The native oxide skin of liquid metal Ga nanoparticles prevents their rapid coalescence during electrocatalysis

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke-poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs drive the electrochemical CO2 reduction reaction (CO2RR) while remaining well-separated from each other. The combination of electrochemical, microscopic and spectroscopic techniques, including operando X-Ray absorption, indicates that the unique properties of the native oxide skin of the Ga NPs account for their resistance to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.

Promotion électrochimique des catalyseurs à base de rhodium et d'iridium

Justyna Eaves

In this thesis, the electrochemical promotion of lr and Rh based catalysts was studied for the complete combustion of ethylene and propylene. The study was performed by using two types of electrocatalytical cells: a classical, pellet type cell and a tubular cell. This work has two major goals. The first one is the optimization of the employed electrocatalytical systems from electrochemical point of view. The second one concerns the study of the mechanism of the electrochemical promotion. The major part of this work concerns the electrochemical characterization of two cell types: a pellet type cell and a tubular cell. On the basis of the obtained results the modélisation of the potential and current distribution was performed, providing key parameters for the development of more efficient cell configurations. On one hand, these configurations ensure an uniform current distribution on the working electrode and enable correct catalyst potential measurement. On the other hand, it minimizes the current bypass in the bipolar tubular cell. The aim of the investigation in a two-compartment reactor was to evaluate and to compare the Au and Ir electrodes used in this work. Under 400°C, gold was found to be much worse material than lrÛ2 both as reference and counter electrode. Its potential was affected by oxygen and propylene adsorption and no equilibrium of electrochemical reaction was obtained at such low temperature. On the base of this knowledge, several materials better adapted to electrochemical promotion conditions have been proposed. Using the two-compartment reactor potential measurements under steady state conditions allowed to verify the mechanism of the propylene combustion on Rh and that of ethylene combustion on lr based catalysts proposed in the literature. Ameliorations of the existent kinetic models were proposed. The kinetic measurements were then analyzed with the additional help of ex situ XPS analysis. This study allowed to link the oxidation state of the catalyst surface with the extent of electrochemical promotion of the catalysts. It also allowed the ex situ observation of the appearance of new oxygen (oxide) species on the gas exposed surface of Ir catalyst. This new type of oxygen was different from the oxygen of Ir02 oxide, and it can represent the promoting species, according to backspiïïover theory. The same oxygen species was observed on well oxidized Ir02 catalyst surface, even before the polarization of this catalyst. Its presence was associated to the catalyst-support effect. It explains the high activity of Ir02 catalyst by comparison with the catalytic activity of lr metal. XPS analysis of Rh based catalysts indicated the possibly formation of an inferior Rh oxide (close to RhO) due to polarization. This may be a possible explanation of the observed permanent electrochemical promotion. Voltammetric charge measurements in very narrow potential window were performed while changing the catalyst thickness. The charge was demonstrated to be proportional to the catalyst thickness, suggesting the presence of the charged species on the gas exposed catalyst surface. The analogy between this situation and that of the emerged electrodes in liquid state electrochemistry was emphasized. The voltammograms obtained in this study were interpreted in the quite new, original way, which allowed to calculate the capacity of the different catalyst interfaces, the gas/catalyst interface, and the triple phase boundary, in good agreement with reference values from impedance spectroscopy measurements. Finally, the reaction rate transients during galvanostatic polarization of lr based catalyst have been moralized. The model fits well the experimental results.

EPFL2004

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

Using petrol as a main source of energy, during the 20th century, has caused considerable amounts of damage among which are atmospheric pollution and global warming. At the same time, many geologists and economists expect that the discovery rate of new petrol sources will not follow the market's demand causing a serious shortage of petrol in the near future. These alarming perspectives have motivated scientists around the world to develop new clean and renewable energy sources. It is in that objective that electrochemists intensified their research in order to develop the fuel cell technology. This latter consists in directly converting chemical energy into electricity. The main advantage of this technology over traditional energy production is that the fuel cell energy efficiency is Carnot cycle independent. The theoretical efficiency of such an energy source is nearly 90%. Direct Alcohol Fuel Cell (DAFC), fed by ethanol on one hand and by air (oxygen) in the other hand, is a very interesting option for such kind of energy production. Ethanol is a renewable energy source of vegetal origin whose CO2 production is consumed by plants during the cycle. Even though Pt has long been known to be a perfect catalyst for DAFC, it has some principal limitations, the most important ones being poisoning by intermediate products (OH on the cathodic side and CO on the anodic side) and a relatively high cost compared to other materials. These problems have pushed the experts in the field to focalize their research on the development of more suitable and better performing materials. Gold nanoparticles have shown a surprising catalytic activity, very different from the bulk, making them a preferential candidate to replace Pt. A system based on using two or more metals as catalysts (example Au-Pt) dispersed on an appropriate substrate seems to be also an interesting candidate to enhance the cell's efficiency. The substrate should be chemically inert, non-oxydable and should not present any interference with the electrochemical and electrocatalytic behaviour of the nanoparticles. In the first part, the employed substrate, which is boron doped diamond (BDD) is characterized. The substrate's main characteristics are its chemical inertness, its weak residual current and its resistance to corrosion. These characteristics contribute to making the substrate an ideal support for nanoparticles. In this work, the substrate is a diamond film, but for fuel cell applications diamond should be used as powder or dispersed particles. In chapters III and IV, we studied the effect of boron incorporation on the morphology and the behaviour of the diamond film. For this purpose, samples with different ratios of boron/carbon (B/C) in the vapour phase during preparation were characterized (B/C = 300, 1000, 2500 and 6000 ppm). Results showed that the grain size decreases with increasing boron content in the film. This is accompanied by a transition towards a quasi-metallic character. It was also found that graphitic impurities in the diamond film increase when the B/C ratio increases. We also established some graphitic impurities increase in the diamond film when the ratio of B/C increases. The identity of these impurities was established in chapter III as being surface redox couples (semi-quinone/semi-hydroquinone). Using cyclic voltametry we were able to observe a total elimination (B/C=300 ppm) or partial elimination (B/C=6000 ppm) of these surface redox couples. We also found in chapter IV that these surface redox couples are responsible of the electrochemical activity of the BDD electrode. BDD electrodes with different doping level B/C=300, 1000 and 6000 ppm) in the vapour phase are modified with IrO2 nanoparticles using the method of thermal decomposition. Our results showed that, at low potentials, the behaviour of the composite electrode IrO2/BDD with respect to the reaction of oxygen evolution is independent of the doping level. On the other hand, at high potentials, the weakly doped electrode (B/C=300 ppm) presents a supplementary overpotential due to the interface BDD-IrO2. An approach based on the double junction is developed in this work describing two distinct behaviours depending on the boron concentration in the diamond film. A new method for Au nanoparticles deposition on highly doped BDD electrodes (B/C= 2500 ppm) is presented in chapter VI. This method involves two steps: the first step consists on sputtering the equivalent of few nanolayers of Au on the diamond surface; the second step is a heat treatment in air at high temperatures. Several parameters that could influence the dispersion, the stability as well as the diameter of the Au nanoparticles are studied. In chapter VII, the behaviour of the composite electrode Au/BDD is studied in a solution containing redox couples in acid media. This electrode was simulated as an arrangement of active Au microelectrodes dispersed on a less active BDD substrate. The overall behaviour of the composite electrode (Au/BDD) has been related to the electrochemical rate constants on each material. In chapter VIII, another new synthesis method for a bimetallic system formed by Au and Pt nanoparticles dispersed on a BDD substrate is developed. This method consists in electrodepositing a small amount of Pt on a composite electrode Au/BDD. Our results showed that Pt was preferentially deposited on Au covering its whole surface. In fact, Au nanoparticles serve as germ of nucleation for Pt. In the following step, the bimetallic PtbAu/BDD is heated to 400°C in air to enhance the interaction between Au and Pt. The experiments showed that Au reappeared on the surface. The electrochemical behaviour of the bi-metallic electrode is characterized by a negative shift in the peak potential of reduction of Pt oxides. This is not the case of heterogeneous alloys obtained by the fusion of two metals at high temperatures. The composite electrode Au/BDD as well as the bi-metallic electrode Pt-Au/BDD are used for electrocatalytic applications. The studied reaction in chapter IX is the oxygen reduction reaction (ORR) in an acid medium. We found that the number of exchanged electrons increases with increasing the amount of Pt at the surface. Oxygen is reduced into H2O2 on the Au/BDD electrode and into H2O on the Pt-Au/BDD electrode. In chapter X we studied the reaction of oxidation of oxalic acid. The behaviour of both Au and Pt electrodes seemed very similar for this reaction. The bi-metallic electrode containing both Pt and Au on its surface has shown an intermediate behaviour between the behaviour of the two metals.

EPFL2006

Two-Dimensional Metal-Organic Networks as a New Class of Electrocatalysts

Benjamin Wurster

This thesis deals with the electrocatalytic properties of two-dimensional metal organic coordination networks (2D-MOCNs) self-assembled on electrode surfaces and catalytic activity of 2D-MOCNs is demonstrated for the first time. The low-coordinated metal centers within these supramolecular structures resemble the catalytically active sites of metallo-enzymes. The activity of the metal centers is determined by the type of the metal and the coordination environment; both parameters can be carefully adjusted in 2D-MOCNs. In order to study and correlate electrocatalytic properties with the atomic structure of 2D-MOCNs, a scanning tunnelling microscope operating in ultra-high vacuum (UHV) was combined with an electrochemical (EC) setup. Sample preparation and scanning tunneling microscopy (STM) were conducted under UHV conditions. By STM the composition and structure of the networks were controlled and characterized prior to EC experiments. A transfer system between UHV and EC instrumentation was constructed to keep the sample at all times in a clean and controlled environment. It is demonstrated that the mechanism of the oxygen reduction reaction (ORR) can be influenced by the type of the metal center in the network. The networks formed by benzene-tricarboxylic acid (TMA) with either Fe or Mn atoms are structurally identical, but their electrochemical signal differs significantly. Both networks catalyze the reduction of O2 to H2O, but on different pathways. These results emphasize the key role of the unsaturated metal centers in the electrocatalytic reduction. The electrocatalytic response in the ORR can also altered by the distinct coordination environment; this is shown for Fe atoms nitrogen-coordinated by either tetracyanoquinodimethane (TCNQ), bis-pyridyl-bipyrimidine (PBP) or phythalocyanine (Pc). Depending on the specific coordination environment formed by the different ligands the the final product (H2O2 or H2O) and the mechanism of the ORR is changed. In the last part of this thesis the two metal binding sites of 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) are used to selectively incorporate different metal centers in a fixed organic environment and create homo- and hetero-bimetallic networks. The coupling of the two metals centers promoted by the organic environment modifies the electrocatalytic response. The correct combination of two metal centers in the right positioning makes an efficient catalyst. In the ORR the peak and onset potential is varied depending on the metal combination. In the case of the oxygen evolution reaction a non-linearly increased catalytic activity by the cooperative bimetallic effect is obtained. The results presented in this thesis demonstrate the high potential of 2D-MOCNs for heterogeneous catalytic chemical conversions. Engineering of the network structure and the distinct coordination environment of the metal centers offers the possibility to tune their catalytic activity. This opens up a new route for the design of a new class of nanocatalyst materials.

EPFL2015