Active sites on copper-based catalysts for electrochemical CO2 reduction selectively to C2+ products

Electrochemical CO2 reduction (eCO2RR) towards value-added chemicals, powered by renewable electricity, is a promising technology for storing the intermittent renewable energy in the form of chemical bonds. Among the various products of eCO2RR, multi-carbon (C2+) molecules, such as ethylene and ethanol, are particularly interesting because they can be either important commodity chemicals or liquid fuels compatible with the current storage and transport facilities. Production of C2+ molecules from eCO2RR still suffers from low selectivity, reactivity, and stability, mainly due to the lack of proper catalysts and optimal reaction conditions. The main objective of this thesis is to develop advanced copper catalysts for and understand the eCO2RR process. First, to reveal the active sties on monometallic copper catalysts for eCO2RR to C2+ products, we used copper phthalocyanine as a precursor, which contains a single copper atom in each molecule, allowing us to observe the evolution of single copper atoms to nanoclusters to nanoparticles. We found that the average size of the copper nanoparticles was correlated to the selectivity of the C2+ products and identified the grain boundaries as the active sites for C2+ products. In addition, the reaction intermediates, such as CO, were found to accelerate the reconstruction of the copper nanoparticles.Next, we investigated the active sites of copper-based bimetallic catalysts. We designed and synthesized Ag@Cu and Ag@C@Cu core-shell nanoparticles to control the interface formation between silver and copper, making it possible to distinguish between the tandem and interface effects. By using this specially designed bimetallic catalysts, we found that the high local concentration of CO, generated by the eCO2RR on silver, suppressed the production of ethylene but improved the selectivity of ethanol compared with that on monometallic copper. These results indicate that the tandem effect in bimetallic catalysts for eCO2RR has the potential to switch the dominant C2+ product from ethylene to ethanol.Afterwards, we developed self-supported copper-based gas diffusion electrodes and achieved a current density up to 300 mA cm-2 with a high selectivity for C2+ products (> 40%). The self-supported gas diffusion electrodes also allowed us to compare the selectivity and activity of eCO2RR directly on the same catalysts in the batch and flow cells. This comparison revealed that the copper catalyst in the batch cell was limited by the mass transfer of CO2. However, the Cu catalyst in the flow cell produced a much higher selectivity of C2+ products at a higher current density due to the fast CO2 diffusion and high pH. The electrode design strategies and the experimental findings presented in this section are valuable for the development of other self-supported electrodes for practical applications.Finally, we found that more than 30% of the liquid products from eCO2RR passed through the gas diffusion electrode and anion exchange membrane (AEM) at commercial current densities from 50 mA cm-2 to 300 mA cm-2. Volatile products pass through the gas diffusion electrode mainly by evaporation, and the migration rate increases linearly with the applied current density and CO2 flow rate. Non-volatile products prefer to cross the AEM through electromigration, and the migration rate is affected by the current density and the flow rate of the catholyte.

Self and induced stabilized nanoparticles for electrocatalysis

Mériadec Limat

Since few decades the search of new catalysts formulations for fuel cell applications has motivated numerous projects especially due to the future lack of fossil fuel, environmental and economical reasons. Consequently, Direct Alcohol Fuel Cell (DAFC) or Direct Formic Acid Fuel Cell domains have received great attention. In spite of the promising results reported recently, these types of fuel cell still exhibit some problems. In fact, platinum, bulk or nanoparticles, undergoes surface poisoning involved by intermediates (as acetaldehyde, acetic acid, carbon monoxide) which are strongly bonded to the surface. These poisoning species block the active sites at the surface electrode and drastically decrease the catalytic activity and also the life-time of the cell. Probable solutions are suggested essentially in Pt-based alloys catalysts or nanocatalysts in order to avoid this phenomenon. Therefore, multicomponent catalysts deposited at carbon electrodes (Boron-Doped Diamond and Highly Oriented Pyrolytic Graphite) has been investigated during this thesis. These two substrates have been chosen due to their outstanding properties. In the first part of this study, self stabilized nanoparticles at BDD substrate have been studied. Ex-situ ternary Pt-Ru-Sn microemulsion-synthesized nanoparticles deposited at BDD substrate were investigated towards both methanol and ethanol electrooxidation. Such type of composite electrode seemed to possess a greater efficiency towards methanol oxidation than observed for ethanol, leading essentially for the latter to C2 oxidation products. The inability of this catalyst to activate the C-C bond scission can explain this observation. However, the major enhancement was observed for the methanol electrooxidation. Recent studies have shown that gold nanoparticles exhibited surprising catalytic activity, very different from the bulk, making them a privileged candidate to replace or be added to platinum. Gold nanoparticles prepared with a twofold technique (sputtering followed by a heat treatment in air) have been characterized with both outer and inner-sphere redox reactions. It has been observed that the resulting electrochemical behavior of such type of composite electrode was similar to a gold microelectrodes array, due to the high difference between the two material electrochemical rate constants. In a morphological point of view, the processes of nucleation and growth engaged in the stabilized and well-dispersed gold nanoparticles formation have especially attracted a great attention. At BDD surface, the gold nanoparticles were located at specific sites (grain boundaries, defects, impurities,…). This phenomenon has motivated further investigations at HOPG substrate, known to possess a quite smooth and perfect surface. In the second part of this thesis, induced stabilized nanoparticles at HOPG surface were studied. In a first time, HOPG was morphologically and electrochemically characterized. The surface state has appeared to be a key parameter, essentially with the presence of the two surface species, the edge plane and the basal plane. High modifications in the electrochemical behavior of HOPG electrode were observed after an anodic pretreatment in acidic media, due to surface concentration increase of the edge plane species, which are highly reactive. Based on these results a tailoring of the HOPG has been performed in order to create nanocontainers for a further gold deposition. HOPG surface has been bombarded by gold clusters and then oxidized (etching step) under well-defined experimental conditions, leading in a tailored pattern of nanopits with high reliability and reproducibility. Gold deposition was performed by evaporation. The gold nanoparticles were located in the created nanocontainers and were found to be stable under cyclic voltammetric measurements in acidic media. A new synthesis method for a bimetallic or alloy system formed by Au and Pt nanoparticles stabilized at tailored HOPG surface was developed. Pt electrodeposited at Au/HOPG electrode surface have been observed to preferentially deposit on Au. In fact, gold nanoparticles were used as nucleation sites for the platinum particles. The resulting Pt-Au/HOPG electrodes were then heated at 400 °C in vacuum conditions to improve interactions between both metals. The appearance of an alloy was observed and characterized by a negative shift in the peak potential of Pt oxides reduction. Thanks to these results, Pt-Au/HOPG composite electrodes were applied for electrocatalytic oxidation. At a surface composition of P55-Au45 alloy, Pt-Au/HOPG after the heat treatment has demonstrated a higher electrocatalytic activity than before the heat treatment towards formic acid electrooxidation. It seemed that electronic and geometrical effect was responsible of such enhancement. Finally, bismuth adsorption at both P55-Au45/HOPG and polycrystalline platinum was investigated towards formic acid electrooxidation. The effect of Bi adatoms has been essentially ascribed to geometrical restrictions for CO to adsorb at platinum active sites. Drastic enhancement in current density at a fixed potential and dramatic decrease of the onset potential of formic acid oxidation were observed. Thus, it seemed that avoiding the CO poisoning can be the most judicious development for new electrocatalysts in fuel cell applications.

EPFL2009

Efficient Base-Metal NiMn/TiO2 Catalyst for CO2 Methanation

Wei Chen, Emanuele Moioli, Min Zhang, Andreas Züttel

Energy storage solutions are a vital component of the global transition toward renewable energy sources. The power-to-gas (PtG) concept, which stores surplus renewable energy in the form of methane, has therefore become increasingly relevant in recent years. At present, supported Ni nanoparticles are preferred as industrial catalysts for CO2 methanation due to their low cost and high methane selectivity. However, commercial Ni catalysts are not active enough in CO2 methanation to reach the high CO2 conversion (>99%) required by the specifications for injection in the natural gas grid. Herein we demonstrate the promise of promotion of Ni by Mn, another low-cost base metal, for obtaining very active CO2 methanation catalysts, with results comparable to more expensive precious metal-based catalysts. The origin of this improved performance is revealed by a combined approach of nanoscale characterization, mechanistic study, and density functional theory calculations. Nanoscale characterization with scanning transmission electron microscopy-energy- dispersive X-ray spectroscopy (STEM-EDX) and X-ray absorption spectroscopy shows that NiMn catalysts consist of metallic Ni particles decorated by oxidic Mn2+ species. A mechanistic study combining IR spectroscopy of surface adsorbates, transient kinetic analysis with isotopically labeled CO2, density functional theory calculations, and microkinetics simulations ascertains that the MnO clusters enhance CO2 adsorption and facilitate CO2 activation. A macroscale perspective was achieved by simulating the Ni and NiMn catalytic activity in a Sabatier reactor, which revealed that NiMn catalysts have the potential to meet the demanding PtG catalyst performance requirements and can largely replace the need for expensive and scarce noble metal catalysts.

2019

Self and induced stabilized nanoparticles for electrocatalysis

Mériadec Limat

EPFL2009