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State-of-the-art water-oxidation catalysts (WOCs) in acidic electrolytes usually 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 Fe2O3/Fe2TiO5 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 Fe2TiO5 and then, CoFe-PBA. The underlying physical mechanism of performance enhancement through formation of the Fe2O3/Fe2TiO5/ 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., This work was supported by the Spanish Ministerio de Economia y Competitividad (MINECO, Grants CTQ2015-71287-R, CTQ2015-71287-R, and CTQ2015-68770-R) and the coordinated Project ValPEC (ENE2017-85087-C3), the BIST Ignite Project inWOC2 and the Generalitat de Catalunya (2017 SGR 90, 2017 SGR 327, 2017 SGR 329, 2017 SGR 1246, and 2017 SGR 1406). ICN2 acknowledges the support from the Severo Ochoa Program (MINECO, Grant SEV-2017-0706). ICN2, ICIQ, and IREC are funded by the CERCA Programme/Generalitat de Catalunya. P.Y.T. acknowledges the scholarship support of DAAD short term grant. H.C.D. acknowledges support from the Deutsche Forschungsgemeinschaft (SFB 917). F.S.H. thanks the “LaCaixa”-Severo Ochoa International Programme (Programa internacional de Becas “LaCaixa”- Severo Ochoa) for a Ph.D. fellowship. P.P. and M.H. thank for the support by the Federal Ministry for Economic Affairs and Energy (BMWi) (Fundingregistration number: 03ET6080E). M.L. thanks the COST Action StableNextSol project MP1307, supported by COST (European Cooperation in Science and Technology). H.X. acknowledges the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2013-0295 for the postdoctoral contract.

Prussian blue and its related compounds are formed by cheap and abundant metals and have shown their importance in the generation of new fuels by renewable sources. To optimize these compounds it is important to understand their electronic structure and thus establish robust structure−activity relationships. To this end, we employed theoretical simulations based on density functional theory, employing functionals of different degree of complexity, including pure generalized gradient approximation (GGA) and GGA+U functionals, which introduce self-interaction correction terms through the Hubbard parameter, and compared those to the hybrid functionals HSE03 and HSE06. With this robust setup, we can identify an appropriate computational scheme that provides the best compromise between computational demand and accuracy. A complete database considering Berlin green and Prussian blue and white for all alkaline cations is presented.

Bismuth vanadate (BiVO4) has emerged as one of the most promising photoanode materials for solar fuel production. Oxygen vacancies play a pivotal role in the photoelectrochemical efficiency, yet their electronic nature and contribution to n-type conductivity are still under debate. Using first-principles calculations, we show that oxygen vacancies in BiVO4 have two distinguishable geometric configurations characterized by either undercoordinated, reduced VIVO3 and BiIIO7 subunits or a VIV−O−VIV/V bridge (split vacancy), quenching the oxygen vacancy site. While both configurations have similar energies in the bulk, the (001) subsurface acts like an energetic sink that stabilizes the split oxygen vacancy by ∼1 eV. The barrierless creation of a bridging V2O7 unit allows for partial electron delocalization throughout the near-surface region, consistent with recent experimental observations indicating that BiVO4(001) is an electron-rich surface.

The efficient integration of photoactive and catalytic materials is key to promoting photoelectrochemical water splitting as a sustainable energy technology built on solar power. Here, we report highly stable water splitting photoanodes from BiVO4 photoactive cores decorated with CoFe Prussian blue-type electrocatalysts (CoFe-PB). This combination decreases the onset potential of BiVO4 by ∼0.8 V (down to 0.3 V vs reversible hydrogen electrode (RHE)) and increases the photovoltage by 0.45 V. The presence of the catalyst also leads to a remarkable 6-fold enhancement of the photocurrent at 1.23 V versus RHE, while keeping the light-harvesting ability of BiVO4. Structural and mechanistic studies indicate that CoFe-PB effectively acts as a true catalyst on BiVO4. This mechanism, stemming from the adequate alignment of the energy levels, as showed by density functional theory calculations, allows CoFe-PB to outperform all previous catalyst/BiVO4 junctions and, in addition, leads to noteworthy longterm stability. A bare 10−15% decrease in photocurrent was observed after more than 50 h of operation under light irradiation

The realization of artificial photosynthesis may depend on the efficient integration of photoactive semiconductors and catalysts to promote photoelectrochemical water splitting. Many efforts are currently devoted to the processing of multicomponent anodes and cathodes in the search for appropriate synergy between light absorbers and active catalysts. No single material appears to combine both features. Many experimental parameters are key to achieve the needed synergy between both systems, without clear protocols for success. Herein, we show how computational chemistry can shed some light on this cumbersome problem. DFT calculations are useful to predict adequate energy-level alignment for thermodynamically favored hole transfer. As proof of concept, we experimentally confirmed the limited performance enhancement in hematite photoanodes decorated with cobalt hexacyanoferrate as a competent water-oxidation catalyst. Computational methods describe the misalignment of their energy levels, which is the origin of this mismatch. Photoelectrochemical studies indicate that the catalyst exclusively shifts the hematite surface state to lower potentials, which therefore reduces the onset for water oxidation. Although kinetics will still depend on interface architecture, our simple theoretical approach may identify and predict plausible semiconductor/catalyst combinations, which will speed up experimental work towards promising photoelectrocatalytic systems.

Catalyst modification of metal oxide photoanodes can result inmarkedly improved water oxidation efficiency. However, the reasons for improvementare often subtle and controversial. Upon depositing a CoFe Prussian blue(CoFe-PB) water oxidation catalyst on BiVO4, a large photocurrent increase andonset potential shift (up to 0.8 V) are observed, resulting in a substantially moreefficient system with high stability. To elucidate the origin of this enhancement, weused time-resolved spectroscopies to compare the dynamics of photogenerated holesin modified and unmodified BiVO4 films. Even in the absence of strong positive bias, a fast (pre-ms), largely irreversible hole transfer from BiVO4 to CoFe-PB is observed.This process retards recombination, enabling holes to accumulate in the catalyst.Holes in CoFe-PB remain reactive, oxidizing water at a similar rate to holes inpristine BiVO4. CoFe-PB therefore enhances performance by presenting a favorableinterface for efficient hole transfer, combined with the catalytic function necessary to drive water oxidation.