Recently, a convenient method has been suggested to produce a highly porous metal structure such as Cu using a single-step electrodeposition method [1]. In this approach, hydrogen bubbles are used as dynamic templates during the metal electrodeposition process to create the micro-porous structure. However, the pore walls of the porous layers often grow in ramified dendritic structure, which results in limited mechanical stability and creates challenged for applications in compact devices.

In this contribution, a novel approach will be discussed to improve the mechanical stability of these porous structures by employing a second electrodeposition step. This process greatly reduces the number of dendrites within the pore walls making the porous layer more compact and thus improving the mechanical stability significantly. Furthermore, a complete separation of the porous layer from the flat substrate can be achieved by ultrasonication to obtain a free-standing porous Cu framework. Some parameters such as the layer thicknesses and pore sizes can be tuned easily by changing the current density and the electrodeposition time, respectively.

The use of thus prepared free-standing porous Cu frameworks will be discussed for photoelectrochemical (PEC) water splitting and CO₂ reduction. For the PEC water splitting application, a photoactive Cu₂O layer is electrodeposited on the free-standing porous Cu framework. A homogenous coating of the Cu₂O can be achieved and the PEC performance shows a significant increase of photocurrents by more than 80 % at 0 V vs. RHE in comparison to the Cu₂O planar substrate [2]. For the CO₂ reduction application, a large sample with10 cm² geometric surface area was successfully prepared. The sample was used in a custom-made electrochemical cell with a cation exchange membrane as a separator. CO₂ gas in a high humidity environment is transported to the free-standing porous Cu working electrode via a membrane pump in a closed-loop system. Our preliminary results show formation of ethylene and ethane gas using gas chromatography which indicates a successful reduction reaction of the CO₂.

[1] H.C. Shin., M. Liu. Chem Mater 16:5460–5464 2004. Doi:10.1021/cm048887b

[2] M. Kurniawan, M. Stich, M. Marimon, M. Camargo, R. Peipmann, T. Hannappel, A. Bund, J. Mater. Sci. 2021, Doi:10.1007/s10853-021-06058-y