Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte.

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

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.

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

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.

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

CO2 conversion and utilization

Observation of a potential-dependent switch of water-oxidation mechanism on Co-oxide-based catalysts

The electrochemical and catalytic reduction of CO2 into valuable chemicals like methanol offers a promising strategy for CO2 utilization while also producing sustainable fuels and feedstocks. The electrochemical reduction of CO2 is the most promising technique for the future, as it has an impact on environmental issues and the advancement of renewable energy. This review summarizes recent advances in both heterogeneous and homogeneous catalytic hydrogenation and electrocatalytic conversion of CO2 to methanol. The fundamentals of both CO2tomethanol pathways are discussed, covering proposed mechanisms, catalyst materials (Cu, Pd alloys, nanostructured metals/metal oxides), reactor configurations, and process conditions. Key catalyst materials are analyzed, including copper, palladium alloys, nanostructured metals/metal oxides, and metalorganic frameworks tailored for selective 6-electron CO2 reduction. Critical performance factors and optimization strategies for these catalysts and reactor setups are analyzed in depth, including electrolyte choice, applied potential, cathode design, and flow cell configuration. Finally, technoeconomic assessments of integrated CO2-to-methanol processes provide vital insight into scale-up challenges and future research priorities across materials design, reactor engineering, and process intensification. This review offers a comprehensive reference on latest developments in catalytic and electrocatalytic CO2 conversion to methanol.

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

Ionic liquid-mediated electrochemical CO2 reduction in a microbial electrolysis cell

Electrochemical CO2 conversion is in the global spotlight as one of the most promising ‘clean’ approach for the conversion of atmospheric and marine CO2 into clean and renewable liquid fuels as well as offering a potential approach to the amelioration of a major greenhouse gas. In particular, carbon monoxide (CO) produced by electrochemical CO2 reduction is a principal industrial feedstock for the production of hydrocarbons and bulk oxygenated products. Consequently, the development of a highly energy efficient and selective heterogeneous catalyst for the conversion of CO2 to CO is of major interest in energy and environmental science research and industry. It is challenging to split CO2 due to the high activation energy required in the reduction process. In order to resolve this, a variety of catalysts have been investigated to reduce the activation energy including precious metals, non-precious metals, transition metal oxides, transition metal chalcogenides and metal complexes. In the presence of molecular catalysts, specifically, CO2 reduction typically proceeds by a proton-coupled electron transfer reaction, resulting in lower overpotentials due to the stabilization of the metal-carbon dioxide complex. Depending on the process under the CO2 atmosphere, a variety of gaseous or liquid products can be generated at various potentials. However, the competitive hydrogen evolution reaction (HER) can concomitantly occur at a similar potential to the electrochemical CO2 reduction reaction.Therefore, selectivity is a vital property in a practical electrocatalyst for this process. Metal complexes (e.g. metalloporphyrins and metallophthalocyanines) have been widely studied as homogeneous catalysts for electrochemical CO2 reduction due to their high selectivity, low cost, and ease of preparation on a large scale. However, the use of the metal complexes as homogeneous catalysts is not ideal for practical applications since the catalytic activities are influenced by a number of factors including (i) diffusion of the catalyst into the diffusion layer adjacent tothe electrode surface, limiting the number of active catalytic species, (ii) catalyst deactivation processes such as dimerization and aggregation and (iii) the use of volatile and flammable organic solvents like dimethylformamide and acetonitrile, which is not ideal.[1,2] In addition, unlike heterogeneous catalysts, it is not easy to separate or reuse the homogeneous catalysts following the reaction, one of the most important requirements in commercial use. For these reasons, there have been a number of studies in which the molecular complexes have been immobilised and used as heterogeneous catalysts for the electrochemical CO2 reduction.[3-5] Although the majority of metal complexes have demonstrated improved catalytic performance through immobilisation, most of the catalysts for the production of CO operate at high overpotentials and show poor catalytic durability of less than 5 h, resulting in a low energy efficiency for the electrochemical CO2 reduction catalysis. Therefore, a stable, efficient metal complex-based heterogeneous catalyst for the electrocatalytic reduction of CO2 at low overpotentials has yet to be realised. We have developed simple and facile self-assembly methods for the fabrication of heterogeneous composite electrocatalysts for CO2 reduction to CO using reduced graphene oxide with both metalloporphyrins and metallophthalocyanines by π-π stacking and electrostatic interactions.[6,7] The resulting electrocatalysts possess a number of advantages including (1) facile and rapid electron transfer, (2) easy electrolyte and CO2 accessibility, (3) structural robustness, (4) excellent faradaic efficiencies (>96%) at overpotentials as low as 280 mV, and (5) superior long-term stability (up to 50 h) under the electrochemical reduction conditions. In contrast to previous work, these catalysts exhibited excellent electrocatalytic CO2 reduction performance and stability. Costentin, C.; Robert, M.; Savéant, J.-M. Curr. Opin. Electrochem. 2017, 2, 26–31.Bullock, M.; Das, A.; Appel, A. Chem. Eur. J. 2017, 23, 7626–7641.Zhang, X.; Wu, Z.; Zhang, X.; Li, L.; Li, Y.; Xu, H.; Li, X.; Yu, X.; Zhang, Z.; Liang, Y.; Wang H. Nat. Commun. 2017, 8, 14675.Tatin, A.; Comminges, C.; Kokoh, B.; Costentin, C.; Robert, M.; Savéant, J.-M. Proc. Nat. Acad. Sci. 2016, 113, 5526–5529.Hu, X.; Rønne, M.; Pedersen, S.; Skrydstrup, T.; Daasbjerg, K. Angew. Chem. Int. Ed. 2017, 56, 6468–6472.Choi, J.; Wagner, P.; Jalili, R.; Kim, J.; MacFarlane, D. R.; Wallace, G. G.; Officer, D. L. Adv. Ener. Mater. 2018, 8, 1801280, doi.org/10.1002/aenm.201801280.Choi, J.; Kim, J.; Wagner, P.; Gambhir, S.; Jalili, R.; Byun, S.; Sayyar, S.; Lee, Y. M.; MacFarlane, D. R.; Wallace, G. G.; Officer, D. L. Ener. Environ. Sci., 2018, submitted for publication.

Electrocatalytic reduction of CO2 has the potential to convert CO2 into carbon-based fuels by using renewable energy. While a wide range of electrocatalytic materials have been investigated, the process is still limited by poor reaction selectivity or large overpotentials. Hence many have studied the effects of surface oxides, crystal facets, and nanoparticles on the activity and selectivity of CO2 reduction. Others have also shown that the reaction is sensitive to conditions such as temperature, CO2 pressure, electrolyte buffer concentration and electrode porosity. These effects mean that it can be challenging to determine the underlying cause for differences in intrinsic catalytic behaviour [1,2], and thus it is important to understand the effect of experimental conditions on CO2reduction. Firstly, we will review the literature and our own experimental work which clearly highlight the importance of interfacial pH on the reaction [2-4]. It is well known that in weak buffers such as KHCO3, the pH at the surface of the cathode is significantly higher than the bulk due to the hydrogen evolution and CO2 reduction reactions. By conducting experiments over a range of KHCO3 concentrations (and thus different interfacial pH values), it is clear that this alters the selectivity and activity of CO2 reduction. As the hydrodynamics at the cathode surface will also alter interfacial pH, we have examined the effect of hydrodynamics on the CO2 reduction reaction using a Cu rotating cylinder electrode [5]. Given that the enhanced mass transport will also increase the CO2 concentration at the electrode surface [6], it seems clear that mass transfer effects should influence the reaction selectivity. We confirm that this is indeed an important factor and suggest that increasing mass transport not only decreases the interfacial pH (closer to bulk values) but also decreases the surface coverage of CO on the cathode (a key immediate during CO2 reduction), which ultimately lowers the current going to the CO2reduction reaction. As these experiments revealed the importance of both KHCO3 concentration and mass transport, we developed a numerical model to predict how the bulk electrolyte composition changes during long term electrolysis experiments. This model was validated against experiment data and shows that changes in the bulk electrolyte (ionic resistance, pH, CO2-electrolyte equilibria) can occur over the course of long-term electrolysis experiments. These changes can complicate the interpretation of long-term electrode behaviour as well as the control of the electrochemical process. From these findings, we suggest a range of strategies to improve the experimental aspects of electrochemical CO2reduction investigations. [1] A.S. Hall, Y. Yoon, A. Wuttig, Y. Surendranath, Mesostructure-Induced Selectivity in CO2Reduction Catalysis, Journal of the American Chemical Society 137 (2015) 14834-14837. [2] R. Kas, R. Kortlever, H. Yılmaz, M.T.M. Koper, G. Mul, Manipulating the Hydrocarbon Selectivity of Copper Nanoparticles in CO2Electroreduction by Process Conditions, ChemElectroChem 2 (2015) 354-358. [3] K.J.P. Schouten, E. Pérez Gallent, M.T.M. Koper, The influence of pH on the reduction of CO and to hydrocarbons on copper electrodes, Journal of Electroanalytical Chemistry 716 (2014) 53-57. [4] A.S. Varela, M. Kroschel, T. Reier, P. Strasser, Controlling the selectivity of CO2electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH, Catalysis Today 260 (2016) 8-13. [5] C.F.C. Lim, D.A. Harrington, A.T. Marshall, Effects of mass transfer on the electrocatalytic CO2 reduction on Cu, Electrochimica Acta (2017) in press. [6] N. Gupta, M. Gattrell, B. MacDougall, Calculation for the cathode surface concentrations in the electrochemical reduction of CO2 in KHCO3solutions, Journal of Applied Electrochemistry 36 (2006) 161-172. [7] K. Hara, A. Tsuneto, A. Kudo, T. Sakata, Electrochemical Reduction of CO2 on a Cu Electrode under High Pressure: Factors that Determine the Product Selectivity, Journal of The Electrochemical Society 141 (1994) 2097-2103.

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

Introduction To reduce the carbon dioxide concentration in the atmosphere and to solve the energy shortage issue, CO2 converting technologies, such as electrochemical, photoelectrochemical, and photocatalytic CO2 reduction, have been paid big attention in the last several decades. Researches in this field aim to convert CO2 into useful fuels such as formic acid, carbon monoxide, methane and methanol using electric or solar energy by means of CO2 reduction catalysis, in which the products can be directly used or easily stored. To achieve high conversion efficiency, the development of CO2reduction catalyst is essential. In the present research, a Cu-Zn intermetallic catalyst was developed on the basis of the understanding of the reaction mechanism of CO2 reduction on the metal surface in an aqueous media. It was reported that the adsorption or desorption strength of CO2 and CO anion radicals on the surface of the metal is the essential factor on the reaction pathway of CO2 reduction. 1,2 Among numerous candidates for catalyst materials, the Cu-Zn intermetallic catalyst is supposed to have a very appropriate adsorption-desorption strengths to achieve high efficiency and selectivity for converting CO2into formic acid. Besides, the Cu-Zn intermetallic catalyst is consisted of abundant and eco-friendly metals, also the synthesis method is very simple and low-cost. Experimental and Evaluation A Cu-Zn intermetallic electrode was prepared by sputtering and vacuum sealing methods. Its electrocatalytic properties were evaluated in an electrochemical cell and the composition was optimized according to the electrochemical performance. The optimized Cu-Zn electrode was then used to construct a photoelectrochemical (PEC) cell with a mesoporous SrTiO3photoanode prepared by screen printing method. According to the performance in the PEC cell, Cu-Zn intermetallic nanoparticles loaded SrTiO3powder photocatalyst was also synthesized by chemical reduction and vacuum sealing methods and its photocatalytic property was evaluated. Results and Discussion According to the electrochemical evaluation, the Cu-Zn intermetallic electrode behaved lower onset potential than pure copper and pure zinc. After the optimization of the Zn concentration, we measured the lowest onset potential (-0.65V vs. Ag/AgCl) when the Zn mass ratio was 53%. In the PEC cell, the optimized Cu-Zn electrode was used as the cathode in a CO2 purged KHCO3 electrolyte. When the SrTiO3 photoanode was irradiated by UV light, CO2 reduction was catalyzed on the surface of Cu-Zn electrode. Figure 1 shows the products detected at the rest potential, and HCOOH, CO, CH4 and H2 were produced. When we purged Ar gas in the electrolyte solution at cathodic side, the products amount was much lower than that of CO2 purged condition, indicating that most of the products were originated from catalytic CO2reduction. Further, products amounts from Cu-Zn intermetallic were two times higher than those from pure Cu. It is noteworthy that the faradic efficiency for HCOOH was as high as 79.72%. The turnover number (TON) of this electrode reached 1458.9, which proved that the Cu-Zn electrode performed high stability. In the photocatalytic evaluation, the Cu-Zn intermetallic nanoparticles loaded SrTiO3 powder was dispersed into CO2 purged KHCO3 solution and irradiated under UV light. Significant amount of HCOOH was also detected, while H2 production was almost negligible, indicating that the Cu-Zn also behaves high selectivity as co-catalyst in a CO2photocatalytic reduction system. The carbon source of the products was also confirmed by our isotope tracing experiment. Conclusion The Cu-Zn intermetallic electrode can catalyze CO2 reduction under low bias-potential in electrochemical and photoelectrochemical CO2 reduction systems with high conversion efficiency and selectivity for HCOOH. Our Cu-Zn electrode was very stable under bias application as well as photon irradiation conditions. The Cu-Zn intermetallic nanoparticles loaded STO powder can also convert CO2into HCOOH and other products under UV irradiation. Acknowledgement This work has been supported by a grant from Advanced Catalytic Transformation program for Carbon utilization (ACT-C), Japan Science and Technology Agency (JST). References Kuhl, Kendra P., et al. J. Am. Chem. Soc., 136,14107-14113 (2014)Hori, Y. Modern aspects of electrochemistry. Springer New York, 2008. 89-189. Figure 1

Electrochemical CO₂ reduction (CO₂R) has emerged as a promising solution to mitigate climate change by enabling net-negative emissions while producing valuable fuels and chemicals. However, advancing CO₂R technologies requires in-depth characterization of electrocatalysts under realistic operating conditions to elucidate performance-structure-property relationships that can guide next generation catalyst and reactor designs. X-ray Absorption Spectroscopy (XAS) is a powerful technique for such studies due to its ability to provide real-time insights into catalyst structures and dynamics. However, the success of an in-situ/operando XAS experiments heavily depends on the design of custom electrodes and electrochemical cells that allow the penetration of X-rays to the catalyst layer while mimicking practical reactor conditions. This study introduces a laser-engraved Kapton electrode as a conductive, X-ray-transparent, and mechanically stable substrate, that is integrated into a custom-designed three-electrode electrochemical cell capable of operating in both fluorescence and transmission XAS modes. The Kapton electrode engraved with both a working and counter electrode, demonstrated excellent performance as a catalyst substrate. Copper nanoparticles, used as a model CO₂R catalyst, achieved reproducible product selectivity and activity in the in-situ/operando XAS cell, matching results obtained with conventional setups. This work provides a robust platform for in-situ/operando studies of electrocatalysts, enabling detailed structural and mechanistic insights across diverse reactions. The laser-engraved Kapton electrode and versatile cell design pave the way for advancing CO₂R and other electrochemical technologies by addressing key limitations in XAS-based catalyst characterization. Keywords: Laser-engraved Kapton electrode, in-situ/operando XAS, electrochemical CO2 reduction