Abstract
Due to the limited availability of noble metal catalysts, such as platinum, palladium, or gold, their substitution by more abundant elements is highly advisable. Considerably challenging is the controlled and reproducible synthesis of stable non-noble metallic nanostructures with accessible active sites. Here, we report a method of preparation of bare (ligand-free) Cu nanostructures from polycrystalline metal in a controlled manner. This procedure relies on heterogeneous localized electrorefining of polycrystalline Cu on indium tin oxide (ITO) and glassy carbon as model supports using scanning electrochemical microscopy (SECM). The morphology of nanostructures and thus their catalytic properties are tunable by adjusting the electrorefining parameters, i.e., the electrodeposition voltage, the translation rate of the metal source and the composition of the supporting electrolyte. The activity of the obtained materials towards the carbon dioxide reduction reaction (CO2RR), oxygen reduction reaction (ORR) in alkaline media and hydrogen evolution reaction (HER), is studied by feedback mode SECM. Spiky Cu nanostructures obtained at a high concentration of chloride ions exhibit enhanced electrocatalytic activity. Nanostructures deposited under high cathodic overpotentials possess a high surface-to-volume ratio with a large number of catalytic sites active towards the reversible CO2RR and ORR. The CO2RR yields easily electrooxidizable compounds – formic acid and carbon monoxide. The HER seems to occur efficiently at the crystallographic facets of Cu nanostructures electrodeposited under mild polarization.
Highlights
The unique properties of nanostructured materials attributed to their increased surface area, altered electronic states, or lattice structure are utilized in many applications
The carbon dioxide reduction reaction (CO2RR) and oxygen reduction reaction (ORR) are the key chemical processes employed in these technologies
We present a method of fabrication of bare copper nanostructures with a tailored morphology and catalytic activity towards the ORR in an alkaline environment and the CO2RR with the generation of compounds, which are electrochemically reoxidizable to CO2 at moderate anodic potentials
Summary
The unique properties of nanostructured materials attributed to their increased surface area, altered electronic states, or lattice structure are utilized in many applications. Two-dimensional Cu microcircuits were fabricated by lateral scanning over an indium-tin oxide (ITO) cathode in a Cu-free electrolyte.[58] Muller et al applied a similar approach with bipotentiostatic control of both electrodes.[60] The Unwin group developed a SICM-based method for the fabrication of three-dimensional Cu structures with a dualchannel nanopipette, with one channel for metal precursor delivery and the second for maintaining a constant distance between the nanopipette and electrodeposited metal.[59] Another micropipette-based method for local electrodeposition has been proposed by Staemmler et al.[61] A capillary lled with Cu salt solution and equipped with auxiliary and reference electrodes was brought close to the substrate working electrode to ensure its contact with the pipette electrolyte This technique, called scanning electrochemical cell microscopy (SECCM),[62] was widely employed by the Unwin group, for local electrodeposition of other metals.[63,64,65,66,67] The same technique was used for visualization of increased activity towards the CO2RR at the grain boundaries of a polycrystalline Au electrode.[68] A very similar approach utilizing scanning meniscus con ned electrodeposition has been applied for the preparation of nanoscale Cu connections,[69] line arrays,[70] and three-dimensional nanostructures.[71]. Micrometer size model samples were fabricated and analyzed at the microscale, one can apply the methodology presented to larger scale fabrication of CuNSs
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