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• W2999683494 abstract "•An electrodeposition strategy produced hierarchically porous Ag-Zn alloys•These Ag-Zn surfaces demonstrated >90% selectivity for CO2 electroreduction to CO•CO2 mass transport limited the CO production rate to ∼−21 mA.cm−2 at 1 atm•At elevated pressure (up to 9.5 bar) of CO2, CO production reached −286 mA.cm−2 To abate atmospheric levels of greenhouse gases, research efforts have sought routes to recycle emitted CO2. This can be achieved through electroreduction of CO2 to generate CO, a precursor to valuable chemical feedstocks currently derived from fossil sources. For CO2 electroreduction to reach industrial applications, electrocatalytic current densities > −300 mA.cm−2 must be achieved, requiring a catalytic surface able to accommodate high CO2 conversion rates. This study reports the development of high-surface-area Ag-Zn alloys that show >90% CO2 reduction selectivity to CO. Their use is first demonstrated under 1 atm of CO2, where catalytic current densities for CO production are limited by the mass transport of CO2. At high CO2 pressures, this limitation is overcome, and the electrodes consequently reach current densities for CO production up to −286 mA.cm−2, representing a step closer to the operation of such catalysts at industrially relevant rates. The electrocatalytic conversion of CO2 into valuable chemical feedstocks is a highly sought-after route to recycle CO2 emissions. Among the expected products, CO is a valuable synthon for organic syntheses and fuel generation. Nevertheless, most current electrocatalytic systems do not generate CO at a sufficient rate or purity for its subsequent direct conversion. Herein, we report the rational design of novel and highly active Ag-alloyed Zn dendritic electrodes with remarkable CO2-to-CO selectivity. Through fine-tuning of the individual electrodeposition parameters, the Ag content, porosity, thickness, and surface area of the electrodes were optimized, leading to a CO2-to-CO selectivity as high as 91%, which could be sustained above an average of 90% over 40 h. Increase of the CO2 pressure (up to 9.5 bar) to enhance the CO2 concentration allowed CO partial current densities as high as –286 mA.cm−2 to be achieved, setting a new record for predominantly Zn-based electrodes operating in neutral pH. The electrocatalytic conversion of CO2 into valuable chemical feedstocks is a highly sought-after route to recycle CO2 emissions. Among the expected products, CO is a valuable synthon for organic syntheses and fuel generation. Nevertheless, most current electrocatalytic systems do not generate CO at a sufficient rate or purity for its subsequent direct conversion. Herein, we report the rational design of novel and highly active Ag-alloyed Zn dendritic electrodes with remarkable CO2-to-CO selectivity. Through fine-tuning of the individual electrodeposition parameters, the Ag content, porosity, thickness, and surface area of the electrodes were optimized, leading to a CO2-to-CO selectivity as high as 91%, which could be sustained above an average of 90% over 40 h. Increase of the CO2 pressure (up to 9.5 bar) to enhance the CO2 concentration allowed CO partial current densities as high as –286 mA.cm−2 to be achieved, setting a new record for predominantly Zn-based electrodes operating in neutral pH. The formation of CO from CO2 emissions and its subsequent conversion to high-added-value chemical feedstocks is a route to many carbon-cycle-closing scenarii.1De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (833) Google Scholar,2Nielsen D.U. Hu X.-M. Daasbjerg K. Skrydstrup T. Chemically and electrochemically catalysed conversion of CO2 to CO with follow-up utilization to value-added chemicals.Nat. Catal. 2018; 1: 244-254Crossref Scopus (270) Google Scholar CO is the most facile intermediate to produce from CO2 and can be transformed either at a large scale by well-mastered thermochemical processes (e.g., the Fischer-Tropsch process, the Cativa process, phosgene synthesis)3Maitlis P.M. Haynes A. Sunley G.J. Howard M.J. Methanol carbonylation revisited: thirty years on.J. Chem. Soc. Dalton Trans. 1996; : 2187-2196Crossref Scopus (315) Google Scholar, 4Jones J.H. The Cativa™ process for the manufacture of acetic acid: iridium catalyst improves productivity in an established industrial process.Platin. Met. Rev. 2000; 4: 94-105Google Scholar, 5Dry M.E. The Fischer–Tropsch process: 1950–2000.Catal. Today. 2002; 71: 227-241Crossref Scopus (1748) Google Scholar or at a smaller scale for fine chemicals synthesis (e.g., hydroformylation, hydroxycarbonylation).6Franke R. Selent D. Börner A. Applied hydroformylation.Chem. Rev. 2012; 112: 5675-5732Crossref PubMed Scopus (1004) Google Scholar, 7Brennführer A. Neumann H. Beller M. Palladium-catalyzed carbonylation reactions of aryl halides and related compounds.Angew. Chem. Int. Ed. 2009; 48: 4114-4133Crossref PubMed Scopus (1166) Google Scholar, 8Foit S.R. Vinke I.C. de Haart L.G.J. Eichel R.A. Power-to-syngas: an enabling technology for the transition of the energy system?.Angew. Chem. Int. Ed. 2017; 56: 5402-5411Crossref PubMed Scopus (179) Google Scholar However, currently, CO production relies almost entirely on fossil-fuel-reforming processes: endothermic reactions that require little energy input but are entirely unsustainable. The past decades have witnessed the emergence of renewably powered electrochemical CO2 reduction, which offers a sustainable and safer route to produce CO on-site, with high flexibility on the small to medium scale.9Hansen H.A. Varley J.B. Peterson A.A. Nørskov J.K. Understanding trends in the electrocatalytic activity of metals and enzymes for CO2 reduction to CO.J. Phys. Chem. Lett. 2013; 4: 388-392Crossref PubMed Scopus (523) Google Scholar,10Zhang W. Hu Y. Ma L. Zhu G. Wang Y. Xue X. Chen R. Yang S. Jin Z. Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals.Adv. Sci. (Weinh.). 2017; 5: 1700275PubMed Google Scholar However, to displace fossil-fuel-based processes, CO2 electrolyzers must not only be cost-competitive but also produce industrially relevant CO tonnage (∼5–10 tCO.h−1).11Mittal C. Hadsbjerg C. Blennow P. Small-scale CO from CO2 using electrolysis.Chem. Eng. World. 2017; 52: 44-46Google Scholar Under realistic cost assumptions (electricity price of $0.04 per kWh and Faradaic efficiency, FE, of 90%), scale-up of such a pathway will require current densities over 300 mA.cm−2 at a power efficiency over 70% (i.e., a cell overpotential lower than 570 mV).1De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (833) Google Scholar To satisfy this demand, management of CO2 mass transport and electrocatalyst design have been identified as key parameters. Mass-transport limitations have been addressed by concentrating CO2 at the surface of the catalyst, through either gas-diffusion electrodes12Higgins D. Hahn C. Xiang C. Jaramillo T.F. Weber A.Z. Gas-diffusion electrodes for carbon dioxide reduction: a new paradigm.ACS Energy Lett. 2019; 4: 317-324Crossref Scopus (292) Google Scholar or high-pressure strategies.13Dufek E.J. Lister T.E. Stone S.G. McIlwain M.E. Operation of a pressurized system for continuous reduction of CO2.J. Electrochem. Soc. 2012; 159: F514-F517Crossref Scopus (112) Google Scholar The latter is highly amenable to industrial CO2 waste streams, for which compression technologies have been extensively explored in the context of CO2 utilization and storage.14Feron P.H. Hendriks C.A. CO2 capture process principles and costs.Oil Gas Sci. Technol. 2005; 60: 451-459Crossref Scopus (122) Google Scholar In addition, many different electrocatalyst designs have been proposed, among which heterogeneous surfaces stand out for their stability and ease of application. Original work by Hori highlighted Ag, Au, and Zn as surfaces with remarkable CO2-to-CO selectivity, which was later confirmed by subsequent studies.8Foit S.R. Vinke I.C. de Haart L.G.J. Eichel R.A. Power-to-syngas: an enabling technology for the transition of the energy system?.Angew. Chem. Int. Ed. 2017; 56: 5402-5411Crossref PubMed Scopus (179) Google Scholar,15Hori Y. Kikuchi K. Suzuki S. Production of CO and CH4 in electrochemical reduction of CO2 at metal electrodes in aqueous hydrogencarbonate solution.Chem. Lett. 1985; 14: 1695-1698Crossref Google Scholar,16Bagger A. Ju W. Varela A.S. Strasser P. Rossmeisl J. Electrochemical CO2 reduction: a classification problem.ChemPhysChem. 2017; 18: 3266-3273Crossref PubMed Scopus (329) Google Scholar Record activities are reported for Au and Ag, owing to both their outstanding intrinsic catalytic performance and amenability to nanostructuration,13Dufek E.J. Lister T.E. Stone S.G. McIlwain M.E. Operation of a pressurized system for continuous reduction of CO2.J. Electrochem. Soc. 2012; 159: F514-F517Crossref Scopus (112) Google Scholar,17Ma S. Luo R. Gold J.I. Yu A.Z. Kim B. Kenis P.J.A. Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide.J. Mater. Chem. A. 2016; 4: 8573-8578Crossref Google Scholar, 18Verma S. Hamasaki Y. Kim C. Huang W. Lu S. Jhong H.-R.M. Gewirth A.A. Fujigaya T. Nakashima N. Kenis P.J.A. Insights into the low overpotential electroreduction of CO2 to CO on a supported gold catalyst in an alkaline flow electrolyzer.ACS Energy Lett. 2018; 3: 193-198Crossref Scopus (288) Google Scholar, 19Dutta A. Morstein C.E. Rahaman M. Cedeño López A. Broekmann P. Beyond copper in CO2 electrolysis: effective hydrocarbon production on silver-nanofoam catalysts.ACS Catal. 2018; 8: 8357-8368Crossref Scopus (96) Google Scholar which provides high electrochemically active surface areas (ECSAs). As such, these noble metals can satisfy the operational specifications for industrial application, but their implementation is hampered by their high and fluctuating prices (currently around $500/kgAg and $45,000/kgAu)20Gold Pricehttps://goldprice.org/Date: 2019Google Scholar and their limited availability, calling for the development of catalysts with low noble-metal content. Zn is the only non-noble metal in the CO-generating class; however, compared to Au and Ag, few Zn-based catalysts have been reported, and difficulties encountered in their nanostructuration have led to lower current densities.21Nguyen D.L.T. Jee M.S. Won D.H. Jung H. Oh H.-S. Min B.K. Hwang Y.J. Selective CO2 reduction on zinc electrocatalyst: the effect of zinc oxidation state induced by pretreatment environment.ACS Sustain. Chem. Eng. 2017; 5: 11377-11386Crossref Scopus (99) Google Scholar, 22Quan F. Zhong D. Song H. Jia F. Zhang L. A highly efficient zinc catalyst for selective electroreduction of carbon dioxide in aqueous NaCl solution.J. Mater. Chem. A. 2015; 3: 16409-16413Crossref Scopus (101) Google Scholar, 23Rosen J. Hutchings G.S. Lu Q. Forest R.V. Moore A. Jiao F. Electrodeposited Zn dendrites with enhanced CO selectivity for electrocatalytic CO2 reduction.ACS Catal. 2015; 5: 4586-4591Crossref Scopus (300) Google Scholar, 24Won da H. Shin H. Koh J. Chung J. Lee H.S. Kim H. Woo S.I. Highly efficient, selective, and stable CO2 electroreduction on a hexagonal Zn catalyst.Angew. Chem. Int. Ed. 2016; 55: 9297-9300Crossref PubMed Scopus (248) Google Scholar, 25Kuhl K.P. Hatsukade T. Cave E.R. Abram D.N. Kibsgaard J. Jaramillo T.F. Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces.J. Am. Chem. Soc. 2014; 136: 14107-14113Crossref PubMed Scopus (1021) Google Scholar Previous work has shown that addition of Cu2+ during the electrodeposition of Zn can greatly increase surface nanostructuration, but this sacrifices the CO selectivity of the cathode.26Lamaison S. Wakerley D. Montero D. Rousse G. Taverna D. Giaume D. Mercier D. Blanchard J. Tran H.N. Fontecave M. et al.Zn–Cu alloy nanofoams as efficient catalysts for the reduction of CO2 to syngas mixtures with a potential-independent H2/CO ratio.ChemSusChem. 2019; 12: 511-517Crossref PubMed Scopus (34) Google Scholar Herein, the analogous use of Ag+ is exploited to promote the growth of ultra-high-surface-area electrodes without introducing the H2-evolution sites intrinsic to Cu. The as-generated Ag-alloyed Zn electrodes proved to be CO selective and highly active, reaching FECO as high as 91% and a CO partial current density (jCO) of −21 mA.cm−2, which is the maximum current achievable based on the mass transport of aqueous CO2 under atmospheric conditions. The electrodes maintained an average selectivity greater than 90% over 40 h and 85% over 100 h of operation with little change in their mesostructure. Further electrolysis at moderately elevated pressures addressed the low concentration of dissolved CO2 that limits the jCO at atmospheric pressure, allowing current densities as high as –286 mA.cm−2 to be reached, outperforming most previously reported noble and non-noble metal catalysts.1De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (833) Google Scholar Superior activity (–350 mA.cm−2 with FECO of 92% to 101%) has only been reported on purely Ag-based catalysts operating at higher pressure and temperature (18 bar CO2 at 60°C)13Dufek E.J. Lister T.E. Stone S.G. McIlwain M.E. Operation of a pressurized system for continuous reduction of CO2.J. Electrochem. Soc. 2012; 159: F514-F517Crossref Scopus (112) Google Scholar or in alkaline media.17Ma S. Luo R. Gold J.I. Yu A.Z. Kim B. Kenis P.J.A. Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide.J. Mater. Chem. A. 2016; 4: 8573-8578Crossref Google Scholar A co-electrodeposition strategy was adopted to prepare the Ag-Zn alloyed electrodes, which used 1 cm2 Zn plates immersed in 1.5 M H2SO4 solutions of metal ion precursors. The total metal salt concentration was set constant (0.2 M), apportioned between X% of Ag+ and (100−X)% of Zn2+ with X% varying between 0% and 10%. The highly acidic conditions were chosen in order to instigate H2 evolution during electrodeposition, which is known to provide architectures with mesoporous channels from the escape of H2 bubbles.27Moreno-García P. Schlegel N. Zanetti A. Cedeño López A. Gálvez-Vázquez M.J. Dutta A. Rahaman M. Broekmann P. Selective electrochemical reduction of CO2 to CO on Zn-based foams produced by Cu2+ and template-assisted electrodeposition.ACS Appl. Mater. Interfaces. 2018; 10: 31355-31365Crossref PubMed Scopus (45) Google Scholar To control the growth of the Ag-Zn structures, galvanic exchange, i.e., the bias-free electron exchange between metallic Zn (E°Zn2+/Zn = ‒0.76 V versus NHE) and Ag+ (E°Ag+/Ag = +0.80 V versus NHE),28Haynes W.M. CRC Handbook of Chemistry and Physics.Ninety-Third Edition. Taylor & Francis, 2012Google Scholar must be avoided during deposition. This could be achieved through the use of sufficiently large cathodic current densities, to ensure deposition was purely governed by diffusion of the metal ions to the electrode. This was demonstrated experimentally by showing how the electrodeposition current density affects the relation between atomic percentage of incorporated Ag in the electrode (henceforth referred to as %Ag, determined by inductively coupled plasma-atomic emission spectroscopy, ICP-AES) and the molar percentage of precursor Ag+ in the deposition solution (henceforth referred to as %[Ag+]). Two deposition current densities were tested, −0.5 A.cm−2 and −4 A.cm−2, but linear dependence between %Ag and %[Ag+] was only achieved at −4 A.cm−2, as displayed in Figure 1A. At −0.5 A.cm−2, Ag incorporation increased more rapidly at higher %[Ag+], indicating that both electrodeposition and galvanic exchange governed the electrode composition, thereby hampering control over the Ag:Zn ratio within the material. At −4 A.cm−2, the incorporated %Ag was always twice as high as its precursor %[Ag+] (Table 1), reflecting the difference in diffusion coefficients between Ag+ and Zn2+ (1.50 cm2.s−1 versus 0.69 cm2.s−1 in water, respectively).29Kariuki S. Dewald H.D. Evaluation of diffusion coefficients of metallic ions in aqueous solutions.Electroanalysis. 1996; 8: 307-313Crossref Scopus (62) Google ScholarTable 1Complete Characterization of the Ag-Alloyed Zn Electrodes with Increasing %AgaDeposition carried out at −4 A.cm−2 for 30 s.Precursor %[Ag+] (%)aDeposition carried out at −4 A.cm−2 for 30 s.Incorporated %Ag (%)bDetermined by inductively coupled plasma-atomic emission spectroscopy.BET-Specific Surface Area (m2.g–1)cSpecific surface area determined by Kr-adsorption measurements and BET analysis.Deposited Mass (mg.cm–2)dDetermined by weighing the electrode before and after deposition.BET-Derived RF (cm2phys.cm–2geo)eBET-derived roughness factor (RF) corresponds to the physical (“phys”) surface area deposited per cm−2 of flat (“geo”) electrode. It is calculated by multiplying the BET-specific surface area by the mass of deposited electrode on the 1 cm2 flat Zn support.ECSA-Derived RF (cm2echem.cm–2geo)fECSA-derived RF corresponds to the electrochemically active (“echem”) surface area available per cm−2 of flat (“geo”) electrode.Thickness (μm)gThickness determined using 45°-tilted SEM of the electrode cross section.0.51.02 ± 0.02N/A10.4N/A172211.94 ± 0.051.5711.2176387735.64 ± 0.036.13137978512859.43 ± 0.149.0713.111881221261020.1 ± 0.2822.713.83133349203a Deposition carried out at −4 A.cm−2 for 30 s.b Determined by inductively coupled plasma-atomic emission spectroscopy.c Specific surface area determined by Kr-adsorption measurements and BET analysis.d Determined by weighing the electrode before and after deposition.e BET-derived roughness factor (RF) corresponds to the physical (“phys”) surface area deposited per cm−2 of flat (“geo”) electrode. It is calculated by multiplying the BET-specific surface area by the mass of deposited electrode on the 1 cm2 flat Zn support.f ECSA-derived RF corresponds to the electrochemically active (“echem”) surface area available per cm−2 of flat (“geo”) electrode.g Thickness determined using 45°-tilted SEM of the electrode cross section. Open table in a new tab These conditions were used to generate a range of Ag-alloyed Zn electrodes fabricated by varying the precursor %[Ag+]. For convenience, the so-generated Ag-Zn electrodes will be referred to hereafter as Y%-Ag-alloyed Zn electrodes, where Y% is the incorporated atomic %Ag (corrected to one decimal place) determined by ICP-AES and taken equal to 1.0%, 1.9%, 5.6%, 9.4%, or 20.1% (Table 1). Scanning electron microscopy (SEM) revealed that even at the lowest %Ag (1.0%), high-surface-area microporous dendritic structures were attained, offering greatly improved structuration over the stacked configuration of pure Zn (Figures 1B and S1–S6). As the %Ag was increased, both the density of the dendritic structure and electrode thickness increased (Table 1; Figure S7), leading to high physical surface areas and associated roughness factors (RFs; further referred as BET-derived RF), as established by Kr-adsorption measurements and subsequent BET analysis (Table 1). BET-derived RF ranged between 176 and 3133 cm2phys.cm−2geo, from the 1.0%- to the 20.1%-Ag-alloyed Zn electrode: a substantial increase over previously reported Zn-based electrode surface areas.23Rosen J. Hutchings G.S. Lu Q. Forest R.V. Moore A. Jiao F. Electrodeposited Zn dendrites with enhanced CO selectivity for electrocatalytic CO2 reduction.ACS Catal. 2015; 5: 4586-4591Crossref Scopus (300) Google Scholar,26Lamaison S. Wakerley D. Montero D. Rousse G. Taverna D. Giaume D. Mercier D. Blanchard J. Tran H.N. Fontecave M. et al.Zn–Cu alloy nanofoams as efficient catalysts for the reduction of CO2 to syngas mixtures with a potential-independent H2/CO ratio.ChemSusChem. 2019; 12: 511-517Crossref PubMed Scopus (34) Google Scholar This enhancement can be assigned to the high homogeneity of the surface compared with past systems as a result of the Ag sites, which trigger growth of the dendrites. As previously shown, the presence of homogeneously mixed phases, rather than regions of pure metal crystals, is key to novel catalytic activity.30Lee S. Park G. Lee J. Importance of Ag–Cu biphasic boundaries for selective electrochemical reduction of CO2 to ethanol.ACS Catal. 2017; 7: 8594-8604Crossref Scopus (218) Google Scholar,31Hansen H.A. Shi C. Lausche A.C. Peterson A.A. Nørskov J.K. Bifunctional alloys for the electroreduction of CO2 and CO.Phys. Chem. Chem. Phys. 2016; 18: 9194-9201Crossref PubMed Google Scholar The alloyed nature of the Ag-alloyed Zn electrodes was proven by combining the spatial resolution of scanning transmission electron microscopy (STEM) with elemental analysis from X-ray energy-dispersive spectroscopy (XEDS) elemental mapping. The data show a homogeneous distribution of Ag and Zn within the structures at the nanoscale (Figures 1C and S8). Powder X-ray diffraction (PXRD) on the powder recovered from the electrodes revealed the presence of two sets of peaks that can be indexed in the hexagonal P63/mmc space group (Figure 1D). The first set of peaks (marked by pink regions) can be indexed with lattice parameters a = 2.67 Ǻ and c = 4.92 Ǻ and corresponds to pure Zn. The intensity of the second set of peaks (blue regions) increases at the detriment of the pure Zn peaks when the incorporated %Ag increases and can be indexed with a = 2.82 Ǻ and c = 4.39 Ǻ, corresponding to a Ag0.13Zn0.87 phase.32Valdez S. Pérez R. Rodriguez-Diaz R.A. Angeles-Chávez C. Casolco S.R. Relationship between silver concentration with microstructural and mechanical properties of rolled AlZn alloy.Mater. Sci. Eng. A. 2010; 527: 3085-3090Crossref Scopus (10) Google Scholar,33Andrews K.W. Davies H.E. Hume-Rothery W. Oswin C.R. The equilibrium diagram of the system silver-zinc.Proc. R. Soc. Lond. A. 1941; 177: 149-167Crossref Google Scholar For a %Ag of 20.1%, only Ag0.13Zn0.87 peaks are observed in the PXRD pattern. Assuming that peak broadening in the spectra originates purely from size effects, the crystallite sizes were estimated to range between 30 and 50 nm. The surface and near-surface compositions (up to a depth of 526 nm, see Supplemental Information for further detail) of each electrode were investigated by X-ray photoelectron spectroscopy (XPS) and 15 kV SEM-XEDS, respectively (Figures 1E and S9–S15). Both experiments revealed the presence of Ag and Zn, even at the lowest %Ag (1.0%). In some cases, low amounts of S were also detected on the as-prepared electrodes that are attributed to residual metal salts from electrodeposition, e.g., namuwite-like zinc-sulfate species.26Lamaison S. Wakerley D. Montero D. Rousse G. Taverna D. Giaume D. Mercier D. Blanchard J. Tran H.N. Fontecave M. et al.Zn–Cu alloy nanofoams as efficient catalysts for the reduction of CO2 to syngas mixtures with a potential-independent H2/CO ratio.ChemSusChem. 2019; 12: 511-517Crossref PubMed Scopus (34) Google Scholar Equivalent measurements after cathodic electrolysis proved the stability of the Ag-Zn alloyed structures and confirmed that the S-based species were removed, as verified by the disappearance of their XPS and XEDS signals (Figures S9 and S12–S15). Electrochemical studies were undertaken in a two-compartment H-cell separated by a bipolar membrane using 0.1 M CsHCO3 as an electrolyte. Cs+ cations were chosen based on their ability to impede the H2 evolution reaction.34Singh M.R. Kwon Y. Lum Y. Ager J.W. Bell A.T. Hydrolysis of electrolyte cations enhances the electrochemical reduction of CO2 over Ag and Cu.J. Am. Chem. Soc. 2016; 138: 13006-13012Crossref PubMed Scopus (460) Google Scholar The cathodic compartment was saturated with CO2 beforehand, and CO2 was continuously flowed at 20 mL.min−1 throughout the electrolysis. Products were analyzed by online gas chromatography (GC) and 1H-NMR after each controlled potential electrolysis (CPE). The potential-dependent activity of the 1.9%-Ag-alloyed Zn electrode was first investigated (Figure 2A). Product analysis during CPE showed remarkable selectivity for CO evolution, particularly between −0.9 V and −1.1 V versus reversible hydrogen electrode (RHE), where FECO reached values >90% and parasitic side reactions were suppressed (FEH2<7% and FEHCOOH<2.5%). Lower overpotentials could not be explored as Zn oxidation and subsequent dissolution takes place at around −0.5 V versus RHE. The electrode morphology and composition proved to be stable throughout the typical 3-h electrolysis experiments, as revealed by SEM- and STEM-XEDS (Figures S12 and S16). The electrode activity proved remarkably steady over extended periods, as an average FECO above 90% could be attained for 40 h of continuous operation at a controlled current density of −10 mA.cm−2 (Figure 2B). A slight decrease in selectivity was seen between 40 h and 100 h, resulting in an average FECO of 85%, with a FEHCOOH of 5.3% and FEH2 <5% over the 100 h of operation. Post-electrolysis characterization of this electrode via SEM and XEDS elemental mapping revealed that the mesostructure remained mostly intact; however, sporadic hexagonal pillars were visible at the topmost layer of the surface (Figure S17). These pillars exhibited a higher Zn content than expected, suggesting a dealloying process and Zn redeposition; means to inhibit this Zn redeposition are currently under investigation. Further electrochemical analyses were performed to establish the influence of the incorporated Ag content on the corresponding Ag-alloyed Zn electrodes. Analysis of the product distribution showed that all electrodes generated CO as the major product (Figure 2C) and that the required overpotential to reach optimal FECO decreased with the %Ag: 1.0%- and 1.9%-Ag-alloyed Zn electrodes showed maximum FECO of 93% and 91%, respectively, at −1.0 V versus RHE; 5.6%- and 9.4%-Ag-alloyed Zn electrodes attained highest FECO of 90% and 97%, respectively, at −0.9 V versus RHE; and the 20.1%-Ag-alloyed Zn electrode reached maximum FECO of 85% at −0.8 V versus RHE. Increasing the Ag content also altered catalytic selectivity for side products, as H2 evolution was slightly increased, while formate selectivity decreased (Figure 2C). In order to best compare these data to past literature, electrocatalysis was repeated in the more commonly used 0.1 M KHCO3 on a model 9.4%-Ag-alloyed Zn electrode. As displayed in Figure S18, switching from Cs+ to K+ showed no discernible effect on either activity or Faradaic efficiency. In 0.1 M KHCO3 electrolyte, a partial current density for CO of −16.8 mA.cm−2 at −1.0 V versus RHE was achieved, which outperforms previously reported Zn-based catalysts operating below the mass-transport limiting current of CO2 reduction (see System Optimization toward Enhanced CO Evolution for further discussion).21Nguyen D.L.T. Jee M.S. Won D.H. Jung H. Oh H.-S. Min B.K. Hwang Y.J. Selective CO2 reduction on zinc electrocatalyst: the effect of zinc oxidation state induced by pretreatment environment.ACS Sustain. Chem. Eng. 2017; 5: 11377-11386Crossref Scopus (99) Google Scholar This indicates that small quantities of Ag offer a route to further boost the CO2 reduction characteristics of Zn-based materials. Figure 2D shows that the catalytic current density (jtotal) increases with %Ag, which is expected based on the enhancement of available physical surface area of the electrodes with %Ag (Table 1). At low overpotentials, the corresponding partial current densities for CO formation (jCO, dashed lines in Figure 2D) comprised most of jtotal and therefore followed a similar trend. However, at high jtotal (>−20 mA.cm−2), discordance between jtotal and jCO arose as jCO plateaued at ∼−21 mA.cm−2 (further referred to as “jCO-1 bar”), while jtotal continued to increase. This plateauing effect is particularly noticeable for Ag-Zn electrodes with the largest surface areas (namely the 9.4%- and 20.1%-Ag-alloyed Zn electrodes) because high currents were attained at lower overpotentials. Upon reaching this jCO-1 bar plateau, FECO decayed in favor of FEH2, as most clearly exemplified by the 20.1%-Ag-alloyed Zn electrode (Figures 2C and 2D). Rather than an intrinsic limitation of the electrode, this is assigned to a CO2-mass-transport limitation in aqueous solution owing to its low solubility at atmospheric pressure. This hypothesis is supported by studies observing a similar peak partial current density for CO2 reduction.35Burdyny T. Graham P.J. Pang Y. Dinh C.-T. Liu M. Sargent E.H. Sinton D. Nanomorphology-enhanced gas-evolution intensifies CO2 reduction electrochemistry.ACS Sustain. Chem. Eng. 2017; 5: 4031-4040Crossref Scopus (99) Google Scholar, 36Hori Y. Ito H. Okano K. Nagasu K. Sato S. Silver-coated ion exchange membrane electrode applied to electrochemical reduction of carbon dioxide.Electrochim. Acta. 2003; 48: 2651-2657Crossref Scopus (91) Google Scholar, 37Suter S. Haussener S. Optimizing mesostructured silver catalysts for selective carbon dioxide conversion into fuels.Energy Environ. Sci. 2019; 12: 1668-1678Crossref Google Scholar In addition to the effect that higher surface areas have on selectivity,38Wang L. Nitopi S. Wong A.B. Snider J.L. Nielander A.C. Morales-Guio C.G. Orazov M. Higgins D.C. Hahn C. Jaramillo T.F. Electrochemically converting carbon monoxide to liquid fuels by directing selectivity with electrode surface area.Nat. Catal. 2019; 2: 702-708Crossref Scopus (104) Google Scholar the observed increase in CO2 reduction activity with increasing Ag content may also be assigned to the larger number of Ag sites at the surface, which function at lower overpotentials than Zn sites. In order to unravel the contribution of Ag in electrode nanostructuration versus its contribution to CO2 reduction kinetics, comparison of two electrodes with different %Ag but comparable surface area (BET-derived RF) was carried out. This was achieved through exploitation of the linear increase in BET-derived RF with electrodeposition time, as discussed in detail in System Optimization toward Enhanced CO Evolution. An extended electrodeposition time was used to grow a 1.9%-Ag-alloyed Zn electrode (deposition duration of 90 s) with a BET-derived RF of 810 cm2phys.cm−2geo, nearly identical to the 797 cm2phys.cm−2geo of the 5.6%-Ag-alloyed Zn electrode described previously. Although the electrodes had a similar physical surface area, their activity and selectivity significantly differed. As displayed in Figure S19, the 1.9%-Ag-alloyed Zn electrode showed catalytic current densities 1.5–2 times lower than the 5.6%-Ag-alloyed Zn electrode at each given potential, suggesting an enhancement in activity owing to the presence of Ag sites. Furthermore, the selectivity for formate was 4 times lower in the 5.6%-Ag-alloyed Zn electrode. This agrees with theoretical analysis that predicts that the stronger Ag–H* bond (compared with Zn–H*) would reduce formate production and agrees with previous experimental evidence.16Bagger A. Ju W. Varela A.S. Strasser P. Rossmeisl J. Electrochemical CO2 reduction: a classification problem.ChemPhysChem. 2017; 18: 3266-3273Crossref PubMed Scopus (329) Google Scholar,39Hori Y. Electrochemical CO2 reduction on metal electrodes.in: Vayenas C.G. White R.E. Gamboa-Aldeco M.E. Modern Aspects of Electrochemistry. Springer, 2008: 89-189Crossref Google Scholar Efforts to understand the interplay between the catalyst morphology, the electrolysis conditions, and the subsequent CO2 electroreduction activity were then undertaken to guide engineering of more active systems. The electrode thickness was first probed as a route to higher current densities. The 1.9%-Ag-alloyed Zn electrode was used as a model, which was prepared with thicknesses between 43 μm and 268 μm and near-identical BET-specific surface areas (Table S1; Figure S20). This was achieved by varying the electrode deposition time from 15 to 90 s in identical electrodeposition conditions, resulting in a linear increase of electrode thickness (Figure S20). The physical surface area also showed a linear correlation with the deposition time, as the mass of electrodeposited material increased linearly while the BET-specific surface area remained constant. The ECSA and associated roughness factor (further referred to as ECSA-derived RF) of the aforementioned electrodes increased between thicknesses of 43 to 150 μm but did not increase further using a 268-μm-thick electrode. This correlated with the catalytic activity, which increased up to an electrode thickness of 150 μm but plateaued for thicker electrodes (Figure S21). It can therefore be concluded that the CO2-saturated electrolyte does not penetrate beyond 150 μm into the electrode structure. The most restrictive parameter for CO2 mass transport is its aqueous solubility, which posed a significant barrier to the electrocatalytic performance of the Ag-alloyed Zn electrodes. This was overcome by performing CO2 reduction at increased CO2 pressures using the 9.4%-Ag-alloyed Zn electrode. This electrode was chosen as it exhibited a “jCO-1 bar” plateau at a low overpotential, allowing mass-transport effects to be probed at reasonable applied biases. The experiments were carried out in a one-compartment high-pressure reactor that contained a sacrificial graphite counter electrode in order to avoid the production of easily reduced O2. A small decrease in pH is expected as the pressure increased by an order of magnitude (c.a. 0.2–1 pH units),40Peng C. Crawshaw J.P. Maitland G.C. Martin Trusler J.P. Vega-Maza D. The pH of CO2-saturated water at temperatures between 308 K and 423 K at pressures up to 15 MPa.J. Supercrit. Fluids. 2013; 82: 129-137Crossref Scopus (111) Google Scholar,41Nitopi S. Bertheussen E. Scott S.B. Liu X. Engstfeld A.K. Horch S. Seger B. Stephens I.E.L. Chan K. Hahn C. et al.Progress and perspectives of electrochemical CO2 reduction on copper in aqueous electrolyte.Chem. Rev. 2019; 119: 7610-7672Crossref PubMed Scopus (1599) Google Scholar which is accounted for in the 10% error of the reported potential. Three CO2 pressures were tested (1, 3, and 6 bar) while passing a constant current density (jtotal) of −200 mA.cm−2. At 1 bar, the applied −200 mA.cm−2 of current was mostly expended on H2 evolution (Figure 2E, FEH2 of 69%) and required a highly cathodic potential of −3.3 ± 0.3 V versus RHE (all reported potentials in the high-pressure cell are ohmic-drop corrected based on a resistance of 21 Ω to correct for solution resistances at high current densities). As the amount of dissolved CO2 increased (with increasing CO2 pressures), jCO values far beyond the −21 mA.cm−2 plateau were achieved, and lower cathodic potentials were required: At 3 bar (E = –2.0 ± 0.2 V) and 6 bar (E = –1.2 ± 0.1 V versus RHE), jCO increased dramatically to −131 mA.cm−2 and −188 mA.cm−2, respectively, the latter corresponding to a FECO of 94%. The catalyst also showed considerable catalytic stability across the experiment and functioned at an average FECO of 82% for 11 h at −200 mA cm−2 at a pressure of 6 bar, corresponding to an overall conversion of 40% of the available CO2 (Figure S22B). Given that the high-pressure cell required the anode and cathode to operate in the same compartment, control experiments were used to confirm all CO was derived from CO2 reduction. 13CO2-labeling experiments at 3 bar in 0.1 M NaH13CO3 in an otherwise identical system produced purely 13CO (Figure S23), indicating that CO derived solely from CO2 reduction. Furthermore. analysis of the anodic graphite oxidation reaction in 0.1 M CsHCO3 under Ar with a Pt cathode at a current density of −200 mA.cm−2 showed only a small amount of CO2 and a trace of CO were produced (Table S2), alongside large amounts of H2 from the cathode. The anodic reaction was therefore predominantly oxidation of the graphite surface functionality to surface-immobilized oxides,42Weinberg N.L. Reddy T.B. Electrochemical oxidation of the surface of graphite fibres.J. Appl. Electrochem. 1973; 3: 73-75Crossref Scopus (62) Google Scholar which may produce some CO2 but very little CO. Future work will focus on the development of a high-pressure reactor with separated anodic and cathodic chambers allowing water oxidation to be employed as the anodic reaction. Additional experiments at higher current density and pressure (−400 mA.cm−2 at 9.5 bar) were tested to further demonstrate the outstanding intrinsic activity of the catalyst (Figure 2E). This allowed jCO as high as −286 mA.cm−2 (E = –2.3 V ± 0.2 V versus RHE, FECO of 72%) to be reached, setting a new record for a predominantly Zn-based electrocatalyst and outperforming most previously reported noble and non-noble metal catalysts, with the exception of those at higher pressures or at alkaline pH.1De Luna P. Hahn C. Higgins D. Jaffer S.A. Jaramillo T.F. Sargent E.H. What would it take for renewably powered electrosynthesis to displace petrochemical processes?.Science. 2019; 364: eaav3506Crossref PubMed Scopus (833) Google Scholar Through addition of small quantities of Ag+ to a solution of Zn2+, growth of ultra-high-surface-area (as high as 3133 cmphys2.cmgeo−2) porous dendritic electrodes was achieved. The combination of Ag and Zn produced catalytic environments highly biased toward CO evolution, showing Faradaic efficiencies over 91%, which functioned for 40 h without substantial loss in selectivity (FECO >90%)." @default.
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