The reduction of protons into dihydrogen is important because of its potential use in a wide range of energy applications. The preparation of efficient and cheap catalysts for this reaction is one of ...the issues that need to be tackled to allow the widespread use of hydrogen as an energy carrier. In this paper, we report the study of an amorphous molybdenum sulfide (MoS x ) proton reducing electrocatalyst under functional conditions, using in situ X-ray absorption spectroscopy. We probed the local and electronic structures of both the molybdenum and sulfur elements for the as prepared material as well as the precatalytic and catalytic states. The as prepared material is very similar to MoS3 and remains unmodified under functional conditions (pH = 2 aqueous HNO3) in the precatalytic state (+0.3 V vs RHE). In its catalytic state (−0.3 V vs RHE), the film is reduced to an amorphous form of MoS2 and shows spectroscopic features that indicate the presence of terminal disulfide units. These units are formed concomitantly with the release of hydrogen, and we suggest that the rate-limiting step of the HER is the reduction and protonation of these disulfide units. These results show the implication of terminal disulfide chemical motifs into HER driven by transition-metal sulfides and provide insight into their reaction mechanism.
The electrochemical CO2 reduction reaction (CO2RR) using Cu-based catalysts holds great potential for producing valuable multi-carbon products from renewable energy. However, the chemical and ...structural state of Cu catalyst surfaces during the CO2RR remains a matter of debate. Here, we show the structural evolution of the near-surface region of polycrystalline Cu electrodes under in situ conditions through a combination of grazing incidence X-ray absorption spectroscopy (GIXAS) and X-ray diffraction (GIXRD). The in situ GIXAS reveals that the surface oxide layer is fully reduced to metallic Cu before the onset potential for CO2RR, and the catalyst maintains the metallic state across the potentials relevant to the CO2RR. We also find a preferential surface reconstruction of the polycrystalline Cu surface toward (100) facets in the presence of CO2. Quantitative analysis of the reconstruction profiles reveals that the degree of reconstruction increases with increasingly negative applied potentials, and it persists when the applied potential returns to more positive values. These findings show that the surface of Cu electrocatalysts is dynamic during the CO2RR, and emphasize the importance of in situ characterization to understand the surface structure and its role in electrocatalysis.
A cobalt-sulfide (Co–S) film prepared via electrochemical deposition on conductive substrates is shown to behave as an efficient and robust catalyst for electrochemical and photoelectrochemical ...hydrogen generation from neutral pH water. Electrochemical experiments demonstrate that the film exhibits a low catalytic onset overpotential (η) of 43 mV, a Tafel slope of 93 mV/dec, and near 100% Faradaic efficiency in pH 7 phosphate buffer. Catalytic current densities can approach 50 mA/cm2 and activity is maintained for at least 40 h. The catalyst can also be electrochemically coated on silicon, rendering a water-compatible photoelectrochemical system for hydrogen production under simulated 1 sun illumination. The facile preparation of this Co–S film, along with its low overpotential, high activity, and long-term aqueous stability, offer promising features for potential use in solar energy applications.
Biological and heterogeneous catalysts for the electrochemical CO2 reduction reaction (CO2RR) often exhibit a high degree of electronic delocalization that serves to minimize overpotential and ...maximize selectivity over the hydrogen evolution reaction (HER). Here, we report a molecular iron(II) system that captures this design concept in a homogeneous setting through the use of a redox non-innocent terpyridine-based pentapyridine ligand (tpyPY2Me). As a result of strong metal–ligand exchange coupling between the Fe(II) center and ligand, Fe(tpyPY2Me)2+ exhibits redox behavior at potentials 640 mV more positive than the isostructural Zn(tpyPY2Me)2+ analog containing the redox-inactive Zn(II) ion. This shift in redox potential is attributed to the requirement for both an open-shell metal ion and a redox non-innocent ligand. The metal–ligand cooperativity in Fe(tpyPY2Me)2+ drives the electrochemical reduction of CO2 to CO at low overpotentials with high selectivity for CO2RR (>90%) and turnover frequencies of 100 000 s–1 with no degradation over 20 h. The decrease in the thermodynamic barrier engendered by this coupling also enables homogeneous CO2 reduction catalysis in water without compromising selectivity or rates. Synthesis of the two-electron reduction product, Fe(tpyPY2Me)0, and characterization by X-ray crystallography, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), variable temperature NMR, and density functional theory (DFT) calculations, support assignment of an open-shell singlet electronic structure that maintains a formal Fe(II) oxidation state with a doubly reduced ligand system. This work provides a starting point for the design of systems that exploit metal–ligand cooperativity for electrocatalysis where the electrochemical potential of redox non-innocent ligands can be tuned through secondary metal-dependent interactions.
Monoclinic scheelite bismuth vanadate (m-BiVO4) is a promising semiconductor photoanode for photoelectrochemical (PEC) water splitting. Despite considerable recent progress in achieving improved ...photocurrents and photovoltages, there remain open questions about the basic optoelectronic properties of this material. Indeed, there is disagreement about the nature of its fundamental bandgap, with theoretical predictions and some experimental observations pointing to an indirect bandgap and other experimental studies to a direct bandgap. Knowledge of this property is critical for understanding light absorption and photocarrier properties, as well as for establishing rational approaches to improved efficiency. Here, experimental spectroscopic techniques are used to resolve this issue and provide a fundamental portrait of the optical properties of the material. Resonant inelastic X-ray scattering proves conclusively that m-BiVO4 is an indirect bandgap semiconductor. These measurements are supported by UV–vis absorption spectroscopy and spectroscopic ellipsometry, which confirm this finding and also indicate the presence of a direct transition located at 200 meV above the indirect one. The spectral dependence of the optical constants is determined by development of a photophysical model for the ellipsometric data. Photogenerated carrier dynamics are probed by transient absorption spectroscopy, which reveals a relatively long lifetime compared to other commonly utilized metal oxide photoanodes and is attributed to the indirect nature of the fundamental gap. The combination of strong visible light absorption and relatively long excited state lifetime provides the basis for the high performance that has been achieved from BiVO4 photoanodes for water splitting.
Transition metal phosphides exhibit high catalytic activity toward the electrochemical hydrogen-evolution reaction (HER) and resist chemical corrosion in acidic solutions. For example, an ...electrodeposited CoP catalyst exhibited an overpotential, η, of −η < 100 mV at a current density of −10 mA cm–2 in 0.500 M H2SO4(aq). To obtain a chemical description of the material as-prepared and also while effecting the HER in acidic media, such electrocatalyst films were investigated using Raman spectroscopy and X-ray absorption spectroscopy both ex situ as well as under in situ and operando conditions in 0.500 M H2SO4(aq). Ex situ analysis using the tandem spectroscopies indicated the presence of multiple ordered and disordered phases that contained both near-zerovalent and oxidized Co species, in addition to reduced and oxygenated P species. Operando analysis indicated that the active electrocatalyst was primarily amorphous and predominantly consisted of near-zerovalent Co as well as reduced P.
A water oxidation catalyst generated via electrodeposition from aqueous solutions containing phosphate and Co2+ (Co−Pi) has been studied by in situ X-ray absorption spectroscopy. Spectra were ...obtained for Co−Pi films of two different thicknesses at an applied potential supporting water oxidation catalysis and at open circuit. Extended X-ray absorption fine structure (EXAFS) spectra indicate the presence of bis-oxo/hydroxo-bridged Co subunits incorporated into higher nuclearity clusters in Co−Pi. The average cluster nuclearity is greater in a relatively thick film (∼40−50 nmol Co ions/cm2) deposited at 1.25 V vs NHE than in an extremely thin film (∼3 nmol Co ions/cm2) deposited at 1.1 V. X-ray absorption near edge structure (XANES) spectra and electrochemical data support a Co valency greater than 3 for both Co−Pi samples when catalyzing water oxidation at 1.25 V. Upon switching to open circuit, Co−Pi undergoes a continuous reduction due to residual water oxidation catalysis, as indicated by the negative shift of the edge energy. The rate of reduction depends on the average cluster size. On the basis of structural parameters extracted from fits to the EXAFS data of Co−Pi with two different thicknesses and comparisons with EXAFS spectra of Co oxide compounds, a model is proposed wherein the Co oxo/hydroxo clusters of Co−Pi are composed of edge-sharing CoO6 octahedra, the structural motif found in cobaltates. Whereas cobaltates contain extended planes of CoO6 octahedra, the Co−Pi clusters are of molecular dimensions.
Vulvovaginal candidiasis (VVC) is a common infection affecting women worldwide. Reports of patterns/risk factors/trends for episodic/recurrent VVC (RVVC) are largely outdated. The purpose of this ...study was to obtain current patient perspectives of several aspects of VVC/RVVC.
Business cards containing on-line survey information were distributed to healthy volunteers and patients seeking standard, elective, or referral gynecologic care in university-affiliated Obstetrics/Gynecology clinics. The internet-based questionnaire was completed by 284 non-pregnant women (78% Caucasian, 14% African American, 8% Asian).
The majority of the participants (78%) indicated a history of VVC with 34% defined as having RVVC. The most common signs/symptoms experienced were itching, burning and redness with similar ranking of symptoms among VVC and RVVC patients. Among risk factors, antibiotic use ranked highest followed by intercourse, humid weather and use of feminine hygiene products. A high number of respondents noted 'no known cause' (idiopathic episodes) that was surprisingly similar among women with a history of either VVC or RVVC. VVC/RVVC episodes reported were primarily physician-diagnosed (73%) with the remainder mostly reporting self-diagnosis and treating with over-the-counter (OTC) medications. Most physician-diagnosed attacks utilized a combination of pelvic examination and laboratory tests followed by prescribed antifungals. Physician-treated cases achieved a higher level of symptom relief (84%) compared to those who self-medicated (57%). The majority of women with RVVC (71%) required continual or long-term antifungal medication as maintenance therapy to control symptoms.
Current patient perspectives closely reflect historically documented estimates of VVC/RVVC prevalence and trends regarding symptomatology, disease management and post-treatment outcomes.
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