Electrode structuring by ultrashort laser pulses : a new tool for the hydrogen economy

Abstract

Our energy system and the energy market have changed radically with the use of energies from the renewable sources solar and wind due to their highly fluctuating power generation. The share of renewable energies in the energy mix can therefore only be further increased, if economically viable solutions for the storage of energy are considered. In the concept of the hydrogen economy, hydrogen will be used as the main energy carrier. The sustainable production of hydrogen achieved by the electrochemical splitting of water into hydrogen and oxygen and their later utilization in fuel cells include the exchange of electrons on an electrode. Due to the fast kinetics of the hydrogen related reactions, the catalysis of the electrochemical oxygen reactions become the actual bottleneck, namely the oxygen evolution reaction—OER (water electrolysis) and the oxygen reduction reaction—ORR (fuel cells). By enlarging the electrochemically usable electrode surface area, the efficiency of these reactions can be increased. In the present thesis, ultrashort laser pulses (shorter than a few picoseconds) were used to generate various surface-rich structures on metal electrodes. For this purpose, the first systematic study of the surface area enlargement of platinum electrodes using the laser-induced surface structures LIPSS, CLP and black metal was examined. Especially, the black metal surface structure exhibited an exceptionally high surface area increase of 1500 times compared to a polished platinum surface. Subsequently, the black metal surface structure was transferred to less expensive electrode materials, especially to nickel. In order to prevent oxidation of the material during the process and therefore to ensure good electrical conductivity of the electrode, the laser structuring was carried out in argon atmosphere. During the investigations, a new highly porous surface structure was discovered and it was called laser-induced nano-foam (LINF). Therefore, a nickel electrode was scanned with a laser beam in a line pattern. Thus, the surface of the LINF nickel electrodes was increased by a factor of 1600. The surface area of the LINF electrodes is adjustable by variation of the distance between the lines. In addition, this thesis shows how ultrashort laser pulses can be used to activate or deactivate certain areas of an electrode surface for gas evolving reactions, e. g. OER. The analysis of the irradiated surfaces with X-ray photoelectron spectroscopy (XPS) showed that the laser treatment produces defect sites close to the surface. The defects promote the formation of catalytically active phases such as the OER-active β-NiOOH phase, which increases wettability and thus facilitates the removal of gas bubbles from the laser-structured electrodes.