Closing the Carbon Loop through CO₂ Electrolysis

CO₂ emissions continue to increase as several industries continue to depend on fossil fuels. The atmospheric CO₂ concentration has almost doubled after the Industrial Revolution (Current: >400 ppm, before the Industrial Revolution: 280 ppm). This increase translated to a global temperature rise of approximately 2 °C. To reduce the adverse effects of global warming to human life and property, the Intergovernmental Panel on Climate Change (IPCC) limits greenhouse gas emissions in such a way that the global temperature rise will decrease to 1.5 °C. 

One promising way to decarbonize industries is to couple the heavy industrial processes with renewable energy. This route necessitates the use of efficient carbon capture and utilization strategies to sequester CO₂ from the atmosphere or any other gaseous streams and then convert it to high-value chemicals and fuels. The most commonly used carbon capture and utilization techniques involve the use of thermal catalysts, electrolysis, and biotechnology. Among these CO₂ conversion technologies,  CO₂ electrolysis proves to be low cost, scalable, and sustainable. It enables the direct storage of renewable electricity in a convenient and high-energy-density form. In contrast to the conventional thermochemical conversion method that requires huge energy input, the electrochemical conversion of CO₂ using a CO₂ electrolyzer can be operated under mild conditions and can be powered using renewable energy sources, such as solar, wind, tide, and geothermal energy. 

As such, electrochemical techniques can be carbon negative, thereby closing the carbon loop and possibly reducing CO₂ atmospheric emission. Additionally, unlike some methods, electrolysis has a single-step dissociation process without moving parts, allowing the separate release of products in the anode and cathode compartments with solid electrolyte. Electrolysis cells are well-suited for mass production and automated maintenance.

Advantages and disadvantages of carbon utilization technologies

What are the key processes involved in carbon dioxide electrolysis?

1. CO₂ reduction reaction. During CO₂ electrolysis, CO₂ is introduced and dissolved in the catholyte. Under the influence of an applied electrical potential, the dissolved CO₂ undergoes reduction reactions at the cathode to form the desired product. 
   
   For example, for CO formation, the reaction is: 
   CO_2(aq) + H₂O + 2 e^– → CO + 2 OH^–. 
   
   
2. Ion and electron transfer. To complete the circuit, the generated OH^– ions move from the cathode to the anode through the ion exchange membrane. On the other hand, the electrons travel along the external wirings of the circuit.
   
   
3. Oxidation. OH^– ions that cross from the cathode to the anode undergoes oxidation or oxygen evolution reaction, which produces water and O₂ gas.
   
   2 OH^– → O_2(g) + 2 H₂O + 4 e^–

High Value-Added Products from CO₂ Conversion

Depending on the energy input, operating and reaction conditions, and metal or alloy electrodes/catalysts employed, the electrolysis or electrochemical reduction of CO₂ can generate different products, such as carbon monoxide (CO), formic acid (HCOOH), hydrocarbons like ethylene (C₂H₄), and alcohols (e.g. CH₃OH).

What are the different types of electrolysis cells used in carbon dioxide reduction processes?

CO₂RR electrolyzers are classified as H-cells or flow cells. 

• H-cells earn their name due to their H-shaped design. In these reactors, the anode and cathode chambers contain liquid electrolytes and are separated by an ion exchange membrane. This setup prevents the unwanted mixing of reactants and the reoxidation of products back into CO₂. H-cells find extensive use in electrochemical CO₂ reduction research, particularly in studies on catalyst design and screening because this setup enables straightforward and easy testing procedures. However, the low solubility of CO₂ in aqueous electrolytes at atmospheric conditions (only 34 mM) causes mass transport limitation when conducting electrolysis in H-type cells. This restricts the current density to values well below 100 mA cm^–2. For reference, target industrial scale throughput is set at 300 mA cm^–2. Results obtained in an H-type cell are not directly applicable to its performance under industrial conditions. Consequently, testing and optimizing catalysts under realistic operating conditions at higher current densities require alternative setups. 
  
  
• Flow cells were designed to perform CO₂ electrolysis at an industrial scale. In contrast to H-cells wherein CO₂ is supplied and dissolved in the liquid catholyte, CO₂ is transported in the gaseous phase in flow cells. This makes the reaction not limited by the solubility of gaseous CO₂ in a liquid electrolyte. Flow cells are continuous reactors in which CO₂ and the electrolyte are continuously cycled, allowing the device to reach current densities >200 mA cm^–2. Depending on the state of the electrolyte used, flow cells can be further classified as liquid-phase or solid phase electrolyzers. In liquid-phase flow cells, the cathode and anode compartments are also separated by an ion Exchange Membrane (IEM), which allows the migration of ions essential for the electrochemical reactions while preventing the undesired crossover of liquid products from the cathode to the anode side and vice versa. In contrast, in solid-phase flow cells, solid materials or components play a significant role in the cell's operation. This could involve the use of solid electrolytes, solid-state electrodes, or other solid components within the electrochemical cell.

Comparison between H-cells and flow cells

What are the primary components of the electrolysis cells used in carbon dioxide reduction processes?

Electrolysis cells used in carbon dioxide reduction processes typically consist of several key components that facilitate the conversion of carbon dioxide into valuable products. The primary components of these cells include:

1. Anode catalyst and substrate. The anode is the electrode where oxidation reactions occur. The anode reaction also affects the overall energy efficiency of the cell. As such, OER or water oxidation catalysts are also critical. Typical anode substrates used are non-porous metal substrates.

2. Cathode catalyst and substrate. In a liquid-phase flow cell, the CO₂ reduction reaction occurs at a gas diffusion electrode. A gas diffusion electrode (GDE) typically consists of a porous and conductive substrate or gas diffusion layer (e.g., graphitized carbon paper or cloth) with a catalyst layer (commonly precious metals like platinum) for enhancing electrochemical reactions. Common CO₂ reduction catalysts include metals (Cu, Ag, Pt, and Au),  metal–organic frameworks (Cu-, and porphyrin-based MOFs), nitrogen-doped carbon materials, and single atom catalysts. In certain applications, some GDEs may contain hydrophobic coating to repel water. 

3. Electrolyte. The electrolyte is the medium through which ions can move between the anode and cathode. It may be a liquid or solid substance that facilitates the ionic conduction necessary for the electrochemical reactions.

4. Ion Exchange Membrane (IEM). Regardless of the type of electrochemical cell, an ion exchange membrane is used. The IEM allows the migration of ions while preventing the unwanted crossover of reactants or products between the two compartments.