Julien Durst

New insights into the electrochemical hydrogen oxidation and evolution reaction mechanism

The effect of pH on the hydrogen oxidation and evolution reaction (HOR/HER) rates is addressed for the first time for the three most active monometallic surfaces: Pt, Ir, and Pd carbon-supported catalysts. Kinetic data were obtained for a proton exchange membrane fuel cell (PEMFC; pH ≈ 0) using the H2-pump mode and with a rotating disk electrode (RDE) in 0.1 M NaOH. Our findings point toward: (i) a similar ≈100-fold activity decrease on all these surfaces when going from low to high pH; (ii) a reaction rate controlled by the Volmer step on Pt/C; and (iii) the H-binding energy being the unique and sole descriptor for the HOR/HER in alkaline electrolytes. Based on a detailed discussion of our data, we propose a new mechanism for the HOR/HER on Pt-metals in alkaline electrolytes.

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 materials 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.

Beyond conventional electrocatalysts: hollow nanoparticles for improved and sustainable oxygen reduction reaction activity

Long-term catalytic performance of electrode materials is a well-established research priority in electrochemical energy conversion and storage systems, such as proton-exchange membrane fuel cells. Despite extensive efforts in research and development, Pt-based nanoparticles remain the only – but an unstable – electrocatalyst able to accelerate efficiently the rate of the oxygen reduction reaction. This paper describes the synthesis and the atomic-scale characterization of hollow Pt-rich/C nanocrystallites, which achieve 4-fold and 5-fold enhancement in specific activity for the oxygen reduction reaction over standard solid Pt/C nanocrystallites of the same size in liquid electrolyte and during real proton-exchange membrane fuel cell (PEMFC) testing, respectively. More importantly, the hollow nanocrystallites can sustain this level of performance during accelerated stress tests, therefore opening new perspectives for the design of improved PEMFC cathode materials.

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.

Influence of PEMFC Operating Conditions on the Durability of Pt3Co/C Electrocatalysts

This work is dedicated to the characterization of structure and composition change of the membrane electrode assembly (MEA) upon on-site proton-exchange membrane fuel-cell (PEMFC) operation (constant current load and start/stop conditions), with special emphasis on the Pt3Co/C cathode electrocatalyst. A fast degradation of the cathode catalytic layer was monitored. It yielded to the redistribution of both Pt and Co elements within the whole MEA, decrease of the MEA thickness and a continuous leaching of the surface/bulk Co atoms. The cell performance decay is discussed in the light of these changes.