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.

In situ x-ray absorption investigations of a heterogenized molecular catalyst and its interaction with a carbon nanotube support.

A highly active heterogenized molecular CO2 reduction catalyst on a conductive carbon support is investigated to identify if its improved catalytic activity can be attributed to strong electronic interactions between catalyst and support. The molecular structure and electronic character of a [Re+1(tBu-bpy)(CO)3Cl] (tBu-bpy = 4,4'-tert-butyl-2,2'-bipyridine) catalyst deposited on multiwalled carbon nanotubes are characterized using Re L3-edge x-ray absorption spectroscopy under electrochemical conditions and compared to the homogeneous catalyst. The Re oxidation state is characterized from the near-edge absorption region, while structural changes of the catalyst are assessed from the extended x-ray absorption fine structure under reducing conditions. Chloride ligand dissociation and a Re-centered reduction are both observed under applied reducing potential. The results confirm weak coupling of [Re(tBu-bpy)(CO)3Cl] with the support, since the supported catalyst exhibits the same oxidation changes as the homogeneous case. However, these results do not preclude strong interactions between a reduced catalyst intermediate and the support, preliminarily investigated here using quantum mechanical calculations. Thus, our results suggest that complicated linkage schemes and strong electronic interactions with the initial catalyst species are not required to improve the activity of heterogenized molecular catalysts.

Dissociation of Pyridinethiolate Ligands during Hydrogen Evolution Reactions of Ni-Based Catalysts: Evidence from X-ray Absorption Spectroscopy.

The protonation of several Ni-centered pyridine-2-thiolate photocatalysts for hydrogen evolution is investigated using X-ray absorption spectroscopy (XAS). While protonation of the pyridinethiolate ligand was previously thought to result in partial dechelation from the metal at the pyridyl N site, we instead observe complete dissociation of the protonated ligand and replacement by solvent molecules. A combination of Ni K-edge and S K-edge XAS of the catalyst Ni(bpy)(pyS)2 (bpy = 2,2'-bipyridine; pyS = pyridine-2-thiolate) identifies the structure of the fully protonated catalyst as a solvated [Ni(bpy)(DMF)4]2+ (DMF = dimethylformamide) complex and the dissociated ligands as the N-protonated 2-thiopyridone (pyS-H). This surprising result is further supported by UV-vis absorption spectroscopy and DFT calculations and is demonstrated for additional catalyst structures and solvent environments using a combination of XAS and UV-vis spectroscopy. Following protonation, electrochemical measurements indicate that the solvated Ni bipyridine complex acts as the primary electron-accepting species during photocatalysis, resulting in separate protonated ligand and reduced Ni species. The role of ligand dissociation is considered in the larger context of the hydrogen evolution reaction (HER) mechanism. As neither the pyS-H ligand nor the Ni bipyridine complex acts as an efficient HER catalyst alone, the critical role of ligand coordination is highlighted. This suggests that shifting the equilibrium toward bound species by addition of excess protonated ligand (2-thiopyridone) may improve the performance of pyridinethiolate-containing catalysts.

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.

Exciton Coherence Length and Dynamics in Graphene Quantum Dot Assemblies.

Exciton size and dynamics were studied in assemblies of two well-defined graphene quantum dots of varying size: hexabenzocoronene (HBC), where the aromatic core consists of 42 C atoms, and carbon quantum dot (CQD) with 78 C atoms. The synthesis of HBC and CQD were achieved using bottom-up chemical methods, while their assembly was studied using steady-state UV/vis spectroscopy, X-ray scattering, and electron microscopy. While HBC forms long ordered fibers, CQD was found not to assemble well. The exciton size and dynamics were studied using time-resolved laser spectroscopy. At early times (∼100 fs), the exciton was found to delocalize over ∼1-2 molecular units in both assemblies, which reflects the confined nature of excitons in carbon-based materials and is consistent with the calculated value of ∼2 molecular units. Exciton-exciton annihilation measurements provided the exciton diffusion lengths of 16 and 3 nm for HBC and CQD, respectively.

Electron Transfer Kinetics at Graphene Quantum Dot Assembly Electrodes.

Electrochemical performance of nanostructured carbon electrodes was evaluated using cyclic voltammetry and a simple simulation model. The electrodes were prepared from soluble precursors by anodic electrodeposition of two sizes of graphene quantum dot assemblies (hexabenzocoronene (HBC) and carbon quantum dot (CQD)) onto a conductive support. Experimental and simulated voltammograms enabled the extraction of the following electrode parameters: conductivity of the electrodes (a combination of ionic and electronic contributions), density of available electrode states at different potentials, and tunneling rate constant (Marcus-Gerischer model) for interfacial charge transfer to ferrocene/ferrocenium (Fc/Fc+) couple. The parameters indicate that HBC and CQD have significant density of electronic states at potentials more positive than -0.5 V versus Ag/Ag+. Enabled by these large densities, the electron transfer rates at the Fc/Fc+ thermodynamic potential are several orders of magnitude slower than those commonly observed on other carbon electrodes. This study is expected to accelerate the discovery of improved synthetic carbon electrodes by providing fast screening methodology of their electrochemical behavior.

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.

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.