## 4 Conclusions and Future Outlook  
Energy harvesting technology from the unlimited source of solar radiation has been considered as a sustainable solution for the global energy challenge. In that respect, the solar-driven water splitting technology has greatly progressed so far, since hydrogen as a product of water splitting reaction is expected to support future societies. Here, we reviewed the enhancement strategies, and recent progress on the device designs toward the efficient and practical PEC solar water splitting. In decades of research, many researchers have endeavored to discover small band gap semiconductors, and this has resulted in the development of BiVO\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\) etc. Introduction of those semiconductors into the photoelectrode have brought the utilization of visible light, which results in increased photocatalytic efficiency. However, there are still problems remaining, such as the poor charge separation in the bulk and the slow oxidation kinetics. Recently, various efforts are being investigated, including the strategies of fabricating heterostructures, loading plasmonic NPs, gradient-doping of foreign elements, and synthesizing OECs on the surface. Due to the above-mentioned efforts, more efficient charge separation efficiency and faster kinetics of oxidation have been achieved, and this enables a deep understanding of PEC water splitting. Moreover, the application of plasmonic NPs into PEC water splitting has made it possible to develop visible-light-responsive photoelectrodes consisting of large band gap oxides. Based on these achievements, it is expected that further improvements of the plasmonic photocatalysts will be available by designing plasmonic NPs with various shapes, structures, and compositions. Most recently, in order to achieve further improvement, very interesting and challenging attempts are being explored. One is so-called facet engineering which requires delicate material growth techniques to expose certain crystal facets on the surface of photoelectrodes. By DFT calculations, it is known that the intrinsic carrier transport properties and the light absorption abilities are different depending on the crystallographic orientations of semiconductors. Therefore, a PEC device with preferentially oriented crystal facets on its surface is crucial for achieving optimization between the carrier transport, the light absorption efficiency, and the water oxidation kinetics. The other one is a ferroelectric coupled PEC device. To date, the positive effect of the built-in potential induced by the spontaneous polarization of the ferroelectrics on charge separation has been reported. However, this state-of-the-art PEC device design is still in its infancy. Although the fabrication of ferroelectrics requires the elaborate deposition process, well known facts on ferroelectrics, such as the different polarization values depending on the different crystallographic orientations, imply that there are numerous of undiscovered fascinating strategies. The various enhancement strategies and the device designs are summarized in this review. We believe that the practical PEC water splitting can be achieved by combining two or more introduced strategies and designs. Especially, the combination of the OECs and the facet engineering or the ferroelectric coupled PEC devices seem more promising and fascinating. In addition, the cost-effectiveness and the long-term stability should be considered for the commercialization of PEC water splitting technology. In that respect, the use of inexpensive materials, for example, semiconductors including BiVO\({}_{4}\), BiFeO\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\), and OECs of NiOOH, FeOOH, CoOOH etc. should be taken into account for future development. [PERSON] supervised the overall project and [PERSON], [PERSON] and [PERSON] co-wrote the manuscript. This research was funded by [Ministry of Education-Korea] grant number [NRF- 2016R1D1A1B03931748] and [Ministry of Science and ICT-Korea] grant number [NRF- 2017M3D1A1040828].