Tuning the donor density and the surface state density of hematite multilayer nanowire photoanodes.

For the design of a beneficial device structure, in which both electrodes are exposed to the same medium, and considering that the hydrogen evolution is most efficiently carried out in acidic electrolyte and the advantages of the proton exchange membrane, a robust photoanode would be highly desirable. [10][11][12][13][14][15] Nonetheless the development of an efficient and affordable photoanode, which is stable in acidic electrolyte, imposes a great challenge and limits the large-scale implementation of economically viable PEC water-splitting. In light of this challenge, much attention has been drawn to the development of efficient and affordable photoanode systems adapted to acidic electrolytes.Hematite is arguably the most desirable photoanode material. On one hand, its relatively small bandgap of 1.9-2.1 eV and its suitably aligned valence band level perfectly match the thermodynamic energy requirements needed to drive water oxidation. [4,10] On the other hand, it is made from the most abundant transition metal on Earth crust, iron. Unfortunately, the bare hematite surface is catalytically very poor, and therefore requires modification with water-oxidation catalysts (WOCs) in order to extract the thermodynamic power stored when light is absorbed. State-of-the-art water-oxidation catalysts (WOCs) in acidic electrolytesusually contain expensive noble metals such as ruthenium and iridium. However, they too expensive to be implemented broadly in semiconductor photoanodes for photoelectrochemical (PEC) water splitting devices. Here, an Earth-abundant CoFe Prussian blue analogue (CoFe-PBA) is incorporated with core-shell Fe 2 O 3 /Fe 2 TiO 5 type II heterojunction nanowires as composite photoanodes for PEC water splitting. Those deliver a high photocurrent of 1.25 mA cm −2 at 1.23 V versus reversible reference electrode in acidic electrolytes (pH = 1). The enhancement arises from the synergic behavior between the successive decoration of the hematite surface with nanolayers of Fe 2 TiO 5 and then, CoFe-PBA. The underlying physical mechanism of performance enhancement through formation of the Fe 2 O 3 /Fe 2 TiO 5 / CoFe-PBA heterostructure reveals that the surface states' electronic levels of hematite are modified such that an interfacial charge transfer becomes kinetically favorable. These findings open new pathways for the future design of cheap and efficient hematite-based photoanodes in acidic electrolytes.

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Regulating the coordination environment via heteroatoms to break the symmetrical electronic structure of M-N 4 active sites provides a promising route to engineer metal-nitrogen-carbon catalysts for electrochemical CO 2 reduction reaction. However, it remains challenging to realize a site-specific introduction of heteroatoms at atomic level due to their energetically unstable nature. Here, this paper reports a facile route via using an oxygen-and nitrogen-rich metal-organic framework (MOF) (IRMOF-3) as the precursor to construct the Fe-O and Fe-N chelation, simultaneously, resulting in an atomically dispersed axial O-coordinated FeN 4 active site. Compared to the FeN 4 active sites without O coordination, the formed FeN 4 -O sites exhibit much better catalytic performance toward CO, reaching a maximum FE CO of 95% at −0.50 V versus reversible hydrogen electrode. To the best of the authors' knowledge, such performance exceeds that of the existing Fe-N-C-based catalysts derived from sole N-rich MOFs. Density functional theory calculations indicate that the axial O-coordination regulates the binding energy of intermediates in the reaction pathways, resulting in a smoother desorption of CO and increased energy for the competitive hydrogen production.

Although the Faraday efficiencies (FEs) obtained on most of the Ni based single-atom catalysts (Ni-N-C) are satisfactory (generally > 90 %) for electrochemical transfer CO2 to CO, the practical application...