Cyclic voltammetry, square wave voltammetry or electrochemical impedance spectroscopy? Interrogating electrochemical approaches for the determination of electron transfer rates of immobilized redox proteins

In this article, we cross-examine three well-established electrochemical approaches, namely cyclic voltammetry (CV), cyclic square-wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) to dissect the electron transfer (ET) rate of electrostatically immobilized cytochrome c on Ag electrodes. A detailed analysis supported by simulations of redox transition provided three distinct values for the heterogeneous electron transfer (HET) rate constant of cyt c interfaced on COOH-terminated C₁₀-long alkanethiol, i.e., k_HET= 47.8 (±2,91) s^-1 in CV, k_HET= 64.8 (±1,27) s^-1 in SWV, and k_HET= 26.5 s^-1 in EIS. We discuss the obtained discrepancies obtained from electrochemical methods and compare them with the data from spectro-electrochemical experiments. A comprehensive selection list is created from which the most applicable approach can be chosen for studying proteins of interest. CV is most applicable to study the interfaced proteins exhibiting k_HET of ca. 0.5 - 70 s^-1, SWV is suitable for a broader range of k_HET of 5 - 120 s^-1 and EIS for k_HET of 0.5 to 5 s^-1 if alkanethiols are used as immobilization strategy.

Switching Catalyst Selectivity via the Introduction of a Pendant Nitrophenyl Group

The conversion of abundant small molecules to value-added products serves as an attractive method to store renewable energy in chemical bonds. A family of macrocyclic cobalt aminopyridine complexes was previously reported to reduce CO₂ to CO with 98% faradaic efficiency through the formation of hydrogen-bonding networks and with the number of secondary amines affecting catalyst performance. One of these aminopyridine macrocycles, (NH)₁(NMe)₃-bridged calix[4]pyridine (L⁵), was modified with a nitrophenyl group to form L^NO2 and metalated with a cobalt(II) precursor to generate CoL^NO2, which would allow for probing the positioning and steric effects on catalysis. The addition of a nitrophenyl moiety to the ligand backbone results in a drastic shift in selectivity. Large current increases in the presence of added protons and CoL^NO2 are observed under both N₂ and CO₂. The current increases under N₂ are ∼30 times larger than the ones under CO₂, suggesting a change in the selectivity of CoL^NO2 to favor H₂ production versus CO₂ reduction. H₂ is determined to be the dominant reduction product by gas chromatography, reaching faradaic efficiencies up to 76% under N₂ with TFE and 71% under CO₂ with H₂O, in addition to small amounts of formate. X-ray photoelectron spectroscopy (XPS) reveals the presence of a cobalt-containing heterogeneous deposit on the working electrode surface, indicating the addition of the nitrophenyl group reduces the electrochemical stability of the catalyst. These observed catalytic behaviors are demonstrably different relative to the tetra-NH bridged macrocycle, which shows 98% faradaic efficiency for CO₂-to-CO conversion with TFE, highlighting the importance of pendant hydrogen bond donors and electrochemically robust functional groups for selective CO₂ conversion.

Understanding active sites in molecular (photo)electrocatalysis through complementary vibrational spectroelectrochemistry

Highlighting vibrational spectroelectrochemistry for the investigation of synthetic molecular (photo) electrocatalysts for key energy conversion reactions.