Unintended cation crossover influences CO2 reduction selectivity in Cu-based zero-gap electrolysers

Membrane electrode assemblies enable CO2 electrolysis at industrially relevant rates, yet their operational stability is often limited by formation of solid precipitates in the cathode pores, triggered by cation crossover from the anolyte due to imperfect ion exclusion by anion exchange membranes. Here we show that anolyte concentration affects the degree of cation movement through the membranes, and this substantially influences the behaviors of copper catalysts in catholyte-free CO2 electrolysers. Systematic variation of the anolyte (KOH or KHCO3) ionic strength produced a distinct switch in selectivity between either predominantly CO or C2+ products (mainly C2H4) which closely correlated with the quantity of alkali metal cation (K+) crossover, suggesting cations play a key role in C-C coupling reaction pathways even in cells without discrete liquid catholytes. Operando X-ray absorption and quasi in situ X-ray photoelectron spectroscopy revealed that the Cu surface speciation showed a strong dependence on the anolyte concentration, wherein dilute anolytes resulted in a mixture of Cu+ and Cu0 surface species, while concentrated anolytes led to exclusively Cu0 under similar testing conditions. These results show that even in catholyte-free cells, cation effects (including unintentional ones) significantly influence reaction pathways, important to consider in future development of catalysts and devices

Comparative Spectroscopic Study Revealing Why the CO2 Electroreduction Selectivity Switches from CO to HCOO– at Cu–Sn- and Cu–In-Based Catalysts

To address the challenge of selectivity toward single products in Cu-catalyzed electrochemical CO2 reduction, one strategy is to incorporate a second metal with the goal of tuning catalytic activity via synergy effects. In particular, catalysts based on Cu modified with post-transition metals (Sn or In) are known to reduce CO2 selectively to either CO or HCOO– depending on their composition. However, it remains unclear exactly which factors induce this switch in reaction pathways and whether these two related bimetal combinations follow similar general structure–activity trends. To investigate these questions systematically, Cu–In and Cu–Sn bimetallic catalysts were synthesized across a range of composition ratios and studied in detail. Compositional and morphological control was achieved via a simple electrochemical synthesis approach. A combination of operando and quasi-in situ spectroscopic techniques, including X-ray photoelectron, X-ray absorption, and Raman spectroscopy, was used to observe the dynamic behaviors of the catalysts’ surface structure, composition, speciation, and local environment during CO2 electrolysis. The two systems exhibited similar selectivity dependency on their surface composition. Cu-rich catalysts produce mainly CO, while Cu-poor catalysts were found to mainly produce HCOO–. Despite these similarities, the speciation of Sn and In at the surface differed from each other and was found to be strongly dependent on the applied potential and the catalyst composition. For Cu-rich compositions optimized for CO production (Cu85In15 and Cu85Sn15), indium was present predominantly in the reduced metallic form (In0), whereas tin mainly existed as an oxidized species (Sn2/4+). Meanwhile, for the HCOO–-selective compositions (Cu25In75 and Cu40Sn60), the indium exclusively exhibited In0 regardless of the applied potential, while the tin was reduced to metallic (Sn0) only at the most negative applied potential, which corresponds to the best HCOO– selectivity. Furthermore, while Cu40Sn60 enhances HCOO– selectivity by inhibiting H2 evolution, Cu25In75 improves the HCOO– selectivity at the expense of CO production. Due to these differences, we contend that identical mechanisms cannot be used to explain the behavior of these two bimetallic systems (Cu–In and Cu–Sn). Operando surface-enhanced Raman spectroscopy measurements provide direct evidence of the local alkalization and its impact on the dynamic transformation of oxidized Cu surface species (Cu2O/CuO) into a mixture of Cu(OH)2 and basic Cu carbonates [Cux(OH)y(CO3)y] rather than metallic Cu under CO2 electrolysis. This study provides unique insights into the origin of the switch in selectivity between CO and HCOO– pathways at Cu bimetallic catalysts and the nature of surface-active sites and key intermediates for both pathways

Cu-Sn Bimetallic CO2 Reduction Catalysts: Assembling the Puzzle of How Composition, Structure, Morphology and Speciation Affect Activity and Selectivity

In the field of electrochemical reduction of CO2 (CO2ER) Cu and oxide derived OD-Cu electrocatalysts have been widely studied due to their unique capability to produce high added value products, such as CO, hydrocarbons and alcohols, albeit with relatively low selectivity.1 Cu-M bimetallic catalysts are a promising approach to break scaling relations among key intermediates and modulate the CO2ER selectivity. In the past 5 years, several studies on the CO2ER activity of Cu-Sn bimetallic catalysts have demonstrated remarkably high selectivities towards CO2,3 or formate.4,5 In general, comparison of several studies employing various Cu-Sn stoichiometries shows that Sn-poor catalysts are typically selective towards CO production, while Sn-rich catalysts favor formate (HCOO⁻). However, the specific optimal compositions leading to high activity towards CO or formate vary significantly among reports. 6–8 Furthermore, the mechanistic origins of the selectivity differences among Cu-Sn catalysts remains a topic of debate. Trends in product selectivity have been ascribed to aspects including composition, lattice effects,7 charge redistribution among metals in alloy structures,9 oxidation states,4,8 and the resulting effects on adsorption strength of key intermediates (e.g. *COOH, *OCHO, *CO, *H) directing selectivity among H2, CO and HCOO⁻. A comparison of the relevant literature has allowed us to establish common trends in CO2ER activity of Cu-Sn of various morphologies, synthetic procedures and speciation (Oxide derived vs Alloy materials) and identify points of controversy and key open questions that might help unifying the understanding of the activation of CO2 on Cu-Sn bimetallics. At the center of the debate is the persistence of oxidized metal sites during CO2ER and the precise nature of the active site. A major challenge in this regard, is the complex dependence of catalyst structure and composition with applied electrochemical bias. In this context, we explore X-ray spectroscopies as powerful tools to investigate the chemical environment and oxidation state of metal sites Sn and Cu in bimetallic electrocatalysts. By correlating diverse X-ray spectroscopy methods (soft and hard X-ray absorption (XAS) techniques, as well as X-ray photoelectron spectroscopy (XPS)), complementary information can be obtained on the chemical environment of metal sites in an electrocatalyst bulk and surface. We report our study on the dependence of structure and composition on applied electrochemical potential in Sn-functionalized Cu catalysts, achieved by combining in situ hard XAS, ex situ soft-XAS and XPS toward building a more complete picture of this model catalyst system. References Nitopi, S. et al. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chem. Rev. 119, 7610–7672 (2019). Schreier, M. et al. Solar conversion of CO2 to CO using Earth-abundant electrocatalysts prepared by atomic layer modification of CuO. Nat. Energy 2, 17087 (2017). Sarfraz, S., Garcia-Esparza, A. T., Jedidi, A., Cavallo, L. & Takanabe, K. Cu-Sn Bimetallic Catalyst for Selective Aqueous Electroreduction of CO2 to CO. ACS Catal. 6, 2842–2851 (2016). Ye, K. et al. In Situ Reconstruction of a Hierarchical Sn-Cu/SnOx Core/Shell Catalyst for High-Performance CO2 Electroreduction. Angew. Chemie - Int. Ed. 59, 4814–4821 (2020). Hou, X. et al. 3D core-shell porous-structured Cu@Sn hybrid electrodes with unprecedented selective CO 2 -into-formate electroreduction achieving 100%. J. Mater. Chem. A 7, 3197–3205 (2019). Vasileff, A., Xu, C., Ge, L., Zheng, Y. & Qiao, S. Z. Bronze alloys with tin surface sites for selective electrochemical reduction of CO2. Chem. Commun. 54, 13965–13968 (2018). Morimoto, M. et al. Electrodeposited Cu-Sn Alloy for Electrochemical CO 2 Reduction to CO / HCOO −. Electrocatalysis 9, 323–332 (2018). Li, Q. et al. Tuning Sn-Catalysis for Electrochemical Reduction of CO2 to CO via the Core/Shell Cu/SnO2 Structure. J. Am. Chem. Soc. 139, 4290–4293 (2017). Vasileff, A. et al. Selectivity Control for Electrochemical CO2 Reduction by Charge Redistribution on the Surface of Copper Alloys. ACS Catal. 9, 9411–9417 (2019)

Determining Structure-Activity Relationships in Oxide Derived CuSn Catalysts During CO2 Electroreduction Using X-Ray Spectroscopy

The development of earth-abundant catalysts for selective electrochemical CO2 conversion is a central challenge. Cu-Sn bimetallic catalysts can yield selective CO2 reduction toward either CO or formate. This study presents oxide-derived Cu-Sn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO2 reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X-ray absorption spectroscopy for in situ study, and quasi in situ X-ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu0 under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO-selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate-selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn0 and Sn oxide species. Density functional theory simulations suggest that Snδ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn0 sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalysts

Poly(ionic liquid) nanovesicles via polymerization induced self-assembly and their stabilization of Cu nanoparticles for tailored CO2 electroreduction

Herein, we report a straightforward, scalable synthetic route towards poly(ionic liquid) (PIL) homopolymer nanovesicles (NVs) with a tunable particle size of 50 to 120 nm and a shell thickness of 15 to 60 nm via one-step free radical polymerization induced self-assembly. By increasing monomer concentration for polymerization, their nanoscopic morphology can evolve from hollow NVs to dense spheres, and finally to directional worms, in which a multilamellar packing of PIL chains occurred in all samples. The transformation mechanism of NVs’ internal morphology is studied in detail by coarse-grained simulations, revealing a correlation between the PIL chain length and the shell thickness of NVs. To explore their potential applications, PIL NVs with varied shell thickness are in situ functionalized with ultra-small (1 ∼ 3 nm in size) copper nanoparticles (CuNPs) and employed as electrocatalysts for CO2 electroreduction. The composite electrocatalysts exhibit a 2.5-fold enhancement in selectivity towards C1 products (e.g., CH4), compared to the pristine CuNPs. This enhancement is attributed to the strong electronic interactions between the CuNPs and the surface functionalities of PIL NVs. This study casts new aspects on using nanostructured PILs as new electrocatalyst supports in CO2 conversion to C1 products

Comparative Spectroscopic Study Revealing Why the CO2 Electroreduction Selectivity Switches from CO to HCOO– at Cu–Sn- and Cu–In-Based Catalysts

To address the challenge of selectivity toward single products in Cu- catalyzed electrochemical CO2 reduction, one strategy is to incorporate a second metal with the goal of tuning catalytic activity via synergy effects. In particular, catalysts based on Cu modified with post-transition metals (Sn or In) are known to reduce CO2 selectively to either CO or HCOO− depending on their composition. However, it remains unclear exactly which factors induce this switch in reaction pathways and whether these two related bimetal combinations follow similar general structure−activity trends. To investigate these questions systematically, Cu−In and Cu−Sn bimetallic catalysts were synthesized across a range of composition ratios and studied in detail. Compositional and morphological control was achieved via a simple electrochemical synthesis approach. A combination of operando and quasi-in situ spectroscopic techniques, including X-ray photoelectron, X-ray absorption, and Raman spectroscopy, was used to observe the dynamic behaviors of the catalysts’ surface structure, composition, speciation, and local environment during CO2 electrolysis. The two systems exhibited similar selectivity dependency on their surface composition. Cu-rich catalysts produce mainly CO, while Cu-poor catalysts were found to mainly produce HCOO−. Despite these similarities, the speciation of Sn and In at the surface differed from each other and was found to be strongly dependent on the applied potential and the catalyst composition. For Cu-rich compositions optimized for CO production (Cu85In15 and Cu85Sn15), indium was present predominantly in the reduced metallic form (In0), whereas tin mainly existed as an oxidized species (Sn2/4+). Meanwhile, for the HCOO−-selective compositions (Cu25In75 and Cu40Sn60), the indium exclusively exhibited In0 regardless of the applied potential, while the tin was reduced to metallic (Sn0) only at the most negative applied potential, which corresponds to the best HCOO− selectivity. Furthermore, while Cu40Sn60 enhances HCOO− selectivity by inhibiting H2 evolution, Cu25In75 improves the HCOO− selectivity at the expense of CO production. Due to these differences, we contend that identical mechanisms cannot be used to explain the behavior of these two bimetallic systems (Cu−In and Cu−Sn). Operando surface-enhanced Raman spectroscopy measurements provide direct evidence of the local alkalization and its impact on the dynamic transformation of oxidized Cu surface species (Cu2O/CuO) into a mixture of Cu(OH)2 and basic Cu carbonates [Cux(OH)y(CO3)y] rather than metallic Cu under CO2 electrolysis. This study provides unique insights into the origin of the switch in selectivity between CO and HCOO− pathways at Cu bimetallic catalysts and the nature of surface-active sites and key intermediates for both pathways. KEYWORDS: CO2 electroreduction, Cu nanostructures, in situ spectroscopy, bimetallic catalysts, electrodepositio

Facile Synthesis of Hierarchical CuS and CuCo2S4 Structures from an Ionic Liquid Precursor for Electrocatalysis Applications

Covellite-phase CuS and carrollite-phase CuCo2S4 nano- and microstructures were synthesized from tetrachloridometallate-based ionic liquid precursors using a novel, facile, and highly controllable hot-injection synthesis strategy. The synthesis parameters including reaction time and temperature were first optimized to produce CuS with a well-controlled and unique morphology, providing the best electrocatalytic activity toward the oxygen evolution reaction (OER). In an extension to this approach, the electrocatalytic activity was further improved by incorporating Co into the CuS synthesis method to yield CuCo2S4 microflowers. Both routes provide high microflower yields of >80 wt %. The CuCo2S4 microflowers exhibit a superior performance for the OER in alkaline medium compared to CuS. This is demonstrated by a lower onset potential (∼1.45 V vs RHE @10 mA/cm2), better durability, and higher turnover frequencies compared to bare CuS flowers or commercial Pt/C and IrO2 electrodes. Likely, this effect is associated with the presence of Co3+ sites on which a better adsorption of reactive species formed during the OER (e.g., OH, O, OOH, etc.) can be achieved, thus reducing the OER charge-transfer resistance, as indicated by X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy measurements

Poly(ionic liquid) nanovesicles via polymerization induced self-assembly and their stabilization of Cu nanoparticles for tailored CO2 electroreduction

Herein, we report a straightforward, scalable synthetic route towards poly(ionic liquid) (PIL) homopolymer nanovesicles (NVs) with a tunable particle size of 50 to 120 nm and a shell thickness of 15 to 60 nm via one-step free radical polymerization induced self-assembly. By increasing monomer concentration for polymerization, their nanoscopic morphology can evolve from hollow NVs to dense spheres, and finally to directional worms, in which a multilamellar packing of PIL chains occurred in all samples. The transformation mechanism of NVs’ internal morphology is studied in detail by coarse-grained simulations, revealing a correlation between the PIL chain length and the shell thickness of NVs. To explore their potential applications, PIL NVs with varied shell thickness are in situ functionalized with ultra-small (1 ∼ 3 nm in size) copper nanoparticles (CuNPs) and employed as electrocatalysts for CO2 electroreduction. The composite electrocatalysts exhibit a 2.5-fold enhancement in selectivity towards C1 products (e.g., CH4), compared to the pristine CuNPs. This enhancement is attributed to the strong electronic interactions between the CuNPs and the surface functionalities of PIL NVs. This study casts new aspects on using nanostructured PILs as new electrocatalyst supports in CO2 conversion to C1 products