Level Alignment as Descriptor for Semiconductor/Catalyst Systems in Water Splitting: The Case of Hematite/Cobalt Hexacyanoferrate Photoanodes

Abstract

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.

A Noble‐Metal‐Free Heterogeneous Photosensitizer‐Relay Catalyst Triad That Catalyzes Water Oxidation under Visible Light

“…[10][11][12][13][14][15][16][17][18][19] Although the use of robust heterogeneous assemblies such as oxides could be apromising solution to improve stability,the lack of complete control on the coordination of metal ions in oxide chemistry does not allow the preparation of oxidebased dyads. [22][23][24][25][26][27][28][29][30][31] PBAs consist of earth-abundant elements,perform at relatively low overpotentials (< 500 mV for ac urrent density of 1mAcm À2 ), exhibit exceptional stabilities in awide pH range (from 1to13), and can operate even with molecular chromophores for light-driven catalysis. [21] Different perspectives of cobalt-based Prussian blue analogues (PBAs) as WOCs have recently been explored by us and other researchers.…”

Tailored Assembly of Molecular Water Oxidation Catalysts on Photoelectrodes for Artificial Photosynthesis

“…Even for CoFe‐PBAs, their integration gave rise to different results depending on the type of an underlying photoanode (Figure c). While there was a significant performance improvement for BiVO 4 , there was a negligible improvement for Fe 2 O 3 (Figure c‐d) due to the undesirable alignment of the VB of Fe 2 O 3 and t 2g level of Co in CoFe‐PBAs for efficient charge transport (Figure e) , . In addition, it is reported that PBA‐modified photoanodes exhibit excellent photochemical stability due to their additional role as a protective coating layer , .…”

“…While there was a significant performance improvement for BiVO 4 , there was a negligible improvement for Fe 2 O 3 (Figure c‐d) due to the undesirable alignment of the VB of Fe 2 O 3 and t 2g level of Co in CoFe‐PBAs for efficient charge transport (Figure e) , . In addition, it is reported that PBA‐modified photoanodes exhibit excellent photochemical stability due to their additional role as a protective coating layer , . Despite their interesting and promising properties, however, their application in photocatalytic water oxidation seems to still be in its infancy, as evidenced by the utilization of only Co‐based PBAs.…”

“…Part (c) is reproduced with permission from ref . Copyright 2017 American Chemical Society, and part (d) is reproduced from ref …”

“…Initially, they were tested for photocatalytic water oxidation in the form of powder dispersion in the presence of homogeneously dispersed molecular photosensitizers such as Ru(bpy) 3 2+ and sacrificial electron scavengers , . Recently, their application in the fabrication of water‐oxidation photoanodes was also reported via the bottom‐up self‐assembly and precipitation on the surface of various photoanodes such as Fe 2 O 3 , BiVO 4 and Si . For PEC water oxidation, PBAs incorporating Co and Fe ions (CoFe‐PBAs) were preferentially utilized rather than those incorporating only Co ions probably due to concerns regarding the energy‐level alignment between semiconductor photoanodes and PBAs.…”