High Surface Area 3D Copper Nanowire Networks for High-Throughput Electrochemical CO2 Reduction

Abstract

An attractive solution towards net-zero carbon emission is the electrocatalytic CO2 reduction with its ability to convert the greenhouse gas CO2 with renewable electricity and appropriate catalytic materials to useful chemicals and fuels to store energy. Depending on the number of electrons transferred a variety of oxygenates and hydrocarbons can be obtained. Among used catalytic materials, such as metals, alloys or composites, copper has shown the unique property to electrocatalytically convert CO2 into a wide variety of valuable C2+ products such as ethylene or alcohols.Apart from the high potential to reshape our carbon economy the electrocatalytic CO2 reduction is still in its early development stage compared to the more mature water electrolysis, with high-throughput operation at practical current densities as well as long-term stability of the catalysts being only scarcely demonstrated. Additionally, selectivity towards high-value (beyond C1) products using copper catalysts has proven to be challenging and many research efforts are directed towards improving product selectivity. For this purpose, mainly nanostructured porous Cu electrodes either based on randomly ordered nanoparticles loaded on porous (carbon-based) supports or low-surface area metal foams, meshes or felts with high porosity are commonly used. While the former electrode architecture typically shows poor long-term stability with often weak adhesion between the nanoparticles and the porous support, the latter electrodes have a low electrochemical surface area and hence are not suitable for high-throughput CO2 reduction at high current densities.In this work we present novel high-surface area porous copper electrodes, so-called copper nanomeshes, that are regular 3D-networks of interconnected Cu nanowires. These unique few µm-thin electrodes show a large surface area enhancement (compared to planar Cu) by a factor of ~80, while providing a high porosity of ~70% together with sufficient mechanical stability, an important aspect towards their practical implementation in electrocatalytic flow cells. Cu nanomesh electrodes with a thickness of 4µm were fabricated through electrochemically plating in 3D-porous anodic aluminum oxide templates and show a mixed surface texture of (111), (100) and (110) Cu. We demonstrate the high potential toward high-throughput CO2 electrolysis of these novel electrodes in comparison to planar copper electrodes in various CO2-containing electrolyte solutions. The CO2 reduction product analysis showed a significant difference in selectivity between planar (polycrystalline or with preferential 111 or 200 texture) and the polycrystalline Cu nanomesh electrodes with CO or C2H4 as major reduction products depending on the potential applied. The beneficial effect of the high electrochemical surface area was demonstrated by a significant increase in the current density on the nanostructured Cu electrodes. Additionally, we characterized the copper nanomesh electrodes before, during and after CO2 reduction with complementary techniques to gain insights on the reaction mechanism and on the electrode stability. Figure 1