Functional Carbon Nanostructures for Green Electrolyzers

Chapter I gives an overall overview of the nanotechnology field. The chapter starts illustrating the historical background progress, followed by the description of nanomaterials with a particular emphasis on CNMs properties, strategies of functionalization and applications. The discussion is then focused on the characteristics of inorganic-organic CNMs hybrids with polyoxometalates (POMs), highlighting the main topics of the dissertation. Chapter II provides a summary about the production of nitrogen-doped carbon nanohorns (N-CNHs). The N-CNHs were produced using a hydrothermal post-synthetic approach to install nitrogen domains. Next, this chapter presents the investigation of the correlation of N-CNHs surface properties, such as surface charge state and basicity with nitrogen-doping and defect density. The N-CNHs synthesis allowed to produce an alkaline linker-free CNMs protonable in acidic pHs, opening new strategies towards the realization of supports for negatively charged particles. Chapter III is dedicated to the preparation of catalysts capable of performing the selective electroesterification of bioethanol to ethyl acetate (EtOAc), in order to be implemented in photoelectrochemical cell prototypes. Exploiting the positively charged N-CNHs discussed in Chapter II, the [Ru4(µ-O)4(µ-OH)2(H2O)4(γ-SiW10O36)2]10-, Ru4POM, a polyanionic compound, well-documented for its outstanding properties in oxidations catalysis, was anchored through an ion metathesis (Ru4POM@N-CNHs). The resulting Ru4POM@N-CNHs was electrochemically tested towards the ethanol oxidation reaction (EOR), demonstrating superior activity and selectivity towards the formation of EtOAc. In addition, mechanistic insights were provided for the electroesterification reaction. Finally, the correlation between N-CNHs nitrogen content, Ru4POM loading and electrocatalytic activity was studied. Chapter IV is centered on a further development on the design of innovative N-CNH based on the materials and properties discovered in the previous Chapters. Indeed, considering the interesting properties of N-CNHs, in this chapter a new pathway is proposed to enhance further the nitrogen-doping. Specifically, the N-CNHs were prepared using N2 and NH3 plasma generated with radio-frequency discharges. The nanomaterials were investigated with sophisticated analytical tools, evidencing improved performances in terms of surface properties and interaction with Ru4POM with respect to the N-CNHs discussed in the Chapter II The overall work reported the successful design and production of self-standing electrodes based on CNMs through a straightforward experimental set-up based on a nitrogen ion deposition synthetic procedure. Chapter V explores the amino functionalization of CNHs to achieve highly positive surfaces, as optimized building block to host Ru4POM with high yields. A branched polyethylenimine (PEI) was grafted on CNHs (C-PEI-CNHs) graphitic surface through an amidation step. Experimental evidence underlined that the high number of primary amines guaranteed a noticeable water solubility of the materials and cationic nature even at neutral pH. These characteristics consented the use of such materials as supports to anchor Ru4POM with high loadings, making the nanohybrids (Ru4POM@C-PEI-CNHs) appealing for the water splitting reaction. Later, their electrocatalytic performances were investigated towards EOR catalytic activity and their performances compared with Ru4POM@N-CNHs. This study provided precious insights about the pivotal role of an appropriate CNMs functionalization to diversify the interaction with Ru4POM and, consequently, its catalytic activity.