• Research

Surface Distortion as a unifying concept and descriptor in Oxygen Reduction Reaction electrocatalysis

A study conducted jointly by the Laboratory of Electrochemistry and Physico-Chemistry of Materials and Interfaces (LEPMI - CNRS / Grenoble INP / UGA / USMB), the Laboratory of Materials Science and Engineering and Processes (SIMaP - CNRS / Grenoble INP / UGA) and the Néel Institute (CNRS), published in Nature Materials on July 16th 2018, shows that structurally disordered catalysts have a very high catalytic activity for the conversion of atmospheric oxygen into water (ORR, Oxygen Reduction Reaction), the key catalytic process for a decarbonized future.
The oxygen reduction reaction is a key reaction for corrosion, sensing and energy conversion systems, such as metal-air batteries and proton-exchange membrane fuel cells (PEMFC). However, in the latter devices, the high platinum (Pt) content needed to activate the oxygen reduction reaction (ORR) and its instability, especially at the cathode, are majors barriers for massive implementation of PEMFC devices in automobiles. Successive strategies to boost the ORR kinetics and minimize the cost of the PEMFC cathode over the last 30 years have involved decreasing the size of the crystallites to nanometre dimensions, alloying Pt with transition metals and controlling the shape of the Pt-based nanocatalysts. Combining these strategies synergistically leads to cubic or octahedral PtNi nanoparticles with 20-30-fold enhancement of the intrinsic activity for the ORR relative to pure Pt nanoparticles. However, recent results on ‘structurally-disordered’ catalysts, such as hollow PtNi nanoparticles, dealloyed porous PtNi nanoparticles or PtNi aerogels, have also shown desirable enhancement of the ORR activity (x 10-12 relative to pure Pt/C), thus raising confusion in academic and industrial research groups. To date, the mechanisms of the ORR activity enhancement remain unclear, and prevent the development of this vital technology for a carbon-free energy future.

The two possible strategies towards enhanced ORR kinetics: implementing structural disorder (SD>0) versus maintaining structural order (SD=0).
Density functional theory (DFT) - inspired schematic highlighting the heterogeneous versus homogeneous catalytic site configurations.

In an article published in Nature Materials on July 16th 2018, Chattot et al., in collaboration with various European laboratories, synthesized a vast library of PtNi nanoalloys with distinct atomic composition, size, shape and degree of surface defectiveness and high ORR activity. By performing Rietveld analysis of synchrotron X-ray diffraction data and ab initio calculations, the authors were able to quantify the fraction of disorder that is chemical in origin and the fraction that is structural. On these bases, they introduced a new and original structural descriptor, the surface distortion (SD), which proved capable to rationalize the ORR activity of this vast library of bimetallic nanocatalysts and to capture their changes in structure, chemistry and morphology in real time in a PEMFC cathode environment. The results of this study show that the surface of ‘structurally ordered’ catalysts buckles under the operating conditions of a PEMFC cathode, i.e. leads to the appearance of an atomically rough surface, surprisingly close to that of the ‘structurally disordered’ catalysts. In other words, these two families of catalysts are in fact not so far apart! By combining the information derived from SD and from electrochemistry, the authors conclude that implementing structural disorder in ORR nanocatalysts offers a more sustainable approach to catalyse this reaction in PEMFC devices.

Published on July 20, 2018