Solvent-induced surface hydroxylation of a layered perovskite Sr3FeCoO7−δ for enhanced oxygen evolution catalysis

Efficient oxygen evolution reaction (OER) catalysts made of earth abundant elements are essential for the cost-effective generation of solar fuels. Surface modification is a novel strategy to enhance the activity of OER catalysts, however, it is commonly done under harsh and energy intensive conditions. Herein, we present a facile approach of solvent treatment to hydroxylate the surface of a layered perovskite, Sr3FeCoO7−δ. The catalytic activity correlates with the degree of surface hydroxylation, which influences the absorption energy of intermediates on the surface. The optimized catalyst exhibits higher activity than the current benchmark perovskite OER catalysts. This work introduces a mild and convenient method for surface modification, which may be applicable for the improvement of other nanoscale electrocatalysts.

Coupling of electrocatalysts to photoabsorbers for solar fuels production

Carlos Gilberto Morales Guio

The direct transformation of sunlight into fuels and chemicals has attracted considerable attention for the potential impact on the storage of solar energies at large scales and the reduction of CO2 emissions. More than 80% of our current global energy consumption comes from the burning of non-renewable fossil fuels which produces enormous quantities of CO2 and contributes to global warming. We believe today that the key to reduce our dependence on exhaustible natural resources and limit the emission of greenhouse gases is the transition to CO2-free renewable energy sources. However, harvesting of energy from renewable sources (i.e. solar, wind, tidal) is intermittent and the deployment of devices for their transformation into useful forms of energy (i.e. electricity, chemicals, fuels, biomass) in a global scale is conditioned upon the improvement in efficiency of the energy storage mechanisms. A promising approach to store energy is the use of sunlight to drive the production of hydrogen fuel from water. Hydrogen produced from water and sunlight can be transformed back to electricity on demand or be used to generate mechanical energy in cars to power the transportation industry. The oxidation of hydrogen generates useable energy and at the same time exhaust clean, carbon-free water as the only product. Therefore, the production of hydrogen using a renewable energy source in an efficient and inexpensive device has the potential to discourage the continued use of fossil fuels and improve our environment. The viability of any mechanism for the storage of sunlight as fuel will depend on the reduction of energy penalties during the transformation process. Electrocatalysts are advanced materials that bring into one active site electrons and molecules in the appropriate orientation to lower activation energy barriers, accelerate rate of transformations and improve overall efficiencies for chemical reactions. The best catalysts for water splitting are precious metals such as Pt, Ir and Ru, which show superior rates of reaction at low overpotentials. However, these noble metals are too expensive and scarce for large scale applications. This thesis summarizes our recent research in the preparation, characterization, and application of Earth-abundant electrocatalysts for solar water splitting. Although in the last decade a great number of efficient and inexpensive electrocatalysts have been reported, methods for the deposition of these materials on the surface of photoabsorbers are rarely reported. This work shows various simple and scalable techniques for the deposition of hydrogen and oxygen evolving electrocatalysts on the surface of photocathodes and photoanodes, respectively. Surface-protected photocahodes were activated for solar hydrogen production in alkaline conditions with MoS2+x, Ni-Mo and Mo2C. A novel method for the deposition of an optically transparent amorphous iron nickel oxide oxygen evolution electrocatalyst is also reported. The catalyst was deposited on both thin film and high-aspect ratio nanostructured hematite photoanodes. The low catalyst loading combined with its high activity at low overpotential results in significant improvement on the onset potential for photoelectrochemical water oxidation. This transparent catalyst further enables the preparation of a stable hematite/perovskite solar cell tandem device, which performs unassisted water splitting.

EPFL2016

Mapping of Chemical Heterogeneity at Surfaces with High-Throughput Wide-Field Second Harmonic Microscopy

Igor Nahálka

Many industrial processes are based on electrochemical reactions using engineered electrocatalysts. However, the current lack of theoretical knowledge obscures efforts to maximize catalysts' efficiency, and the optimization process is mostly empirical. To fully understand the structure/chemical function relationship on complex surfaces, new experimental tools are needed that can evaluate electrochemical processes on nm to tens of ÎŒm length scales. These tools have to be surface-specific and ideally label-free in order not to influence the reactions. In this thesis, we apply high-throughput wide-field second harmonic (SH) microscopy together with in-situ electrochemical techniques to i) study the extent of surface heterogeneity, ii) surface changes during electrochemical cycling, and iii) to identify and characterize surface active sites. We first investigated surface heterogeneity at the glass/electrolyte interface with the novel SH microscope developed in our laboratory, as this interface has been widely discussed in the literature with inconsistent results. The spatial resolution of our SH microscope, ~400 nm, together with the temporal resolution of 250 ms, allowed us to spatially resolve the surface pKa,s for the silica deprotonation reaction with values ranging from 2.3 to 10.7. The average value, 6.7, coincides well with the reported values from experimental studies lacking spatial resolution. Furthermore, we demonstrated the ability of SH microscopy to image the orientation of bulk water molecules under an externally-applied electric field in a confined 1 ÎŒm-sized glass pore. We further applied SH imaging coupled in-situ with cyclic voltammetry to characterize the surface of gold electrodes during electrochemical cycling. Analyzing the voltage dependence of the SH signal and utilizing a novel correlation coefficient procedure for wide-field SH imaging, we have identified two types of surface areas on the polycrystalline gold. The first type remains stable during electrochemical cycling, while the second type undergoes surface reconstruction. We assign the second type of gold surface areas to domains of higher roughness, where anion adsorption occurs at lower potentials than expected. Lastly, we focused on studying the oxygen evolution reaction (OER) on polycrystalline gold working electrodes by coupling SH imaging with electrochemical methods. We observe a large spatial heterogeneity of the OER, and we quantify that only < 1 % of the electrode surface is responsible for oxygen production. We further identify two types of active sites. The first type is observed at potentials > 2 V vs. the reverse hydrogen electrode (RHE), which is the onset of the OER. This type is stable under potential cycling, presumably extending multiple layers deep into the gold electrode. The second, anomalous type, is observed at potentials < 2 V vs. RHE. This type is removed by potential cycling, suggesting it involves a structural motif only 1-2 gold layers deep. The findings in this thesis highlight the importance of surface heterogeneity on a sub-ÎŒm level, showing that the surface structure influences the chemical properties and reactivity. This heterogeneity needs to be considered and utilized in the industrial context to establish more efficient electrochemical processes with lower energy requirements.

EPFL2020

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

The main objectives of this work are 2-fold : The development of a novel type of composite material based on carbon nanofibers supported on sintered metal fibers filters (CNF/SMF) and to explore the use of this material as a catalytic support for a model reaction: selective hydrogenation of acetylene. The advantages of this novel support are: Due to carbon nanofibers, it combines a graphitic structure with a high specific surface area without microporosity. Supported carbon nanofibers can be used in a fixed bed reactor without very high pressure drop. Combination of two materials with high thermal conductivity gives a composite which keeps this property. It is suitable for highly exo/endothermic reactions, diminishing temperature gradients in the catalytic bed. The synthesis of carbon nanofibers on SMF filters was carried out by ethane catalytic decomposition at 655°C directly on metallic filters. An oxidation of SMF followed by a reduction in H2 create surface roughness, leading to a larger area for CNF nucleation. The CNF formation mechanism includes several steps: it starts with carbon deposition on metallic surface followed by its diffusion into the metal leading to carbides formation. When the limit of carbon solubility is attained, a layer of carbon is formed on the surface of metallic fibers. High tension within the bulk metal leads to carbide dissociation and by phase separation to graphite formation, which pushes the metallic particles through the carbon layer. Nickel, stainless steel an Inconel (an alloy of mainly nickel, iron and chromium) filters were used. The latter gave the highest uniformity of the carbon nanofibers layer showing an excellent mechanical stability. In order to control carbon deposition on the filters, a formal kinetic analysis of the decomposition reaction was undertaken. Partial orders of 1 for ethane, -2 for hydrogen and an apparent activation energy of 235 kJ/mol were obtained. The observed kinetics suggests strongly that the C – C bond scission is the rate determining step and not carbon diffusion into the metal which is often reported in the literature. The synthesis procedures like time on stream and reaction temperature were optimized: 1 hour of a mixture C2H6:H2:Ar (3:17:80) decomposition over SMFInconel at 655°C lead to ∼6% of carbon, leading to a CNF layer of 1 µm thickness covering the metallic fibers entirely. This material presents a high specific surface area (22.2 m2/g) and a low pressure drop for the gas flowing through. The carbon nanofibers have a diameter between 20 and 50 nm and a crystalline structure with platelets morphology where the graphene layers are stacked one above the other. Their surface is composed almost exclusively of carbon and hydrogen. Amongst the multiple possible applications of this novel composite material, its use as a catalyst support was assessed. Selective hydrogenation of acetylene was carried out over palladium supported on CNF/SMF and compared with commercial supports (ACF, EGF and AGF). The activated carbon fibers cloth (ACF) led to a Pd activity at least twice as E-type glass fibers tissue (EGF) or EGF covered by an alumina layer (AGF). Reaction kinetics was studied for Pd/ACF, leading to acetylene and hydrogen partial orders of -0.5 and 2.4, respectively and apparent activation energy of 50 kJ/mol. The negative partial order for C2H2 suggests high adsorption strength of this compound compared to other reagents on palladium. This particularity allows Pd to be selective. Hydrogenations are known to be structure sensitive. Therefore the catalyst activity for the model reaction depends on Pd particle size. It increases for large particle sizes. The control of this parameter was a priority. Variations of the palladium precursor, of the support and catalyst treatment were carried out. Micro-wave plasma was used since it is fast and has low energy consumption allowing chemically modifying the carbon support surface and activating catalyst after impregnation. This controls not only palladium sintering, but also decomposes tetraaminopalladate more efficiently, resulting in a catalyst activity of one order of magnitude higher. In order to control the palladium deposition on CNF/SMFInconel composite, it is important to have surface functional groups. The activation of carbon nanofibers was carried out with a 4 hours treatment in a boiling aqueous solution of hydrogen peroxide. This led to an amount on surface oxygen containing groups, per mass of carbon, higher than in ACF. Palladium particle size on the composite was controlled by support activation, the use of several metal precursors or different loadings or by a thermal treatment in argon at 500°C. Optimisation of the procedure allowed to obtain in a controlled way an average palladium particle sizes between 3.6 and 25 nm. The activity and selectivity towards ethylene of Pd/CNF/SMFInconel were observed to be higher compared to palladium supported on activated carbon, alumina or barite. Graphitic nature of carbon nanofibers, leading to strong metal-support interaction, was accounted as responsible for these improvements. The high thermal conductivity of the composite CNF/SMF ensured good heat transfer released due to the exothermicity of the reaction and allowed to work close to isothermal conditions in the catalytic bed. Finally it can be concluded, that for the first time carbon nanofibers synthesis was carried out in one step on sintered metal fibers filters, resulting in a novel structured composite material combining different specific properties and having promising potential applications.

EPFL2006