Boosting hydrogen production via urea electrolysis on an amorphous nickel phosphide/graphene hybrid structure

Overall urea electrolysis has favorable thermodynamic properties and can complement traditional water splitting by simultaneously purifying urea-rich waste streams. However, the kinetics of the reaction is slow and more efficient catalysts are required. In this work, we describe a facile way to fabricate amorphous nickel phosphides on graphene nanosheets (a-Ni2P/G), which shows high efficient performance for overall urea electrolysis. The a-Ni2P/G catalyst has abundant surface area for abundant catalytic active sites and high proportion of Ni3+ active sites, realizing a superior intrinsic activity for both hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Impressively, the a-Ni2P/G catalyst requires only much lower potentials of 1.28 V and - 0.1 V for UOR and HER at 10 mA cm(-2). The assembled a-Ni2P/G parallel to a-Ni2P/G electrolyzer needs a small cell voltage of 1.39 V to deliver the current density of 10 mA cm(-2). This work highlights a promising pathway to develop a cost-efficient catalyst for energy-saving H-2 generation combined with waste water purification.

Nanostructured Electrocatalysts for Water Splitting Reactions

Lucas-Alexandre Stern

At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.

EPFL2017

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

Emission reduction by targeted transient operations in three-way catalysts for heavy-duty natural gas applications

Moyu Wang

To curb the severe effects of climate change, our society needs to radically reduce its CO2 footprint. For the heavy-duty sector, where electrification is difficult, alternative fuels can be the solution. Methane-fueled engines have lower energy-specific CO2 emissions than conventional Diesel-fueled engines. Furthermore, they have the potential of becoming carbon-neutral, when combined with bio-methane/synthetic methane. However, oxidation of unburnt methane in the exhaust gas poses a challenge for aftertreatment systems. The aim of this thesis is to investigate the mechanism of methane abatement and to reveal methods to reduce methane and other hazardous gas emissions. The majority of the experiments were conducted directly on an engine test bench, which is rarely seen in studies focusing on catalytic reaction pathways. Initial investigations focused on methane abatement under steady state and Î»-step transitions. Under steady state, the presence of oxygen was identified as a prerequisite for methane conversion. Reacting with oxygen is the only methane conversion pathway. However, after step transition from oxygen excess conditions (slightly lean) to oxygen-poor conditions (slightly rich), high methane conversion was observed under rich conditions with no oxygen available. This high conversion was attributed to steam reforming (SR), which was activated by the step transition. The SR reaction rate decreased over time when staying at rich conditions, until full deactivation. Investigations in the lab-scale model gas reactor confirmed this analysis. In addition, the reason for SR deactivation was identified by DRIFTS measurements as the accumulation of carbonates on the catalytic surface, blocking the active sites. Based on the identified importance of the SR reaction, targeted Î» oscillations across stoichiometry were introduced, in order to repeatably activate SR and achieve sustainable high methane conversion. During the rich parts of the oscillations, methane was converted via SR, while, during lean parts, the carbonates were periodically removed from the catalyst surface. With these oscillations, methane conversion has been significantly improved, in comparison to steady state. In parallel, a numerical model has been developed in order to simulate the catalyst behavior under oscillating conditions. The model provided insights on the reaction pathways and their distribution along the catalyst axis. The catalytic activity of the different Platinum-group metals has been investigated for the identified reactions. Various catalysts of different compositions were tested under cold start, Î» oscillations and quasi-steady state conditions. Both Pt and Pd activated SR reactions, however SR attenuation was faster in Pt catalysts. In lean conditions, Pt exhibited higher methane oxidation. Rh was identified as important for enhancing NOx reduction and lowering NH3 emissions. The combination of all three metals has improved the overall catalyst performances. In the final part of the thesis, a special aftertreatment system was investigated. It combines a Pd/Rh catalyst subject to stoichiometric conditions with a Pt oxidation catalyst subject to lean conditions. In the Pd/Rh catalyst, methane was removed via Î» oscillations. In the Pt catalyst, the remaining CO, H2 and NH3 were oxidized. The setup provided a novel perspective in reducing the overall environmental impacts.

EPFL2022