Deciphering Structure -ORR Activity -Stability Relationships Thanks to Physical and Electrochemical Markers
Résumé
The electrochemical activation of oxygen is the cornerstone of energy storage and conversion devices, such as metal-air batteries, electrolysers and fuel cells. In proton-exchange membrane fuel cells, the high cost and scarcity of platinum (Pt) drives intense research to enhance its mass activity (the current produced per gram of Pt) towards the oxygen reduction (ORR). It has been shown that the ORR kinetics is maximal on Pt-alloys and at (111) facets, hence PtNi/C octahedra are envisioned as the dream ORR catalyst. Strikingly, it has also been found that structurally-disordered PtNi nanocatalysts feature highly desirable and sustainable ORR activity. However, to date, the reasons of this unexpected ORR activity enhancement remain unclear. To shed fundamental light onto this, state-of-the art PtNi/C nanocatalysts with distinct atomic composition, size, shape and degrees of disorder were synthesized. The contribution of chemical and structural disorders were quantified experimentally, by introducing a new parameter, the surface distortion (SD). The SD descriptor was used to rationalize the ORR activity enhancement of the two classes of materials (structurally-ordered and structurally-disordered), and to probe their stability in simulated PEMFC cathode operating conditions 5 . Besides, we will also show how the ratio of the CO ads monolayer stripping charge (QCO) to the HUPD charge (QH) allows one to semi-quantitatively estimate a catalyst’s surface disorder just as synchrotron X-rays do, but in a more facile and accessible way.