Paulo Brito

Cathodic oxygen reduction on noble metal and carbon electrodes

Abstract Because of their high electrocatalytic activity and stability noble metals were thoroughly investigated for the cathodic oxygen reduction. Emphasis has also been placed on the study of oxygen reduction on carbon electrodes in alkaline media due to their relative stability, easy modification of surface properties, large availability and low price. The observed behaviour of oxygen reduction on noble metal and carbon electrocatalysts is surveyed here. The common mechanisms under Langmuir conditions of adsorption are also examined in an attempt to account for the reaction kinetics of oxygen reduction on metaldoped polymer carbons in alkaline solutions.

Advanced biodiesel production technologies: novel developments

Biodiesel, i.e. a mixture of monoalkyl esters of long chain fatty acids derived from renewable biological sources such as vegetable oils has in recent years emerged as an alternative fuel for transportation sector. The conventional method of producing biodiesel is through homogeneous catalytic transesterification; however, increased production costs associated with downstream purification steps have led to the development of more cost-effective and environmental friendly technologies. These advanced production technologies involve heterogeneous or enzymatic catalysts to produce biodiesel, as well as no catalysts in supercritical conditions. Heterogeneous catalytic systems can ease the separation of biodiesel from the reaction mixture along with the possibility of catalyst recovery, potentially leading to lower production costs; enzymatic catalysts give the same advantages, but transesterification can be carried out in milder conditions and with a wider range of feedstocks. Biodiesel synthesis in supercritical conditions composes another alternative to conventional methods due to higher reaction rates, shorter reaction times, and simpler biodiesel separation steps. Nevertheless, mass transfer limitations caused by diffusion problems between phases represent an hindrance for future establishment of these technologies, calling for the development of novel methods to intensify the process. These process intensification technologies include ultrasound irradiation, microwave heating, use of co-solvents, and membrane reactors. The main focus of this review is to discuss recent advances as regards to biodiesel production technologies, devoting a special attention to the use of novel catalysts, diversified feedstocks, besides an analysis of main operational parameters of transesterification processes.

Nickel coated electrodes for oxygen evolution in alkaline solution

Abstract Ni, Ni/Ir and Ni/Ru electrodeposited electrodes on AISI 316 stainless steel gauze have been studied as electrocatalysts for the oxygen evolution reaction in 5M KOH solution at room temperature. Oxygen evolution on the coated electrodes occurs only after the oxidation of the electrode surface. The Ni/Ir electrode offers a large catalytic activity and a better corrosion resistance in alkaline solution. The electrocatalytic performances obtained by using Ru and Ir elements are discussed.

Reactively deposited cobalt electrodes for hydrogen evolution in alkaline solution

Abstract The hydrogen evolution reaction was conducted on reactively electrodeposited cobalt electrodes in a 6M KOH solution at room temperature. For an electrode with 100 mg cm–2 Co loading, the utilisation factor was above 75%, and the polarisation potential was –1·30 V (versus HgO/Hg) for H2 evolution at 1 A cm–2. The electrochemical efficiency of the electrodes was proportional to the Co loading in the 10–200 mg cm–2 range studied. The electrodes have very high surface area and well developed porous structure, showing high activity for the H2 evolution reaction.

Electrocatalysis of oxygen reduction by lanthanum-strontium manganate

The electrocatalysis of the oxygen reduction reaction by lanthanum-strontium manganate La0.5Sr0.5MnO3 (LSM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium. From the ring-disc data and other kinetic parameters it was concluded that the oxygen reduction occurs by dissociative chemisorption at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2− in this case) on the electrocatalyst has been observed. The apparent exchange current density for oxygen reduction on LSM has been found to be 2 × 10−7 A cm−2, while the corresponding Tafel slope is 0.100 V per decade. The possible reaction mechanism for electroreduction of oxygen on this oxide catalyst has been discussed.

Electrodeposition of Zn-Mn alloys from recycling battery leach solutions in the presence of amines

The recovery of metal ions by electrodeposition from solutions resulting from the lixiviation of spent Zn-MnO2 batteries was studied. It was attempted to optimise the electrodeposition process, the selectivity of ion-separation, the morphologic characteristics, and the anticorrosive and galvanic properties of metallic deposits. The simultaneous deposition of zinc and manganese on different ferrous substrates under various experimental conditions was tested. This allowed us to access the efficiency of the electrodeposition, the morphology and composition of the metallic deposits, as well as their performance as galvanic coating layers. The effect of amine additives, namely, of methylamine and ethylenediamine, on the properties of the coatings was also studied. It was shown that the amines with buffering or passivating effects improve the simultaneous deposition of Mn.

Electrochemical generators and the environment. Fuel Cells and metal/air batteries

Publisher Summary This chapter presents an overview of the state of the art and short-term perspectives of fuel cells and metal/air batteries, and analyzes their environmental benefits and drawbacks. Fuel cells and metal/air batteries are a set of energy generators which appear to hold considerable potential for future efficient and cleaner, generation of electricity. These systems are presently being developed for stationary power plants and for transport applications. The main advantage of fuel cells is to produce electricity and heat with negligible emissions of nitrogen oxides, carbon monoxide and dioxide, and others airborne pollutants. A further advantage of fuel cells and metal/air batteries over all other methods of energy conversion is that chemical energy is converted directly into electrical energy. Other advantages of fuel cells are the modular construction. Cells can be made in any shape or size, which allows them to be used for a wide range of applications, adaptability to operate over any desired temperature range and simple operation compared to other energy conversion devices. The major limitation of metal/air batteries, that has prevented their more wide-spread applications, is their low specific power. This limitation arises from the air performance electrode, which suffers from considerable polarization losses on discharge, largely due to mass transport limitations.

Production of High Calorific Value Biochars by Low Temperature Pyrolysis of Lipid Wastes and Lignocellulosic Biomass

Low quality waste oils and fats, with elevated levels of acidity, water and other contaminants are not appropriate for biodiesel production or other material recycling processes. Nevertheless, they can be converted to energy-dense bio-oils by pyrolysis, in an oxygen deficient atmosphere. These bio-oils may be used in combustion or upgraded to yield liquid biofuels for internal combustion engines although their chemical stability still creates some limitations during storage. This work aimed at the evaluation of biochar production by pyrolysis of mixtures of lipid wastes and pine sawdust. For this purpose, thermal conversion was performed with initial vacuum, at temperatures between 573.15 and 673.15 K and a reaction time of 60 min. Biomass incorporation varied between 0 and 38% (w/w). Biochar formation was favored by biomass incorporation and by the increase in temperature. The biochar yield varied from 1–28% and its high calorific value ranged between 24.3 to 36.6 MJ/kg. For incorporations of pine sawdust higher than 23% the carbon content and the calorific value of the biochar decreased due to a higher oxygen content in the raw materials. Highly calorific bio-oils were also produced in these conditions with yields from 24 to 83% (w/w), and high calorific values from 37.8 to 43.4 MJ/kg. These bio-oils have a carbon content higher than 75% and contain high molecular weight components. As such, they could be used in direct combustion in boilers, or as pellet additives. This approach contributes to the implementation of the renewed hierarchy for wastes as defined in the Directive 2008/98/EC namely by identifying alternatives to the deposition of the used cooking oils in landfills.

Composition of Producer Gas Obtained by Gasification of Pellet Mixtures Produced with Residual Lignocellulosic Biomass, Cork Wastes, Polymers and Polymer Derived Chars

In this work, the gasification of pellets produced from residual pine biomass and their mixtures with cork wastes, polymeric wastes and polymer derived char pellets was studied. The gasification tests were performed in a fixed bed downdraft gasifier at the temperatures of 700 °C and 800 °C. The influence of pellet combinations and gasification temperature in the composition and high heating value of the product gas was evaluated. The results were compared with commercially available pine pellets. At 800 °C, all the tested pellet mixtures exhibited higher CO and H2 concentrations than the commercial pellets, with 9.3% mol CO and 6.2% mol H2 for the mixture with cork wastes pellets, for example. The high heating value of the product gas for the different pellet mixtures presented values between 1.04 and 3.84 MJ/m3, for different gasification conditions. Residual lignocellulosic biomass and mixed wastes have the potential to be used as sustainable raw materials for energy production by gasification. Gasification operational parameters, such as temperature and equivalence ratio are decisive factors in product gas output, that need to be optimized in order to efficiently take advantage of the energy potential from these raw materials. Furthermore, this approach can contribute to the coupling of waste management and energy production through endogenous resources, reducing the deposition of these waste materials in landfills.

Characterization of Municipal, Construction and Demolition Wastes for Energy Production Through Gasification - A Case Study for a Portuguese Waste Management Company

Gasification of wastes is considered a promising alternative for energy generation due to its lower environmental impacts when compared with conventional landfilling and incineration. Valorisation of such wastes improves sustainability of resource management and of energy production. However, an appropriate characterisation of wastes in terms of physical and chemical properties is essential for the prediction of their behaviour during gasification, allowing to identify possible problems for the environment and installed equipment and also to define which materials present a greater energy potential. This study aimed to characterise 10 different fractions from municipal, construction and demolition wastes received in different fluxes by a Portuguese waste management company. These fractions included wood (44.83 wt%), plastic (22.15 wt%), paper/card (0.04 wt%), mixtures of paper and plastic (14.67 wt%) and sewage sludge (18.31 wt%). For this purpose, determination of density, proximate and ultimate analysis, higher heating value (HHV), thermogravimetric profiles and inorganic composition of ashes were performed for each fraction. Analysis revealed that plastics and their mixtures with paper/card possess the highest HHV’s (25–45 MJ/kg db), thus exhibiting a greater capacity for energy production. High levels of ashes found in dried sewage sludge (50 wt% db) indicate that a lot of by-product will be generated after gasification, possibly increasing the treatment costs. A gasification unit operating at 50 kg/h and admitting a mixture of all these wastes would generate 109.7 kW of total power, having capacity to receive more waste fluxes along the year.