the agglomeration of Au nanoparticles. Although the reducibility of the Au/LaMnO3-AE catalyst calcined at 250 ℃ is lower than that of Au/LaMn1.2O3 and Au/LaMnO3 catalysts calcined at the same temperature, the former exhibited higher reducibility when the catalyst was calcined at high temperatures (500 and 900 ℃). In the CO oxidation reaction, the catalytic activity of all the prepared catalysts decreased when the calcination temperature was increased from 250 to 500, 700, and 900 ℃. However, the catalytic activity of the Au/LaMn1.2O3 catalyst was significantly higher than those of LaMnO3- and LaMnO3-AE-supported Au catalyst, when calcination temperature was below 500 ℃, while the activity of the Au/LaMnO3-AE catalyst was significantly higher than those of the Au/LaMnO3 and Au/LaMn1.2O3 catalysts when the calcination temperature was more than 700 ℃. As shown in characterization results, after the catalyst was calcined at high temperatures (700 and 900 ℃), the Au nanoparticle size on the Au/LaMnO3-AE catalyst was lower than those on Au/LaMnO3 and Au/LaMn1.2O3 catalysts, leading to high reducibility and catalytic activity of the Au/LaMnO3-AE catalyst. The Au/LaMnO3-AE catalyst also exhibited high stability in CO oxidation. The catalytic activity of the Au/LaMnO3-AE catalyst can be maintained for 20 h at 130 ℃.
2019, 35(4): 431-441
doi: 10.3866/PKU.WHXB201805211
Abstract:
Dimethyl ether (DME) is considered a promising energy source and clean fuel for the next generation, with its high hydrogen content, and non-toxicity compared with methanol. In addition, it is easy to store and transport. DME steam reforming (SR) has received considerable attention for its applicability in the production of hydrogen for fuel cell applications. Generally, DME SR consists of two steps: DME hydrolysis and methanol SR. DME hydrolysis often occurs on an acidic catalyst, such as γ-Al2O3. Methanol SR in Cu-based catalysts requires both Cu0 and Cu+ as the active sites; moreover, the relative ratios of Cu0 and Cu+ can influence the catalytic performance. In addition, the byproduct of CO also commonly exists in DME SR, and a small amount of CO can poison Pt electrodes of fuel cells. Therefore, it is necessary to reduce the concentration of the generated CO in DME SR. Herein, using an ammonia-evaporation method, we synthesized a Cu/SiO2 catalyst, which can simultaneously generate the dual copper species of Cu0 and Cu+ by reduction. After modification with Ga2O3, the xGa-Cu/SiO2 catalysts show much improved catalytic activity and decreased CO selectivity. The Cu/SiO2 catalyst shows a DME conversion of 90.7% and CO selectivity of 11.5% at 380 ℃. The 5Ga-Cu/SiO2 catalyst, with a loading amount of Ga2O3 of 5% (w, based on the weight of Cu), shows the best performance, with a DME conversion of 99.8% and CO selectivity of 4.8% under the same conditions. The measurement of apparent activation energies shows that the addition of Ga2O3 cannot change the reaction path. By multiple characterization methods, we demonstrated that the improved performance can be ascribed to the following two aspects. First, our characterization results show that the loaded Ga2O3 is highly dispersed on the Cu/SiO2 catalyst, which can increase the interaction between Ga and Cu species. This can not only improve the dispersion of copper species (Cu0 and Cu+) on the catalysts, but can also adjust the ratios of Cu+/(Cu0 + Cu+). The H2 production rate shows a typical volcano curve owing to the ratio of Cu+/(Cu0 + Cu+), and reaches a maximum of 5.02 mol·g-1·h-1 at Cu+/(Cu0 + Cu+) = 0.5 for the 5Ga-Cu/SiO2 catalyst. We conclude that the interaction between Ga and Cu species and the synergistic effect between Cu0 and Cu+ result in the promoted catalytic activity for DME SR. Second, by using a temperature-programmed surface reaction (TPSR), we showed that the addition of Ga2O3 can efficiently promote the water–gas shift reaction, thereby reducing the CO selectivity in DME SR. Thus, Ga2O3 suppresses the generation of CO, leading to the low CO selectivity and high CO2 selectivity. In summary, the Ga2O3-modified Cu/SiO2 catalyst yields reformates with low CO selectivity and high catalytic activity for DME SR. Our work provides a novel approach to designing a highly efficient Cu-based catalyst for catalytic SR systems. ![]()
Dimethyl ether (DME) is considered a promising energy source and clean fuel for the next generation, with its high hydrogen content, and non-toxicity compared with methanol. In addition, it is easy to store and transport. DME steam reforming (SR) has received considerable attention for its applicability in the production of hydrogen for fuel cell applications. Generally, DME SR consists of two steps: DME hydrolysis and methanol SR. DME hydrolysis often occurs on an acidic catalyst, such as γ-Al2O3. Methanol SR in Cu-based catalysts requires both Cu0 and Cu+ as the active sites; moreover, the relative ratios of Cu0 and Cu+ can influence the catalytic performance. In addition, the byproduct of CO also commonly exists in DME SR, and a small amount of CO can poison Pt electrodes of fuel cells. Therefore, it is necessary to reduce the concentration of the generated CO in DME SR. Herein, using an ammonia-evaporation method, we synthesized a Cu/SiO2 catalyst, which can simultaneously generate the dual copper species of Cu0 and Cu+ by reduction. After modification with Ga2O3, the xGa-Cu/SiO2 catalysts show much improved catalytic activity and decreased CO selectivity. The Cu/SiO2 catalyst shows a DME conversion of 90.7% and CO selectivity of 11.5% at 380 ℃. The 5Ga-Cu/SiO2 catalyst, with a loading amount of Ga2O3 of 5% (w, based on the weight of Cu), shows the best performance, with a DME conversion of 99.8% and CO selectivity of 4.8% under the same conditions. The measurement of apparent activation energies shows that the addition of Ga2O3 cannot change the reaction path. By multiple characterization methods, we demonstrated that the improved performance can be ascribed to the following two aspects. First, our characterization results show that the loaded Ga2O3 is highly dispersed on the Cu/SiO2 catalyst, which can increase the interaction between Ga and Cu species. This can not only improve the dispersion of copper species (Cu0 and Cu+) on the catalysts, but can also adjust the ratios of Cu+/(Cu0 + Cu+). The H2 production rate shows a typical volcano curve owing to the ratio of Cu+/(Cu0 + Cu+), and reaches a maximum of 5.02 mol·g-1·h-1 at Cu+/(Cu0 + Cu+) = 0.5 for the 5Ga-Cu/SiO2 catalyst. We conclude that the interaction between Ga and Cu species and the synergistic effect between Cu0 and Cu+ result in the promoted catalytic activity for DME SR. Second, by using a temperature-programmed surface reaction (TPSR), we showed that the addition of Ga2O3 can efficiently promote the water–gas shift reaction, thereby reducing the CO selectivity in DME SR. Thus, Ga2O3 suppresses the generation of CO, leading to the low CO selectivity and high CO2 selectivity. In summary, the Ga2O3-modified Cu/SiO2 catalyst yields reformates with low CO selectivity and high catalytic activity for DME SR. Our work provides a novel approach to designing a highly efficient Cu-based catalyst for catalytic SR systems.
2019, 35(4): 442-450
doi: 10.3866/PKU.WHXB201805163
Abstract:
Graphitic carbon nitride (g-CN), as a nonmetal semiconductor material, has been widely used in various fields, such as photocatalysis, electrocatalysis, batteries, light-emitting diodes, and solar cells, owing to its unique electronic and photophysical properties. However, the application of g-CN in practical devices remains limited because of the difficulties in fabricating g-CN films of high quality. In this work, we report a method for preparing a g-CN film with high optical quality on a substrate of indium tin oxide (ITO) glass and/or soda lime (NaCa) glass by using melamine as a precursor. First, we prepared the bulk g-CN from melamine in a muffle furnace via thermal polymerization. Then, we fabricated the g-CN film on the ITO and/or NaCa glass substrate with fine-milled, bulk g-CN in a tube furnace using thermal vapor deposition. With this two-step method, a yellow, transparent g-CN film with high optical quality was successfully fabricated on both the ITO and/or NaCa glass substrates. To check the quality of the film, we used scanning electron microscopy (SEM) to study the morphology of the fabricated g-CN film on the ITO glass substrate. Both the high-resolution and low-resolution SEM image results show that the obtained g-CN film on the ITO glass substrate had a homogeneous and dense structure without a corrugated surface, illustrating that it had good surface roughness. Then, we investigated the thickness and surface roughness of the g-CN film via atomic force microscopy (AFM). The AFM results show that the thickness of the g-CN film deposited on the ITO glass substrate was around 300 nm and that the surface roughness of the g-CN film deposited on the ITO glass substrate was less than 40 nm. To verify the chemical composition of the obtained g-CN film on the ITO glass substrate, we performed X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS) analyses. Both the XPS and EDS results demonstrate that the chemical composition of the g-CN film deposited on the ITO glass substrate was similar to that of bulk g-CN powder. More importantly, we determined the band structure for the g-CN film deposited on the ITO glass substrate by using a combination of steady-state absorption and high-resolution valence band XPS analysis. It was found that the determined band structure for the g-CN film deposited on the ITO glass substrate was close to that of bulk g-CN powder, also indicating that its chemical composition was similar to that of bulk g-CN. Meanwhile, we also found that the prepared g-CN film on the ITO glass substrate effectively degraded methylene blue dye under Xe lamp irradiation, which was similar to the effect of bulk g-CN powder. All analyses performed demonstrate that the two-step method presented in this study could successfully fabricate a g-CN film with high optical quality. In addition, we also analyzed the fluorescence lifetime of the g-CN film deposited on the ITO glass substrate by using a homemade time-correlated single-photon counting apparatus and found that it was much shorter than that of bulk g-CN. ![]()
Graphitic carbon nitride (g-CN), as a nonmetal semiconductor material, has been widely used in various fields, such as photocatalysis, electrocatalysis, batteries, light-emitting diodes, and solar cells, owing to its unique electronic and photophysical properties. However, the application of g-CN in practical devices remains limited because of the difficulties in fabricating g-CN films of high quality. In this work, we report a method for preparing a g-CN film with high optical quality on a substrate of indium tin oxide (ITO) glass and/or soda lime (NaCa) glass by using melamine as a precursor. First, we prepared the bulk g-CN from melamine in a muffle furnace via thermal polymerization. Then, we fabricated the g-CN film on the ITO and/or NaCa glass substrate with fine-milled, bulk g-CN in a tube furnace using thermal vapor deposition. With this two-step method, a yellow, transparent g-CN film with high optical quality was successfully fabricated on both the ITO and/or NaCa glass substrates. To check the quality of the film, we used scanning electron microscopy (SEM) to study the morphology of the fabricated g-CN film on the ITO glass substrate. Both the high-resolution and low-resolution SEM image results show that the obtained g-CN film on the ITO glass substrate had a homogeneous and dense structure without a corrugated surface, illustrating that it had good surface roughness. Then, we investigated the thickness and surface roughness of the g-CN film via atomic force microscopy (AFM). The AFM results show that the thickness of the g-CN film deposited on the ITO glass substrate was around 300 nm and that the surface roughness of the g-CN film deposited on the ITO glass substrate was less than 40 nm. To verify the chemical composition of the obtained g-CN film on the ITO glass substrate, we performed X-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS) analyses. Both the XPS and EDS results demonstrate that the chemical composition of the g-CN film deposited on the ITO glass substrate was similar to that of bulk g-CN powder. More importantly, we determined the band structure for the g-CN film deposited on the ITO glass substrate by using a combination of steady-state absorption and high-resolution valence band XPS analysis. It was found that the determined band structure for the g-CN film deposited on the ITO glass substrate was close to that of bulk g-CN powder, also indicating that its chemical composition was similar to that of bulk g-CN. Meanwhile, we also found that the prepared g-CN film on the ITO glass substrate effectively degraded methylene blue dye under Xe lamp irradiation, which was similar to the effect of bulk g-CN powder. All analyses performed demonstrate that the two-step method presented in this study could successfully fabricate a g-CN film with high optical quality. In addition, we also analyzed the fluorescence lifetime of the g-CN film deposited on the ITO glass substrate by using a homemade time-correlated single-photon counting apparatus and found that it was much shorter than that of bulk g-CN.
2019, 35(4): 378-384
doi: 10.3866/PKU.WHXB201805031
Abstract:
Self-aggregated quaternary ammonium polysulfone (aQAPS) is a high-performance alkaline polymer electrolyte that has been applied in alkaline polymer electrolyte fuel cells (APEFCs). For a long time, N, N-dimethyl formamide (DMF) has been considered the best solvent to dissolve aQAPS, but the high boiling point of DMF makes it hard to remove from the electrodes, which potentially poisons the electrocatalysts. Our recent experiments have shown that although aQAPS is unable to dissolve in ethanol, n-propanol, or water, it can dissolve in the mixture of these alcohols and water. This peculiar dissolution behavior significantly facilitates the fabrication of the membrane electrode assembly (MEA) for APEFCs, even though it has not been understood. In this work, atomistic molecular dynamics (MD) simulations were employed to study the dissolution behavior of aQAPS in different solvents, including water, methanol, ethanol, n-propanol, DMF, and the mixture of these non-aqueous solvents and water. The conformation of the aQAPS chain in pure solvents agreed well with the dissolution behavior observed in the experiments, even though in the water-containing mixed solvents, the aQAPS chain tended to be in a more contracted state. The simulations further revealed that the water component in the mixed solvents played dual roles. On one hand, the hydrocarbon chain of aQAPS was compressed to a contracted state upon the addition of water, because of the hydrophobic effect. On the other hand, water can drive the dissociation of the counterion (Cl– ), which led to an enhancement in the solute-solvent interaction energy and thus facilitated the dissolution of aQAPS. In most mixed solvents, the compensation of these two interactions resulted in a general increase in the total solute-solvent interaction energy; therefore, the addition of water was energetically favorable for the dissolution of aQAPS. This study not only furthers our fundamental understanding of the dissolution behavior of polyelectrolytes but also is technologically significant for the development of better APEFCs. ![]()
Self-aggregated quaternary ammonium polysulfone (aQAPS) is a high-performance alkaline polymer electrolyte that has been applied in alkaline polymer electrolyte fuel cells (APEFCs). For a long time, N, N-dimethyl formamide (DMF) has been considered the best solvent to dissolve aQAPS, but the high boiling point of DMF makes it hard to remove from the electrodes, which potentially poisons the electrocatalysts. Our recent experiments have shown that although aQAPS is unable to dissolve in ethanol, n-propanol, or water, it can dissolve in the mixture of these alcohols and water. This peculiar dissolution behavior significantly facilitates the fabrication of the membrane electrode assembly (MEA) for APEFCs, even though it has not been understood. In this work, atomistic molecular dynamics (MD) simulations were employed to study the dissolution behavior of aQAPS in different solvents, including water, methanol, ethanol, n-propanol, DMF, and the mixture of these non-aqueous solvents and water. The conformation of the aQAPS chain in pure solvents agreed well with the dissolution behavior observed in the experiments, even though in the water-containing mixed solvents, the aQAPS chain tended to be in a more contracted state. The simulations further revealed that the water component in the mixed solvents played dual roles. On one hand, the hydrocarbon chain of aQAPS was compressed to a contracted state upon the addition of water, because of the hydrophobic effect. On the other hand, water can drive the dissociation of the counterion (Cl– ), which led to an enhancement in the solute-solvent interaction energy and thus facilitated the dissolution of aQAPS. In most mixed solvents, the compensation of these two interactions resulted in a general increase in the total solute-solvent interaction energy; therefore, the addition of water was energetically favorable for the dissolution of aQAPS. This study not only furthers our fundamental understanding of the dissolution behavior of polyelectrolytes but also is technologically significant for the development of better APEFCs.
2019, 35(4): 394-400
doi: 10.3866/PKU.WHXB201805091
Abstract:
Non-fullerene electron acceptors have attracted enormous attention of the research community owing to their advantages of optoelectronic and chemical tunabilities for promoting high-performance polymer solar cells (PSCs). Among them, fused-ring electron acceptors (FREAs) are the most popular ones with the good structural planarity and rigidity, which successfully boost the power conversion efficiencies (PCEs) of PSCs to over 14%. In considering the cost-control of future scale-up applications, it is also worthwhile to explore novel structures that are easy to synthesize and still maintain the advantages of FREAs. In this work, we design and synthesize a new electron acceptor with an unfused backbone, 5, 5'-((2, 5-bis((2-hexyldecyl)oxy)-1, 4-phenylene)bis(thiophene-2-yl))bis(methanylylidene)) bis(3-oxo-2, 3-dihydro-1H-indene-2, 1-diylidene))dimal-ononitrile (ICTP), which contains two thiophenes and one alkoxy benzene as the core and 2-(3-oxo-2, 3-dihydroinden-1-ylidene) malononitrile (IC) as the terminal groups. The synthetic route to ICTP involves only three steps, with high yields. Density functional theory calculations indicate that the non-covalent interactions, O…H and O…S, help reinforce the space conformation between the central core and the terminals. ICTP shows broad and strong absorption in the long-wavelength range between 500 and 760 nm. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of ICTP were measured to be -5.56 and -3.84 eV by cyclic voltammetry. The suitable absorption and energy levels make ICTP a good acceptor candidate for medium bandgap polymer donors. The best devices based on PBDB-T:ICTP showed a PCE of 4.43%, with an open circuit voltage (VOC) of 0.97 V, a short circuit current density (JSC) of 8.29 mA∙cm-2, and a fill factor (FF) of 0.55, after adding 1% 1, 8-diiodooctane (DIO) as the solvent additive. Atomic force microscopy revealed that DIO could ameliorate the strong aggregation in the blended film and lead to a smoother film surface. The hole and electron mobilities of the optimized device were measured to be 9.64 and 2.03 × 10-5 cm2∙V-1∙s-1, respectively, by the space-charge-limited current method. The relatively low mobilities might be responsible for the moderate PCE. Further studies can be performed to enlarge the conjugation length by including more aromatic rings. This study provides a simple strategy to design non-fullerene acceptors and a valuable reference for the future development of PSCs. ![]()
Non-fullerene electron acceptors have attracted enormous attention of the research community owing to their advantages of optoelectronic and chemical tunabilities for promoting high-performance polymer solar cells (PSCs). Among them, fused-ring electron acceptors (FREAs) are the most popular ones with the good structural planarity and rigidity, which successfully boost the power conversion efficiencies (PCEs) of PSCs to over 14%. In considering the cost-control of future scale-up applications, it is also worthwhile to explore novel structures that are easy to synthesize and still maintain the advantages of FREAs. In this work, we design and synthesize a new electron acceptor with an unfused backbone, 5, 5'-((2, 5-bis((2-hexyldecyl)oxy)-1, 4-phenylene)bis(thiophene-2-yl))bis(methanylylidene)) bis(3-oxo-2, 3-dihydro-1H-indene-2, 1-diylidene))dimal-ononitrile (ICTP), which contains two thiophenes and one alkoxy benzene as the core and 2-(3-oxo-2, 3-dihydroinden-1-ylidene) malononitrile (IC) as the terminal groups. The synthetic route to ICTP involves only three steps, with high yields. Density functional theory calculations indicate that the non-covalent interactions, O…H and O…S, help reinforce the space conformation between the central core and the terminals. ICTP shows broad and strong absorption in the long-wavelength range between 500 and 760 nm. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of ICTP were measured to be -5.56 and -3.84 eV by cyclic voltammetry. The suitable absorption and energy levels make ICTP a good acceptor candidate for medium bandgap polymer donors. The best devices based on PBDB-T:ICTP showed a PCE of 4.43%, with an open circuit voltage (VOC) of 0.97 V, a short circuit current density (JSC) of 8.29 mA∙cm-2, and a fill factor (FF) of 0.55, after adding 1% 1, 8-diiodooctane (DIO) as the solvent additive. Atomic force microscopy revealed that DIO could ameliorate the strong aggregation in the blended film and lead to a smoother film surface. The hole and electron mobilities of the optimized device were measured to be 9.64 and 2.03 × 10-5 cm2∙V-1∙s-1, respectively, by the space-charge-limited current method. The relatively low mobilities might be responsible for the moderate PCE. Further studies can be performed to enlarge the conjugation length by including more aromatic rings. This study provides a simple strategy to design non-fullerene acceptors and a valuable reference for the future development of PSCs.
2019, 35(4): 401-407
doi: 10.3866/PKU.WHXB201803131
Abstract:
Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention owing to their high absorption coefficient and ambipolar charge transport properties. With only several years of development, the power conversion efficiency (PCE) has increased from 3.8% to 22.7%. In general, PSCs have two types of structural architecture: mesoporous and planar. The latter possesses higher potential for commercialization due to its simpler structure and fabrication process, especially the inverted planar structure, which possesses negligible hysteresis. In an inverted PSC, the electron transport materials (ETM) are deposited on a perovskite film. Only a few ETMs can be used for inverted PSCs as the perovskite film is easily damaged by the solvent used to dissolve the ETM. Furthermore, the energy levels of the ETM should be well aligned with that of the perovskites. Normally it is difficult to use inorganic ETMs as they require high temperatures for the annealing process to improve the electron conductivity; the perovskite film cannot sustain these high temperatures. To date, the fullerene derivative, [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM), is the most commonly used organic ETM for high efficiency inverted planar PSCs. However, the high manufacturing cost due to its complex synthesis retards the industrialization of the PSCs. Here, we introduce a fullerene pyrrolidine derivative, N-methyl-2-pentyl-[60]fullerene pyrrolidine (NMPFP), synthesized via the Prato reaction of C60 directly with cheap hexanal and sarcosine. Then the NMPFP electron transport layer (ETL) was prepared by a simple solution process. The properties of the resulting NMPFP ETLs were characterized using UV-Vis absorption spectroscopy, cyclic voltammetry measurements, atomic force microscopy, and conductivity test. From the results of the UV-Vis absorption spectroscopy and cyclic voltammetry measurements, the LUMO level of NMPFP ETL was calculated to be 0.2 eV higher than that of the PCBM ETL. This contributes to a higher open-circuit photovoltage. In addition, the NMPFP film presented higher conductivity than the PCBM film. Thus, the photo-generated charge carriers in the perovskite films should be transported more efficiently to the NMPFP electron transport layer (ETL) than to the PCBM ETL. This was confirmed by the results of the steady-state photoluminescence spectroscopy. Finally, the NMPFP as an alternative low-cost ETL was employed in an inverted planar PSC to evaluate the device performance. The device made with the NMPFP ETL yielded an efficiency of 13.83% with negligible hysteresis, which is comparable to the PCBM counterpart devices. Moreover, since stability is another important parameter retarding the commercialization of PSCs, the stability of the PCBM and NMPFP base PSCs were investigated and compared. It was found that the NMPFP devices possessed significantly improved stability due to the higher hydrophobicity of the NMPFP. In conclusion, this research demonstrates that NMPFP is a promising ETL to replace PCBM for the industrialization of cheap, efficient and stable inverted planar PSCs. ![]()
Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted significant attention owing to their high absorption coefficient and ambipolar charge transport properties. With only several years of development, the power conversion efficiency (PCE) has increased from 3.8% to 22.7%. In general, PSCs have two types of structural architecture: mesoporous and planar. The latter possesses higher potential for commercialization due to its simpler structure and fabrication process, especially the inverted planar structure, which possesses negligible hysteresis. In an inverted PSC, the electron transport materials (ETM) are deposited on a perovskite film. Only a few ETMs can be used for inverted PSCs as the perovskite film is easily damaged by the solvent used to dissolve the ETM. Furthermore, the energy levels of the ETM should be well aligned with that of the perovskites. Normally it is difficult to use inorganic ETMs as they require high temperatures for the annealing process to improve the electron conductivity; the perovskite film cannot sustain these high temperatures. To date, the fullerene derivative, [6, 6]-phenyl-C61-butyric acid methyl ester (PCBM), is the most commonly used organic ETM for high efficiency inverted planar PSCs. However, the high manufacturing cost due to its complex synthesis retards the industrialization of the PSCs. Here, we introduce a fullerene pyrrolidine derivative, N-methyl-2-pentyl-[60]fullerene pyrrolidine (NMPFP), synthesized via the Prato reaction of C60 directly with cheap hexanal and sarcosine. Then the NMPFP electron transport layer (ETL) was prepared by a simple solution process. The properties of the resulting NMPFP ETLs were characterized using UV-Vis absorption spectroscopy, cyclic voltammetry measurements, atomic force microscopy, and conductivity test. From the results of the UV-Vis absorption spectroscopy and cyclic voltammetry measurements, the LUMO level of NMPFP ETL was calculated to be 0.2 eV higher than that of the PCBM ETL. This contributes to a higher open-circuit photovoltage. In addition, the NMPFP film presented higher conductivity than the PCBM film. Thus, the photo-generated charge carriers in the perovskite films should be transported more efficiently to the NMPFP electron transport layer (ETL) than to the PCBM ETL. This was confirmed by the results of the steady-state photoluminescence spectroscopy. Finally, the NMPFP as an alternative low-cost ETL was employed in an inverted planar PSC to evaluate the device performance. The device made with the NMPFP ETL yielded an efficiency of 13.83% with negligible hysteresis, which is comparable to the PCBM counterpart devices. Moreover, since stability is another important parameter retarding the commercialization of PSCs, the stability of the PCBM and NMPFP base PSCs were investigated and compared. It was found that the NMPFP devices possessed significantly improved stability due to the higher hydrophobicity of the NMPFP. In conclusion, this research demonstrates that NMPFP is a promising ETL to replace PCBM for the industrialization of cheap, efficient and stable inverted planar PSCs.
