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Photochemical Conversion and Storage of Solar Energy
ACS Energy Letters ( IF 19.3 ) Pub Date : 2019-01-08 00:00:00 , DOI: 10.1021/acsenergylett.8b02411 Xin Xia 1 , Jia Hong Pan 1 , Xu Pan 2 , Linhua Hu 2 , Jianxi Yao 1 , Yong Ding 1 , Defa Wang 3 , Jinhua Ye 3, 4 , Songyuan Dai 1
ACS Energy Letters ( IF 19.3 ) Pub Date : 2019-01-08 00:00:00 , DOI: 10.1021/acsenergylett.8b02411 Xin Xia 1 , Jia Hong Pan 1 , Xu Pan 2 , Linhua Hu 2 , Jianxi Yao 1 , Yong Ding 1 , Defa Wang 3 , Jinhua Ye 3, 4 , Songyuan Dai 1
Affiliation
The 22nd International Conference on Photochemical Conversion and Storage of Solar Energy (IPS-22) was held in Hefei, China, July 29–August 2, 2018. “Every two years, after the Olympic Games or after the FIFA World Cup, IPS is held,” said Detlef W. Bahnemann (Leibniz University of Hannover, Germany), Chairman of the 20th and 21st IPS. This humorous statement about the meeting time also demonstrates that the 44 year long tradition of the IPS conference series has been once again continued. According to the recall of Prashant V. Kamat (University of Notre Dame, U.S.A.), IPS-0 was initiated as a workshop on “The Current State of Knowledge of Photochemical Formation of a Fuel” in 1974 and chaired by Norman N. Lichtin (Boston University, U.S.A.).(1,2) To date, the IPS series has been one of the most important conferences in the fields of photochemistry and solar energy utilization, offering a brilliant platform to discuss the latest advances in renewable energy research and to enhance regional and international collaborations. The IPS torch was passed to China again 26 years later after the ninth IPS conference in Beijing,(2) 1992, chaired by Songyuan Dai (North China Electric Power University, China) and Jinhua Ye (Tianjin University, China and National Institute for Materials Science (NIMS), Japan). Figure 1 is a group photo of the representatives of local organizers and international organizing committee at the conference opening ceremony. IPS-22 attracted over 800 delegates, 14 plenary talks, 35 keynote talks, 105 invited and oral presentations, and 233 poster presentations. A broad range of topics was covered and organized into the following sessions:
Solar water splitting and CO2 conversion Environmental and synthetic photocatalysis Photoelectrochemical conversion and devices Molecular and biomimetic photosynthesis Photoinduced charge carrier transfer, mechanism, and modeling Hybrid organic–inorganic perovskite solar cells Dye-sensitized solar cells/organic solar cells Semiconductor quantum dots and perovskite materials for luminescence Novel photovoltaic materials and devices Energy storage materials, devices, and applications Figure 1. Local organizer and international organizing committee members at the conference opening ceremony, IPS-22 meeting in Hefei, China. (Photo courtesy of Xin Xia.) Sessions 1–5 focused on photo(electro)catalysis, while topics 6–9 were mainly about photoelectrochemical devices, typically novel thin-film solar cells. Regarding session 10, it is the first time that IPS listed it as an individual topic, in view of particular importance of solar energy storage. Perovskite solar cells (PSCs, session 6) were the dominant topic of IPS-22 due to their rapid rise as a promising future photovoltaic technology. Besides, research on semiconductor photocatalysis for solar fuel synthesis and environmental applications (sessions 1 and 2) retained strong interest. Figure 2 gives a snapshot of a plenary talk at IPS-22. Figure 2. Prof. Michael Grätzel delivering the Plenary Lecture at the IPS-22 meeting, Hefei, China. (Photo courtesy of Xin Xia.) The first plenary lecture given by Michael Grätzel (École Polytechnique Fédérale de Lausanne (EPFL), Switzerland) provided an overview of the history of semiconductor photoelectrochemistry since the 1970s when artificial photosynthesis was in its infancy. Later in 1991, following the progressive development of nanotechnology and synthetic dyes with broad spectral response, strategical coupling of mesoscopic TiO2 films with Ru dyes led to his great invention of the dye-sensitized solar cell (DSC), and the engineering installation of colorful semitransparent DSC panels in the SwissTech Convention Center on the EPFL campus was shown. Then the recent advances in PSC materials and devices were addressed in detail. The design concept was extended to the field of solar fuel synthesis, and high-efficiency photoelectrochemical cells for H2 generation and CO2 reduction were highlighted. The power conversion efficiency (PCE) of PSC increased sharply from 3.8% in 2009 to 23.3% in 2018, which is now comparable to that of crystalline silicon solar cells, while stability remains an issue. From another angle, the presentation by Prashant V. Kamat ranged from semiconductor quantum dot (QD) solar cells to hybrid and all-inorganic (CsPbBr3) PSCs. The light-induced charge carrier generation and transport across interfaces and the influences of the qualities of films and interfaces on cell stability were discussed. In addition, perovskite light-emitting diodes (LEDs) with strong and narrow emission were addressed for promising industrial applications. Hybrid Organic–Inorganic Perovskite Solar Cells. In this highlighted session of IPS-22, the golden triangle, namely, efficiency, cost, and stability, of hybrid pervoskite solar cells was comprehensively discussed. Efficiency was the hottest subject, and a world record of 23.3% has been reached. The cell efficiency value can be independently certified by different bodies. However, the definition of stability has not been standardized. Various different measurement conditions have been reported. Ultimately, survival of the device at a high temperature in sunlight is the only relevant condition, and Michael Grätzel set that hurdle as 1000 h at 60 °C, the practical temperature of a solar cell in use. Sang-Il Seok shared his experiences in high-efficiency PSC fabrication. During the past years, he has set several world record performances. Both Nam-Gyu Park and Liyuan Han presented their fabrication techniques for large-area perovskite films and modules with low defect density. A large-area, stable, and hysteresis-free PSC has been produced by Park through the combination of a Lewis acid–base adduct, interfacial engineering, and coating techniques. Han reported the development of a new graded heterojunction structure of PSCs using formamidinum perovskite (PCE of 19.2%) and fabricated large-area perovskite films via a soft-cover deposition method. The research of Yi-Bing Chen has been focused on chemical modification and charge recombination of the interface between the perovskite layer and the hole transport material (HTM) layer. It was suggested that the band alignment at the perovskite–HTM interface can be tuned to improve the solar cell efficiency via adsorption of para-substituted benzenethiol molecules on the surface of the perovskite layer. A PCE of 20.2% can be achieved through this method, and more importantly, benzenethiol modification resulted in a significant improvement in stability of PSCs. A state of art PSC with a certified PCE of 22.7% was achieved through manipulation of the defects at grain boundaries and engineering of the interconnecting heterojunction, as presented by Yang Yang. In consideration of thermal stability and output voltage, Tsutomu (Tom) Miyasaka proposed that metal oxide electron transport layers (ETLs) could significantly benefit the thermal stability and elevate the output voltage of the PSC. TiO2 ETL-based multication perovskite cells yield PCEs over 21% and Voc values above 1.15 V and in ambient air exhibit thermal stability in a wide temperature range (−80 to 100 °C). Such performance enables perovskite devices to work as a power source of high-output voltage even under weak light. Joseph Luther produced cubic-phase CsPbI3 QDs with a high open-circuit voltage of >1.2 V and PCE over 13%, which has potential application in both photovoltaics and LEDs. As well as competing on the PCE and thermal stability, removing the lead is another important issue that may affect the acceptance of PSCs. Shuzi Hayase proposed halogenated SnPb and SnGe PSCs to reduce the use of Pb. The lead-free SnGe mixed metal PSC shows good stability in ambient atmosphere and yields a PCE of 7.9%, while the SnPb PSC offers an energy efficiency of 18.9%. Eric Wei-Guang Diau and his group investigated the role of ethylenediammonium diiodide (EDI2) additives in FASnI3 perovskite. They concluded that the presence of EDI2 can serve to control the film morphology, minimize the Sn2+/Sn4+ oxidation, and reduce the defect states at the surface for better charge separation. Several speakers discussed the degradation of PSC devices in more detail under the extrinsic conditions of high temperature, strong light, oxygen, and moisture and the intrinsic conditions of phase changes, surface chemistry, grain boundaries, twinning, and composition variation. A variable in the design of the cells was the range of structures, p–i–n planar, p–i–n mesoporous, and n–i–p inverted. From an intrinsic viewpoint, the mesoporous designs were seen as less stable due to the higher surface area susceptible to recombination and chemical degradation, while there were a range of inert capping concepts and materials suggested to protect the PSC such as perylene. However, it was recognized that the classical MAPbI3 and FAPbI3 formulations may be inherently unstable as a result of the breakdown of the molecules when subject to attack by moisture, a likely event over long-term exposure to the atmosphere. This breakdown explained why additional PbI2 can enhance or partially stabilize efficiency as the formation of this compound is the first step in humidity-based degradation. The second degradation step of formation of HI from the MAI leads to creation of iodine that is mobile in the structure. Hence, there were some presenters who effectively proposed that every component of the most well-known formulation, MAPbI3, is unstable. This was the reasoning behind the significant number of papers dealing with solid-state devices. Here the Goldschmidt tolerance factor was the subject of discussion along with recognition that the cubic phase of the perovskite is the preferred phase for high stability. Meanwhile, Wanjian Yin proposed an additional criterion, the octahedral factor, the relationship between the radii of the B and X ions. Songyuan Dai systematically studied the mechanism of the instability of the APbI3 compounds and designed and prepared new mixed-dimensional [(NH4)2.4(FA)n−1PbnI3n+1.4]0.85(MAPbBr3)0.15 hybrid perovskites and used these as an absorber in solar cells. Notably, when the unsealed mixed-dimensional perovskite device was stored under humidity over 90% for 24 days, the PCE showed no apparent decrease, demonstrating an excellent long-term stability under this test condition. With the long-term stability and high-efficiency properties, the mixed-dimensional perovskites might replace the traditional perovskite materials and show good commercialization prospects. Returning to performance enhancement, those who had a previous history in DSC could recognize some well-known chemical techniques to improve both efficiency and stability such as tBP and guanadimium. Those who based their designs on the tolerance factor had found that addition of K+ to the A site helped to stabilize and enhance the performance. A number of other cation modifications were reported, particularly by those who had based the perovskite on tin as an alternative to the more toxic lead, in which case one of the challenges to be addressed is to stop the oxidation of the divalent tin. From the wide selection of component ions presented, there are still many openings for perovskite compositions, including modifying the X component by partially or fully removing the iodide, with bromide being currently the preferred alternative, while performance suffers. Of course, the PSC is not simply the perovskite layer, and each other element, electron conductor, ETL, hole transport layer (HTL), and hole conductor (HTM) attracted the attention of speakers. Enhancement of the interface connection to the ETL involved a distinctly broad range of additives, coatings, and dopants, with no particular winner in this race yet, though the argument for polystyrene was persuasive. The HTM has attracted a lot of attention, with the recognition that the classical spiro from the organic photovoltaic (OPV) era is both too expensive and unstable. Both organic and inorganic hole conductors were demonstrated, some going back to the early days of solid-state DSCs at EPFL, with the likely winners coming from inorganic candidates such as CuSCN and LiCoO2 and high-stability organics such as the phthalocyanines, maybe compounded with nickel. However, the door is still wide open for new candidates. These alternatives also eliminate the need to protect the Au conductor from metal ion migration through the HTM. However, more appropriate treatment may be to eliminate gold as the conductor and replace it with a cheap and stable alternative such as either Cu or carbon, even though the initial “hero” efficiency may suffer. The promoters of high stability for outdoor long life contend that stable PSCs at 20% will compete effectively in the electricity market. At this conference, there was not much attention on low light advantages of PSCs that make it such a good candidate for BIPV and any city condition for which the conference major sponsor SunGrow is developing systems. Several talks addressed manufacturing, including the recognition that spin coating with only about 1% material utilization and non/continuous deposition must be replaced by blade or slot die printing and that the coating solutions must be resistant to atmospheric conditions. One approach is to utilize solvents such as acetonitrile that have a high vapor pressure to displace water. Antisolvents also attracted attention as did soft-cover technology in both cases to produce smooth perovskite films of controlled thickness. However, it seems that the current technology status depends on capping of the perovskite with a hydrophobic organic material or with a thin layer of an inorganic such as lead sulfate. In statistical summary, the number of papers currently published on PSCs is more than 9000 and still growing rapidly. It seems that AI will become a necessary tool to analyze the relationships between the vast array of materials, designs, and processes. Photocatalysis, Photoelectrochemical Conversion, and Devices were extensively discussed in sessions 1–5. Six plenary presentations were delivered by Michael R. Hoffman, Detlef W. Bahnemann, Wonyong Choi, Licheng Sun, James R. Durrant, and Bunsho Ohtani, respectively. Michael R. Hoffman’s lecture emphasized the use of atomic layer deposition (ALD) in precisely tuning the surface charge densities of electrocatalysts for efficient oxygen and chlorine evolution reactions (OER and CER). Bahnemann addressed the urgent challenges in photocatalytic organic synthesis. The key issue lies in how to control the interfacial charge transfer process so as to achieve a high selectivity and yield. Especially, a nonselective photo-oxidation process by a hole and derivatives should be well controlled. Choi presented strategies for designing coupled semiconductors with heterojunctions at the interface for efficient photo(electro)catalytic solar fuel synthesis and environmental remediation, with special emphasis on the photoreductive O2 activation for H2O2 production. Sun summarized their research progress in artificial photosynthesis during the past 20 years in Sweden with his colleagues. A range of dye molecules were synthesized for various photochemical conversion systems. Durrant provided illumination on photocatalysis kinetics based on LEDs, rather than lasers. Transient absorption spectroscopy (TAS) has been employed to measure the photogenerated hole lifetime and investigate the kinetic dependences of photocatalysis in TiO2 films of varying morphology. Ohtani developed reversed double-beam photoacoustic spectroscopy as a powerful characterization tool for charge carriers (using TiO2 photocatalysis as the baseline). He captivated the audience through a personal image analogy with Steve Jobs, how the current criteria for defining a photocatalyst is inadequate, concluding that active photocatalysts are all alike but every inactive one is inactive in its own way. At IPS-22, several presentations concentrated on the active photocatalysts and related strategies to photocatalytic activity enhancement. TiO2 and its coupled systems were still the most popular photocatalysts. Fundamental studies mainly focused on crystal and band engineering, surface modification, charge carrier transfer, and the underlying photocatalytic reaction mechanism. Their applications in photocatalytic solar fuel production, CO2 reduction, and environmental purification were comprehensively discussed, and novel characterization techniques have been developed to improve the deep understanding of the relationship between surface properties and the photocatalytic mechanism of nanosized TiO2. Polymeric g-C3N4 was another hot photocatalyst owing to its metal-free nature, nontoxicity, processability, and stability. Photocatalytic Cr(VI) reduction, organic degradation, CO2 reduction, H2 generation, and bacterial inactivation were addressed by several scientists. Bi-containing (e.g., BiVO4, Bi2O3, Bi2Sn2O7, BiOBr, and BiOCl) photocatalysts attracted attention. It may be easy to see that using single-component photocatalysts is insufficient to achieve high photocatalytic activity. Several novel coupling systems have been developed to suppress the large quantity of charge carrier recombination widely occurring in single semiconductor systems. Various narrow-band semiconductors, plasmonic metal nanoclusters, surface grafting, Ru-complex sensitizers, cobalt sulfide, and phosphosulfide were developed to extend the light absorption spectrum and to improve the charge carrier separation. Typically, when two different band structure semiconductors are coupled, a heterojunction, p–n homojunction, and Z-scheme photocatalyst can be built up. The last mentioned presents a new class of photocatalyst in which charge carriers separate through recombination between electrons in the lower conduction band and holes in the higher valence band, and these were heatedly discussed by Akihiko Kudo, Jiaguo Yu, Hiroshi Irie, Ji-Jun Zou, Xiwang Zhang, Qiuye Li, and Zaicheng Sun. However, the methodology of the underlying interfacial charge carrier transfer was still ambiguous. Moreover, 2D nanomaterials, such as the graphene family and MoS2, were used as electron collectors for coupled photocatalysts, efficient water splitting, CO2 reduction, and environmental disinfection by Xiwang Zhang, Jungang Hou, and Xuanhua Li. Potential applications of TiO2 photocatalysis discussed were largely in the environmental area, which included:
更新日期:2019-01-08
1. | Solar water splitting and CO2 conversion | ||||
2. | Environmental and synthetic photocatalysis | ||||
3. | Photoelectrochemical conversion and devices | ||||
4. | Molecular and biomimetic photosynthesis | ||||
5. | Photoinduced charge carrier transfer, mechanism, and modeling | ||||
6. | Hybrid organic–inorganic perovskite solar cells | ||||
7. | Dye-sensitized solar cells/organic solar cells | ||||
8. | Semiconductor quantum dots and perovskite materials for luminescence | ||||
9. | Novel photovoltaic materials and devices | ||||
10. | Energy storage materials, devices, and applications |
- metal inks photodeposited on the surface of various commercial TiO2 films for metal micropatterns and photocatalytic activity assessments by Andrew Mills
- self-cleaning and plasmon-free surface-enhanced Raman scattering (SERS) and substrate fabrication by grafting chiral carbon nanotubes with TiO2 nanocrystals by Jinlong Zhang
- conversion of ethylbenzene to (R)-1-phenylethanol with highly selective activation of C–H bonds by strategic cooperation of photocatalytic-generated H2O2 with enzymes by Jonathan Z. Bloh
- two very interesting applications in the biological field, production of rare sugars and improvement of germination of seeds, both using photocatalysis by Kazuya Nakata.
- Hong Li summarized current developments in energy storage technology at the Institute of Physics, Chinese Academy of Sciences. A long-term effort has been devoted to the commercialization of their solid-state lithium ion battery.
- Guozhong Cao compared the electrochemical properties of Mn-based (MnO, MnS, and MnNCN) electrodes for Li-ion capacitors. MnO mesocrystals as promising anodes offer a high capacity of 637 mAh/g at 100 mA/g and excellent cycling performance against degradation. For MnNCN, the more covalent bonding nature of Mn–N leads to a lower discharge potential in Li-ion batteries. Metastable MnS was demonstrated to be a better choice for Li-ion battery anodes than the stable MnS.
- Shihe Yang presented preparation strategies in interfacing and assembling various nanostructures by solution processes. The resulting architectures have been explored as electrocatalysts and photoelectrochemical electrodes.
- Guoxiu Wang presented their progress in materials nanoarchitecture design of graphene nanosheets, tin-graphene and silicon-graphene nanocomposites for high-rate or high-capacity lithium/sodium/potassium batteries.
- Jana Timm shared their cell and stack design for their newly developed photoelectrocatalytic redox flow batteries by strategically integrating the technologies of TiO2 photoelectrocatalysis and vanadium redox batteries.
- Yanglong Hou delivered a talk to introduce the rational design of carbon nanoarchitectures as cathodes for lithium–sulfur batteries. A high sulfur loading up to 90% was achieved for novel hollow graphene nanoshells, with very stable cycling life.
- Haobin Wu discussed the application of nanostructured materials derived from metal–organic frameworks (MOFs) as electrode materials for batteries and electrocatalysts for generation of fuels, as well as composite electrolytes developed with the assistance of MOFs.
- Weiqing Yang utilized two novel materials of hierarchical tubular carbon nanotubes and 2D Ti3C2Tx as electrodes of a flexible microsupercapacitor.
- Ying Wang showed a porous heterostructured MoO2/Mo2N nanobelt cathode for rechargeable zinc-ion batteries.