Carbon Capture and Utilization

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Chemical Feedstock and Fuel Production

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Carbon Dioxide Electrolyzers

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. To achieve this goal, CO₂ emissions must be reduced to half of their 2015 levels by 2030.

One effective way to reduce carbon emissions in industries is by combining heavy industrial processes with renewable energy. This approach requires efficient methods to capture CO₂ from the air or other gas streams and convert it into valuable chemicals and fuels. The main techniques for carbon capture and utilization include biomass with carbon capture and storage (BECCs), CO₂ hydrogenation using thermal activated catalysts, and CO₂ electrolysis.

Bioenergy with carbon capture and storage (BECCS) is a technology that combines biomass energy production with CO₂ sequestration to achieve net-negative emissions. Plants absorb CO₂ as they grow, and when biomass is burned for energy, the CO₂ released is offset by the carbon absorbed during growth, making the process carbon neutral. By capturing and storing the CO₂, BECCS removes it from the atmosphere, creating a carbon-negative impact. As a scalable solution, BECCS could play a key role in reducing emissions. However, its commercial implementation requires huge land areas—potentially as much land as is currently used for global food production—which presents a significant challenge.

PROS: Can be carbon negative, scalable

CONS: Large land required, high energy input for capturing and storing CO₂

Carbon dioxide hydrogenation

CO₂ hydrogenation, i.e. reacting CO₂ with hydrogen gas, is a process that can convert emissions into useful chemicals and fuels. Now, the process can be carbon-neutral or even carbon-negative when renewable hydrogen gas is used. Initially, CO₂ is converted into CO and water, a mixture called syngas, through the reverse water–gas shift (RWGS) reaction:

CO₂ + H₂ → CO + H2O

Syngas can then be further processed into liquid fuels through the Fischer–Tropsch synthesis (FTS) or converted into methanol through methanol synthesis:

Fischer–Tropsch (for liquid fuels): CO + H₂ → Liquid fuels + Water

Methanol synthesis: CO + 2 H₂ → Methanol (CH₃OH)

The Fischer–Tropsch and methanol synthesis reactions are useful for making fuels, but they need high temperatures and pressures, which uses a lot of energy. These processes can produce valuable fuels, but they are not very selective and require complicated catalysts, making them less efficient and more costly.

PROS: Can produce carbon-neutral/negative products, useful for creating fuels/chemicals

CONS: High temperatures and pressures needed, high energy demand, complex catalysts required

Electrochemical CO₂ reduction reaction (CO₂R) turns CO₂ into chemicals and fuels using only water and renewable electricity. If CO₂ is captured from the air, CO₂R can help close the carbon cycle. It produces chemicals like ethylene or methanol in one step, unlike traditional methods that require multiple reactions. CO₂ electrolysis operates at lower temperatures and pressures, using renewable energy sources like solar or wind, making it more energy-efficient than conventional processes. It also has no moving parts, allowing easy separation of products and making it ideal for mass production and automated maintenance. These features make CO₂R a scalable, sustainable, and potentially carbon-negative solution for reducing CO₂ emissions.

PROS: Operates under mild conditions, can be powered by renewable energy, scalable and low-cost, carbon-negative potential

CONS: Energy efficiency may vary, still under development for large-scale deployment

Key Processes involved in Carbon Dioxide Electrolysis

High Value-added Products from CO₂ Electrolyzers

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).

Product		Reaction	Reaction Potential [V vs. RHE]
	Carbon Monoxide	CO₂(g) + 2 H⁺ + 2e^– → CO + H₂O	–0.10
	Formic Acid	CO₂(g) + 2 H⁺ + 2e^– → HCOOH	–0.12
	Methanol	CO₂(g) + 6 H⁺ + 6e^– → CH₃OH + H₂O	0.03
	Acetic Acid	2 CO₂(g) + 8 H⁺ + 8e^– → CH₃COOH + H₂O	0.11
	Acetaldehyde	2 CO₂(g) + 10 H⁺ + 10e^– → CH₃CHO + 3 H₂O	0.06
	Vinyl Alcohol	2 CO₂(g) + 12 H⁺ + 12e^– → C₂H₃OH + 3 H₂O	0.09
	Ethylene	2 CO₂(g) + 12 H⁺ + 12e^– → C₂H₄ + 4 H₂O	0.08
	Ethane	2 CO₂(g) + 14 H⁺ + 14e^– → C₂H₆ + 4 H₂O	0.14
	N-propionaldehyde	2 CO₂(g) + 16 H⁺ + 16e^– → C₂H₅CHO + 4 H₂O	0.09
	Ethanol	3 CO₂(g) + 18 H⁺ + 18e^– → C₂H₅OH + 5 H₂O	0.10

Primary Components of 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.

Frequently Asked Questions about Carbon Dioxide Electrolyzers

What is the efficiency of carbon dioxide electrolyzers?

Efficiency varies based on factors such as the catalyst, operating conditions, and target product. Generally, the faradaic efficiency (the portion of current used to produce a specific product) ranges from 50% to 90% depending on these factors.

What is the typical operating temperature of a CO₂ electrolyzer?

CO₂ electrolysis can occur across a range of temperatures, categorized as low-temperature electrolysis (LTE) and high-temperature electrolysis (HTE). LTE operates below 100 °C, while HTE can reach up to 600 °C, boosting reaction rates and energy efficiency, and making it suitable for producing complex hydrocarbons.

Are there commercial CO₂ electrolyzers available on the market?

A few companies are developing commercial CO₂ electrolyzers, though the technology is still emerging, with most applications in pilot or early commercial phases.

How are CO₂ electrolyzers different from water electrolyzers?

In water electrolysis, H₂O is split into hydrogen (H₂) and oxygen (O₂) gas at the cathode and anode, respectively.

In CO₂ electrolysis, CO₂ is reduced to form carbon-based products like CO, formic acid, methanol, or ethylene, depending on the catalyst and reaction conditions. CO₂ electrolysis involves multiple complex reactions with competing pathways, often producing a mix of products. It requires catalysts that can stabilize CO₂ intermediates and selectively promote the desired reduction pathways, making catalyst design and reaction control more challenging.