Dynamic surface self-reconstruction is the key of highly active perovskite nano-electrocatalysts for water splitting.

The growing need to store increasing amounts of renewable energy has recently triggered substantial R&D efforts towards efficient and stable water electrolysis technologies. The oxygen evolution reaction (OER) occurring at the electrolyser anode is central to the development of a clean, reliable and emission-free hydrogen economy. The development of robust and highly active anode materials for OER is therefore a great challenge and has been the main focus of research. Among potential candidates, perovskites have emerged as promising OER electrocatalysts. In this study, by combining a scalable cutting-edge synthesis method with time-resolved X-ray absorption spectroscopy measurements, we were able to capture the dynamic local electronic and geometric structure during realistic operando conditions for highly active OER perovskite nanocatalysts. Ba0.5Sr0.5Co0.8Fe0.2O3-δ as nano-powder displays unique features that allow a dynamic self-reconstruction of the material's surface during OER, that is, the growth of a self-assembled metal oxy(hydroxide) active layer. Therefore, besides showing outstanding performance at both the laboratory and industrial scale, we provide a fundamental understanding of the operando OER mechanism for highly active perovskite catalysts. This understanding significantly differs from design principles based on ex situ characterization techniques.

Electrochemical CO2 Reduction - A Critical View on Fundamentals, Materials and Applications.

The electrochemical reduction of CO(2) has been extensively studied over the past decades. Nevertheless, this topic has been tackled so far only by using a very fundamental approach and mostly by trying to improve kinetics and selectivities toward specific products in half-cell configurations and liquid-based electrolytes. The main drawback of this approach is that, due to the low solubility of CO(2) in water, the maximum CO(2) reduction current which could be drawn falls in the range of 0.01-0.02 A cm(-2). This is at least an order of magnitude lower current density than the requirement to make CO(2)-electrolysis a technically and economically feasible option for transformation of CO(2) into chemical feedstock or fuel thereby closing the CO(2) cycle. This work attempts to give a short overview on the status of electrochemical CO(2) reduction with respect to challenges at the electrolysis cell as well as at the catalyst level. We will critically discuss possible pathways to increase both operating current density and conversion efficiency in order to close the gap with established energy conversion technologies.

Reversibility of Pt-Skin and Pt-Skeleton Nanostructures in Acidic Media.

Following a well-defined series of acid and heat treatments on a benchmark Pt3Co/C sample, three different nanostructures of interest for the electrocatalysis of the oxygen reduction reaction were tailored. These nanostructures could be sorted into the "Pt-skin" structure, made of one pure Pt overlayer, and the "Pt-skeleton" structure, made of 2-3 Pt overlayers surrounding the Pt-Co alloy core. Using a unique combination of high-resolution aberration-corrected STEM-EELS, XRD, EXAFS, and XANES measurements, we provide atomically resolved pictures of these different nanostructures, including measurement of the Pt-shell thickness forming in acidic media and the resulting changes of the bulk and core chemical composition. It is shown that the Pt-skin is reverted toward the Pt-skeleton upon contact with acid electrolyte. This change in structure causes strong variations of the chemical composition.

Impact of metal cations on the electrocatalytic properties of Pt/C nanoparticles at multiple phase interfaces.

Proton-exchange membrane fuel cells (PEMFCs) use carbon-supported nanoparticles based on platinum and its alloys to accelerate the rate of the sluggish oxygen-reduction reaction (ORR). The most common metals alloyed to Pt include Co, Ni and Cu, and are thermodynamically unstable in the PEMFC environment. Their dissolution yields the formation and redistribution of metal cations (M(y+)) within the membrane electrode assembly (MEA). Metal cations can also contaminate the MEA when metallic bipolar plates are used as current collectors. In each case, the electrical performance of the PEMFC severely decreases, an effect that is commonly attributed to the poisoning of the sulfonic acid groups of the perfluorosulfonated membrane (PEM) and the resulting decrease of the proton transport properties. However, the impact of metal cations on the kinetics of electrochemical reactions involving adsorption/desorption and bond-breaking processes remains poorly understood. In this paper, we use model electrodes to highlight the effect of metal cations on Pt/C nanoparticles coated or not with a perfluorosulfonated ionomer for the CO electrooxidation reaction and the oxygen reduction reaction. We show that metal cations negatively impact the ORR kinetics and the mass-transport resistance of molecular oxygen. However, the specific adsorption of sulfonate groups of the Nafion® ionomer locally modifies the double layer structure and increases the tolerance to metal cations, even in the presence of sulphate ions in the electrolyte. The survey is extended by using an ultramicroelectrode with cavity and a solid state cell (SSC) specifically developed for this study.