Improving O_2 production of WO_3 photoanodes with IrO_2 in acidic aqueous electrolyte

WO_3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO_3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO_3 surface can improve the kinetics for water oxidation and increase the branching ratio for O_2 production. Ir-based OECs were attached to WO_3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO_3 photoanodes in 1 M H_2SO_4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO_3/electrolyte interface. Sputtering of IrO_2 layers on WO_3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O_2 yield at 1.2 V vs. RHE from ~0% for bare WO_3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO_2/WO_3 junction, which provided a shunt pathway for electrocatalytic IrO_2 behavior with the WO_3 photoanode under reverse bias. Although other OECs were tested, only IrO_2 displayed extended stability under the anodic operating conditions in acid as determined by XPS

Study of different pretreatments on Spirulina platensis biomass for bioethanol production

Aquatic biomass presents a large variety of compounds that can be used for the production of third generation (3G) biofuels, mainly carbohydrates, lipids, proteins and co-products, which can be obtained and used in the production of biofuels such as bioethanol from rich carbohydrate biomass [1]. Nowadays Spirulina platensis biomass can be considered as an alternative since it has a great capacity to produce carbohydrates [2]. This work presents a study of 3G biorefinery process from Spirulina platensis biomass; diverse types of hydrothermal pretreatments (autoclave 121 °C 20 min; freezing/thawing -4 °C and gelatinization 100 °C 10 min; gelatinization 100 °C 20 min; microwave 121 °C 20 min; ultrasound bath 20 min) and their effects on enzymatic hydrolysis with -amylase and amyloglucosidase in order to obtain fermentable sugars were evaluated. Moreover, two fermentation strategies were evaluated; simultaneous saccharification and fermentation (SSF) and pre-saccharification and fermentation (PSF), the conditions used for the fermentation were pH 4.5, 35 ° C, 150 rpm and Saccharomyces cerevisiae yeast was employed, all strategies were used as alternatives in 3G bioethanol process. Results showed that the pretreatment with autoclave (121 ° C 20 min 5% solids) was better for the cellular breakdown and accessibility of enzymes to cellular matrix in the enzymatic hydrolysis. The treatment of pre-saccharification and fermentation (PSF) with 5 % solids pretreated with autoclave at 121 ° C for 20 min and pre-hydrolyzed with -amylase and amyloglucosidase after fermentation obtained a maximum yield of conversion of glucose to bioethanol of 79.34 %. Simultaneous saccharification and fermentation (SSF) was the best strategy for the obtention of bioethanol from pretreatment biomass of Spirulina platensis with a yield of 81.12 %. These results are good since there are no previously reported studies of the use of SSF for bioethanol from microalgae biomass production.info:eu-repo/semantics/publishedVersio

Microwave Near-Field Imaging of Two-Dimensional Semiconductors

Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS_2 and n- and p-doped WSe_2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants. By further performing tip bias-dependent point spectroscopy together with finite element simulations, we distinguish the effects of the quantum capacitance and determine the local dominant charge carrier species and dopant concentration. These results underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects

Microwave Near-Field Imaging of Two-Dimensional Semiconductors

Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS_2 and n- and p-doped WSe_2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants. By further performing tip bias-dependent point spectroscopy together with finite element simulations, we distinguish the effects of the quantum capacitance and determine the local dominant charge carrier species and dopant concentration. These results underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects

Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS_2 and MoSe_2

Single crystals of n-type MoS_2 and n-MoSe_2 showed higher electrocatalytic activity for the evolution of H_2(g) in alkaline solutions than in acidic solutions. The overpotentials required to drive hydrogen evolution at −10 mA cm^(–2) of current density for MoS^2 samples were −0.76 ± 0.13 and −1.03 ± 0.21 V when in contact with 1.0 M NaOH(aq) and 1.0 M H_2SO_4(aq), respectively. For MoSe_2 samples, the overpotentials at −10 mA cm^(–2) were −0.652 ± 0.050 and −0.709 ± 0.073 V in contact with 1.0 M KOH(aq) and 1.0 M H_2SO_4(aq), respectively. Single crystals from two additional sources were also tested, and the absolute values of the measured overpotentials were consistently less (by 460 ± 250 mV) in alkaline solutions than in acidic solutions. When electrochemical etching was used to create edge sites on the single crystals, the kinetics improved in acid but changed little in alkaline media. The overpotentials measured for polycrystalline thin films (PTFs) and amorphous forms of MoS_2 showed less sensitivity to pH and edge density than was observed for single crystals and showed enhanced kinetics in acid when compared to alkaline solutions. These results suggest that the active sites for hydrogen evolution on MoS_2 and MoSe_2 are different in alkaline and acidic media. Thus, while edges are known to serve as active sites in acidic media, in alkaline media it is more likely that terraces function in this role

A scanning probe investigation of the role of surface motifs in the behavior of p-WSe_2 photocathodes

The spatial variation in the photoelectrochemical performance for the reduction of an aqueous one-electron redox couple, Ru(NH_3)_6^(3+/2+), and for the evolution of H_2(g) from 0.5 M H_2SO_4(aq) at the surface of bare or Pt-decorated p-type WSe_2 photocathodes has been investigated in situ using scanning photocurrent microscopy (SPCM). The measurements revealed significant differences in the charge-collection performance (quantified by the values of external quantum yields, Φ_(ext)) on various macroscopic terraces. Local spectral response measurements indicated a variation in the local electronic structure among the terraces, which was consistent with a non-uniform spatial distribution of sub-band-gap states within the crystals. The photoconversion efficiencies of Pt-decorated p-WSe_2 photocathodes were greater for the evolution of H_2(g) from 0.5 M H_2SO_4 than for the reduction of Ru(NH_3)_6^(3+/2+), and terraces that exhibited relatively low values of Φ_(ext) for the reduction of Ru(NH_3)_6^(3+/2+) could in some cases yield values of Φ_(ext) for the evolution of H_2(g) comparable to the values of Φ_(ext) yielded by the highest-performing terraces. Although the spatial resolution of the techniques used in this work frequently did not result in observation of the effect of edge sites on photocurrent efficiency, some edge effects were observed in the measurements; however the observed edge effects differed among edges, and did not appear to determine the performance of the electrodes

Methods of photoelectrode characterization with high spatial and temporal resolution

Materials and photoelectrode architectures that are highly efficient, extremely stable, and made from low cost materials are required for commercially viable photoelectrochemical (PEC) water-splitting technology. A key challenge is the heterogeneous nature of real-world materials, which often possess spatial variation in their crystal structure, morphology, and/or composition at the nano-, micro-, or macro-scale. Different structures and compositions can have vastly different properties and can therefore strongly influence the overall performance of the photoelectrode through complex structure–property relationships. A complete understanding of photoelectrode materials would also involve elucidation of processes such as carrier collection and electrochemical charge transfer that occur at very fast time scales. We present herein an overview of a broad suite of experimental and computational tools that can be used to define the structure–property relationships of photoelectrode materials at small dimensions and on fast time scales. A major focus is on in situ scanning-probe measurement (SPM) techniques that possess the ability to measure differences in optical, electronic, catalytic, and physical properties with nano- or micro-scale spatial resolution. In situ ultrafast spectroscopic techniques, used to probe carrier dynamics involved with processes such as carrier generation, recombination, and interfacial charge transport, are also discussed. Complementing all of these experimental techniques are computational atomistic modeling tools, which can be invaluable for interpreting experimental results, aiding in materials discovery, and interrogating PEC processes at length and time scales not currently accessible by experiment. In addition to reviewing the basic capabilities of these experimental and computational techniques, we highlight key opportunities and limitations of applying these tools for the development of PEC materials

Operando Synthesis of Macroporous Molybdenum Diselenide Films for Electrocatalysis of the Hydrogen-Evolution Reaction

The catalytically inactive components of a film have been converted, through an operando method of synthesis, to produce a catalyst for the reaction that the film is catalyzing. Specifically, thin films of molybdenum diselenide have been synthesized using a two-step wet-chemical method, in which excess sodium selenide was first added to a solution of ammonium heptamolydbate in aqueous sulfuric acid, resulting in the spontaneous formation of a black precipitate that contained molybdenum triselenide (MoSe_3), molybdenum trioxide (MoO_3), and elemental selenium. After purification and after the film had been drop cast onto a glassy carbon electrode, a reductive potential was applied to the precipitate-coated electrode. Hydrogen evolution occurred within the range of potentials applied to the electrode, but during the initial voltammetric cycle, an overpotential of ~400 mV was required to drive the hydrogen-evolution reaction at a benchmark current density of −10 mA cm^(–2). The overpotential required to evolve hydrogen at the benchmark rate progressively decreased with subsequent voltammetry cycles, until a steady state was reached at which only ~250 mV of overpotential was required to pass −10 mA cm^(–2) of current density. During the electrocatalysis, the catalytically inactive components in the as-prepared film were (reductively) converted to MoSe_2 through an operando method of synthesis of the hydrogen-evolution catalyst. The initial film prepared from the precipitate was smooth, but the converted film was completely covered with pores ~200 nm in diameter. The porous MoSe_2 film was stable while being assessed by cyclic voltammetry for 48 h, and the overpotential required to sustain 10 mA cm^(–2) of hydrogen evolution increased by <50 mV over this period of operation