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

Efficiency gains for thermally coupled solar hydrogen production in extreme cold

Hydrogen produced from water using solar energy constitutes a sustainable alternative to fossil fuels, but solar hydrogen is not yet economically competitive. A major question is whether the approach of coupling photovoltaics via the electricity grid to electrolysis is preferential to higher levels of device integration in artificial leaf designs. Here, we scrutinise the effects of thermally coupled solar water splitting on device efficiencies and catalyst footprint for sub freezing ambient temperatures of amp; 8722;20 C. These conditions are found for a significant fraction of the year in many world regions. Using a combination of electrochemical experiments and modelling, we demonstrate that thermal coupling broadens the operating window and significantly reduces the required catalyst loading when compared to electrolysis decoupled from photovoltaics. Efficiency benefits differ qualitatively for dual and triple junction solar absorbers, which has implications for the general design of outdoor located photoelectochemical devices. Similar to high efficiency photovoltaics that reached technological maturity in space, application cases in polar or alpine climates could support the scale up of solar hydrogen at the global scal

Generalized Method to Extract Carrier Diffusion Length from Photoconductivity Transients Cases of BiVO4, Halide Perovskites, and Amorphous and Crystalline Silicon

Long diffusion lengths of photoexcited charge carriers are crucial for high power conversion efficiencies of photoelectrochemical and photovoltaic devices. Time resolved photoconductance measurements are often used to determine diffusion lengths in conventional semiconductors. However, effects such as polaron formation or multiple trapping can lead to time varying mobilities and lifetimes that are not accounted for in the conventional calculation of the diffusion length. Here, a generalized analysis is presented that is valid for time dependent mobilities and time dependent lifetimes. The diffusion length is determined directly from the integral of a photoconductivity transient and can be applied regardless of the nature of carrier relaxation. To demonstrate our approach, photoconductivity transients are measured from 100 fs to 1 s by the combination of time resolved terahertz and microwave spectroscopy for BiVO4, one of the most studied metal oxide photoanodes for photoelectrochemical water splitting. The temporal evolution of charge carrier displacement is monitored and converges after about 100 ns to a diffusion length of about 15 nm, which rationalizes the photocurrent loss in the corresponding photoelectrochemical device. The presented method is further validated on a amp; 8722;Si H, c amp; 8722;Si, and halide perovskite, which underlines its potential to determine the diffusion length in a wide range of semiconductors, including disordered material

Interfacial Oxide Formation Limits the Photovoltage of Alpha SnWO4 NiOx Photoanodes Prepared by Pulsed Laser Deposition

alpha SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar to hydrogen STH efficiencies above 20 . It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid electrolyte junction. In this study, an extensive analysis of the alpha SnWO4 NiOx interface is performed by synchrotron based hard X ray photoelectron spectroscopy HAXPES . NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2 to Sn4 at the interface. By combining the HAXPES data with open circuit potential OCP analysis, density functional theory DFT calculations, and Monte Carlo based photoemission spectra simulation using SESSA, the presence of a thin oxide layer at the alpha SnWO4 NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid state interfaces in multi layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splittin

Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin film metal oxide photoanodes for solar water splitting

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, xi lamba , represents the wavelength dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract xi lamba through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo oxidation, and observed that it is self consistent for different illumination conditions and applied potentials. We found agreement between the extracted xi lamba spectrum and the photoconductivity spectrum measured by time resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite s absorption spectrum. Low energy non contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining xi lamb

Author Correction Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin film metal oxide photoanodes for solar water splitting

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, xi lamba , represents the wavelength dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract xi lamba through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo oxidation, and observed that it is self consistent for different illumination conditions and applied potentials. We found agreement between the extracted xi lamba spectrum and the photoconductivity spectrum measured by time resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite s absorption spectrum. Low energy non contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining xi lamb

Different Photostability of BiVO4 in Near pH Neutral Electrolytes

Photoelectrochemical water splitting is a promising route to produce hydrogen from solar energy. However, corrosion of photo electrodes remains a fundamental challenge for their implementation. Here, we reveal different dissolution behaviors ofBiVO4photoanode in pH-buffered borate, phosphate, and citrate (hole-scavenger)electrolytes, studied in operandoemploying an illuminated scanning flow cell. We demonstrate that decrease in photocurrents alone does not reflect the degradation of photo electrodes. Changes in dissolution rates correlate to the evolution of surface chemistry and morphology. The correlative measurements on both sides of the liquid−semiconductor junction provide quantitative comparison and mechanistic insights into the degradation processes

Comparability of Raman Spectroscopic Configurations: A Large Scale Cross-Laboratory Study

This is the final version. Available on open access from the American Chemical Society via the DOI in this recordThe variable configuration of Raman spectroscopic platforms is one of the major obstacles in establishing Raman spectroscopy as a valuable physicochemical method within real-world scenarios such as clinical diagnostics. For such real world applications like diagnostic classification, the models should ideally be usable to predict data from different setups. Whether it is done by training a rugged model with data from many setups or by a primary-replica strategy where models are developed on a 'primary' setup and the test data are generated on 'replicate' setups, this is only possible if the Raman spectra from different setups are consistent, reproducible, and comparable. However, Raman spectra can be highly sensitive to the measurement conditions, and they change from setup to setup even if the same samples are measured. Although increasingly recognized as an issue, the dependence of the Raman spectra on the instrumental configuration is far from being fully understood and great effort is needed to address the resulting spectral variations and to correct for them. To make the severity of the situation clear, we present a round robin experiment investigating the comparability of 35 Raman spectroscopic devices with different configurations in 15 institutes within seven European countries from the COST (European Cooperation in Science and Technology) action Raman4clinics. The experiment was developed in a fashion that allows various instrumental configurations ranging from highly confocal setups to fibre-optic based systems with different excitation wavelengths. We illustrate the spectral variations caused by the instrumental configurations from the perspectives of peak shifts, intensity variations, peak widths, and noise levels. We conclude this contribution with recommendations that may help to improve the inter-laboratory studies.COST (European Cooperation in Science and Technology)Portuguese Foundation for Science and TechnologyNational Research Fund of Luxembourg (FNR)China Scholarship Council (CSC)BOKU Core Facilities Multiscale ImagingDeutsche Forschungsgemeinschaft (DFG, German Research Foundation