Developing active and selective catalysts that convert CO2 into valuable products remains a critical challenge for further application of the electrochemical CO2 reduction reaction (CO2RR).... Show moreDeveloping active and selective catalysts that convert CO2 into valuable products remains a critical challenge for further application of the electrochemical CO2 reduction reaction (CO2RR). Catalytic tuning with organic additives/films has emerged as a promising strategy to tune CO2RR activity and selectivity. Herein, we report a facile method to significantly change CO2RR selectivity and activity of copper and gold electrodes. We found improved selectivity toward HCOOH at low overpotentials on both polycrystalline Cu and Au electrodes after chemical modification with a poly(4-vinylpyridine) (P4VP) layer. In situ attenuated total reflection surface-enhanced infrared reflection-adsorption spectroscopy and contact angle measurements indicate that the hydrophobic nature of the P4VP layer limits mass transport of HCO3- and H2O, whereas it has little influence on CO2 mass transport. Moreover, the early onset of HCOOH formation and the enhanced formation of HCOOH over CO suggest that P4VP modification promotes a surface hydride mechanism for HCOOH formation on both electrodes. Show less
This work describes several studies into the electroreduction of carbon dioxide (CO2RR), both regarding mechanistical aspects and catalyst stability considerations. Mechanistic insights into carbon... Show moreThis work describes several studies into the electroreduction of carbon dioxide (CO2RR), both regarding mechanistical aspects and catalyst stability considerations. Mechanistic insights into carbon-carbon bond formation on a silver catalyst are described in Ch 2, were we find an acetaldehyde-like surface adsorbate to be a key intermediate. Gold stability in alkaline environment under strongly reducing conditions and the effect of near-surface CO on said stability is reported on in Ch 3, showing catalyst degradation under reductive conditions which is furthermore exacerbated by CO adsorption. Chapter 4 details the voltammetric behavior of variously oriented copper surfaces when prepared via a non-standard methodology, showing good agreement with prior literature. The last chapter (Ch 5) contains investigations into the stability of copper under reductive conditions in near-neutral buffered electrolyte and in alkaline electrolyte, where it was observed that copper is morphologically stable under strongly alkaline conditions and neutral conditions in the presence of CO2 and buffer ions, so long as strictly reducing potentials are applied. Show less
Though copper is a capable electrocatalyst for the CO2 reduction reaction (CO2RR), it rapidly deactivates to produce mostly hydrogen. A current hypothesis as to why this occurs is that potential... Show moreThough copper is a capable electrocatalyst for the CO2 reduction reaction (CO2RR), it rapidly deactivates to produce mostly hydrogen. A current hypothesis as to why this occurs is that potential-induced morphological restructuring takes place, leading to a redistribution of the facets at the interface resulting in a shift in the catalytic activity to favor the hydrogen evolution reaction over CO2RR. Here, we investigate the veracity of this hypothesis by studying the changes in the voltammetry of various copper surfaces, specifically the three principal orientations and a polycrystalline surface, after being subjected to strongly cathodic conditions. The basal planes were chosen as model catalysts, while polycrystalline copper was included as a means of investigating the overall behavior of defect-rich facets with many low coordination steps and kink sites. We found that all surfaces exhibited (perhaps surprisingly) high stability when subjected to strongly cathodic potentials in a concentrated alkaline electrolyte (10 M NaOH). Proof for morphological stability under CO2RR-representative conditions (60 min at -0.75 V in 0.5 M KHCO3) was obtained from identical location scanning electron microscopy, where the mesoscopic morphology for a nanoparticle-covered copper surface was found unchanged to within the instrument accuracy. Observed changes in voltammetry under such conditions, we found, were not indicative of a redistribution of surface sites but of electrode fouling. Besides impurities, we show that (brief) exposure to oxygen or oxidizing conditions (i.e., 1 min) leads to copper exhibiting changing morphology upon cathodic treatment which, we posit, is ultimately the reason why many groups report the evolution of copper morphology during CO2RR: accidental oxidation/reduction cycles. Show less
refers toAlisson H.M. da Silva, Stefan J. Raaijman, Cássia S. Santana, José M. Assaf, Janaina F. Gomes, Marc T.M. KoperElectrocatalytic CO2 reduction to C2+ products on Cu and CuxZny...Show morerefers toAlisson H.M. da Silva, Stefan J. Raaijman, Cássia S. Santana, José M. Assaf, Janaina F. Gomes, Marc T.M. KoperElectrocatalytic CO2 reduction to C2+ products on Cu and CuxZny electrodes: Effects of chemical composition and surface morphologyJournal of Electroanalytical Chemistry, Volume 880, 1 January 2021, Pages 114750The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for producing multi carbon compounds using only CO2 and H2O at room temperature. Significant advances have already been achieved in understanding how some characteristics of copper electrodes, the current state-of-the-art catalyst for multi carbon formation via CO2RR, affect the product spectrum. Advances and insights have been reported for, among others, the effect of crystallographic orientation, active surface area, and composition of M copper (M = Au, Ag, Zn, etc.) materials, and how these alter the distribution of CO2RR products. However, a systematic study evaluating the significance of these variables in the CO2RR to C2+ products is still lacking in the literature and represents an important step in the development of new materials with optimized properties that can be more selective to C2+ compounds. In this paper, we have systematically investigated the effect of the roughness factor, chemical composition, and surface morphology of CuxZny electrocatalysts on the product distribution during CO2RR. Firstly, Cu, Cu90Zn10, and Cu75Zn25 electrodes were exposed to oxidation-reduction cycles to produce Cu and CuxZny electrodes with different morphologies, roughness factors, and chemical composition. Our results show that an increase in the roughness factor and Zn content lead to higher faradaic efficiency (FE) to C2+ products. Furthermore, the influence of the nanoscale morphology is imperative for the production of C2+ compounds. Specifically, nanocubes of Cu and CuxZny presented the highest FE to C2+ products among the different surface morphologies studied in this work (polished flat surface, nanosheres, nanocubes, nanodendrites, and nanocauliflowers), showing that C-C coupling during CO2RR is mainly shape dependent. Show less
Although copper is widely used as an electrocatalyst for the CO2 reduction reaction, often little emphasis is placed on identifying exactly the facet distribution of the copper surface. Furthermore... Show moreAlthough copper is widely used as an electrocatalyst for the CO2 reduction reaction, often little emphasis is placed on identifying exactly the facet distribution of the copper surface. Furthermore, because of differing surface preparation methodologies, reported characaterization voltammograms (where applicable) often vary significantly between laboratories, even for surfaces of supposedly the same orientation. In this work, we describe a surface preparation methodology involving the combination of induction annealing and well-documented electrochemical steps, by which reproducible voltammetry for copper surfaces of different orientations can be obtained. Specifically, we investigated copper surfaces of the three principal orientations: {111}, {100} and {110}, and a representative polycrystalline surface. We compared these surfaces to surfaces reported in the literature prepared via either electropolishing or UHV-standard methodologies, where we find induction preparation to yield improvements in surface quality with respect to electropolished surfaces, though not quite as good as those obtained by UHV-preparation. Show less
Reducing CO2 to long-chain carbon products is attractive considering such products are typically more valuable than shorter ones. However, the best electrocatalyst for making such products from CO2... Show moreReducing CO2 to long-chain carbon products is attractive considering such products are typically more valuable than shorter ones. However, the best electrocatalyst for making such products from CO2, copper, lacks selectivity. By studying alternate C2+ producing catalysts we can increase our mechanistic understanding, which is beneficial for improving catalyst performance. Therefore, we investigate CO reduction on silver, as density functional theory (DFT) results predict it to be good at forming ethanol. To address the current disagreement between DFT and experimental results (ethanol vs. no ethanol), we investigated CO reduction at higher surface coverage (by increasing pressure) to ascertain if desorption effects can explain the discrepancy. In terms of product trends, our results agree with the DFT-proposed acetaldehyde-like intermediate, yielding ethanol and propanol as C2+ products-making the CO2 electrochemistry of silver very similar to that of copper at sufficiently high coverage. Show less
The electroreduction of CO2 (CO2RR) is a promising strategy toward sustainable fuels. Cu is the only Earth-abundant and pure metal capable of catalyzing CO2-to-hydrocarbons conversion with... Show moreThe electroreduction of CO2 (CO2RR) is a promising strategy toward sustainable fuels. Cu is the only Earth-abundant and pure metal capable of catalyzing CO2-to-hydrocarbons conversion with significant Faradaic efficiencies; yet, its dynamic structure under operando CO2RR conditions remains unknown. Here, we track the Cu structure operando by electrochemical scanning tunneling microscopy and Raman spectroscopy. Surprisingly, polycrystalline Cu surfaces reconstruct forming Cu nanocuboids whose size can be controlled by the polarization potential and the time employed in their in situ synthesis, without the assistance of organic surfactants and/or halide anions. If the Cu surface is covered by a graphene monolayer, smaller features with enhanced catalytic activity for CO2RR can be prepared. The graphene-protecting layer softens the 3D morphological changes that Cu-based catalysts suffer when exposed to aggressive electrochemical environments and allows us to track the kinetic roughening process. This novel strategy is promising for improving Cu long-term stability, and consequently, it could be used as a platform to ultimately control product selectivity. Show less
The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for producing multi-carbon compounds using only CO2 and H2O at room temperature. Significant advances have already been... Show moreThe electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for producing multi-carbon compounds using only CO2 and H2O at room temperature. Significant advances have already been achieved in understanding how some characteristics of copper electrodes, the current state-of-the-art catalyst for multi-carbon formation via CO2RR, affect the product spectrum. Advances and insights have been reported for, among others, the effect of crystallographic orientation, active surface area, and composition of M-copper (M = Au, Ag, Zn, etc.) materials, and how these alter the distribution of CO2RR products. However, a systematic study evaluating the significance of these variables in the CO2RR to C2+ products is still lacking in the literature and represents an important step in the development of new materials with optimized properties that can be more selective to C2+ compounds. In this paper, we have systematically investigated the effect of the roughness factor, chemical composition, and surface morphology of CuxZny electrocatalysts on the product distribution during CO2RR. Firstly, Cu, Cu90Zn10, and Cu75Zn25 electrodes were exposed to oxidation-reduction cycles to produce Cu and CuxZny electrodes with different morphologies, roughness factors, and chemical composition. Our results show that an increase in the roughness factor and Zn content lead to higher faradaic efficiency (FE) to C2+ products. Furthermore, the influence of the nanoscale morphology is imperative for the production of C2+ compounds. Specifically, nanocubes of Cu and CuxZny presented the highest FE to C2+ products among the different surface morphologies studied in this work (polished flat surface, nanosheres, nanocubes, nanodendrites, and nanocauliflowers), showing that CC coupling during CO2RR is mainly shape dependent. Show less
