Electrochemical CO2 Reduction: A Classification Problem

Description

In this work we propose four non-coupled binding energies of intermediates as descriptors, or 'genes', for predicting the product distribution in CO₂ electroreduction. Simple reactions can be understood by the Sabatier principle (catalytic activity vs. one descriptor), while more complex reactions tend to give multiple very different products and consequently the product selectivity is a more complex property to understand. We approach this, as a logistical classification problem, by grouping metals according to their major experimental product from CO₂ electroreduction: H₂, CO, formic acid and beyond CO* (hydrocarbons or alcohols). We compare the groups in terms of multiple binding energies of intermediates calculated by density functional theory. Here we find three descriptors to explain the grouping: the adsorption energies of H*, COOH* and CO*. To further classify products beyond CO*, we carry out formaldehyde experiments on Cu, Ag and Au and combine these results with the literature to group and differentiate alcohol or hydrocarbon products. We find that the oxygen binding (adsorption energy of CH₃O*) is an additional descriptor to explain the alcohol formation in reduction processes. Finally, the adsorption energy of the four intermediates, H*, COOH*, CO* and CH₃O*, can be used to differentiate, group and explain products in electrochemical reduction processes involving CO₂, CO and carbon-oxygen compounds.

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(a) the three dimensional space of descriptors are plotted with the coupled binding energy ellipsoids that represent the first σ confidence interval of the calculations with respect to Au, while the Au error ellipsoid is given with respect to the slab and the gas phase (see script how this is carried out). Each background plane has been giving a color to match the 2d projections of energies and ellipses in (b), (c) and (d) for the ∆E HCOO* vs ∆E COOH* , ∆E COOH* vs. ∆E H* and ∆E HCOO* vs. ∆E H* planes, respectively. In (b), (c) and (d) the solid black lines shown the thermodynamic line (see script for value) and the diagonal dashed black lines indicate the relative stability of the two adsorbates.