Mechanistic Studies of Electrode-Assisted Catalytic Oxidation by Flavinium and Acridinium Cations

Small Organic Molecular Electrocatalysts for Fuels Production

In recent years, heterocyclic organic compounds have been explored as molecular electrocatalysts in relevant reactions for energy conversion and storage. Merging mimetics of biological systems that perform hydride transfer with rational synthetic chemical design has opened many opportunities for organic molecules to be tuned at the atomic level conferring them interesting reactivities. These molecular electrocatalysts represent an alternative to traditional metallic materials and metal complexes employed for water oxidation, hydrogen production, and carbon dioxide reduction. This minireview describes recent reports concerning design, catalytic activity and the mechanism of synthetic molecular electrocatalysts towards solar fuels production.

Triarylmethyl cation redox mediators enhance Li-O2 battery discharge capacities

A major impediment to Li-O2 battery commercialization is the low discharge capacities resulting from electronically insulating Li2O2 film growth on carbon electrodes. Redox mediation offers an effective strategy to drive oxygen chemistry into solution, avoiding surface-mediated Li2O2 film growth and extending discharge lifetimes. As such, the exploration of diverse redox mediator classes can aid the development of molecular design criteria. Here we report a class of triarylmethyl cations that are effective at enhancing discharge capacities up to 35-fold. Surprisingly, we observe that redox mediators with more positive reduction potentials lead to larger discharge capacities because of their improved ability to suppress the surface-mediated reduction pathway. This result provides important structure-property relationships for future improvements in redox-mediated O2/Li2O2 discharge capacities. Furthermore, we applied a chronopotentiometry model to investigate the zones of redox mediator standard reduction potentials and the concentrations needed to achieve efficient redox mediation at a given current density. We expect this analysis to guide future redox mediator exploration.

Efficient Voltage-Driven Oxidation of Water and Alcohols by an Organic Molecular Catalyst Directly Attached to a Carbon Electrode

The integration of heterogeneous electrocatalysis and molecular catalysis is a promising approach to designing new catalysts for the oxygen evolution reaction (OER) and other processes. We recently showed that the electrostatic potential drop across the double layer contributes to the driving force for electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. Here, we report high current densities and low onset potentials for water oxidation attained using a metal-free voltage-assisted molecular catalyst (TEMPO). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine faradic efficiencies for the generation of H₂O₂ and O₂. The same catalyst was employed for efficient oxidations of butanol, ethanol, glycerol, and H₂O₂. DFT calculations show that the applied voltage alters the electrostatic potential drop between TEMPO and the reactant as well as chemical bonding between them, thereby increasing the reaction rate. These results suggest a new route for designing next-generation hybrid molecular/electrocatalysts for OER and alcohol oxidations.

Toward a mechanistic understanding of electrocatalytic nanocarbon

Electrocatalytic nanocarbon (EN) is a class of material receiving intense interest as a potential replacement for expensive, metal-based electrocatalysts for energy conversion and chemical production applications. The further development of EN will require an intricate knowledge of its catalytic behaviors, however, the true nature of their electrocatalytic activity remains elusive. This review highlights work that contributed valuable knowledge in the elucidation of EN catalytic mechanisms. Experimental evidence from spectroscopic studies and well-defined molecular models, along with the survey of computational studies, is summarized to document our current mechanistic understanding of EN-catalyzed oxygen, carbon dioxide and nitrogen electrochemistry. We hope this review will inspire future development of synthetic methods and in situ spectroscopic tools to make and study well-defined EN structures.

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

Cocatalysis is a promising approach toward enhanced electrocatalytic activity. We report such synergic catalysis involving organic xanthylium-based catalyst, Xan²⁺, and oxides formed on the electrode surface. The oxygen evolution reaction (OER) was observed on some working electrodes (gold, platinum, glassy carbon, boron-doped diamond), while others (titanium and fluorine-doped tin oxide) exhibited no OER activity. On the basis of experimental data and supported by calculations, we propose a mechanism in which oxidized Xan²⁺ activates electrode toward the rate-determining O-O bond formation. In light of our findings, efficient OER electrocatalysis can be achieved using materials that strongly bind oxygen species and electron-deficient organic cations.

Novel Fully Organic Water Oxidation Electrocatalysts: A Quest for Simplicity

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.

Ultrafast Spectroscopy Reveals Electron-Transfer Cascade That Improves Hydrogen Evolution with Carbon Nitride Photocatalysts

Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C₃N₄) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C₃N₄ electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 μmol h^-1 g^-1 when we infuse a suspension of bulk g-C₃N₄ with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-C₃N₄ at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-C₃N₄ decays with a time constant of 1/k_e' = 660 ps in the mixture versus 1/k_e = 4.1 ns in g-C₃N₄ alone. Our TA measurements suggest that the charge generation efficiency in g-C₃N₄ is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-C₃N₄, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts.

Metal-Free Motifs for Solar Fuel Applications

Metal-free motifs, such as graphitic carbon nitride, conjugated polymers, and doped nanostructures, are emerging as a new class of Earth-abundant materials for solar fuel devices. Although these metal-free structures show great potential, detailed mechanistic understanding of their performance remains limited. Here, we review important experimental and theoretical findings relevant to the role of metal-free motifs as either photoelectrodes or electrocatalysts. First, the light-harvesting characteristics of metal-free photoelectrodes (band energetics, exciton binding energies, charge carrier mobilities and lifetimes) are discussed and contrasted with those in traditional inorganic semiconductors (such as Si). Second, the mechanistic insights into the electrocatalytic oxygen reduction and evolution reactions, hydrogen evolution reaction, and carbon dioxide reduction reaction by metal-free motifs are summarized, including experimental surface-sensitive spectroscopy findings, studies on small molecular models, and computational modeling of these chemical transformations.