9 results found
Bedoya-Lora FE, Hankin A, Kelsall GH, 2019, In situ determination of polysulfides in alkaline hydrogen sulfide solutions, Electrochimica Acta, ISSN: 0013-4686
A method was developed to determine low concentrations of polysulfide ions (Sn2- expressed as zero-valent sulfur) in situ and in the presence of high concentrations (0.5 mol dm-3) of hydrogen sulfide ions, HS-, at pH 14. UV-visible spectrophotometry was used to determine absorbances at 295 and 420 nm using an immersion probe, designed for highly corrosive environments. Three absorbance trends were found, corresponding to three concentration ranges of zero-valent sulfur: low (0 – 1.2 10-3 mol dm-3), medium (1.2 – 3.6 10-3 mol dm-3) and high (3.6 – 10 10-3 mol dm-3). The non-linear dependence of absorbance on concentration over the range studied was due to disproportionation of polysulfides. Determination of these species is well known to be problematic at low concentrations due to the effects of adventitious oxygen in solution, meta-stability and speciation of polysulfide species: S22- – S82-. Oxygen concentrations must be minimised in the inert gas used to de-oxygenate sulfide solutions and for the same reason, their contact with atmospheric oxygen should be minimised. During potentiostatic oxidation of alkaline solutions containing HS- ions in the anolyte of electrochemical reactors incorporating cation-permeable membranes, temporal changes in anolyte absorbance and charge were used to estimate polysulfide concentrations. Charge yields for sulfide to polysulfide oxidation were close to unity, confirming the utility of the technique developed. Molar attenuation coefficients of the predominant polysulfide ions S32- at 420 nm and S42- at 295 nm were also estimated as 289 and 3609 dm3 mol-1 cm-1, respectively, and comparable to values of (190, 206) and (3420, 3690) dm3 mol-1 cm-1 reported previously.
Bedoya Lora FE, Hankin A, Kelsall G, 2017, En route to a unified model for photo-electrochemical reactor optimization. I - Photocurrent and H₂ yield predictions, Journal of Materials Chemistry A, Vol: 5, Pages: 22683-22696, ISSN: 2050-7496
A semi-empirical model was developed for prediction of photocurrent densities and implemented to predict the performance of a photo-electrochemical reactor for water splitting in alkaline solutions, using Sn-doped α-Fe₂O₃ photo-anodes produced by spray pyrolysis. Photo-anodes annealed at different temperatures were characterized using photo-electrochemical impedance spectroscopy, cyclic voltammetry in the presence and absence of a hole scavenger and also the open circuit potential under high intensity illumination. Mott-Schottky analysis was used cautiously to estimate charge carrier concentration and the flat band potential. In addition to overpotential/current distribution and ohmic potential losses, the model also accounts for absorbed photon flux, surface and bulk electron-hole recombination rates, gas desorption, bubble formation and (H₂-O₂) cross-over losses. This allows the model to estimate the total yield of hydrogen, charge and gas collection efficiencies. A methodology is presented here in order to evaluate parameters required to assess the performance of a photo-electrochemical reactor in 1D and 2D geometries. The importance of taking into account bubble generation and gas desorption is discussed, together with the difficulties of measuring charge carrier concentration and electron-hole recombination in the bulk of the semiconductor, which are of major importance in the prediction of photocurrent densities.
Bedoya-Lora FE, Hankin A, Holmes-Gentle I, et al., 2017, Effects of low temperature annealing on the photo-electrochemical performance o tin-doped hematite photo-anodes, Electrochimica Acta, Vol: 251, Pages: 1-11, ISSN: 0013-4686
The effects of post-deposition annealing at 400 and 500 °C on the photo-electrochemical performance of SnIV-doped α-Fe2O3 photo-anodes are reported. Samples were fabricated by spray pyrolysis on fluorine-doped tin oxide (FTO) and on titanium substrates. Photo-electrochemical, morphological and optical properties were determined to explain the shift in photocurrent densities to lower electrode potentials and the decrease of maximum photocurrent densities for alkaline water oxidation after annealing. Annealing at 400 and 500 °C in air did not affect significantly the morphology, crystallinity, optical absorption or spatial distributions of oxygen vacancy concentrations. However, XPS data showed a redistribution of SnIV near SnIV-doped α-Fe2O3 | 1 M NaOH interfaces after annealing. Thus, electron-hole recombination rates at photo-anode surfaces decreased after annealing, shifting photocurrents to lower electrode potentials. Conversely, depletion of SnIV in the α-Fe2O3 bulk could increase recombination rates therein and decrease photon absorption near 550 nm, due to an increased dopant concentration in the semiconductor depletion layer. This accounted for the decrease of maximum photocurrents when electron-hole recombination rates were suppressed using HO2− ions as a hole scavenger. The flat band potential of SnIV-doped α-Fe2O3 remained relatively constant at ca. 0.7 V vs. RHE, irrespective of annealing conditions.
Hankin A, Bedoya-Lora FE, Ong CK, et al., 2016, From millimetres to metres: the critical role of current density distributions in photo-electrochemical reactor design, Energy & Environmental Science, Vol: 10, Pages: 346-360, ISSN: 1754-5706
0.1×0.1 m2 tin-doped hematite photo-anodes were fabricated on titanium substrates by spray pyrolysis and deployed in a photo-electrochemical reactor for photo-assisted splitting of water into hydrogen and oxygen. Hitherto, photo-electrochemical research focussed largely on the fabrication, properties and behaviour of photo-electrodes, whereas both experimental and modelling results reported here address reactor scale-up issues of minimising inhomogeneities in spatial distributions of potentials, current densities and the resultant hydrogen evolution rates. Such information is essential for optimising the design and photon energy-to-hydrogen conversion efficiencies of photo-electrochemical reactors to progress their industrial deployment. The 2D and 3D reactor models presented here are coupled with a modified micro-kinetic model of oxygen evolution on hematite thin films both in the dark and when illuminated. For the first time, such a model is applied to a scaled-up photo-electrochemical reactor and validated against experimental data.
Bedoya FE, Bermúdez Á, Castaño JG, et al., 2016, Electrochemical impedance study for modeling the anticorrosive performance of coatings based on accelerated tests and outdoor exposures, Journal of Coatings Technology and Research, Vol: 13, Pages: 895-904, ISSN: 1547-0091
Bedoya-Lora F, Hankin A, Kelsall GH, 2016, Photo-electrochemical Hydrogen Sulfide Splitting using SnIV-doped Hematite Photo-anodes, Electrochemistry Communications, Vol: 68, Pages: 19-22, ISSN: 1388-2481
Spray-pyrolysed SnIV-doped α-Fe2O3 photo-anodes were used for photo-assisted splitting of HS- ions in alkaline aqueous solutions, producing polysulfide (Sn2-) ions together with hydrogen at the cathode. Subsequent aerial oxidation of polysulfide could be used to produce elemental sulfur. At an applied electrode potential of 1.07 V (RHE) and an irradiance of 5.6 kW m-2, stable photocurrents of ca. 11 A m-2 (210-3 A W-1) were recorded over 75 hours, polysulfide concentrations increasing linearly with time. Despite being predicted thermodynamically to form iron sulfide(s) in sulfide solutions, such photo-anodes appeared to be stable. In comparison with conventional water splitting under alkaline conditions, the coupled processes of hydrogen sulfide ion oxidation and water reduction had a lower energy requirement.
Hong J, Chen C, Bedoya FE, et al., 2016, Carbon nitride nanosheet/metal–organic framework nanocomposites with synergistic photocatalytic activities, Catalysis Science & Technology, Vol: 6, Pages: 5042-5051, ISSN: 2044-4753
Heterogeneous photocatalysis plays a key role in the implementation of novel sustainable technologies, e.g. CO2 conversion into fuel, H2 production from water or organics degradation. The progress of photocatalysis relies on the development of tuneable photocatalysts and particularly the ability to build nanocomposites exhibiting synergistic properties with reduced electron–hole recombination rates. We report for the first time the in situ synthesis of nanocomposites of carbon nitride nanosheets (CNNSs) and metal–organic frameworks (MOFs) for application as photocatalysts. This approach leads to the ‘nano-scale mixing’ of the components, thereby enabling a greater performance compared to other types of 2D materials/MOF composites typically obtained via physical mixing. The objective is to take advantage of the complementary features of the materials while forming a heterojunction. The structural, chemical, photophysical and electrochemical properties of the nanocomposites are characterized and compared to those of the parent materials and their physical mixture. The nanocomposites retain the high specific surface area and strong visible light absorbance of MIL-100(Fe). The intimate contact between the CNNSs and the MOF particles is found to promote the electron–hole separation significantly due to the formation of a heterojunction. Hence, more efficient photocatalytic dye degradation is achieved over the composites than the physical mixture.
Bedoya FE, Gallego LM, Bermúdez A, et al., 2014, New strategy to assess the performance of organic coatings during ultraviolet–condensation weathering tests, Electrochimica Acta, Vol: 124, Pages: 119-127, ISSN: 0013-4686
Calderon J, Bedoya F, 2014, Barrier property determination and lifetime prediction by electrochemical impedance spectroscopy of a high performance organic coating, DYNA, Vol: 81, Pages: 97-97, ISSN: 0012-7353
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