Enhanced Molecular CO₂ Electroreduction Enabled by a Flexible Hydrophilic Channel for Relay Proton Shuttling

Abstract

The effects of primary and second coordination spheres on molecular electrocatalysis have been extensively studied, yet investigations of third functional spheres are rarely reported. Here, an electrocatalyst (ZnPEG8T) was developed with a hydrophilic channel as a third functional sphere that facilitates relay proton shuttling to the primary and second coordination spheres for enhanced catalytic CO2 reduction. Using foot‐of‐the‐wave analysis, the ZnPEG8T catalyst displayed CO2‐to‐CO activity (TOFmax) thirty times greater than that of the benchmark catalyst without a third functional sphere. A kinetic isotopic effect (KIE) study, in conjunction with voltammetry and UV/Vis spectroscopy, uncovered that the rate‐limiting step was not the protonation step of the metallocarboxylate intermediate, as observed in many other molecular CO2 reduction electrocatalysts, but rather the replenishment of protons in the proton‐shuttling channel. Controlled‐potential electrolysis using ZnPEG8T displayed a faradaic efficiency of 100 % for CO2‐to‐CO conversion at −2.4 V vs. Fc/Fc+. A Tafel plot was also generated for a comparison to other reported molecular catalysts. This report validates a strategy for incorporating higher functional spheres for enhanced catalytic efficiency in proton‐coupled electron‐transfer reactions.

Outer-coordination sphere in multi-H+/multi-e–molecular electrocatalysis

“…Our group studied the effects of PEG units in the higher coordination sphere, in conjunction with eight proton-donating triazole groups in the second coordination sphere (ZnPEG3T8 in Figure 6B). (Williams et al, 2020) The catalytic activity of ZnPEG3T8 was 30-fold higher than that of the control compound with alkyl chains in the higher coordination sphere (ZnC8T8). Another control experiment using a PEGylated, triazole-free complex (ZnPEG3T0) a Estimated based on the reported data as described in Table 2.…”

“…Several hydrophilic groups have been utilized to achieve this shuttling effect, including phenol (Margarit et al, 2018;Nichols et al, 2020;Sinha and Warren, 2018), triazole (Lashgari et al, 2020;Sen et al, 2019), polyethylene glycol (PEG) (Chaturvedi et al, 2021;Lashgari et al, 2020), imidazole (Sung et al, 2019), urea (Gotico et al, 2020;Haviv et al, 2018), and amine (Jakobsen et al, 2021). The relay can be as short as a single OH group in the second coordination sphere, as achieved using an asymmetric Fe-porphyrin (Amanullah et al, 2021), (B) Fe-porphyrin bearing eight pendant proton relays (FeTPPOH 8 ) (Costentin et al, 2012), (C) Co-bipyridine with a bridging pendant amine group (CoBpy-amine) (Chapovetsky et al, 2018), (D) Zn-porphyrin containing 8 triazole units each with PEG proton relays (Williams et al, 2020), (E) Feporphyrin with a single PEG proton relay (Chaturvedi et al, 2021), (F) Mn-bipyridine with imidazole in the second coordination sphere (Sung et al, 2019).…”

“…Figure 4. Enzymatic and synthetic CO 2 RR models (A) Active site of [NiFe]CO dehydrogenases(Amanullah et al, 2021), (B) Fe-porphyrin bearing eight pendant proton relays (FeTPPOH 8 )(Costentin et al, 2012), (C) Co-bipyridine with a bridging pendant amine group (CoBpy-amine)(Chapovetsky et al, 2018), (D) Zn-porphyrin containing 8 triazole units each with PEG proton relays(Williams et al, 2020), (E) Feporphyrin with a single PEG proton relay(Chaturvedi et al, 2021), (F) Mn-bipyridine with imidazole in the second coordination sphere(Sung et al, 2019).…”

Electrodeposited Copper Nanocatalysts for CO2 Electroreduction: Effect of Electrodeposition Conditions on Catalysts’ Morphology and Selectivity

“…Additionally, the catalytic activity of DAT-derived electrodes may be also linked with the partial incorporation of DAT molecules into the deposit structure, as observed both from EDX and XPS analyses (Figures 4 and 5b). Catalytic activity towards ERCO2 was previously observed on azole-metal complexes [41] so the simultaneous catalytic action of metallic copper and DAT-copper compounds cannot be excluded. Catalytic properties of DAT-mediated electrodeposited electrodes were already observed in literature [27] but those structures were only deposited on to flat supports such as metallic foils or carbon nanoparticles sheets.…”

Hydrodechlorination of Dichloromethane by a Metal-Free Triazole-Porphyrin Electrocatalyst: Demonstration of Main-Group Element Electrocatalysis

“…Therefore, we hypothesized that the inner nitrogen of free-base porphyrin represented a site for substrate binding, while the triazole units formed a second coordination sphere for proton delivery, as was previously observed for other triazole systems. [37][38][39][40] Both control compounds exhibited no traces of liquid products in the corresponding 1 H NMR spectra (Figs. S14 and S15).…”