Electrochemical CO₂ reduction (CO2RR) holds immense potential for mitigating climate change by converting CO₂ into valuable chemicals and fuels. Over the last decade, significant advancements in research and development have accelerated this field with the discovery of new catalysts and new process concepts. Advances in electrolyzer designs and operating conditions have improved reaction rates, product yields and overall energy efficiency. These developments have brought CO2RR closer to industrial-scale implementation, offering a promising pathway to a carbon-neutral future.

Here the Redox-Flow electrochemical flow cells are unique because they are designed as building blocks where different electrodes, membranes, materials, process concept and active areas can be tested built from the same platform.

Below is are some considerations on materials selections followed by examples Redox-Flow electrochemical CO₂ electrolysers.

Materials for CO₂ electrolysers

Material choices for the components in the CO₂ reduction cells are extremely important. Several considerations have to be taken into account and is in the end a trade-off between different parameters. Not considering catalysts, the typical materials (current collectors and electrodes) used are stainless steel (316 L), titanium, nickel and graphite based composites. The overall considerations are

Side reactions: For the CO₂ reducing side of the cell it is preferable to use materials that do not catalyse H2 evolution (HER) as this will be a competing parasitic reaction. Here typically stainless steel and nickel are relatively good catalysts for HER, while titanium and carbon has high overpotential for HER and are the preferred materials for both current collectors and porous electrodes.

Corrosion due to pH: Depending on the exact CO₂ reduction reaction process, it can be in liquid phase under different pH conditions ranging from acidic to alkaline. Here it is only titanium and graphite that are corrosion resistant in the full pH range, while stainless steel and nickel are only stable from neutral/mild alkaline to strongly alkaline solutions. Typically, the corrosion stability of materials can be seen from their respective Pourbaix diagrams, however, they are typically calculated based on thermodynamic properties and do not take the reactions kinetics/rates into account. However, more importantly during electrolysis processes the local pH close to the electrode surface will change due to concentration polarisation. Or in more simple terms, the CO₂ reducing side consumes protons whereby the local environment becomes more alkaline. On the oxygen evolution side (OER) hydroxide ions are consumed or protons are produced, whereby the local pH close to the electrode or bipolar plate surface decreases. As an example, a typical Pourbaix diagram for nickel suggest that it is stable in pH down to 7. However, if it is used as electrode for oxygen evolution it will most likely corrode do the pH in the electrode surface is decreased into the acidic range where it dissolves into the solution. Setting exact pH ranges is impossible as the corrosion rates depends on many parameters such as temperature, operation conditions (high/low electrical currents), flow rates, formulation of electrolyte etc.

Anodic protection: As can also be seen from Pourbaix diagrams, all materials becomes stable against corrosion if a sufficiently low potential is applied to them. This can be exploited for materials for the negative side, where the negative potential can lead to ‘anodic protection’. As an example, nickel is not stable in acidic solutions, however, if a potential of about -0.5 V_SHE is applied, it becomes anodically protected. However, this protection will only work if the potential is applied constantly (which can be difficult), so it is in general not recommended to use anodic protection to reduce corrosion.

Oxidative stress: On the positive side the materials can also be corroded by oxidative stress due to the applied positive potential that will oxidise the materials. Here, typically all metals (stainless steel, nickel and titanium) forms protective metal oxide layers, while graphite is prone to corrosion. However, the rate of graphite corrosion depends on the applied half cell potential (e.g. vs SHE) and can in some cases be used as bipolar plate material and electrode (but only for relatively low potentials)

The above considerations have been compiled into the table below where the green is recommend materials for the specific condition, red are not recommended, while black are possible to use it. It is noted that nickel and stainless steel are not recommended as electrode material for the negative side as they catalyse parasitic HER. Under certain conditions they can be used as current collectors (as the surface area here in many cases is much smaller than that of the electrodes). As an example, a nickel current collector can in some cases be combined with a carbon based electrode.

Below are three examples of CO₂ reduction cells that can be built from Redox-Flow components. The examples are inspired by some the cell concepts from the following articles (https://doi.org/10.1016/j.cherd.2024.07.014, https://doi.org/10.1016/j.jcou.2019.09.007, https://doi.org/10.1016/j.isci.2022.104011 )They have been divided into 2, 3 and 4 compartment cells and all come with Titanium current collectors with flow field. Titanium is an all-round material that is corrosion resistant in almost full pH range. However, it can also be replaced with a carbon based interface (from the S-cell) on e.g. the negative side only. Also current collectors with flat surface can be used (e.g. for thicker electrodes).

Example 1 – Standard two compartment cell

This example is based on our X-cell with additional ports. In the negative side CO₂ is fed to a (thin) metal or carbon based electrode coated with CO2R catalyst. Here the gas phase products (e.g. CH₄) leaves the cell from the outlet. The positive side is constructed similarly with a (thin) electrode coated with oxygen evolution reaction (OER) catalyst, where O₂  is produced from H₂O. The two chambers are separated by an ion exchange membrane where ions are transported from one to the other side to ensure charge balance. For both sides a titanium current collector with flow field is used.

The example is shown with gas phase reactants, however, it can equally well be used for liquid phase reactants. E.g. carbonated (CO3^2-) dissolved in (mild) alkaline solutions.

If the cell with additional ports is used, it is possible to form a Luggin capillary (shown by the red dotted line going through the cell) to the membrane surface inside the cell. It can be used for measuring the potential between the membrane and the negative side current collector, which is typically associated with overpotential for CO₂ reduction. Here Redox-Flow also offers an electrode holder that will make the measurement easy.

Example 2 – Three compartment without membranes

Some of the shortcomings of using the two compartment cell is that liquid phase products (e.g. formic acid HCOOH) continuously will concentrate in the electrode (if gas phase CO₂ is used). Depending on the membrane is can also cross-over to the positive side where it potentially will be oxidized back to CO₂.

To mitigate this (to the best of Redox-Flow’s knowledge) the three compartment CO2 reduction cell was developed. Here gas phase CO2 is reduced on Gas Diffusion Electrode (GDE) coated with a catalyst coated gas/liquid interphase. CO2 is reduced into a water soluble molecule that diffuses into the liquid phase of a central chamber of  the cell. On the electrochemically positive side of the cell oxygen is produced through a catalyst coated GDE.

The Redox-Flow concept for this cell is based on a X-cell with additional ports to form a three compartment cell. The hydraulic circuit is shown by the red dotted line and enters/exits at the end of the positive side. The outer compartments (half-cell) are for electrochemical CO2 reduction and oxygen evolution reactions, while the central chamber supplies water for the reactions and also becomes the solvent for the reduced CO2 molecule. The GDEs are kept tight from leaking to the surrounding of the cell by a ‘GDE gaskets’, while the internal leak integrity is provided by sealing the GDE together with the ‘GDE gasket’ with a thin tape.

As for the other examples, there is full freedom to choose other current collector materials (nickel, stainless steel and carbon based) with and without flow field. However, the most important is to underline that it can the gas diffusion electrodes can also be replaced with ‘normal’ electrodes and membranes. The only difference being slightly different gaskets.

For three compartment cells a reference electrode can be connected to the central chamber liquid stream by the Redox-Flow flow though electrode holder. This can be used for measuring the CO₂ reduction overpotential.

Example 3 – Four compartment cell with both GDEs and membranes

This example is a four compartment cell based on the X-cell with additional ports. The two outermost compartments (half-cells) are gas-phase for CO2 reduction and OER, respectively. These two compartments are separated by GDEs with catalyst coating for CO2 reduction and OER, respectively. Gas phase products (e.g. CH₄ and O₂) exits through the two outmost chambers. The hydraulic circuit of the two inner chambers are shown by the red dotted lines. The negative side central chamber exits on the negative and vice versa. The two central chambers are separated by an ion exchange membrane.

By Redox-Flow it is anticipated that the four compartment cell is a further development of the three compartment cell. Here the liquid products enters into the single central chamber and can then be oxidized back to CO₂ when it gets in contact with the positive side GDE. In the four compartment cell this is mitigated by including a membrane that effectively ensures that the carbon products do not get in contact the the positive side GDE.

As for the other cells, the GDEs are kept leak free by a special gasket that is sealed with tape. Also any of the GDEs can be replaced with standard electrodes (e.g. carbon paper) and ion exchange membrane by using different gaskets. Also there is full flexibility on the choice of materials.

As for the three-compartment cell, a reference electrode can be connected to the central chamber (negative side chamber) liquid stream by the Redox-Flow flow though electrode holder. This can be used for measuring the CO₂ reduction overpotential.