Novel Fully Organic Water Oxidation Electrocatalysts: A Quest for Simplicity

Abstract

Despite the growing need for readily available and inexpensive catalysts for the half-reactions involved in water splitting, water oxidation and reduction electrocatalysts are still traditionally based on noble metals. One long-standing challenge has been the development of an oxygen evolution reaction catalyzed by easily available, structurally simple, and purely organic compounds. Herein, we first generalize the performance of the known N-ethyl-flavinium ion to a number of derivatives. Furthermore, we demonstrate an unprecedented application of different pyridinium and related salts as very simple, inexpensive water oxidation organocatalysts consisting of earth-abundant elements (C, H, O, and N) exclusively. The results establish the prospects of heterocyclic aromatics for further design of new organic electrocatalysts for this challenging oxidation reaction.

Cocatalysis: Role of Organic Cations in Oxygen Evolution Reaction on Oxide Electrodes

“…12 More recently, Bachmann and co-workers observed OER catalysis by pyridinium ions (MePy + , MePyz + , and MePym + , Scheme 1) and found that the catalytic efficiency directly correlates with the electrophilic character of the iminium ion (the best behavior was observed with MePy + ). 13 Considering that the electrophilic nature of iminium cations appears to play a crucial role in cocatalysis, we extended our investigation to an electron-deficient xanthylium cation Xan 2+ (Scheme 1). Generally, xanthylium cations exhibit low pseudobase pK a , 15 which reflects their Lewis acidity.…”

“…While the oxidation potential of Xan 2+ is significantly higher than the thermodynamic potential for OER, the potential can be lowered using iminium-based cations, such as flavinium or pyridinium systems studied previously. 11,13 The list of oxides that strongly bind *O species is available through computational 24,25 and experimental 29 studies. In synergy with Lewis acid cocatalysts presented in Scheme 2, these oxides are expected to show accelerated OER activity.…”

“…The performance of heterogeneous electrocatalysts can be modulated using molecular cocatalysts to lower the overpotential or to improve the product selectivity. − For example, electrochemical reduction of carbon dioxide on metal electrodes produces carbon monoxide with high selectivity if imidazolium cations are used as cocatalysts, − while the formation of methanol is favored if pyridinium ions are employed. − Ongoing studies provide crucial insights into the mechanism of these cocatalysts, − but general consensus is yet to be achieved. The electrocatalytic OER can also be affected by organic cocatalysts. − Our group has shown that flavinium and acridinium cations (Et-Fl + and Acr + , Scheme ) , both exhibit electrode-dependent behavior, but while Et-Fl + significantly accelerates OER, Acr + inhibit electrocatalysis. On the basis of spectroelectrochemical studies and observed electrode dependence, it was hypothesized that hydroxylated flavin (Et-FlOH) interacts with the oxides formed on the electrode surface.…”

“…It is possible that inhibition of OER by Acr + is due to the lower electrophilic character of Acr + . More recently, Bachmann and co-workers observed OER catalysis by pyridinium ions (MePy + , MePyz + , and MePym + , Scheme ) and found that the catalytic efficiency directly correlates with the electrophilic character of the iminium ion (the best behavior was observed with MePy + ) …”

Investigating the Photochemistry of C7 and C8 Functionalized N(5)-Ethyl-flavinium Cation: A Computational Study

“…The redox and photochemical versatility of flavins has been extensively exploited for their use outside of a biological environment as molecular catalysts in organic synthesis, , targeted drug delivery, , sensing applications, ,, optoelectronics, ,, and as catalysts for water oxidation. − Moreover, synthetic flavins have gained an elevated interest as functionalization of the flavin isoalloxazine backbone provides insight to, and possible improvement of, the flavin redox and photochemical properties. The C6-C9 positions on the benzene moiety and the N3 and N10 positions on the isoalloxazine unit have been the primary areas for functionalization and have been shown to alter the photophysical properties of flavins (see Scheme for numbering).…”