Operando soft and hard X-ray spectroscopic techniques
were used
in combination with plane-wave density functional theory (DFT) simulations
to rationalize the enhanced activities of Zn-containing Cu nanostructured
electrocatalysts in the electrocatalytic CO2hydrogenation
reaction. We show that at a potential for CO2hydrogenation,
Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic
Zn segregated; at the interface, low reducible Cu(I)–O species
are consumed. Additional spectroscopic features are observed, which
are identified as various surface Cu(I) ligated species; these respond
to the potential, revealing characteristic interfacial dynamics. Similar
behavior was observed for the Fe–Cu system in its active state,
confirming the general validity of this mechanism; however, the performance
of this system deteriorates after successive applied cathodic potentials,
as the hydrogen evolution reaction then becomes the main reaction
pathway. In contrast to an active system, Cu(I)–O is now consumed
at cathodic potentials and not reversibly reformed when the voltage
is allowed to equilibrate at the open-circuit voltage; rather, only
the oxidation to Cu(II) is observed. We show that the Cu–Zn
system represents the optimal active ensembles with stabilized Cu(I)–O;
DFT simulations rationalize this observation by indicating that Cu–Zn–O
neighboring atoms are able to activate CO2, whereas Cu–Cu
sites provide the supply of H atoms for the hydrogenation reaction.
Our results demonstrate an electronic effect exerted by the heterometal,
which depends on its intimate distribution within the Cu phase and
confirms the general validity of these mechanistic insights for future
electrocatalyst design strategies.