Electrochemical reduction of carbon dioxide on gas diffusion electrodes : optimization of electrode preparation and reaction conditions

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Electrochemical investigations in low conductivity organic solvents using microelectrodes

Alain Fankhauser

Electrochemical acetoxylation has been widely investigated about 40 years ago. However, there are only a limited number of studies in non-aqueous electrolytes, mainly because of difficulties related with the extremely low electrical conductivity of organic media. This work focuses on electrochemistry in organic media and deals with the problem of low conductivity by using ultramicroelectrodes which allow neglecting the ohmic drop. Precise electrochemical measurements were thus accessible and electrochemical acetoxylation of aromatic compounds in a glacial acetic acid media has been investigated using three electrode materials: platinum (electrocatalytic), graphite (industrial electrodes) and boron-doped diamond (BDD) (non-electrocatalytic). In the first part of this work, a glacial acetic acid medium has been characterized on the three electrode materials. Strong similarities in the behavior of the electrodes in glacial acetic acid and in aqueous electrolytes have been outlined. Furthermore an oxidation mechanism of acetate depending on the electrode material has been proposed. Methane and methyl acetate are the main side-products formed during this oxidation but only in negligible quantities. The proposed mechanisms for the formation of these side-products are based on a model similar to that for the oxidation of organic substrates in aqueous solutions on BDD. In the second part of this work, the electrochemical behavior of p-methylanisole (p-MA) in acetonitrile has been investigated. It has been shown that two oxidation peaks appear on the three electrode materials. The main anodic reaction of p-MA is polymerization and both peaks are related to this reaction. The first peak is attributed to the oxidation of p-MA to the corresponding radical cation, whereas the second one is supposed to correspond to the oxidation of a cationic intermediate. Finally it has been shown that the electrode deactivation due to the deposition of polymeric material on the electrode surface occurs at different potentials on the three investigated electrode materials. A mechanism involving two polymerization routes has been proposed. Acetoxylation of substituted aromatic substrates has been investigated using different substrates: p-MA, toluene and nitrobenzene. It has been shown that mainly three types of reactions occur: acetoxylation, methylation and polymerization. Acetoxylation is favored on platinum, while on BDD electrodes polymerization is privileged. Concerning methylation, it has been shown to occur only if the oxidation potential of the substrate is close to that of the solvent. A general reaction scheme depending on the electrode material has been proposed. In the fourth part of this work, acetoxylation of p-MA and toluene using graphite electrodes has been performed in a lab-scale monopolar electrolyzer and in a bench-scale bipolar cell. It has been found that due to the cell design, acetoxylation of p-MA can be conducted at high current efficiencies and at high selectivities even for a conversion as high as 90%. Finally, first milestones have been set for the continuation of this work. A kinetic model based on the previously proposed mechanism of acetate oxidation, acetoxylation and polymerization depending on the electrocatalytic activity of the electrode has been simulated and compared to the obtained results in the first parts of this work. Furthermore, the importance of the supporting electrolyte has been highlighted. Moreover, it has been shown that dimensionally stable anodes (DSA) can be used in glacial acetic acid media. Finally, a bipolar reactor with a sub-mm interelectrode gap and internal recirculation has been constructed.

EPFL2007

Effectiveness factor of thin-layer IrO2 electrocatalyst

Erika Herrera Calderon

Dimensionally Stable Anodes (DSA®) are electrodes composed of a metal support and an oxide coating (catalyst). Particularly in this thesis, the Ti/IrO2 electrode was used for the study of the fraction of the surface which participates effectively in the investigated reactions (effectiveness factor, Ef ). Four reactions with different kinetics have been investigated: (a) Fe3+/Fe2+ (fast reaction), (b) O2 and (c) Cl2 evolutions (slow reactions), and (d) isopropanol oxidation (complex reaction involving redox catalysis). This study has allowed us to apply for the first time in electrocatalysis the term, effectiveness factor, Ef , used before in other fields like in heavy metals recovery with 3D electrodes and in heterogeneous catalysis. On the other hand, in order to achieve the main objective of this thesis, an analytical and a qualitative approach was proposed. Furthermore, the Ti/IrO2 electrodes were characterized electrochemically using the cyclic voltammetry (CV) technique in which the voltammetric charge was measured at different potential scan rates, potential windows, and electrolyte temperatures using various Ti/IrO2 loadings. From the voltammetric charge, two contributions were found. The first contribution is related to the double layer capacitance and the second with the redox couples present on the electrode surface. The first reaction investigated in this work was the Fe3+/Fe2+ redox couple. In the first part of the investigation of this reaction, the CV technique was used; however, some issues related to the uncompensated electrolyte resistance were encountered. Therefore the effectiveness factor for this particular case was not possible to obtain. On the other hand, using rotating Ti/IrO2 disk electrodes (RDE) for the same reaction, the effectiveness factor, Ef , has been estimated, however, this value is much lower than the predicted one. The other investigated reactions in this work were O2 and Cl2 evolutions. The investigated kinetics of both reactions showed that the evolution of chlorine is much faster than that of oxygen. The effectiveness factor, Ef , for the oxygen evolution was close to the unity for all IrO2 loadings, contrary to the chlorine reaction that decreased with the loading. This behaviour was predicted according to what is given in the analytical approach. The last reaction investigated in this work was the oxidation of isopropanol. In this case a model was proposed based on three main reactions: Electrochemical IrO2 oxidation to IrO3, chemical oxidation of isopropanol via IrO3, and chemical decomposition of IrO3 to IrO2. The effectiveness factor, for the electrochemical reaction seemed not to correlate with the loading, whereas for the chemical one, it diminished with the loading.

EPFL2008

Monitoring and understanding Cu catalyst dissolution during CO2 electroreduction

Jan Vávra

Electrochemical reduction of carbon dioxide is a promising approach to decrease the amount of anthropogenic CO2 being released into the atmosphere, provided that a surplus of renewable electrical energy is available. Copper based materials are at the center of attention as a catalyst to this reaction. The exact atomic arrangement of the Cu surface determines the selectivity and activity of the electrochemical reaction. However, recent evidence reports morphology changes of the Cu catalyst surface during the course the reaction. This thesis investigates the operational stability of Cu catalysts with various in-situ techniques, particularly liquid cell TEM, which was adapted to electrochemical CO2 reduction reaction so that it provides morphological and structural information on the catalyst in real-time. Two distinct stages contributing to the reconstruction of Cu catalysts are identified. In the first stage, the catalyst dissolves and spontaneously oxidizes during its contact with the reaction environment. As the reducing potential is applied, the initially dissolved Cu deposits back onto the working electrode. In the second stage, during the catalyst operation at reducing potentials, the dissolution/redeposition continues in the close vicinity of the catalyst surface. Formation of transient [CuCO]+ complexes is identified as the major process responsible for the operational dissolution of Cu catalysts. The provided mechanistic understanding of Cu catalyst (in)stability during electrochemical reduction of CO2 rationalizes the efforts in designing catalysts and electrolyzers with long-term stable operation and motivates further investigation on how to control the operational redeposition of Cu to continuously refresh the active surface motives.