Unveiling selectivity trends for CO ₂ reduction reaction over Ti ₃ C ₂ T _<i>x</i> MXene: the key role of less-stable intermediate states and coadsorbates

The electrochemical conversion of carbon dioxide <i>via</i> the CO₂ reduction reaction (CO₂RR) is an attractive strategy for the production of value-added chemicals. However, the CO₂RR suffers from a selectivity problem due to the large number of carbon-based products that can be obtained and the competing hydrogen evolution reaction (HER). It has been experimentally shown that the ratio and chemical nature of terminal groups, T_<i>x</i>, present on the Ti₃C₂T_<i>x</i> (T_<i>x</i> = O, OH or F) surface under electrochemical conditions affect selectivity and activity trends of the MXene electrocatalyst. In the present manuscript, we use electronic structure theory calculations to comprehend the selectivity trends in the CO₂RR over Ti₃C₂T_<i>x</i> with different terminal groups, including *OH and *F adsorbates. We show that the traditional modeling approach used in calculations to derive activity and selectivity trends, which only includes the most stable intermediate state in the analysis, is not consistent with experimental observations. Rather, it is necessary to include energetically less favorable intermediate states and coadsorbates in the analysis of mechanistic pathways. Remarkably, the inclusion of less-stable intermediates, although stable on the electrode surface, and coadsorbates opens up new reaction channels that are energetically more favorable, and only by considering these extensions are we able to map our results to the experimental data. We believe that the reported finding is not only limited to the CO₂RR or MXene systems, but likely also plays an important role in other catalytic transformations under applied bias.