Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes

Abstract

Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag–S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverage and less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not only strictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

Thiophene‐Based Conjugated Acetylenic Polymers with Dual Active Sites for Efficient Co‐Catalyst‐Free Photoelectrochemical Water Reduction in Alkaline Medium

“…Orientation effects on this peak cannot be excluded. [41] Noteworthy, the 2181 cm −1 peak downshifted by approximately 20 cm −1 (2161 cm −1 at 0.4 V, Figure S24b). This drastically lowered the energy required to break the −C≡C− bond reaching an even lower value than pDET /Cu (2170 cm −1 ), thus explaining the highest HER of pDTT /Cu amongst the CAPs in alkaline medium.…”

“…Changes in the diacetylenic peaks were also observed (green highlights) with alternating increase in the intensities of the −C≡C−/Cu (1933 cm −1 ) and −C≡C− (2181 cm −1 ) vibrations, mainly in the potential region of 0.4–0.2 V (Figure 4 d). Orientation effects on this peak cannot be excluded [41] . Noteworthy, the 2181 cm −1 peak downshifted by approximately 20 cm −1 (2161 cm −1 at 0.4 V, Figure S24b).…”

Molecular Control of Carbon‐Based Oxygen Reduction Electrocatalysts through Metal Macrocyclic Complexes Functionalization

“…The most common conventional modification strategy involves simply adsorbing the MMC on a glassy carbon or pyrolytic graphite electrode by drop‐casting or immersion. [ 146,147 ] For example, Ye et al. [ 148 ] irreversibly adsorbed cobalt porphyrins prepared with different substituents on pyrolytic graphite by impregnation to study the effect of introducing a π–π conjugated system to cobalt porphyrin molecules on the ORR catalytic activity.…”

Enzyme‐Inspired Iron Porphyrins for Improved Electrocatalytic Oxygen Reduction and Evolution Reactions

“…For example, efficient electron transfer to Fe porphyrin sites and proper hydrogen bonding interactions with Fe‐O 2 adducts can facilitate the 4e reduction of O 2 to water [11, 19–24] . However, despite these achievements in improving selectivity, Fe porphyrins in general showed moderate electrocatalytic ORR activity with half‐wave potential <0.80 V versus reversible hydrogen electrode (RHE) [10, 11, 14, 25, 26] . Thus, ORR activity improvement of molecular Fe porphyrins is required.…”