The solvent dependence of the formal redox potentials of the ferrocenium/ferrocene (Fc+/0) and 1,2,3,4,5-pentamethylferrocenium/1,2,3,4,5-pentamethylferrocene (Me5Fc+/0) couples versus the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) couple indicates that the latter is a superior redox standard for studying solvent effects on the thermodynamics of electron transfer. The couples were studied in 29 solvents and the differences in formal redox potentials between the Me n Fc+/0 (n = 5, 10) and Fc+/0 couples are surprisingly solvent dependent. In the case of the Fc+/0 couple versus the Me10Fc+/0 couple, the potential difference ranges from +583 mV in 2,2,2-trifluoroethanol to +293 mV in water. The positive shifts for the Me5Fc+/0 couple versus the Me10Fc+/0 couple were about half of these values. The Me10Fc+/0 redox couple can also be used in easily oxidized solvents, such as N-methylaniline and N,N-dimethylaniline, or in conjunction with a Hg working electrode. Statistical multiparameter analysis of the differences in potential versus empirical solvent parameters indicate that the redox potential of the Fc+/0 couple is more solvent dependent than that of the Me5Fc+/0 couple. The latter, in turn, is notably more solvent dependent than that of the Me10Fc+/0 couple. These results contradict the widely used “ferrocene assumption” that the redox potential of the Fc+/0 couple is not very solvent dependent. The data show that the Me10Fc+/0 couple is better suited than the Fc+/0 couple as a redox standard for studies of the thermodynamics of solvation of other redox couples. The data also enable previous measurements using the Fc+/0 couple to be corrected to values referenced against the Me10Fc+/0 couple.

The previously published solvent dependence of the electrochemistry of C60 has been extended and reanalyzed. The potentials have been measured against the decamethylferrocenium/decamethylferrocene (Me10Fc+/0) couple, which is much less solvent-dependent than the ferrocenium/ferrocene (Fc+/0) couple, and hence gives a more accurate reflection of the solvent dependence of the C60 n -/( n +1)- couples. The formal redox potentials of the C60 n -/( n +1)- couples exhibit substantial dependences on the solvent and become more solvent-dependent as the charge increases. This dependence arises from two main interactions with the solvents:  hydrogen-bonding interactions with acidic functionality and electrostatic interactions with solvent dipoles. The strength of the hydrogen-bonding interaction increases as the square of the solute charge due to the increase in basicity of the fullerides. In addition to the contribution of the hydrogen-bonding acidity and the solvent dipoles, the potentials of the C60 0/1- couple are influenced by the Lewis basicities of the solvents. The latter is likely to be due to ion-pair formation between the fullerides and the Bu4N+ ion and/or interactions between the Lewis basicity of the solvent and the Me10Fc+ ion of the reference couple. This interaction is much smaller compared to the solvent/solute interaction. π-Stacking interactions between C60 and aromatic solvents results in a significant contribution of polarizability to the first reduction. The solvent dependences of the fullerenes can be used to model electrode/solvent interactions in which the surface (albeit a distorted one) goes from the point-of-zero-charge (C60) to a variety of discrete negative surface charges (fullerides).

The future of fuel cells that convert chemical energy to electricity relies mostly on the efficiency of oxygen reduction reaction (ORR) due to its sluggish kinetics. By effectively bypassing the use of organic surfactants, the postsynthesis steps for immobilization onto electrodes, catalytic ink preparation using binders, and the common problem of nanoparticles (NPs) detachment from the supports involved in traditional methodologies, we demonstrate a versatile electrodeposition method for growing anisotropic microstructures directly onto a three-dimensional (3D) carbon felt electrode, using platinum NPs as the elementary building blocks. The as-synthesized materials were extensively characterized by integrating methods of physical (thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, inductively coupled plasma, and X-ray photoelectron spectroscopy) and electroanalytical (voltammetry, electrochemical impedance spectrometry) chemistry to examine the intricate relationship of material-to-performance and select the best-performing electrocatalyst to be applied in the model reaction of ORR for its practical integration into a microbial fuel cell (MFC). A tightly optimized procedure enables decorating an electrochemically activated carbon felt electrode by 40-60 nm ultrathin 3D-interconnected platinum nanoarrays leading to a hierarchical framework of ca. 500 nm. Half-cell reactions reveal that the highly rough metallic surface exhibits improved activity and stability toward ORR (E ∼ 1.1 V vs reversible hydrogen electrode, p(HO) < 0.1%) and the hydrogen evolution reaction (-10 mA cm for only 75 mV overpotential). Owing to its unique features, the developed material showed distinguished performance as an air-breathing cathode in a garden compost MFC, exhibiting better current and faster power generation than those of its equivalent classical double chamber. The enhanced performance of the material obtained herein is explained by the absence of any organic surfactants on the surface of the nanoarrays, the good metal-support interaction, particular morphology of the nanoarrays, and the reduced aggregation/detachment of particles. It promises a radical improvement in current surface reactions and paves a new way toward electrodes with regulated surface roughness, allowing for their successful application in heterogeneous catalysis.

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