Synthesis and characterization of np-Ag

The monolithic np-Ag catalyst was obtained by two-step dealloying of an Ag–Al precursor using an aqueous HCl solution. By selectively etching Al through dealloying, the remaining Ag atoms were reorganized to form a three-dimensional interconnected nanoporous structure. The dealloying process has been reported in a few alloyed systems26,27 and the resulting materials have shown unique catalytic performance such as in fuel cells28,29,30 and alcohol oxidation31. However, there is still no report on their catalytic properties for CO 2 reduction. A typical scanning electron microscopy (SEM) image of the as-synthesized np-Ag catalyst is shown in Fig. 1b. The ligament size of np-Ag is ~50–200 nm, while the size of the pores extends to a few hundred nanometres. The high-resolution transmission electron micrograph exhibits uniform lattice fringes, and the Fourier transform of the image shows a clear crystallographic symmetry (Fig. 1c). Both techniques suggest that the resulting np-Ag is highly crystalline (single-crystal-like), which is further confirmed by powder X-ray diffraction (XRD) data (Supplementary Fig. 1). Additional SEM studies confirm that the resulting nonporous structure is coherent throughout the material (cross-section SEM, Supplementary Fig. 2), and no Al residue was detected in either X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 3) or energy-dispersive spectroscopic analysis.

CO 2 reduction performance of the np-Ag catalyst

The CO 2 electroreduction activity of np-Ag and polycrystalline Ag was measured using constant-potential electrolysis experiments in a CO 2 -saturated high purity aqueous KHCO 3 electrolyte (0.5 M). In the case of polycrystalline Ag, the electrolyte was treated using a pre-electrolysis process, although no significant difference in activity between treated and untreated electrolytes was observed (Supplementary Fig. 4). Testing was performed in an airtight electrochemical cell with three electrodes and two compartments separated by an anion exchange membrane with electrolyte in each chamber. Gas-phase products from the headspace of the electrochemical cell were measured using gas chromatography (GC) every 30 min. In addition, liquid-phase products were measured using 1H NMR. CO and hydrogen (H 2 ) were the primary products detected using the GC; however, trace amounts of formate were detected using NMR at potentials more negative than −0.60 V (versus RHE).

The electrocatalytic CO 2 reduction results collected at −0.60 V (versus RHE) are presented in Fig. 2a. The geometric current density is calculated based on electrode area. The operating voltage corresponds to an overpotential of 490 mV since the CO 2 /CO equilibrium potential is at −0.11 V versus RHE. At this moderate overpotential, the np-Ag electrode exhibited a long-term stable current of ~18 mA cm−2 during the 2-h electrolytic CO 2 reduction. A high initial current was observed (Fig. 2a), which stems from the reduction of a thin surface oxide layer, with an estimated thickness of about 0.5 nm, formed during the material handling in atmospheric air. Faradaic efficiency for CO was maintained at ~92% throughout the electrocatalytic process, further confirming that the np-Ag catalyst was stable under electrocatalytic conditions once the surface oxide layer was removed electrochemically. In a sharp contrast, polycrystalline Ag exhibits a very low current density of 470 μA cm−2 with a poor CO Faradaic efficiency of ~1.1% at the same potential.

Figure 2: Electrocatalytic performance of np-Ag. CO 2 reduction activity of np-Ag and polycrystalline silver at (a) −0.60 V, and np-Ag at (b) −0.50 V and (c) −0.40 V versus RHE. Total current density versus time on (left axis) and CO Faradaic efficiency versus time (right axis). Full size image

The performances of np-Ag at more positive potentials (−0.50 V and −0.40 V versus RHE, corresponding to overpotentials of 390 mV and 290 mV, respectively) are shown in Fig. 2b,c. With lower overpotentials, smaller currents were observed as expected, while CO Faradaic efficiencies also decreased as potential decreased. At −0.50 V versus RHE, a stable current of ~9.0 mA cm−2 was observed with a CO efficiency of ~90%, whereas at −0.40 V versus RHE a current of ~3.3 mA cm−2 was reached with a CO efficiency of ~79%. The decrease in CO Faradaic corresponds to an increase in the relative rate of hydrogen evolution. This is most likely from the fact that the overpotential is relatively low to drive CO 2 reduction quickly enough to compete with the hydrogen evolution reaction, which only requires a small overpotential to occur.

To investigate the maximum CO current density that can be achieved, experiments at higher overpotentials were conducted. The maximum current density of np-Ag for CO production is reached at potentials more negative than −0.8 V versus RHE. A further increase in overpotential does not significantly affect the CO partial current density but instead promotes other product formation (primarily hydrogen but also trace amounts of formate). This can be further seen by the decrease of CO Faradaic efficiency, as potentials are scanned more negative than −0.8 V (Supplementary Fig. 5). Similar behaviour has also been observed in other reports and is most probably due to mass transport limitations of CO 2 at high current densities and not the intrinsic activity of np-Ag16,32.

The structural integrity and surface conditions of post-reacted np-Ag were examined using powder XRD, SEM and XPS techniques. No obvious crystal structure or nanoporous morphology change was observed in either powder XRD (Supplementary Fig. 1) or SEM analysis (Supplementary Fig. 6). The XPS result also revealed that the surface of np-Ag remained stable under these electrocatalytic conditions (Supplementary Fig. 3). To further investigate the long-term stability of the catalytic activity of np-Ag, an 8-h electrolysis was performed under the working condition of −0.50 V (versus RHE). No significant decrease in current density was observed and the CO Faradaic efficiency was maintained above 87% throughout the process (Supplementary Fig. 7). The np-Ag catalyst remained remarkably stable during the extended reaction period, as confirmed by the post-reaction XPS (Supplementary Fig. 3) and SEM (Supplementary Fig. 7) studies.

Electrochemical surface area measurement was performed to elucidate the origin of the high activity of np-Ag. The electrochemically active surface area of the np-Ag catalyst was measured by forming an oxide monolayer on the surface electrochemically (Supplementary Fig. 8)33. The np-Ag shows an electrochemical surface area (normalized to electrode area) 150 times larger than that of polycrystalline Ag. As the current densities obtained at −0.60 V (versus RHE) for np-Ag is 3,000 times higher than polycrystalline Ag, there is another 20 times difference that cannot be simply explained by the surface area effect alone. This suggests that the intrinsic activity of catalytic sites on the nanoporous surface is much higher than those on a flat surface. Since the activity of np-Ag is mass transport limited at −0.60 V (versus RHE), later shown by the Tafel analysis (Fig. 3), the intrinsic activity for np-Ag is at least 20 times higher than the polycrystalline counterpart.

Figure 3: Tafel analysis. Overpotential versus CO production partial current density on polycrystalline silver and np-Ag. Full size image