Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook Zhaosheng Li, * Wenjun Luo, Minglong Zhang, Jianyong Feng and Zhigang Zou Harnessing solar energy for the production of clean hydrogen fuels by a photoelectrochemical (PEC) cell represents a very attractive but challenging alternative. This review focuses on recent developments of some promising photoelectrode materials, such as BiVO 4 , a-Fe 2 O 3 , TaON, and Ta 3 N 5 for solar hydrogen production. Some strategies have been developed to improve PEC performances of the photoelectrode materials, including: (i) doping for enhancing visible light absorption in the wide bandgap semiconductor or promoting charge transport in the narrow bandgap semiconductor, respectively; (ii) surface treatment for removing segregation phase or surface states; (iii) electrocatalysts for decreasing the overpotentials; (iv) morphology control for enhancing the light absorption and shortening transfer distance of minority carriers; (v) other methods, such as sensitization, passivating layer, and band structure engineering using heterojunction structures, and so on. Photochemical durability of the photoelectrodes is also discussed, since any potential PEC technology must balance efficiency against cost and photochemical durability. Photochemical durability may be amended by optimizing the photoelectrode, electrocatalyst, and electrolyte at the same time. In addition, solar seawater splitting is briefly introduced because it has received attention recently. Finally, trends in research in PEC cells for solar hydrogen production are detailed. Broader context One essential issue facing humanity today is powering the Earth without producing additional CO 2 in the atmosphere. Solar energy can easily provide sufficient power for all of our energy demands if it can be efficiently harvested. Natural photosynthesis can store energy from sunlight in the chemical bonds of carbohydrates. Articial photosynthetic routes, for example, photoelectrochemical solar energy conversion, can use solar energy to split water for hydrogen production, thus attracting more and more interest. However, the solar energy conversion efficiency is very low. This review assesses critically recent progress on some promising photoelectrodes with high potential efficiency, such as BiVO 4 , a-Fe 2 O 3 , TaON, and Ta 3 N 5 , and discusses the approaches to improve their solar energy conversion efficiency and photoelectrochemical durability. 1 Introduction Harvesting sunlight to produce clean hydrogen fuel remains one of the main challenges for solving the energy crisis and ameliorating global warming. 1–3 Photoelectrochemical (PEC) H 2 production, using solar energy to split water, is a promising method for providing clean energy carriers in the future. Since titanium dioxide (TiO 2 ) was reported to exhibit the ability for PEC water splitting, intensive and growing attention has been paid to photoelectrode materials for solar H 2 production, because they make use of the Earth's abundant, long lasting and clean solar energy. 4–8 Early studies on photoelectrode materials for PEC cells have been focused on TiO 2 . However, TiO 2 only absorbs ultraviolet light, which accounts for only 4% of the incoming solar energy on the Earth's surface, owing to its relatively large bandgap of about 3.0–3.2 eV. The PEC properties of TiO 2 photoelectrodes can be improved by tuning the morphology. 9–12 For example, Mor et al. reported that incident photon-to-electron conversion efficiency (IPCE) of a nano-array TiO 2 photoelectrode is up to 90% at 337 nm and 1.0 V vs. reversible hydrogen electrode (RHE). 9 Hydrogen treatment has also been used to upgrade fundamentally the performance of TiO 2 nanowires (i.e. H:TiO 2 ) for PEC water splitting. 13 However, owing to the large bandgap, only the ultraviolet part of the solar irradiation can be absorbed by TiO 2 , and the theoretical solar-to-hydrogen efficiency (STH) of rutile TiO 2 cannot be over 2.2% (1.3% for anatase TiO 2 ) under air mass 1.5 global (AM 1.5 G) illumination. To enhance the STH, it is necessary to extend the light absorption edge of the National Laboratory of Solid Microstructures, College of Engineering and Applied Sciences, Ecomaterials and Renewable Energy Research Center (ERERC), Nanjing University, 22 Hankou Road, Nanjing, 210093, PR China. E-mail: zsli@nju.edu.cn Cite this: Energy Environ. Sci., 2013, 6, 347 Received 21st June 2012 Accepted 1st November 2012 DOI: 10.1039/c2ee22618a www.rsc.org/ees This journal is ª The Royal Society of Chemistry 2013 Energy Environ. Sci., 2013, 6, 347–370 | 347 Energy & Environmental Science REVIEW Downloaded by NANJING UNIVERSITY on 28 January 2013 Published on 07 December 2012 on http://pubs.rsc.org | doi:10.1039/C2EE22618A View Article Online View Journal | View Issue