Composition-structure-activity relations in Cu-Sn and Cu-S based electrocatalysts for CO2 conversion

The development of our society relying on utilization of raw materials from Earth has left unprecedented marks on our planet’s environment. A key issue is the climate change phenomenon caused by the continuous increase in the atmospheric concentration of the greenhouse gas CO2 due to combustion of fossil fuels as main energy source. The mitigation of the CO2 emissions via its capture and conversion, increase in the utilization of renewable energy and recycling technologies, and eliminating the dependence from fossil fuels is a strategy for building sustainable society. A promising concept for tackling the CO2 emission via its conversion into valuable products (hydrocarbons and alcohols etc.) is the electrochemical reduction of CO2 (CO2ER), that has many advantages over the other conversion concepts. Cu is unique in terms of material that can intrinsically catalyze CO2 reduction into hydrocarbons and alcohols. However, there are many Cu catalyst/experimental conditions/engineering - related challenges and other issues of various nature that affect the product selectivity and therefore still hinder the large-scale application of the CO2ER. Regarding the catalyst and experimental conditions challenges, possible alternative for overcoming the selectivity issues is step- wise CO2ER i.e., two-electron electrochemical reduction of CO2 into CO and subsequent conversion of CO into hydrocarbons, alcohols and other valuable products. Furthermore, another two-electron product, that is formic acid (HCOOH) or formate (HCOO–) that find various industrial applications and are also promising alternative as fuel in fuel cells, together with CO can be produced with high selectivity on various cheap and abundant electrocatalysts. Namely, the Cu rich Cu-Sn materials appear to be promising catalysts for CO2ER into CO, while Sn rich Cu-Sn and Cu-S for production of HCOO–, and therefore they are worth and inspiring to be more thoroughly studied in terms of their composition- structure relations with the catalytic activity for electrochemical conversion of CO2. Hence, the first main goal of this thesis is dedicated to study of the composition-structure-CO2ER activity relations in the Cu-Sn and Cu-S based electrocatalyst materials. On the other hand, the second main goal encompasses providing simple, cheap and fast synthesis methods for both Cu-Sn and Cu-S based materials, and moreover, including a successful proof-of-concept for recycling/repurposing waste for deriving CO selective Cu-Sn electrocatalyst, which are prerequisites toward possible application of these materials for large-scale conversion of CO2 and building a sustainable society based on recycling in order to mitigate and finally cease the extraction of natural resources. The thesis is divided into three studies, from which the first study represents determination of the composition and speciation of Cu and Sn in Cu-Sn electrocatalysts under CO2 electrolysis in order to reveal the relationship between these parameters and the CO2ER selectivity alteration between CO and HCOO– at various applied potentials. For the purpose of this study, SnO2 functionalized CuO nanowires with varying thickness of surface SnO2 layers (low and high Sn), were synthesized. The CO2ER product quantification was performed using chromatography, while the material characterization methods comprised of mainly spectroscopy-based techniques including ex-situ soft x-ray XAS, in-situ hard x-ray XAS and quasi in-situ XPS, supported by microscopy/electron diffraction (EF-TEM, HR-TEM and SAED) and computational modeling (DFT). The results show that thin layer of SnO2 (low Sn) functionalized CuO nanowires electrocatalysts that are selective for CO2ER into CO, reaching maximal FE of ~80% at –0.7 V, undergo surface transformation generating Cu0 and SnOx (Snd+) species under all examined potentials. The presence of Snd+ is supporting the Sn to Cu charge redistribution mechanism and therefore promoting desorption of the Cu bound *CO intermediate, leading to significantly higher CO evolution, compared to the activity of pristine Cu. On the other hand, the results show that the increase in the surface Sn content is beneficial for CO2ER into HCOO–, achieving the highest FE (80%) at –0.9 V for the catalyst with highest Sn content. Altering the potential toward more negative values is leading to increase in the surface fraction of metallic Sn specie that readily bind the *OCHO* intermediate following the HCOO– pathway, accompanied with significant suppression of the competitive hydrogen evolution reaction (HER) due to weak binding of the *H intermediate. Even though these Cu-Sn materials can reach very high selectivity for both CO and HCOO– in dependence of the surface Sn content, sophisticated, expensive and time-consuming approach, that includes atomic layer deposition (ALD) of SnO2, was used for their synthesis. An important requirement for future practical application of the CO2ER catalysts is definitely simple, cheap and fast synthesis. Therefore, in the scope of the second study, facile one-step electrochemical method was developed for deriving Cu-Sn foam with low Sn content from waste bronze. The bronze derived Cu-Sn foam reached 80% FE for CO at –0.8 V, competing with the best catalysts for this purpose, which makes it promising for future large-scale application. This study is showing that recycling/repurposing waste material for CO2ER catalyst synthesis is achievable, which is an important step towards sustainable supply of materials for this purpose. The third study is based on investigation of the composition-structure relations in Cu-S catalysts selective for CO2ER into HCOO–, and moreover presenting a facile method for synthesis of these materials based on direct reaction between elemental Cu and S dissolved in toluene, hence avoiding usage of expensive and extremely toxic precursors. The most important finding in this study, based on examination of the Cu-S catalysts with quasi in-situ XPS, reveals that under CO2 electrolysis the materials do not undergo complete reduction and Cu+ surface species persist at all examined potentials (–0.5 to –0.9 V), compared to pristine Cu which is completely reduced to metallic under identical conditions. The presence of residual surface sulfur species is most probably stabilizing the Cu+ with oxophilic nature on which the *OCHO* intermediate favorably binds and further converts into HCOO–. However, the HCOO– selectivity that can reach up to 70-75% is dependent on activation of the electrocatalyst that is related to the Cu:S surface composition and various electrode-electrolyte interface effects. Namely, besides the S2–, presence of unexpected SO42– specie is found on the surface of the electrocatalysts that are subjected to applied potential of –0.9 V, most probably due to local pH increase effects. These local effects are not fully understood from this study which is inspiring for further research that involve probing the electrode-electrolyte interface with other surface sensitive methods under in- situ conditions such as Raman and infrared spectroscopy. Finally, the future challenges include an adaptation of the facile synthesis methods developed in this work to prepare gas-diffusion electrodes loaded with Cu-Sn and Cu-S catalysts. Examining their CO2ER activity in gas-diffusion electrolyzers is important to achieve high current densities and, hence, industrial relevant conversion rates that are required for future large-scale applications

Cost Effective Microscale Gas Generation Apparatus

A new method for microscale gas generation is described. The best way of performing this method is as a hands-on experiment. The microscale gas generation apparatus consists of a plastic test tube closed with half-cut pipette bulb and a syringe with a needle. In order to observe the reaction of the generated gas with some reagent, the pipette tubing can be introduced into a second test tube. Using this method, many gases such as carbon dioxide, chlorine, oxygen, hydrogen, ammonia, sulfur dioxide, hydrogen halogenides, hydrogen cyanide, some hydrides and acetylene can be generated. The proposed method for microscale gas generation is safe, simple, cheap and attractive. Keywords: gas generation, hands-on experiments, teaching chemical experiment, microscale experimentatio

Non–enzymatic Amperometric Sensor for H2O2 Based on MnCO3 Thin Film Electrodes

The present study describes development of a non–enzymatic amperometric sensor for detection of H2O2 based on MnCO3 thin film electrodes. The film was deposited on electroconductive FTO coated glass substrates using simple chemical bath deposition method. The phase composition of the thin film was confirmed by X-ray diffraction analysis. The electrochemical properties and the sensor sensitivity towards H2O2 were examined using cyclic voltammetry and chronoamperometry in 0.1 M phosphate buffer solution with pH = 7.5. It was revealed that the sensing mechanism is based on electrocatalytic oxidation of H2O2, involving Mn species as redox mediators. According to the results, the best sensor response towards H2O2 was found at E = +0.25 V, with detection limit and sensor sensitivity of 10.0 µM and 2.64 µA cm–2 mM–1 (for the range of 0.09–1.8 mM), respectively, associated with R2 = 0.999. This work is licensed under a Creative Commons Attribution 4.0 International License

Chemical Bath Coating and Characterisation of Electrochromic Manganese(II) Carbonate Thin Films

A chemical bath technique for manganese(II) carbonate thin films deposition on electroconductive FTO – layered glass substrates is described in this report. Homogeneous thin films were obtained from an aqueous solution containing H2NCONH2 and MnCl2. The deposition is performed at temperature of 98 °C. The chemistry background of the process is the hydrolysis of urea [1,2]. Thin films were studied using X-ray diffraction, Profilometry, Cyclic Voltammetry (CV) and UV/VIS spectrophotometry. A combination of electrochemical and optical measurements has revealed electrochromic behaviour. By means of X-ray diffraction measurements the structure, crystallinity and the chemical composition, corresponding to manganese(II) carbonate, have been determined. Thin films thicknesses were determined using Profilometry. Keywords: manganese(II) carbonate, thin films, electrochromic materials, chemical bath deposition method

Design of Nonenzymatic Amperometric Sensor for H2O2 Based on Electrodes Modified with Nanoscaled MnCO3 Thin Films

The present study is related to the development of nonenzymatic amperometric sensors for detection of hydrogen peroxide (H2O2). The designed sensors are based on manganese(II) carbonate thin film modified electrodes. The films are deposited on electroconductive fluorine doped SnO2-coated glass substrates using chemical bath deposition method. Thin film chemical composition and structural analysis are studied using XRPD and FTIR. The electrochemical properties and sensitivity towards H2O2 are examined using cyclic voltammetry and chronoamperometry. Thin films with three different thicknesses of 75 and 100 nm are used. The electrochemical experiments are carried out in a phosphate buffer solution with c(K2HPO4/KH2PO4) = 0.1 M and pH = 7.5 and wide concentration range of hydrogen peroxide from 0.1 to 25 mM is investigated. The best results are obtained under oxidation potential when using 75 nm MnCO3 thin film and concentrations of H2O2 from 0.1 up to 10 mM. The lowest detection limit was 90 μM and the sensitivity of the sensor was 2.00 μA·cm-2·mM-1. The calibration plot is associated with a linear regression line and coefficient of R2 = 0.99

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

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)

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

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