Speaker
Description
Transition-metal carbides (TMCs) are generally anticipated to be effective electrocatalysts for the carbon dioxide reduction reaction (CO2RR) to useful chemicals. Computational screening would then be the method of choice to validate this expectation and identify specific TMCs with particularly promising properties. On the other hand, characteristic for compound materials, TMCs generally exhibit a wide range of geometric and compositional motives that could act as active sites. We address this inherent complexity by extending computational materials screening over a diverse set of such sites [1]. For Mo carbides, this active-site screening indeed confirms that varying adsorption modes break many of the known scaling relations that limit CO2RR at parent transition metals. Despite the resulting inherently rich reduction chemistry, clear trends emerge. Notably, this includes a product selectivity governed by the metal/carbon ratio of the active site.
In contrast to these high expectations, we experimentally find Mo2C to be completely inactive for the CO2RR in aqueous electrolytes. In a multi-method experiment and theory characterization we rationalize this discrepancy with the formation of surface oxides that are stable down to potentials as low as −1.9 V versus the standard hydrogen electrode [2]. The benefit of a possibly suitable reactive carbide surface chemistry for CO2RR will thus only become accessible under conditions where the competing hydrogen evolution reaction (HER) already proceeds at extremely high overpotentials.
Intriguingly, the surface oxide formation and resulting change of the active sites depend sensitively on the local pH. We show via a 1D transport model that the pH increases towards the electrode surface following proton consumption during the occurring HER. Using ab initio thermodynamics, we find that the resulting high interfacial pH subsequently stabilizes the surface oxide. This understanding is further experimentally confirmed as we indeed find Mo2C active for CO2RR when surface oxide formation is prevented by a protective atmosphere and using non-aqueous electrolytes. Our results thus point to the absolute imperative of recognizing the true interface which establishes under working conditions. This understanding is particularly relevant as a boundary condition during computational screening of catalyst materials.
[1] H. Li and K. Reuter, ACS Catal. 10, 11814 (2020).
[2] C. Griesser et al., ACS Catal. 11, 4920 (2021).
| Abstract Number (department-wise) | TH 17 |
|---|---|
| Department | TH (Reuter) |
