Selectivity and Active State Formation During Pulsed CO ₂ Electroreduction in Cation‐Modified Electrolyte

Pulsed CO 2 reduction reaction (CO 2 RR) is a viable approach to steer selectivity between C 1 and C 2+ products as well as hydrocarbons and oxygenates via intermittent Cu oxidation. Here, we report on the catalytic influence of alkali metal cations using oxide‐derived Cu nanocrystals based on comprehensive electrocatalytic testing and time‐resolved in situ X‐ray absorption spectroscopy and diffraction. Typical cation‐dependent selectivity trends were observed at optimal potentiostatic conditions, while pulsed operation significantly reduces hydrogen and formate selectivity and introduces a discontinuous shift in product selectivity not directly following the ionic radius. K + /Cs + exhibits enhanced ethanol formation with a weaker enhancement of methane compared to Li + /Na + . Our in situ studies revealed dynamic structural catalyst responses to potential modulation and cation identity, with a quantitative link of the lattice parameter to the cation size but not their effective ionic radius, which includes the hydration shell. Importantly, this trend is also linked to the enhanced C 2+ formation under potentiostatic CO 2 RR, while under pulsed CO 2 RR, not the Cu(I) but Cu(II) formation kinetics, facilitating Cu dissolution, influences selectivity. Overall, our results demonstrate that cations modulate both the electrode (surface) structure and thus, the electronic environment of Cu, but also its dynamicity, thereby governing the mechanistic pathway and selectivity of CO 2 RR.