Lecture

Van Marum Colloquium: High-Rate CO2 Electrolysis: From Electrocatalysts to Electrolyzers / Multiple reactivity descriptors for the Catalytic Activity of Molecular Catalysts

Prof. Kazuhide Kamiya (Osaka University), Prof. José Zagal (University of Santiago de Chile)

Date: Monday 15 September 2025
Time: 15:00 - 17:00
Location: Gorlaeus Building
Einsteinweg 55
2333 CC Leiden
Room: CM.3.23

Kazuhide Kamiya: High-Rate CO2 Electrolysis: From Electrocatalysts to Electrolyzers

CO₂ electrolysis to produce value-added products is a promising technology for closing the carbon cycle and converting anthropogenic CO₂ into chemical feedstocks. Increasing the current density for multi-carbon products is essential for practical implementation.¹ The use of gas diffusion electrodes (GDEs) facilitates CO₂ reduction reactions (CO₂RR) at the solid catalyst/liquid electrolyte/gaseous CO₂ triple-phase interface. This approach effectively accelerates CO₂RR by overcoming mass transport limitations caused by the inherently low diffusion and solubility of CO₂ in aqueous electrolytes. However, CO₂RR at the three-phase interface is a complex process, and comprehensive guidelines for catalyst design to achieve high activity are yet to be fully established. This presentation summarizes our recent studies on high-rate CO₂ reduction, encompassing perspectives ranging from electrocatalysis, GDEs, to electrochemical cells.

We have employed various single metal-doped covalent triazine frameworks (M-CTFs) as platforms for CO₂RR electrocatalysts on GDEs and systematically investigated them to derive sophisticated design principles using a combined computational and experimental approach.^2-4 The molecular-scale design of M-CTFs can affect CO₂RR activity with a current density greater than 100 mA/cm². In addition to sophisticated design of electrocatalysts, enlarging the three-phase interface at the GDE is essential for high-rate gaseous CO₂RR. We successfully increased the partial current density for multicarbon products (C₂₊) over cupric oxide nanoparticles on gas diffusion electrodes in neutral electrolytes to a record value of j_C2+=1.8 A/cm² by maximizing the area of the CO₂RR active interface.^5-8 Furthermore, we investigated the operating principles of membrane electrode assembly electrolyzers (MEAs) with anion exchange membrane electrolytes for CO₂RR. As a result, we clarified that alkali metal cations crossing over from the anolyte to the cathode surface play a critical role in C₂₊ generation in the MEAs.^9,10

References

K. Kamiya et al. Chem. Lett. 2021, 50, 166.
S. Kato, K. Kamiya et al. Chem. Sci., 2023, 14, 613
P. Su, K. Kamiya et al., Chem. Sci., 2018, 9, 3941
K. Kamiya Chem. Sci. 2020, 11, 8339–8349.
A. Inoue, K. Kamiya et al. EES Catal. 2023, 1, 9.
T. Liu, K. Kamiya et al. Small 2022, 18, 2205323.
R. Kurihara, K. Kamiya et al. Adv. Mater. Interfaces, 2024, 11, 2300731.
A. Inoue, K. Kamiya et al. Small. 2025, 21, 2500693.
S. Kato, K. Kamiya et al. ChemSusChem, 2024, 17, e202401013.
R. Kurihara, K. Kamiya et al. EES Catal., 2025, accepted.

José Zagal: Multiple reactivity descriptors for the Catalytic Activity of Molecular Catalysts

The binding energy of intermediates to the active sites is a well know reactivity descriptor in electrocatalysis, especially metal electrodes. However, for MN₄ or MN_x molecular catalysts for ORR several reactivity descriptors have been proposed: (i) The M-O₂ binding energy, (ii) the M(III)OH/(II) redox potential, (iii) the number of d electrons and the donor (M)-acceptor intermolecular hardness. The activity (log j)_E for ORR at constant potential plotted versus the binding energy and versus E°’M_(III)/(II) have both the shape of a volcano. However, when (log TOF)_E is used as a measure of the activity the volcano becomes a linear correlation, with a slope ca. -0.120 V/decade.

In this work we report a new reactivity descriptor: the electrochemical hardness, DEºh, which is the gap between the Eº_M(II)/(I)and the Eº_M(III)/(II)redox processes for FeN4 and CoN4 complexes. A plot of (log j)_E (current density at constant potential) versus DEºh give linear correlations for FeN4 and CoN4 for ORR in alkaline media. For FeN4 these correlations are observed in a wide range of pH. In all case the activity decreases as DE^h increases. This is also observed for the oxidation of 2-mercaptoethanol catalyzed by several Fe phthalocyanines, suggesting that this descriptor could be universal.