A Tantalum Nitride Photoanode Modified with a Hole‐Storage Layer for Highly Stable Solar Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting is an ideal approach for renewable solar fuel production. One of the major problems is that narrow bandgap semiconductors, such as tantalum nitride, though possessing desirable band alignment for water splitting, suffer from poor photostability for water oxidation. For the first time it is shown that the presence of a ferrihydrite layer permits sustainable water oxidation at the tantalum nitride photoanode for at least 6 h with a benchmark photocurrent over 5 mA cm À2 , whereas the bare photoanode rapidly degrades within minutes. The remarkably enhanced photostability stems from the ferrihydrite, which acts as a holestorage layer. Furthermore, this work demonstrates that it can be a general strategy for protecting narrow bandgap semiconductors against photocorrosion in solar water splitting.The direct conversion of solar energy to chemical fuels is not only of scientific interest but also a highly desirable approach to power the planet. [1] Photoelectrochemical (PEC) water splitting is a promising strategy for renewable solar fuel production using photoelectrodes. [2] However, trade-offs between light-harvesting and photostability of photoanodes have greatly limited the performance of PEC water splitting devices. [3] Typically, tantalum nitride (Ta 3 N 5 ) has a desirable band alignment for water splitting and a theoretical solar to hydrogen conversion efficiency of 15.9 %, but it is not stable in the highly oxidative environments of water oxidation. [4] To solve this problem, efforts have been made on alleviating accumulation of photogenerated holes by facilitating water oxidation. [5] For example, modification of Ta 3 N 5 with IrO 2 , an excellent water oxidation catalyst (WOC), could maintain half of the initial photocurrent in less than 10 min at 1.2 V versus the reversible hydrogen electrode (RHE). [5b, 6] Recently, continuous production of oxygen in 100 min was reported on a cobalt phosphate (CoPi) WOC modified Ba-Ta 3 N 5 photoanode at a moderate potential (0.9 V vs. RHE). [7] As yet, the endurance of the Ta 3 N 5 photoanode in harsh oxidative environments for long-term performance still remains a challenging task.Herein, for the first time, we employed a ferrihydrite (Fh) layer for protecting the unstable Ta 3 N 5 photoanode against photocorrosion. With overlying Co 3 O 4 nanoparticles (NPs), the resulting photoanode (designated as Co 3 O 4 /Fh/Ta 3 N 5 ) yielded a photocurrent up to 5.2 mA cm À2 at a potential of 1.23 V vs. RHE under AM 1.5 G simulated sunlight (100 mW cm À2 ) and remained at about 94 % of the initial activity after 6 h irradiation, which is the highest durability of the Ta 3 N 5 based photoanodes reported to date. [5b,e] The porous cubic Ta 3 N 5 electrode shown in Figure 1 a and b was formed by nitridation of NaTaO 3 film by an anodization and hydrothermal process on Ta substrate (Supporting Information, Figure S1) developed from our previous work. [8] The Ta 3 N 5 electrode was then modified with a ferrihydrite layer (Figure 1 c). The X...

A novel Bi 2 S 3 nanowire @ TiO 2 nanorod heterogeneous nanostructure for photoelectrochemical hydrogen generation

“…The photoelectrochemical (PEC) splitting of H 2 O or H 2 S to generate hydrogen is one of highly promising methods to produce hydrogen, which can directly capture solar energy and convert it to hydrogen energy [2][3][4][5][6][7][8]. As an undesirable byproduct, large quantities of H 2 S are produced in the coal, petroleum and metallurgical industry.…”

Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta3N5 Photocatalyst

“…1,4 Among the photocatalysts investigated to date, tantalum(V) nitride (Ta 3 N 5 ) has emerged as one of the most promising candidates and has been extensively investigated for more than a decade. [4][5][6][7][8][9][10][11][12][13][14][15] Ta 3 N 5 has a visible-light response (approximately 600 nm, ~2.1-eV band gap) and is capable of producing hydrogen or oxygen in the presence of appropriate sacrificial reagents and surface modifications under visible-light irradiation. 1,[5][6][7][8][9][10][11][12][13][14][15][16] Our recent work on Ta 3 N 5 thin films for the photoelectrochemical oxygen evolution reaction (OER) noted that despite having suitable absorption in the visible-light range, Ta 3 N 5 suffered from very low transport properties and fast carrier recombination (<10 ps) without any surface decoration.…”

Thin-Film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators

“…Li et al reported that a ferrihydrite only operated for several min under the same conditions. The ferrihydrite was hypothesized to be a hole storage layer, which captured and transmitted the photogenerated holes readily to the electrolyte or to the Co 3 O 4 catalyst 103. BaTaO 2 N treated with pre-loaded CoO x particles and post-loaded with RhO x , along with a necking treatment that involved impregnating a TaCl 5 solution followed by treatment with H 2 , showed at least 1 h stability for water oxidation in pH = 8 phosphate buffer 104.…”