Fundamentals of the Electroreduction of CO2

Fundamentals of the Electroreduction of CO₂: Comparison

Please note this is a comparison between Version 1 by Laxia Wu and Version 2 by Jessie Wu.

Carbon dioxide (CO₂) is one of the main greenhouse gases and the major factor driving global climate change. From the viewpoint of abundance, economics, non-toxicity, and renewability, CO₂ is an ideal and significant C1 resource, and its capture and recycling into fuels and chemical feedstocks using renewable energy is of great significance for the sustainable development of society. Electrochemical CO₂ reduction reactions (CO₂RRs) are an important pathway to utilize CO₂ resources. Zinc has been demonstrated as an effective catalyst for CO₂RRs.

CO₂
electrode
metals
electroreduction

1. Electrochemical Behavior of Electrocatalysts

1.1. Cyclic Voltammetry

Cyclic voltammetry experiments require the setting of the three most basic parameters: the potentials of the upper limit, the potentials of the lower limit, and the scan rate. Upper and lower potential limits are determined based on the electrochemical window of the solvent (e.g., water) and the stability of the electrode material. The potential scan rate is determined according to the reaction type and testing method. The scan rate can usually be above 50 mV·s⁻¹ for the liquid phase; however, it should not exceed 20 mV·s⁻¹ during steady-state measurements.

The electrochemical voltametric behavior of electrocatalysts is generally determined as follows: A three-electrode system with platinum mesh is used as a counter electrode. A saturated calomel electrode (SCE) is used as the reference electrode, whereas a catalyst-coated substrate electrode (such as a glassy carbon electrode) is used as the working electrode. These electrodes are used in an undivided cell at normal temperature and pressure. The voltammograms are recorded using an electrochemical workstation under sequential N₂ and carbon dioxide (CO₂) bubbling.

1.2. Electrochemical Activity Surface Area (ECSA) Characterization

The surface structure of the electrode significantly influences its catalytic performance. Electrodes are usually solid, though the structure of the solid surface is complex. Moreover, there are many types of surface sites (such as platforms, steps, kinks and vacancies, and different structures of the atomic arrangements). The structural information of the electrode surface can be obtained using electrochemical measurements. Atoms, molecules, and ions (such as H, O, CO) that interact strongly with the surface are selected to characterize the surface structure of the electrodes by using their adsorption–desorption characteristics and oxidative removal.

Hydrogen adsorption on the platinum surface consists of monolayer adsorption, which means that one platinum atom corresponds to one adsorbed hydrogen atom. Therefore, the ECSA of platinum can be calculated based on the amounts of charge on the adsorbed and desorbed hydrogen. For platinum alloys, the values of the ECSA calculated using this method tend to be small, because alloying elements can inhibit the adsorption and desorption of hydrogen. Moreover, CO can produce strong adsorption on a variety of metal surfaces. Therefore, CO stripping curves are often used to measure the ECSA of metals, especially platinum group metals and their alloys. For coin elements such as gold, silver, and copper, the adsorption capacities of both H and CO are not strong, and the underpotential deposition of metals such as Pb and Cu is often used to calculate the ECSA.

2. Experimental Procedures and Product Analysis

2.1. Experimental Procedures

Three types of electrocatalytic reactors are used for CO₂RRs and include the H-cell, the flow cell, and the membrane electrode assembly (MEA) cell ^[1]. Among them, the H-cell is the most commonly used in fundamental studies mainly because of its low cost and simple operation ^[2]. In this section, the setup of the H-cell and the experimental procedure for electrocatalysis are briefly described. Typically, a CO₂RR is carried out using potentiostatic electrolysis in a two-compartment electrochemical cell using a standard three-electrode system. The working electrode is usually a catalyst-coated carbon paper, a glassy carbon electrode, a glassy carbon plate, or a carbon fiber paper. The working and reference electrodes are placed in the cathode compartment, whereas the counter electrode is placed in the anode compartment. The two compartments are separated by an ion exchange membrane. A proton exchange membrane is taken as an example in Figure 1. Aqueous solutions of NaHCO₃ or KHCO₃ are often chosen as electrolytes. When saturated with CO₂, an electrolyte can effectively buffer the change in pH of the bulk solution and keep it to near-neutral. The reduction in CO₂ occurs at the cathode, whereas the oxidation of oxygen (coming from water) occurs at the anode. H⁺ ions migrate to the cathode through a proton exchange membrane under the action of an electric field, thereby providing a source of hydrogen for the reduction of the carbon dioxide. Thermodynamically, for the electroreduction of CO₂ at different potentials, different multiple electron transfer reactions can occur and include the transfer of 2e⁻, 4e⁻, 6e⁻, 8e⁻, 12e⁻, and so on while also generating different reduction products. At present, the reported products of the electroreduction of CO₂ mainly include carbon monoxide (CO) ^[3][4][5], methane (CH₄) ^[6], methanol (CH₃OH) ^[7][8], formic acid/formate (HCOOH/HCOO^-) ^[9][10], ethylene (C₂H₄) ^[11][12], ethane (C₂H₆), ethanol (C₂H₅OH) ^[13][14][15], acetic acid/acetate (CH₃COOH/CH₃COO^-) ^[16], and n-propanol (CH₃CH₂CH₂OH) ^[17]. The electrochemical half reactions generating these products, along with the corresponding standard redox potentials, are listed in Table 1 ^[18].

Figure 1.

Schematic of electroreduction of CO

₂

in the H-cell.

Table 1.

Electrochemical potentials of possible CO

₂

RRs in aqueous solutions

^[18]

Products	Equation	E^⊖ (V vs. RHE)
Hydrogen	2H⁺ + 2e⁻ → H₂	0.000
Carbon monoxide	CO₂ + 2H⁺ + 2e⁻ → CO + H₂O	−0.104
Methane	CO₂ + 8H⁺ + 8e⁻ → CH₄ + H₂O	0.169
Methanol	CO₂ + 6H⁺ + 6e⁻ → CH₃OH + H₂O	0.016
Formic acid/formate	CO₂ + 2H⁺ + 2e⁻ → HCOOH	−0.171
Ethylene	CO₂ + 12H⁺ + 12e⁻ → C₂H₄ + 4H₂O	0.085
Ethane	CO₂ + 14H⁺ + 14e⁻ → C₂H₆ + 4H₂O	0.144
Ethanol	2CO₂ + 12H⁺ + 12e⁻ → CH₃CH₂OH + 3H₂O	0.084
Acetic acid/acetate	2CO₂ + 8H⁺ + 8e⁻ → CH₃COOH + 2H₂O	0.098
n-Propanol	3CO₂ + 18H⁺ + 18e⁻→ CH₃CH₂CH₂OH + 5H₂O	0.095

2.2. Qualitative and Quantitative Analyses of Products

After electrolysis, the catholyte is transferred to a headspace sample injector, while the liquid products, such as methanol, ethanol, and acetone, are detected using gas chromatography. Comparing the product’s peak position with that of the standard sample allows for qualitative judgment of the liquid products. The liquid product can also be quantified using ¹H NMR spectroscopy.

The gas products (such as H₂, CO, CH₄, and C₂H₄) generated during electrolysis are collected using a gas sampling bag. At the time of detection, gas is injected into the gas chromatograph with a syringe. Comparing the retention times of products obtained using gas chromatography with those of standards allows for qualitative judgment of the gas products. A standard curve is plotted according to the peak area of the produced gas chromatogram of the standard gas and the concentration of each component in the gas. The Faraday efficiency of the products can be quantitatively calculated according to Equation (1) ^[19].

F E = ϕ v t z F Q V_{m}

where φ is the volume fraction of gas products in the total gas, which can be obtained from the standard curve, v is the flow rate of CO₂ (L·min⁻¹), t is the electrolysis time (min), z is the number of electrons transferred in a specific electrode reaction, as shown by the data presented in Table 1 (for example, z = 2 for a CO₂RR to CO), F is the Faraday constant with the value of 96,485 C·mol⁻¹, Q is the total amount of electricity in the electrolysis process (C), and V_m is the molar volume of gas at 25 °C and standard pressure.

2.3. Reaction Mechanism of CO₂RRs

The electroreduction of carbon dioxide is a process in which reduction occurs by CO₂ molecules or CO₂-solvated ions acquiring electrons from the electrode’s surface within the solution. Electroreduction is a multistep process involving the transfer of multiple electrons, and it consists of CO₂ adsorption, electron transfer, and product desorption at the electrode surface. A large number of studies have shown that the current density, species, and selectivity of the CO₂RR are largely dependent on the electrode material and the reduction potential. The electrocatalysis of carbon dioxide undergoes different reaction pathways to generate different products. Figure 2 shows the main pathways for the electroreduction of CO₂ ^[20].

Figure 2.

Possible reaction pathways for the electroreduction of CO

₂

The CO₂ molecule is first adsorbed onto the surface of the catalyst, and, then, it is activated to absorb carbon dioxide (*CO₂⁻), which generates into intermediate *COOH through proton transfer. The intermediate *COOH undergoes another proton transfer and eventually generates HCOOH. The formation pathway of CO is similar to that of formic acid. Meanwhile, the intermediate *COOH is further reduced to form adsorbed CO (*CO). *CO is a relatively important intermediate that undergoes a series of electron transfer and protonation processes to generate different reduction products. For the generation of CH₄, the *CO is hydrogenated in C or O to generate *CHO or *COH. In the *CHO pathway, the configurations of adsorbed intermediates change from C binding in *CHO to O binding in *OCH₂. Moreover, *OCH₃, and gaseous CH₄ with *O are obtained on the surfaces of the catalysts. The other pathway of *COH involves the formation of adsorbed C (*C). The *C is further reduced to *CH, *CH₂, *CH₃, and finally to CH₄. The formation of C₂H₄ or other hydrocarbons requires controlled coupling reactions between CHO* and CH₂O* into *OCH-CHO*, *OCH-CH₂O*, or *OCH₂-CH₂O*, which is then followed by hydrogenation/dehydration reactions ^[21][22].

It is generally believed that the selectivity of the products of the electroreduction of CO₂ depends on the binding energy between the electrocatalytic materials and the reaction intermediates such as *CO, CO₂⁻, *COOH. When CO₂ is reduced to CO on the surface of electrodes, the binding energy between the electrode material and the CO determines the selectivity of the products generated during electrocatalytic reduction. The electrocatalytic products of electrodes (such as Ag, Au, and Zn) with weak CO binding tend to have high CO selectivity. Moreover, the CO generated during the reduction reaction is easily separated from the electrode surface and does not enter into the next reduction reaction. Various electrode materials (such as Pt, Fe, Co, and Ni) have stronger binding energies for CO, and, therefore, almost no CO₂ reduction products are produced when using these materials. This is because, after its generation, CO forms strong interactions with the metal sites on the surface, and the next reduction reaction cannot proceed as a result. Meanwhile, H⁺ reduction dominates, and a hydrogen evolution reaction occurs. Some electrode materials have moderate binding energies for the intermediates (such as *CO, CO₂⁻, and *COOH) due to the coincidence that the intermediates are stabilized. This, in turn, prompts the generation of reduction products with more than two electron transfers. C-C coupling reaction may also occur to yield C₂₊ products.

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