ELECTROLYTE-DRIVEN NANOSTRUCTURING IMPROVES C-C COUPLING DURING CO2 ELECTROREDUCTION
An electrolyte-driven nanostructuring strategy is used to synthesise highly selective Cu catalysts for CO2 electrochemical reduction to valuable multicarbon products. Operando X-ray spectroscopy measurements showed that not only catalyst roughness but also the presence of subsurface oxygen, Cu+ species and adsorbed halides play a beneficial role for the C-C coupling.
ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS
The electrochemical production of fuels and chemical feedstock from CO2 and water using electricity derived from renewable energy holds promise as a sustainable process that might help mitigate current energy and climate challenges. The CO2 electrochemical reduction reaction (CO2RR) is a complicated catalytic process due to the broad range of products obtained when operating at the high over-potentials required to get high market-value products in significant yields. Multicarbon oxygenates and hydrocarbons (C2+) with high energy density are highly desirable, but production is severely limited by the slow kinetics of multiple proton and electron transfer steps during C-C coupling. Copper is the only metal to yield hydrocarbons and alcohols in considerable amounts. However, polycrystalline Cu usually suffers from high over-potentials and low C2+ product selectivity. Interestingly, the formation of C2+ products during CO2RR is very sensitive to the catalyst structure [1-4]. It is desirable, therefore, to develop nanostructured electrocatalysts through rational design that could be capable of efficient generation of multicarbon products from CO2RR.
Surface reconstructions are commonly observed in heterogeneous electrocatalysis, with the most active catalysts dynamically adjusting to the electrolyte, applied potential and other reaction conditions [5,6]. Here, an efficient electrochemical strategy has been developed for the synthesis of high-surface-area nanostructured Cu catalysts for selective C2+ generation from CO2RR, namely, electrolyte-
driven nanostructuring. This approach consists of an electrochemical modification of an electro- polished Cu foil by successive oxidation and reduction pre-treatment cycles in different electrolytes. The originally flat surface is transformed into well-defined nanostructures with dimensions and shape that are strongly dependent on the electrolyte anion employed (Figure 92). Interestingly, the newly created rough Cu catalysts show suppressed methane formation but improved selectivity towards ethylene and multicarbon alcohols. The iodine- modified Cu catalyst displays the highest C2+ Faradaic Efficiency of ~80% at -0.9 V vs. RHE (reversible hydrogen electrode).
It is well known that the activity and selectivity of CO2RR catalysts strongly depend on the precise control of their structure, such as the defect density, content of subsurface oxygen or Cu+ species, the specific shape/facet of the nanocrystals, or the surface composition and atomic ordering in bimetallic nanostructures. To uncover the reaction mechanism behind the improved CO2RR selectivity of halide-modified Cu catalysts, operando high-energy-resolution fluorescence-detected X-ray absorption near- edge structure (HERFD-XANES) measurements were conducted at beamline BM20. This technique is very sensitive to the chemical state and coordination environment of Cu and allows the detection of residual Cu(I) species under reaction conditions. Operando Cu K-edge spectra acquired in grazing incidence show that some Cu(I) species can still survive in the form of Cu-halide and/or Cu2O at the negative potentials relevant to CO2RR (Figure 93). Of all the halide- modified Cu catalysts, the iodine-modified one has the highest Cu+ content (a mixture of Cu2O or CuI species), which is consistent with the highest C2+ Faradaic Efficiency observed. These important high-resolution synchrotron experiments provided direct evidence of the positive correlation between the presence of Cu+ species during CO2RR and the improved generation of C2+ products. Thus, it can be postulated that Cu+/Cu0 species coexisting at the catalyst surface under reaction conditions serve to facilitate the C-C coupling step during CO2RR. Nonetheless, the specific reaction pathways must be investigated in close collaboration with theorists.
Fig. 92: Maximum Faradaic efficiency and partial current density for C2+ products over halide-and carbonate- modified Cu catalysts and an electro- polished Cu foil.