Fast operando spectroscopy tracking in situ generation of rich defects in silver nanocrystals for highly selective electrochemical CO2 reduction

Electrochemical CO2 reduction (ECR) is highly attractive to curb global warming. The knowledge on the evolution of catalysts and identification of active sites during the reaction is important, but still limited. Here, we report an efficient catalyst (Ag-D) with suitable defect concentration operando formed during ECR within several minutes. Utilizing the powerful fast operando X-ray absorption spectroscopy, the evolving electronic and crystal structures are unraveled under ECR condition. The catalyst exhibits a ~100% faradaic efficiency and negligible performance degradation over a 120-hour test at a moderate overpotential of 0.7 V in an H-cell reactor and a current density of ~180 mA cm−2 at −1.0 V vs. reversible hydrogen electrode in a flow-cell reactor. Density functional theory calculations indicate that the adsorption of intermediate COOH could be enhanced and the free energy of the reaction pathways could be optimized by an appropriate defect concentration, rationalizing the experimental observation.

Debye-Waller factor (DW) to account for both thermal and structural disorders; ΔE 0 : inner potential correction; R factor: the goodness of the fit. The S 0 2 values, an amplitude reduction factor, was determined as 0.714 from the Ag foil reference.

Supplementary note 1. The Adsorption Energy Calculation
The adsorption energy was calculated as 19 : where E ads/slab , E slab and E ads are the energy of the adsorbate on the surface, the energy of the clean surface and the energy of the isolated adsorbate, respectively. The negative ∆E ads indicates exothermic adsorption. The more negative of ∆E ads , the more stable is the adsorption.

Supplementary note 2. The Free Energy Calculation
The free energy of each state is calculated through eq. S2: Where E is the electronic energy obtained directly from DFT calculation. The zero point energy The free energy of gaseous CO was obtained through the following reaction at standard state: This is because the energy of CO obtained using the pseudopotential method lead to un-negligible error of the reaction free energy (∆Gr) of re. S1 with respect to the experimental value (∆Gr =0.24 eV). Then we have: G(CO(g))= ∆Gr-G(H2O(l))+ G(CO2(g))+ G(H2(g)) (eq. S3) The computational hydrogen electrode (CHE) model was used to include the electrode potential correction and the pH correction to the free energy of each state 21 . In this model, the free energy of a proton-electron pair at U vs RHE is defined to be: G(4) = G(CO(g)) + G (H2O(l)) + G(surf) (eq. S8) Based on eqs. S5-S8, we can obtain the free energy profile which provides the thermodynamical information of the reaction.
The limiting potential (UL) is defined as the potential at which the electrochemical elementary step becomes exergonic (or downhill in free energy). It can be calculated as: The difference between the equilibrium potential of the overall electrochemical reduction of CO2 to CO and the min{UL_1, UL_2…UL_M} (M is the number of the electrochemical elementary steps) is the overpotential 22 .

Supplementary note 3. DFT calculations on the Ag (100), and (110) related facets.
The Ag (111), (100) and (110) facets were corresponding to the three intensive peaks observed in the XRD pattern (Fig. 1a), and are often considered as the representative models in theoretical studies 23 . Based on the result on Ag (111), we constructed proper vacancy models for Ag (100) and Ag (110). The most stable binding geometry of COOH on pristine Ag (100) (∆Eads = -1.98 eV) is similar to that on pristine Ag (111) (∆Eads = -2.0 eV) (Fig. 5, and Supplementary Fig. 22). The computed reaction free energy diagrams (Supplementary Fig. 24) depict that on all three studied Ag crystal surfaces, the activation of carbon dioxide by protonation to form *COOH is the potential determining step. The Ag (110) surface has the highest limiting potentials (UL) among three surfaces, revealing that Ag (110) has the highest CO evolution activity, which is consistent with the previous study 23 . The strengthened binding of COOH results in higher limiting potentials on the vacancy containing surfaces than that on the pristine surfaces.