Mediated Fuel Cells: Soluble Redox Mediators and Their Applications to Electrochemical Reduction of O2 and Oxidation of H2, Alcohols, Biomass, and Complex Fuels.

Mediated fuel cells are electrochemical devices that produce power in a manner similar to that of conventional proton exchange membrane fuel cells (PEMFCs). They differ from PEMFCs in their use of redox mediators dissolved in liquid electrolyte to conduct oxidation of the fuel or reduction of the oxidant, typically O2, in bulk solution. The mediators transport electrons (and often protons) between the electrode and the catalysts or chemical reagents in solution. This strategy can help overcome many of the challenges associated with conventional fuel cells, including managing complex multiphase reactions (as in O2 reduction) or the use of challenging or heterogeneous fuels, such as hydrocarbons, polyols, and biomass. Mediators are also commonly used in enzymatic fuel cells, where direct electron transfer from the electrode to the enzymatic active site can be slow. This review provides a comprehensive survey of historical and recent mediated fuel cell efforts, including applications using chemical and enzymatic catalysts.

Oxidative Catalytic Fractionation of Lignocellulosic Biomass under Non-alkaline Conditions.

Biomass pretreatment methods are commonly used to isolate carbohydrates from biomass, but they often lead to modification, degradation, and/or low yields of lignin. Catalytic fractionation approaches provide a possible solution to these challenges by separating the polymeric sugar and lignin fractions in the presence of a catalyst that promotes cleavage of the lignin into aromatic monomers. Here, we demonstrate an oxidative fractionation method conducted in the presence of a heterogeneous non-precious-metal Co-N-C catalyst and O2 in acetone as the solvent. The process affords a 15 wt% yield of phenolic products bearing aldehydes (vanillin, syringaldehyde) and carboxylic acids (p-hydroxybenzoic acid, vanillic acid, syringic acid), complementing the alkylated phenols obtained from existing reductive catalytic fractionation methods. The oxygenated aromatics derived from this process have appealing features for use in polymer synthesis and/or biological funneling to value-added products, and the non-alkaline conditions associated with this process support preservation of the cellulose, which remains insoluble at reaction conditions and is recovered as a solid.

Kinetic and Mechanistic Characterization of Low-Overpotential, H2O2-Selective Reduction of O2 Catalyzed by N2O2-Ligated Cobalt Complexes.

A soluble, bis-ketiminate-ligated Co complex [Co(N2O2)] was recently shown to catalyze selective reduction of O2 to H2O2 with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(N2O2)-catalyzed O2 reduction reaction (ORR) with decamethylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O2 binding stoichiometry and kinetic studies support an O2 reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(N2O2)] and [AcOH], but no dependence on [Fc*] or [O2]. Differential pulse voltammetry and computational studies support CoIII-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR systems, which commonly feature rate-limiting protonation of a CoIII-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the CoIII(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the p Ka values of the protonated CoIII(OOH) species across a series of Co(N2O2) catalysts provides a rationale for the unusually low overpotential observed for O2 reduction to H2O2.

Cooperative Electrocatalytic O2 Reduction Involving Co(salophen) with p-Hydroquinone as an Electron-Proton Transfer Mediator.

The molecular cobalt complex, Co(salophen), and para-hydroquinone (H2Q) serve as effective cocatalysts for the electrochemical reduction of O2 to water. Mechanistic studies reveal redox cooperativity between Co(salophen) and H2Q. H2Q serves as an electron-proton transfer mediator (EPTM) that enables electrochemical O2 reduction at higher potentials and with faster rates than is observed with Co(salophen) alone. Replacement of H2Q with the higher-potential EPTM, 2-chloro-H2Q, allows for faster O2 reduction rates at higher applied potential. These results demonstrate a unique strategy to achieve improved performance with molecular electrocatalyst systems.

Co(salophen)-Catalyzed Aerobic Oxidation of p-Hydroquinone: Mechanism and Implications for Aerobic Oxidation Catalysis.

Macrocyclic metal complexes and p-benzoquinones are commonly used as co-catalytic redox mediators in aerobic oxidation reactions. In an effort to gain insight into the mechanism and energetic efficiency of these reactions, we investigated Co(salophen)-catalyzed aerobic oxidation of p-hydroquinone. Kinetic and spectroscopic data suggest that the catalyst resting-state consists of an equilibrium between a Co(II)(salophen) complex, a Co(III)-superoxide adduct, and a hydrogen-bonded adduct between the hydroquinone and the Co(III)-O2 species. The kinetic data, together with density functional theory computational results, reveal that the turnover-limiting step involves proton-coupled electron transfer from a semi-hydroquinone species and a Co(III)-hydroperoxide intermediate. Additional experimental and computational data suggest that a coordinated H2O2 intermediate oxidizes a second equivalent of hydroquinone. Collectively, the results show how Co(salophen) and p-hydroquinone operate synergistically to mediate O2 reduction and generate the reactive p-benzoquinone co-catalyst.

Quinone 1 e- and 2 e-/2 H+ Reduction Potentials: Identification and Analysis of Deviations from Systematic Scaling Relationships.

Quinones participate in diverse electron transfer and proton-coupled electron transfer processes in chemistry and biology. To understand the relationship between these redox processes, an experimental study was carried out to probe the 1 e- and 2 e-/2 H+ reduction potentials of a number of common quinones. The results reveal a non-linear correlation between the 1 e- and 2 e-/2 H+ reduction potentials. This unexpected observation prompted a computational study of 134 different quinones, probing their 1 e- reduction potentials, pKa values, and 2 e-/2 H+ reduction potentials. The density functional theory calculations reveal an approximately linear correlation between these three properties and an effective Hammett constant associated with the quinone substituent(s). However, deviations from this linear scaling relationship are evident for quinones that feature intramolecular hydrogen bonding, halogen substituents, charged substituents, and/or sterically bulky substituents. These results, particularly the different substituent effects on the 1 e- versus 2 e-/2 H+ reduction potentials, have important implications for designing quinones with tailored redox properties.

Influence of substituents on the through-space shielding of aromatic rings.

A series of naphthalene derivatives, bearing a methyl group and a substituted phenyl ring in a 1,8-relationship, have been synthesized. The chemical shifts of the protons of the methyl group, which are pointed toward the shielding zone of the phenyl ring, were monitored as the phenyl substituents were varied. This work indicates that the shielding effect of the phenyl ring is not so severely altered by the substituents as to significantly influence the chemical shift of the methyl group. Nonetheless, within the small changes observed experimentally, there appears to be a tendency for electron-withdrawing X to shift the methyl signal downfield, whereas electron-donating X-groups cause a more upfield shift. Polarization and field effects are discussed as possible causes for this phenomenon. Chemical shifts computed for selected members of the series, using the recently published procedures of Rablen and Bally, are in agreement with the experimentally observed trends.

Anthraquinone-Mediated Fuel Cell Anode with an Off-Electrode Heterogeneous Catalyst Accessing High Power Density when Paired with a Mediated Cathode.

The development of processes for electrochemical energy conversion and chemical production could benefit from new strategies to interface chemical redox reactions with electrodes. Here, we employ a diffusible low-potential organic redox mediator, 9,10-anthraquinone-2,7-disulfonic acid (AQDS), to promote efficient electrochemical oxidation of H2 at an off-electrode heterogeneous catalyst. This unique approach to integrate chemical and electrochemical redox processes accesses power densities up to 228 mW/cm2 (528 mW/cm2 with iR-correction). These values are significantly higher than those observed in previous mediated electrochemical H2 oxidation methods, including those using enzymes or inorganic mediators. The approach described herein shows how traditional catalytic chemistry can be coupled to electrochemical devices.