PT-FREE NANO- AND MICRO-STRUCTURED CARBONS FOR ELECTROCHEMICAL OXYGEN REDUCTION REACTION

Oxygen reduction reaction (ORR) catalysts are of crucial importance in developing low- and medium-temperature fuel cells, as PEMFCs (Polymer Electrolyte Membrane Fuel Cells), from which sizeable energy saving and reduction of greenhouse gas emission are expected in comparison with the use of coal and oil based fuels in thermal engines. The same electrochemical oxygen reduction reaction takes place in oxygen depolarized cathodes (ODC) in chlor-alkali electrolysis, replacing the conventional hydrogen evolving cathode, gaining about 30% energy consumption reduction in the overall process. At present carbon-supported Pt and Pt-rich alloys are best credited to the ORR purpose. However, Pt-based catalysts are not free from certain drawbacks, such as oxide formation and Pt particle coarsening through Ostwald ripening, that decrease the overall cell energy conversion efficiency. Furthermore, attendant problems concerning natural availability, geographic distribution and cost of platinum, render platinum supply strategic and fuel cells hardly scalable to mass production. At present, projections on platinum usage for PEMFCs are estimated at ~15 ton y-1 in addition to the current ones, at a cost of ~40 $ g-1. Therefore, non-precious metal catalysts are actively searched for, such as to meet already established operational benchmarks for conventional platinum PEMFC vehicular requirements (0.5 W cm-2; 5500 h durability) with the additional target of significant cost reduction. Several papers on non-precious ORR catalysts have been published after a first report by Jasinski in 1964 demonstrating the ORR activity of metal substitutes-phthalocyanines. Then, research on metal-nitrogen macrocycles significantly expanded, leading to the picture that ORR catalytic activity can be related to N4-Me and N2-Me moieties. However, for precursors cost and unsatisfactory lifetime performance, research was steered toward more simple nitrogen-containing reactants and preparation procedures. Significant steps in this direction were obtained by Dodelet et al. who demonstrated that ORR overpotentials almost linearly decrease with increasing nitrogen content in carbon. Positive results were obtained on a series of samples prepared by high temperature treatment of carbon precursors in NH3/H2/N2 mixtures; doping of these modified carbons with iron rather than cobalt salts was shown to be preferable for better efficiency in oxygen reduction, even though still lower than that of platinum. Further improvements both in terms of incipient ORR potentials and currents were obtained by Maruyama et al. using carbons from hemoglobin and adenine-glucose pyrolysis in the presence of added Fe(II) and Cu(II)/Fe(II) mixtures, respectively. The ORR promoting role of nitrogen in carbon was independently demonstrated both theoretically and experimentally. Indeed, it was found that substitutional nitrogen at a few, specific, peripheral positions of graphene layers in well-ordered carbon nanostructures is in itself able to promote ORR activity even in the absence of accompanying metal centers. Besides the above examined composition-dependent factors, catalyst activity also depends on structural and morphological carbon support features. In fact, many electrocatalytic reactions show faster kinetics on carbon edge planes compared with basal ones. This is related to the ability of the edges to more readily chemisorb O2 (this is the same reason why O2 combusts faster from edges and defects). On the other hand, an optimized porosity of carbon supports is beneficial for an easy access of the oxygen to the catalyst layer in contact with the proton exchange electrolyte membrane. Despite carbon materials of different textural morphology are widely used at an industrial level as supports for precious metal catalysts, in the PEMFC field, electrocatalysts are by far supported on the same VULCAN XC72 carbon. Given that improved catalytic activity of Pt-based catalysts has been achieved by the use of carbons with pore size centered in the mesoporous region, even developed with advanced synthesis, including template methods, such strategy should be pursued also for Pt-free catalytic systems. In this project a number of Pt-free N-doped C-based catalysts have been synthesized on the basis of different synthetic and templating strategies aiming to understand how compositional, morphological and textural aspects of the end material can affect the electrochemical behaviour of ORR. Materials have been characterized using different physico-chemical methods including a study of the kinetics and mechanism of the electrochemical oxygen reduction reaction. Electrochemical results were obtained by rotating disk electrode (RDE) and rotating ring disk electrode (RRDE). Surface and bulk analyses have been performed by BET technique, XPS and XRPD (sometimes data were recorded at synchrotron facilities). (HR) TEM, SEM (combined with FIB milling) imaging was also performed to characterize samples morphology. Many types of samples have been synthesized, starting with mesoporours N-, Fe- doped carbons obtained by heat treatment of a solution of precursors, using silica as a templating agent. Outstanding results in terms of ORR electroactivity in acidic and alkaline conditions have been recorded. Some samples, especially in alkaline media, catalyze ORR even better than commercial Pt-based catalysts. Then, attempting to prepare materials with a precise and defined order and trying to emphasize some of their properties such as surface area and conductivity, ordered carbonaceous nano- and microstructures were synthesized. By chemical vapor deposition N-, Fe- doped carbon nanotubes (N-CNTs) were prepared and some interesting aspects related to the aging of the Fe-doped MgO catalyst used to grow N-CNTs were evidenced. Then, a modified method, but very similar to that used in the synthesis of nanotubes, surprisingly allowed the synthesis of innovative N-doped hollow carbon nanocubes (N-CNCs). This is the great novelty of the work. Due to nanocubes endothermal transformation, happening at 37°C, as detected by DSC, and to the empty space available in the internal part of each cube, many applications can be thought for example involving cubes as nano-reactors that can be opened/close in correspondence of body’s temperature changes. This feature could be taken into considerations for medical applications, after testing and verifying the biocompatibility of nanocubes. Finally, a completely different technique, an ultraspray pyrolysis method (USP) was used to obtain N-, Fe- doped carbon microspheres. The inherent scalability of continuous flow methods such as USP represents a significant advantage compared to alternative synthetic strategies requiring batch processing or surface catalyzed deposition of nanostructured carbon materials (e.g. CVD growth), this feature might be useful in order to improve electrode packing and, consequently, mass transport electrocatalytic applications. The last results section, apparently diverging from the main goals of the present work, was thought to better understand the electronic C-surface behavior in charge transfer reactions. This is actually strongly connected to the oxygen reduction reaction, for which all the catalysts, hereby synthesized, were designed. However, instead of starting from complicated systems involving porous and doped-carbons, the choice was addressed to the simplest but closest material: annealed and non-annealed amorphous carbon thin films prepared by DC-magnetron sputtering technique. Part of the work described was carried out at Trinity College Dublin in the laboratory of Prof. Colavita as a part of an academic collaboration and 5 months exchange granted by the European LLP Erasmus Program.