How Polymer Coatings Could Help Accelerate CO2 Conversion

By Cvetelin Vasilev, Ph.D.Sep 8 2021

Quick and efficient conversion of atmospheric CO₂ into chemical feedstock is one of the most promising approaches to mitigate anthropogenic CO₂ emissions, a primary driver of climate change. Recently, researchers from the University of Tsukuba in Japan created a polymer coating that, when applied to a standard metal catalyst, dramatically accelerates the electrochemical CO₂ conversion. 

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Mainly due to the extensive use of fossil fuels, CO₂ concentrations in the atmosphere have risen from 280 parts per million (ppm) in the pre-industrial era to today's 410 ppm. With the ever-increasing share of polymer materials in the modern economy, addressing CO₂ emissions associated with plastic production has become a strategic focal point.

CO₂ Conversion into Value-Added Chemicals

Life-cycle analyses of fossil-based feedstocks for the petrochemical and chemical industries, such as ethylene, propylene, and others, show that their production emits approximately 0.4–1.2 kg of CO₂ per kg of feedstock produced by petrochemical cracking processes. Data like this has urged scientists to explore new carbon capture methods that can convert atmospheric CO₂ into chemicals traditionally sourced from oil or natural gas. Such a scheme, together with the use of recyclable feedstock, has the potential to circularize the polymer production chain.

Besides, electrochemical carbon capture offers an attractive opportunity to store renewable energy in a chemical form by producing carbon-based fuels and chemical feedstock from CO₂. The electrochemical reduction of CO₂ is a complex conversion process comprising multiple proton-electron reaction steps, ultimately generating products with high commercial value, like ethylene, methane, or formic acid.

What Are the Limitations for Electrochemical Carbon Capture?

In the traditional fuel cell-based electrochemical reactor designs, electrodes are separated by a polymer membrane. Due to the low specific surface area, these designs possess intrinsic mass transfer limitations between gas-liquid-solid phases, limiting the overall conversion efficiency.  To overcome these issues and enable highly efficient electrochemical CO₂ conversion, researchers have focused their attention on the development of porous gas diffusion metal electrocatalysts.

As one of the most critical components in a CO₂ conversion system, the gas diffusion catalytic electrodes have been the subject of intensive research in previous years. One of the most promising avenues to explore is enhancing the CO₂ selectivity of the catalyst/electrode assembly. The use of CO₂ adsorbing materials as an electrode coating proved a promising approach to improve the selectivity of CO₂ conversion. Unfortunately, the design and synthesis of dedicated compounds with high selectivity and permeability for CO₂ are complex and expensive.

Polymer Coatings Accelerate Electrocatalysis

Recently, a research group led by Dr. Yoshikazu Ito, an associate professor at the University of Tsukuba, demonstrated how an inexpensive polymer coating applied to a porous tin (Sn) electrocatalyst can increase CO₂ conversion efficiency several-fold. Their research was published in the journal ACS Catalysts on  July 26^th, 2021.  

Some polymers like polyethyleneimine (PEI) and polyethylene glycol (PEG) can effectively capture CO₂ molecules. The idea of Dr. Ito's team was that such CO₂-absorbing polymers could effectively increase the number of CO₂ molecules reaching the catalyst surface and create a CO₂-rich local environment. The result will be an accelerated catalytic reaction on the catalyst's surface.

The scientists developed an original deposition method that enabled them to create a special conformal PEG (a low-cost and abundant polymer with a wide range of applications) coating on a porous tin electrode without affecting the porosity of the electrocatalyst. The thickness of the polymer layers was optimized in a series of detailed investigations while monitoring the performance of the polymer-coated metal catalyst.

Optimized Polymer-Coated Metal Catalyst for Future Clean Technology Applications

Conventionally, tin-based catalysts are used to convert CO₂ to formic acid or formate (HCOO⁻). The experiments of Dr. Ito's colleagues demonstrated that the PEG-coated electrocatalyst could produce 1.5 times more formate compared to a similar non-coated porous electrode. Compared to a conventional tin plate electrode, the formate production rate was 24 times higher, with no byproducts present (with 99% formate yield).

Next, the scientist devised a series of experiments to understand how the CO₂ molecules are transferred through the polymer coating and delivered to the catalytically active site. They have compared the CO₂ conversion rates of the PEG-coated catalyst and the same catalyst coated with another CO₂-capturing polymer (PEI), which has a stronger affinity to CO₂. The PEG-coating still outperformed the PEI-coating in terms of CO₂ conversion efficiency.

Further theoretical modeling revealed that PEI-coating held the CO₂ molecules too tightly, whereas PEG-coating provided the right balance between capture, transport, and release at the electrocatalyst surface. Another critical factor in that balance was the morphology of the polymer coating itself. The PEG-coating was too dense and thick, which also had a detrimental effect on CO₂ conversion.

According to the team, the newly-developed polymer coating can be employed to improve the existing electrosynthesis processes where an initial CO₂ adsorption step drives further electrochemical reactions. Such developments enable the production of many other valuable chemicals, such as methane, methanol, ethane, ethanol, and olefins, from atmospheric CO₂.

References and Further Reading

Jeong, S., et al., (2021) Polyethylene Glycol Covered Sn Catalysts Accelerate the Formation Rate of Formate by Carbon Dioxide Reduction. ACS Catalysis 11 (15), 9962-9969. Available at: https://doi.org/10.1021/acscatal.1c02646  [Accessed on September 1^st 2021].

University of Tsukuba (2021) Polymer coating accelerates fuel production [Online] Science Daily. Available at: https://www.sciencedaily.com/releases/2021/08/210805133758.htm [Accessed on September 1^st 2021].

Adamu, A., et al., (2020) Process intensification technologies for CO₂ capture and conversion – a review. BMC Chem. Eng. 2, 2. Available at: https://doi.org/10.1186/s42480-019-0026-4 [Accessed on September 1^st 2021].

Pappijin, C. A. R, et al., (2020) Challenges and Opportunities of Carbon Capture and Utilization: Electrochemical Conversion of CO₂ to Ethylene. Front. Energy Res. Available at: https://www.frontiersin.org/articles/10.3389/fenrg.2020.557466/full [Accessed on September 1^st 2021].

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