Dottorato in PROCESSI CHIMICI PER L'INDUSTRIA E PER
L'AMBIENTE

Titolo della tesi: Synthesis of copper and zinc-based electrodes by electrodeposition for the catalytic electroreduction of carbon dioxide

The use of renewable energy sources and the detachment from fossil fuels represent the key to the decarbonization process of the modern industrial society. In this scenario, the industrial development of the electrochemical reduction of CO2 (ERCO2) could represent a fundamental step. This process accumulates electric energy as chemical energy, reducing CO2 into light carbon compounds. Cell’s cathode acts as a heterogeneous catalyst, focusing ERCO2 towards the desired product. Catalyst’s chemical composition resulted to play a critical role into reaction selectivity. Amongst the different metals tested as ERCO2 catalysts, copper is known for being the only hydrocarbon-selective metal, while zinc express a strong selectivity towards CO and formic acid. Copper and zinc are non-noble, inexpensive, and non-critical metals, and may be directly manipulated without risk of toxicity for human. Catalyst’s selectivity is also influenced by morphology, that should be controlled during catalyst synthesis. A synthetic approach could be represented by electrodeposition, whose structuring potential is widely documented. This process is easily scalable, may allow polymetallic deposition and does not require complex technical equipment. Electrodeposition of copper and/or zinc was evaluated as a synthesis method of supported catalytic electrodes for the ERCO2 process. A carbon-based, gas permeable, conductive material was chosen as a deposition support and an electrochemical pretreatment of this support was developed and discussed. Copper-based electrodes were electrodeposited from admixed deposition baths. The nature of the additives resulted to be crucial for deposition morphology. Microspheres were obtained by the use of a cationic surfactant, while hemispherical aggregates of nanowires were observed using an organic complexing agent. Deposition current density resulted to affect structures’ dimension. Co-deposition of copper and zinc was attempted using pH-controlled, alkaline deposition bath. Bath’s pH resulted to have an influence on the amount of zinc deposited. Zinc electrodes were deposited by boric acid admixed baths. Most of the deposition conditions resulted in hexagonal microstructures, while an interaction between high levels of zinc precursor and presence of boric acid resulted in the formation of needle-like structures. Catalytic tests were performed on the deposited electrodes to evaluate their selectivity towards the production of carbon monoxide and other ERCO2 products in an aqueous environment. Microspheres and nanowires aggregated resulted to be three to four times more selective towards carbon monoxide than a copper electrode deposited without additives or a non-nanostructured copper foil. No correlation was found between copper speciation on catalyst’s surface and selectivity. Copper-zinc electrodes, composed mostly of copper, resulted to be as selective towards CO as the sole copper electrodes. ERCO2 selectivity resulted to be higher for zincless electrodes. Methane was 2 obtained in traces when a cathodic potential of -2.1V vs Ag/AgCl was applied. Electrodes maintained their selectivity and activity even after 105 minutes of testing. Zinc based catalysts resulted to be the most selective electrodes towards the production of CO, without measured presence of hydrocarbons. The most CO-selective electrode from each bath was tested under a range of operative potentials. While electrodes composed of hexagonal microstructures reached their maximum faradaic efficiency towards CO (FECO) of 69 % at -1.8 V vs Ag/AgCl and maintaining it at more cathodic potentials, needle-like structures expressed a decrease in FECO when -2.0 V vs Ag/AgCl were applied. Selectivity and activity resulted to be stable after 80 minutes of test. Consideration on the electrode and cell design were made. The use of gas diffusion electrode design was proposed as the most suitable approach for a large-scale application of the process.