Sustainability of small reservoirs and large scale water availability under current conditions and climate change

Semi-arid river basins often rely on reservoirs for water supply. Small reservoirs may impact on large-scale water availability both by enhancing availability in a distributed sense and by subtracting water for large downstream user communities, e.g. served by large reservoirs. Both of these impacts of small reservoirs are subject to climate change. Using a case-study on North-East Brazil, this paper shows that climate change impacts on water availability may be severe, and impacts on distributed water availability from small reservoirs may exceed impacts on centralised water availability from large reservoirs. Next, the paper shows that the effect of small reservoirs on water availability from large reservoirs may be significant, and increase both in relative and absolute sense under unfavourable climate change

Photocurrent Enhancement by Spontaneous Formation of a p n Junction in Calcium Doped Bismuth Vanadate Photoelectrodes

The application of bismuth vanadate BiVO4 photoelectrodes for solar water splitting is hindered by the poor carrier transport. To overcome this, multiple donor doping strategies e.g. dual doping, gradient doping have been explored. Here, we show for the first time the successful introduction of calcium Ca as an acceptor type dopant into BiVO4 photoelectrodes. Interestingly, instead of generating cathodic photocurrents, the Ca doped BiVO4 photoelectrodes show anodic photocurrents with an enhanced carrier separation efficiency. Hard X ray photoelectron spectroscopy HAXPES shows that this enhancement is caused by out diffusion of Ca during the deposition process, which spontaneously creates a p n junction within the BiVO4 layer. Overall, a significant two fold improvement of the AM1.5 photocurrent is obtained upon Ca doping. This study highlights the importance of controlled doping beyond simply modifying carrier concentration and may enable new device architectures in photoelectrode material

Evaluation of electrodeposited alpha Mn2O3 as a catalyst for the Oxygen Evolution Reaction

alpha Mn2O3 is of interest as a low cost and environmentally benign electrocatalyst for the Oxygen Evolution Reaction OER in the process of water splitting. Mechanically stable alpha Mn2O3 electrodes are prepared by annealing of galvanostatically deposited MnOOHx layers on F SnO2 coated glass. The overpotential eta to achieve a current density of j 10 mA cm2 decreases from 590 to 340 mV with increasing layer thickness. Differential capacitance measurements reveal that this high OER activity can be attributed to the large electrochemically active surface area ECSA , which scales linearly with the thickness of these highly porous and electrolyte permeable films. The oxide layers exhibit a reversible oxidation behavior from Mn III to Mn IV , whereas only about 25 of the Mn III is oxidized to Mn IV before the OER reaction takes off. Although the intrinsic activity is small compared to that of other OER catalysts, such as NiFeOx, the combination of high ECSA and good electrical conductivity of these amp; 945; Mn2O3 films ensures that high OER activities can be obtained. The films are found to be stable for gt;2 h in alkaline conditions, as long as the potential does not exceed the corrosion potential of 1.7 V vs. RHE. These findings show that amp; 945; Mn2O3 is a promising OER catalyst for water splitting device

Light Induced Surface Reactions at the Bismuth Vanadate Potassium Phosphate Interface

Bismuth vanadate has recently drawn significant research attention as a light absorbing photoanode due to its performance for photoelectrochemical water splitting. In this study, we use in situ ambient pressure X ray photoelectron spectroscopy with Tender X rays 4.0 keV to investigate a polycrystalline bismuth vanadate BiVO4 electrode in contact with an aqueous potassium phosphate KPi solution at open circuit potential under both dark and light conditions. This is facilitated by the creation of a 25 to 30 nanometers thick electrolyte layer using the dip and pull method. We observe that under illumination bismuth phosphate forms on the BiVO4 surface leading to an increase of the surface negative charge. The bismuth phosphate layer may act to passivate surface states observed in photoelectrochemical measurements. The repulsive interaction between the negatively charged surface under illumination and the phosphate ions in solution causes a shift in the distribution of ions in the thin aqueous electrolyte film, which is observed as an increase in their photoelectron signals. Interestingly, we find that such changes at the BiVO4 KPi electrolyte interface are reversible upon returning to dark conditions. By measuring the oxygen 1s photoelectron peak intensities from the phosphate ions and liquid water as a function of time under dark and light conditions, we determine the timescales for the forward and reverse reactions. Our results provide direct evidence for light induced chemical modification of the BiVO4 KPi electrolyte interfac

In situ XAS study of CoBi modified hematite photoanodes

Xi L, Schwanke C, Zhou D, Drevon D, van de Krol R, Aziz-Lange K. In situ XAS study of CoBi modified hematite photoanodes. DALTON TRANSACTIONS. 2017;46(45):15719-15726.Solar water splitting is a potentially scalable method to store solar energy in the form of renewable hydrogen gas. In this study, we demonstrate that the photoelectrochemical (PEC) performance of hematite photoanodes can be improved by modification with the oxygen evolution catalyst CoBi. The current density at 1.23 V of the pristine hematite under one sun is 0.88 mA cm(-2) and it increases to 1.12 mA cm(-2) after CoBi modification (similar to 27% improvement). The presence of a CoBi cocatalayst layer is proposed to improve the oxygen evolution reaction (OER) kinetics and also to prevent electron-hole recombination at the surface via passivating surface defects as well as suppressing the tunneling of electrons from the hematite core, thus improving the photocurrents and resulting in a negative shift of photocurrent onset potentials. These effects of CoBi modification are supported by experimental data obtained by performing electrochemical impedance spectroscopy (EIS), PEC and incident photon-to-current efficiency (IPCE) measurements. To investigate the electronic structure of the CoBi cocatalyst deposited on hematite, XPS and in situ X-ray absorption spectroscopy (XAS) are employed. Co K-edge spectra at different potentials and light conditions are recorded. This makes the present work different from most of the previous studies. Using a quantitative analysis method, information on the mean oxidation state of Co in the CoBi film under applied potential and illumination is revealed. We also compare different methods for determining the oxidation state from the edge position and find that the integral method and half height methods are most suitable. In summary, the present work underlines the improvement of the semi-conductor/cocatalyst interface of oxygen evolving photoanodes and strengthens the importance of in situ XAS spectroscopy when studying catalysts. This study is the first report so far combining the studies of the PEC performance of a CoBi modified hematite nanorod array photoanode and in situ XAS at the Co K-edge

Combined soft and hard X ray ambient pressure photoelectron spectroscopy studies of semiconductor electrolyte interfaces

The development of solar fuel generating materials would greatly benefit from a molecular level understanding of the semiconductor/electrolyte interface and changes in the interface induced by an applied potential and illumination by solar light. Ambient pressure photoelectron spectroscopy techniques with both soft and hard X-rays, AP-XPS and AP-HAXPES respectively, have the potential to markedly contribute to this understanding. In this paper we initially provide two examples of current challenges in solar fuels material development that AP-XPS and AP-HAXPES can directly address. This will be followed by a brief description of the distinguishing and complementary characteristics of soft and hard X-ray AP-XPS and AP-HAXPES and best approaches to achieving monolayer sensitivity in solid/aqueous electrolyte studies. In particular we focus on the detection of surface adsorbed hydroxyl groups in the presence of aqueous hydroxide anions in the electrolyte, a common situation when investigating photoanodes for solar fuel generating applications. The paper concludes by providing an example of a combined AP-XPS and AP-HAXPES study of a semiconductor/aqueous electrolyte interface currently used in water splitting devices specifically the BiVO4/aqueous potassium phosphate electrolyte interface