Converting emitted carbon dioxide into chemical feedstocks by electrochemical routes

The traditional Metrohm Poster Award was first presented 29 years ago at the Conference for Electroanalytical Chemistry (ELACH). This tradition continues to this day at the Electrochemistry Conference in Berlin, Germany. Most recently, this prize was awarded at Electrochemistry 2022. After a break due to the COVID-19 pandemic, Electrochemistry 2022 was able to continue in Berlin under the motto «At the Interface between Chemistry and Physics». In this conference, scientists from various fields of electrochemistry discuss forward-looking trends and applications. Over 600 scientists came together to speak about research topics in the fields of energy storage, electrocatalysis, CO₂ reduction, bioelectrochemistry, electrosynthesis, corrosion, photoelectrochemistry, electrochemical analytics, and sensor technology.

Winners of the Metrohm Poster Award 2022

Out of over 300 posters presented, the two best posters were selected by the poster committee, which consists of the members of the scientific panel. Both poster prizes are endowed with €500 each and were presented during the award ceremony. 

This article covers the research of one of the two winners, Dr. Gumaa A El-Nagar. Dr. El-Nagar’s poster was titled: «Effects of cation crossover through anion exchange membranes on the operation of zero-gap CO₂ electrolysers».

CO₂ emissions and global warming 

Since the Industrial Revolution, Earth’s atmosphere has experienced rapidly rising levels of carbon dioxide (CO₂) where it acts as a greenhouse gas, trapping heat and contributing to global warming. This increase in CO₂ is primarily due to anthropogenic activities—mainly the combustion of fossil fuels for energy. The impacts from climate change are potentially catastrophic for mankind, especially if certain «tipping points» are exceeded, leading to irreversible change [1]. 

Addressing this threat will require a rapid and massive system change. This would involve a transition to renewable energy sources (e.g., solar and wind) and reinventing industries to become «carbon neutral», meaning that they operate without contributing additional CO₂ to the atmosphere. Currently, our society is highly dependent on carbon-based chemicals and fuels which largely depend on the use of fossil fuels. In particular, the chemical industry contributes approximately 7% of the global greenhouse gas emissions and consumes 10% of global energy [2]. Therefore, developing routes for the sustainable synthesis of chemicals powered by renewable energy sources is a critical challenge.

Achieving carbon neutrality will require that we stop extracting carbon from the ground, and instead develop technologies to recycle carbon that is already available. An attractive source of carbon is CO₂ itself, which could be captured from stationary sources (e.g., factories or power plants) or from the air, and then converted back into useful chemicals and fuels. This approach would help to both prevent further CO₂ emissions and provide the carbon-based chemicals on which we rely.

Electrochemical CO₂ conversion

While burning fuels to release energy produces CO₂, converting CO₂ back into valuable commodities requires the input of energy. One way to do this is to use electrochemistry driven by renewable energy sources. With this technique, electricity facilitates bond breaking and bond formation steps which convert CO₂ into various important small molecules such as hydrocarbons (e.g., ethylene, ethane, methane) and oxygenates (e.g., ethanol, methanol, propanol, acetate, formate) which can be used as chemical feedstocks and fuels. Conventional methods for synthesizing these chemicals usually involve high temperatures and pressures, whereas electrochemical approaches can be operated under milder conditions, with CO₂, water, and electricity as the only inputs.

While promising, the electrochemical CO₂ reduction (eCO₂R) approach suffers from challenges with regards to stability, selectivity, and production rates. At the Helmholtz-Zentrum Berlin, Dr. El-Nagar and his colleagues are working to overcome such challenges in eCO₂R with their research into designing new catalyst materials, using spectroscopy to study reaction mechanisms, and examining various electrolyzer configurations which can generate valuable chemical products at practical rates.

Regarding this last point, in order to feed CO₂ gas to the electrode at high rates – thus enabling industrially relevant current densities – researchers are studying the use of gas diffusion electrodes (GDEs) for this application [3]. Generally speaking, a GDE-based eCO₂R cell is composed of an anode, cathode, and electrolyte. The latter may include a liquid electrolyte and/or an ion-exchange membrane intended to impart selectivity to ion transport through the device. Interfacing the cathode directly with the membrane (without a gap of liquid electrolyte) results in a so-called «zero-gap» configuration, also referred to as a membrane electrode assembly (MEA). This configuration, depicted in Figure 1, helps minimize resistive losses and thus maximizes cell efficiency at high currents [4]. Regarding the choice of membrane, it is generally observed in MEA eCO₂R studies that catalysts interfaced with anion-exchange membranes (AEMs) are better suited to suppress the rates of the competitive and undesired hydrogen evolution reaction.

Researchers studying these types of cells for eCO₂R purposes often observe a common problem: the device can operate in a stable manner for a while, but eventually the performance starts to decrease, accompanied by impeded gas flow. After stopping the reaction and opening the cell, one often finds that salt crystals have formed in the gas flow path and on the GDE (Figure 2). These salts are alkali metal carbonates and bicarbonates resulting from the chemical reaction of the CO₂ gas with alkaline electrolyte. Due to the highly alkaline environment generated at the electrolyzer cathode when operating at high current density, this behavior presents a particular challenge for the design of GDE-based eCO₂R reactors. In addition to the performance degradation due to salt formation, these reactions also result in low CO₂ conversion efficiencies, as a considerable amount of CO₂ is lost in this chemical reaction [5].

Cation crossover through anion-exchange membranes (AEMs)

As stated above, it is very common during investigations of eCO₂R with MEA cells to observe potassium-containing precipitates which often result in cell failure. Various strategies have been proposed for minimizing this problem, including periodically rinsing the electrode, pulsing the cell potential, or modifying the membrane [6]. However, many prior investigations failed to address a fundamental problem: cations readily pass through AEMs under conditions commonly employed in eCO₂R studies. This caused Dr. El-Nagar and his team to wonder why the AEM does not effectively exclude cations (which it is designed to do), and what are the factors influencing this behavior. Perhaps with a better understanding of this behavior, they could develop ways of mitigating the negative impacts of unintentional cation crossover.

In their recently published study in Nature Communications [7], Dr. El-Nagar’s group tested a range of different electrolyte concentrations. They expected that lower concentrations would lead to a smaller degree of cation crossover, reducing the possibility of detrimental formation of carbonate salts at the cathode. The group observed that typical electrolyte concentrations of 0.1 mol/L or greater eventually resulted in salt precipitation and performance degradation, while lower concentrations led to devices which operated stably without salt formation. During the study, the team made a striking discovery when lowering the concentration—the product selectivity drastically changed!

As shown in Figure 3, experiments using more concentrated KOH anolytes led to the production of predominantly C₂₊ products (primarily ethylene), which is typical behavior on copper electrocatalysts. However, when using dilute electrolytes the production of multi-carbon products nearly disappeared, replaced by generation of carbon monoxide (CO) with nearly 80% faradaic efficiency (FE). This led the team to conclude that electrolyte cations, which unintentionally cross through anion-exchange membranes, can significantly impact electrocatalytic CO₂ conversion selectivity, even in zero-gap MEA cells with no discrete liquid electrolyte at the cathode.

Measurements of cations reaching the cathode showed that the degree of ion crossover directly correlated with the anolyte concentration – a behavior which can be explained by the dependence of the Donnan exclusion effect on electrolyte concentration relative to the membrane’s ion exchange capacity. While this behavior is probably no surprise to ion-exchange membrane experts, the team considers this an important lesson to the eCO₂R community: one should not expect an AEM to be very effective at blocking electrolyte cations when using concentrated electrolytes, and the reaction selectivity can be very sensitive to these cations reaching the cathode.

Understanding and controlling product selectivity

Control over product selectivity is one of the key challenges in eCO₂R. The present study revealed how the selectivity can be greatly influenced by simply adjusting the electrolyte concentration, even for MEA configurations without a discrete liquid electrolyte layer at the cathode. The observations suggest that alkali metal cations like K⁺ play a key role in dictating the eCO₂R reaction pathway, whether it follows the two-electron route to form CO or continues on toward more reduced products with C-C bond formation. While others in the research field have shown the importance of cations in aqueous eCO₂R conditions [8,9], this study concludes that these effects are also critical in practical cell configurations.

Many different possible small-molecule products can be formed from electrochemical CO₂ conversion, from single-carbon (C₁) products (e.g., carbon monoxide, formate) to multi-carbon (C₂₊) products (e.g., hydrocarbons, oxygenates). Copper, the catalyst used in this study, is unique as the only metal catalyst capable of reducing CO₂ to C₂₊ products with appreciable reaction rates. However, since Cu tends to produce a mixture of many different products at once, a significant issue is developing methods that can control its product selectivity [10]. The results of the present study show that in zero-gap MEA cells, the effects of alkali metal cations are necessary for activating C₂₊ product pathways, but this can also make the devices susceptible to salt precipitation and failure. When K⁺ crossover was prevented, Cu mainly produced CO. Hence, there is a trade-off between selectivity and stability which must be considered in reactor development.

Nonetheless, although researchers are working hard to design devices capable of high yields of C₂₊ products, recent techno-economic analyses suggest that C₁ products (e.g., CO) produced electrochemically are likely the closest to being economically competitive with conventional production routes [11,12]. The results shown here suggest that Earth-abundant copper may potentially be exploited as a catalyst for CO production – an attractive alternative to silver and gold catalysts typically used for this reaction.