Silicon nanoparticles are used to enhance the anode specific capacity for the lithium-ion cell technology. Due to the mechanical deficiencies of silicon during lithiation and delithiation, one of the many strategies that have been proposed consists of enwrapping the silicon nanoparticles with graphene and creating a void area between them so as to accommodate the large volume changes that occur in the silicon nanoparticle. This work aims to investigate the electrochemical performance and the associated kinetics of the hollow outer shell nanoparticles. To this end, we prepared hollow outer shell silicon nanoparticles (nps) enwrapped with graphene by using thermally grown silicon dioxide as a sacrificial layer, ball milling to enwrap silicon particles with graphene and hydro fluorine (HF) to etch the sacrificial SiO₂ layer. In addition, in order to offer a wider vision on the electrochemical behavior of the hollow outer shell Si nps, we also prepared all the possible in-between process stages of nps and corresponding electrodes (i.e., bare Si nps, bare Si nps enwrapped with graphene, Si/SiO₂ nps and Si/SiO₂ nps enwrapped with graphene). The morphology of all particles revealed the existence of graphene encapsulation, void, and a residual layer of silicon dioxide depending on the process of each nanoparticle. Corresponding electrodes were prepared and studied in half cell configurations by means of galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy. It was observed that nanoparticles encapsulated with graphene demonstrated high specific capacity but limited cycle life. In contrast, nanoparticles with void and/or SiO₂ were able to deliver improved cycle life. It is suggested that the existence of the void and/or residual SiO₂ layer limits the formation of rich Li_XSi alloys in the core silicon nanoparticle, providing higher mechanical stability during the lithiation and delithiation processes.

High Temperature Polymer Electrolyte Membrane Fuel Cells (HTPEMFCs) commercialization, is impeded by the sluggish, power intensive and elusive cathodic Oxygen Reduction Reaction (ORR) kinetics. In this work we provide solid experimental evidence that the simple three elementary step Dissociative Adsorption (DA) pathway, with two intermediate species (O_ad and OH_ad), can accurately describe the steady state (IV) and the Electrochemical Impedance Spectra (EIS) response of a HTPEMFC. Deconvoluted EIS outlined the important role of Intrinsic Kinetic Inertia, which dominated both EIS and polarization resistance. The Degree of Rate Control (DRC) analysis, identified the O_2(g) dissociative adsorption as the rate limiting step. Finally, Transition State Theory (TST) allowed the extraction and analysis of ORR energetics, demonstrating that the high ORR overpotential losses originate from the combined strength of both kinetically and thermodynamically imposed barriers, due to the high bonding strength of O_ad on Pt and the high activation energy of O_2(g) adsorption.

Unraveling the elusive electrokinetics and energetics of the sluggish Oxygen Reduction Reaction (ORR) is critical for the development of efficient Proton Exchange Membrane Fuel Cells (PEMFCs). Physical microkinetic modeling, Transition State Theory (TST), Transmission Line Modeling and Degree of Rate Control (DRC) analysis was employed to simulate analytically both the experimental Electrochemical Impedance Spectra (EIS) and the polarization curve data of a High Temperature PEMFC. The excellent fitting results revealed that: i) O2(g) dissociative adsorption is the main limiting step responsible for the high ORR overpotentials, ii) both EIS and polarization resistance are dominated by the intrinsic ORR kinetic inertia due to the competitive nature of the elementary reaction steps on the coverages of the adsorbed species (Oad and OHad). Finally, from TST the activation energies (kinetics) and the reaction steps free energies (thermodynamics) were extracted, which can serve as a guide for catalyst design optimizing the oxygen binding strength.

Owing to their notable power efficiency and zero greenhouse emissions, High Temperature Polymer Electrolyte Membrane Fuel Cells (HTPEMFCs) hold immense promise as a prospective candidate for generating clean energy. However, their commercialization is still impeded by the cathodic Oxygen Reduction Reaction (ORR) kinetics, which after almost half a century not only remain elusive, but constitute the most energy-intensive process, requiring large amounts of the expensive Pt catalyst to mitigate. In this work we provide solid experimental evidence, that the simple three elementary step Dissociative Adsorption Pathway (DAP) with two intermediate species (O_ad and OH_ad), can accurately capture both the steady state (IV) and the Electrochemical Impedance Spectra (EIS) as well. Detailed EIS and IV experimental data were recorded at 180 °C under differential conditions, using a single serpentine HTPEMFC with a 4 cm² active area. The fuel cell operated at the activation regime, where ORR potential losses dominate but mass transport limitations are negligible. The experimental data were successfully fitted with the derived analytic IV and EIS microkinetic model. The use of the DAP in accordance with the Transitions State Theory (TST), allowed the extraction and analysis of ORR kinetics and energetics. The deconvoluted EIS outlined the important role of the intrinsic kinetic inertia , i.e. the outcome of the competitive nature of the elementary reaction steps on the adsorbed intermediate species. Kinetic inertia not only dominated EIS and the polarization resistance, but demonstrated that the commonly used Butler Volmer and apparent Tafel slope kinetic approach is incapable of describing the multistep ORR kinetics. With the help of the Degree of Rate Control (DRC) analysis, we successfully identified the O_2(g) dissociative adsorption as the rate limiting step. Finally, from an energetics perspective, by employing the TST kinetics formulation, we were able to calculate the free Gibbs’ transition states activation (kinetics) and reaction steps’ energies (thermodynamics). These results clarified that, the high ORR overpotential losses stem from the combined strength of both kinetically and thermodynamically imposed barriers, which originate from the high bonding strength of O_ad on the Pt surface and the rather high activation energy of the O_2(g) adsorption step.

The scope of this work is to shed some light to the oxygen reduction reaction mechanism for HTPEMFC utilizing polarization and EIS data, in order to extract the elusive ORR kinetics. To do so first we propose a simple three step reaction scheme, then based on the proposed mechanism we derive an analytic electrode impedance equation at the activation regime of operation and develop a Monte Carlo type algorithm to estimate the values of the respective kinetic constants, by simultaneously fitting polarization-dc bias and the corresponding galvanostatic AC Spectra. All data in this study were produced by a 1-D finite element macro-homogeneous physical model of the fuel cell. Our results clearly demonstrate that 1) when the fuel cell is operated at the activation regime two arcs at most, (charge transfer and relaxation), constitute ORR impedance with the latter being dominant and 2) the effectiveness of the proposed algorithm.