Comparative Study between Pristine Ag and Ag Foam for Electrochemical Synthesis of Syngas with Carbon Dioxide and Water

Abstract

The electrosynthesis of syngas (H₂ + CO) from CO₂ and H₂O can reduce greenhouse gas emissions and address the energy crisis. In the present work, silver (Ag) foam was employed as a catalytic electrode for the electrochemical reduction of CO₂ in aqueous solution to design different syngas ratios (H₂:CO). In addition to H₂ and CO, a small amount of formic acid was found in the liquid phase. By contrast, the planar polycrystalline Ag yields CO, formic acid, methane and methanol as the carbon-containing products. During the potential-controlled electrolysis, the Ag foam displayed a relatively higher activity and selectivity in the electroreduction of aqueous CO₂ to CO compared with its smooth surface counterpart, as evidenced by the lower onset potential, higher partial current density and Faradic efficiency at the same bias voltage. Moreover, the electrode remained stable after three successive cycles. Based on the characterization using X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, potential step determination and density functional theory calculations, superior performance was credited to the three-dimensional structure of Ag foam constructed with coral-like Ag particles, in which the numerous edge sites are beneficial for the stabilization of the surface adsorbed COOH species and the exposed {111} facets favor the desorption of adsorbed CO species.

1. Introduction

The exponential increase in anthropogenic carbon dioxide (CO₂) emissions to the Earth’s atmosphere, has become one of the most urgent problems for the human society [1,2]. Many feasible methods for decreasing the concentration of CO₂ in the atmosphere have been carried out, such as CO₂ capture, utilization and storage [3,4,5,6,7,8,9]. Among these methods, the conversion of CO₂ to useful chemicals with the aid of renewable energy sources (wind, solar, tidal, etc.) plays an important role in addressing current environmental and energy challenges associated with the continued use of fossil fuels [10,11]. However, a major challenge in the implementation of direct renewable energy source-driven CO₂ reduction is the intermittency of energy input, leading to a mismatch between energy supply and demand [12]. Since solar energy, as well as wind and tidal energy, can be stored in the form of electricity, the electrochemical reduction of CO₂ to value-added chemicals is recognized as a potential way to minimize CO₂ emissions and replace fossil fuels [13,14]. In this manner, CO₂ can be transformed to various carbon compounds, including CO, methane, formic acid, alcohols and higher molecular weight hydrocarbons, such as oxalic acid [12,15]. In particular, CO, as a main component of syngas, is an attractive product because of the two-electron reduction process of CO₂, which seems to provide the best chance for potential industrial application [16,17].

Syngas is a versatile feedstock for producing bulk chemicals and synthetic fuels. The H₂:CO ratio in syngas mixtures can be tuned to 0.6, 0.3–1.0 and 2.0 for Fischer−Tropsch synthesis of hydrocarbons, emerging fermentation and short-chain thermochemical and methanol syntheses, respectively [18]. The co-electrolysis of aqueous CO₂ is desired for large-scale processes and offers a viable alternative to adjust the syngas mixture composition [19].

One bottleneck in the CO₂-water electrochemical reduction to syngas is the preference of the hydrogen evolution reaction (HER). This is because the standard reduction potential for H⁺/H₂O reduction to H₂ at pH 7.0 is −0.41 V versus NHE at pH 7.0, higher than that for CO₂ electroreduction to CO (−0.53 V vs. NHE at pH 7.0) [13,20]. To overcome this limitation, the selection of a suitable electrode that could selectively produce CO seems to be an effective approach for the controllable preparation of syngas. Gold (Au) and silver (Ag) are the two most favorable metal electrodes, with a preference to reduce CO₂ to CO [21]. Compared with Au, Ag is less expensive and thus is considered as a promising metallic material for translating CO₂ into a tunable syngas by varying the electrolyzing potential [22]. Another challenge in the electroreduction of CO₂ is that the reactor should operate at a current density of at least 100 mA cm⁻², while still maintaining the CO Faradic efficiency at > 50% [23]. To this end, numerous efforts have been devoted to resolving the mass transfer and reaction kinetics of the electroreduction of CO₂ by improving the electrode, electrolyte and reactor configuration [24,25].

Ag foam is a low density solid with a highly porous structure, providing a large specific area and enabling high permeability. Compared with the flat electrode, the electrons in Ag foam can disperse in a three-dimensional space rather than confined within a two-dimensional space [26]. It is expected that the Ag foam would yield syngas with excellent selectivity at a relatively high current density. Thus, detailed work is required to determine the electroreduction of CO₂ in aqueous solution on Ag foam cathodes.

In the current work, we compare the electrochemical performance of Ag foam and pristine Ag in a CO₂-saturated KHCO₃ solution with a concentration of 0.1 M. The catalytic electrode was characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and potential step determination. The Faradic efficiency (FE), partial current density (PCD) and cyclic use of the electrode were determined via potentiostatic electrolysis. Finally, the possible advantage of Ag foam over pristine Ag was elucidated with the help of density functional theory (DFT) calculations, as well as structural and morphological characterization.

2. Results and Discussion

2.1. Electrode Characterization

2.1.1. XRD

The crystallographic structure of Ag electrodes was investigated by XRD. All the peaks for the Ag foam and pristine Ag were indexed to the cubic phase of Ag (JCPDS card 01-1167). The peaks at 38.1°, 44.37°, 64.18° and 77.55° corresponded to (111), (200), (220) and (311) diffraction planes. No additional obvious characteristic peak was obtained, suggesting the purity of Ag electrodes. Compared with pristine Ag, the relatively sharp peaks of the Ag foam indicate the high crystallinity of the sample (Figure 1). In addition, 2θ values of peak maxima remained unchanged before and after the reaction either for Ag foam or for pristine Ag, suggesting that the crystal structure of the catalyst was sustained during the electro-reduction of CO₂.

2.1.2. FESEM and High-Resolution TEM (HR-TEM)

The morphology of Ag electrodes was visualized by FESEM. Figure 2a displayed the image of pristine Ag with a rather flat surface and no special morphology. By contrast, a cross-linked grid structure was observed for the Ag foam, as shown in Figure 2b. Furthermore, the typical high magnification scanning electron microscopy (SEM) image of the inset of Figure 2b showed that the Ag foam was constructed of coral-like Ag particles with a particle size of 3–15 μm. Furthermore, it can be observed that the morphology of pristine Ag and Ag foam (Figure 2(a’,b’)) remained unchanged after the reaction, indicating that the electrodes exhibit excellent structural stability.

To gain a deeper understanding, Ag foam and pristine Ag were further analyzed using HR-TEM. As depicted in Figure 3a, the measured d spacing of 0.24 nm in the HR-TEM image corresponds to the (111) plane of Ag and no other lattice fringes were measured [27,28], indicating that the Ag foam was completely bound by {111} facets. Contrarily, in addition to (111) facet, the d-spacing of 0.21 and 0.15 nm in the pristine Ag can be observed in Figure 3b, corresponding to the (100) and (110) facet, respectively. In other words, the surface of pristine Ag was enclosed by a mix of {111}, {100} and {110} facets. It is worthwhile noting that no obvious change in exposed facet was found for the fresh and spent electrodes. These results demonstrated that the facet restructure induced by the electro-reduction process hardly occurred.

2.1.3. Roughness Factor and Uncompensated Solution Resistance

The electrochemical area of the electrodes was evaluated by calculating the roughness factor. The roughness factor values for pristine Ag and Ag foam were calculated to be 33.4 and 44.5 cm² cm⁻², respectively. Obviously, the former is slightly less than the latter. It should be noted that the pore size at the range of <2.0 nm (micropores) and 2–50 nm (mesopores) contribute mostly to the specific surface area of a material. In this work, the Ag foam electrode contained large amount of pores in micrometer level. Therefore, the roughness factor of the Ag foam was not obviously greater than that of pristine Ag.

Figure 4 presents the Nyquist plots of Electrochemical Iimpedance Spectroscopy (EIS) for the Ag foam and pristine Ag. The Ru of pristine Ag and Ag foam, derived from the fitting with ZView software (version 3.3, Scribner Associates, Southern Pines, NC, USA), was 28.9 and 32.5 ohm, respectively.

2.2. Electrode Selectivity

The applied potential is usually used for controlling the selectivity of an electrochemical reaction. The FE was examined to investigate the effect of applied potential on the overall CO₂ conversion process. A typical variation in FE of CO, methane, formic acid, methanol and H₂ as a function of potential is shown in Figure 5a,b, in which the potentials ranged from −0.53 to −2.97 V for Ag foam and −0.56 to −2.26 V for pristine Ag. It can be seen that Ag foam produces CO and H₂ as major products, accompanied by less solution phase product of formic acid over the entire investigated potential range. Different to pristine Ag, in addition to H₂, CO and formic acid, methane and methanol were accurately detected when the potential was greater than −1.07 and −1.41 V, respectively. During the electroreduction of CO₂ in aqueous solution, the HER inevitably occurred as hydrogen evolution becomes thermodynamically possible at 0 V versus RHE (at any pH) [20,29,30,31]. Consequently, the HER would compete with the CO₂ reduction at all negative potentials. The evolved H₂ combining with the major product CO became the main composition of so-called syngas [12,15,18,19,22,32,33,34]. For both electrodes, the total calculated FE of (H₂ + CO) gaseous products was above 90% and other products, such as formic acid, methane and methanol, accounted for no more than 10% of the total FE. In other words, the Ag electrode seems to be a proper alternative for supplying syngas via the electroreduction of aqueous CO₂.

For Ag foam, CO and formic acid were detected at the starting potentials of −0.53 and −0.68 V, respectively. Methane or methanol was not detected in this work. FE_CO and FE_HCOOH showed maximum values of 82.9% at −1.12 V and 4.0% at −1.46 V in the potential range of −0.53–−2.97 V. The pristine Ag afforded different onset potentials with respect to Ag foam, which were −0.56 and −0.72 V for CO and formic acid, respectively. The variation in FE_CO and FE_HCOOH with applied potential exhibited a trend similar to that for Ag foam with maximum values of 79.0% and 3.1% at −1.12 and −1.35 V, respectively. Obviously, the onset potential for the reduction of CO₂ to CO and formic acid on Ag foam shifted ~30 and ~40 mV and was more positive than that on pristine Ag, suggesting that the Ag foam electrode was more active than the polycrystalline Ag electrode.

It should be noted that the onset potential for CO, formic acid, methanol and methane formation on the pristine Ag electrode was at variance with the trend in their redox potentials. The redox potentials of the two-electron reduction of CO₂ to form CO or formic acid, the six-electron reduction of CO₂ to methanol and the eight-electron reduction of CO₂ to methane are −0.53, −0.42, −0.38 and −0.24 V, respectively [22,27,33,35,36,37]. Therefore, the CO₂ electrochemical reduction reaction was truly under kinetic control compared with thermodynamic control in this study.

Greater differences in the CO₂ reduction selectivity between Ag foam and pristine Ag were observed at the potential window for providing synthetic gas (H₂ + CO). From Figure 6, it can be seen that an increased H₂ to CO ratio was obtained at more negative potentials. This is because the mass transfer of CO₂ molecules from the bulk solution to the electrode surface dominated the CO₂ electroreduction process with gradually increasing overpotentials, whereas the water reduction to H₂ was rarely mass transfer limited. As an example, if we want to obtain syngas with a H₂:CO ratio of 0.3–1.0 (corresponding to 50.0–76.9% selectivity of CO), the potential ranges would be approximately −0.65–−2.77 V and −0.72–−1.56 V for Ag foam and pristine Ag, respectively. Obviously, Ag foam afforded a wider potential window than pristine Ag in the electrochemical reduction of aqueous CO₂ to syngas.

The electrochemical reduction of CO₂ on the Ag electrode proceeded via a series of steps as follows:

Step 1: CO₂ (g) + H⁺ (aq) + e⁻ + * → *COOH

Step 2: *COOH + H⁺ (aq) + e⁻ → *CO + H₂O (l)

Step 3: *CO → CO (g) + *

*COOH and *CO are important intermediates during the reduction of CO₂ [21,22,24,32,38,39,40]. The binding energy of reaction intermediate plays a vital role in the reaction selectivity [41]. Therefore the binding energy of COOH and CO on different Ag surfaces were calculated and summarized in

Table 1. The binding energy of COOH and CO on Ag surfaces.

Facets	ECO (eV)	ECOOH (eV)
Ag(111)	−0.15	−1.15
Ag(110)	−0.33	−1.44
Ag(100)	−0.35	−1.62

. As shown in Table 1, COOH binds the strongest with Ag(100), followed by Ag(110) and Ag(111) surfaces. The strong interaction of COOH on Ag surfaces allowed COOH to stay on the surface and undergo the following reaction steps, which would facilitate the formation of *CO. However, as shown in Table 1, Ag(100) and Ag(110) present strong binding energy of CO among the three Ag surfaces, preventing the desorption of CO from Ag(100) and Ag(110) surface. Different from the case on Ag(110) and Ag(100), once *CO was formed, it would desorb from the Ag(111) surface rapidly due to its weak binding energy on this surface, leading to a higher reaction selectivity on Ag(111).

In addition, the number of edge sites and corner sites of Ag is another factor influencing the CO₂ reduction reaction. The catalytic activity of Ag differed significantly with active sites of edge, corner and facet, in the order of edge > corner> facet [21,38,42]. The Ag foam consisted of coral-like particles on the micrometer scale, providing more low-coordinated edge and corner atoms than pristine Ag for CO₂ electroreduction. This property combined with the positive effect of Ag(111) facets for liberating *CO from the surface resulted in higher selective reduction of CO₂ to syngas by Ag foam than pristine Ag.

2.3. Electrode Activity

The catalytic activity of the electrochemical reduction of CO₂ can be compared by the current density. In Figure 7a,b, the PCD_CO, PCD_H2, PCD_syngas (PCD_CO + PCD_H2) and the total current density are plotted as a function of cathode potential. Obviously, the total current density gradually increased with increasing applied negative potential for pristine Ag and Ag foam electrodes. In addition, Ag foam displayed ~1.2-fold higher total current density compared with pristine Ag in the range investigated.

At a H₂:CO molar ratio of 0.3–1.0, PCD_syngas of pristine Ag and Ag foam showed a similar increasing trend with negatively shifted the electrolysis voltage, as shown in Figure 8. Pristine Ag produced the maximum current density of 69.58 mA cm⁻² at −1.56 V, at the same current density, the potential on the Ag foam cathode was shifted positively by 180 mV. As a consequence, Ag foam displayed an elevated preference for the electroreduction of CO₂ to syngas. It should be noted that the maximum current density of 237.58 mA cm⁻² for PCD_syngas production was reached on Ag foam at a potential of −2.77 V, which was ~3.4-fold greater than that observed on pristine Ag (69.58 mA cm⁻² at −1.56 V). Therefore, Ag foam resulted in a much higher current density in the electrochemical reduction of CO₂ to CO and thus seems to be a promising electrode with great potential for industrial applications.

To exclude the influence of surface area, the values of the PCD per unit of electrochemical surface area of electrode were calculated and the results are shown in

Table 2. The specific activity for the formation of CO on Ag foam and pristine Ag.

Sample	Roughness Factor (cm2 cm−2)	Potential (V vs. RHE)	PCDCO (mA cm−2)	SACO (10−5 A cm−2)
Ag foam	44.5	−0.53	1.20	2.70
		−0.60	2.97	6.68
		−0.65	7.38	16.59
pristine Ag	33.4	−0.56	0.34	1.02
		−0.61	1.15	3.46
		−0.68	3.11	9.32

and Figure 9. As one of the components of syngas, CO was produced on Ag foam with a SA greater than that on pristine Ag. Consequently, the intrinsic properties other than surface area, such as exposed crystal faces, edges and corner sites, contributed to the improved activity of the electroreduction of CO₂ to CO.

As discussed previously, Ag(111) is a suitable electrocatalyst for stabilizing *COOH and liberating *CO. Furthermore, the edge sites of Ag would enhance the activation of CO₂ to *COOH. Consequently, the Ag foam electrode with fully exposed {111} facets and more edge sites, in comparison with polycrystalline pristine Ag electrode, quickly reduced CO₂ to *COOH and *CO and released *CO from the electrode surface in the form of CO. A comparison of the activity and selectivity achieved with these Ag electrocatalysts with previous references is provided in

Table 3. The comparison of the activity and selectivity with different Ag electrodes.

Morphology of Ag Electrodes	Electrolyte	Selective Production of CO
		Applied Potential V (vs. RHE)	Faradaic Efficiency (%)	Current Density (mA cm−2)	Ref.
Nanoparticles	0.5 M KHCO3	−0.75	79	1	43
Nanoporous	0.5 M KHCO3	−0.60	92	18	30
Nanocorals	0.1 M KHCO3	−0.60	95	6.62	39
Oxide-derived	0.1 M KHCO3	−0.80	89	1.15	44
Compact grains	0.1 M KHCO3	−1.1	88.87	5.73	7
Truncated hexagonal bipyramidal	0.1 M KHCO3	−0.93	89.4	4.92	27
Plate	0.1 M KHCO3	−1.12	79.0	22.93	In our work
Foam	0.1 M KHCO3	−1.12	82.9	27.43	In our work

. It demonstrated that the performance of Ag foam have a superior activity and comparable selectivity of CO production to similar reported materials [7,27,30,39,42,43]. In other words, the Ag foam afforded a higher PCD_syngas than pristine Ag in the aqueous electrochemical reduction of CO₂.

2.4. Electrode Stability

The stability of an electrode is a critical challenge in commercial applications. Taking the first applied potential with the syngas H₂:CO molar ratio of 1 as an example, the variation in FE_CO and FE_H2 as a function of time are plotted in Figure 10. During the 4-h reaction period, the variation in FE_CO and FE_H2 on Ag foam compared with pristine Ag showed a different tendency. For Ag foam, the FE_CO rapidly increased to the maximum value of 56.94% in the first 30 min of electrolysis and then slowly weakened to 54.96% in the next reaction period. By contrast, the FE_CO reached the maximum value of 54.39% at 60 min and promptly dropped to 39.78% at the end of the reaction with pristine Ag. The slight decrease in FE_CO with the Ag foam electrode may have been due to the mass transfer limitation of CO₂ in the batch experiment, as seen previously [9,20,23,24,35]. Different to Ag foam, we believe that the striking decrease in FE_CO on pristine Ag stemmed from a poisoning effect rather than the limitation of mass transfer.

The Ag foam had a higher PCD_CO at −0.65 V (7.38 mA cm⁻²) than pristine Ag at −0.72 V (6.23 mA cm⁻²). Thus, Ag foam consumed more soluble CO₂ in the catholyte than pristine Ag. Theoretically, the concentration decrease in CO and the corresponding concentration increase in H₂ on pristine Ag should be slower than those on the Ag foam electrode. Nonetheless, the curves in Figure 10 clearly display the opposite trend.

As shown in previous studies, the overbinding of *CO and/or protonated *CO can lead to surface poisoning of the Ag electrode [20,44,45]. It is well known that the stability of adsorbed species is closely related to the exposed facets of the crystals, which directly affect the properties of the catalysts. As previously mentioned, the binding energy of CO for different crystal planes of Ag follows the order: Ag(110) > Ag(100) > Ag(111). Therefore, *CO was desorbed more easily on Ag(111) than on Ag(110). Thus, relative to pristine Ag enclosed with mixed (111), (110) and (100) facets, Ag foam with only a single exposed facet of (111) could maintain its activity and prolong the electrolysis time.

To further elucidate the high stability of the Ag foam electrode, we performed recycling experiments at a high potential of −2.67 V, where the molar ratio of H₂ to CO was ~1. As displayed in Figure 11, no obvious difference in the variation of FE_CO and FE_H2 was observed for Ag foam over three runs of electrochemical reduction of CO₂. After each run, FE_CO returned to its original activity upon resaturation of the catholyte with CO₂. The results of the time-dependent experiments confirmed that the Ag foam was not deactivated during the course of electroreduction of CO₂ in aqueous solution.

3. Experimental

3.1. Materials and Reagents

Ag foam and pristine Ag (99.99% purity) were purchased from Kunshan Jiayisheng Electronics Co., Ltd., Kunshan, China and Jinjiang Dongyin Jewelry Co., Ltd., Jinjiang, China, respectively. CO₂ (purity 99.995%), CO (purity 99.99%), H₂ (purity 99.999%) and methane (purity 99.99%) were provided by Hangzhou Jingong Special Gas Co., Ltd., Hangzhou, China. All the reagents used in the present study, including NaOH, KHCO₃, KCl, acetone and ethanol, were of analytical grade and were obtained from Huadong Medical Co., Ltd., Hangzhou, China. Milli-Q water (18.2 MΩ) was used in all experiments.

3.2. Electrode Characterization

XRD patterns were recorded by a PANalytical X’Pert Pro X-ray diffractometer using Cu Kα radiation (k = 1.5406 Å) with a scan step of 0.05° between 10° and 80°. FESEM was performed on a Hitachi S-4800 operating at an accelerating voltage of 5 kV. TEM and high-resolution TEM (HR-TEM) images were observed with a Philips-FEI Tecnai G2 F30 S-Twin microscope with an accelerating voltage of 300 kV.

The surface roughness of the electrodes was obtained by calculating the roughness factor (ρ) according to Equation (1).

$$\rho =\frac{{{\displaystyle \int }}_0^t\left(I-I_{\infty }\right)dt}{\mathsf{\Delta }\phi \times C\times S}$$

(1)

where I and I_∞ denote the current and stable current value after a step, respectively, S is the geometric area of the electrode, which is 0.25 cm² (0.5 cm × 0.5 cm in length and width) for both pristine Ag and Ag foam and C is equal to 20 µF cm⁻², representing the double-layer capacitance for an ideally smooth electrode. I and I_∞ were obtained by using the multi-potential step mode of an electrochemical workstation (CHI760E, CH Instrument, Bee Cave, TX, USA) in a 0.5 mol L⁻¹ NaClO₄ aqueous solution at 20 °C. The potential of the electrodes was held at 0.10 V versus the saturated calomel electrode (SCE) for 10 s and then stepped to 0.095 V versus SCE for 10 s. Consequently, the t and Δφ in Equation (1) were 10 s and 5 mV, respectively.

The uncompensated solution resistance (R_u) were determined from EIS measurements. The EIS experiments were conducted in the dark at room temperature, with a frequency range of 1000 kHz to 0.01 Hz and AC amplitude of 0.005 V.

The DFT calculations of E_CO and E_COOH were performed by the Vienna Ab initio Simulation Package with PW-91 functions in the generalized gradient approximation calculation. A kinetic cutoff energy of 396 eV was used for the plane wave truncation and a periodic 3 × 3 unit cell with a 3 × 3 × 1 Monkhorst-Pack k-point grid were used for all calculations. All surfaces were modelled by adding six equivalent layers of vacuum onto four layers of metal atoms corresponding to the most close-packed configurations. The two bottom layers of the slab were fixed, a lattice parameter of 4.09 Å was used for Ag, whereas the top two layers were allowed to relax to reach the lowest energy configuration. The binding energy was calculated using the equation:

$${\mathrm{E}}_{\mathrm{adsorbate}/\mathrm{slab}}={\mathrm{E}}_{\mathrm{adsorbate}/\mathrm{slab}}-{\mathrm{E}}_{\mathrm{slab}}-{\mathrm{E}}_{\mathrm{adsorbate}}$$

(2)

where E_{adsorbate/slab} is the binding energy of adsorbate on the given slab, E_{adsorbate/slab} is the energy of the surface with adsorbate, E_slab is the energy of the slab in vacuum and E_adsorbate is the energy of adsorbate in the gas phase. Different adsorption sites, such as atop, bridge, fcc and hcp, were calculated for CO. E_COOH investigated the formate chemisorption in a symmetric bidentate configuration at the bridge on Ag surfaces.

3.3. Electrochemical Reduction of CO₂

All electrochemical reduction experiments were conducted using a three-electrode configuration controlled by a CHI 760E potentiostat. The reactor was a gas-tight two-compartment electrochemical cell with a cation-exchange membrane (Nafion-117) as the separator. Ag foam or pristine Ag (50 mm × 50 mm × 0.3 mm) was used as the working electrode, which was cleaned and degreased with acetone, ethanol and Milli-Q water for 1.5 h, washed thoroughly and dried by N₂. A platinum sheet was employed as the counter electrode. All potentials were measured against the SCE and converted to a reversible hydrogen electrode (RHE) scale based on the Nernst equation. The anolyte and catholyte were 0.5 M NaOH and 0.1 M KHCO₃ with solution saturated with CO₂ (pH 6.8), respectively.

Potential conversion to a RHE scale and iR_u corrections were made to assess the activity and selectivity of the electrode on the same bias, as shown in Equation (3).

E = E (vs. SCE) + 0.2412 V + 0.0591 V × pH − iR_u

(3)

where E and E (vs. SCE) are the final reported potential and applied potential, respectively, i is the average current and R_u is the uncompensated solution resistance.

The FE value of the electrode catalyst for CO₂ electroreduction was calculated using the following equation:

$$\mathrm{FE}=\frac{z\times n\times \mathrm{F}}{Q}$$

(4)

where z represents the number of electrons exchanged (z is equal to 2 for the reduction of CO₂ to HCOOH and H₂, respectively), n is the number of moles of a given product, F is Faraday’s constant (96485 C mol⁻¹) and Q is the charge passed (C).

The PCD of productions was calculated from the gas chromatograph (GC) peak area as follows:

$$\mathrm{PCD}=j_{\mathrm{total}}\times \mathrm{FE}=j_{\mathrm{total}}.\frac{z\times n\times \mathrm{F}}{Q}$$

(5)

where j_total is the total current density, n is the molar of productions and Q is the amount of charge passed during the reaction.

3.4. Analytical Procedures

The GC (7890B, Agilent, Santa Clara, CA, USA) was equipped with a HP-MOLESIEVE capillary column (30 m × 0.32 mm × 12 mm). Helium (99.999%) was used as the carrier gas. A thermal conductivity detector was used to quantify the H₂ and CO concentrations. The amount of methane in the gas phase was measured using GC (GC-2014, Shimadzu, Tokyo, Japan) equipped with a flame ionization detector (FID) and an HPPLOT/Q column (30 m × 0.32 mm × 20 mm). The concentration of methanol volatilized from the liquid phase to the gas phase was quantified by GC (7890B, Agilent, USA) equipped with a FID and an HP-INNOWAX capillary column (30 m × 0.25 mm × 25 mm). The liquid phase product formic acid dissolved in the catholyte was determined using ion chromatography (ICS2000, Dionex, Sunnyvale, CA, USA) equipped with a conductivity detector (DS6, Dionex), a guard column (IonPac AG19, Dionex, Sunnyvale, CA, USA), an analytical column (IonPac AS19, Dionex, Sunnyvale, CA, USA) and a dual-piston (in series) pump. Suppression of the eluent was achieved using a Dionex anion ASRS electrolytic suppressor (4 mm) in the auto-suppression external water mode.

4. Conclusions

We demonstrated that Ag foam is an efficient catalyst for the electrochemical reduction of CO₂ to syngas in aqueous solution. Ag foam was superior to pristine Ag in terms of the onset potential of CO, Faradic efficiency and partial current density of syngas. When producing CO (one of the main components of syngas), Ag foam showed a ~30 mV more positive onset potential than pristine Ag. The FE_CO and PCD_syngas of Ag foam were maximum at −1.12 and −2.77 V (82.9% and 237.58 mA cm⁻², respectively), compared to a maximal FE_CO value of 79.0% at −1.12 V and PCD_syngas of 69.58 mA cm⁻² at −1.56 V for pristine Ag, respectively. For the molar ratio of H₂:CO at 0.3–1.0, Ag foam showed a wider potential window of 2.12 V than pristine Ag (0.84 V). In addition, Ag foam displayed fairly stable electrolysis performance in the three cyclic durability tests. DFT calculations demonstrated that the low-index Ag (111) plane favors the desorption of *CO. Furthermore, the edge sites govern the stabilization of *COOH. Namely, the abundant edge sites of coral-like Ag with large number of {111} facets could increase the selectivity, activity and stability of the Ag foam electrode. This Ag foam electrode provides an option for obtaining a flexible ratio of H₂:CO in the production of syngas to meet the requirements of downstream products.