Date Published: January 08, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Hao‐Lin Wu, Xu‐Bing Li, Chen‐Ho Tung, Li‐Zhu Wu.
The increasing demand for sustainable and environmentally benign energy has stimulated intense research to establish highly efficient photo‐electrochemical (PEC) cells for direct solar‐to‐fuel conversion via water splitting. Light absorption, as the initial step of the catalytic process, is regarded as the foundation of establishing highly efficient PEC systems. To make full use of visible light, sensitization on photoelectrodes using either molecular dyes or semiconducting quantum dots provides a promising method. In this field, however, there remain many fundamental issues to be solved, which need in‐depth study. Here, fundamental knowledge of PEC systems is introduced to enable readers a better understanding of this field. Then, the development history and current state in both molecular dye‐ and quantum dot‐sensitized photocathodes for PEC water splitting are discussed. A systematical comparison between the two systems has been made. Special emphasis is placed on the research of quantum dot‐sensitized photocathodes, which have shown superiority in both efficiency and durability towards PEC water splitting at the present stage. Finally, the opportunities and challenges in the future for sensitized PEC water‐splitting systems are proposed.
Concerning the rapid consumption of fossil fuels and the emergence of consequent environmental problems, it is urgent for mankind to explore low‐cost, sustainable, and environmentally friendly energy sources.1, 2 Solar energy, one of the inexhaustible natural resources, has great potential to replace traditional energy forms of fossil fuels for its high‐power delivery. Theoretically, solar energy reaching the surface of Earth in 1 h is comparable to the whole energy consumption of the whole world in one year.3, 4 However, the intermittent property of solar irradiation makes solar energy difficult to utilize directly. To avoid this drawback, enormous efforts have been paid to convert solar energy to other kinds of usable energy forms, such as thermal energy, electric power, and chemical fuel, to acquire persistent and steady power supply.5, 6, 7, 8, 9, 10
Similar to photocatalytic water splitting process, PEC water‐splitting process can also be divided into three critical steps: (i) solar light harvesting; (ii) charge separation and charge diffusion; (iii) surface reduction/oxidation reaction. Considering the composition of solar spectrum, nearly half of the solar energy incident on the Earth’s surface lies in visible‐light region (400 nm < λ < 800 nm). The ultraviolet light region, however, accounts for only 4% of the full spectrum.101, 102 So, it is very practically important to take full advantage of the visible light to improve the overall efficiency of PEC systems. Nevertheless, there always exists a dilemma between the light response scope and electrochemical reaction dynamics. It means that almost no semiconductor can own both adequately narrow bandgaps for exceptional light response and appropriate energy levels for catalytic reactions simultaneously, thus leading to either poor absorption performance or additional undesirable external bias. Therefore, the first step of photon absorption can be a big limiting factor, which is regarded as the foundation of establishing high‐efficient PEC systems.103, 104, 105, 106 PEC water splitting offers a great potential to provide sustainable and clean energy and has received great attentions in the past years. The above‐mentioned results demonstrate that sensitized‐photocathodes, either molecular dye‐ or quantum dot‐sensitized photocathodes, have showed extremely important application prospects for solar‐to‐fuel conversion via water splitting. With the aid of cocatalysts, the exploration of electrode construction and the modification of photosensitizers, both the efficiency and stability of PEC water‐splitting systems have been greatly enhanced. However, as an emerging research field with confused understanding of fundamental catalytic principles, and the lack of suitable electrode materials, sensitized photocathodes can only run in laboratory scale for PEC water splitting. We also have to admit that both molecular dyes and quantum dot working as photosensitizers have their own drawbacks. In terms of molecular dyes, poor light stability and instability on electrode surface are major disadvantages that limit their rapid development in dye‐sensitized photocathodes. Presently speaking, compared to molecular dyes counterparts, PEC systems based on QDs‐sensitized photocathodes have showed much better performance in H2 evolution, because of their inherent advantages in visible light response, exciton generation, charge separation, etc. However, the types of quantum dots used in sensitized photocathodes are very limited, and the loading amount of quantum dots on the electrode surface is low. Therefore, there is a long way to realize the practical application of sensitized photocathodes. And from our point of view, further efforts to improve the performance of such systems from the following aspects can be made.i) Design and synthesis of light absorbers with exceptional solar‐light response. As the initial step, light absorption, especially the absorption of visible light, can greatly influence the photo‐electrocatalytic performance because light harvesting determines the theoretical uppermost efficiency for water splitting. Therefore, further exploitation of new and highly efficient visible‐light responsive materials should be made. As far as molecular dye photosensitizer is concerned, it is very important to design and develop molecular dyes with excellent visible‐light response. Moreover, in the design of molecular dyes, how to improve their stability and how to firmly anchor them onto the electrode surface are also of great significance. For the realm of colloidal quantum dots, it is very urgent to exploit new kinds of quantum dots with excellent visible‐light response, excellent photo stability, and low toxicity.ii) Exploration of materials with excellent hole transfer properties. As a counterpart of photogenerated electrons, the capture and migration of photogenerated holes often determine the energy conversion efficiency of PEC system for its large barrier of migration. Up to date, less attention has been paid to this aspect. We think that in the future more and more attention should be paid to exploring new kinds of hole transfer materials for photocathode construction. At present, as the most frequently studied p‐type semiconductor material, NiO has a series of unique advantages as mentioned above. However, its poor hole transfer and unfavorable interfacial electron‐transfer kinetics affect the further improvement of PEC performance. Therefore, it is necessary to optimize the structure and composition of NiO semiconductor. In addition, although some other p‐type semiconductor materials, such as p‐Si and Cu2O, have also been developed, each of them has its own limitations such as photocorrosion or scarcity. So, developing new materials with excellent hole transfer capability, chemical stability and photostability, and also low cost are of great importance. In recent years, carbon materials have shown great potential applications in this field. Their well electrical conductivity, good carrier transport and separation capability, stability, and low cost have attracted wide attentions and researches.iii) Development of highly efficient proton reduction cocatalysts and coupling with absorption units. In general, the introduction of the cocatalyst can significantly reduce the reaction active energy. So, developing more efficient and stable cocatalysts, especially non‐noble metal based cocatalysts, is also one of the key points of future research for photocathode construction. In addition, it is also one of the key points of future research to combine the cocatalyst and photosensitizers through reasonable design. The rational coupling between cocatalyst and photosensitizers can favor the capture of photoelectrons and the process of proton reduction. For dye sensitized photocathodes, the established coupling methods include covalent bond interaction, coordination bond interaction, ion connection, etc. The systems constructed by covalent bond interaction often show higher stability, while the building‐up process is usually complex. The other two methods, coordination bond interaction and ion connection, are easier to operate and more attentions should focus on how to further improve their stability and efficiency. As for quantum dots, because of their extremely small sizes, it is challenging to control preparation of photosensitizer‐catalyst coupling systems. Currently, few attentions have been paid to this aspect in the field of quantum dot sensitized photocathode. How to controllably synthesize QDs‐cocatalyst coupling systems and to efficiently anchor them onto electrode surface maybe one of the future research directions in sensitized PEC systems.iv) Exploration of new methods for system construction. As a multi‐interface catalytic process, the catalytic efficiency of PEC water‐splitting system is affected by various interfaces. Therefore, it is very important to understand the kinetics of charges migration at the interface and to simplify the catalytic process of electrode interface, which can greatly promote the transfer and utilization of photogenerated electrons and holes. More importantly, a balance between the reduction and oxidation reactions should be achieved to avoid charge accumulation in the electrode, thus the stability of the PEC system can be improved. The authors declare no conflict of interest. Source: http://doi.org/10.1002/advs.201700684