Solar assisted water splitting in a PEC is an attractive concept to store solar energy as hydrogen fuel but the effective efficiency of the process is too low for it to be a serious contender for commercialization. The most important component of the PEC to achieve efficient water splitting is a photo active anode that could effectively absorb photons and deliver holes for the oxygen evolution reaction. Hematite has many attributes that make it a good candidate material for a photoanode but it also has some deficiencies. This article reviews the state-of-the-art hematite-based photoanodes, with special emphasis on attempts made by researchers to overcome its drawbacks. The numerous research reports are categorized under distinct strategies such as nanostructuring, elemental doping, surface passivation, cocatalyst application, conducting template incorporation and heterostructures which are possible pathways to improve the performance of hematite. The scientific understandings of the operating mechanisms for each strategy are systematically presented and discussed, and the improvements achieved by different approaches are compared. Some cutting-edge strategies, such as heterojunctions, could be important and hematite based heterostructures are discussed. A developing interest in an emerging material, Fe2TiO5 is also discussed and some of the important benefits of this material are presented. Finally, the importance of scaling up the technology is discussed and attention is drawn to some possible challenges on scaling up. The paper concludes with some future technology directions and prospects for hematite-based photoanodes.

Iron pyrite (FeS2) holds an enormous potential as a low cost and non-toxic photoelectrochemical and energy-harvesting material owing to its interesting optical, electronic, and chemical properties along with elemental abundance. In this Review, low cost and scalable processing techniques to synthesize phase-pure pyrite thin films and nanocubes are described, and the application of this material in various energy-harvesting devices such as dye-sensitized solar cells, photodiodes, and heterojunction solar cells is discussed. A detailed analysis of the electron transport in single-crystal iron pyrite is presented to shed light on its bulk- and surface-conduction properties, which could be useful in designing better pyrite solar cells and could be exploited for novel device architectures. Finally, future prospects and directions are discussed.

The origin of p-type conductivity and the mechanism responsible for low carrier mobility were investigated in pyrite (FeS2) thin films. Temperature-dependent resistivity measurements (10-400 K) were performed on polycrystalline and nanostructured thin films prepared by three different methods: (1) spray pyrolysis, (2) hot-injection synthesized and spin-coated nanocubes, and (3) pulsed laser deposition. The films have a high hole density (1018-1019) cm-3 and low mobility (0.1-4 cm2 V-1 s-1) regardless of the method used for their preparation. The charge transport mechanism is determined to be thermally activated conduction (TAC) at near room temperature, with Mott-type variable-range hopping (VRH) of holes via localized states occurring at lower temperatures. The density functional theory (DFT) predicts that sulfur vacancy induces localized defect states within the band gap and the charge remains localized around the defect. The data indicates that the electronic properties including hopping transport in pyrite thin films can be correlated to sulfur vacancy-related defects. The results provide insights into the electronic properties of pyrite thin films and their implications for charge transport.

Fabrication of catalytically active electrodes by electrodeposition is attractive due to its in situ nature, easy controllability, and large-scale operation capability. Most recently, modifying the electrodes with phosphate ligands through electrodepositing electrode materials has shown promising results in improving the kinetics of some reactions. However, it is unclear how the presence of phosphate anions affects the electrodeposition process and functions in catalyzing reactions. Here, we report a systematic study on electrodeposition of Cu in the presence of phosphate anions. The phosphate anions form a complex with free Cu(ii) cations, competing with the electrodeposition process. The competition between the two processes results in an insufficient supply of free Cu(ii) for electrodeposition, especially at the diffusion layer. This is evidenced by the calculation of free Cu(ii) concentration and the electrodeposition current at identical applied potentials. We also found that the electrodeposition of Cu in the presence of phosphate generates Cu-oxyo/hydroxyo-phosphate species on the deposited copper surface. The modified electrodes with phosphate species exhibit higher selectivity for HCOOH formation (faradaic efficiency ∼80%) from the electrochemical reduction of CO2 as compared with Cu foil (faradaic efficiency ∼33%). The effect of phosphate ligands promoting HCOOH selectivity is further verified by stripping off the ligands and regenerating the ligands.