A formalism to compare electrocatalysts for the oxygen reduction reaction by cyclic voltammetry with the thin-film rotating ring-disk electrode measurements

This report describes a general method to correlate the features determining the performance of an electrocatalyst (EC), including the accessibility of O2 to the active sites and the kinetic activation barrier, with the outcome of conventional electrochemical experiments. The method has been implemented for oxygen reduction reaction ECs by cyclic voltammetry with the thin-film rotating ring-disk electrode setup. The method (i) does not rely on the simplifications associated with the Butler-Volmer kinetic description of electrochemical processes and (ii) does not make assumptions on the specific features of the EC, allowing to compare accurately the kinetic performance of oxygen reduction reaction ECs with completely different chemistry. Finally, with respect to other widespread figures of merit (e.g. the half-wave potential E1/2), the figure of merit here proposed, for example, E(jPt[5%]), allows for much more accurate comparisons of the kinetic performance of ECs

Interplay between Conductivity, Matrix Relaxations and Composition of Ca-Polyoxyethylene Polymer Electrolytes

This article also appears in: In Memoriam: Prof. Jean-Michel Savéant.In this report, the conductivity mechanism of Ca2+-ion in polyoxyethylene (POE) solid polymer electrolytes (SPEs) for calcium secondary batteries is investigated by broadband electrical spectroscopy studies. SPEs are obtained by dissolving into the POE hosting matrix three different calcium salts: CaTf2, Ca(TFSI)2 and CaI2. The investigation of the electric response of the synthetized SPEs reveals the presence in materials of two polarization phenomena and two dielectric relaxation events. It is demonstrated that the nature of the anion (i. e., steric hindrance, charge density and ability to act as coordination ligand) and the density of “dynamic crosslinks” of SPEs is fundamental in the establishment of ion-ion/ion-polymer interactions. The long-range charge migration processes occurring along the two revealed percolation pathways of the electrolytes are generally coupled with the polymer host dynamics and depend on the temperature and the anion nature. This study offers the needed tools for understanding Ca2+ conduction in POE-based electrolytes.This work has been supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 829145(FETOPEN-VIDICAT).V. Di Notothanks the University CarlosIII of Madrid for the “Catedras de Excelencia UC3M-Santander” (Chairof Excellence UC3M-Santander)

Towards 'Pt-free' Anion-Exchange Membrane Fuel Cells: Fe-Sn Carbon Nitride-Graphene 'Core-Shell' Electrocatalysts for the Oxygen Reduction Reaction

We report on the development of two new Pt-free electrocatalysts (ECs) for the oxygen reduction reaction (ORR) based on graphene nanoplatelets (GNPs). We designed the ECs with a core-shell morphology, where a GNP core support is covered by a carbon nitride (CN) shell. The proposed ECs present ORR active sites that are not associated to nanoparticles of metal/alloy/oxide, but are instead based on Fe and Sn sub-nanometric clusters bound in coordination nests formed by carbon and nitrogen ligands of the CN shell. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new Pt-free ECs for the ORR having ample room for further performance improvement beyond the state of the art

Solid-liquid equilibria of multicomponent lipid mixtures under CO2 pressure: Measurement and thermodynamic modeling

A method for evaluating solid-liquid equilibria of mixtures of lipids and carbon dioxide (CO2) under pressure is developed and presented in this article. Experimental measurements by high-pressure differential scanning calorimetry (DSC) are performed to determine solid-liquid transition temperatures as a function of composition. These data are used to develop a simplified approach for thermodynamic modeling, i.e. correlation and prediction, of the solid-liquid equilibria. The model requires fewer pure component properties in comparison with cubic equations of state: the solid-state fugacity is determined with reference to the subcooled liquid state while regular solution theory is applied for the calculation of liquid-phase activity coefficients. Application of the model is demonstrated for mixtures of ceramide 3A and cholesterol in compressed CO2 in the range of pressure from 0.1 to 6.1 MPa. A satisfactory correlation of solid-liquid equilibria is obtained for binary systems consisting of either the lipid mixture or one lipid in compressed CO2. With the fitted binary interactions parameters, the eutectic temperature of the ternary mixture is predicted to within 1\ub0C of the experimental value in the range of pressure considered. The proposed evaluation method looks sufficiently accurate for representing multiphase equilibria of lipid mixtures in compressed CO2 and has direct application to gas-assisted micronization processes such as PGSS (Particles from Gas-Saturated Solution)