Strategic Modification of BiVO₄ for Improving Photoelectrochemical Water Oxidation Performance

Abstract

To improve the photoelectrochemical (PEC) performance of BiVO4, three different modifications (doping, heterojunction, and catalyst deposition) using earth-abundant elements are performed and their effects are compared in a 0.1 M phosphate electrolyte at pH 7 under AM1.5 light (100 mW/cm2). When a hexavalent element (Cr6+, W6+, or Mo6+) is doped at various levels, the Mo6+-doping effect is most significant at 10 atomic % with about two times higher photocurrent generation at the oxygen evolution potential (1.23 VRHE). Such enhancement is attributed to a decrease in charge transfer resistance (R ct) by donor doping, resulting in an approximate 2-fold increase in charge separation efficiency (ηsep) to about 25%. W6+ is less effective than Mo6+, whereas Cr6+ has a detrimental effect. To further improve the charge separation efficiency of Mo6+-doped BiVO4 (Mo-BiVO4), a approximate 600 nm thick WO3 layer is deposited under a similarly thick Mo-BiVO4 layer. This binary heterojunction (WO3/Mo-BiVO4) exhibits ηsep of about 50% along with more than 3 times higher photocurrent generation. On the other hand, an oxygen evolving cobalt-phosphate (Co-Pi) catalyst electrodeposited to Mo-BiVO4 (Mo-BiVO4/Co-Pi) enhances charge injection efficiency (ηinj) from ∼50 to ∼70% at 1.23 V RHE. These two binaries are coupled into a ternary heterojunction (WO3/Mo-BiVO4/Co-Pi) in order to improve the charge transfer efficiencies (ηsep and ηinj). The PEC performance of this ternary is significantly high with photocurrent density of about 2.4 mA/cm2 at 1.23 VRHE (corresponding to the solar-to-hydrogen efficiency of ca. 3%) due to ηsep and ηinj of ∼60 and 90%, respectively.

Factors affecting bismuth vanadate photoelectrochemical performance

“…[15][16][17][18][19][20][21] However, typical photocurrents of unmodied BiVO 4 photoanodes are limited by high electron-hole recombination and poor charge transport properties, 16,[22][23][24] and inadequate water oxidation kinetics.…”

Improving the photoactivity of bismuth vanadate thin film photoanodes through doping and surface modification strategies

“…The concept of water splitting concept into oxygen and hydrogen was first demonstrated by Fujishima and Honda in 1972 using a TiO2 photoanode under ultraviolet light irradiation and a dark platinum cathode [6]. Due to the slow kinetics associated with multi-electron and multi-proton transfers (PEC water splitting has approximately three orders of magnitude lower rate than the hydrogen evolution reaction in conventional water electrolysis [7]), water oxidation is particularly demanding and requires high overpotentials. Therefore, many efforts have been made to develop efficient and practical photoanode systems that can oxidize water to O2 in a stable manner as this is a critical point for the successful construction of high performance and commercially viable PEC cells [3][4][5].…”

“…All these facts allow this compound to have a theoretical solar-to-hydrogen (STH) conversion efficiency close to 9.2%. Nevertheless, the typical STH conversion efficiencies of unmodified BiVO4 photoanodes are disappointingly low (lower than 1%) as they suffer from excessive electron-hole recombination and poor photogenerated carrier transport properties [7]. The slow water oxidation kinetics, due to the high kinetic barrier for water oxidation reaction, is another important limitation for the use of BiVO4 in photoelectrochemistry [26].…”

Improvement of the Photocatalytic Water Splitting Activity of Silver Tantalate by Photodeposited Platinum and Cobalt-Oxide Nanoclusters

“…24 After this report, although quite a number of researches related to the Co-based cocatalyst have been performed, most of them are reported electrochemically or photoelectrochemically using electrode systems. [25][26][27] A few of them are related to photocatalytic water oxidation activity using powder systems, but only for O 2 evolution from the half-reaction of water. 28,29 Therefore, we attempted to deposit the oxidation cocatalyst, CoO x and the reduction cocatalyst, Pt on AgTaO 3 powder photocatalyst by photodeposition.…”