ClAg14(C≡CtBu)12 Nanoclusters as Efficient and Selective Electrocatalysts Toward Industrially Relevant CO2 Conversion

Abstract Atomically precise metal nanoclusters (NCs) have emerged as a promising frontier in the field of electrochemical CO2 reduction reactions (CO2RR) because of their distinctive catalytic properties. Although numerous metal NCs are developed for CO2RR, their use in practical applications has suffered from their low‐yield synthesis and insufficient catalytic activity. In this study, the large‐scale synthesis and electrocatalytic performance of ClAg14(C≡CtBu)12 + NCs, which exhibit remarkable efficiency in catalyzing CO2‐to‐CO electroreduction with a CO selectivity of over 99% are reported. The underlying mechanisms behind this extraordinary CO2RR activity of ClAg14(C≡CtBu)12 + NCs are investigated by a combination of electrokinetic and theoretical studies. These analyses reveal that different active sites, generated through electrochemical activation, have unique adsorption properties for the reaction intermediates, leading to enhanced CO2RR and suppressed hydrogen production. Furthermore, industrially relevant CO2‐to‐CO electroreduction using ClAg14(C≡CtBu)12 + NCs in a zero‐gap CO2 electrolyzer, achieving high energy efficiency of 51% and catalyst activity of over 1400 A g−1 at a current density of 400 mA cm−2 is demonstrated.

The equivalent electric circuit [2] used to fit the electrochemical impedance spectra is shown in the inset of (a).Rs, solution resistance; Rct, charge-transfer resistance; Qdl, constant phase element for double layer.The jCO of ClAg14/GDE linearly increased with increasing the NC loading and levelled off at the loading of over 106 nmol/cm 2 (0.28 mg/cm 2 ).Since the jCO value levelled off at higher loading, the CO2RR activities were evaluated up to their levelling points; 0.56 mg/cm 2 and 0.23 mg/cm 2 for the ClAg14/GDE and Ag25/GDE, respectively.[a] Agocta and Agcubic denote octahedral Ag6 and cubic Ag8, respectively.
[b] The values in square brackets are fractional expressions of each value.

Figure S2 .
Figure S2.(a) Digital photograph showing single crystals of [ClAg14(C≡C t Bu)12] + [BF4] -NCs grown at 25 °C by layering diethyl ether over a CH2Cl2 NC solution.(b) Core structure of the ClAg14 NCs obtained using SC-XRD analysis.(c) Dissecting representations of the core framework.

Figure S4 .Figure
Figure S4.Comparison of the powder XRD pattern of the cluster product synthesized over a 10-gram scale (red line) with the simulated diffractogram based on the crystal structure of ClAg14 NCs (green line).

Figure S6 .Figure S7 .
Figure S6.Resistance (R) components obtained from fitting of the Nyquist plots in Figure 2b.Rs, solution resistance; Rct, chargetransfer resistance.Rs value remains unchanged, but Rct value largely decreased with activation.

Figure S8 .
Figure S8.Representative TEM images of ClAg14/GDE before and after electrochemical activation at -0.96 V for 1 h.The insets show histograms of core diameters measured in several TEM images.The average diameters were determined to be 1.0 ± 0.2 nm for both ClAg14 NCs before and after activation.

Figure S9 .
Figure S9.Representative fitted R space (azure) and k space (orange) Ag K-edge EXAFS spectra of the (a) pristine and (b) activated ClAg14 NCs.(c) The optimized structure of the ClAg14 NCs after losing four ligands.Alkyl chains are shown in wireframe.

Figure S10 .Figure
Figure S10.(a) Comparison of turnover frequency (TOF) values of various Ag-and Au-based NCs measured in H-cell electrolyzers.The NC loadings were in the range of 0.03-0.5 mg/cm 2[1] and the ClAg14 loading was 0.28 mg/cm 2 (10.6 nmol/cm 2 ).(b) Comparison of jCO and CO selectivity values of ClAg14/GDE measured in H-cell and gas flow cell.pH-independent standard hydrogen electrode (SHE) scale was used to compare data measured in various pH conditions.The potentials were iR-corrected.

Figure S14 . 2 GDEFigure S15 .
Figure S14.Schematic of the operando ATR-FTIR experimental setup.The electrochemical cell was filled with 1.0 M NaClO4 electrolyte solution and the solution was purged with CO2 gas for 30 min before conducting operando experiments.The CO2 gas was kept blowing to the headspace of the cell during the experiments.The activated ClAg14/GDE was tightly mounted on the ATR crystal with the MPL side facing down the crystal, allowing a very thin layer of electrolyte solution between them.

Figure S20 .Figure S21 .
Figure S20.LSV traces recorded at 20 mV/s on the NF electrode in a one-compartment three-electrode cell containing 1.0 M KOH electrolyte solution.The potentials were iR-corrected.

Table S1 .
Summary of the Ag K-edge fitting results of EXAFS for pristine and activated ClAg14 NCs.

Table S2 .
Benchmarks of CO2RR performance of metal NCs in H-cells