Nanostructured Tin Catalysts for Selective Electrochemical Reduction of Carbon Dioxide to Formate

Efficient Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich Cu Surface

Up to date copper is the only electrocatalyst with relevant activity for the reduction of CO2 and CO to value added hydrocarbons and alcohols1. However, CO reduction studies over nanostructured copper catalysts, which are believed to have a high abundancy of active sites, were hindered by coppers instability in alkaline conditions. This instability makes Cu-based catalysts prone to dissolution during immersion into the electrolyte. Recently, we reported on an experimental methodology for immersing catalysts under potential control in reactors generally used for CO2 and CO reduction2. Compared to experiments without electrocatalyst immersion under potential control our method increases the CO reduction activity by four orders of magnitude, showing that small, mass-selected Cu nanoparticles are active catalysts for electrochemical CO reduction. This improvement in activity is attributed to the inhibition of Cu dissolution during immersion into the electrolyte as demonstrated by subsequent Cu stripping experiments.We now utilize the above described methodology to study the pulsed electrochemical CO reduction on 5.2 nm mass-selected Cu nanoparticles. Pulsed electrolysis has shown promise to improve CO(2) reduction activity and steer product selectivity by potential oscillations3–5. Nevertheless, detailed mechanistic understanding of the dynamic reactivity upon potential pulsing is still lacking. Using highly sensitive electrochemical mass-spectrometry we demonstrate a highly active transient activity over mass-selected Cu nanoparticles. By conducting pulsed electrolysis in different electrolytes we ascribe the high transient activity to an initial presence and local depletion of proton donors. Our results highlight the importance of proton donor nature and its local concentration to guide activity and selectivity. We believe that similar strategies can be of importance for the selective conversion of more complex biomass molecules and electrosynthesis.References(1) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.; Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F.; Chorkendorff, I. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chemical reviews 2019, 119 (12), 7610–7672. DOI: 10.1021/acs.chemrev.8b00705. Published Online: May. 22, 2019.(2) Hochfilzer, D.; Sørensen, J. E.; Clark, E. L.; Scott, S. B.; Chorkendorff, I.; Kibsgaard, J. The Importance of Potential Control for Accurate Studies of Electrochemical CO Reduction. ACS Energy Lett. 2021, 6 (5), 1879–1885. DOI: 10.1021/acsenergylett.1c00496.(3) Kimura, K. W.; Fritz, K. E.; Kim, J.; Suntivich, J.; Abruña, H. D.; Hanrath, T. Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential. ChemSusChem 2018, 11 (11), 1781–1786. DOI: 10.1002/cssc.201800318. Published Online: May. 22, 2018.(4) Bui, J. C.; Kim, C.; Weber, A. Z.; Bell, A. T. Dynamic Boundary Layer Simulation of Pulsed CO 2 Electrolysis on a Copper Catalyst. ACS Energy Lett. 2021, 1181–1188. DOI: 10.1021/acsenergylett.1c00364.(5) Arán-Ais, R. M.; Scholten, F.; Kunze, S.; Rizo, R.; Roldan Cuenya, B. The role of in situ generated morphological motifs and Cu(i) species in C2+ product selectivity during CO2 pulsed electroreduction. Nat Energy 2020, 5 (4), 317–325. DOI: 10.1038/s41560-020-0594-9.

Bubble Formation in the Electrolyte Triggers Voltage Instability in CO2 Electrolyzers.

Tuning the preferentially electrochemical growth of carbon at the “gaseous CO2-liquid molten salt-solid electrode” three-phase interline

Advantages of CO over CO2 as reactant for electrochemical reduction to ethylene, ethanol and n-propanol on gas diffusion electrodes at high current densities

Electrochemical reduction of CO 2 in methanol with aid of CuO and Cu 2O

The electrochemical reduction of carbon dioxide (CO2) to hydrocarbons and alcohols using copper catalysts is a promising way to produce valuable carbon fuels and chemical feedstocks. However, the selectivity for this process is still rather poor. Here, we present our recent efforts to develop Cu-based catalysts for the electroreduction of CO2 to target molecules such as ethylene, ethanol, and propanol. In the case of CO2 reduction to ethylene and ethanol, we found that their faradaic yields can be systematically tuned by changing the thickness of the deposited Cu2O overlayers. 1.7-3.6 μm thick films exhibited the best selectivity for these C2 compounds at -0.99 V vs. RHE, with faradaic efficiencies of 34-39% for ethylene and 9-16% for ethanol. Less than 1% methane was formed. A high C2H4/CH4 products’ ratio of up to ~100 could be achieved. Scanning electron microscopy, X-ray diffraction and in-situ Raman spectroscopy revealed that the Cu2O films reduced rapidly and remained as metallic Cu0 particles during the CO2 reduction. The selectivity trends exhibited by the catalysts during CO2 reduction in phosphate buffer and KHCO3 electrolytes suggest that an increase in local pH at the surface of the electrode is not the only factor in enhancing the formation of C2 products. An optimized surface population of edges and steps on the catalyst is also necessary to facilitate the dissociation of CO2 and the dimerization of the pertinent CHxO intermediates to ethylene and ethanol. We also demonstrate in this presentation the importance of defects in the formation of propanol from CO2 reduction. Reference: Ren D, Deng Y, Handoko AD, Chen CS, Malkhandi S and Yeo BS. Selective Electrochemical Reduction of Carbon Dioxide to Ethylene and Ethanol on Copper (I) Oxide Catalysts. ACS Catal. 2015, 5, 2814-2821

Solid oxide electrolysis cells (SOECs) have emerged as an attractive approach for electrochemical reduction of CO2 and/or H2O to high energy molecules, such as CO and/or H2, and demonstrated high faradic efficiency (good selectivity), high energy efficiency (low cell voltage), and high operating current density (high productivity). Conventionally SOECs based on YSZ as electrolyte material operate approximately 800oC in order to achieve a practical cell performance and to mitigate carbon deposition. However, high operating temperature leads to high cost and fast degradation of the SOEC system. Meanwhile, it becomes impossible to produce chemical fuels, such as methane, from the co-electrolysis of CO2 and H2O through in situ methanation.In this contribution, we studied electrochemical reduction of H2O and CO2 at reduced temperatures using anode supported SDC cells, with Ni-SDC anode and SSC cathode of effective area of 2 cm2, and the SDC electrolyte about 10-15 um in thickness. We found that the SDC cell performance (Cell#1) under electrochemical reduction of H2O reaches 0.5 A/cm2 at 1.15 V at 500oC with inlet gas composition of 50% H2O + 50% H2, at a flow rate of 100 ml/min/cm2. We repeated electrochemical reduction of H2O test using another SDC cell (Cell#2), and also conducted electrochemical reduction of CO2 at reduced temperatures. It was noticed that the cell performance under electrochemical reduction of H2O showed slightly better than, but quite similar to, that under CO2 reduction. The SDC cell (Cell#2) performance under electrochemical reduction of CO2 showed 0.5 A/cm2 at 1.17 V at 500oC with inlet gas composition of 80% CO2 + 20% H2, at a flow rate of 10 ml/min. This cell also showed no degradation during a short term (72hr) stability test under the CO2 reduction condition.Interestingly, under the tested conditions, SDC cells showed much better performance under the electrolysis cell (EC) mode than under the fuel cell (FC) mode. For example, at 500 oC, with 100mlpm H2-100mlpm H2O, the cell OCV is about 0.844V, the cell only reached 0.25A/cm2 at 0.4V under the FC mode, while under the EC mode it reached 1.26V at 1 A/cm2, which is approximately 4 times higher with the same polarization value of the cell voltage in comparison to the FC mode. The cause for the difference are investigated.

Coupling covariance matrix adaptation with continuum modeling for determination of kinetic parameters associated with electrochemical CO2 reduction

Electrochemical and photoelectrochemical CO2 reduction technologies offer the promise of zero-carbon-emission renewable fuels needed for heavy-duty transportation. However, the inert nature of the CO2 molecule poses a fundamental challenge that must be overcome before efficient (photo)electrochemical CO2 reduction at scale will be achieved. Optimal catalysts exhibit enduring stability, fast kinetics, high selectivity, and low manufacturing cost. Identifying catalytic mechanisms of CO2 reduction in (photo)electrochemical systems could accelerate design of efficient catalysts. In recent decades, numerous theoretical studies have contributed to our understanding of CO2 reduction pathways and identifying rate-limiting steps. Although a significant body of work exists regarding homogeneous electrocatalysis for CO2 reduction, this review focuses specifically on the theory of heterogeneous (photo)electrochemical reduction. We first give an overview of the relevant thermodynamics and semiconductor physics. We then introduce important, widely used theoretical techniques and modeling approaches to catalysis. Recent progress in elucidating mechanisms of heterogeneous (photo)electrochemical CO2 reduction is discussed through the lens of two experimental systems: pyridine (Py)-catalyzed CO2 (photo)electrochemical reduction at p-GaP photoelectrodes and electrochemical CO2 reduction at Cu electrodes. We close by proposing strategies and principles for the future design of (photo)electrochemical catalysts to improve the selectivity and reaction kinetics of CO2 reduction.

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

This chapter introduces copper (Cu) catalysts for the electrochemical reduction of carbon dioxide (CO2) in aqueous media. Cu is the only metallic electrode capable of electrochemically converting CO2 into hydrocarbons and alcohols with significant faradaic efficiencies. However, there are still challenges pertaining to reaction selectivity, efficiency and catalyst stability that need to be overcome before Cu can be applied to industrial-scale CO2 reduction. Previous experimental and theoretical works have suggested that tuning the binding energy of the key reaction intermediates by nanostructuring the Cu surface can play an important role in achieving this end. Therefore, this chapter focuses on the role of nanostructured Cu catalysts such as nanoparticles, oxide-derived Cu and Cu composites for the efficient and selective CO2 reduction to target products.

Advances in electrochemical reduction of carbon dioxide to formate over bismuth‐based catalysts

Photoctalytic reduction of CO2 and H2O to CO and H2 has merited increasing attention, because synthesis gas (a fuel gas mixture consisting primarily of CO and H2) can be converted to liquid hydrocarbon fuels by Fischer-Tropsch processes.[1-3] Because the CO2 reduction to CO competes with the proton reduction to H2 as well as CO2 reduction to HCOOH, selective CO production in the photocatalytic reduction of CO2in water has been a challenging issue. We report herein photoelectrochemical reduction of CO2 to CO with high Faradaic efficiency using a cobalt chlorin complex (CoII(Ch)) adsorbed on multi-walled carbon nanotubes (MWCNTs) as a cathode and a surface-modified BiVO4 photoanode with iron(III) oxide(hydroxide) (FeO(OH)) for oxidation of water in a CO2-saturated aqueous solution (pH 4.6). The photoelectrochemical reduction of CO2 was performed in a two-compartment cell composed of an FeO(OH)/BiVO4/FTO photoanode and a CoII(Ch)/MECNTs cathode, where the two electrodes were connected with conducting wire as an external circuit and separated by a Nafion membrane (Figure 1). Electrocatalytic reduction of CO2 occurred efficiently using the CoII(Ch)/MECNTs electrode at an applied potential of –1.1 V vs. NHE to yield CO with a Faradaic efficiency of 89% with hydrogen production accounting for the remaining 11% at pH 4.6.[4] CO was produced with 83% Faradaic efficiency at an applied potential of –1.3 V vs the potential of the photoanode under visible light irradiation in a CO2-saturated aqueous solution (pH 4.6). The amount of O2 produced in the photoelectrochemical oxidation of water is one-half of the amounts of the sum of CO and H2 produced in the electrochemical reduction of CO2 and H2O on the cathode. The difference in the oxidation potential of the FeO(OH)/BiVO4/FTO electrode under dark and that under light illumination was ca. 1.5 V, indicating that the FeO(OH)/BiVO4/FTO photoanode lowered the total bias that enabled simultaneous water oxidation and CO2 reduction. Faradaic efficiency for CO production was much improved by adsorption of CoII(Ch) on MWCNTs, because two [CoI(Ch)]– are located close to each other when the two-electron reduction of CO2to CO occurs.[4,5] Photocatalytic reduction of CO2 and H2O with triethylamine also occurred efficiently using CoII(Ch) adsorbed on MWCNTs as a CO2 reduction catalyst and [RuII(Me2phen)3]2+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline) as a photocatalyst to yield CO and H2with a ratio of 2.4:1 and the high turnover number of 710.[5] The present study provides a unique strategy for selective photoelectrocatalytic reduction of CO2 to CO over proton reduction to H2using an earth-abundant metal cathode.

The development of low-cost and stable catalysts is important for lowering the capital and operational cost of CO2 electro-reduction (ECR). Zinc (Zn) is an earth-abundant metal, with promising performance for the CO2-to-CO conversion.1 Zinc oxide (ZnO) has been recently employed for the CO2-to-CO conversion, recording promising selectivity (FECO) but short-term stability, in Flow-Cell configuration.2,3 ZnO phase has been proven critical for competent ECR performance, since both the oxidation state of Zn and the Zn/ZnO interface are proven critical for high FECO. 4,5In our work 6, we have synthesised various ZnO allotropes, the properties of which induced differences in their ECR performance. We have identified the ZnO nanorods (ZnO-NR) as the best performing catalyst. The latter was implemented in a zero-gap ECR electrolyser (MEA), recording partial current density for CO (jCO) of 160 mA cm-2 at cell voltage of 3.6 V. We have correlated the depletion of the ZnO phase in the MEA with the degradation of the performance (initially 15 h stability). We applied a periodic oxidation protocol in the MEA, causing the regeneration of ZnO-phase, allowing us to prolong the life-time of the catalyst. Through our strategy we were able to record 82% CO selectivity (FECO) for over 100 h, at -160 mA cm-2. This work provides an approach of practical use of inexpensive Zn-based catalysts for large-scale ECR applications.(1) Luo, W.; Zhang, J.; Li, M.; Züttel, A. Boosting CO Production in Electrocatalytic CO2 Reduction on Highly Porous Zn Catalysts. ACS Catal. 2019, 9 (5), 3783–3791. https://doi.org/10.1021/acscatal.8b05109.(2) Zeng, J.; Fontana, M.; Sacco, A.; Sassone, D.; Pirri, C. F. A Study of the Effect of Electrode Composition on the Electrochemical Reduction of CO2. Catalysis Today 2021. https://doi.org/10.1016/j.cattod.2021.07.014.(3) Zong, X.; Jin, Y.; Li, Y.; Zhang, X.; Zhang, S.; Xie, H.; Zhang, J.; Xiong, Y. Morphology-Controllable ZnO Catalysts Enriched with Oxygen-Vacancies for Boosting CO2 Electroreduction to CO. Journal of CO2 Utilization 2022, 61, 102051. https://doi.org/10.1016/j.jcou.2022.102051.(4) Nguyen, 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 Sustainable Chem. Eng. 2017, 5 (12), 11377–11386. https://doi.org/10.1021/acssuschemeng.7b02460.(5) Geng, Z.; Kong, X.; Chen, W.; Su, H.; Liu, Y.; Cai, F.; Wang, G.; Zeng, J. Oxygen Vacancies in ZnO Nanosheets Enhance CO2 Electrochemical Reduction to CO. Angewandte Chemie International Edition 2018, 57 (21), 6054–6059. https://doi.org/10.1002/anie.201711255.(6) Stamatelos, I.; Dinh, C.-T.; Lehnert, W.; Shviro, M. Zn-Based Catalysts for Selective and Stable Electrochemical CO2 Reduction at High Current Densities. ACS Appl. Energy Mater. 2022. https://doi.org/10.1021/acsaem.2c02557. Figure 1