Catalytic Multilayers for Efficient Solar Water Oxidation through Catalyst Loading and Surface-State Passivation of BiVO₄ Photoanodes

Abstract

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO4 modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO4 photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO4 due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

Modulating Charge Separation Efficiency of Water Oxidation Photoanodes with Polyelectrolyte‐Assembled Interfacial Dipole Layers

“…The measured EIS spectra were well fitted and analyzed using a 2‐RC equivalent circuit ( Figure a, b and Figure S9a, Supporting Information). The suggested equivalent circuit is composed of the space‐charge capacitance ( C SC ) of the bulk Fe 2 O 3 , carrier migration resistance ( R BULK ) from the bulk Fe 2 O 3 to the surface, surface‐state capacitance ( C SS ) associated with charge separation, and charge transfer resistance ( R CT ) by the electrochemical reactions at the photoelectrode–electrolyte interface . As expected, there were negligible variations in the bulk properties of the Fe 2 O 3 photoanode, such as R bulk and C SC , after the deposition of 3 BLs (data not shown).…”

“…It can be applied to almost all types of photoelectrodes by taking advantages of versatile LbL assembly. Furthermore, our approach can be readily combined with conventional approaches to improve the performance of photoelectrodes, such as catalyst loading, doping, defect engineering, and heterojunction formation…”

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar‐Driven Water Splitting

“…Copyright 2016, The Royal Society of Chemistry. d) J – V curves of the multilayer Co‐POM‐modified BiVO 4 in comparison with the Co‐Pi and NiOOH‐modified BiVO 4 and the bare BiVO 4 . e) Scheme of Co‐POM and conventional catalysts‐modified BiVO 4 and the illustration of the mechanisms of charge transport.…”

“…e) Scheme of Co‐POM and conventional catalysts‐modified BiVO 4 and the illustration of the mechanisms of charge transport. Reproduced with permission . Copyright 2019, The American Chemical Society.…”

“…Interestingly, not only heterogenous catalysts but POM (polyoxometalates) based molecular catalysts can also be stabilized on photoelectrodes by SILAR. Recently, Bae et al reported the fabrication of a BiVO 4 photoanode decorated with catalytic multilayers (CMs) composed of a Co‐based POM catalyst and poly(diallydimethylammonium chloride) (PDDA) polyelectrolyte . The electrode was prepared by alternately dipping in the PDDA and POM solutions followed by washing.…”