Hematite-based photoelectrochemical interfaces for solar fuel production

The production of solar fuels, i.e. energy-rich molecules obtained from sunlight-driven processes, represents one of the most pursued strategies to satisfy the increasing global energy demand in a sustainable way. Indeed, solar energy possesses the appealing features of being abundant, inexhaustible and widely geographically distributed, yet intermittent. Thus, a common approach consists in the development of specific devices (e.g. photoelectrochemical cells, PECs) allowing for its storage in the form of chemical energy. In order to produce viable PEC systems, the main focus should be the optimization of the light absorbing component(s), which generally comprise the exploitation of semiconductors (SCs). These materials must be efficient, durable, cheap and energetically suitable to perform the reaction(s) leading to the desired solar fuel(s). Hematite (α-Fe2O3) possesses these features, being composed by earth abundant elements, as well as being capable of harvesting a sizable portion of the solar spectrum. Its valence band maximum, more positive than the redox potential of water oxidation, makes it an interesting candidate for the photoinduced oxygen production via water splitting. In this short review, we will mainly discuss the capitalization on hematite properties for such process, while implementing suitable optimization techniques in its synthesis (namely nanostructuring, doping and surface functionalization). Examples of case studies also from our laboratory will be discussed, in which various hematite-based interfaces are probed using advanced characterization techniques (e.g. electrochemical impedance spectroscopy and transient photocurrent analysis). These studies aimed at gaining insights into the key processes involved in the photocurrent generation, thus contributing to the rational design of future more efficient photoactive interfaces. This is a challenging goal since at present all the reported hematite-based photoanodes display performances that are far below the maximum thermodynamically attainable photocurrent (i.e. 12.6 mA/cm2). Finally, we will report on recent examples of hematite-based PEC systems yielding value-added organic compounds as the photoinduced oxidation products. This latter strategy, even if currently at its infancy, is believed to be a groundbreaking approach towards the production of organics exploiting sunlight energy in a sustainable electrochemical process.