Moving Mountains: Reshaping the Activity Volcano of Electrocatalysis with Fluxional Subnano Clusters

Abstract

The activity volcano derived from Sabatier analysis provides intuitive guide for catalyst design, but it also imposes fundamental limitations on the maximal activity and the pool of high-performance elements. Here we show that the activity volcano for oxygen reduction reaction (ORR) can be shifted and reshaped in the subnano regime. The fluxional behavior of subnano clusters, in both isolated and graphite-supported forms, not only breaks the linear scaling relationships but also causes an overall strengthening in adsorbate binding. The metals with optimal adsorbate binding in the bulk form (Pt/Pd) thus suffer over-binding issues, while the metals that under-bind in the bulk form (Ag/Au) gain optimal reaction energetics. In addition, the potential-dependence of isomer energies differ, causing non-linear reaction free energy-potential relations and enabling population-tuning of specific isomers, thereby surpassing the apex of the activity volcano. The shift of the volcano that puts under-binding elements closer to the top is likely general in fluxional cluster catalysis, and can be used for cluster catalyst design.

Ensemble representation of catalytic interfaces: soloists, orchestras, and everything in-between

“…This explains the exceptional activity of downsized Au NPs and provides a rational route to utilize cluster fluxionality as a design tool, and to activate elements in catalysis usually considered inert. 33 Intimately related to the size-dependent ability of clusters to restructure and break LSRs is the phenomenon of the reaction thermodynamics and kinetics not linearly scaling with cluster size (the "each atom counts" effect 34 ). The non-directional nature of metal-metal bonds makes the geometric and electronic rearrangements unpredictable as the size increases.…”

Theoretical Perspective on Operando Spectroscopy of Fluxional Nanocatalysts

“…In electrochemical conditions, fluxionality and ensemble size are additionally affected by the electrode charging and solvation of the interface. 67 Larger ensembles would lead to more convoluted ensembleaveraged operando spectra and likely greater dynamics observed with localized in situ techniques. In these cases, a combination of ensemble and localized operando techniques would be valuable, as time-resolved localized techniques can provide insight into the extent of the dynamic behavior, as well as how it changes under varying conditions.…”

“…Operando XANES in this case showed the presence of both valences of Cu in reaction conditions, and theory was needed to decide on the more plausible active site. In electrochemical conditions, fluxionality and ensemble size are additionally affected by the electrode charging and solvation of the interface …”