An overview on the recent developments of Ag-based electrodes in the electrochemical reduction of CO₂ to CO

Abstract

Conversion of aqueous CO₂ into CO through the electrochemical reduction reaction (ERR) suffers from poor selectivity and low current density with the classic Ag-based catalysts due to the inert nature of CO₂ and the low activity of the active sites. Recently, it has been reported that nanostructured catalysts can display very distinctive behavior compared to their bulk counterparts. To explore the reasons behind this, the latest developments in Ag-based nanostructured materials in the CO₂ ERR are summarized into six aspects in this review: (1) the size effect, (2) the morphology effect, (3) the porous effect, (4) the alloying effect, (5) the support effect, and (6) the surface modification effect. In particular, Ag-based alloy catalysts could exhibit a superior catalytic activity and long-term stability owing to the synergistic effect. Besides, the morphology of Ag-based alloy catalysts could also be controlled through adjusting their compositions. From the current literature survey we find that the maximum FE for CO obtained from Ag-based nanocatalysts reaches as high as 99.3%, which is a breakthrough over that from the traditional Ag foil. This review aims to offer a comprehensive understanding of Ag-based nanostructured catalysts in the CO₂ ERR and consequently improve the catalyst design for active and selective electro-reduction of CO₂ to CO.

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Samah A. Mahyoub

Samah A. Mahyoub is a Ph.D. candidate under the supervision of Dr Prof. Zhenmin Cheng in the School of Chemical Engineering at the East China University of Science and Technology. She obtained her Master's degree from the School of Chemical Engineering at Tianjin University in 2017. Her research is focused on CO2 electrochemical reduction.

Fahim A. Qaraah

Fahim A. Qaraah is a Ph.D. candidate under the supervision of Dr Prof. Guangli Xiu in the School of Environmental Science and Engineering at the East China University of Science and Technology. He obtained his Master's degree from the School of Chemical Engineering at Tianjin University in 2018. His research is focused on nanomaterials synthesis and their applications.

Chengzhen Chen

Chengzhen Chen received his Ph.D. degree under the supervision of Prof. Zhenmin Cheng in the Department of Chemical Engineering at the East China University of Science and Technology. His research is focused on catalyst materials synthesis and CO2 electrochemical reduction.

Fanghua Zhang

Fanghua Zhang is a Ph.D. candidate under the supervision of Prof. Zhenmin Cheng in the Department of Chemical Engineering at the East China University of Science and Technology. He obtained his Master's degree from the School of Chemical Engineering at the Ocean University of China in 2015. His research is focused on novel electrochemical reactor development for CO2 reduction.

Shenglin Yan

Shenglin Yan is a Ph.D. candidate under the supervision of Prof. Zhenmin Cheng in the School of Chemical Engineering at the East China University of Science and Technology. He received his B.S. degree from the College of Chemistry & Environmental Engineering, Yangtze University, in 2017. His research is highly focused on the synthesis of various metal nanostructures for CO2 electroreduction.

Zhenmin Cheng

Zhenmin Cheng has been a Professor of Chemical Engineering at the East China University of Science and Technology since 1999 and he received his Ph.D. degree from the same university in 1993. He was a visiting scholar in the Laboratoire des Sciences du Genie Chimique in France and the University of Queensland in 2001 to 2003. So far, he has published over 130 academic papers in SCI-indexed journals. His research covers multiphase chemical and electrochemical reaction engineering, transport process simulation, and interfacial engineering.

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1. Introduction

CO₂ reduction by electrochemical catalysis has attracted considerable attention because of the following unique advantages: (1) CO₂ is reduced by electrons instead of hydrogen; (2) the electrocatalytic process occurs under mild conditions; (3) the reduction products can be adjusted using the redox potential, reaction temperature, electrolyte, etc.; and (4) by improving the electrocatalysts, side products can be minimized.^1–8 Electrochemical CO₂ reduction could be achieved through multiple electron transfer in an aqueous solution with appropriate electrocatalysts to produce different products (Table 1) such as carbon monoxide (CO), methane (CH₄), ethylene (C₂H₄), and even liquid products, for example, formic acid (HCOOH), methanol (CH₃OH) and ethanol (C₂H₅OH).^6,9 CO, as one of the valuable gases produced by CO₂ electroreduction, can be efficiently separated from other reaction products for the production of a wide variety of chemicals such as acetic acid,¹⁰ phosgene,¹¹ and aldehydes.¹² Moreover, the mixture of H₂ and CO (“syngas”) can be used to produce higher hydrocarbons to be used as vehicle fuels via the Fischer–Tropsch process.^13,14 To date, Au, Ag, Zn, Pd, and Ga have been revealed to be active in the catalysis of the selective conversion of CO₂ to CO.^15–21 In particular, Ag has received the most particular consideration due to its excellent catalytic activity and moderate price. Consequently, the synthesis and characterization of Ag catalysts have been explored extensively.^15,22–26 This review thus aims to summarize the recent progress in the CO₂ electrochemical reduction reaction (CO₂ ERR) to CO using nanostructured Ag catalysts.

Table 1 CO₂ electrolysis products at different standard redox potentials

Reaction	E (V) vs. SHE
2H^+ + 2e^− → H2	−0.41
CO2 + 2H^+ + 2e^− → HCOOH	−0.61
CO2 + 2H^+ + 2e^− → CO + H2O	−0.53
CO2 + 4H^+ + 4e^− → C + 2H2O	−0.20
CO2 + 4H^+ + 4e^− → HCHO + H2O	−0.48
CO2 + 6H^+ + 6e^− → CH3OH + H2O	−0.38
CO2 + 8H^+ + 8e^− → CH4 + 2H2O	−0.24
2CO2 + 12H^+ + 12e^− → C2H4 + 4H2O	0.064
2CO2 + 12H^+ + 12e^− → C2H5OH + 3H2O	0.084

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2. Ag-based nanostructured electrode materials

In a typical CO₂ electrolyzer, the anode and cathode are set in two chambers isolated with an ion-conducting membrane. At the anode, water is oxidized to molecular oxygen, whereas CO₂ is reduced at the cathode, which is a solid electrocatalyst, while the electrolyte is normally an aqueous solution saturated with CO₂ by bubbling. The CO₂ ERR procedure on a catalyst surface involves three main steps: (1) CO₂ adsorption and interaction with atoms on the catalyst surface; (2) CO₂ activation and reduction through catalyst-initiated electron/proton transfer; and (3) product desorption and recovery of the catalyst surface for the next round of the reaction.^8,27 Conversion of CO₂ into CO over an Ag-based electrocatalyst in an aqueous electrolyte is accomplished through the following steps, as shown in eqn (1)–(4):²⁸

COOH* + H⁺_aq + e⁻ → CO* + H₂O (3)

CO* → CO + * (4)

where * represents the vacant catalytically active site. In the above reaction process, one electron transfers to CO₂ in order to produce , which is considered to be the rate-controlling step for most transition metal-based catalysts owing to the high energy barrier required to start the process.^8,27,29 Theoretically, the standard reduction potential for reducing CO₂ in aqueous solution (i.e., pH 7, 1 M electrolyte, 25 °C, 1 atm) into CO is in the range of −0.2 V to 0.2 V (vs. the reversible hydrogen electrode (RHE)).²² The reason for such a high overpotential is often attributed to the high equilibrium potential associated with the formation of the radical intermediate .^30,31 Such a circumstance becomes serious in an aqueous electrolyte-based CO₂ electrolyzer because the side reaction of hydrogen evolution also takes place at a similar potential (Table 1) and competes with CO₂ reduction at the cathode surface. Thus, an efficient electrocatalyst should not only accelerate the CO₂ ERR with significant productivity and selectivity but also suppress the competing HER.³²

2.1 The size effect

A direct strategy to increase the number of active sites in the CO₂ ERR is to use nanoparticles (NPs) as the catalyst. By applying crystal-growth theory to metal nanocrystals, particles with different sizes could be manufactured. The size of the particle directly affects the ratio of the atoms at the corner, edge, and surface positions. Atoms at the corner or edge positions have a low coordination number and high surface energy. Accordingly, NPs with a small size can exhibit remarkable performance in CO₂ reduction.^8,33,34 Consequently, it is, in principle, promising to adjust NP catalysts by controlling their surface structure and size. For example, the conversion rate of CO₂ into CO was 10 times higher on 5 nm Ag NP_S than on a bulk silver electrode in anionic liquid EMIM-BF₄ solution.³⁵ To understand the size effect of Ag nanocatalysts, Hwang et al. reported a facile one-pot synthesis of Ag NP_S supported over carbon via employing a cysteamine anchoring agent to control the monodispersed particle sizes, as represented in Fig. 1.³⁴ Among 3, 5 and 10 nm Ag NPs, the 5 nm Ag/C catalyst exhibits the best CO₂ reduction reaction performance due to the smallest interaction between cysteamine and the COOH* intermediate.

Fig. 1 TEM images of (a) 3 nm, (b) 5 nm and (c) 10 nm-sized Ag NPs/C. (d) HR-TEM image of 5 nm Ag/C and (e) CO partial current densities of the three typical Ag/C catalysts with different NP sizes and different potentials.³⁴ Copyright 2015 American Chemical Society.

In another study, Zhang et al.³⁶ synthesized a porous silver catalyst via electrochemical growth/reduction of an AgCl film on a silver substrate. The as-prepared catalyst comprised 30–50 nm particles and a considerable electrochemical surface area, displaying a size effect on the reduction of CO₂ to CO.

2.2 The morphology effect

Besides the effects of size and local electric field, the morphological effect of metal NPs on the CO₂ ERR is another factor in increasing the activity of Ag for CO₂ electrochemical reduction to CO. Previous studies on single crystal Ag electrodes showed that CO₂ conversion to CO is favorable over Ag (110) compared to Ag (111) or Ag (100).³⁷ Density functional theory (DFT) simulation as represented in Fig. 2 shows that the Ag (110) or Ag (211) surface has a lower energy barrier for the first proton-coupled electron transfer for COOH˙ stabilization than Ag (111) or Ag (100), resulting in a better catalytic activity for the reduction of CO₂ to CO on Ag (110) or Ag (211).²⁵

Fig. 2 Free energy diagrams for the electrocatalytic conversion of CO₂ into CO on flat (Ag (100) and Ag (111)) and edge (Ag (221) and Ag (110)) surfaces at −0.11 V [RHE]. Proton-coupled electron transfer is involved in the first two reaction steps. Sphere colors: white, H; black, C; red, O; silver, Ag.²⁵ Copyright 2015 American Chemical Society.

It is worth noting that preferentially oriented Ag nanoparticles were capable of attaining a very high faradaic efficiency of 96.7% at a low overpotential of 0.58 V, along with a very high areal CO partial current density of 4.4 mA cm⁻². Since the preferentially oriented Ag nanoparticles are self-supported and produced in situ under the reaction conditions, it is likely to be possible to exclude utilization of ionic polymers for electrode manufacturing in the future, which would decrease the cost and difficulty for commercial application.³⁸

Hsieh et al.²³ synthesized a high-surface-area Ag nanocoral catalyst via an oxidation–reduction process in the presence of chloride anions in an aqueous medium. In the CO₂ ERR to CO, the Ag nanocoral catalyst achieved a current efficiency of 95% at a potential below −0.6 V and a current density of 2 mA cm⁻².

In contrast to Ag foil, the Ag nanocoral catalyst brought about a 32-fold improvement in surface-area normalized activity, at an overpotential of 0.49 V. They observed that the adsorbed chloride anions played a crucial role in the improved activity and selectivity of the Ag nanocoral electrocatalyst for CO₂ reduction. Liu et al. prepared triangular silver nano-plate (Tri-Ag-NP) catalysts through a direct chemical reduction process for a highly active CO₂ reduction reaction to CO. The as-prepared Tri-Ag-NPs showed a significantly improved faradaic efficiency (96.8%) and energy efficiency (61.7%), together with a substantial durability (7 days) compared to similarly sized Ag nanoparticles (SS-Ag-NPs) and bulk Ag. DFT simulation indicated that the considerably improved electrocatalytic activity and selectivity at decreased overpotentials are a result of both the optimum edge-to-corner ratio and the prevalent Ag (100) facet in Tri-Ag-NPs, which require less energy to initiate the rate-limiting step (Fig. 3).³⁹

Fig. 3 (a) Free energy diagrams for CO₂ to CO on different facets and the Ag₅₅ cluster at −0.11 V; active adsorption site density on (b) Tri-Ag-NPs and (c) SS-Ag-NPs as a function of particle size; (d) proposed mechanism of the CO₂ ERR to CO on Tri-Ag-NPs.³⁹ Copyright 2017 American Chemical Society.

To understand the relation between the crystallographic structures of Ag catalysts and the CO formation efficiency, a dendrite-type Ag catalyst was synthesized through an electrodeposition process in an ammonia-based Ag deposition solution comprising ethylenediamine (EN) as the additive. The effect of electrodeposition variables on the features of this catalyst was established and linked with CO formation efficiency. The results showed that the addition of EN during Ag electrodeposition altered the diffraction peak intensity ratio of the (111) to (220) planes, along with their electronic states, which was also found to be related to the CO formation activity. The as-prepared catalysts exhibited high faradaic efficiency and activity for CO through CO₂ electrolysis in 0.5 M KHCO₃ under optimized deposition conditions (−0.45 V vs. Ag/AgCl, 40 mM EN) (Fig. 4).⁴⁰ The one dimensional Ag nanowire (NW) is another specialized geometry that can have a large portion of specific facets exposed.^41–43 5-fold twinned Ag nanowires (NWs) with diameters less than 25 nm (D-25) and 100 nm (D-100) were obtained using the bromide-mediated polyol technique. In contrast to the D-100 Ag NWs and Ag NPs, the D-25 Ag NWs were electrochemically more active in the CO₂ ERR with higher selectivity and superior stability. Furthermore, a comparable low onset η in CO₂ ERR initiation, together with a good stability for more than 24 hours, was accomplished, representing the outstanding performance of the D-25 Ag NWs in the CO₂ ERR. DFT calculations show that the greater catalytic activity of the twinned Ag NWs mostly originates from the effects of diameter and length, in addition to the fully enclosed nanostructures (100) and the increased edge-to-corner ratio (Fig. 5).⁴⁴ To exploit the CO₂ ERR activity on an active crystal facet, ultrathin 2D nanosheets were synthesized and established as a new form of the Ag catalyst. The hierarchical structure of Ag nanosheets afforded an enriched surface area and promising gas transport/diffusion and exhibited improved FE (95%) at an overpotential as low as 0.29 V. The author thought that residual Cl⁻ and a noteworthy number of grain boundaries might participate in the extremely efficient reduction of CO₂ to CO using this material.⁴⁵

Fig. 4 (a) FESEM image of AgEN40 fabricated by electrodeposition at −0.45 V and (b) FE for CO and its conversion to mass activity for Ag catalysts fabricated at −0.45 V with various EN concentrations.⁴⁰ Copyright 2017 Elsevier B.V.

Fig. 5 (a) CO and H₂ partial current densities of different catalysts; (b) Tafel plots of Ag NPs, D-25, and D-100 Ag NWs; (c) EFs of Ag NPs, D-25 and D-100 Ag NWs at various potentials; (d) long-term stabilities of Ag NPs, D-25 and D-100 Ag NWs at a potential of −0.9 V.⁴⁴ Copyright 2018 Elsevier.

2.3 The nanoporous effect

In addition to the well-designed forms of the catalyst, nanostructures with porous or two-dimensional (2D) morphologies have also been developed to increase the atom utilization efficiency of the catalyst. For example, a monolithic nanoporous-Ag (np-Ag) catalyst is produced by a two-step dealloying of an Ag–Al precursor with an aqueous HCl solution. The as-prepared catalyst has a considerably large surface area (around 150 times larger) and high activity (about 20 times higher) compared to polycrystalline silver in CO₂ electroreduction. The improved activity is due to the superior stabilization of COOH* intermediates on the extremely curved surfaces, which leads to a smaller overpotential for overcoming the kinetic barrier for producing CO.¹⁵

Fig. 6 (a) SEM image and (b) ligament size distribution of np-Ag (21 nm). (c) SEM image and (d) ligament size distribution of np-Ag (87 nm).⁴⁶ Copyright 2018 Royal Society of Chemistry.

Fig. 7 (a) Cathodic LSV curves of bare carbon paper, Np-Ag (87 nm) and Np-Ag (21 nm). (b) FEs of CO, (c) CO partial current density, and (d) mass activity at different potentials of Ag foil, Np-Ag (87 nm) and Np-Ag (21 nm). (e) Stability of Np-Ag (87 nm) and np-Ag (21 nm) at −0.9 V vs. RHE.⁴⁶ Copyright 2018 Royal Society of Chemistry.

Nanoporous Ag network catalysts (Np-Ag) with an average ligament size of 21 nm Ag (np-Ag 21) and 87 nm (np-Ag 87) were synthesized by dealloying of the precursor Mg₈₀Ag₂₀ (wt%) alloy ribbon with 1 wt% citric acid and 5 wt% phosphoric acid, separately. The results showed that the Np-Ag (21 nm) catalyst exhibited a considerably improved selectivity with a faradaic efficiency for CO formation of 85.0% at −0.8 V [RHE], about two times that (41.2%) of Np-Ag 87. The superior catalytic activity of Np-Ag 21 can be ascribed to the enriched electrochemical surface area and higher local pH resulting from the ligament size effect, in addition to more defect sites (i.e., grain boundaries) in ligaments (Fig. 6 and 7).⁴⁶ In another study, an extremely active, selective, and stable Ag electrocatalyst for the reduction of CO₂ to CO was fabricated through an easy and rapid anodic-etching process. The as-prepared catalyst showed a high FE > 92% at −0.4 V vs. RHE and stability for more than 100 h. The enhanced CO₂ reduction achievement could be ascribed to the increased number of active sites and the better intrinsic CO₂ reduction activity by fast initial electron transfer.⁴⁷ Dutta et al.⁴⁸ reported an additive-assisted metal-foam deposition method for preparing novel Ag-foam for CO₂ electrochemical reduction. The as-prepared catalyst with a thickness of 17 μm in the Ag film shows that the pore sidewalls are composed of highly anisotropic, needle-shaped Ag on different length scales Fig. 8(e and f). The electrocatalytic CO₂ reduction results showed remarkable activity in aqueous solution. For example, under low and moderate overpotentials these Ag foam samples offer a high activity and selectivity to CO production with a maximum Faraday efficiency of 90% in the potential range of −0.3 to −1.2 V versus the reversible hydrogen electrode (RHE). Most interesting is the ability of the novel Ag foam catalyst to produce hydrocarbons in noteworthy quantities, showing CH₄ and C₂H₄ efficiencies of FECH₄ = 51% and FEC₂H₄ = 8.6%, respectively, at <−1.1 V versus RHE. This notable Cu-like activity was justified in expression of the CO* binding energy, which is considerably improved on the novel Ag foam compared to the polycrystalline Ag-foil as the reference. Nonetheless, degradation is observed to be more rapid in the EC-CO₂RR extended at −1.5 V versus RHE. FECH₄ values were reduced to 32% within 5 hours of electrolysis. Likewise, the HR-SEM examination demonstrates morphological changes, mostly at higher applied over-potentials (−1.5 V) (Fig. 9).

Fig. 8 Morphology of the Ag foam. (a) Side-view SEM image of the Ag foam. (b–d) Top-down SEM images showing the primary macroporosity of the Ag foam. (e and f) Top-down SEM images showing the secondary mesoporosity of the Ag foam.⁴⁸ Copyright 2018 American Chemical Society.

Fig. 9 (a) Potential (E)-dependent product distribution of the CO₂ ERR displayed as partial current densities normalized to the geometric surface area (multi-catalyst approach, 1 h of potentiostatic CO₂ ERR carried out at each potential applied using a freshly prepared Ag foam catalyst). (b) Corresponding plot of the faradaic efficiencies (FEs) as a function of the applied electrolysis potential (E). (c) Potential (E)-dependent FE plot with respect to the total hydrocarbon formation. (d) Potential (E)-dependent product distribution of the EC-CO₂RR on an Ag foil electrode, displayed as faradaic efficiencies.⁴⁸ Copyright 2018 American Chemical Society.

2.4 The alloying effect

Alloying is known as a widely used technique to alter the catalytic performance of nanomaterials. The addition of foreign atoms can alter not only the geometric but also the electronic structure of the parent nanostructures and induce synergistic effects. Ag-based alloy catalysts prepared by alloying some transition metals such as Cu,^49–51 In,^52–54 Zn,⁵⁵ Co⁵⁶ and Sn⁵⁷ revealed that the composition has a substantial impact on the CO faradaic efficiency and partial current density. Theoretical studies manifest that alloying could modify the suboptimal linear relation between CO and COOH binding energies on transition metals and that the partial covalency imposed by some p-block atoms as dopants may decrease the onset overpotential for CO on Ag.^58–61 Choj et al. synthesized dendritic Ag–Cu catalysts through an electrodeposition technique and investigated their activities in electrochemical CO₂ reduction. Among the synthesized dendritic catalysts, the Ag₁₀₀ dendritic catalyst exhibits a CO faradaic efficiency of 64.6% at a constant potential of −1.7 V_SCE in a CO₂-saturated 0.5 M KHCO₃ solution. However, the catalytic activity of the Ag₅₇Cu₄₃ dendritic catalyst was 2.2 times higher than that of the Ag₁₀₀ dendritic catalyst in terms of silver unit mass activity. These results showed that the direct production of syngas was possible by adjusting the composition of Ag and Cu.⁵⁰ In addition, Ag/Cu foam with manageable pore sizes and wall thicknesses was synthesized through bubble-template Cu electrodeposition followed by Ag metal galvanic replacement for the formation of CO via the electrochemical conversion of CO₂. At lower overpotentials, the foams demonstrated morphologically advantages that were created from their rough surfaces; nevertheless, this positive influence was missing at higher overpotentials due to the uncovered Cu.⁶² Furthermore, a recent study found that the combination of Ag with inexpensive metals like Cu not only led to a rise in the unit mass activity with regard to CO production but also reduced the overpotential of the reaction to a ∼300 mV lower value compared with bulk Ag and Ag foam, respectively (Fig. 10). The bimetallic Ag–Cu nanofoam also preserved a stable selectivity over 5 hours of testing. Raman study showed that the existence of Ag moved the peak intensity of the intramolecular CO band (CO surface coverage) to more positive potentials compared to that of Cu foam. This also illustrates the faster desorption of CO from the Ag–Cu surface at low overpotentials which enhanced the activity for CO₂ reduction to CO.⁵¹ Park et al.⁵³ studied the catalytic performance of the electrodeposited Ag and Ag–In dendrites in the electrochemical reduction of CO₂ to CO. In the sample of electrodeposited Ag dendrites, the CO faradaic efficiency was intensely affected by the Ag (220)/Ag (111) ratio, whereas the CO partial current density exhibited a reliance on the roughness of the catalyst. The addition of an optimized amount of In into the Ag dendrites enhanced the efficiency of CO production. This is due to the increased Ag (220)/Ag (111) intensity ratio in the Ag–In catalyst and the suppression of the hydrogen evolution reaction (HER) versus the diminution in the quantity of active Ag with the increasing addition of In. So as to additionally improve catalytic performance, bubble-assisted Cu foam (CF) was utilized to provide a vast electrochemical surface area (ECSA) for the Ag/In catalyst. Compared with the reported activities, noteworthy performance was detected for Ag₆₀₀/In/CF in terms of the CO faradaic efficiency and partial current density, particularly at a low overpotential which can be attributed to the supressed hydrogen evolution by In and morphological advantage of Cu foam.⁵⁴ Polycrystalline Ag–Zn foil was also utilized for CO₂ electroreduction in an aqueous bicarbonate electrolyte, and CO is the major product, likely because of the weak CO binding energy of the surface, with methane and methanol evolved as minor products. In contrast to pure silver and pure zinc foil, improvement in activity and selectivity for methane and methanol is observed which demonstrates the presence of a synergistic impact among silver and zinc at the surface of the composite.⁵⁵

Fig. 10 SEM images of Cu (a and b), Ag (c and d), AgCu-10 (e and f) and AgCu-50 (g and h) foam electrodeposited at 1 A cm⁻²; (i) steady-state current as a function of applied potential during 1 h of electrolysis; (j) current ratio of CO₂ reduction to the HER at different potentials; faradaic selectivities of the major products CO and formate along with H₂ of electrodeposited Cu, electrodeposited Ag, AgCu-10, AgCu-50, and Ag foil as a function of applied potential vs. RHE (k–m). The error bars correspond to the average of two individual measurements on two different samples.⁵¹ Copyright 2019 American Chemical Society.

Fig. 11 (a) The partial current density for CO and (b) faradaic efficiency of Ag/TiO₂ catalysts with different Ag loadings: 5, 10, 20, 40, and 60 wt%; (c) partial current densities for CO vs. cathode potential relative to the cathode Ag loading for different catalysts. The error bars represent the standard deviation of the average of three experiments (N = 3).⁶³ Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

2.5 The support effect

The catalyst support can have a great effect on catalytic performance and can reduce the catalyst loading while improving its performance and durability.^34,52,61,63 It was demonstrated that Ag supported on TiO₂ outperforms Ag supported on black carbon in the reduction of CO₂ to CO. However, Ag/TiO₂ performs at the almost identical level as unsupported Ag nanoparticles, for which the faradaic productivity for CO surpasses 90% and the current density surpasses 100 mA cm⁻². Compared to carbon black, TiO₂ is an excellent support for Ag catalysts for the electrochemical reduction of CO₂ to CO. By utilizing TiO₂ as the support, the Ag loading can be minimized without sacrificing performance to selective CO formation. 40 wt% Ag/TiO₂ can generate a similar quantity of CO as unsupported Ag NPs but at 2.5 fold less Ag loading. In view of the cyclic voltammetry data, a reaction pathway is suggested that involves the contribution of (Ti⁺⁴/Ti⁺³) from the support material, which acts as the redox couple and stabilizes the reaction intermediates (Fig. 11).⁶³ Larrazábal et al.⁵² fabricated catalysts with different silver loadings (5–40 wt% Ag) supported on In(OH)₃ and In₂O₃, respectively, for electroreduction of CO₂ to CO. The Ag/In₂O₃ and Ag/In(OH)₃ systems exhibited an improved current efficiency for CO at a modest overpotential compared both to the pristine supports and to catalysts with analogous Ag loadings on a carbon black support. This finding suggests the presence of a synergistic effect between Ag and the In₂O₃ support, which was especially apparent between Ag and In(OH)₃ at higher Ag loadings. CO₂-TPD measurements showed a rise in the CO₂ uptake capacity of In(OH)₃ and In₂O₃ originating from the interaction of Ag with the supports. In a recent study, three-dimensional copper foam was chosen as the substrate for coating silver dendrites through electrodeposition. The cyclic voltammograms showed that the coupling of Ag dendritic catalysts with the high active surface area of the Cu foam effectively raised the electrocatalytic performance toward CO formation and resulted in an increase to a stable maximum faradaic efficiency of 95.7% at −0.8 V [RHE] compared to flat Cu foil with the Ag catalyst and a flat Ag foil cathode. This great achievement was ascribed to mainly two advantages of the Cu foam-based cathode: (i) reduced mass transport restriction due to the enlarged 3D electrochemically active contact area between the homogeneously coated Cu foam and the electrolyte, and (ii) the high selectivity of the deposited Ag catalyst to CO formation (Fig. 12).⁶¹

Fig. 12 (a) SEM images of the Ag dendritic catalyst electrodeposited on Cu foam for 15, 30, and 60 s at constant potentials of −0.05, −0.2, −0.35, and −0.5 V_RHE, respectively, in a 0.1 M AgNO₃ containing plating solution; (b) faradaic efficiency of CO and H₂ production as a function of the applied potential for the cathodes prepared at −0.2 V [RHE] with Ag electrodeposition at 60 s; (c) applied potential versus CO production partial current density for Ag dendrites and Ag foil; (d) stability of total CO₂ reduction current density (left ordinate) for Ag dendrites and Ag foil at −0.8 V [RHE] in 0.5 M KHCO₃. The CO faradaic efficiency of both tested cathodes as a function of operating time is plotted on the right ordinate.⁶¹ Copyright 2018 American Chemical Society.

2.6 The surface modification effect

Surface property modification is an effective approach to improve the performance of the catalyst. Generally, the surface properties of catalysts can be adjusted by improvement with functional molecules. For instance, in an early study the authors observed the covalency-aided electrochemical reaction (CAER) mechanism in which p-block dopants have a significant impact on modifying the reaction energetics via enforcing partial covalency in the metal catalysts, thus boosting their catalytic activity well beyond modifications originating from d-block dopants. In particular, sulfur or arsenic doping can efficiently reduce the overpotential with the desired structure and electrochemical durability.⁵⁸ Additionally, another study showed that particular interactions among Ag nanoparticles and the attaching agents (Ag–S) affected the durability of the intermediate COOH*, leading to boosted catalytic activity. These studies suggested that functional groups of the organic molecules can notably minimize the overpotential or enhance CO selectivity for the electrochemical CO₂ reduction.³⁴ To discover the direct relationship among the functional groups and CO₂ reduction efficiency of the Ag nanoparticle, Kim et al. established a strategy to adjust CO₂ reduction activity by modifying the binding energies of the intermediates on the electrocatalyst surfaces with the support of molecules that have functional groups. Therefore, they synthesized three kinds of Ag nanoparticles through utilizing various surface capping agents including oleylamine, oleic acid, and dodecanethiol having amine, carboxyl, and thiol functional groups. The amine capped Ag/C exhibited higher selectivity with 94.2% CO faradaic efficiency owing to the efficient inhibition of the HER and appropriate activity of the CO₂ reduction reaction. Density functional theory (DFT) calculations indicated that the amine-capped Ag nanoparticles stabilized the COOH* intermediate whereas a reverse effect was found with thiol molecules. Furthermore, ethylenediamine exhibited a higher selectivity towards CO formation, in contrast with cysteamine, when it was applied as an anchoring agent to immobilize the Ag NPs on the carbon support (Fig. 13).⁶⁴ Recently, in order to understand the relation between the catalytic performance and the N/S doping ratio in Ag-based electrocatalysts. An all-water-based solution method was established to synthesize extremely active Ag electrocatalytic films through a seed-mediated metal growth technique on aminopropyltriethoxysilane (APTES)-deposited substrates. The obtained Ag films showed excellent durability throughout electrochemical CO₂ reduction as the Ag film was robustly attached to the APTES layer. The N- and S-doped Ag film with an adjusted N/S doping ratio of 1.75 achieved a CO FE of 75.7% at a low overpotential of 0.19 V [RHE] which is greater than those before reported for Ag-based electrocatalysts.⁶⁵

Fig. 13 (a) CO faradaic efficiency depending on the applied potential. (b) CO faradaic efficiency depending on the applied potential for CA Ag/C, amine-treated CA Ag/C, and thiol-treated CA Ag/C. (c) CO faradaic efficiency depending on the applied potential for CA Ag/C and EDA Ag/C. (d) CO faradaic efficiency at fixed potentials of −0.70 V and −1.0 V (vs. RHE).⁶⁴ Copyright 2017, American Chemical Society.

3. Outlook on surface engineering

The surface science of catalysts is fundamental to heterogeneous catalysis. Great research efforts have contributed to the surface designing of catalysts to improve the reaction performance by increasing the surface area and catalytically active sites. Zhou et al. reported a simple and efficient method by anodization to modify a polycrystalline Ag electrode. The anodized Ag electrode exhibited a great improvement in the electroreduction activity toward CO under a moderate potential (0.5 V, 92.8%) as shown in Fig. 14. These results indicate that a thin silver oxide layer is produced on the electrode surface. These results indicate that the production of a thin silver oxide layer on the electrode surface and then reducing this oxide layer display an important role in enhancing carbon dioxide reduction.⁶⁶ Surface-sensitive evaluation systems were used to understand the mechanism of the improvement of the electrocatalytic CO₂ reduction to CO on electrochemically treated Ag surfaces where the CO₂ERR efficiencies changed depending on the electrochemical treatment of the Ag surface. The samples, CVAg-5, and pulsed Ag showed the highest improvement in CO₂ reduction efficiency for selective CO formation in contrast to the bare Ag foil and CVAg-50. The excessive work function exists at the nanostructure boundaries of the effective Ag electrode, which strongly indicates localized O concentration on the nanostructured Ag surface. The stable surface O adjusts the electronic territory of Ag to have stronger collaboration with COOH* and CO* intermediates.⁶⁷ Recent studies showed that the total CO FE of oxide-derived silver was positively shifted by more than 0.4 volts compared to the polycrystalline silver electrode. Electrokinetics analyses demonstrate that the boosted catalytic activity is attributed to the improved stabilization of the COOH* intermediate. Moreover, it is very likely that extremely nanostructured Ag was able to create a high local pH close to the catalyst surface, which may also simplify the catalytic activity towards the reduction of CO₂ with curbed H₂ evolution (Fig. 15).⁶⁸ The variances in the surface structure between a pure silver film and oxide-derived silver catalysts were investigated by operando extended X-ray absorption fine structure (EXAFS) analysis. The results showed the existence of a long Ag–O bond where oxygen may be the main contributor to the reduction of CO₂ to CO on oxide-derived silver catalysts (Fig. 16). The silver needs a long and consequently atomic oxygen bond in order to change its reactivity and enhance its tendency to CO₂ binding. Both the existence of oxygen and nanostructuring of the catalyst which raise the local pH boosting the activity and selectivity for CO formation on silver electrocatalysts are consequently crucial factors of an active and selective CO₂ reduction catalyst.⁶⁹ Oxygen plasma-treated Ag exhibited more than 90% faradaic efficiency for CO production at −0.6 V versus RHE. The secret behind the upgraded reactivity was discovered through DFT calculations, which suggested that the greatly defective surface produced by plasma oxidation in the presence of local electric fields was able to decrease the thermodynamic barriers, permitting CO₂ reduction to CO at a low overpotential. DFT calculations suggested that the defect-rich surface of the plasma-oxidized silver foil in the presence of local electric fields radically decreased the overpotential of CO₂ electroreduction.⁷⁰ Furthermore, it has been observed that halide anions adsorbed onto the surfaces of the electrode can increase the capture capacity of CO₂ and suppress the adsorption of protons. Zhang et al.⁷¹ found that an iodide derived nanostructured silver (ID-Ag) catalyst is proficient in reducing CO₂ to CO with a faradaic efficiency (FE) of 94.5% at a potential of −0.7 V. It should be noted that the improved catalytic activity and selectivity are associated with a high surface area with a huge amount of electrochemically active sites for CO₂ conversion. In addition, the combination of halide (X) anions on the catalyst surface plays a vital role in enhancing the selectivity for CO production by suppressing the competing hydrogen evolution reaction (HER). The activity of X–Ag catalysts is in the order Cl > Br > I, which is in agreement with the suggested model of the active site. The Tafel analysis of electrochemical CO₂ reduction points to the sensitivity of the mechanism of electrocatalysts to the nature of X (Fig. 17).⁷²

Fig. 14 Electrocatalytic performance of anodized Ag electrodes. (a) CO faradaic efficiency for Ag electrodes anodized at different potentials against Ag/AgCl. The anodization charge was fixed at 5 coulombs in a neutral (pH 7) 0.1 M NaNO₃ electrolyte. (b) CO faradaic efficiency for Ag electrodes anodized at different charges. The anodization potential was fixed at 0.6 V against Ag/AgCl in a neutral 0.1 M NaNO₃ electrolyte. (c) CO faradaic efficiency (at 0.5 V overpotential) and current density (when 10 coulombs of total charge were passed through) for Ag electrodes anodized under various conditions in a neutral 0.1 M NaNO₃ electrolyte. (d) Current density during 2 hour electrocatalysis for the an-Ag electrode in comparison with poly-Ag (solid lines, left axis) and CO faradaic efficiency (at 0.5 V overpotential) versus time for the an-Ag electrode. (e) Tafel plots of the Ag electrode anodized at a potential of 0.6 V with 5.5 coulombs of charge in a neutral 0.1 M NaNO₃ electrolyte and unanodized poly-Ag.⁶⁶ Copyright 2015 Royal Society of Chemistry.

Fig. 15 Comparison of the electrocatalytic activity of polycrystalline Ag and oxide-derived Ag. Faradaic efficiency for CO (a) and H₂ (b) on polycrystalline Ag and oxide-derived Ag at various potentials in CO₂-saturated 0.1 m KHCO₃ (pH 6.8). (c) The current density for CO at various potentials; (d) Tafel plots of the CO partial current density for polycrystalline Ag and oxide-derived Ag. (e) Bicarbonate concentration at a constant potential for oxide-derived Ag.⁶⁸ Copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 16 (a) DFT calculations and Bader analysis for O on an Ag (110) slab give Ag–O bond lengths very similar to those found in the EXAFS data fitting. (b) Left: A CO₂ molecule binds to an Ag (110) facet in the presence of a water molecule. Right: The presence of a water molecule. Right: The presence of oxygen in the cavities of a silver (110) facet can facilitate the binding of CO₂ to silver through the presence of a shared proton. (c) Left: A CO₂ molecule binding to an Ag (111) surface. Right: Subsurface O below Ag (111) facets shift the d-band of silver toward the Fermi level (indicated by a change in color of the affected Ag atoms), increasing the reactivity of the subsurface O–Ag (111) facet compared to Ag (111). Adapted from ref. 69. Copyright 2019 Royal Society of Chemistry.

Fig. 17 Tafel plots (iR-corrected overpotentials η as a function with the CO partial current density j_CO) for CO₂ electroreduction on Ag foil, Ag nanoparticles, Cl–Ag, Br–Ag, and I–Ag.⁷² Copyright 2019 American Chemical Society.

Recently, a phosphate silver-derived silver (PD-Ag) electrocatalyst was synthesized through a low-cost and quite simple electrochemical technique, displaying a promising performance for the selective reduction of CO₂ to CO. The high FE of PD-Ag could reach 97.3% with a potential of −0.7 V vs. RHE, and a current density of 2.93 mA cm⁻². Moreover, the PD-Ag catalyst's activity can be maintained for a long period without noticeable deactivation for at least 10 h (Fig. 18).²⁴ To better understand the recent developments of Ag-based nanostructured catalysts in the CO₂ ERR, a summarization is given for some Ag-based nanocatalysts and their faradaic efficiencies in Table 2. At present, the maximum FEs of CO obtained with Ag-based nanocatalysts reach as high as 99.3%, which is a significant improvement over those with the traditional Ag foil.

Table 2 Summary of the performance of some representative Ag-based nanocatalysts in the CO₂ ERR

Electrocatalysts	Potential	Electrolytes	CO FE [%]	Ref.
5 nm Ag/C	−0.75 V [RHE]	0.5 M KHCO3	84.4%	34
PONs Ag	−0.69 V [RHE]	0.5 M KHCO3	96.7%	38
Ag nanocorals	<−0.6 V [RHE]	0.1 M KHCO3	95%	23
Ag TNP	−0.206[RHE]	0.1 M KHCO3	96.8%	39
Dendritic Ag	−1.5 V [SCE]	0.5 M KHCO3	>60%	40
Ag-200 nm NWs	−0.6 [RHE]	0.5 M KHCO3	91%	42
D-25 Ag NWs	−0.95 V [RHE]	0.1 M KHCO3	99.3%	44
Ag nanosheets	−0.29 V [RHE]	0.5 M NaHCO3	95%	45
Ag nanoporous	−0.490 V [RHE]	0.5 M KHCO3	92%	15
Ag nano foam	−0.3–1.2 V [RHE]	0.5 M KHCO3	90%	48
Dendritic Ag–Cu	−1.5 V [SCE]	0.5 M KHCO3	<40%	50
Ag dendrites on Cu	−0.8 V [RHE]	0.5 M KHCO3	95.7%	61
Ag/TiO2	−1.8 V [Ag/AgCl]	1 M KOH	<90%	63
OD-Ag	−0.8 [RHE]	0.1 M KHCO3	80%	68
ID-Ag	−0.7 [RHE]	0.5 M KHCO3	94.5%	71
PD-Ag	−0.7 [RHE]	0.5 M KHCO3	∼97.3%	24

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Fig. 18 (A) Total current density at different potentials on Ag foil (black trace), PD-Ag (red trace), and Ag nanoparticles (blue trace). (b) The current density for CO at different potentials on Ag foil (black trace), PD-Ag (red trace), and Ag nanoparticles (blue trace). (c) CO faradaic efficiencies versus applied potentials catalyzed by Ag foil (black trace), PD-Ag (red trace), and Ag nanoparticles (blue trace). (d) Stability performance of PD-Ag for CO₂ reduction operated at a potentiostatic potential of −0.7 V vs. RHE for 10 h. Total current density vs. time and FE for CO vs. time.²⁴ Copyright 2019 American Chemical Society.

4. Conclusion

In this article, we summarized the progress of recent silver-based electrocatalysts for CO₂ reduction, focused on morphology control, composition manipulation, and support effects on the catalytic performance towards CO formation compared to that of Ag foil or bulk electrodes. Adjustment of precisely exposed facets and the surface electronic structure seems to be effective to improve catalytic activity and selectivity. A multi-metal system with synergistic effects from different metals can be optimized in terms of CO faradaic efficiency and partial current density, particularly in the low overpotential region. In addition, owing to the fast deactivation of the Ag-based catalysts, the competitive hydrogen evolution reaction (HER) causes a low conversion efficiency and product selectivity. The catalyst support has a remarkable effect on the catalyst performance and durability, which is a critical step in the enhancement of the commercial feasibility of this technology. It is essential to establish a new stabilization approach.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support from the National Natural Science Foundation (21676085) is gratefully acknowledged.

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