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Photoelectrochemical water splitting represents an attractive method of capturing and storing the immense energy of sunlight in the form of hydrogen, a clean chemical fuel. Given the large energetic demand of water electrolysis, and the defined spectrum of photons available from incident sunlight, a two absorber tandem device is required to achieve high efficiencies. The two absorbers should be of different and complementary bandgaps, connected in series to achieve the necessary voltage, and arranged in an optical stack configuration to maximize the utilization of sunlight. This latter requirement demands a top device that is responsive to high-energy photons but also transparent to lower-energy photons, which pass through to illuminate the bottom absorber. Here, cuprous oxide (Cu 2 O) is employed as a top absorber component, and the factors influencing the balance between transparency and efficiency toward operation in a tandem configuration are studied. Photocathodes based on Cu 2 O electrodeposited onto conducting glass substrates treated with thin, discontinuous layers of gold achieve reasonable sub-bandgap transmittance while retaining performances comparable to their opaque counterparts. This new high-performance transparent photo-cathode is demonstrated in tandem with a hybrid perovskite photovoltaic cell, resulting in a full device capable of standalone sunlight-driven water splitting.

Given the intermittent nature of solar radiation, the large-scale use of solar energy requires an efficient energy storage solution. So far, the only practical way to store such large amounts of energy is in the form of a chemical energy carrier, i.e., a fuel. Photoelectrochemical (PEC) cells offer the ability to convert solar energy directly into chemical energy in the form of hydrogen. Cuprous oxide (Cu2O) is being investigated for photoelectrochemical solar water splitting since it has a band gap of 2.0 eV with favorable energy band positions for water cleavage; it is abundant and environmentally friendly. A major challenge with Cu2O is its limited chemical stability in aqueous environments. We present a simple and low-cost treatment to create a highly stable photocathode configuration for H2 production, consisting of steam treatment of the multilayer structures. The role of this treatment was investigated and the optimized electrodes have shown photocurrents over-5mA cm-2 with 90% stability over more than 50 h of light chopping (biased at 0 VRHE in pH 5 electrolyte). 2

Photoelectrochemical (PEC) water splitting offers an elegant approach for solar energy conversion into hydrogen fuel. Large-scale hydrogen production requires stable and efficient photoelectrodes and scalable PEC cells that are fitted for safe and cost-effective operation. One of the greatest challenges is the collection of hydrogen gas from millions of PEC cells distributed in the solar field. In this work, a separate-cell PEC system with decoupled hydrogen and oxygen cells was designed for centralized hydrogen production, using 100 cm 2 hematite (-Fe 2 O 3) photoanodes and nickel hydroxide (Ni(OH) 2) / oxyhydroxide (NiOOH) electrodes as redox mediators. The operating conditions of the system components and their configuration were optimized for daily cycles, and ten 8.3 h cycles were carried out under solar simulated illumination without additional bias at an average short-circuit current of 55.2 mA. These results demonstrate successful operation of a decoupled PEC water splitting system with separate hydrogen and oxygen cells.

The influence of the substrate on the performance of WO3 photoanodes is assessed as a function of the temperature. Two samples were studied: WO3 deposited on a FTO glass and anodized on a tungsten foil. Current-voltage curves and electrochemical impedance spectroscopy techniques were used to characterize these samples between 25 °C and 65 °C. The photocurrent-density increased with temperature for both samples and the onset potential shifted to lower potentials. However, for WO3/FTO, a negative shift of the dark current onset was also observed. The intrinsic resistivity of this substrate limits the photocurrent plateau potential range. On the other hand, this behavior was not observed for WO3/metal. Therefore, the earlier dark current onset observed for WO3/FTO was assigned to the FTO layer. The optimal operating temperatures observed were 45 °C and 55 °C for WO3/FTO and WO3/metal, respectively. For higher temperatures, the bulk electron-hole recombination phenomenon greatly affects the overall performance of WO3 photoanodes. The stability behavior was then studied at these temperatures over 72 h. For WO3/FTO, a crystalline-to-amorphous phase transformation occurred during the stability test, which may justify the current decrease observed after the aging period. The WO3/metal remained stable, maintaining its morphology and good crystallinity. Interestingly, the preferential orientation of the aged crystals was shifted to the (-222) and (222) planes, suggesting that this was responsible for its better and more stable performance. These findings provide crucial information for allowing further developments on the preparation of WO3 photoanodes, envisaging their commercial application in PEC water splitting cells.

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The influence of temperature on the performance of a photoelectrochemical (PEC) cell prepared with Si-doped hematite photoanode was studied for water splitting. The cell performance was characterized by photocurrent-voltage (J-V) characteristic curves and electrochemical impedance spectroscopy at different cell operating temperatures, from 25 ºC to 65 ºC. A standard three-electrode configuration comprehending the photoelectrode of hematite, the counterelectrode of pure platinum wire (99.9 %) and the reference electrode of Ag/AgCl/Sat.KCl was used. The identification of possible degradation pathways was addressed. It was observed that the generated photocurrent-density 2 increased with temperature. However, the photoelectrode became unstable above 50 ºC. The experiments performed concerning the study of the temperature effect and the aging showed that the optimal operation temperature of the PEC cell is ca. 45 ºC; this temperature ensures simultaneously the highest photocurrent-density and stability. This study is important for understanding the behavior of hematite photoelectrodes operating under real outdoor conditions.

A photoelectrochemical (PEC) device capable of splitting water into storable hydrogen fuel by the direct use of solar energy is becoming a very attractive technology since it is clean and sustainable. Indeed, real field experiments are being developed in order to assess technological issues for large-scale usage under outdoor conditions. Following the need for developing photoelectrochemical devices with an optimized design that allows reaching a commercial performance level, the present works describes an innovative PEC cell for testing different photoelectrodes configurations, suitable for continuous operation and for easily collect the evolved gases. Moreover, a porous Teflon ® diaphragm useable for a wide range of aqueous electrolyte solutions is tested. Two semiconductors were investigated: tungsten trioxide and undoped hematite. The WO3 photoelectrodes were deposited in two different substrates: i) anodized WO3 photoelectrodes on a metal substrate and ii) WO3 deposited by blade spreading method on a TCO glass substrate.The undoped-Fe2O3 photoanode was deposited by ultrasonic spray pyrolysis technique in a TCO glass substrate. The material deposited on glass substrates allows to obtain transparent photoelectrodes. Photocurrent-voltage characteristics were obtained for all samples characterized under three different conditions: i) no membrane separating the anode and the cathode evolution; ii) using a Teflon ® diaphragm and iii) using a Nafion ® 212 membrane. The transparent samples (photoanodes deposited on glass substrates) produced the highest values of photocurrent when the Teflon ® diaphragm was used. This