Copper Electrocatalyst Designs For Electrochemical Reduction and Utilization of Carbon Dioxide

Abstract

Carbon dioxide (CO₂) utilization is a technique that can be used to mitigate the current climate crisis by capturing industrially generated carbon dioxide from the atmosphere and converting it to useful chemicals. Research on carbon dioxide utilization has prompted scientists to explore the topic of carbon dioxide reduction through electrolysis, a technique used to generate and convert CO₂ to multi-carbon complexes through electrical currents. These complexes, such as ethanol and ethylene, include much wider beneficial applications as renewable fuel sources and feedstock than single-carbon compounds. This review explores the special properties of various designs of copper electrocatalysts used in the formation of multi-carbon complexes. Studies show that the surface of copper contains unique characteristics in its active sites, which enhances the faradaic efficiency of the multi-carbon complexes and catalyzes key reactions in the CO₂ reduction mechanism [1-3]. They also found that modifying the surface of copper catalysts with bi-pyrimidine organic films and indium atoms can enhance the selectivity and faradaic efficiency of multi-carbon compounds. These findings can help researchers better develop electrocatalyst structural and chemical altering methods that will enhance the selectivity for desired chemicals.

Introduction

In 2021, the world’s annual emission of carbon dioxide from fossil fuel sources was 37.12 billion tons [9]. CO₂ waste products and other greenhouse gasses released by the rapid usage of fossil fuels worldwide are stored in the Earth’s atmosphere, creating the greenhouse effect. The greenhouse effect traps heat in Earth’s atmosphere, causing the surface to reach undesirably high temperatures. Recent statistics regarding global surface temperatures show that 2022’s temperature has increased by 1.55 degrees Fahrenheit higher than in the 1880s [10]. To minimize the amount of CO₂ in the atmosphere, carbon capture is a solution that has been extensively researched to mitigate the current global climate change crisis. 

Carbon capture and utilization is a process that converts carbon dioxide from the atmosphere to useful chemicals as renewable fuel sources and energy storage applications. One step in the process of carbon capture is through carbon dioxide reduction reactions, which use various electrocatalysts that accelerate electrolytic reaction rates to efficiently produce the desired products. Electrolysis reactions require a cathode, and an anode, which is where reduction & oxidation take place respectively.  A reduction reaction occurs when electrons are added to a species while an oxidation reaction occurs when a substance loses electrons. Recent research on carbon reduction has determined that lab-synthesized copper-derived catalysts increase the efficiency in the formation of multi-carbon products over single-carbon compounds [1-8]. 

The resulting multi-carbon products, such as ethanol and ethylene, can be used as efficient and environmentally sustainable fuel sources [1-7]. Copper catalysts also enhance Carbon-Carbon (C-C) coupling reactions by combining carbon atoms to catalyze multi-carbon products [3]. These are more useful than single-carbon compounds, like methanol, which lack high energy density and have fewer industrial applications. Multi-carbon compounds can be used in a wider scale of essential reactions such as the synthesis of various detergents and many medicinal compounds. Ethanol-derived fuels are considered efficient due to their high octane number, which measures how much fuel is wasted by an engine [11]. The multi-carbon products of these CO₂ reactions can also provide alternative energy sources to fossil fuels and help mitigate climate change as reliable energy sources. 

Many of these copper catalysts/copper oxide catalysts undergo structural or chemical changes to increase their stability and faradaic efficiency for desired compounds [1-5]. Faradaic efficiency measures electron transfer between the electrodes, which details the efficiency of product formation. Current research evaluates the mechanisms of CO₂ reduction through density functional theory and cyclic voltammetry [1]. Density function theory emphasizes the electronic structure and analyzes the transfer of electrons between multiple compounds to create a more visualized picture of CO₂ reduction reactions. Cyclic voltammetry is a chemistry technique that measures the ability of an electrode to generate current during a CO₂ reduction reaction when voltage is applied.  Copper can also be modified to form desired products by implementing organic molecular films on the copper surface and mixing in molecular atoms to enhance intrinsic properties [1,3,4]. These intrinsic properties of copper include grain boundaries, which are non-smooth areas on the surface of copper catalysts, that enhance carbon dioxide absorption onto its surface [1,4].

Non-copper metal catalysts, such as nickel, hydrogen, nitrogen, and iron have also been extensively researched for CO₂ reduction to carbon monoxide, which is a single-carbon compound [6]. When modified with nitrogen, nickel catalysts create a higher amount of electric charge and more active sites for CO₂ reduction reaction instead of hydrogen evolution reactions for the formation of carbon monoxide at low voltages. However, electrocatalytic research has also shown that these catalysts are not capable of forming multi-carbon compounds. They do not contain the grain boundaries/activation sites that copper catalysts contain to fully reduce carbon monoxide to multicarbon fuel sources. Moreover, hydrogen evolution reactions will not be discussed here because they create undesired flammable hydrogen gas and do not promote efficient renewable energy. Additionally, the non-copper catalyst faradaic efficiency for carbon monoxide production was about 99% through carbon dioxide reductions, revealing their limiting high affinity for single-carbon formation. Copper-based catalysts will generate more efficient results of multi-carbon compounds compared to nickel catalysts due to their higher potential to create more active sites and selectivity for C-C coupling reactions. Furthermore, the non-copper catalysts lack the intrinsic properties of various copper-based catalysts, such as grain boundaries on the active sites, that lower the energy barriers/activation energy for various key reactions in the CO2 reduction mechanism for multi-carbon complexes [1-3,5].  Energy barriers/ activation energy constitute the minimum amount required to generate a desired chemical reaction. Overall, non-copper catalysts are inefficient due to their lack of surface activation sites to catalyze CO2 reduction reactions through electrolysis. Copper catalysts tend to be more selective towards carbon dioxide reduction reactions compared to other reactions of non-copper catalysts. In this review, we will describe a few prominent chemical designs of copper-derived electrocatalysts by analyzing their mechanistic impact, chemical properties, and their ability to change physical/chemical structure for the formation of multi-carbon complexes.

Designs of Copper-Derived Electrocatalysts

Copper Oxide (Mechanism and Chemical Properties) 

Research of the mechanisms between CO₂ and the surface of copper oxide analyzes Density Functional Theory which focuses on the unique binding sites that copper catalysts create and on the selectivity of carbon dioxide reduction products [1, 2]. Copper oxide electrocatalysts also contain special chemical properties that can completely reduce carbon dioxide into multi-carbon compounds [1-5,8]. Generally, the process of CO₂ reduction reactions during electrolysis involves three main steps. The first step is the bonding of carbon dioxide with the surface of the electrocatalyst. Then, the second step involves the reduction of carbon dioxide. The final step is the removal of the multi-carbon products from the surface of copper oxide [1]. According to density functional theory computation, the unique grain boundaries of copper oxide help catalyze the reduction of carbon dioxide and facilitate efficient binding [1,3]. Because grain boundaries aid in carbon dioxide bonding to the copper catalyst surface, this results in relatively high faradaic efficiencies at 57% overall at -0.95 volts for multi-carbon products [1]. The increased efficiency of copper oxide catalysts to reduce carbon dioxide molecules to multi-carbon products can have beneficial applications to society, such as in the generation of renewable energy. Compared to other metal catalysts which have a 0% faradaic efficiency for ethanol [6], Copper is much more applicable.

One research lab experimented with reducing CO₂ to 1-butanol using copper electrocatalysts [1]. The unique binding sites of modified copper electrocatalysts also form valuable intermediates. One of the intermediate products formed during this reaction is 3-hydroxybutanal. The 3-hydroxybutanal compound undergoes a dehydration reaction by losing a water molecule from its structure. The non-uniform sites of copper catalysts catalyze these reactions, unlike other metal catalysts that do not have these binding sites. [1]. The main reaction, aldol condensation, is then catalyzed by the copper catalysts to reduce the 3-hydroxybutanal intermediate product to 1-butanol [1]. 1-butanol is an important multi-carbon compound that copper catalysts can produce through the copper binding sites because it has the highest faradaic efficiency during the electrolysis reaction at 9.6%. This is one of the first times in a research experiment that 1-butanol was synthesized using a metal catalyst [1].

According to density functional theory, a crucial step in the mechanism of carbon dioxide reduction on the surface of copper nanoparticle electrocatalysts is the hydrogenation of carbon monoxide. This reaction increases the selectivity for desired multi-carbon products through copper’s unique active sites. The reaction is challenging because it has to overcome a high energy barrier. Copper catalysts can lower reaction energy barriers, which is valuable because they create more efficient carbon dioxide reduction reactions for multi-carbon compounds [2,3]. The multi-carbon products also require C-C coupling steps and the dimerization of carbon monoxide. Dimerization and C-C coupling reactions enhance carbon dioxide reduction reactions, which helps lower the amount of greenhouse gasses in the atmosphere by creating more commercially useful multi-carbon compounds in various industries.

Copper Organic Films (Morphology)

The morphology (the change of structure) of copper heightens the molecular bonding of carbon dioxide intermediates onto the catalyst surface for reduction [1, 3, 4]. Organic chemical films are one modification that can enhance copper’s chemical properties for carbon dioxide reduction by boosting C-C coupling reactions [3, 5]. Carbon adsorption properties are also enhanced due to the modification of the copper catalysts by T-bipyridine organic films. The increased thickness of the organic films decreased the roughness factors on the surface of the foil, which allowed more active sites to be accessed. The modification of the copper foil by bipyridine films also increased the electrochemical surface area for ethylene when compared to pristine copper [3]. Due to the T-bipyridine, the larger electrochemical surface area increases carbon dioxide adsorption onto the copper catalyst surface and generates greater efficiency of carbon dioxide reduction in electrolytic reactions. During this experiment, it was determined that copper foil had the greatest selectivity and faradaic efficiency for the formation of ethylene at 46.1% for -0.96 V, while also suppressing other competitive reactions [3].  Future research involves understanding the impact of various other organic films on the surface of copper catalysts [3] to help increase the faradaic efficiencies of carbon dioxide reduction reactions, and potentially reduce greenhouse gasses in the atmosphere. 

Copper Nanotubes (Morphology)

Another modification technique that has been extensively researched is the implementation of molecular atoms onto the surface of copper nanotube catalysts to enhance the selectivity for multi-carbon compounds [4, 8]. Differing from organic films, copper nanotubes are materials made out of tiny nanoparticles to enhance electrical conductivity. One of the experiments conducted to enhance the faradaic efficiency of copper catalysts involved the implementation of indium atoms into copper nanotubes [4]. Copper is the only metal catalyst so far with a negative energy adsorption for carbon monoxide formation and a positive adsorption energy for hydrogen formation, which is why carbon dioxide reduction is favored instead of hydrogen evolution [2]. Negative adsorption energy in copper catalysts is relevant for CO₂ reduction reactions since it correlates to a more efficient voltage in electrolysis reactions allowing CO₂ adsorption to take place on the surface of the copper catalyst, while positive adsorption energy decreases the efficiency of carbon dioxide reduction electrolytic reactions. This is due to a lack of active sites appearing on the copper surface when positive adsorption energy is present. One research topic confirmed that the electrical current density for the indium-modified copper nanotube during electrolysis was higher than the pristine copper at -53mm-2 for -0.96 voltage [4]. A greater current density correlates to a greater faradaic efficiency for Indium-modified copper catalysts. Compared to pristine copper catalysts, the density would require a much higher voltage from pristine copper to generate a similar faradaic efficiency to modified copper catalysts [4]. Indium-modified copper nanotubes can reduce carbon dioxide to multi-carbon products more efficiently than other experimented catalysts. The binding sites on the copper surface generated by the indium also decreased the energy barrier for the formation of carboxylic acid, which is a key intermediate for carbon monoxide formation [4]. Future research on the modifications of copper catalysts for increased faradaic efficiency of multi-carbon compounds could explore the addition of indium particles on three-dimensional molecular materials, such as copper foams [4]. 

Conclusion

Copper-based catalysts have important properties in creating active sites on their surface and in reducing CO₂ to multi-carbon compounds. The research on these catalysts analyzes how morphology and faradaic efficiencies impact CO₂ reduction selectivity. Comparisons on the efficiency of multi-carbon compound generation between modified copper catalysts and modified non-copper catalysts suggest that copper catalysts are more beneficial due to their unique, non-uniform binding sites. It was discovered that the copper surface contains grain boundaries which creates more active sites for CO₂ reduction—a property that most non-copper catalysts do not have. The active sites further catalyze specific reactions by lowering their energy barrier in the CO₂ reduction mechanism, which leads to the dimerization of carbon monoxide to form multi-carbon compounds. Selectivity, faradaic efficiency, and current density for forming multi-carbon compounds can be further enhanced by improving the morphology of carbon catalysts. These findings have applications for mitigating CO₂ in the atmosphere, while also creating viable multi-carbon compounds for applications of generating renewable energy.

About the Author: Hani Sobhi

My name is Hani Sobhi and I am from Richmond, California. My major is Chemistry and I am currently a 4th year. My ethnicity is half Chinese and half Egyptian. I like basketball and playing piano in my spare time. In the McCormack lab, I am researching the synthesis and analysis of ultra-high-temperature ceramic materials for space exploration applications, including Hafnium and Tantalum Carbides.

References

1. Ting LRL, García-Muelas R, Martín AJ, Veenstra FLP, Chen ST-J, Peng Y, Per EYX, Pablo-García S, López N, Pérez-Ramírez J, et al. Electrochemical reduction of carbon dioxide to 1-butanol on oxide-derived copper. Angewandte Chemie (International ed. in English). 2020;59(47):21072–21079. http://dx.doi.org/10.1002/anie.202008289. 

2. Zahid A, Shah A, Shah I. Oxide derived copper for electrochemical reduction of CO2 to C2+ products. Nanomaterials (Basel, Switzerland). 2022;12(8):1380. http://dx.doi.org/10.3390/nano12081380. 

3. Zhao S, Christensen O, Sun Z, Liang H, Bagger A, Torbensen K, Nazari P, Lauritsen JV, Pedersen SU, Rossmeisl J, et al. Steering carbon dioxide reduction toward C–C coupling using copper electrodes modified with porous molecular films. Nature communications. 2023;14(1). http://dx.doi.org/10.1038/s41467-023-36530-z. 

4. Niorettini A, Mazzaro R, Liscio F, Kovtun A, Pasquini L, Caramori S, Berardi S. Indium-modified copper nanocubes for syngas production by aqueous CO2electroreduction. Dalton transactions (Cambridge, England: 2003). 2022;51(28):10787–10798. 

5. Gustavsen KR, Johannessen EA, Wang K. Sodium persulfate pre‐treatment of copper foils enabling homogenous growth of cu(OH) 2 nanoneedle films for electrochemical CO 2 reduction. ChemistryOpen. 2022;11(10). http://dx.doi.org/10.1002/open.202200133.

6. Hao Q, Liu D-X, Deng R, Zhong H-X. Boosting electrochemical carbon dioxide reduction on atomically dispersed nickel catalyst. Frontiers in chemistry. 2021;9:837580. 

7. Li H, Zhou H, Zhou Y, Hu J, Miyauchi M, Fu J, Liu M. Electric-field promoted C–C coupling over Cu nanoneedles for CO2 electroreduction to C2 products. Cuihua Xuebao/Chinese Journal of Catalysis. 2022;43(2):519–525. 

8. Ma W, Xie S, Liu T, Fan Q, Ye J, Sun F, Jiang Z, Zhang Q, Cheng J, Wang Y. Electrocatalytic reduction of CO2 to ethylene and ethanol through hydrogen-assisted C–C coupling over fluorine-modified copper. Nature catalysis. 2020.

9. Ritchie, H. (2020, May 11). CO₂ and Greenhouse Gas Emissions. Our World in Data. https://ourworldindata.org/co2-emissions.

10. NASA. Global Temperature. Climate Change: Vital Signs of the Planet. https://climate.nasa.gov/vital-signs/global-temperature/. 

11. Ethanol Basics. https://afdc.energy.gov/files/u/publication/ethanol_basics.pdf#:~:text=Ethanol%20has%20a%20higher%20octane%20number%20than%20gasoline%2C  (accessed 2024-02-07). 

12. Kim D, Kley CS, Li Y, Yang P. Copper nanoparticle ensembles for selective electroreduction of CO₂ to C₂–C₃ products. PNAS. 2017;114(40):10560–10565.