Electrochemical syntheses Assisted by new nano-structured Catalysts

Abstract

Nowadays, the necessity of a full transition towards green chemistry has become of fundamental importance. In order to guarantee such transition, an increasing reduction of fossil-based sources and, consequently, a growing shift towards recycled and renewable resources would be the most feasible and desirable idea. Among renewables, molecular hydrogen gas (H2) is one of the most promising energy carriers for the future. Although 95% of the global hydrogen is currently produced from fossil fuels, turning to the so-called sustainable “green hydrogen” is of such fundamental relevance for the development of a carbon-neutral economy. Water electrolysis enables the generation of H2 at its highest level of purity and at the lowest temperatures when compared to all the other hydrogen generation processes, starting from an easily available and abundant source such as water, and with the possibility of receiving as input energy the one from renewables, hence overcoming also the issues related with them. Platinum (Pt)-based electrocatalysts and their derivatives are by no means the most efficient catalysts towards the cathodic hydrogen evolution reaction (HER) in acid electrolyte solutions. However, their industrial-scale production and application have been significantly limited by the scarce availability and high price of Pt. In view of all this, as well as by considering that platinum is anyhow indispensable in the formulation of an efficient HER electro-catalysts, the primary objective of the following PhD research project has been to design, with the help of nanotechnology, new nano-structured catalysts for the hydrogen evolution reaction (HER) with the purpose of (i) using reduced amount of platinum and (ii) studying new combinations of small quantities of other metals with platinum, as well as by adopting more economic and, at the same time, highly efficient supports. Therefore, two new nano-electrocatalysts were synthesized, PtRh and PtRh/MoS2, with a scalable, simple, and cost-effective bottom-up wet chemistry synthesis approach. Subsequently, the samples were extensively characterized and their electrocatalytic properties for HER were evaluated, proving to be more efficient than most of the current HER catalysts reported in literature. Throughout this study, carried out during the first year of the PhD course, another important issue emerged: distilled water adopted in the electrolytic cell is obtained only after a series of expensive and complex purification steps, followed by the addition of either acid or basic electrolytes, which obviously increase the total costs of the process. Considering this, an interesting and attractive alternative is represented by the electrochemical generation of H2 directly from seawater, about which, however, a mature scientific literature background is still missing. Furthermore, in the unpurified seawater there are hundreds of different impurities which might lead to catalyst poisoning, especially with platinum-based catalysts. That is why the catalysts prepared for the “traditional” HER could not be adopted for seawater electrolysis. Therefore, throughout the second year of the following PhD project, efforts have been devoted to design two new and efficient nano-structured catalysts for HER in seawater, such as a trimetallic alloy (NiRuIr_G) and a quaternary nanostructure (RuOs_G), both supported on graphene and exhibiting high stability in the new electrolytic environment. Once again, they were synthesized through a wet-chemistry, easily reproducible and scalable synthesis approach, and after being broadly characterized, they were tested in an electrolytic cell with seawater, showing remarkable performance when compared to literature and Pt itself. Eventually, during the third year, a further step forward towards sustainability has been taken, based on the following rationale: combining water and renewable sources to produce clean and pure hydrogen in a green perspective is appealing, but it would be even more fascinating if also a detrimental greenhouse gas, such as carbon dioxide (CO2), could be employed in the same process along with the same reactant with the double purpose of reducing the carbon footprint and obtaining a valuable chemical, such as a ”greener” syngas, with a tunable CO/H2 ratio, enabling the integration of a sustainable process into production lines of different chemicals. From an accurate search and evaluation of the available scientific literature on the most efficient catalysts thus far proposed for the simultaneous HER and carbon dioxide reduction reaction to carbon monoxide (CO2RR to CO), it emerged that catalytic activity and stability can be significantly increased by turning from metal-based nano-sized catalysts to metal single atom catalysts (SACs) in the M-N-C form. However, literature on this cutting-edge topic is still limited and there are still many routes to explore such as, for instance, regulating the H2 and CO production

by preparing two metals on the same carbonaceous support with dual N-M active sites. Following this idea,

throughout the last period of PhD research, a novel dual-active sites single atom catalyst for the syngas electro- production has been prepared through a simple pyrolysis method, adopting nontoxic glucose as carbon

precursor and very small quantities of two economic metals, i.e. zinc (Zn) and cobalt (Co). The as-prepared catalyst, named as ZnCo-NC, showed higher performance than most of the current catalysts reported in literature. [edited by Author]