Electrodeposited 3D Nano-Porous High Surface Area Metal Electrodes for Electrocatalytic Cells

Abstract

Over the past decades the use of renewable energy for the conversion of readily available resources to valuable chemicals (power-to-X) was found to be a key factor in enabling the transition to a more sustainable future. This can for example be the reduction of CO2 or N2 to CO, synthetic fuels, formic acid, alcohols, ammonia, and other more complex chemicals. The most prominent type of reaction, however, is the electrolysis of water for the formation of H2, which can serve as a medium for energy storage or as building block for the further conversion to a variety of different molecules. While serving different purposes, all these reactions share the general requirement of an energy efficient conversion to be economically viable. In many areas where green electricity is inexpensive, increasing gas prices already make green hydrogen (from electrolysis) cheaper than grey hydrogen (from steam reforming of methane). In order to reach Europe’s ambitious goal of 2251 TWh of energy consumption that could be covered by hydrogen in 2050 (24% of the total), however, current electrolysis technology is in need of improvements.As in any electrochemical system, the electrodes play an essential role in the efficiency of an electrolysis cell. Some factors that influence the performance of an electrolytic cell are the catalytic activity of the electrodes to reduce the overpotential, the electrochemically active surface area (ECSA), and mass transport of electrolyte and produced gases through the electrodes. The mass transport becomes especially detrimental in polymer electrolyte membrane electrolysis (PEM) and hydroxyl exchange membrane electrolysis (HEM). A zero-gap architecture in these types of cells demands electrodes with an open porosity. HEM allows furthermore the use of more cost-efficient materials, such as nickel, and can thus facilitate the transition to green hydrogen.To meet the above-mentioned requirements for a new generation of electrode materials, we developed 3D nano-porous electrodes with an ECSA of about 26 m2/cm3, leading to an area enhancement of about 130x compared to the geometric electrode area over an electrode thickness of only 5µm. Furthermore, these electrodes offer a tunable high porosity of more than 75% and mechanical stability as a fully freestanding electrode that is given through interconnected nanowires and an integrated porous support structure.We were able to demonstrate the drastically improved performance as compared to classical Ni-foam electrodes in HEM-type electrolyzers. The surface area enhancement in combination with a porosity and tortuosity that facilitate mass transport leads to a low overpotential, even without the application of additional catalytic coatings.While the improved electrochemical behavior is fundamental for the application of novel electrode materials, it alone is not sufficient for their successful application in real systems. Also scalability is an important factor, which can often be difficult to reach when nanomaterials are involved. Especially electrochemical processes, such as anodization of the templates and electrodeposition of the nano-structured electrodes, have to be carefully controlled. Here, we were able to leave the typical lab-scale and bring the 3D nano-porous nickel electrodes to an industrially relevant size. Figure 1