Biofuel Cells. Группа авторов

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СКАЧАТЬ de Investigación y Desarrollo Tecnológico en Electroquímica S.C., Pedro Escobedo, Mexico

       2Centro de Investigación y Desarrollo Tecnológico en Electroquímica S.C., Pedro Escobedo, Mexico

       Abstract

      Bioelectrocatalysis is the acceleration of reactions that occur on an electrode via a biological component, be it an enzyme, a cellular organelle or a whole cell. Enzymatic reactions on the anode are mainly the oxidation of saccharides and alcohols, while the oxidative metabolism of bacteria is exploited for removal of short-chain organic acids. In the cathode, the main enzyme-controlled reaction is the reduction of dioxygen, while microbial catalysis tends to obtain hydrogen and methane-like energy vectors. One of the challenges in bioelectrocatalysis is the preparation of electrodes. The techniques for immobilization of enzymes and organelles include the use of polymers and composites and the naturallyoccurring adhesion of bacteria to the solid material forming a biofilm on the electrode. Given the importance of the support material, numerous efforts have been directed to modifying materials that improve the adhesion of enzymes and bacteria, as well as electron transfer. The control of electron transfer is performed by the modification of the pH in the medium, the use of mediators, and the application of a potential difference in an electrolytic cell. The applications of electrochemical cells in bioelectrocatalytic operation include energy conversion, enzymatic sensors and gaseous fuel production in microbial bioelectrochemical systems.

      Keywords: Bioelectrocatalysis, biofuel cell, enzymatic electrocatalysis, microbial electrocatalysis

1.1 Introduction: Generalities of the Bioelectrocatalysis

      Electrochemical catalysis or electrocatalysis is used to describe charge transfer-based reactions occurring on an electrode. This term was employed for first time in 1936 by Santos and Schimickler [1]. The electrocatalysis is focused on increasing the reaction rate of an electrochemical process (oxidation/reduction), involving a dissociative chemisorption or a reaction step on an electrode surface and thus, the electrocatalysis depends on the ad/desorption of reactants and products, and on the formation of an electrochemical double layer. An electrocatalytic cycle is composed of three stages: 1) mass transport of electroactive species from bulk to the interface, 2) the electrocatalytic reaction, and 3) transport of products to bulk. Additionally, stage 2 involves the adsorption of reactants, the electron transfer, and the desorption of products. Consequently, the art of electrocatalysis consists of identifying the barriers of an electrochemical reaction to adjust the properties of the electrochemical interface (electrode and/or solution) with the aim of remove or at least, decrease the energy barriers (activation energies).

      The practical role of electrocatalysis implies the science of designing the electrochemical interface properties. Hence, the morphological and electronic properties of the electrocatalyst, together with the electrolyte characteristics, become important to analyze. On the other hand, the activation energy of electrocatalytic reactions also depends on the electrode potential, thus enabling a fine control of the reactions. Consequently, electrocatalysis focuses on minimizing electrode overpotential, and increasing the reaction rate via the decrease of activation energies for a specific reaction.

The relation between electrocatalysis and microorganisms was presented in 1910 when a yeast was used as catalyst in a fuel cell. In the 60s microbial fuel cells (MFC) were used to exploit human waste from spacecraft, and only about 40 years later, the MFCs gained worldwide attention due to the use of industrial wastewater as fuel [2]. Entire microbial cells, organelles and biological molecules have been utilized as catalyst in biofuel cells. The molecules for energy conversion in living eukaryotic cells are utilized as biocatalyst and as model reactions. The reactions are complex and involve the action of nucleotides nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The nucleotides in the cell are reduced to NADH and FADH₂ by protons coming from a chain of oxidation reactions belonging to the microbial catabolic metabolism. The cyclic oxidation and reduction of nucleotides enables the transport of charge in the mitochondria and thus in the microbial cell. The energy pathways in prokaryote cells involve a chain of transmembrane enzymatic proteins. The c-type cytochromes in the outer cell membrane enable direct contact cell-electrode and research in molecular biology shows that cytochromes are responsible for extracellular electron transfer (EET).

      The reactions occurring in the living cells involve different catalytic proteins or enzymes; thus charge transfer through biological molecules has required many years of investigation. Enzymes can act in the electrolyte, or be immobilized at the electrode, and electron transfer achieved via either mediated or direct form. The contact of the enzyme with the substrate is achieved via physical or covalent adsorption. The type of contact is a function of the location of the active site in the enzyme, which can be in the periphery or in the core of the catalytic protein. The electrode material for immobilization of the bioelectrocatalyst is one of the main issues. Thus, the intrinsic properties of the electrode such as porosity and conductivity must be improved via doping, template construction or addition of nanomaterials. Another concern in bioelectrocatalysis is the lifetime of the enzymatic electrodes, which are very sensitive to environmental conditions. Plenty of strategies using polymers have been proposed, including encapsulation, cross-linking, anchoring, and self-assembly with the aim of improving the electron transfer between the enzyme and the electrode. This process can be explained by different mechanisms like percolation though immobile redox centers, collision of mobile centers, and conduction through a conjugated backbone. The direct transfer occurs via electron tunneling from the active site in the enzyme and the electrode.

      In the following sections, reactions of general interest in cells catalyzed by enzymes and microorganisms are described in the first instance. The next section focuses on advances in electrode material development, as well as enzyme immobilization and bacterial biofilm preparation strategies. Finally, in the last sections the phenomena that occur in the transfer of electrons at the enzymatic and bacterial level are described, and two cases of application of bioelectrocatalysis are presented.

1.2 Reactions of Interest in Bioelectrocatalysis

      1.2.1 Enzyme Catalyzed Reactions

Oxidoreductases (EC group 1^†) are enzymes that are capable of catalyzing reactions in which electron transfer is involved and have been used as fuel cell components since the early 1960s. In 1962, Davis and Yarborough reported an increase in the potential of a cell in which glucose oxidase was present in one of the electrode compartments (the other being a Pt/O₂ one) [3]. Two years later, Yahiro et al. reported the first polarization curves using glucose oxidase (GOx), D-amino acid oxidase and yeast alcohol dehydrogenase in bioanodes that they coupled with Pt cathodes for O₂ reduction [4]. Since then, most of the enzymatic biofuel research has been centered in the oxidation of glucose and the reduction of oxygen. This has been driven by the abundance of both substances in our biosphere; while oxygen is abundant in the atmosphere, glucose is used as a source of energy by almost all living beings. Because of their predominance in the literature, this section will focus on the mechanistic description of enzymatic oxidation of glucose and reduction of oxygen by glucose oxidase and laccase, respectively. Other enzymes, like glucose dehydrogenase [5, 6] and bilirubin oxidase [5] can perform similar reactions through different mechanisms and can be useful in some situations. Furthermore, a variety of other fuels (e.g. carbohydrates [7, 8], alcohols [9, 10], lipids [11] and organic acids [12] and oxidants (mainly H₂O₂ [13]) have been employed in enzymatic biofuel cells. However, it is out of the scope of this work to review them all in detail. Rather, it is expected that the information presented here will allow the readers to perform similar literature search for their particular enzyme of interest.

Glucose can be oxidized by a variety of enzymes including glucose oxidase and СКАЧАТЬ