Photoanode For Solar Water Oxidation Research Articles

Metal-Organic Frameworks as Promising Photosensitizers for Photoelectrochemical Water Splitting.

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

BiVO4 Photoelectrodes for Use in Solar Water Splitting

Many recent efforts have focused on the development of n-type semiconductors that can serve as efficient photoanodes for solar water oxidation. Desirable photoanodes should have a small bandgap to utilize a significant portion of visible light, and a valence band (VB) edge that is positive enough to provide sufficient overpotential for the water oxidation reaction. The position of the conduction band (CB) edge or the flatband potential of the photoanode is also important because it determines the potential of the photoexcited electrons that will be used for the cathode reaction (i.e., water reduction). If these electrons do not have sufficient overpotential to reduce water, an external bias needs to be provided, effectively reducing the cell efficiency. Bismuth vanadate (BiVO4) has recently emerged as a promising material for use as a photoanode in water splitting photoelectrochemical cells. It is because it absorbs a substantial portion of the visible spectrum (bandgap energy, ca. 2.4 eV) and has a favorable conduction band (CB) edge position very near the thermodynamic H2 evolution potential. Significant advancement in the understanding and construction of efficient BiVO4-based photoanode systems has been made within a short period of time owing to various newly developed ideas and approaches. In this presentation, we will discuss our recent efforts in improving the photoelectrochemical properties of BiVO4 photoanodes, which include an enhancement in photon absorption and charge transport properties. These efforts resulted in an applied bias photon-to-current efficiency (ABPE) for water splitting higher than 2% using all oxide-based electrodes.

Efficient photoelectrochemical water splitting using three dimensional urchin-like hematite nanostructure modified with reduced graphene oxide

Herein, we present a highly photoactive photoanode for solar water oxidation using three dimensional (3D) urchin-like hematite (α-Fe2O3) nanostructures modified with ultra-thin reduced graphene oxide (rGO). rGO acts as both electron conducting scaffold and surface passivation layer. By virtue of these combined effects, the composite photoanode exhibits 1.47 times higher photocurrent density (1.06 mA cm−2, at 1.23 V vs. reversible hydrogen electrode (RHE)) and two-fold enhancement in the photoconversion efficiency than that of pristine α-Fe2O3. The dual effect of rGO as both electron conducting scaffold and surface passivation layer is further evidenced from the 1.82 and 1.67 fold enhancements in charge separation and charge injection efficiencies at 1.23 and 1 V vs. RHE respectively. To get further evidence about the origin of the improved photoactivity of the rGO modified photoanode, a series of electrochemical, photoelectrochemical and impedance spectroscopy measurements were carried out. Our results demonstrate the benefits of a noble metal free highly promising photoanode for photoelectrochemical water oxidation.

Depth-reduction induced low onset potential of hematite photoanodes for solar water oxidation

The pressure-induced depth-reduction of hematite in a crucible results in a cathodic shift of the onset potential by 70 mV.

Facile synthesis of tungsten oxide nanostructures for efficient photoelectrochemical water oxidation

We develop a facile and low-cost hydrothermal method to prepare WO3 nanostructures and demonstrate their applications as high-performance photoanodes for solar water oxidation. This present method allow us readily tune the morphology of WO3 nanostructures. By simply adjusting the constitution of the solution, WO3 nanoparticles (NPs-a), cubic-like nanoparticles (NCs) and another nanoparticles (NPs-b) can be obtained on F doped SnO2 glass substrates. Moreover, these WO3 nanostructures exhibit remarkable photoelectrochemical activities under the light irradiation and the WO3 NCs have the best performance than the others due to the enhanced light absorption ability and increased carrier density.

Al-doped ZnO inverse opal networks as efficient electron collectors in BiVO4 photoanodes for solar water oxidation

A conductive Al-doped ZnO inverse opal network has been introduced into BiVO4 photoanode to improve photogenerated electron transport and collection.

Dynamics of photogenerated holes in undoped BiVO4 photoanodes for solar water oxidation

We use transient absorption spectroscopy and photoelectrochemical methods to study the dynamics of photogenerated holes in BiVO4 for solar water oxidation. The back electron/hole recombination is found to be slow and therefore competes with water oxidation, limiting water oxidation efficiency.

Synthesis and characterization of high surface area CuWO4 and Bi2WO6 electrodes for use as photoanodes for solar water oxidation

Two n-type W-containing ternary oxides, CuWO4 and Bi2WO6, were prepared as high surface area electrodes and characterized for use as photoanodes in a water-splitting photoelectrochemical cell. The synthesis involved electrochemical preparation of porous WO3 electrodes and annealing them with Cu2+- or Bi3+-containing solutions on their surfaces to form the respective electrodes. The resulting CuWO4 electrode had a bandgap of 2.3 eV, and showed excellent photostability and photocurrent-to-O2 conversion efficiency (ca. 100%) in 0.1 M borate buffer solution (pH 9). Bi2WO6 had a bandgap of 2.8 eV but, regardless of its higher bandgap energy, Bi2WO6 showed an earlier photocurrent onset and much higher photocurrent than CuWO4 due to its more favorable CB edge and flatband potential position for water splitting. Bi2WO6 also showed chemical stability over a wide pH range (−0.26 ≤ pH ≤ 9.0). The photocurrent-to-O2 conversion efficiency of Bi2WO6 was in the range of 50–75% and its photocurrent decayed over time, indicating photocorrosion. However, stable photocurrent was obtained when H2O2, which has faster oxidation kinetics than water, was introduced into the electrolyte as a hole scavenger. This suggests that the photocorrosion of Bi2WO6 can be suppressed when an oxygen evolution catalyst is placed on its surface to improve interfacial hole transfer kinetics. With proper oxygen evolution catalysts and improved charge transport properties, both CuWO4 and Bi2WO6 have the possibility of achieving better photoelectrochemical performances than WO3.

Correlating long-lived photogenerated hole populations with photocurrent densities in hematite water oxidation photoanodes

Photogenerated charge carrier dynamics are investigated as a function of applied bias in a variety of different hematite photoanodes for solar water oxidation. Transient absorption spectroscopy is used to probe the photogenerated holes, while transient photocurrent measures electron extraction. We report a general quantitative correlation between the population of long-lived holes and the photocurrent amplitude. The yield of long-lived holes is shown to be determined by the kinetics of electron-hole recombination. These recombination kinetics are shown to be dependent upon applied bias, exhibiting decay lifetimes ranging from ca 5 μs to 3 ms (at −0.4 and +0.4 V versus Ag/AgCl, respectively). For Si-doped nanostructured hematite photoanodes, electron extraction and electron-hole recombination are complete within ∼20 ms, while water oxidation is observed to occur on a timescale of hundreds of milliseconds to seconds. The competition between electron extraction and electron-hole recombination is electron-density-dependent: the effect on recombination of applied bias and excitation intensity is discussed. The timescale of water oxidation is independent of the concentration of photogenerated holes, indicating that the mechanism of water oxidation on hematite is via a sequence of single-hole oxidation steps.

A new electrochemical synthesis route for a BiOI electrode and its conversion to a highly efficient porous BiVO4 photoanode for solar water oxidation

A new electrodeposition condition utilizing p-benzoquinone reduction was developed to produce BiOI electrodes composed of extremely thin 2D BiOI crystals. These electrodes served as precursors to form porous BiVO4 electrodes via mild chemical and thermal treatments. The resulting porous BiVO4 electrodes showed outstanding photoelectrochemical performance for sulfite oxidation reaching 1.25 mA cm−2 at 0.5 V vs. RHE in 0.1 M potassium phosphate buffer (pH 7) containing 0.1 M sodium sulfite. When a ca. 100 nm thick FeOOH layer was deposited on the surface of BiVO4 as an oxygen evolution catalyst, the kinetics of water oxidation was improved to the level of sulfite oxidation and the maximum power point for solar water oxidation was achieved at a bias as low as 0.55 V vs. RHE with a photocurrent density of 1.17 mA cm−2. The remarkable solar water oxidation performance achieved by the porous BiVO4–FeOOH system strongly encourages further morphological and compositional optimizations of the BiVO4-based photoanode systems to realize highly efficient and practical solar water oxidation.

Photo-assisted electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation

A photo-assisted electrodeposition approach was used to deposit a cobalt–phosphate water oxidation catalyst (“Co–Pi”) onto recently improved dendritic mesostructures of α-Fe2O3. A comparison between this approach, electrodeposition of Co–Pi, and Co2+ wet impregnation showed that photo-assisted electrodeposition of Co–Pi yields superior α-Fe2O3 photoanodes for photoelectrochemical water oxidation. Stable photocurrent densities of 1.0 mA cm−2 at 1.0 V and 2.8 mA cm−2 at 1.23 V vs. RHE measured under standard illumination and basic conditions were achieved. By allowing deposition only where visible light generates oxidizing equivalents, photo-assisted electrodeposition provides a more uniform distribution of Co–Pi onto α-Fe2O3 than obtained by electrodeposition. This approach of fabricating catalyst-modified metal-oxide photoelectrodes may be attractive for optimization in conjunction with tandem or hybrid photoelectrochemical cells.