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Beyond Copper in CO Electrolysis: Effective Hydrocarbon Production on Silver Nano-Foam Catalysts Abhijit Dutta, Carina Elisabeth Morstein, Motiar Rahaman, Alena Cedeño López, and Peter Broekmann ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01738 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Beyond Copper in CO2 Electrolysis: Effective Hydrocarbon Production on Silver Nano-Foam Catalysts Abhijit Dutta*, Carina Elisabeth Morstein, Motiar Rahaman, Alena Cedeño López, and Peter Broekmann* Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, Bern 3012 Switzerland

ABSTRACT: Ag foam catalysts have been developed for the electrochemical CO2 reduction reaction (ec-CO2RR) based on a concerted additive- and template-assisted metal deposition process. In aqueous media (CO2 saturated 0.5 M KHCO3 electrolyte) these Ag foams show high activity and selectivity towards CO production at low and moderate overpotentials. Faradaic efficiencies for CO (FECO) never fell below 90% within an extremely broad potential window of ~900 mV starting at -0.3 V and reaching up to -1.2 V vs RHE. An increased adsorption energy of CO on the Ag foam is discussed as the origin of the efficient suppression of the competing hydrogen evolution reaction (HER) in this potential range. At potentials < -1.1 V vs RHE the FEH2 values significantly increase at the expense of FECO. Superimposed on this anticorrelated change in the CO and H2 efficiencies is the rise in the CH4 efficiency to the maximum of FECH4 = 51% at -1.5 V vs RHE. As a minor by-product, even C-C coupled ethylene could be detected reaching a maximum Faradaic efficiency of FEC2H4 = 8.6% at -1.5 V vs RHE. Extended ec-CO2RR reveals an extremely high long-term stability of the Ag foam catalysts with CO efficiencies never falling below 90% for more than 70 h electrolysis at -0.8 V vs RHE (potential regime of predominant CO production). However, a more rapid degradation is observed for extended ec-CO2RR at -1.5 V vs RHE (potential regime of predominant CH4 production) where the FECH4 values drop down to 32% within 5 h of electrolysis. The degradation behavior of the Ag foam catalyst is correlated to time-resolved identical location scanning electron microscopy investigations that show severe morphological changes particularly at higher applied overpotentials (current densities) at -1.5 V vs RHE. This study reports on the first ecCO2RR catalyst beyond copper that demonstrates a remarkably high selectivity towards hydrocarbon formation reaching a maximum of ~60% at -1.5 V vs RHE. Experimental observations presented herein strongly suggest that this newly designed Ag foam catalyst shares in part mechanistic features with common Cu catalysts in terms of ec-CO2RR product selectivity and catalyst degradation behavior. KEYWORDS: CO2 reduction, Ag nano foam, nano needle, hydrocarbons, methane formation 1. INTRODUCTION The electrochemical reduction of CO2 (in the following denoted as ec-CO2RR) has great potential to substantially con-

tribute to the closing of the so-called anthropogenic carbon 1-3 cycle. What renders the ec-CO2RR particularly appealing is 4 that value-added products such as high energy density fuels 5,6 7-10 10-12 (e.g., methanol , ethanol , and n-propanol ) or chemical 13 7,9,14-16, feedstock (e.g., formic acid , ethylene and syn17-21 gas(CO/H2) ) can be produced by using the surplus of renewable electricity originating from solar, hydro, and wind 22, 23 sources. Key to the ec-CO2RR process is the use of specific catalyst materials which control both the overall ec-CO2RR rate and the resulting product distribution. Early work by 24 Hori et al. already demonstrated that the pure elemental ec-CO2RR catalysts can be categorized into three major groups: A first group of catalysts (Pb, Hg, In, Sn, Cd, and Tl) exclusively produces formate, a second group predominantly yields CO (Au, Ag, and Zn) whereas Cu was found to be the only ec-CO2RR catalyst which can produce alcohols and 24 6 hydrocarbons in significant amounts. Kuhl et al. , however, pointed out that methanol and methane can in principle be produced also on other transition metals such as Au, Ag, Zn, Ni, Pt, and Fe, but only with extremely low Faradaic efficiencies below 0.1%. One key descriptor among others rationalizing the profound differences in the observed ec-CO2RR product distributions is the binding strength of *CO (the asterisk * refers to an adsorption state). CO can be either the final reaction product of the CO2 electroconversion (e.g. on Au, Ag, and Zn) or a key intermediate (e.g. on Cu) which then undergoes further coupled electron-proton transfer reactions yielding alcohols or hydrocarbons of various carbon chain lengths. For a num6 ber of ec-CO2RR catalyst materials, Kuhl et al. derived a Volcano-like interrelation between the *CO binding strength and the ec-CO2RR current densities at -0.8 V vs RHE with Au being closest to the Volcano maximum whereas Ag and Cu were found next to the maximum on the low and high binding energy side of the volcano, respectively. This explains why CO is the exclusive reaction product when the ecCO2RR is performed e.g. on Ag catalysts. In case of Cu it is the moderately increased *CO binding strength which leads to a prolonged *CO mean residence time on the catalytically active surface site and therefore also to an increased *CO surface coverage. These two characteristics mark crucial mechanistic pre-requisites not only for methane production but in particular also for the C2 hydrocarbon reaction path6,8,12-14 7-9 way, e.g. yielding C-C coupled ethylene or ethanol . Additionally important for the hydrocarbon formation are a

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2.2 Electrolytes – (i) The standard plating bath for the Ag foam deposition was composed of 1.5 M H2SO4 (prepared from 96% H2SO4, ACS grade, Sigma-Aldrich) serving as supporting electrolyte, 0.02 M Ag2SO4 (Sigma Aldrich, purity ≥ 99.5%), and 0.1 M Na3C6H5O7 ⋅ 2 H2O (tri-sodium citrate dihydrate, ACS grade > 99.7%, Merck). (ii) The electrochemically active surface area (denoted as ECSA) was determined by means of cyclic voltammetry (CV) using di-methyl vio2+ 27, logens (DMV ) as reversible redox-probe (see Figure S14). 28 Scan-rate dependent CVs were measured in aqueous 1 M Na2SO4 (ACS grade, Sigma-Aldrich) solution containing 10 mM DMVCl2 (Sigma-Aldrich). (iii) ec-CO2RR experiments were carried out in 0.5 M KHCO3 (ACS grade, Sigma-Aldrich) electrolyte solutions saturated with CO2 gas (99.999%, Carbagas, Switzerland).

low kinetic energy barrier for the subsequent *CO hydrogenation (often considered as the rate-limiting step) and the stabilization of the *CHO species as the key intermediate in 25 the course of further reductive hydrogenation reactions. 25 Based on density functional theory calculations, Liu et al. recently discovered a clear scaling relation between the CO adsorption energy and the related transition state energies for a variety of transition metal catalysts. These results rationalize why Cu is so far the optimum catalyst for hydrocarbon production in terms of reaction rate and product selectivity. In this study, low-coordinated surface sites, available e.g. on stepped Cu(211) surfaces, were identified to be superi25 or over more compact surfaces such as (111) facets. Following the generalized catalyst design criteria discussed by Liu 25 et al. , it should in principle be possible to transform a predominantly CO producing catalyst (e.g. Ag) into a hydrocarbon producing one by creating a high density of lowcoordinated surface sites that are capable (i) to stabilize chemisorbed *CO, (ii) to reduce the kinetic energy barrier for *CO hydrogenation, and (iii) to further stabilize the formed *CHO intermediate.

2.3 Electrochemical experiments – (i) The Ag foam deposition was carried out in a glass-beaker containing 100 mL of electrolyte. For the galvanostatic deposition process, a nomi-2 nal current density of J = -3.0 A cm (referred to the geometric surface area) was applied. The standard cell geometry used is indicated in Figure S2. (ii) All voltammetric and ecCO2RR experiments were carried out in a custom-built, air28 tight glass-cell (H-type) described elsewhere. The threeelectrode arrangement consisted of a leakless Ag/AgCl3M reference electrode (EDAQ), a bright Pt-foil (15mm x 5mm) serving as counter electrode, and the Ag foam (deposited on Ag foils) catalysts serving as working electrodes. Possible chloride cross-contaminations in the working electrolytes originating from the Ag/AgCl3M reference electrode were monitored and excluded by ion exchange (IC) chromatography. Technical details of the ec-CO2RR product analysis based on online gas-chromatography and IC chromatography 28 were described elsewhere.

In this current study we present an electrodeposition approach which yields a novel Ag-foam type of ec-CO2RR catalyst that reveals porosity on different length scales and shows superior activity in aqueous media towards CO production at low and moderate overpotentials. The almost quantitative suppression of the competing hydrogen evolution reaction (HER) within an extended potential range of ~900 mV (-0.3 to -1.2 V vs RHE, FECO > 90%) is strongly supporting our working hypothesis of a substantially increased CO adsorption energy and related to that of an increased *CO coverage with chemisorbed CO serving as an effective suppressor of the parasitic HER. In agreement with this reasoning we will demonstrate that such an Ag catalyst showing improved CO binding characteristics is indeed capable of carrying out C1 hydrocarbon formation and even C-C coupling reactions at higher overpotentials.

2.4 Structural characterization – (i) The crystallinity of the Ag was studied prior to and after the CO2 electrolysis by means of powder XRD techniques. An STOE Stadi P system with a Cu Kα radiation source (λ = 0.1540 nm, 40 mA) generated at 40 keV was used. XRD spectra were recorded in reflection mode (Bragg-Brentano geometry) between 10 to 90 –1 degrees 2θ in steps of 1° min . To exclude Ag diffraction peaks originating from the Ag foil substrate, the silver foam was deposited onto a graphite substrate (99.8%, Alfa Aesar, 0.13 mm thickness) for this measurement. The obtained XRD pattern were analyzed and compared to JCPDS (Joint Committee on Powder Diffraction standards) for Ag, Ag2O, and AgO. (ii) X-ray photoelectron spectroscopy (XPS) studies were carried out using a PHI VersaProbeII scanning XPS micro-probe (Physical Instruments AG, Germany) equipped with a monochromatic Al Kα X-ray source operated at 24.8 W with a spot size of 100 µm. Peak positions were referenced to the carbon C1s peak at 285.5 eV and curve fitting was performed using the Casa-XPS software. (iii) The morphology of the Ag foams was characterized by means of SEM. For the high-resolution IL-SEM imaging a Zeiss DSM 982 instrument was used. (iv) The three-dimensional characterization of the Ag foams was carried out using a 3D digital microscope with focus variation (VHX600, Keyence).

2. EXPERIMENTAL SECTION 2.1 Materials - Ag foils were purchased from Goodfellow Cambridge Limited and Sigma Aldrich (thickness 0.25 mm, purity ≥ 99.95%). The as received silver foils were cut into pieces having dimensions of ~ 8 mm x 22 mm. Initial cleaning was performed by boiling the Ag slices in Milli-Q water for 30 min. followed by drying in an Ar gas stream (99.99%, Carbagas, Switzerland). To yield a well-defined total geomet2 ric surface area of 1 cm , the Ag foil slices were masked with an inert Teflon tape (PTFE Thread Seal Tape, for details see Figure S2). No extra mechanical polishing treatment was applied to the foil samples prior to the foam deposition. Also note that, unless otherwise mentioned, a new Ag foil substrate was used for each deposition/electrolysis experiment in order to ensure that all experiments were actually started from identical experimental conditions. The morphology of the Ag foil substrate was characterized by means of AFM (Figure S1). As a proof for the suitability of this preparation protocol we refer to ec-CO2RR reference measurements 26 (Figure S18a) reproducing results from the literature.

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ACS Catalysis

Figure 1. Scheme demonstrating the basic principle of the template- and additive-assisted metal foam deposition. 3. RESULTS AND DISCUSSION

parably low acidity of the tribasic citric acid (pka1 = 3.13, pka2 = 4.76, and pka3 = 6.39) one can safely assume that the dissolved citrate anions are nearly quantitatively transformed -2 ) = 9.98 . 10 M) so that only minor into citric acid ( amounts of its corresponding anions are present -4 -8 (c( = 2.29 ⋅ 10 M, c( = 1.23 ⋅ 10 M, and -14 c( ) = 1.51 ⋅ 10 M). However, under reactive condi-2 tions at such high current densities of J = -3 A cm the local pH at the electrode turns from strongly acidic to neutral or even alkaline, thus resulting in a pronounced interfacial pH gradient (Figure 1). Such drastic local pH changes can be monitored using a pH probe placed in proximity to the electrode surface during the electrodeposition process (Figure S8). Even when positioned in a distance of approximately ~ 1 mm the pH probe detects a raise in the pH from ~ 0.49-0.57 to 6.9-7.67 in the course of the concerted electrodeposition/HER process. Note that although the solution is stagnant (no extra forced convection is applied) the near-surface electrolyte is not fully motionless. The growing H2 gas bub38 bles induce radial motion in the liquid. In addition, detached gas bubbles cause convection and thereby disturb to some extend the formation of the pH gradient along the surface normal. A pH of 6.9 is, however, already sufficient to -2 yield a citrate concentration of = 6.84 ⋅ 10 M. Yet, a further increase of the pH directly at the electrode surface would only cause a minor additional raise in the respective citrate concentration (e.g. at pH = 9: c( )= -2 8.97 ⋅ 10 M). As indicated in Figure 1, the massive HER creates a reactive boundary layer in which a cascade of reactions takes place prior to the actual Ag electrodeposition. Due to the pH-dependent deprotonation of the citric acid, an extra gradient of the citrate concentration appears along with the pH gradient (Figure 1). In this scenario it is the HER which ultimately activates the additive action of the citric acid/citrate at the location where it is actually desired. Fur+ thermore, Ag ions that need to diffuse through this reactive boundary layer undergo a complexation reaction with the chelating citrate ligands prior to their deposition on the -2 substrate. Given the high current density of J = -3 A cm and + the relatively low Ag concentration of 40 mM in the bulk electrolyte, the Ag deposition becomes readily mass + transport limited with an Ag concentration that drops down + to zero at the electrode surface. As a consequence, an Ag ion gradient appears under such harsh conditions as further

3.1 Metal foam electrodeposition – Figure 1 demonstrates the basic concept of the hydrogen template assisted Ag foaming process that was adopted from previous deposition approaches developed by Shin et al. for Cu and Cu based 29, 30 31, 32 alloys and recently extended to Ag based systems. Key element of the Ag catalyst deposition is the hydrogen evolution reaction (HER) which is superimposed on the respective metal deposition when applying extraordinarily high current -2 29, 30 densities of J = -3 A cm . These experimental conditions lead to the temporary appearance of gas bubbles on the electrode which serve as transient geometric template for the desired metal foaming process. Primary source of the HER in + acidified aqueous solutions is the H reduction (Eq. 1). +

-

2H + 2e → H2

(1) +

However, at such high current densities the H reduction 33-35 so already reaches mass transport (diffusion) limitations that H2O electroreduction inevitably gets initiated as secondary source of the HER (Eq. 2). -

-

2H2O + 2e → H2 + 2OH

(2)

Both partial HERs (Eq. 1 and 2) contribute to severe changes of the pH in close proximity to the electrode surface as con+ sequence of the massive H consumption and OH production which also affect the additive-assisted metal deposition itself. A major difference to the standard Cu foaming pro28-30, 36 cesses studied so far is related to the use of particular plating additives. For the deposition of Ag foams Cherevko et 31 al. already used plating formulations which contained KSCN (1.5 M), various amounts of NH4Cl (0-2.5 M) and minor amounts of citrate (0.01-0.03 M Na3C6H5O7. 2 H2O) as additives. Particularly, the presence of NH4Cl has been demonstrated to have a strong influence on the resulting Ag foam morphology whereas the citrate was added to the bath solely to 31 avoid undesired precipitation side-reactions in this study. -2 Besides the Ag precursor (2 ⋅10 M Ag2SO4) and the acidic supporting electrolyte (1.5 M H2SO4) our plating bath con-1 tained extra 10 M tri-sodium citrate di-hydrate (Na3C6H5O7 ⋅ 2 H2O) whereas KSCN and NH4Cl were excluded from the formulation. In electroplating applications citrate ) is often used as additive and complexing agent due to its 37 strong chelating capabilities. Given the resulting pH of 0.49 in the ‘bulk’ of the Ag plating bath and considering the com-

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Figure 2. Morphology of the Ag foam: a) Side-view SEM inspection of the Ag foam; b) – d) Top-down SEM images showing the primary macro-porosity of the Ag foam; e) 3D optical micrograph introducing a depth resolution into the Ag foam analysis; f) Height-section (Δh = 3.75 µm) of the 3D optical micrograph in e) showing the topmost level of the pore architecture; g) – h) Top-down SEM images showing the secondary meso-porosity of the Ag foam. constituent part of such reactive boundary layer (Figure 1). In + this scenario the highest ratio of the citrate/Ag concentrations is expected directly at the electrode surface. Extra rotating disk electrode (RDE) experiments on the Ag deposition under more defined mass transport conditions indeed prove + the complexation of Ag in particular at elevated pH (Figure S9).

The novel Ag foam reveals a distinct gradient in the pore diameter along the surface normal with the smallest pores close to the substrate level and the largest ones at the outer28, 29 This most surface of the foam (Figure S7 and Figure S10). characteristic can be rationalized in terms of a highly dynamic template with initially smaller H2-gas bubbles that tend to coalesce to larger ones in the course of the concerted electrodeposition/HER process. An increased residence time of the gas bubbles on the surface thereby also increases the bubble break-off diameter d0. Such multilevel characteristics of the Ag foam can be deduced from the SEM image shown in panel d as well as from the 3D optical microscopy inspection (panel e). The latter provides an additional depth information.

3.2 Structural characterization - The morphological characteristics of the Ag foam yielded after 20 s of deposition are depicted in Figure 2. A side-view SEM inspection (panel a) allows a clear differentiation between the compact Ag foil substrate and the covering Ag foam that reveals a thickness of ~17 µm. Compared to the corresponding additive-free deposition process (Figure S7) this Ag foam shows a rather uniform appearance even on the larger scale with an opencell architecture of interconnected macro-pores (panel c and d). However, these macro-pores are significant smaller than in case of the citrate-free metal-plating process (Figure S7). This observation can be seen as a clear prove of a citratemediated additive action at the interface which affects not only nucleation and growth of the Ag itself, but also the formation and growth of the gaseous H2-template at the 38 emerging foam. As discussed by Vogt et al. and Nicolic et 39 al. , the key descriptor for the evolution of macro-porosity during hydrogen-assisted metal growth is the so-called ‘bubble break-off diameter’, d0, which ultimately determines the resulting pore diameter. d0 depends on the contact angle (ϑ), the densities of the involved liquid and gaseous phases (ϱL and ϱG), and the potential-dependent liquid-electrode inter40 facial tension (γ) according to :

The deepest pores (highlighted blue in panel e) which nearly reach down to the Ag foil substrate (depth of ~17.8 µm) are close to round-shaped and indeed reveal the smallest diameters. For the quantitative analysis more extended areas of the Ag foam were analyzed by the 3D optical microscopy particularly focusing on three distinct height intervals (∆h1 = 3.75 – 0 µm, ∆h2 = 7.75 - 3.75 µm , and ∆h3 = 9.25 - 7.75 µm) which represent the three major height levels of the pores inside the 3D foam (Figure S10). As an example of this analysis, Figure 2f shows the topmost pore level (highlighted in red). Compared to the smallest pores, which form in the initial stage of the foaming process, the larger pores reveal a more irregular shape. In addition to the primary macro-porosity originated by the gaseous H2-template, the scaffold of the 3D foam itself is porous, too. The macro-pore sidewalls in Figure 2g-h are composed of randomly distributed needle-like Ag particles of about 400 nm length and diameters below 50 nm. The comparison with the additive-free foaming process (Figure S7) reveals a clear involvement of the citrate additive in this action (Figure 1) which causes such highly anisotropic Ag growth on the nm length scale. An XPS analysis of the as

(3) Citrate additives (Figure 1) interacting with the Ag surface obviously decrease d0 most likely due to an altered interfacial 40 tension.

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deposited Ag foam indicates the presence of a surfaceconfined oxide layer. The Ag3d5/2 photo-emission peak at BE = 367.9 eV is slightly downward-shifted (∆BE ~ 0.3 eV) with regard to the Ag foil reference (BE(Ag3d5/2) = 368.2 eV) and the respective Ag foam analyzed after the ec-CO2RR (BE(Ag3d5/2) = 368.2 eV) (Figure S12). The Ag3d emission of the as received Ag foam needs to be fitted by two components which can be assigned to metallic Ag (BE(Ag3d5/2) = 41 368.2 eV) and Ag2O (BE(Ag3d5/2) = 367.6 eV), respectively. Obviously, the Ag foam is sensitive towards oxidation right after its emersion from the plating bath similar to what is 28 known from the electrodeposition of Cu foams. These sur41 face oxides (most likely Ag2O) get readily reduced under 9, 28, 36 the harsh conditions of the ec-CO2RR. Therefore we can consider the metallic Ag foam catalyst to be ‘oxidederived’ even without any extra thermal annealing treat42, 43 14, 44 ment or exposure to oxygen plasma . Based on the XPS analysis we cannot exclude the presence of sub-surface oxygen species which remain embedded into the Ag catalyst after completion of the oxide reduction as reported by Mistry .21 et al. . Of eminent importance for the in-depth discussion on the anomalous hydrocarbon activity of the Ag foam catalyst reported herein is the absence of any kind of Cu contamination on its surface. Cu might be a trace impurity possibly originating from the electroplating process. However, the Cu2p region in the respective XPS spectra shows only background noise (Figure S11).

regimes (Figure 3b, region I and region II). CO is the dominant ec-CO2RR product in the initial potential range starting from -0.3 V and extending to -1.2 V vs RHE. CO formation on the novel Ag foam catalyst sets in at comparably low overpotentials which are ~300 mV less negative as compared to the ec-CO2RR onset on the polycrystalline Ag foil reference (Figure 3d). The observed ec-CO2RR onset on the Ag foam is indicative for a significantly improved catalytic activity and comparable or even superior to the best Ag-based CO forming catalysts 20 reported so far in literature for aqueous environments. Note that the Ag nano-coral electrodes studied in the work 20 of Polyansk et al. are highly efficient also because of the presence of halide anions, which act as co-suppressor for the HER (co-catalyst for the ec-CO2RR) at those low overpotentials, whereas no halides were present in our electrolysis experiments. Corresponding voltammetric data measured in Ar- and CO2-saturated bicarbonate electrolytes further confirm an increased activity of the Ag foam (Figure S15). Note that such low onset potentials for CO production were re45 ported so far only for Au nano-needle catalysts. -2

The CO partial current increases from jCO = -0.02 mA cm to -2 jCO = -14.72 mA cm when going from -0.3 V to -1.2 V vs RHE. The partial current densities on the Ag foam are about one order of magnitude higher than the Ag foil reference system. This can be rationalized by a convolution of a surface area effect and an increased electrocatalytic activity (note the ECSA increases only by a factor of 4 when going from the planar Ag foil to the Ag foam catalysts, see Figure S14). The FECO values thereby never fall below 90% (Figure 3b in the region I). In particular at lowest overpotentials the FECO values reach 99% (-0.3 V vs RHE). This extended plateau region in the FECO vs E plot of about ~ 900 mV is insofar unique for an aqueous reaction environment as polycrystalline Ag electrocatalysts typically show a peak-like behavior in their FECO vs E plot regarding the CO formation. Figure 3d exemplarily compares the potential-dependent FECO evolution of the novel Ag foam catalyst with a representative Ag foil reference, which is in full agreement with the litera26, 46, 47 ture. The reference shows a maximum in the FECO evolution at about E = -1.1 V vs RHE whereas the CO efficiencies at low overpotentials drop down to values even below 1%. It is the HER which typically dominates the FE characteristics of the Ag foil catalyst at low (E > -0.6 V vs RHE) and high overpotentials (E < -1.2 V vs RHE), respectively, with FECO and FEH2 values that are strongly anti-correlated (Fig26, 48 ure 3d, region I). The same peak-like behavior was recently reported by Wang et al. for an electrodeposited Ag 32 foam catalyst. The general peak-like behavior in the FECO characteristics was conserved in their study, but significantly upward-shifted with respect to Ag foil references reaching peak values in the FECO of 94% at -1.03 V vs RHE (converted 32 from SCE scale used in their study). These results were 46 recently reproduced by Daiyan et al.. The major differences between the Ag foam catalyst presented herein and the ones 32, 45 reported in the literature are related to the respective macro- and meso-porosities of the foams. In particular the nanoscale morphology of the Ag crystallites forming the pore side-walls is different. Only the Ag foam deposited in the presence of the citrate reveals such highly anisotropic nee-

The powder XRD analysis of the as deposited Ag foam reveals dominant fcc diffraction pattern of polycrystalline Ag bulk material as expected from the random distribution of the needle-shaped Ag crystallites in the pore side-walls (Figure 2g-h). Only minor traces of oxidic Ag phases are visible in the XRD spectrum at 2θ values below 38° (Figure S13), thus confirming the lack of 3D translational order of the surface confined oxide phases. Important to note is that the polycrystalline Ag foil (reference) shows a somewhat anomalous texturing with relative peak intensities of I(111) : I(200) : I(220) = 1 : 0.79 : 4.76 which differ from regular polycrystalline Ag samples (I(111) : I(200) : I(220) = 1 : 0.4 : 0.15). The electrochemically active surface area (ECSA) of the Ag foam was estimated by means of voltammetric analyses of the reversible first viologen redox transition (Figure S14) of a di-methyl viologen redox probe following approaches de27, 28 scribed in the literature. The Ag foam reveals an electro2 chemically active surface area of ECSAfoam = 4.77 cm , which is higher only by a factor of 4 with respect to the correspond2 ing Ag foil (ECSAfoil = 1.17 cm ). This moderately increased surface area might be an indication that the electrolyte can penetrate into the macro-pores but does not entirely interfuse the meso-porous sidewalls of the 3D foam architecture. 3.3 CO2 electrolysis – Major results of the potentialdependent ec-CO2RR using the Ag foam as electrocatalyst are summarized in Figure 3. Figure 3a shows the resulting product distribution in terms of partial currents (normalized to the geometric surface area). Respective data normalized to the ECSA can be found in the supporting information file (Figure S16). ec-CO2RR experiments were carried out in the potential range from -0.3 V to -1.6 V vs RHE. The potential window studied herein can be subdivided into two distinct

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Figure 3. a) Potential (E) dependent product distribution of the ec-CO2RR displayed as partial current densities normalized to the geometric surface area (multi catalyst approach, 1h potentiostatic ec-CO2RR carried out at each potential applied using a freshly prepared Ag foam catalyst); b) Corresponding plot of the Faradaic efficiencies (FEs) as 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 (FEs). dle-like Ag crystallites (Figure 2), whereas the side-walls of the Ag foam in work by Wang et al. are more compact and composed of more irregularly shaped Ag particles like the Ag 32 foam obtained in the absence of any additive (Figure S7).

reaches a maximum in its Faradaic efficiency of remarkable FECH4 = 51% at -1.5 V vs RHE before its decreases again (Figure 3b and Figure 3c, region II). This maximum FECH4 value corresponds to an extraordinarily high partial current density -2 of jCH4 = -18.8 mA cm . The peak-like behavior in the potential-dependent FECH4 evolution indicates that a critical rate of CO and H2 production is mandatory for the observed CH4 formation. Such high C1 hydrocarbon production rates and efficiencies were reported so far only for Cu based electrocat17, 49-52 alysts , whereas Ag was considered as a predominantly CO forming catalyst due to its weak interaction with the formed *CO, which therefore gets readily released from the 6, 17 53 Ag surface. A recent mechanistic study by Singh et al. based on a multiscale physics approach predicted that the majority of intermediates chemisorbed on Ag model catalysts are * COOH, and *H species. *CO is discussed only as a minority species due to its weak affinity to Ag. In particular, the observed hydrocarbon formation points to an increased *CO binding strength in case of the Ag foam catalyst. The observed onset potential of about -1.1 V vs RHE agrees well with the one reported for Cu-based CH4-producing cata17,50 lysts. Hydrocarbon formation typically requires a more balanced interaction of the *CO intermediate with the catalyst surface (Sabatier principle) in conjunction with a sufficiently high residence time on the reactive catalyst site as

The extended ~900 mV wide plateau in the FECO vs E plot (Figure 3b) can be considered as a first experimental hint for a *CO binding strength that is considerably increased with respect to the Ag foil reference (Figure 3d). CO adsorption therefore becomes favored over the competitive H adsorption in particular at those lower overpotentials. In this sense, the more strongly bound *CO acts as an effective suppressor for the HER. An increased *CO binding energy presumably goes along with an increase in the *CO surface coverage and an extended residence time on the catalyst surface. Both effects are essential for the second characteristic potential range in the FECO vs E plot (Figure 3b, region I) that starts at E < -1.1 V vs RHE and involves a drastic drop-down of the FECO down to 11% at -1.6 V vs RHE accompanied by the simultaneous raise of the FEH2 values up to 64%. Such drastic decrease of the FECO is typically associated with the onset of the CO2 transport limitation in the course of the ec-CO2RR at 6, 32 higher reaction rates. A unique feature of the novel Ag foam catalysts, never reported before for any other Ag based ec-CO2RR catalyst, concerns the appearance of CH4 as reaction product. CH4 formation sets in at E < -1.1 V vs RHE and

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Figure 4. Scheme demonstrating the proposed mechanistic pathways of CO (panel a) and hydrocarbon formation (panel b) on the Ag foam catalyst. mechanistic pre-requisites for further reductive hydrogena6, 54 For a number of ec-CO2RR catalyst materition reactions. 6 als, Kuhl et al. derived a Volcano like interrelation between the *CO binding strength and the ec-CO2RR current densities at -0.8 V vs RHE with Au being closest to the Volcano maximum and Ag and Cu being next to it on the low and high binding energy side of the Volcano maximum, respectively. From the observed superior CO selectivity at lower overpotentials and in particular from the remarkable CH4 selectivity at higher overpotentials it can be concluded that the novel Ag foam has to be positioned close to the Cu in such a Volcano plot. Further support for this hypothesis comes from the intriguing observation that the novel Ag foam catalyst is also capable of C-C coupling reactions (Figure 3b, region II). C2H4 is formed in the same potential window between -1.1 V and -1.6 V vs RHE with a maximum of FEC2H4 = 8.6% at -1.5 V vs RHE. The total hydrocarbon efficiency therefore amounts to 59.6% at -1.5 V vs RHE (Figure 3c). Note that on Cu, the C2H4 formation typically sets in at 2,9, 10, 14, 15 lower overpotentials than the CH4 formation. It can be assumed that it is the dominance of the CO formation and related to that the extremely low HER rate which prevent C2H4 formation at lower overpotentials on the Ag foam as observed for the Cu based catalysts.

tained extra 0.1 M sodium citrate served as electrolyte. The respective time-resolved GC head-space analysis clearly confirms the absence of any gaseous hydrocarbon product that might be formed during cathodic stressing of the citrate additive (Figure S20). Note that a certain amount of CO was formed in this control experiment, which originates either from direct bicarbonate reduction or from the reduction of minor amounts of physically dissolved CO2 from the bicarbonate/CO2 equilibrium reaction (Figure S20). Further blank experiments were carried out at -1.5 V vs RHE in citrate-free 0.1 M K2SO4 electrolytes that were saturated either with Ar (pH = 7.76) or with CO2 (pH = 7.15). In these control experiments, the electrodeposited Ag foam served as a catalyst (Figure S20). Neither CO nor hydrocarbons were detected in the CO2-free electrolyte and only H2 was found as the reaction product in the Ar-saturated electrolyte. As expected, CO and CH4 were detected in the CO2-containing electrolyte at 1.5 V vs. RHE (Figure S20). The CH4 production on the Ag foam catalyst can therefore clearly be assigned to the ecCO2RR and undesired additive effects during CO2 electrolysis can be excluded. To further rule out any side-effects from the chosen Ag support material, foils from different suppliers were tested but yielded the same result (Figure S18b). Furthermore, variations in the initial cleaning treatment of the Ag foil support have only minor effects on the resulting Ag foam performance in the ec-CO2RR (Figure S20). As a working hypothesis, Figure 4 summarizes possible mechanistic pathways of the CO2 conversion on the improved Ag foam catalyst. The experimentally observed hydrocarbon formation clearly indicates that the novel Ag foam in part shares essential electrocatalytic properties with Cu-based 55, 56 catalysts. On the Ag foam, CO is the only and final ecCO2RR product at low and moderate overpotentials (CO pathway). This is in agreement with Cu-based catalysts, where CO forms at low and moderate overpotentials, but with significant lower efficiencies as compared to Ag-based catalysts with maximum FECO values ranging from ~20% (at 0.65 V vs RHE on electro-polished Cu foil) to ~50% (at -0.6 V

Also note that such C-C coupling requires the stabilization of the *CO intermediates at the catalyst surface, thus resulting in a sufficiently high surface concentration and mean resi14, 16, 55 dence time of *CO as discussed for Cu. From our XPS analysis of the Ag foam catalyst prior and after ec-CO2RR we safely exclude any trace contaminations of Cu as origin of the observed anomalous hydrocarbon activity of the Ag foam (Figure S11). Furthermore, extra blank electrolysis experiments were performed to rule out the possibility of undesired citrate residuals or decomposition products thereof from the initial electrodeposition step which might contribute to the observed massive CH4 formation (Figure 3). For this purpose, a 1h (blank) electrolysis experiment was carried out at -1.5 V vs RHE using an Ag foil electrode as catalyst. An Ar-saturated 0.5 M KHCO3 solution (CO2-free) which con-

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Figure 5. a) – b) Potential (E) dependent product distribution displayed as Faradaic efficiencies (FEs) yielded by a single catalyst approach using one single Ag foam catalyst for the entire electrolysis campaign. vs RHE on solvo-thermally prepared Cu2O) and ~61% (at -0.3 58 - -0.4 V vs RHE on CuO nanowires) .

presented in Figure 3a and Figure 3b, were based on a socalled multi catalyst approach where freshly prepared Ag foams were used for each ec-CO2RR experiment. In contrast to this and to demonstrate the particular robustness of the Ag foam catalyst, Figure 5a-b shows continuous electrolysis experiments, in which the same Ag foam catalysts was used for all the electrolysis potentials applied (single catalyst approach). This ec-CO2RR campaign started at highest overpotentials (-1.6 V vs RHE, Figure 5a) and continued stepwise towards lower overpotentials as indicated by the grey arrow (forward scan). Figure 5b summarizes the results of the corresponding reverse electrolysis campaign starting at -0.3 V vs RHE and proceeding gradually towards the final electrolysis potential of -1.6 V vs RHE (backward scan). The forward scan almost reproduces the characteristics and the FE values of the multi catalyst approach (Figure 3a-b).

In particular the oxide-derived Cu catalysts with an increased density of low-coordinated reaction sites show an enhanced 57, 58 selectivity towards CO. As a second important ec-CO2RR product, formate often appears on Cu at moderate and low overpotentials with efficiencies reaching values of FEformate = 10 50% at -0.7 V vs. RHE at the expense of FECO. With respect to that, the novel Ag foam is clearly superior to Cu due to its higher selectivity towards CO in this potential regime. The Ag foam catalyst preserves the Ag characteristics at low and moderate overpotentials and switches to a Cu like behavior only at potentials < -1.1V vs RHE (Figure 3). The high selectivity towards CO observed at moderate and low overpotentials (Figure 3) needs to be considered as interplay of various effects on multiple length scales. Our results suggest a high abundance of low-coordinated reaction sites on the needlelike Ag particles. Early work by Hoshi et al. already demonstrated a profound sensitivity of the CO formation rate on 59 the atomic scale structure of the Ag-based catalysts. More open (110) facets were shown to be much more active towards CO formation than the more compact (100) and the 59 hexagonal (111) surface. Liu et al. recently discussed the effect of high local electric fields on the selectivity enhance60 ment observed for highly curved needle-like Au catalysts. All these effects might also rationalize the high selectivity of the Ag foam towards CO, but they do not explain the appearance of an extra hydrocarbon pathway at higher overpotentials, which further splits into a dominant C1 (CH4) and a minor C2 (C2H4) sub-pathway (Figure 4) as known from Cu. Key for this unique characteristic must be a substantially increased *CO binding strength. How important the particular meso-porosity of the Ag foam (Figure 2) is for the observed hydrocarbon formation is subject of current research.

In this single catalyst approach slightly lower CH4 and C2H4 -2 efficiencies of FECH4 = 49.2% (jCH4 = -15.89 mA cm ) and -2 FEC2H4 = 5.3% (jC2H4 = 1.33 mA cm ), respectively, were detected at -1.5 V vs RHE, whereas CO efficiencies of FECO = -2 98.1% (jCO = -1.83 mA cm ) reached at -0.6 V vs RHE were of the same values as observed for the multi catalyst approach (Figure 3b). Additionally, the FECO values remain well above 90% at potentials > -1.0 V vs RHE in the backward scan (Figure 5b). This is indicative for a CO production that remains largely unaffected by such extended electrolyses experiments, covering the entire spectrum of electrolysis potentials. However, the C1 hydrocarbon pathway is more sensitive to this catalyst stressing. After CO2 electrolysis under variable potential conditions, a maximum CH4 efficiency of only FECH4 = 37.2% is observed at -1.5 V vs RHE in the respective backward scan (Figure 5b) along with an increased H2 efficiency of 32.2%, thus pointing to a certain hysteresis in this cyclic stressing experiment. In Figure 6a a further continuous (single catalyst) stressing experiment is presented, in which the applied potential was repeatedly changed between -1.5 V and -0.8 V vs RHE corresponding to the optimum conditions for CH4 and CO production, respectively. The potential hold time on each selected electrolysis potential was 20 min only.

3.4 Catalyst stressing experiments - Not only are the general electrocatalytic activity and product selectivity important for the overall performance evaluation of the catalyst but also its stability under reactive conditions. It should therefore be noted that the ec-CO2RR product distributions

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Figure 6. a) Catalyst stressing experiment: the ec-CO2RR was carried out at -0.8 V and -1.5 V vs RHE with potential hold times of 20 min.; b) – c) Extended catalyst stressing experiments at -0.8 V vs RHE (b) and -1.5 V vs RHE (c). Both the total geometric current density and the FEs are plotted as function of the electrolysis time. From an inspection of Figure 5a it becomes evident that after an initiation period the FE values periodically switch in an almost reversible manner between FECO = 92% ± 3% (-0.8 V) and FECO = 15% ± 2% (-1.5 V) for CO and between FECH4 = 0% (-0.8 V) and FECH4 = 44% ± 2% (-1.5 V) for CH4 production, respectively. Figure 6b demonstrates a notable stability of the Ag foam catalyst during CO production at -0.8 V vs RHE. Gaseous products were analyzed in intervals of 30 min. for an electrolysis time of 72 h. With respect to the total current -2 density, a slight decrease from initial jtot = -6.9 mA cm to jtot -2 = -5.9 mA cm at the end of the long-term electrolysis experiment was observed. The total charge transferred through the interface during the electrolysis amounts to 1528C (Figure S21). A minor catalyst deactivation can be deduced from the FECO values, which fall from initial 98% to final 90% during the 72h electrolysis, whereas the anti-correlated FEH2 values rise from 2% to 10%. These results further indicate a superior long-term stability of the Ag foam catalyst in the CO forming r egion. Nonetheless, the stability is decreased at higher overpotentials than -1.5 V vs. RHE, where CH4 is the major ec-CO2RR product (Figure 6c). In this case, CO2 electrolysis of only 8 h was carried out which corresponds to a total charge of 1298C transferred during the electrolysis, which is comparable to the more extended electrolysis experiment at lower overpotentials (Figure 6b). Interestingly, the total current density increases over time from initial jtot. = -32.2 -2 -2 mA cm to final jtot. = -45.1 mA cm . Both the FECO and the FECH4 show an anti-correlated characteristic in their timedependence in particular in the initial stage of the electrolysis experiment (< 2 h electrolysis time) starting with a low CH4 (FECO = 17% at 20 min.) and high CO (FECO = 63% at 20 min.) efficiency, respectively. After 1 h the CO efficiency reaches a quasi-plateau while the FECO only marginally drops down from 19% to 14% in the course of further electrolysis. FECH4 reaches a maximum after 1.5 h of FECH4 = 53 % before it

drops down in an almost linear manner to a final FECH4 = 32 % at the end of this experiment. These time-dependent electrolysis results indicate that the Ag foam catalyst requires an ‘operando’ activation before ideal conditions for the CH4 production are obtained. The as synthesized Ag foam is therefore only the precursor for the active catalyst, at least for the CH4 production. A further conclusion that can be drawn from these extended electrolysis experiments is that, similar to what is known from polycrystalline Cu catalysts, the C1 hydrocarbon production on the Ag foam is only a 10, 61-63 transient phenomenon. The C1 hydrocarbon pathway, which might involve the formation of chemisorbed carbon (Figure 4), has been discussed as one possible origin for the 10 observed fast catalyst degradation. According to Hori et al., Akhade et al., and Dewulf et al., this C1 pathway might involve a non-reversible chemisorption of *C surface species on 50, 61, 62 the catalyst surface (see Figure 4). Recent experiments have shown that a suppression of the CH4 formation, e.g. by particular thermal annealing treatment of a Cu catalyst, can 10 lead so a significantly improved degradation behavior. We believe that the degradation behavior discussed above also supports the working hypothesis that both Cu and the novel Ag foam share at least in parts common mechanistic features of the ec-CO2RR (Figure 4). 3.5 Identical location inspection (IL-SEM) – Morphological changes of the Ag foam catalyst caused by the ec-CO2RR were monitored as a function of the electrolysis time by socalled identical location (IL) HR-SEM analysis. As already demonstrated for Cu foam catalysts, the macro-porosity remains largely unaffected by the ec-CO2RR and the involved 28 massive gas evolution. In particular the macro-porous structure does not collapse even under drastic electrolysis 28 conditions at -1.5 V vs RHE (Figure 7a, d and g). Changes in the catalyst morphology, however, take place on the nm length scale and strongly depend on the applied electrolysis

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Figure 7. Identical location (IL) SEM inspection of the Ag foam catalyst prior (a-c) and after (d-i) the ec-CO2RR at -1.5 V vs RHE; IL-SEM inspection of the Ag foam catalyst prior (j-l) and after (m-o) the ec-CO2RR at -0.8 V vs RHE potential (current density) and thereby also on the resulting ec-CO2RR product distribution. The most drastic catalyst degradation is observed when CH4 is produced (e.g. at -1.5 V vs RHE) as demonstrated in Figure 7a-i when comparing the macro- and meso-porosities of the as-deposited Ag foam and the ones after 4 and 8 h electrolysis, respectively (see also Figure S22). The anisotropic Ag crystallites formed by the citrate-assisted Ag electrodeposition tend to coalesce upon electrolysis at -1.5 V vs RHE. It is therefore most likely that an irreversible carbonization of the Ag surface (*C poisoning, see Figure 4) contributes to the observed catalyst degradation when the C1 hydrocarbon pathway is active. Additionally, an enhanced surface mobility of Ag under these harsh conditions seems to lead to the drastic (non-desired) electrochemical annealing phenomena. The latter apparently involve the loss of low-coordinated surface sites, which are particularly important for the stabilization of *CO/*CHO intermediates. When the CO2 electrolysis is carried out at low or moderate overpotentials, only slight morphological changes are visible in the respective IL-SEM inspection (Figure 7j-o). In accordance to this observation the Ag foam catalyst remains active even during extended ec-CO2RR. Yet, after 72 h. of electrolysis the needle-like Ag particles become more roundish (Figure S23).

surface area) in this CO potential regime. Most intriguing is the capability of the novel Ag foam catalyst to produce hydrocarbons in significant amounts, reaching CH4 and C2H4 efficiencies of FECH4 = 51% and FEC2H4 = 8.6%, respectively, at -1.5 V vs RHE. This remarkable Cu-like behavior has been rationalized in terms of the *CO binding energy which is significantly increased on the novel Ag foam with respect to polycrystalline Ag foil references. This leads to a substantially increased *CO surface concentration and mean *CO residence time at the active catalytic sites. These effects are crucial mechanistic pre-requisites for the further reductive hydrogenation reactions of the *CO towards CH4 formation and C-C coupled reaction products, such as ethylene. A key structural feature of the novel Ag foam catalyst is the particular meso-porosity with pore sidewalls that are composed of highly anisotropic, needle-shaped Ag features having dimensions in the nm range. This particular morphology is obtained only by means of the citrate plating additive controlling the Ag growth on the nm-scale. Long term electrolysis experiments indicate a more severe degradation of the Ag foam catalyst in the CH4 forming region similar to Cu. IL HR-SEM analysis also confirms morphological changes which are more severe when CH4 is produced at higher overpotentials/current densities as compared to the CO production at lower overpotentials/current densities.

4. CONCLUSION

This work demonstrates that the tailored design of ec-CO2RR catalysts can transform a predominantly CO producing catalyst, e.g. Ag, into a Cu-like catalyst being capable of producing hydrocarbons.

An additive-assisted metal foam deposition process has been developed, yielding an Ag-based ec-CO2RR catalyst with superior activity and selectivity towards CO production at particularly low and moderate overpotentials. Not only is the onset potential for CO formation of ~ -0.3 V vs RHE considerably low but also do the resulting CO efficiencies never fall below 90% within an extraordinarily broad potential window of ~ 900 mV ranging from -0.3 V to -1.2 V vs RHE. The corresponding partial CO currents increase from jCO = -0.02 mA -2 -2 cm to jCO = -14.72 mA cm (normalized to the geometric

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 00.000/acs.000000

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Experimental procedure for Ag nano foam synthesis, and analytical methodology for quantification of gaseous products, AFM, SEM, XRD patterns, deconvoluted XPS spectra, and control experiments.

12.

AUTHOR INFORMATION 13.

Corresponding Author *[email protected] *[email protected]

Notes

14.

The authors declare no competing financial interests.

ACKNOWLEDGMENT The financial support by the CTI Swiss Competence Center for Energy Research (SCCER Heat and Electricity Storage) is gratefully acknowledged. PB acknowledges the financial support by the Swiss National Science Foundation (SNSF) via the project No. 200020_172507. This study was performed with the support of the interfaculty Microscopy Imaging Centre (MIC) of the University of Bern.

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28. Dutta, A.; Rahaman, M.; Luedi, N. C.; Mohos, M.; Broekmann, P., Morphology Matters: Tuning the Product Distribution of CO2 Electroreduction on Oxide-Derived Cu Foam Catalysts. ACS Catal. 2016, 6, 3804-3814. 29. Shin, H. C.; Liu, M., Copper foam structures with highly porous nanostructured walls. Chem. Mater. 2004, 16, 5460-5464. 30. Shin, H. C.; Dong, J.; Liu, M., Porous Tin Oxides Prepared Using an Anodic Oxidation Process. Adv. Mater. 2004, 16, 237-240. 31. Cherevko, S.; Chung, C. H., Impact of key deposition parameters on the morphology of silver foams prepared by dynamic hydrogen template deposition. Electrochim. Acta 2010, 55, 63836390. 32. Wang, H.; Han, Z.; Zhang, L.; Cui, C.; Zhu, X.; Liu, X.; Han, J.; Ge, Q., Enhanced CO selectivity and stability for electrocatalytic reduction of CO2 on electrodeposited nanostructured porous Ag electrode. J. CO2 Util. 2016, 15, 41-49. 33. Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Topalov, A. A.; Biedermann, P. U.; Auinger, M.; Mayrhofer, K. J. J., The effective surface pH during reactions at the solid–liquid interface. Electrochem. Comm. 2011, 13, 634-637. 34. Grozovski, V.; Vesztergom, S.; Lang, G. G.; Broekmann, P., Electrochemical Hydrogen Evolution: H+ or H2O Reduction? A Rotating Disk Electrode Study. J. Electrochem. Soc. 2017, 164, E3171-E3178. 35. Wiberg, G. K. H.; Arenz, M., On the influence of hydronium and hydroxide ion diffusion on the hydrogen and oxygen evolution reactions in aqueous media. Electrochim. Acta 2015, 159, 66-70. 36. Dutta, A.; Rahaman, M.; Mohos, M.; Zanetti, A.; Broekmann, P., Electrochemical CO2 Conversion Using Skeleton (Sponge) Type of Cu Catalysts. ACS Catal. 2017, 7, 5431-5437. 37. Frank, A. C.; Sumodjo, P. T. A., Electrodeposition of cobalt from citrate containing baths. Electrochim. Acta 2014, 132, 75-82. 38. Vogt, H.; Balzer, R. J., The bubble coverage of gas-evolving electrodes in stagnant electrolytes. Electrochim. Acta 2005, 50, 2073-2079. 39. Nikolic, N. D.; Brankovic, G.; Pavlovic, M. G.; Popov, K. I., The effect of hydrogen co-deposition on the morphology of copper electrodeposits. II. Correlation between the properties of electrolytic solutions and the quantity of evolved hydrogen. J. Electroanal. Chem. 2008, 621, 13-21. 40. Fritz, W., Maximum Volume of Vapor Bubbles, Phys. Z. 1935, 36, 379-384. 41. Hoflund, G. B.; Hazos, Z. F.; Salaita, G. N., Surface characterization study of Ag, AgO, and Ag2O using x-ray photoelectron spectroscopy and electron energy-loss spectroscopy. Phys. Rev. B 2000, 62, 11126-11133. 42. Li, C. W.; Ciston, J.; Kanan, M. W., Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper. Nature 2014, 508, 504-507. 43. Li, C. W.; Kanan, M. W., CO2 Reduction at Low Overpotential on Cu Electrodes Resulting from the Reduction of Thick Cu2O Films. J. Am. Chem. Soc. 2012, 134, 7231-7234. 44. Gao, D.; Zegkinoglou, I.; Divins, N. J.; Scholten, F.; Sinev, I.; Grosse, P.; Roldan Cuenya, B., Plasma-Activated Copper Nanocube Catalysts for Efficient Carbon Dioxide Electroreduction to Hydrocarbons and Alcohols. ACS Nano 2017, 11, 4825-4831. 45. Zhu, W.; Zhang, Y.-J.; Zhang, H.; Lv, H.; Li, Q.; Michalsky, R.; Peterson, A. A.; Sun, S., Active and Selective Conversion of CO2 to CO on Ultrathin Au Nanowires. J. Am. Chem. Soc. 2014, 136, 46, 16132-16135. 46. Daiyan, R.; Lu, X.; Ng, Y. H.; Amal, R., Highly Selective Conversion of CO2 to CO Achieved by a Three-Dimensional Porous Silver Electrocatalyst. ChemistrySelect 2017, 2, 879-884. 47. Liu, S.; Tao, H.; Zeng, L.; Liu, Q.; Xu, Z.; Liu, Q.; Luo, J.-L., Shape-Dependent Electrocatalytic Reduction of CO2 to CO on Triangular Silver Nanoplates. J. Am. Chem. Soc. 2017, 139, 21602163.

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48. Kim, C.; Jeon, H. S.; Eom, T.; Jee, M. S.; Kim, H.; Friend, C. M.; Min, B. K.; Hwang, Y. J., Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles. J. Am. Chem. Soc. 2015, 137, 13844-13850. 49. Hori, Y., Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry, Springer: New York, 2008; pp 89-189. 50. Hori, Y.; Murata, A.; Takahashi, R., Formation of Hydrocarbons in the Electrochemical Reduction of Carbon-Dioxide at a Copper Electrode in Aqueous-Solution. J. Chem. Soc., Faraday Trans. 1 1989, 85, 2309-2326. 51. Varela, A. S.; Ju, W.; Reier, T.; Strasser, P., Tuning the Catalytic Activity and Selectivity of Cu for CO2 Electroreduction in the Presence of Halides. ACS Catal. 2016, 6, 2136-2144. 52. Manthiram, K.; Beberwyck, B. J.; Alivisatos, A. P., Enhanced Electrochemical Methanation of Carbon Dioxide with a Dispersible Nanoscale Copper Catalyst. J. Am. Chem. Soc. 2014, 136, 13319-13325. 53. Singh, M. R.; Goodpaster, J. D.; Weber, A. Z.; Head-Gordon, M.; Bell, A. T., Mechanistic insights into electrochemical reduction of CO2 over Ag using density functional theory and transport models. PNAS 2017, 114, E8812-E8821. 54. Peterson, A. A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Norskov, J. K., How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels. Energy Environ. Sci. 2010, 3, 1311-1315. 55. Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M., Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073-4082. 56. Nie, X.; Esopi, M. R.; Janik, M. J.; Asthagiri, A., Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps. Angew. Chem. Int. Ed. 2013, 52, 2459-2462. 57. Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F., New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050-7059. 58. Raciti, D.; Livi, K. J.; Wang, C., Highly Dense Cu Nanowires for Low-Overpotential CO2 Reduction. Nano Lett. 2015, 15, 68296835. 59. Hoshi, N.; Kato, M.; Hori, Y., Electrochemical reduction of CO2 on single crystal electrodes of silver Ag(111), Ag(100) and Ag(110). J. Electroanal. Chem. 1997, 440, 283-286. 60. Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.; Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P. G.; Safaei, T. S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.; Kelley, S. O.; Sargent, E. H., Enhanced electrocatalytic CO2 reduction via field-induced reagent concentration. Nature 2016, 537, 382. 61. Akhade, S. A.; Luo, W.; Nie, X.; Bernstein, N. J.; Asthagiri, A.; Janik, M. J., Poisoning effect of adsorbed CO during CO2 electroreduction on late transition metals. Phys. Chem. Chem. Phys. 2014, 16, 20429-20435. 62. Dewulf, D. W.; Jin, T.; Bard, A. J., Electrochemical and Surface studies of Carbon-Dioxide Reduction to Methane and Ethylene at Copper Electrodes in Aqueous-Solutions. J. Electrochem. Soc. 1989, 136, 1686-1691. 63. Christian, R.; Ralf, K.; Elena, V.; Bernhard, S.; Sebastian, N.; Andreas, R.; Manfred, S.; Günter, S., Selective Electroreduction of CO2 toward Ethylene on Nano Dendritic Copper Catalysts at High Current Density. Adv. Energy Mater. 2017, 7, 1602114.

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ACS Catalysis

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TOC graphic. Beyond copper – selective production of CO and hydrocarbons on silver foam catalysts.

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Beyond Copper in CO2 Electrolysis ... - ACS Publications

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