Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy

Abstract

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO 4 ) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO 4 with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO 4 and BiVO 4 /NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (k h+ 0 ) to electron (k e− 0 ) via the photocatalyst of BiVO 4 /NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO 4 under illumination. In this case, the ratio of the rate constants k h+ 0 /k e− 0 stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuO x onto BiVO 4 further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.

A highly enhanced photoelectrochemical performance of BiVO4 photoanodes modified with CoPi groups by increasing energy band bending, accelerating hole separation and improving water oxidation kinetics

“…17 It is worth noting that the loading of highly active oxygen evolution co-catalysts has been proven to be a promising strategy for building an efficient PEC photoanode. 18 A number of oxygen evolution co-catalysts such as CoPi, 19 CoBi, 20 MCo 2 O 4 (M = Mn, Zn), 21 CoOOH, 22 and FeOOH/NiOOH 23,24 have been researched to decrease the onset potential and accelerate the oxygen evolution reaction (OER) at the electrode/electrolyte interface. In the Co based co-catalyst studies, cobalt phosphate (CoPi) has been widely used due to the low cost, convenient preparation and excellent OER activity in PEC water oxidation.…”

Insights into the Enhanced Photoelectrochemical Performance through Construction of the Z-Scheme and Type II Heterojunctions

“…Notably, compared to dark conditions, more warm color is clearly seen, revealing that holes from BWO can be in situ -captured and take part in the oxidation reaction of probe molecules, as confirmed by the positive feedback curves. Based on the simplest kinetic model, − the fitting kinetic constant ( K eff ) of relevant photoanodes can be calculated. As shown in Figure c and Table , the K eff increases in the following order: BWO < BWO/BS < BWO/BS/Au < BWO/Au/BS.…”

Imaging Analysis of Scanning Electrochemical Microscopy in Energy Catalysis

“…Tan et al studied monolayer chemical-vapor deposited graphene defects by the SECM feedback mode (Figure B), and the results showed that the reactivity of graphene defect sites to electrochemical reactions was about one order of magnitude higher than that of more primitive graphene surfaces . Yu et al investigated the effect of NiFe layered double hydroxide (LDH) cocatalyst on BiVO 4 photoanode by SECM feedback mode (Figure C) . Studies on the photocatalyst and redox reaction of BiVO 4 /NiFe-LDH show that the rate constant of the photoluminescent hole ( k h+ 0 ) and electron ( k e– 0 ) is five times larger than that of BiVO 4 under illumination.…”