Boosting Photoelectrochemical Water Oxidation of Hematite in Acidic Electrolytes by Surface State Modification

Abstract

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.

Janus‐Structured Co‐Ti3C2 MXene Quantum Dots as a Schottky Catalyst for High‐Performance Photoelectrochemical Water Oxidation

“…To the best of our knowledge, the PEC water oxidation and stability of as constructed Co‐MQD photoelectrocatalyst are comparable or even superior to the works on other classical high performance photoanode materials, such as TiO 2 , [ 44,45 ] BiVO 4 , [ 46 ] Fe 2 O 3 , [ 47–52 ] ZnO, [ 53 ] and MoS 2 . [ 54 ] As shown in Table S5, Supporting Information, at a bias potential of 1.23 V versus RHE, the Co‐MQD‐48 photoanode achieves almost the highest current density of 2.99 mA cm −2 , making it among the state‐of‐the‐art performance in PEC water oxidation.…”

The effect of nanoparticulate PdO co-catalysts on the faradaic and light conversion efficiency of WO3 photoanodes for water oxidation

“…Transient spikes in the chopped light LSV measurements are not observed for our systems ( Figure S8a), in contrast to what is commonly observed for BiVO 4 and Fe 2 O 3 photoanodes. 29,30 Such spikes are indicative of back electron-hole recombination of photogenerated charge pairs, [31][32][33] and their absence herein indicates that this recombination process is masked by sufficiently fast water oxidation kinetics, or that it occurs before the timescale of our measurements.…”

Photoelectrochemical water splitting: a road from stable metal oxides to protected thin film solar cells

“…101 CoFe-PBA decoration modied surface states electronic levels enhancing Fe 2 O 3 /Fe 2 TiO 5 interfacial charge transfer kinetics. 102 Also, partially electrochemical reduction to Fe 2 O 4 (a more conductive phase) can form benecial conduction paths and reduce recombination. 103 BiVO 4 has been modied both with W and Mo dopants and by hydrogen treatment to form oxygen vacancies.…”