GLUCOSE ELECTROOXIDATION

The electrooxidation of glucose has attracted a lot of interest due to its applications in blood glucose sensors and biological fuel cells. Glucose sensors optimization is highly necessary to improve the treatment of Diabetes Mellitus, a chronic disease affecting millions of people around the world, while biological fuel cells have been studied in order to explore new, renewable energy sources alternative to fossil fuels. There are three main ways to perform glucose electrooxidation, depending on the active oxidant agent or mediator employed: enzymatic electrooxidation utilizes enzymes such as glucose oxidase and glucose dehydrogenase in their isolated forms; abiotical electrooxidation makes use of non-biological catalysts e.g., noble metals and microbial electrooxidation employs the whole enzymatic system of an electroactive microorganism. Many researchers in the past focused on the utilization of enzymes to facilitate the process of glucose oxidation, however the limited enzyme stability and difficult immobilization procedures impede long term applications. During my PhD training I worked on both abiotical and microbial electrocatalysis; the former (at gold electrodes) applied to glucose sensing and glucose-gluconate fuel cells, the latter for the development of microbial fuel cells (MFCs). The complex oxidation of glucose at the surface of gold electrodes was studied in detail in different conditions of pH, buffer and halide concentration. As observed in previous studies, an oxidative current peak occurs during the cathodic sweep showing a highly linear dependence on glucose concentration, when other electrolyte conditions are unchanged. The effect of the different conditions on the intensity of this peak has stressed the limitations of the previously proposed mechanisms. A mechanism able to explain the presence of this oxidative peak was proposed in which the key step is the competitive adsorption at the active sites of the ionic species present in the solution (phosphate buffer, chlorides and OH-) and the substrate (glucose). Simulations of the proposed mechanism have supported the plausibility of the mechanism. The application of the above mentioned peak in blood glucose sensing has been hindered by the presence of inhibitors: chlorides amino acids and human albumin. Among them chlorides are the most problematic because of their high concentration in the blood (about 0.1 M) and the difficulty inherent in trying to separate them from glucose. In order to overcome this problem we developed a four-step, three electrode (silver gauze, gold pin and platinum counter electrode) technique. In the first step a silver gauze working electrode is oxidized to silver chloride, while water is reduced at the platinum counter electrode. In the overall reaction, for every chloride removed, a hydroxide ion is generated shifting the solution pH from 7.4 to 11.5. In the second step the gold pin electrode surface is oxidized to gold hydroxide and subsequently reduced in the third step: once the gold surface is regenerated, glucose can be re-adsorbed and oxidized giving rise to the sensing peak. In the last (fourth) step, the silver gauze (partially covered with silver chloride from step 1) is reduced and regenerated, ready for the next sensing. For the first time, an electrochemical glucose sensor able to work in the presence of chlorides and with higher accuracies and sensitivities than an enzymatic device was proposed. All the materials used in the prototype (silver, platinum and gold) are fully bio-compatible thus prefiguring an application in implantable glucose sensors, the future glucose meters technology. The direct oxidation of glucose to produce electrical energy has been widely investigated because of renewability, abundance, high energy density and easy handling of the carbohydrate. Most of the previous studies were conducted in extreme conditions in order to achieve complete glucose oxidation to CO2, neglecting the carbohydrate chemical instability that generally leads to useless by-products mixtures. Instead the partial oxidation to gluconate, originally studied for implantable fuel cells, has the advantage of generating a commercially valuable chemical. For this reason, we started optimizing fuel composition and operating conditions in order to selectively oxidize glucose to gluconate, maximizing the power density output of a standard commercial platinum based anode material. A deep electrochemical characterization concerning reversible potential, cyclic voltammetry and overpotential measurements have been carried out at 25°C in the D-(+)-glucose concentration range 0.01 to 1.0M. NMR and EIS investigation clarified the role of the buffer (Na2HPO4/NaH2PO4) in enhancing the electrochemical performance: it changes the reaction rates and steps; it increases the amount of β form of glucose; it increases the conductivity of the solution; it may adsorb at the platinum surface of platinum and subtract active sites for the electrooxidation of glucose as already highlighted for gold electrodes. Moreover the presence of the buffer not only stabilizes the potential, but also improves the electrochemical performances of the anode in term of exchange current density. Such behavior is not ascribable to the chemical interaction with glucose, as demonstrated by NMR measurements, but to the interaction with the anode material as indicated by the decrease of all the resistive components in the EIS measurement. In order to improve the anode performance of the previously discussed glucose-gluconate FCs, the electrocatalytic properties of nanostructured gold electrodes were investigated, by cyclic voltammetry, and compared with commercially available polycrystalline gold electrodes. These nanostructured electrodes were prepared by depositing gold nanoparticles from a colloidal dispersion (sol) onto different carbonaceous conductive supports: glassy carbon, carbon cloth and graphite paper. The gold sol was prepared reducing an aqueous solution of tetrachloroauric acid with sodium borohydride. The gold particles (average size 100 nm) exhibit better electrocatalytic properties with respect to commercially available polycrystalline electrode for glucose oxidation. A surface treatment of the carbonaceous conductive supports with warm concentrated nitric acid resulted in an improved adhesion of the gold nanoparticles. Gold on treated carbon cloth turned out to be a very promising anode for glucose electrooxidation. On the basis of the information acquired in the above mentioned studies, we came out with a new anode for glucose-gluconate direct oxidation fuel cells prepared by electrodeposition of gold nanoparticles on what we called a “conductive energy textile” realized by conformally coating polyester textile substrates with single walled carbon nanotubes (SWNT). The electrodeposition conditions have been optimized in order to obtain uniform distribution of gold nanoparticles in the 3D porous structure of the conductive textile. The electrochemical characterization, carried out by means of cyclic voltammetry, showed higher current densities with respect to the previously reported materials. As previously mentioned I also worked on microbial glucose electrooxidation applied to microbial fuel cells. As a new member in the fuel cell family, microbial fuel cells (MFCs) are devices that convert chemical energy into electrical energy by the catalytic activity of microorganisms. The most promising application of this technology is to harvest energy from undesirable fuel sources, such as the organic matter in domestic wastewater, marine sediment, or human excrement in space. A novel carbon nanotube-cotton (CNT-cotton) composite material with high conductivity and high porosity was proposed to be used as anode for achieving high-performance MFCs. Scanning electron microscope (SEM) images of microorganisms growing on the CNT layer provided the direct evidence to support the biocompatibility of the CNTs and their feasibility to be used as the anode in MFCs. The randomly intertwined CNT-cotton fibers create a 3D active space for biofilm growth, and meanwhile, the incompact macroporous structure allows efficient mass transfer for microbial metabolism inside the anode. Furthermore, the coated CNTs significantly improve the mechanical binding as well as the electrical conductivity between the exoelectrogenic microorganisms and the CNT-cotton anode. Compared to commercial carbon cloth anode, the CNT-cotton anode achieves 64% higher power density and 75% higher energy recovery efficiency in MFCs. Air is considered to be the most suitable oxidant for field scale MFCs, because it is free and inexhaustible. However, the oxygen reduction efficiency is highly constrained by the specific operating conditions of MFCs, such as ambient temperature and mostly neutral pH. Thus, cathode performance often limits the power output of MFCs. Moreover, cathode usually accounts for the greatest part of the total capital cost of a MFC, mainly because of the use of precious metal catalyst like Pt. Therefore, improving the cathode performance decreasing the catalyst loading represents a critical issue for researchers working on MFCs. A new CNT-textile-Pt cathode specially designed for aqueous-cathode MFCs was obtained by electrochemically depositing Pt nanoparticles on a macroporous CNT-textile substrate. This CNT-textile-Pt cathode shows two orders higher of surface area utilization efficiency than the commercial carbon cloth (CC)-Pt cathode. Assisted by the additional catalytic activity of CNTs, the MFCs equipped with CNT-textile-Pt cathodes achieve higher power density (2.14-fold) with lower Pt loading (19.3%). Moreover, the synthesis process of CNT-textile-Pt is simple and scalable. Thus, CNT-textile-Pt is promising to function as cathodes for large scale high performance aqueous-cathode MFCs. In parallel to glucose electrooxidation a new stretchable, porous conductive energy textile has been developed. Recently there is strong interest in lightweight, flexible and wearable electronics to meet the technological demands of modern society. Integrated energy storage devices of this type are a key area that is still significantly underdeveloped. We developed wearable power devices using everyday textiles as the platform. With an extremely simple “dipping and drying” process using single-walled carbon nanotube (SWNT) ink, we produced highly conductive textiles with conductivity of 125 S cm-1 and sheet resistance less than 1 Ω/sq. Such conductive textiles show outstanding flexibility and stretchability, and demonstrate strong adhesion between the SWNTs and the textiles of interest. Supercapacitors (SC) made from these conductive textiles show high areal capacitance, up to 0.48 F cm-2, and high specific energy. We demonstrated that the loading of pseudocapacitor materials into these conductive textiles leads to a twenty four-fold increase of the areal capacitance of the device. Moreover, supercapacitors have been fabricated using the conductive energy textile as both active material and current collector (resistance lower than 1 Ω/sq). The device has excellent cycling performance (good capacity retention after 35,000 cycles) and high specific capacitance (70–80 F g-1 at 0.1 mA cm–2). The as prepared device is fully wearable since both the textile (cotton) and the lithium sulfate electrolyte are compatible with the human body. It can also be integrated into wearable devices. By means of impedance spectroscopy and differential curves, we have highlighted an additional reversible capacitance due to a slow ionsorption process.