In the past 10–15 years, the microbial fuel cell (MFC) technology has captured the attention of the scientific community for the possibility of transforming organic waste directly into electricity through microbially catalyzed anodic, and microbial/enzymatic/abiotic cathodic electrochemical reactions. In this review, several aspects of the technology are considered. Firstly, a brief history of abiotic to biological fuel cells and subsequently, microbial fuel cells is presented. Secondly, the development of the concept of microbial fuel cell into a wider range of derivative technologies, called bioelectrochemical systems, is described introducing briefly microbial electrolysis cells, microbial desalination cells and microbial electrosynthesis cells. The focus is then shifted to electroactive biofilms and electron transfer mechanisms involved with solid electrodes. Carbonaceous and metallic anode materials are then introduced, followed by an explanation of the electro catalysis of the oxygen reduction reaction and its behavior in neutral media, from recent studies. Cathode catalysts based on carbonaceous, platinum-group metal and platinum-group-metal-free materials are presented, along with membrane materials with a view to future directions. Finally, microbial fuel cell practical implementation, through the utilization of energy output for practical applications, is described.

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Among the most appealing features of oligothiophenesÐwhich are actively investigated for a broad range of electronic applications [1±7] Ðare their chemical stability and ease of functionalization, which allows the fine tuning of relevant properties. Unsubstituted oligothiophenes are p-type, hole-transporting, semiconductor materials and one of the research objectives in the field of organic-based devices is to achieve the kind of functionalization capable of giving stable derivatives displaying n-type semiconductor properties [8±10] for the preparation of electron-transporting layers. One of the possible ways to achieve this objective is functionalization with strongly electron-withdrawing groups, which can be designed to increase the electron affinity of the material.[11]We report here a new strategy towards functionalization of oligothiophenes that consists of the chemical transformation of thienyl sulfurs into the corresponding S,S-dioxides. We show that this kind of functionalizationÐwhich implies the dearomatization of the thiophene ringsÐleads to very stable oligomers with increased electron delocalization and has also a dramatic effect on their electron affinity as deduced from their electrochemical reduction potential. It is worth mentioning that the synthesis and the characterization of unsubstituted thiophene S,S-dioxide has been reported only very recently [12] and that only S,S-dioxides derived from polysubstituted thiophene are stable at room temperature.[13±15] So far, no electrochemical data on this kind of compound have yet been published. The oligothiophene S,S-dioxides were prepared either by action of m-chloroperoxybenzoic acid (m-CPBA) on the parent oligothiophenes [16] or by assembly of the appropriate building blocks via the Stille reaction, according to the improved procedure recently described by us. [17] As an example, the synthesis of a,w-bis(dimethyl(t-butyl)silyl)-2,2¢:5¢,2²:5²,2²¢-quaterthiophene-1,1-dioxide, 5, is illustrated in Scheme 1.As shown in Table 1, which includes the main optical and electrochemical data for a series of structurally correlated tetramers and pentamers, the maximum absorption wavelength of 5 (OTTT) is red shifted by 48 nm with respect to that of the parent quaterthiophene (TTTT), indicating a greater delocalization of the electronic charge. Moreover, while the oxidation potential of OTTT only increases by 0.09 V with respect to that of TTTT, the reduction potential is shifted by 0.84 V towards less negative values, resulting in a substantial increase of the electron affinity and also a decrease of the energy gap, DE el , of OTTT with respect to that of the precursor quaterthiophene.A similar trend in the variation of the oxidation and reduction potentials evaluated by cyclic voltammetryÐfol-lowing the dearomatization of one terminal ring by formation of the corresponding S,S-dioxideÐwas observed for the entire series of oligomers from the dimer to the pentamer bearing ±SiMe 2 t Bu or ±(CH 2 ) 5 CH 3 groups at the a,w positions. This trend is shown in Figu...

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Six substituted 5-pyrimidinols were synthesized, and the thermochemistry and kinetics of their reactions with free radicals were studied and compared to those of equivalently substituted phenols. To assess their potential as hydrogen-atom donors to free radicals, we measured their O-H bond dissociation enthalpies (BDEs) using the radical equilibration electron paramagnetic resonance technique. This revealed that the O-H BDEs in 5-pyrimidinols are, on average, about 2.5 kcal mol(-1) higher than those in equivalently substituted phenols. The results are in good agreement with theoretical predictions, and confirm that substituent effects on the O-H BDE of 5-pyrimidinol are essentially the same as those on the Obond;H BDE in phenol. The kinetics of the reactions of these compounds with peroxyl radicals has been studied by their inhibition of the AIBN-initiated autoxidation of styrene, and with alkyl and alkoxyl radicals by competition kinetics. Despite their larger O-H BDEs, 5-pyrimidinols appear to transfer their phenolic hydrogen-atom to peroxyl radicals as quickly as equivalently substituted phenols, while their reactivity toward alkyl radicals far exceeds that of the corresponding phenols. We suggest that this rate enhancement, which is large in the case of alkyl radical reactions, small in the case of peroxyl radical reactions, and nonexistent in the case of alkoxyl radical reactions, is due to polar effects in the transition states of these atom-transfer reactions. This hypothesis is supported by additional experimental and theoretical results. Despite this higher reactivity of 5-pyrimidinols towards radicals compared to phenols, electrochemical measurements indicate that they are more stable to one-electron oxidation than equivalently substituted phenols. For example, the 5-pyrimidinol analogues of 2,4,6-trimethylphenol and butylated hydroxytoluene (BHT) were found to have oxidation potentials approximately 400 mV higher than their phenolic counterparts, but reacted roughly one order of magnitude faster with alkyl radicals and at about the same rate with peroxyl radicals. The 5-pyrimidinol structure should, therefore, serve as a useful template for the rational design of novel air-stable radical scavengers and chain-breaking antioxidants that are more effective than phenols.

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