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To create a functional photoelectrochemical cell (PEC) for solar fuels production, a cathode and anode reaction must both be chosen and optimized. In order to generate clean energy, a renewable resource such as water, nitrogen, or biomass, is typically reduced at the photocathode to generate H2, ammonia, or carbon-based fuel. In aqueous media, the anode reaction is typically water oxidation, as water is renewable and available in the cell. However, the product of water oxidation, O2, is not very valuable. As such, in recent years attention has been directed to finding a more attractive oxidation reaction that forms a value-added product that can be paired with reduction reactions of interest. Ideally, this reaction would consist of renewable, abundant, and affordable feedstock, such as biomass, that can be readily oxidized to a more valuable compound. The photoelectrochemical oxidation of glycerol to value-added commodity chemicals fits these criteria. Glycerol is the main byproduct of biodiesel production, and as such is cheap and abundant. As investment in clean fuels increases, the production of biodiesel will continue to increase, leading to an overabundance of glycerol, which can create storage and environmental issues. Developing systems to valorize glycerol will ensure sustainable biodiesel production and advance the transition to renewable fuels. Glycerol consists of three alcohol groups, and many oxidation pathways are possible, with the most profitable being the selective oxidation of the secondary alcohol to a ketone to make dihydroxyacetone (DHA). This conversion yields an over 200-fold value increase from $0.66 to $150 per kilogram. Thus, photoelectrochemical DHA production can provide an opportunity to make PEC operation more profitable while improving the sustainability of the biodiesel industry. BiVO4 is a good candidate for photoelectrochemical glycerol oxidation as it has a favorable band gap (2.4eV), excellent charge separation, and low water oxidation selectivity. In this presentation, we report our recent results obtained for photoelectrochemical glycerol oxidation using a BiVO4 photoanode. We will discuss various surface modifications of BiVO4 that we employed to elucidate the reaction mechanisms and to maximize the efficiency and selectivity for DHA production.

Solar water splitting in a photoelectrochemical cell (PEC) is an attractive approach to generate hydrogen fuel from the abundant solar energy source in a benign and environmentally friendly manner. In this field, TiO2 and Fe2O3 are some of the most extensively studied candidates for use as photoanodes. This is because they consist of naturally abundant elements and are photostable in a wide pH range. However, the major drawback of TiO2 for water splitting is its wide bandgap (3.2 eV). Though Fe2O3 has a narrow bandgap (2.0–2.1 eV), its conduction band minimum (CBM) is still ~200 mV below the water reduction potential. Recently, Fe2TiO5, one of the Fe-Ti-O ternary oxide compounds, has received a considerable interest as a new candidate material to overcome the intrinsic limitations of TiO2 and Fe2O3 while maintaining their strengths. That is, Fe2TiO5 has a favorable bandgap (2.1 eV), chemical robustness, and band edges that straddle the water oxidation and reduction potentials. These properties make Fe2TiO5 a promising photoanode candidate for solar water splitting. In this presentation, we report a new electrochemical method to prepare a high quality Fe2TiO5 film to systematically investigate its photoelectrochemical properties. The synthesis is based on the interaction between metals and catechol where catechol acts as a chelating agent to solubilize metal ions in a plating solution. When the catechol ligands are removed by applying electrochemical bias, the resulting insoluble metal ions or metal-catechol complexes are deposited onto the FTO surface. Using this method, we deposit a film that contains atomically well mixed Fe and Ti, which are then converted to crystalline Fe2TiO5 after annealing at a moderate temperature. This method also allows facile doping of the Fe2TiO5 film, enabling us to investigate the effect of both bulk and surface composition tuning on its photoelectrochemical properties.

Hydrogen production by photoelectrochemical water splitting is a challenging and exciting research topic that continues to receive much attention as it is a promising option for meeting the global energy demand. For this technology to compete with fossil fuels, a solar-to-hydrogen (STH) efficiency above 10% must be achieved using an inexpensive and durable photoelectrochemical cell (PEC). Accordingly, our group has been investigating oxide-based photoelectrodes that can be produced via practical solution-based syntheses using inexpensive raw materials. We are specifically interested in developing ternary oxides, which provide more opportunities to tune the composition and atomic/electronic structures of the photoelectrode materials relative to their corresponding binary oxides. In this presentation, the synthesis and investigation of BiFeO3 photoelectrodes will be discussed. BiFeO3 has a bandgap of ~2.2 eV and appropriate band positions for water splitting; however, it has not been extensively studied for photoelectrochemical applications. BiFeO3 has previously shown both n-type and p-type behavior, meaning it can serve as either a photoanode for water oxidation or photocathode for water reduction, respectively. Here, we will first introduce electrochemical methods to prepare high-quality n-type and p-type BiFeO3 films. We will then discuss the photoelectrochemical properties of the resulting films and present strategies to further optimize the performance of BiFeO3 as both a photoanode and photocathode for use in a water-splitting PEC.

As the performance of photoelectrodes used in in a photoelectrochemical cell (PEC) for water splitting continues to improve, concurrently improving their photostabilities has become an important issue. In this presentation, we report a new strategy to suppress photocorrosion of photoelectrodes using a BiVO4 photoanode as an example. BiVO4 has been recently identified as a promising oxide-based photoanodes for solar water splitting owing to many of its uniquely advantageous features. Anodic photocorrosion of BiVO4 photoanodes involves the loss of V5+ from the BiVO4 lattice by dissolution. Any composition change caused by the anodic photocorrosion of BiVO4 must be coupled with the oxidation of one or more of the elements present in BiVO4. Since V5+ in the BiVO4 lattice cannot be further oxidized, the dissolution of V5+ must be linked with the photooxidation of Bi3+ and/or lattice O2- ions. We show that the use of a V5+-saturated electrolyte, which inhibits the photooxidation-coupled dissolution of BiVO4, can slow down the rate of photocorrosion considerably and serve as a simple yet effective method to suppress anodic photocorrosion of BiVO4. We also discovered that the V5+ species in the solution can incorporate into the oxygen evolution catalyst (OEC) layer present on the BiVO4 surface during water oxidation, further enhancing water oxidation kinetics. The effect of the V5+ species in the electrolyte on both the long-term photostability of BiVO4 and the performance of the OEC layer will be discussed in detail.

A photoelectrochemical water splitting technology (PEC) using sustainable solar energy has been considered as one of the most promising methods to produce directly renewable energy (i.e. H2) from water. This system has still lots of challenges in improving water-splitting PEC efficiency. In particularly, development of electrode materials to covert efficiently solar energy to hydrogen is one of the most challenges facing many scientists and engineers in this field. Among the electrode materials for use in a water-splitting PEC cell, n-type bismuth vanadate, or BiVO4, has recently been identified as a promising metal oxide photoanode for O2 evolution, because of a narrow band gap (2.4-2.5 eV) for absorbing substantial position of visible spectrum and a favourable conduction band edge position which is very near the thermodynamic hydrogen reduction potential. Most of the BiVO4 studies for water-splitting PECs have mainly been investigated for use under neutral conditions (pH ∼7) because BiVO4 is chemically unstable and gradually dissolves in strong basic and acidic solutions. When the operating conditions of BiVO4 can be extended to basic or acidic media, BiVO4 can be coupled to more diverse catalysts or photocathodes, which perform optimally only under basic or acidic conditions. Additionally, using basic or acidic media may offer an advantage of achieving higher solution conductivities without using additional supporting electrolytes or buffers for PEC operation. In order to solve such weakness, we tried to add spinel zinc ferrite (ZnFe2O4) as a protection layer to use BiVO4 photoanode in basic condition. A thin layer of ZnFe2O4 was placed on the surface of a nanoporous BiVO4 electrode using following process: (1) photodeposition of iron oxyhydroxide (FeOOH), (2) a mild chemical and thermal treatment of FeOOH with Zn precursor. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that a ZnFe2O4 layer formed a uniform and conformal coating layer on a BiVO4 particles, and the thickness of the layer was 10 -15 nm. The ZnFe2O4 coating layer was very thin and was x-ray amorphous structure as evidenced by a conventional powder x-ray diffractometer. But, the selected area electron diffraction (SAED) clearly indicated that ZnFe2O4 was spinel structure. The effect of the ZnFe2O4 layer on the prevention of chemical dissolution of BiVO4 in basic media in the dark was first tested by immersing BiVO4 and BiVO4/ZnFe2O4 electrode in a 0.1 M KOH solution (pH 13) for 72 h. The SEM images taken after 72 h of immersion showed that the ZnFe2O4-free BiVO4 electrode was considerably dissolved, whereas the ZnFe2O4-coated BiVO4 electrode did not show any detectable sign of dissolution. The effect of the ZnFe2O4 layer on the photoelectrochemical properties and photostabilities of BiVO4 was tested by measuring J−V and J-t plots in 0.1 M KOH (pH 13) under simulated AM 1.5G irradiation (100 mW/cm2), using a three-electrode configuration. The obtained BiVO4/ZnFe2O4 electrode generated a photocurrent density of 2.76 mA/cm2 at 1.23 V vs. RHE with a significantly improved stability compared to the pristine BiVO4 electrode (ca. 1.04 mA/cm2 at 1.23 V vs. RHE). The incident and absorbed photon-to-current conversion efficiencies along with absorption spectra suggested that the ZnFe2O4 protection layer also contributes to photocurrent generation by increasing photon absorption and electron-hole separation of the BiVO4 layer. In addition, when the surface of the ZnFe2O4 layer was modified with Co2+ ions as oxygen evolution reaction catalyst, the resulting BiVO4/ZnFe2O4/Co2+ electrode generates a more improved photocurrent density (ca. 2.83 mA/cm2 at 1.23 V vs. RHE) with a more significantly improved stability. These results suggest that further investigation of protection and catalyst layers can enable more stable and efficient operation of BiVO4-based photoanodes in basic media.

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.

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An electrodeposition condition to anodically prepare CuO films using slightly acidic solutions of copper acetate (pH 5.5-6.4) is established. The plating solution and solutions with related compositions were investigated using cyclic and linear sweep voltammetry to elucidate the deposition mechanism. The key oxidation reaction that provides for CuO deposition appears to be the oxidation of Cu(II) ions to a Cu(III) species, which subsequently oxidizes water while being reduced to CuO. The solution pH appeared to be critical for these reactions as CuO forms only in solutions with pHs higher than 5.5. The use of acetate ions made it possible to elevate solution pHs to a proper range without chemically precipitating Cu(OH) 2 before the initiation of electrodeposition. The resulting films are translucent black and possess smooth surfaces on the microlevel. The as-deposited films are highly crystalline and showed well-defined optical and p-type photoelectrochemical properties that do not improve upon annealing at 300 and 500°C. The bandgap and flatband potential are estimated to be 1.37 eV and 0.26 V in a 0.1 M Na 2 HPO 4 solution (pH 9.2), respectively.