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- Aerogel cathodes for electrochemical CO2 reduction [Comunicação oral]Publication . Messias, Sofia; Fialho, Maria T.; Paninho, A. B.; Branco, Luis C; Nunes, A. V. M.; Martins, Rodrigo; Mendes, Manuel Joao; Nunes, D.; Rangel, C. M.; Machado, AnaABSTRACT: Electrochemical reduction of carbon dioxide powered by renewable energy to produce fuels and chemicals is a technology with potential to contribute to an economy based on a carbon neutral cycle. The development of cost effective, highly active and stable catalysts for CO2 electroreduction is being intensively researched. This work addresses the development of aerogel supported copper-zinc bimetallic catalysts[1]. Aerogels are substances with exceptional properties with many current and potential applications [2-3]. Due to their high surface area, stability in corresponding gaseous or liquid phases, transport through large meso and macropores they are especially suited as catalysts and carrier materials for catalysis and, when electric conductive for electro-catalysis. Aerogels prepared by the sol gel method and impregnated with metallic particles will be tested as cathodes for the co-electrolysis of CO2 and water to produce syngas at temperatures near room temperature and high-pressure. In this way this process can be directly coupled to other high pressure processes, such as Fischer-Tropsch that use high pressure syngas as raw material. Productivities and faradaic efficiencies will be evaluated. The characterization of the aerogel-based cathodes will be undertaken by surface analysis techniques. BET surface areas will be determined. The catalytic cathodes will be tested in an ionic liquid-based electrolyte as a way to increase current densities, due to the high CO2 solubilities exhibited by some ionic liquid families.
- Electrochemical production of syngas from CO2 at pressures up to 30 bar in electrolytes containing ionic liquidPublication . Messias, Sofia; Sousa, Miguel M.; Ponte, Manuel Nunes; Rangel, C. M.; Pardal, T.; Machado, AnaABSTRACT: Electrochemical CO2 reduction in a reactor that can operate up to 100 bar and 80 degrees C, with a configuration similar to that of an alkaline electrolyser, for hydrogen production suitable to be used industrially is reported for the first time. The effect of pressure on the co-electrolysis of CO2 and water was studied. The successful scale-up from a previously reported batch process to electrodes of ca. 30 cm(2) geometrical area (30-fold factor) that combines the use of pressure and an ionic liquid-based electrolyte is presented. Also for the first time, the potential of the system under study to achieve high conversions of CO2 to avoid a purification step of syngas from unreacted CO2 is shown. An inexpensive commercial foil of the common metal zinc was employed. A semi-continuous operation yielded syngas productivities in the range of 0.02-0.04 mmol cm(-2) h(-1) at ca. -1.2 V vs. QRE Ag/Ag+. When an electrolyte consisting of 90 wt% H2O and 10 wt% 1-ethyl-3-methylimidazolium trifluoromethanesulfonate was used, selectivities for CO in the range of 62% to 72% were obtained at 10 bar pressure, whereas selectivities of 82% were obtained at 30 bar pressure. H-2/CO ratios in the range of 1/1 to 4/1 at 10 bar pressure suitable for the synthesis of a variety of fuels, such as hydrocarbons, methanol, methane and chemical building blocks, were observed. An energy efficiency of 44.6% was calculated for a H-2/CO ratio of 2.2 suitable for the synthesis of methanol.
- Advances in electrochemical reduction of CO2 in ionic liquid-based electrolytes [Resumo]Publication . Machado, Ana; Messias, Sofia; Paninho, A. B.; Nunes, A. V. M.; Rangel, C. M.; Branco, Luis CABSTRACT: Electrochemical reduction of CO2 was for the first time reported in 1870 [1], but it was only after 2010 that this field was the subject of intense research efforts. The use of renewable electricity to convert CO2 into products that are currently derived from fossil products and have high carbon footprint will certainly make this technology one pillar of a sustainable chemical industry. The scepticism towards the availability of cost effective products derived from CO2 electro-reduction that customers will be willing to buy has shifted to the belief that they can be commercially viable. Turning electrochemical CO2 reduction into a commercial technology will depend on economics, on the price of electricity, efficiency of the process and the value of the products. One way to improve the economics and improve the efficiency of the process is to integrate CO2 capture with conversion [2,3]. In this way the energy intensive regeneration step of the capture media can be eliminated and also CO2 transportation and storage. Ionic liquids are ideal media to achieve this integration, due to high CO2 adsorption capacity, high selectivity, wide electrochemical windows and nearly zero vapour pressure. The present work reports the progress of electrochemical reduction of CO2 in ionic liquids and the work of the authors in this field. It has been recognized that ionic liquids promote CO2 electro-reduction through lowering the reduction potential, the suppression of the competing hydrogen evolution reaction and by increasing the selectivity towards the target products. However, the understanding of the interactions between ionic liquids, CO2 and catalyst is still quite limited, but fundamental for synthetizing more efficient electrolytes for CO2 electro-reduction [4]. Thus, current cation and anion effects will be analysed and an overview of the current performance of heterogeneous electro-catalysts in ionic liquid- based electrolytes for CO2 electro-reduction will be provided.
- Scale-up of a system for hydrocarbon production by electrochemical reduction of CO2Publication . Fernandes, T. R. C.; Machado, Ana; Condeço, J.; Rangel, C. M.; Pardal, T.This work addresses the scaling up of a system for electrochemical reduction of CO2 to produce hydrocarbons that can be used as fuel for a regenerative energy storage cycle. Challenges involved in such a task are mentioned. Scalingup results of a system based on electrodes of high surface area with modified copper deposits are described. Current densities around 100 mA/cm2 were obtained. This corresponds to the current density threshold that enables technological applications. At potentials as negative as -1.6 V it was observed that CO2 reduction still dominated over hydrogen evolution reaction.
- Electrochemical CO2 reduction at room temperature and mild pressuresPublication . Messias, Sofia; Sousa, Miguel M.; Ponte, Manuel Nunes; Rangel, C. M.; Pardal, T.; Machado, AnaABSTRACT: Carbon capture and utilization technologies (CCU) and electrolytic hydrogen production are closely interconnected technologies and necessary for a sustainable energy system. This work describes the development of a process for room temperature co-electrolysis of CO2 and water to produce syngas, at mild pressures. The influence of several parameters in the performance of the process is reported.
- Tuning cathode porosity for electrochemical reduction of CO2 at high pressure [Resumo]Publication . Messias, Sofia; Fialho, Maria T.; Paninho, A. B.; Nunes, A. V. M.; Branco, Luis C; Nunes, D.; Martins, Rodrigo; Mendes, Manuel Joao; Rangel, C. M.; Machado, AnaABSTRACT: The development of active and stable catalytic cathodes is critical for advancing electrochemical carbon dioxide reduction into fuels and chemicals from Lab to market. This is a technology with a high potential to contribute to combat climate changes by using captured CO2, water and renewable energy [1]. The use of pressures higher than atmospheric pressure to carry out the co-electrolysis of CO2 and water has been recognized as an important process intensification parameter to increase productivities and energy efficiency [2]. Ongoing work addresses the preparation of aerogels by the sol gel method and impregnation with zinc and copper metallic particles to be used as cathodes for the co-electrolysis of CO2 and water to produce syngas at temperatures near room temperature and high-pressure. Ionic liquid-based electrolytes are used to increase CO2 concentration at the surface of the electrode and consequently productivities, as some ionic liquid families are known to solubilize high amounts of CO2. Aerogels have been investigated for many different applications including as catalyst supports, due to their high surface area, stability in gaseous or liquid phases, and efficient transport through large meso and macropores. The present work reports a strategy to tune the pore sizes of the catalytic electrodes by the use of reticulating agents and supercritical CO2 drying. Productivities and faradaic efficiencies of the porous materials with the different reticulating agents are compared and interpreted in respect to their surface characterization e.g. BET surface areas and morphologies determined by SEM. The potential of new aerogel-based catalytic cathodes on the efficiency of the electrochemical CO2 reduction will be discussed and its impact in fostering supercritical fluids technology through its use in processes for the mitigation of climate changes.
- Pressurized electrochemical process for syngas production [Resumo]Publication . Messias, Sofia; Sousa, Miguel M.; Rangel, C. M.; Pardal, T.; Ponte, Manuel Nunes; Machado, AnaABSTRACT: Carbon Capture and Utilisation (CCU) comprise technologies that capture carbon dioxide and convert it into chemical products, such as fuels, chemicals and building materials that can replace the same products derived from fossil resources. These technologies can thus contribute to reach the ambitious 2030 targets set by the European Union for decarbonisation of the economy of 40% CO2 reduction, introduction of 27% renewable energies in the energy mix and 27% energy efficiency. CCU technologies are at different stages of maturity and some are already commercially deployed. One of these commercial technologies is the production of methanol by CO2 hydrogenation at high temperature using hydrogen produced by water electrolysis. Another CCU technology that is still at laboratory stage is electrochemical CO2 reduction. In this process carbon dioxide is co-electrolyzed with water near room temperature. Hydrogen is produced in situ. Depending on process conditions and catalysts CO2 reduction originates a series of products, such as CO, formic acid, hydrocarbons, alcohols etc. at the cathode. Oxygen is produced at the anode resulting from water oxidation reaction. Figure 1 illustrates the process. Electrochemical CO2 reduction presents several technological challenges that have not been surpassed yet, such as low CO2 conversions, poor stability of electrodes and low energy efficiencies. This work reports not yet published results of the development of a pressurized electrochemical process for producing syngas (CO + H2) by electrochemical reduction of carbon dioxide. Green syngas produced using renewable energy can be an important platform for introducing renewable energy in the chemical industry. The development of a process that works at pressures higher then atmospheric pressure seems promising to circumvent the aforementioned challenges, namely low conversions and low energy efficiencies.
- Structural features of electrodeposited copper electrodes for CO2 conversionPublication . Machado, Ana; Fernandes, T. R. C.; Pardal, T.; Rangel, C. M.Direct electrochemical reduction of CO2 is a process that could contribute to the reduction of the emission of greenhouse gases by using CO2 as a raw material for fuel production. This paper focuses on voltammetric studies of functionalized electrodes for the electrochemical conversion of CO2 and reports on its use as a tool for electrode screening and optimization. Nickel substrates modified with copper and ruthenium/copper electrodeposits were studied. Voltammetric experiments indicate that CO2 electroreduction follows a nickel type mechanism in which this electrochemical reaction occurs simultaneously and in competition with hydrogen evolution. A significant inhibition of hydrogen evolution reaction is observed in nickel modified electrodes. Inhibition characteristics and the onset of carbon dioxide conversion are dependent of the type of electrode functionalization. Voltammetry is thus a powerful tool to evaluate electrode modifications and for tuning electrodes for an optimized electrocatalytic performance.
- Prediction of sunlight-driven CO2 conversion: producing methane from photovoltaics, and full system design for single-house applicationPublication . Vieira, F.; Sarmento, B.; Machado, Ana; Facão, Jorge; Carvalho, Maria João; Mendes, Manuel Joao; Fortunato, Elvira; Martins, RodrigoABSTRACT: CO2 capture and utilization (CCU) technologies are being immensely researched as means to close the anthropogenic carbon cycle. One approach known as artificial photosynthesis uses solar energy from photovoltaics (PV), carbon dioxide and water to generate hydrocarbon fuels, being methane (CH4) a preferential target due to the already in place infrastructures for its storage, distribution and consumption. Here, a model is developed to simulate a direct (1-step) solar methane production approach, which is studied in two scenarios: first, we compare it against a more conventional 2-step methane production route, and second, we apply it to address the energetic needs of concept buildings with usual space and domestic hot water heating requirements. The analysed 2-step process consists in the PV-powered synthesis of an intermediate fuel - syngas - followed by its conversion to CH4 via a Fischer -Tropsch (methanation) process. It was found that the 1-step route could be adequate to a domestic, small scale use, potentially providing energy for a single-family house, whilst the 2-step can be used in both small and large scale applications, from domestic to industrial uses. In terms of overall solar-to-CH4 energy efficiency, the 2-step method reaches 13.26% against the 9.18% reached by the 1-step method. Next, the application of the direct solar methane technology is analysed for domestic buildings, in different European locations, equipped with a combination of solar thermal collectors (STCs) and PV panels, in which the heating needs that cannot be fulfilled by the STCs are satisfied by the combustion of methane synthesized by the PV-powered electrolyzers. Various combinations of situations for a whole year were studied and it was found that this auxiliary system can produce, per m(2) of PV area, in the worst case scenario 23.6 g/day (0.328 kWh/day) of methane in Stockholm, and in the best case scenario 47.4 g/day (0.658 kWh/day) in Lisbon.
- Modified electrodes for electrochemical reduction of carbon dioxidePublication . Machado, Ana; Fernandes, T. R. C.; Pardal, T.; Rangel, C. M.The efforts to constrain greenhouse gas emissions and concerns over security of fossil fuels have led to increased attention for renewable energy for the past decade. Renewable energy is one of the key solutions to the actual energy challenges. Omnidea in collaboration with Research Institutes is developing a technology based upon a regenerative energy storage cycle that could be a contribution to a low-carbon energy future. In this cycle the recharge system, which is composed of an electrochemical cell, converts CO2 into fuel (hydrocarbons and hydrogen) using an external source of power (e.g. solar power). The discharge system produces electric energy when hydrocarbons and oxygen from the recharge system are directly supplied to a Solid Oxide Fuel Cell (SOFC). Currently state of the art systems for direct electrochemical reduction of CO2 exhibit low current densities and or low Faradaic efficiencies. Thus considerable research activity is still needed to develop electrodes with a performance suitable for an industrial application. This paper describes the progress to date and the work carried out with the aim of achieving this goal. It addresses particularly the modification of electrodes for electrochemical conversion of CO2 and reports voltammetric studies as a tool for screening and optimizing electrodes for CO2 conversion