The crises persist in the demand for oil as the concerns of society with environmental preservation are the main questions that lead governments and society to seek strategies for greater production and consumption of renewable fuels. In view of this, biofuels represent an alternative way of replacing the use of fossil fuels, reducing the incessant need for resources of non-renewable origin and effectively contributing to the reduction of emissions of toxic gases into the environment. Ethanol obtained from sugarcane is the fuel capable of meeting the growing world demand for low-cost, low-polluting renewable energy. The gaseous emissions with the burning of the ethanol are of the order of 60% smaller compared to the emissions of the burning of the gasoline. This work develops a study with catalysts of Pt: TRs to oxidize ethanol in acidic and alkaline media. Thus, after the synthesis of the materials, characterization analyzes were performed. Thermal Gravimetric Analysis showed that the synthesized electrocatalysts presented a metallic charge close to 20% in mass, however the Electron Microscopy Transmission that realizes the study of the catalytic surface evidenced that the particles became quite dispersed in the activated carbon surface. The electrochemical analysis techniques showed that the binomial electrocatalysts that presented the best performance were those synthesized from rare earth chlorides and the results obtained qualify the use of these materials for the oxidation of ethanol.

Throughout this dissertation, the examination of hydrogen as an energy carrier encompasses its current state and future potential. The significance of addressing climate change and reducing greenhouse gas emissions is emphasised as the driving force behind the discussion. Although fossil fuels still dominate global energy consumption, it is anticipated that renewable energy sources assume a progressively vital role in the future. A comprehensive overview of hydrogen’s physical and chemical properties is presented, encompassing safety considerations and associated ISO standards. Furthermore, the concept of Technology Readiness Level (TRL) is introduced, which serves as a framework for assessing a technology’s readiness to fulfil energy policy objectives. The focus then shifts to the production of green hydrogen, with water electrolysis technologies taking centre stage. Various methods of hydrogen storage are evaluated, including underground storage, physical storage, and material-based storage. Transportation technologies are examined, taking into account different storage methods, volume requirements, and transportation distances. Lastly, an assessment of well-known technologies for harnessing energy from hydrogen is undertaken, including gas turbines, internal combustion engines, and fuel cells. By delving into these topics, this dissertation aims to provide a comprehensive understanding of hydrogen as an energy carrier, covering its properties, production processes, storage methods, transportation technologies, and energy recovery options. In the conclusions, it is highlighted that the foreseeable future holds significant potential for hydrogen as an energy carrier and its associated technologies. The widespread adoption of hydrogen as a clean and versatile energy carrier presents immense opportunities for various sectors. As the demand for hydrogen continues to grow, particularly in transportation, building heating, and power generation, new business prospects emerge. These opportunities encompass the development and deployment of hydrogen production facilities, storage infrastructure, transportation networks, and the manufacturing of hydrogen-based technologies. The dissertation concludes by emphasising the importance of further research, innovation, and collaboration among industry, academia, and policymakers to unlock the full potential of hydrogen and foster its transition into a mainstream energy solution.

This work focuses on the preparation and densification of a ceramic protonic conductor, hexagonal HZr2(PO4)3, for applications as an electrolyte in fuel cells operating in the temperature zone between 300 and 400 ˚C, which, to date, is marked by a gap where there is an absence of suitable materials that meet these operating requirements. To this end, a new hydrothermal synthesis route for the formation of its precursor, hexagonal NH4Zr2(PO4)3, was developed and its resultant crystallographic system was analyzed. More precisely, this compound can exist in two phases, cubic and hexagonal, and its initial production resulted in a material with both of these phases. Thus, as hydrothermal synthesis essentially involves three variables: temperature, pH and pressure, these parameters were studied individually in order to better understand and discover a mechanism that would allow the desired hexagonal product to be synthesized. In this sense, keeping the total solution volume constant at 75 mL, it was found that for a temperature of 150 ˚C and a volume of aqueous ammonia of (2,10 ± 0,05) mL, the pure cubic phase was obtained, while for 200 ˚C and a volume between (2,30 ± 0,05) mL and (2,40 ± 0,05) mL, the desired hexagonal phase was obtained. Regarding the formation of the protonic conductor, this was conceived from the calcination of the precursor at 600 ˚C for 5 hours in air atmosphere and examined by thermogravimetric analyses. X-ray diffraction and infrared spectroscopy were then carried out on samples of the precursor calcinated at different temperatures and for various lengths of time to better understand the influence of these two parameters on the phase and functional groups of the resultant powder. As a result, it was concluded that at temperatures below 600 ˚C the compound retained the precursor, with the infrared vibrational bands associated with the ammonium cation, NH4+, being present in the sample. On the other hand, above and including 700 ˚C, new bands appeared, which are characteristic of the decomposition of HZr2(PO4)3 into other compounds, such as ZrP2O7. Only at 600 ˚C a pure HZr2(PO4)3 could be obtained. Finally, the cold sintering process was utilized to attempt to densify the HZr2(PO4)3 ceramic. Here, solutions with different pH values and boiling points were tested to check their influence on the process of the solid dissolution and solvent precipitation during the cold sintering process. Despite this work, the highest density obtained was only (73,15 ± 0,09) %. Even so, scanning electron microscopy showed that some pellets had densified regions, despite the general separation of the particles, suggesting that further work is necessary to uniformize local conditions during cold sintering.

Tuning cation nonstoichiometry is an effective approach to modify stability and functional properties and to assist surface redox engineering of perovskite oxides. This work addresses the possibility of the introduction of cation vacancies into the Ln sublattice of perovskite-related Ruddlesden-Popper Ln2NiO4+δ nickelates. La2-xNiO4±δ (x = 0-0.10) and Nd1.95NiO4±δ were selected as model compositions. Ceramic materials were sintered in air at 1350-1450°C for 10-40 h and characterized by the combination of experimental (XRD, ND, SEM, EDS, TGA, measurements of electrical transport properties) and computational (static lattice and molecular dynamics simulations) methods. All nominally A-site deficient materials comprised nickel oxide as a secondary phase. The fraction of NiO impurity in the La2-xNiO4±δ series increased with x, while the parameters of the orthorhombic crystal lattice remained composition-independent. Refinement of neutron diffraction patterns of La2NiO4+δ and La1.95NiO4±δ yielded the cation ratio La:Ni = 2:1 in the Ruddlesden-Popper phase for both materials. The results indicate that the concentration of cation vacancies that can be tolerated in the A sublattice of Ruddlesden-Popper La2NiO4+δ structure is ≪1 at.%, if any. The experimental findings are supported by the computer simulations showing that the formation of lanthanum-deficient La1.95NiO4 is energetically less favorable compared to cation-stoichiometric La2NiO4+δ co-existing with NiO or La4Ni3O10 secondary phases, and that introduction of lanthanum vacancy results in enhanced diffusivity of A-site cations at elevated temperatures and destabilization of Ruddlesden-Popper structure. Within experimental error, nominal cation deficiency had no effect on the electrical conductivity and oxygen permeability of La2-xNiO4±δ ceramics.

The path to decarbonization requires a substantial reduction of global greenhouse gas emissions. This challenge encourages the research of alternative energy sources. Fuel cell devices are one of the most promising hydrogen-based technological solutions for energy conversion. Nowadays, proton-conducting polymer electrolyte membrane fuel cells are typically used for temperatures below 100 °C, while solid oxide fuel cells are used between 800 and 1000 °C. Intermediate temperature fuel cells to bridge this temperature gap remain under development since new capable electrolytes operating between 200 and 500 °C are still needed. Nonetheless, this class of intermediate fuel cells can potentially overcome some of the challenges in low and high-temperature fuel cells nowadays, such as avoiding non-metal catalysts, decreased costs, and higher longevities. Inorganic solid acids such as KH₂PO₄, CsHSO₄, CsH₂PO₄, and their derivations, have drawn considerable attention due to their high protonic conductivity under temperatures ranging from 150 °C to 300 °C. The electrical conductivity of CsH₂PO₄ shows a steep increase around 235 °C due to a phase transition from monoclinic to a superprotonic phase with cubic symmetry. Above this temperature, the conductivity exceeds 10⁻²S/cm, making it a promising material for electrolytes in fuel cells. However, the material is dehydrated beyond this temperature when the atmosphere is not sufficiently humidified. In this respect, two different approaches can be made to extend the narrow operating temperature range of this material: 1) improve the thermal stability of the material to restrict dehydration; 2) increase the conductivity of the material below the superprotonic phase transition. In the present study, we studied the CsH₂PO₄ properties as a function of grain size and processing route. The particle size was decreased by wet ball-milling, while the grain growth was achieved by the cold sintering process. The electrical conductivity of the superprotonic phase was found to be independent of the grain size, which suggests that transport through grain interiors is the dominant process. As for the electrical conductivity of the monoclinic phase, the brick layer analysis revealed that, for the as-grown and ball-milled samples, the conductivity is governed by the conduction pathways along the parallel grain boundaries. Additionally, we prepared several CsH₂PO₄ – CsH₅(PO₄)₂ composite systems to enhance the electrical conductivity of the electrolyte below the superprotonic phase transition. The addition of polycrystalline CsH₅(PO₄)₂, even in small amounts (4 wt %), has a significant impact on the electrical conductivity of the electrolyte. After the first reverse transition from the superprotonic state to the monoclinic phase, the proton conductivity was found to increase by over one order of magnitude in the low-temperature phase (below 235 °C). Conversely, at higher temperatures, the superprotonic nature of the sample remained unchanged, and the conductivity was found to be very close to the pure CsH₂PO₄ material. On adjusting the amount of CsH₅(PO₄)₂, we found that 26 wt % was the optimal quantity to provide superprotonic conductivity over a wide temperature range (160 – 270 °C). The enhanced conductivity at lower temperatures could be explained by forming a liquid CsH5(PO₄)₂ layer along with the interfaces and grain boundaries of the material. Above 151 °C the melting of this highly conductive CsH₅(PO₄)₂ phase was found to assist the proton conductivity of the composite.

Polymer electrolyte fuel cells (PEFCs) have been widely used for their ability to generate electricity in a sustainable way. However, the materials used in PEFCs present themselves with clear negative environmental impacts. Thus, the search for alternative materials is a critical aspect in order to improve the sustainability of such technologies. This dissertation aimed at the development of polymeric membranes of silk fibroin, a protein produced by the domestic silkworm, not thoroughly explored for fuel cell related applications. In an initial phase of this work, two methods for fibroin extraction and purification were tested and the one that proved to be more reproductive was chosen. After that, the membrane preparation conditions were studied and optimized, using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and a significant impact on the structure of fibroin by the evaporation process of water was verified. Considering the application in PEFCs, the ion-exchange capacity (IEC), dynamic water vapor sorption (DVS), thermal and mechanical properties were also studied. The obtained results demonstrated that when the -sheet fraction is higher in the membrane structure, water sorption is up to 28% of its mass, at 25 °C. Finally, the ionic conductivity of the membranes was evaluated by impedance spectroscopy, at different temperature (T) and relative humidity (RH) conditions. Silk fibroin membranes present ionic conductivity values ranging from 5.7 107 S.cm1, at lower temperatures and relative humidities, to 1.8 103 S.cm1 at RH = 98% and T = 94°C. The obtained values for conductivity are lower when compared with commercial membranes based on Nafion®, however, the silk fibroin membranes still demonstrate promising results regarding thermal and mechanical properties, with degradation temperatures up to 260 °C. As a biopolymer with such promising properties, silk fibroin is a material with potential application in PEFCs

Heavy-duty vehicles are a major contributor to CO2 emissions in Europe. Today, there are several technological alternatives to decarbonize the industry, one of which is fuel cells. This thesis sought to understand the impact of fuel cell heavy-duty trucks (FC-HDT) on the long-distance road freight industry in Europe in 2031. The results were obtained by conducting expert interviews and analyzing pilot projects, industry reports, press releases, and scientific journals. Compared to heavy-duty trucks with internal combustion engines, the disadvantages of FC-HDTs in areas such as range, infrastructure, and cost outweigh the advantages. However, zero tailpipe emissions can become a decisive advantage if there is stronger regulation of CO2. Therefore, the experts agreed that in the future, only the use of FC-HDTs and battery electric heavy-duty trucks (BE-HDTs) would be a viable option. FC-HDTs, compared with BE-HDTs, currently offer shorter refueling times and longer ranges but are falling behind in terms of costs. A scenario planning approach indicates that FC-HDTs will have moderate adaptation and split the market with BE-HDTs. Scenarios in which FC-HDTs or BE-HDTs became the only prevailing technology were assessed as less likely.