F. I. Lizama-Tzec

Electrodeposition and characterization of nanostructured black nickel selective absorber coatings for solar–thermal energy conversion

Selective coatings consisting of a bright nickel interlayer and black nickel overlayer for solar-to-thermal energy conversion have been electrodeposited onto stainless steel substrates from a nickel(II) chloride bath. During electrodeposition of black nickel, multiple reduction processes combined with oxidative processes result in a complex film composition, containing α-Ni(OH)2, NiOOH, Ni2O3, NiO, water and metallic Ni, as indicated by a detailed X-ray photoelectron spectroscopy study. Reflectance measurements confirm that the selective coatings have a high absorbance in the wavelength range of the solar spectrum, and that the emissivity in the near to mid infrared is low. Electron microscopy showed that the films consist of nanostructured flakes, oriented perpendicular to the surface. The best coatings were obtained by using a two-pulse galvanostatic deposition method.

Mechanisms of Electron Transport and Recombination in ZnO Nanostructures for Dye‐Sensitized Solar Cells

ZnO is an attractive material for applications in dye-sensitized solar cells and related devices. This material has excellent electron-transport properties in the bulk but its electron diffusion coefficient is much smaller in mesoporous films. In this work the electron-transport properties of two different kinds of dye-sensitized ZnO nanostructures are investigated by small-perturbation electrochemical techniques. For nanoparticulate ZnO photoanodes prepared via a wet-chemistry technique, the diffusion coefficient is found to reproduce the typical behavior predicted by the multiple-trapping and the hopping models, with an exponential increase with respect to the applied bias. In contrast, in ZnO nanostructured thin films of controlled texture and crystallinity prepared via a plasma chemical vapor deposition method, the diffusion coefficient is found to be independent of the electrochemical bias. This observation suggests a different transport mechanism not controlled by trapping and electron accumulation. In spite of the quite different transport features, the recombination kinetics, the electron-collection efficiency and the photoconversion efficiency are very similar for both kinds of photoanodes, an observation that indicates that surface properties rather than electron transport is the main efficiency-determining factor in solar cells based on ZnO nanostructured photoanodes.

Influence of morphology on the performance of ZnO-based dye-sensitized solar cells

ZnO nanomaterials with different morphologies, obtained by a sonochemical synthesis method at pH values of 5.5, 8, 10 and 12, have been used as starting materials for the fabrication of dye-sensitized solar cells. The morphology of the nanomaterials and the texture of the films deposited using screen printing depend on the synthesis pH, and the various exposed surface facets interact in a different manner with the dye and electrolyte solutions. The best cell performance was obtained with the morphology that resulted from the synthesis at pH 10, where the {100} and {110} crystal forms are predominant, and where dye coverage was largest. Interestingly, the BET total surface area was lowest for this nanomaterial illustrating the importance of morphology. The influence of the synthesis pH was also evident in the energetics and recombination kinetics of the solar cells. For the ZnO material synthesized at pH 5.5, the band edges appear to be shifted to more negative potentials, which could have resulted in a larger open circuit potential based on thermodynamic considerations. However, the electron life time for the pH 5.5 ZnO material is significantly smaller than for the other three synthesis pH values, indicating that the recombination kinetics are significantly faster for these cells as well, resulting in a smaller open circuit potential based on kinetics arguments. The balance between these two effects determines the experimentally observed open circuit potential. Overall, the results indicate that the dependence of the dye adsorption characteristics on ZnO nanomaterial morphology and film texture are the dominating factors that determine the solar cell performance.

Optical and thermal properties of selective absorber coatings under CSP conditions

Concentrating solar power (CSP) systems use solar absorbers to convert sunlight into thermal electric power. In CSP systems, a high reflective surface focuses sunlight onto a receiver that captures the solar energy and converts it into heat. The operation of high efficiency CSP systems involves improvements in the performance of the coatings of the solar absorption materials. To accomplish this, novel, more efficient selective coatings are being developed with high solar absorptance and low thermal losses at their operation temperature. Heat losses in a CSP system occur by three mechanisms: conduction, convection and radiation. It has been widely documented that energy losses increase with increasing operating temperature of CSP systems, and the precise knowledge of the thermophysical properties of the materials involved in CSP systems may allow us to increase the efficiency of systems. In this work, we applied the pulsed photoradiometry technique (PPTR) to evaluate the changes in the thermophysical properties of selective coatings on a variety of substrates as a function of temperature. Three types of coatings deposited with two different techniques on three types of substrate were examined: commercial coatings based on titanium oxynitride deposited by sputtering on substrates of copper and aluminum, coatings based on black nickel deposited by electrochemical methods on substrates of steel, and coatings based on black cobalt deposited by electrochemical methods on substrates of steel and copper. Values of the thermal diffusivity and thermal conductivity were obtained in the temperature range of 25 to 550 °C. Optical reflectance measurements have been performed in order to provide an estimate of the dependence of the thermal emittance on temperature using the black body radiation theory.Concentrating solar power (CSP) systems use solar absorbers to convert sunlight into thermal electric power. In CSP systems, a high reflective surface focuses sunlight onto a receiver that captures the solar energy and converts it into heat. The operation of high efficiency CSP systems involves improvements in the performance of the coatings of the solar absorption materials. To accomplish this, novel, more efficient selective coatings are being developed with high solar absorptance and low thermal losses at their operation temperature. Heat losses in a CSP system occur by three mechanisms: conduction, convection and radiation. It has been widely documented that energy losses increase with increasing operating temperature of CSP systems, and the precise knowledge of the thermophysical properties of the materials involved in CSP systems may allow us to increase the efficiency of systems. In this work, we applied the pulsed photoradiometry technique (PPTR) to evaluate the changes in the thermophysical prope...