Alkali metal cation effects on electrocatalytic CO2 reduction with iron porphyrins

Developing highly efficient electrocatalysts for the CO₂ reduction reaction has attracted increasing interests in the past decade. Fe porphyrins have been shown to be highly efficient and selective for the CO₂-to-CO conversion. Extensive efforts have been made recently to investigate structural effects of Fe porphyrins on the CO₂ reduction reaction (CO₂RR), and fundamental knowledge of structure-activity relationships has been obtained. For example, it is suggested that intramolecular proton relay sites and also electrostatic interaction substituents can significantly improve catalytic efficiency for CO₂RR. Despite these achievements, however, the effect of alkaline metal cations on CO₂RR has been rarely studied, although practical CO₂RR electrocatalysis is required to be carried out in aqueous solutions containing electrolytes of alkaline metal cations, such as sodium and potassium.

Recently, a research team led by Prof. Rui Cao from Shaanxi Normal University, China reported the effect of alkaline metal cations, including Na⁺ and K⁺, on electrocatalytic CO₂ reduction reaction using Fe porphyrins. The results were published in Chinese Journal of Catalysis.

First, the authors synthesized a simple iron porphyrin: tetra-(3,4,5-trimethoxyphenyl)-iron porphyrin (FeP). The structure of FeP was characterized by nuclear magnetic resonance, mass spectrometry and single crystal X-ray diffraction. By testing the CO₂RR activities in a DMF solution containing electrolyte, the experimental results showed that FeP can achieve CO₂RR with high efficiency. The catalytic currents increased linearly with FeP, suggesting a first-order dependence of the catalytic rates on the concentrations of FeP. Controlled potential electrolysis confirmed the stability of FeP by functioning as a CO₂RR electrocatalyst, showing stable currents during the electrolysis and almost identical UV-vis spectra of FeP before and after electrolysis. The evolved CO was analyzed by using gas chromatography, giving a Faradaic efficiency of 95% for the CO₂-to-CO conversion.

Based on the backdrop, the authors found that the addition of Na⁺ and K⁺ can significantly improve the catalytic activity, and the addition of K⁺ makes the catalytic current increase more significantly. This may be due to the fact that the migration speed of K⁺ in the solution is faster than that of Na⁺. Based on this experimental phenomenon, the authors synthesized ^N18C6-FeP, which bears a tethered ^N18C6 group to bind Na⁺ and K⁺ ions. The results showed that with the tethered ^N18C6 to bind Na⁺ and K⁺, ^N18C6-FeP is more active than FeP for electrocatalytic CO₂RR. Particularly, by binding Na⁺ for intramolecular catalysis, the CO₂RR activity can be significantly boosted compared to that with free Na⁺ ions. This work thus provides new insights into the design of efficient catalysts for CO₂RR, which will be also valuable for designing catalysts of other small molecule activation reactions.

###

This work was supported by the Natural Science Foundation of China (Grant Nos. 21573139 and 21773146), Fok Ying-Tong Education Foundation for Outstanding Young Teachers in University, Fundamental Research Funds for the Central Universities, and Research Funds of Shaanxi Normal University.

About the Journal

Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top six journals in Applied Chemistry with a current SCI impact factor of 6.146. The Editors-in-Chief are Profs. Can Li and Tao Zhang.

At Elsevier http://www.journals.elsevier.com/chinese-journal-of-catalysis

Manuscript submission https://mc03.manuscriptcentral.com/cjcatal

---

---