On the stability enhancement of cuprous oxide water splitting photocathodes by low temperature steam annealing

Given the intermittent nature of solar radiation, the large-scale use of solar energy requires an efficient energy storage solution. So far, the only practical way to store such large amounts of energy is in the form of a chemical energy carrier, i.e., a fuel. Photoelectrochemical (PEC) cells offer the ability to convert solar energy directly into chemical energy in the form of hydrogen. Cuprous oxide (Cu2O) is being investigated for photoelectrochemical solar water splitting since it has a band gap of 2.0 eV with favorable energy band positions for water cleavage; it is abundant and environmentally friendly. A major challenge with Cu2O is its limited chemical stability in aqueous environments. We present a simple and low-cost treatment to create a highly stable photocathode configuration for H-2 production, consisting of steam treatment of the multilayer structures. The role of this treatment was investigated and the optimized electrodes have shown photocurrents over -5 mA cm(-2) with 90% stability over more than 50 h of light chopping (biased at 0 V-RHE in pH 5 electrolyte).

Solar-driven reduction of CO₂

Marcel Roland Schreier

Rapidly increasing levels of atmospheric carbon dioxide and their damaging impact on the global climate system raise doubts about the sustainability of the fossil resource based energy system. Meanwhile, raising living standards and increasing global population lead to an ever growing need for energy. Renewable energy sources are believed to present a solution to these problems with the sheer abundance of solar energy showing particular promise to fulfill the world's energy needs. However, for large scale application of solar energy to be possible, the problem of its storage has to be addressed. The insufficient flexibility of present-day storage technologies has led to the quest for producing solar fuels, centering on hydrogen as a fuel in a prospective hydrogen economy. Nevertheless, the gaseous state, low volumetric energy density and explosive nature of hydrogen makes it a challenging fuel for practical applications. Using solar energy to produce carbon-based liquid fuels solves these challenges, closes the anthropogenic carbon cycle and allows for the continued utilization of existing infrastructures. A promising method for the production of such fuels consists in the photoelectrochemical and electrochemical conversion of carbon dioxide. In this thesis, both methods are investigated using molecular homogeneous catalysts and heterogeneous systems. The photoelectrochemical reduction of carbon dioxide on TiO2-protected Cu2O photocathodes was investigated using a rhenium bipyridyl catalyst in solution. Important charge transport limitations were encountered, which could be overcome by the addition of protic additives to the electrolyte. Improving on this result, the molecular catalyst was covalently immobilized on the TiO2 surface of the photocathode by modifying the bipyridyl ligand with a phosphonate binding group. A nanostructure of TiO2 was needed to support sufficient catalyst to sustain the photocurrent generated by the Cu2O photoelectrode. The complete device showed photocurrents exceeding 2.5 mA cm-2 and large faradaic efficiency for the production of CO. Moving toward heterogeneous catalysis, the promotion of the CO2 to CO conversion reaction on silver surfaces by imidazolium cations was investigated. Replacing the imidazolium C2 proton with a phenyl substituent led to an enhancement of the co-catalytic effect. Replacing the C4 and C5 protons with methyl groups, however, suppressed the catalysis-promoting effect of the imidazolium salt for different C2 substituents and led to new insights into the role of imidazolium. The unassisted solar-driven splitting of CO2 into CO and O2 was demonstrated using water as electron source. This was achieved by the use of a porous gold cathode and an IrO2 anode, driven by three methylammonium lead iodide perovskite solar cells in series. Extended operation over 18 h was shown, achieving a solar to CO efficiency exceeding 6.5 %. Atomic layer deposition (ALD) modification of CuO nanowire cathodes with SnO2 was investigated, leading to striking impacts on the catalytic selectivity of this system. In an aqueous electrolyte, bare CuO led to the production of a wide spectrum of products, which was modified to the production of CO with high selectivity upon ALD modification. By exploiting the oxygen evolving activity of SnO2-coated CuO, a low cost bifunctional system was constructed, achieving sustained solar-driven production of CO with up to 13.4% efficiency.

EPFL2017

On the Texturing and Nanostructuring of Iron Oxide Photoanodes for Solar Water Splitting

Maurin Cornuz

The world needs to think about the after fossil fuel era and while the sun baths the earth with a tremendous amount of energy, man must learn how to harvest and store it in order to dispose of a continuous supply meeting his needs. Water photo-electrolysis is a route toward the conversion of solar energy and its storage into chemical energy. Iron oxide, in the form of hematite, has been identified as a promising material, showing an excellent bandgap coupled with a high stability and abundance. However, its use is impeded by a very short charge carrier lifetime and poor oxygen evolving reaction kinetics. The former point creates the incentive for a fine nanostructuring, allowing the absorption of light close enough to the semiconductor-liquid interface for the photo-generated hole to be able to diffuse and react with water. The kinetics limitations can be overcome by mean of catalysis. In this thesis, I use a technique called atmospheric pressure chemical vapor deposition (APCVD), developed by Kay et al. in 2006 in order to fabricate hematite photoanodes that are highly nanostructured and show record solar-to-hydrogen conversion efficiencies. My work focuses on the improvement of the nanostructure in order to reduce the energy loss by deep light absorption in the material. First, the deposition parameters of the APCVD are optimized in order to improve the crystallinity of the photoanodes. The key parameter modified is the residence time of the precursor in the gas stream above the substrate. The process results in an improved overall solar energy conversion and a better electric conductivity of the material. The further application of the known best oxygen evolving catalyst – Iridium oxide – results in a record photocurrent of 3.0 mA cm−2 at 1.23 V bias vs. RHE for this class of material. In a second phase, the electrodes prepared using the newly developed conditions are analyzed by X-Ray diffraction and I find that the reduction of the residence time leads to an increased orientation of the iron oxide crystal planes. Since hematite presents a strong conduction anisotropy, and the better conducting basal planes are found perpendicular to the substrate, a correlation is made between the improved conductivity of the photoanodes prepared by reducing the residence and the preferential orientation. In order to better control the morphology of the hematite photoanodes, I present strategies of directed nucleation for the particle-assisted chemical vapor deposition. A flat model substrate is prepared as a platform for the elaboration of directing patterns. Gold nanoparticles are homogeneously deposited on the substrate with a controlled surface density. However, the chosen material reveals to be unable to initiate the iron oxide precursor decomposition and the nanoparticles show no effect on the morphology. Another patterning technique – the nanosphere lithography – is employed to create highly ordered nano-islands of a FePt alloy on the flat substrate. The dimensions of the patterning nanospheres used for the lithography controls the size and spacing of the imprints. The deposition of hematite by APCVD is successfully affected by the pattern and the size of the iron oxide nanostructures is controlled by the imprinted substrate. Finally, I combine the knowledge acquired on the directed nucleation and modified deposition conditions in order to use the APCVD in the so-called host-guest strategy. A transparent, conductive and macroporous host is built using templating TiO2 nanoparticles and a conductive niobium doped tin oxide scaffold. The host is then infiltrated by hematite using different combinations of parameters in the APCVD setup. A maximum photocurrent – equivalent to the state-of-the-art hematite host-guest strategy – is reached with a hybrid nanostructure. A dendritic formation growing atop of the macroporous host while Nb-SnO2 is conformally coated with a thin layer of hematite. By mean of parameter tuning in the APCVD system, a selective improvement of the conformal hematite layer is achieved, showing a better performance when the sample is illuminated from the substrate side as well as a modified surface morphology.

EPFL2013

Thermo-Economic Optimisation of Solar Tower Thermal Power Plants

James Spelling

Amidst a backdrop of rapidly increasing world-wide electricity demand, solar thermal power generation shows great potential for supplying the electricity needs of numerous countries in the Sun-belt regions of the world. In these regions, the absence of significant biomass, hydrological or geothermal reserves makes solar thermal power the most promising solution for meeting that most fundamental of demands: energy. Amidst the different options available for harnessing the Sun’s power, solar thermal technologies have been shown capable of producing electricity at the most economically viable rates [32]. However, all currently existing solar thermal power plants have been based around the use of Rankine, or steam turbine, cycles which are limited in the efficiencies they can achieve. In order to make better use of the Sun’s energy, higher efficiency cycles should be used. With their high concentration ratios, solar thermal power tower systems are amongst the best options for making use of the Sun’s potential. As the energy delivery temperature of central receiver systems is higher, they harness the solar radiation at a higher exergy level [4]. This higher temperature also opens the road to the use of more advanced thermodynamic cycles. Hybrid fossil-fuel – solar systems are also imaginable [13], as the energy delivery temperature is compatible with standard gas turbine cycles. Hybrid systems can help mitigate the requirements of thermal energy storage, as well as reducing the perceived risk when investing in new solar technology, which stimulates research and development, as well as providing employment. Recent developments in the field of high temperature volumetric receivers [10] along with rock- based packed-bed storage systems have opened up an interesting possibility. High temperature receivers allow the use of higher-efficiency combined-cycle setups, whereas packed-bed units offer the possibility of cheap storage. Larger storage volumes allow pure-solar systems to extend their power production into the night, making hybrid options less attractive. This is compounded by the common practice of guaranteeing a preferred tariff for electricity sales from pure-solar sources With this in mind, a complete dynamic model of a power plant based on the pure-solar concept has been elaborated. The performance of the setup is evaluated over a range of days, using solar insolation profiles obtained from satellite data. In order to examine different design options and their impact on the performance of the power plant, a multi-objective, thermo-economic optimisation of both the power plant superstructure and operating conditions was performed using the new, dynamic models. By means of an evolutionary algorithm [17], a family of Pareto-optimal points were obtained, representing the trade-off between increased power plant efficiency, and lower levelised energy costs. Through use of optimisation, it was shown that exergetic efficiencies in the region of 21-27% can be achieved, for relatively low power outputs of between 3 and 11 MWe. These configurations correspond with levelised electricity costs in the region of 14-21 UScts/kWhe. In most regions, the use of solar power is encouraged by the guarantee of a preferential tariff for electricity sales from renewable sources. Sale of electricity at a price of around 24 UScts/kWhe [32] should therefore be imaginable, ensuring the economic viability of solar thermal power tower systems. It can be concluded that, when properly designed, solar thermal power plants based on combined cycles are both economically and thermodynamically promising. By raising the efficiency of the plant, the size of the solar collector field is diminished, increasing the otherwise low energy densities of solar power systems. By reducing the levelised electricity costs, solar thermal power plants become more economically viable, accelerating their construction and thus, hopefully, reducing our dependence on fossil-fuels.

2009