Energy Budget of Electrochemical CO₂ Conversion

Introduction

Fig. 1: The greenhouse gas effect in the atmosphere (Source: Wikimedia Commons).

The accumulation of anthropogenic carbon dioxide (CO₂) in the atmosphere (Fig. 1) has intensified global efforts to capture CO₂ and convert it into useful chemical feedstocks, including gaseous fuels such as carbon monoxide (CO) and methane (CH₄), and liquid products such as methanol (CH₃OH) and ethanol (C₂H₅OH). Electrochemical CO₂ reduction (E-CO₂R) has emerged as a leading conversion process due to its potential for renewable electricity usage, potential for scale-up due to its compactness, modularity, and ease of use, and ability to use recyclable materials.

Despite the promise of this technology, it remains technically challenging to make electrochemical CO₂ reduction both energetically and economically efficient. This report analyzes the energy budget of electrochemical CO₂ conversion, focusing on CO and ethylene (C₂H₄) as representative products, and compares their performance against conventional fossil-based production.

Conventional Manufacturing and Environmental Impact

The conventional production of bulk chemicals like CO and ethylene is highly dependent on the burning of fossil fuels, resulting in significant greenhouse gas emissions.

CO is conventionally produced as a component of synthesis gas (syngas) via steam methane reforming (SMR). In this process, natural gas (methane) is reacted with high-temperature steam (∼700-1,000°C) over a catalyst. [1] The SMR process is highly endothermic, meaning it requires a massive and continuous energy input to maintain its high operating temperature. This heat is supplied by burning fossil fuels, releasing large quantities of CO₂ as a byproduct of both the chemical reaction and the heating process. The main reaction pathway for CO production via SMR is

While highly optimized, this process is CO₂-intensive, as it uses CH₄ as both a feedstock and a fuel to generate the high temperatures required for the reaction, leading to substantial indirect and direct CO₂ emissions. [2]

Ethylene is manufactured by the steam cracking of hydrocarbon feedstocks, such as ethane and naphtha. This process uses high temperatures (∼850°C) to "crack" the larger hydrocarbon molecules into smaller ones, primarily ethylene. [3] Steam cracking is one of the most energy-intensive processes in the entire chemical industry. [3,4] Its environmental impact is substantial; approximately 90% of the process's CO₂ emissions, which can be as high as 1.1 to 1.6 kg-CO₂e per kg of ethylene, come directly from the fossil fuel combustion needed to heat the cracking furnaces. [5,6]

Electrochemical Reaction Pathways

Electrochemical CO₂ reduction is driven in an electrolyzer, typically divided into a cathode (where CO₂ is reduced) and an anode (where water is oxidized). CO₂ is fed as a gas or dissolved species and is typically sourced from direct air capture or industrial sources.

At the cathode, the reduction reactions proceed as follows:

At the anode, the competing reaction is typically the oxygen evolution reaction (OER):

This leads to the following overall, full-cell reactions:

The setup of electrochemical cells for CO₂ reduction varies widely based on the design of the cell. Electrolytes are typically prepared by dissolving specific salts in a chosen solvent before saturating the solution with CO₂ gas. Common electrode materials include metals such as copper, tin, or silver, as well as graphene and graphitic carbon nitride as metal-free catalysts. These variables affect the operating voltages used.

Furthermore, using E-CO₂R for CO and ethylene production is challenged by the competing hydrogen evolution reaction (HER). Tuning the catalyst and local reaction environment can help minimize this.

Energy Budget Comparison

The viability of E-CO₂R is determined by its specific energy consumption (SEC), measured in kWh per kg of product. This value is dictated by the thermodynamics of the reaction and the efficiency of the electrolyzer. The minimum electrical work (W_min) required to drive the reaction is equivalent to the change in Gibbs Free Energy (ΔG) and is related to the standard cell potential (E⁰_cell) by the Nernst equation

where n is the number of electrons transferred, F is the Faraday constant (96,485 C/mol), and E⁰_cell is the standard cell potential (V). The theoretical minimum SEC (SEC_min) in kWh/kg is calculated by the following equation:

where M is the molar mass of the product (kg/mol). The practical SEC (SEC_actual) is always higher than SEC_min due to inefficiencies, primarily:

• Overpotential: The extra voltage (V_{overpotential}) needed to overcome activation barriers. The actual cell voltage is

V_cell = E⁰_cell + V_{overpotential}

• Faradaic efficiency (FE): The percentage of electrons that produce the desired product rather than unwanted byproducts through competing reactions, such as hydrogen in CO production.

Thus, SEC_actual can be calculated by:

Comparison of Energy Consumption

The energy budget for E-CO₂R is dominated by the electricity consumption, which far exceeds the theoretical minimum due to overpotentials, parasitic reactions, and product separation costs. For conventional processes, the SEC is dominated by thermal energy input. Table 1 compares the specific energy consumption for both pathways.

For CO production, current E-CO₂R technology shows SEC values comparable to or better than conventional SMR. This proximity in energy consumption, coupled with the ability of E-CO₂R to achieve near-zero GHG emissions when powered by renewable electricity, makes it a potentially viable solution.

For ethylene, the comparison is currently unfavorable for electrochemical CO₂ reduction. The current best technologies have an SEC of ∼45.6 kWh/kg, which is over six times higher than the ∼7.5 kWh/kg required for conventional steam cracking. This excessive energy demand, which is currently the main reason for its high carbon intensity when using grid electricity, is a major barrier to economic viability. [5]

Furthermore, it's important to consider the energy required to remove CO₂ from the air or from direct emission sources. This affects the overall energy budget.

Conclusion

Electrochemical reduction of CO₂ offers a crucial pathway for decarbonizing the chemical industry, but its current energy budget presents significant challenges, particularly for multi-carbon products like ethylene. While the SEC for CO production is approaching parity with fossil-based methods, the SEC for ethylene remains uncompetitive.

To make E-CO₂R a commercially and energetically efficient solution, several key areas require intensive research and development:

1. Reduce operating voltage (minimize overpotential): The large gap between theoretical and actual SEC is primarily due to the cell overpotential. This requires the development of highly selective, durable, and active electrocatalysts that can operate at high current densities while maintaining a low voltage. [7]

2. Improve Faradaic efficiency and product selectivity: For C₂H₄, energy is wasted on forming unwanted by-products (e.g., H₂, C1 products). High FE for the desired product is necessary to reduce the energy intensity and simplify the downstream separation process, which is itself energy-intensive. [5]

3. Advanced electrolyzer design: Moving beyond lab-scale flow cells to industrial-scale membrane electrode assemblies (MEAs) or solid oxide electrolysis cells (SOECs) is critical for achieving high current densities and minimizing mass transport limitations. [2]

4. Decarbonize electricity generation: The electrochemical route is highly sensitive to the carbon intensity of the electricity source. E-CO₂R can only provide a net climate benefit when coupled with extremely low-carbon electricity, such as dedicated solar or wind power. [2]

In summary, E-CO₂R is a thermodynamically viable concept for a circular carbon economy. However, until energy efficiency can be substantially improved through materials and engineering advances and CO₂ can be sourced in a low-energy way, it will struggle to displace the energy-optimized conventional methods on a purely economic basis.

© Hannah McCollum. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

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