Advancing the Anode Compartment for Energy Efficient CO₂ Reduction at Neutral pH

Abstract

Energy efficient CO 2 reduction in neutral pH requires the development of electrochemical devices operating with minimal energy input. To reach this objective, we propose an original approach focused on the anode compartment where the oxygen evolution reaction (OER) takes place. Different anode catalysts, anolytes and membranes were tested, and the components resulting in the most energy efficient and stable CO 2 reduction reaction device were selected. A stainless-steelbased anode with significant OER activity and stability in a wide pH range was used to compare symmetric (neutral catholyte and anolyte) and asymmetric (neutral catholyte and alkaline anolyte) configurations. The gain in energy efficiency for carbon products of 6-8 % in the asymmetric configuration highlighted the advantages of a pH gradient. Although a Nafion membrane gave the highest energy efficiencies, the pH was unstable. Therefore, bipolar membranes that can maintain a pH gradient emerge as the best option for long-term electrolysis. This work paves the way for the development of highly energy efficient CO 2 -reduction devices with cathodes operating in low pH conditions.

Local Chemical Environment Governs Anode Processes in CO2 Electrolyzers

“…Ni foil, mesh, and foam modified by various methods like etching, laser ablation, , transition metal electrodeposition, − and spray coating of nanoparticles, − are commonly investigated as OER electrodes. Ni has already been employed as anode catalyst in recent studies on CO2RR. − Concentrated electrolyte solutions (e.g., 1 M KOH) were used in most of these studies, and stability was mostly demonstrated in electrolyzer cells operating with liquid catholyte for relatively short time periods. High electrolyte concentration, however, leads to the formation of alkali metal carbonates/bicarbonates in the gas diffusion electrode (GDE) cathode, leading to the clogging of the gas channels and the concurrent selectivity decrease for CO2RR (with the simultaneous increase of the hydrogen evolution reaction (HER) rate). − Ni foam has also been applied in non-zero-gap flow cells with a bipolar membrane or paired with a molecular catalyst for CO2RR. ,, Additionally, NiO nanoparticles were employed to oxidize 5-(hydroxymethyl)­furfural to 2,5-furandicarboxylic acid, paired with CO2RR .…”

“…Ni has already been employed as anode catalyst in recent studies on CO2RR. 24 − 32 Concentrated electrolyte solutions (e.g., 1 M KOH) were used in most of these studies, and stability was mostly demonstrated in electrolyzer cells operating with liquid catholyte for relatively short time periods. High electrolyte concentration, however, leads to the formation of alkali metal carbonates/bicarbonates in the gas diffusion electrode (GDE) cathode, leading to the clogging of the gas channels and the concurrent selectivity decrease for CO2RR (with the simultaneous increase of the hydrogen evolution reaction (HER) rate).…”

“… 28 Ni foam and stainless steel fiber felt were tested in OER under alkaline (1 M KOH, pH = 14) and neutral (1 M potassium phosphate buffer, pH = 7) conditions, and Ni foam was highly unstable compared to stainless steel under neutral conditions. 37 …”

From Nickel Foam to Highly Active NiFe‐based Oxygen Evolution Catalysts

“…The use of a flow setup where electrolytes were recirculated in both anode and cathode compartments enabled minimisation of ohmic losses and efficient removal of bubbles produced at the surface of the electrodes. This work therefore focuses on studying novel OER catalysts not only in a static setup, but also in a flow setup closer to industrial operating conditions, where higher currents can be maintained for longer durations [37,38] . Elemental analysis of the electrolyte by inductively coupled mass spectroscopy (ICP‐MS) confirmed the absence of irreversible Ni and Fe dissolution over the course of the experiment, reflecting the high stability of the surface (Figure S6B).…”

“…Interestingly, the average composition of the resulting material (66.2 % Fe, 22.5 % Ni and 11.3 % Cr) was very close to that of stainless steel (SS), which contains a minimum of 10.5 % of Cr, providing anti‐corrosion properties, and a minimum of 10 % of Ni. As SS is a trimetallic alloy containing transition metals that are known as active centres for OER catalysis (such as Ni and Fe) and since it is a cheap and widely available material, several authors, including ourselves, have considered it as an interesting potential OER catalyst [37,53–56] . The bottom‐up approach and multi‐parameter analysis of the Cr leaching phenomenon presented here enables a deeper understanding of the important characteristics of trimetallic NiFe‐based catalysts.…”