Novel inorganic material for hydrogen evolution reaction in electrochemical water splitting

Sunlight is the most abundant renewable source of energy available on earth, providing an amount more than several hundred times sufficient to satisfy human’s need. However, it is diffusive and inconsistent over time, leading to problems related to its harvesting and storage. Devices able to perform splitting of water by the use of sunlight can be used to overcome these problems by producing hydrogen, which is an excellent fuel. Nevertheless, those devices need to be cheap to produce, and therefore an investment in research of earth abundant electrocatalysts for water splitting is necessary to make them available in a larger scale. This thesis focussed on the development of new inorganic catalysts for the half-reaction responsible for hydrogen production known as hydrogen evolution reaction. The prepared catalysts were morphologically characterized by the use of various techniques and their performances were then probed electrochemically. The first synthesis carried out was the preparation of Co-doped molybdenum sulphide materials by hydrothermal synthesis. This catalyst exhibited a disordered structure, however it could be probed by Raman spectroscopy. The second project was to investigate the role of replacing nickel with molybdenum within the Ni2P structure. A MoxNi2-xP series were prepared using the solid-state method, where nickel was selectively substituted. Optimisation of the method allowed the synthesis of phase pure materials by time programmed reduction. Finally, the potential of the nitride class as hydrogen evolution reaction catalysts was investigated with the synthesis of a ternary nitride using a modified Pechini method. In general, all the prepared materials exhibited good electrochemical performances in acidic media, while the overall performance in basic media was lower. It was possible to deposit the Co-doped molybdenum sulphide in a semi-transparent film on a Fluorine-doped SnO2 substrate without the use of any binder. The material exhibited good electrochemical performance in acid media in line with what was already reported in the literature. The compounds belonging to the MoxNi2-xP series were successfully synthesised and showed enhanced catalytic performance proportional to the number of molybdenum within the formula. All the prepared compounds showed a high stability of the performance over time in acidic media. Moreover, the used synthesis strategy had the advantage of being easy to perform, opening up to the possibility of being scaled up for industrial purposes. The obtained ternary nitride exhibited great performance in acidic media if compared to most of the nitrides already reported in the literature. The high activity was tentatively attributed to the zero state of the nickel within the structure. Further theoretical studies, however, will be needed to fully understand the reason behind this enhanced activity. In conclusion, all the catalysts reported in this work showed good electrochemical performance in acidic media similar to what was already reported in the literature, with the additional synthetic advantage of being simple and cheap to produce

An investigation of medium effects on the one and two-dimensional electronic spectroscopy of chlorophyll a

The study of the photosynthetic processes has always had a particular relevance in spectroscopy because of its possible applications in light harvesting and energy storage. Since the light-harvesting pigment-protein complexes have this functional role in nature, many of studies have been focused on understanding photophysical processes occurring in these systems. Our work aims to study the spectroscopic properties of the best known pigment involved in the photosynthesis, Chlorophyll a, and their changes with different solvents. The study of the pigment was conducted by linear spectroscopic studies for water-solvent mixtures, while by the nonlinear technique 2D spectroscopy was applied for pure solvent. The theory and technique of 2D spectroscopy is described. The properties of solvents strongly influence the environment felt by the molecule and its capability of aggregate formation. The solvent properties that were found to be most relevant for our study were viscosity, H-bonding formation and polarity. While the first two were shown to be responsible for a persistent inhomogeneity in the 2D spectra, the kinetics of aggregate formation in water solvent mixtures were found to be influenced by the polarity and the capability of H bonding formation

The direct hydrothermal deposition of cobalt-doped MoS2 onto fluorine-doped SnO2 substrates for catalysis of the electrochemical hydrogen evolution reaction

Metal chalcogenides, and doped molybdenum sulfides in particular, have considerable potential as earth-abundant electrocatalysts for the hydrogen evolution reaction. This is especially true in the case of solar-to-hydrogen devices, where an ability to deposit these materials on transparent substrates is therefore desirable. Hydrothermal methods are perhaps the most common route by which metal chalcogenide materials suitable for the hydrogen evolution reaction are produced. Such methods are simple and scalable, but the direct hydrothermal deposition of metal chalcogenides on transparent oxide electrodes has hitherto never been reported. Such an advance would greatly facilitate the expansion of the field by removing the requirement for separate hydrothermal-synthesis and catalyst-deposition steps. In this paper, we show that the ternary chalcogenide Co2Mo9S26 can be synthesised on a fluorine-doped tin oxide substrate by hydrothermal methods directly from solutions of the simple metal salts. These films display good activity for the hydrogen evolution reaction from acid solution, achieving current densities of 10 mA cm−2 at 260 mV overpotential with a Tafel slope of 64 mV per decade. Moreover, the resulting films can be made to be translucent, a very useful property which would allow light to be transmitted through the catalyst to an underlying light-harvesting array in any solar-to-hydrogen device employing this material at the cathode

Two-Dimensional Electronic Spectroscopy of Chlorophyll a: Solvent Dependent Spectral Evolution

The interaction of the monomeric chlorophyll Q-band electronic transition with solvents of differing physical-chemical properties is investigated through two-dimensional electronic spectroscopy (2DES). Chlorophyll constitutes the key chromophore molecule in light harvesting complexes. It is well-known that the surrounding protein in the light harvesting complex fine-tunes chlorophyll electronic transitions to optimize energy transfer. Therefore, an understanding of the influence of the environment on the monomeric chlorophyll electronic transitions is important. The Q-band 2DES is inhomogeneous at early times, particularly in hydrogen bonding polar solvents, but also in nonpolar solvents like cyclohexane. Interestingly this inhomogeneity persists for long times, even up to the nanosecond time scale in some solvents. The reshaping of the 2DES occurs over multiple time scales and was assigned mainly to spectral diffusion. At early times the reshaping is Gaussian-like, hinting at a strong solvent reorganization effect. The temporal evolution of the 2DES response was analyzed in terms of a Brownian oscillator model. The spectral densities underpinning the Brownian oscillator fitting were recovered for the different solvents. The absorption spectra and Stokes shift were also properly described by this model. The extent and nature of inhomogeneous broadening was a strong function of solvent, being larger in H-bonding and viscous media and smaller in nonpolar solvents. The fastest spectral reshaping components were assigned to solvent dynamics, modified by interactions with the solute

Epidemiological profile and north-south gradient driving baseline systemic involvement of primary Sjogren's syndrome

Objective: To characterize the systemic phenotype of primary Sjögren’s syndrome at diagnosis by analysing the EULAR-SS disease activity index (ESSDAI) scores. Methods: The Sjögren Big Data Consortium is an international, multicentre registry based on worldwide data-sharing cooperative merging of pre-existing databases from leading centres in clinical research in Sjögren’s syndrome from the five continents. Results: The cohort included 10 007 patients (9352 female, mean 53 years) with recorded ESSDAI scores available. At diagnosis, the mean total ESSDAI score was 6.1; 81.8% of patients had systemic activity (ESSDAI score ≥1). Males had a higher mean ESSDAI (8.1 vs 6.0, P < 0.001) compared with females, as did patients diagnosed at <35 years (6.7 vs 5.6 in patients diagnosed at >65 years, P < 0.001). The highest global ESSDAI score was reported in Black/African Americans, followed by White, Asian and Hispanic patients (6.7, 6.5, 5.4 and 4.8, respectively; P < 0.001). The frequency of involvement of each systemic organ also differed between ethnic groups, with Black/African American patients showing the highest frequencies in the lymphadenopathy, articular, peripheral nervous system, CNS and biological domains, White patients in the glandular, cutaneous and muscular domains, Asian patients in the pulmonary, renal and haematological domains and Hispanic patients in the constitutional domain. Systemic activity measured by the ESSDAI, clinical ESSDAI (clinESSDAI) and disease activity states was higher in patients from southern countries (P < 0.001). Conclusion: The systemic phenotype of primary Sjögren’s syndrome is strongly influenced by personal determinants such as age, gender, ethnicity and place of residence, which are key geoepidemiological players in driving the expression of systemic disease at diagnosis.

The Direct Hydrothermal Deposition of Cobalt-Doped MoS2 onto Fluorine-Doped SnO2 Substrates for Catalysis of the Electrochemical Hydrogen Evolution Reaction

Metal chalcogenides, and doped molybdenum sulfides in particular, have considerable potential as earth-abundant electrocatalysts for the hydrogen evolution reaction. This is especially true in the case of solar-to-hydrogen devices, where an ability to deposit these materials on transparent substrates is therefore desirable. Hydrothermal methods are perhaps the most common route by which metal chalcogenide materials suitable for the hydrogen evolution reaction are produced. Such methods are simple and scalable, but the direct hydrothermal deposition of metal chalcogenides on transparent oxide electrodes has hitherto never been reported. Such an advance would greatly facilitate the expansion of the field by removing the requirement for separate hydrothermal-synthesis and catalyst-deposition steps. In this paper, we show that the ternary chalcogenide Co2Mo9S26 can be synthesised on a fluorine-doped tin oxide substrate by hydrothermal methods directly from solutions of the simple metal salts. These films display good activity for the hydrogen evolution reaction from acid solution, achieving current densities of 10 mA cm−2 at 260 mV overpotential with a Tafel slope of 64 mV/decade. Moreover, the resulting films can be made to be translucent, a very useful property which would allow light to be transmitted through the catalyst to an underlying light-harvesting array in any solar-to-hydrogen device employing this material at the cathode

UV Photodissociation of Pyrroles: Symmetry and Substituent Effects

H (Rydberg) atom photofragment translational spectroscopy and ab initio electronic structure calculations are used to explore ways in which ring substituents affect the photofragmentation dynamics of gas phase pyrroles. S₁ ← S₀ (σ* ← π) excitation in bare pyrrole is electric dipole forbidden but gains transition probability by vibronic mixing with higher electronic states. The S₁ state is dissociative with respect to N–H bond extension, and the resulting pyrrolyl radicals are formed in a limited number of (nontotally symmetric) vibrational levels (Cronin et al. <i>Phys. Chem. Chem. Phys.</i> <b>2004</b>, <i>6</i>, 5031–5041). Introducing σ-perturbing groups (e.g., an ethyl group in the 2-position or methyl groups in the 2- and 4-positions) lowers the molecular symmetry (to <i>C</i>_<i>s</i>), renders the S₁–S₀ transition (weakly) allowed, and causes some reduction in N–H bond strength; the radical products are again formed in a select subset of the many possible vibrational levels but all involve in-plane (<i>a</i>′) ring-breathing motions as expected (by Franck–Condon arguments) given the changes in equilibrium geometry upon σ* ← π excitation. The effects of π-perturbers are explored computationally only. Relative to bare pyrrole, introducing an electron donating group like methoxy (at the 3- or, particularly, the 2-position) is calculated to cause a ∼10% reduction in N–H bond strength, while CN substitution (in either position) is predicted to cause a substantial (∼3000 cm^–1) increase in the S₁–S₀ energy separation but only a modest (∼2%) increase in N–H bond strength