Frontiers in Ceramic Science

Hydrogen seems to be the fuel of the future since it is clean-burning and its only by-product is water. Currently, around 95% of the hydrogen global production is accomplished by non-renewable energy sources, 4% is obtained from water and only 1% from biomass. Hydrogen production from renewable energy sources such as biomass represents an important challenge for the future. Nowadays, steam reforming is the cheapest way to produce hydrogen. This chapter summarizes data regarding hydrogen production by steam reforming of biomass renewable sources and biomass tar, emphasizing the catalysts development for this process. The development of high active catalysts with good stability and selectivity continues to be a challenge. For this purpose, the reactivity of different catalytic systems as well as their advantages and disadvantages will be discussed.

Sorbents with different CaO/Ca12Al14O33 ratios were prepared by microwaveassisted self-combustion. Catalysts were prepared by Ni wet impregnation of sorbents and a catalyst with an optimized composition was also prepared by microwave-assisted self-combustion method. Then, the CO2 sorption capacity and catalytic activity in sorption enhanced steam methane reforming (SE-SMR) for production of hydrogen were evaluated. X-ray diffraction (XRD) analysis of reduced catalysts confirmed Ni, CaO and Ca12Al14O33 phases. Catalysts prepared by wet impregnation favor high surface area, and consequently the CO2 sorption. The CO2 sorption by CaO modifies the equilibrium of the water gas shift reaction (WGS) and consequently the hydrogen production is improved at 650 ºC. The time of breakthrough for H2, CO, CO2, and CH4 depends on both the excess of CaO and operating conditions (H2O/CH4 ratio). Ni-CA75 catalyst obtained by impregnation method with a CaO excess around 75% leads to the optimum activity. It exhibits high sorption capacity and hydrogen selectivity of 100% during 2 h and 16 h for H2O/CH4 ratios of 3 and 1, respectively. Therefore, Ni-CA75 is the most active and stable catalyst during 50 h in CO2 sorption enhanced steam methane reforming, even at an unusually low temperature (650 ºC).

Dry reforming (DR) processes consist of a reaction between an adequate feedstock and carbon dioxide to produce syngas. In the case of a renewable feedstock (biogas, bioalcohols, wood tar,…), the DR processes become very interesting since they consume greenhouse gases (CO2, CH4,…) and produce hydrogen and syngas mixtures that can be considered as renewable alternatives to fossil fuels. The DR processes involve endothermic reactions accompanied by side reactions that decrease the overall process efficiency. The use of a catalytic material is expected to reduce the energy required for the process and to favor the selectivity towards syngas production. Thus, in the last decades, many studies considered the synthesis of catalytic materials that are active, selective and stable in DR reactions. This chapter considers the recent advances in the catalytic DR of methane, alcohols and biomass tar. The most recent catalytic materials are discussed in terms of their preparation, physico-chemical characteristics, and intrinsic properties that serve the purpose of the DR reactions. A special attention is paid to the carbon deposition problem and the different strategies that are adopted to minimize it. A final part of the chapter discusses the most recent developments in plasma, microwaves, solar energy and electrical current technologies for dry reforming reactions. Some examples of the developed reactor technologies are also presented including chemical looping reforming, membrane reactors and ceramic counter flow reactor.

Ni1-xCoxO-CGO, Ni1-xCuxO-CGO, CuOCGO and NiO-CGO composites powders were obtained by a one-step synthesis method and their catalytic activities in dry reforming of methane (CH4+CO2) for hydrogen production were evaluated. X-ray diffraction (XRD) analysis of composite powders confirmed NiO, Co3O4, CuO and Ce0.9Gd0.1O1.95 cubic phases. XRD results also showed the formation of nanocrystalline materials. Cobalt-based materials showed higher surface area values (SBET) than copper-based materials. According to temperature programmed reduction (TPR) analyses, Ni0.6Co0.4O-CGO (NiCo0.4) and Ni0.4Co0.6O-CGO (NiCo0.6) catalysts have higher reduction capacity and stronger metal/support interaction than Ni0.6Cu0.4O-CGO (NiCu0.4) and Ni0.4Cu0.6O-CGO (NiCu0.6) materials. Rietveld refinement analyses for NiCo0.4, NiCo0.6 and NiCu0.4 reduced catalysts, confirmed the presence of Ni-Co and Ni- Cu alloys. These factors are important for enhanced catalytic activity avoiding carbon deposition. NiCo0.4 and NiCo0.6 catalysts had higher conversions of CH4 and CO2 than Ni-CGO and Cu-based catalysts. NiCo-based catalyst showed a better resistance to carbon deposition. NiCo0.4 had high H2/CO ratio and the best reaction selectivity below 600 ºC. The electrocatalytic activity of NiCo0.4/CGO/NiCo0.4 screen-printed symmetrical cells from hydrogen and synthetic biogas (CH4+CO2) electro-oxidation reaction was studied by impedance spectroscopy in the temperature range between 650 and 850 ºC. The polarization resistance was influenced by the atmosphere conditions. Total polarization resistances (Rp) of 0.96 and 36.10 Ω cm2 were obtained at 750 °C for H2 and biogas atmospheres, respectively. The activation energy (Ea) was lower when H2 was used (0.92 eV). The hydrogen oxidation reaction occurs more easily than the dry reforming of methane.

The anode of a Solid Oxide Fuel Cell (SOFC) is the component responsible for releasing electrons. It must present some prerequisites such as porosity (30-40 wt.%) and mixed conductivity (ionic and electronic). To achieve that, it often consists of a ceramic-metal composite (cermet) material. Ni-YSZ cermet is the material most commonly used as anode SOFC, however it presents low electrochemical efficiency below 750 °C. With this in mind, new materials have been studied, and Ni-GDC cermet has shown promising results. This chapter focused mainly on a brief review of NiO-Ce0.9Gd0.1O1.95 (NiO-GDC) composites and their Ni-GDC cermets as SOFC anode materials. The chapter reports the history, concept, operation principle, reaction mechanisms and components of SOFCs, addressing recent works in the field. The synthesis of NiO-GDC composites by a one-step synthesis method (polymer-based method) is compared with the conventional mechanical mixing method. Impedance spectroscopy is shown to be a crucial technique to investigate the electrical properties of SOFC functional materials. The chapter ends with a literature review on the electrical properties of NiO-GDC composites and their cermets.

The present chapter is devoted to comparison of oxide-based anode materials developed during the last 10-15 years, with a particular focus on relationships between their functional characteristics, such as phase and structural stability, electronic and ionic conductivity, thermal and chemical expansion, and the electrochemical activity of the corresponding anodes. In most studies, the strategy of selection of the anode material composition is unclear while no obvious correlations can be revealed between the anode activity and other properties of the material. The situation is complicated by the presence of catalytically active phases in the anode layers, such as CeO2-based compounds or metallic phases (Pt, Ag, Ni, etc). The latter are frequently introduced as current collecting coatings for optimizing the interface contact between the anode and interconnect; however, their influence on the catalytic activity cannot a priori be considered to be negligible. For this reason, the electrochemical characteristics of different anode materials studied by the same research group and, consequently, prepared and modified by the same route, frequently appear to be similar. The purpose of this review is to critically discuss the results where the origins of the observed anode performance are arguable, and to emphasize the studies where a reliable analysis of the performance-determining factors has been done.