Photocatalysis in solar fuel production

With the increasing concerns on energy and environmental problems caused by the combustion of fossil fuels, solar energy becomes vitally important because it is the only viable renewable energy source that can replace the fossil fuels in vast volume. The research on photocatalysis for solar fuel production, especially H2 production by photocatalytic splitting of water and conversion of CO2 to chemical fuels such as CO, CH3OH, CH4, etc., is regarded as the most ideal way for solar energy conversion, storage and utilization. In view of the current research status and the near future practical applications, particulate semiconductor photocatalyst (PSP), photoelectrochemical cell (PEC) cell and photoelectrochemical cell coupled with photovoltaics (PEC-PV) system are the three most promising solutions for solar fuel production. The best PSP reported so far for water splitting under UV light irradiation is La doped NaTaO3 loaded with Ni/NiOx cocatalyst, reaching quantum efficiency (QE) of 56% [1]. Because UV light takes only less than 5% of the solar spectrum, solar energy conversion by UV-active photocatalyst is rather limited. Since visible light occupies more than 40% in the solar spectrum, development of visible light active photocatalyst is necessary. The best visible light photocatalyst for water splitting is (Ga1-xZnx)(N1-xOx) semiconductor photocatalyst loaded with Rh2-yCryO3 cocatalyst, which can reach QE of 5.6% under visible light irradiation (λ > 420 nm) [2]. Visible light QE of 6.3% has also been reported for a ZrO2/TaON and PtOx/WO3 coupled system with I−/IO3− as the shuttle redox mediator. Though it is still far from the practical application requirement of at least 10% solar energy conversion efficiency (ηSTH), these results already demonstrated that photocatalytic water splitting is a promising solution for solar energy to chemical conversion and storage. In order to achieve high solar energy conversion efficiency, the efficiencies of light absorption, charge separation and surface redox reactions should be harmonically and simultaneously achieved. Among them, efficient charge separation is the vital factor for solving the energetically uphill solar energy conversion reactions. Interface (junction) engineering has been demonstrated to be efficient approach for achieving high charge separation. For example, CaFe2O4/TaON heterojunction photoanode has been demonstrated for efficient photoelectrochemical water oxidation [3]. Formation of surface phase junction between α- and β-phase Ga2O3 can also enhance overall water-splitting activity under visible light irradiation (Fig. 1a) [4]. (a) α/β-Ga2O3 surface junction photocatalyst for overall water splitting under UV light irradiation (Reprinted with permission from [5]. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim), (b) dual cocatalyst approach for the assembly of efficient photocatalytic system and (c) the conceptual diagram showing the crystal based artificial photosynthesis system. Cocatalysts [5] play important roles in the assembly of efficient semiconductor photocatalyst (Fig. 1b). It was found that photogenerated electrons and holes may be spatially separated on the {010} and {110} facets of BiVO4 crystal. Upon selective photodeposition of reduction cocatalysts (Pt, Au, Ag, etc.) on {010} facet and oxidation cocatalysts of transition metal oxides (MnOx, CoOx, NiOx, etc.) on {110} facets, water oxidation activity was increased more than two orders of magnitude higher [6]. The intrinsic nature of charge separation between different facets of BiVO4 together with the synergetic effect of dual cocatalysts plays key role in photocatalytic activity enhancement. This opens up a new avenue for the assembly of semiconductor-crystal-based artificial photosynthesis system by selectively loading of dual cocatalysts on the different facets (Fig. 1c). Alternative approaches other than PSP should be also highly considered, such as PEC and PEC-PV systems. A Ta3N5 photoanode coated with ferrihydrite (Fh) layer on which Co3O4 water oxidation cocatalyst (Co3O4/Fh/Ta3N5) was deposited, could yield a photocurrent of 5.2 mA cm−2 at a potential of 1.23 V versus RHE under AM 1.5G simulated sunlight irradiation. And remarkably, about 94% of the initial activity could be maintained even after 6 h irradiation, which is due to the avoidance of the Ta3N5 corrosion via efficient hole storage and transfer by the Fh layer [7]. A self-biased PEC-PV system consisting of FeOOH/Mo:BiVO4 photoanode and a Pt/p-Si solar-cell-based photocathode showed ηSTH of 2.5% under parallel irradiation [8]. A triple junction amorphous silicon photovoltaic coupled with NiMoZn alloy and cobalt borate cocatalysts can achieve ηSTH of 4.7% for a wired configuration and 2.5% for a wireless configuration when illuminated with 1 sun of air mass 1.5 simulated sunlight [9]. Based on these results, concept of ‘artificial leaf ’ has been proposed. Recently, it has been also reported that a solution processed perovskite tandem solar cell coupled with a NiFe-layered double hydroxide electrocatalyst exhibited a water-splitting photocurrent density of around 10 mA cm–2, corresponding to a ηSTH of 12.3% [10], but the stability of the perovskite is a problem. It can be seen that great progresses in solar fuel production have been achieved in the past decades. However, there is still no promising solution for practical application in terms of efficiency, stability and cost effectiveness. In order to further improve the solar fuel production efficiency, in-depth understanding the nature of the energetically uphill photocatalytic reactions is necessary. We should learn from the mother nature, since the natural photosynthesis system has well-resolved all of the uphill energy conversion problems. And addition to the current researches focusing on material engineering for efficient light harvesting and charge separation, the thermodynamic and kinetic aspects of water activation should be also extensively investigated. Compared to the other approaches of solar energy utilization, such as conversion of solar energy into electricity (photovoltaics) and thermal energy (solar heat), solar energy to chemical energy conversion to produce solar fuels by photocatalysis has some unique advantages in terms of energy storage, transportation and utilization, because the photocatalytically converted chemicals (solar fuels) can be conveniently activated for use. Though current research on this field is still in the stage of laboratory research, it can be envisioned that some breakthroughs may be achieved in the near future with extensive research input. And it is necessary to take the research on solar fuel production in a high priority to secure the national renewable energy development. This work was supported by the National Natural Science Foundation of China (21473189, 21361140346), the National Basic Research Program of China (2014CB239403), and the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM).