Efficient Electrochemical CO2 Reduction Using AgN3 Single-Atom Sites Embedded in Free-Standing Electrodes for Flow Cell Applications

Abstract

The electrochemical reduction of CO₂ into valuable chemicals presents a promising strategy for carbon utilization; however, it remains challenging due to low activity, poor selectivity and stability of existing catalysts. In this study, we report the fabrication of free-standing silver single-atom catalysts (Ag SACs) designed for the efficient conversion of CO₂ to carbon monoxide (CO) at high current densities in a bicarbonate electrolyzer. The Ag single atoms dispersed within a carbon matrix, forming Ag-N₃ active sites for the electrocatalytic CO₂ reduction reaction (CO₂ RR). The catalytic activity and stability of the free-standing Ag SACs are evaluated at a current density of 100 mA cm^-2, demonstrating prolonged electrolysis with consistent Faradaic efficiency for CO production. Density functional theory calculations revealed that the Ag-N₃ active site significantly lowers the energy barriers for the CO₂ absorption step compared to Ag-Ag and Ag-Ni sites, facilitating CO₂ activation and contributing to enhanced catalytic activity and stability during CO₂ reduction. Detailed analysis of the electronic structure and coordination environment further validates the superior performance of the Ag-N₃ site in the free-standing Ag SACs, underscoring their effectiveness in CO₂ electroreduction. These findings highlight the potential of the free-standing Ag SACs to advance CO₂ reduction technologies, offering enhanced efficiency and selectivity for CO₂ conversion.

Keywords: CO2 reduction; electrolysis; free‐standing electrodes; silver; single‐atom catalysts.

Figure 1

Schematic representation of a MEA for a bicarbonate electrolyzer, comprising flow plates, an anode, a BPM, and a cathode. Oxygen is produced at the anode through the OER, while the BPM facilitates water dissociation. At the cathode, sequential reactions occur: bicarbonate combines with protons to generate CO₂ in situ, which is then electrochemically converted into CO and OH⁻.

Figure 2

Schematic depiction of the synthesis process for the free‐standing Ag SAC electrode.

Figure 3

Characterization of the free‐standing Ag SAC electrode: A) SEM image of the Ag SAC electrode structure (inset shows the completely fabricated electrode). B) High‐magnification SEM image of the Ag SAC electrode surface. C) Aberration‐corrected HAADF‐STEM image of the Ag SAC electrode. D) Elemental mapping illustrating the distribution of silver (Ag) in green, carbon (C) in cyan, and nitrogen (N) in yellow.

Figure 4

High‐resolution XPS spectra of the free‐standing Ag SAC electrode. A) Ag 3d and B) N 1s.

Figure 5

Electronic states of the Ag atom in the free‐standing Ag SAC electrode: A) Ag K‐edge XANES spectra comparing the Ag SAC and reference Ag foil. B) Enlarged view of the rising edge. C) Fourier‐transformed (FT) k²‐weighted EXAFS spectra of the Ag SAC and Ag foil. D) Corresponding EXAFS R‐space fitting curves for the Ag SAC with a schematic model inset illustrating the Ag SAC structure, where Ag (light blue) is bonded with three pyridinic nitrogen atoms (N in blue); carbon atoms are shown in gray.

Figure 6

Electrocatalytic performances of various electrodes in the CO₂ RR: A) LSV curves obtained at a scan rate of 20 mV s⁻¹, B) CA curves recorded at −0.35 V (vs RHE), and C) CP curves measured at −100 mA cm⁻². Broken lines represent Ar‐saturated electrolytes, and solid lines represent CO₂‐saturated electrolytes. FE for CO and H₂ at various applied current densities are shown for D) bare Ni foam, E) free‐standing Ni/C foam without the Ag SAC, and F) free‐standing Ag SAC electrode.

Figure 7

Stability test of the free‐standing Ag SAC electrode: A) CP recorded at −100 mA cm⁻² in CO₂‐saturated electrolyte over 20 h. B) Corresponding FE for CO and H₂ measured during the first and last two hours. Free energy profiles for C) CO₂ reduction and D) hydrogen evolution reaction with Ag—N₃ and Ag—Ag.