Non noble transition metal elements for the modification of Ni based electrodes in solid oxide fuel and electrolysis cells applications

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Σουβαλιώτη, Αθηνά
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Water electrolysis via fuel cell technology is an efficient technique, which through the electro-chemical reaction produces H2 and O2. Alkaline water electrolysis takes place in low temperatures (<100oC), the electrodes are made of metal, the electrolyte is liquid usually KOH and the two electro-lyser chambers are divided by a diaphragm. This technology produces low purity hydrogen, is expen-sive and has low efficiency due to high operating voltage [8]. Proton Exchange Membrane (PEM) electrolysis cells/stacks operate at low temperature (e.g. 70 oC), have a proton conductive polymer membrane as electrolyte and comprise noble metal-containing electrodes (Pt/C-based and IrOx-based). They generally, succeed higher current densities, compared to the alkaline electrolyzer, but they are more expensive and inadequate for long term operation [9]. Another pathway is the high-temperature electrolysis of steam, using solid oxide electrolysis cell (SOEC). The latter technological proposal is more efficient due to more favorable thermodynamic and electrochemical kinetic condi-tions for the water splitting and exhibits the highest tolerance to CO contamination among all fuel cell technologies. In the present, alkaline and proton electrolysers are commercial but confront many issues. In order to be used as interconnectors for power balancing and energy storage, these systems need two separate features, one operating in the fuel cell mode and the other in the electrolysis mode. These configurations are complex and difficult to manage. On the other hand, solid oxide cells (SOCs) can operate as electrolysers and/or fuel cells, depending on the production needs [10]. Re-versible solid oxide cells have lower activation losses at lower current densities (in comparison to alkaline and PEM electrolytes) which indicates higher power generation during the fuel cell mode and decreased energy demands during electrolysis [8]. In general, RSOCs have many advantages in comparison with the other cell technologies. Poten-tially, such reversible SOCs can be combined with already existing energy technologies. RSOCs is the only system that can operate bi-directionally [10]. In the SOE mode, hydrogen is produced via Power-to-Gas process (P2G) and, the same module is able to operate in the reverse mode as SOFC and produce power, via Gas-to-Power process (G2P). It has been demonstrated that a SOFC system can achieve low electrode overvoltages even for high current densities. Furthermore, at SOEC operation an electrical-to-hydrogen conversion efficiency above 100% is feasible [8]. The RSOC is a very prom-ising technology, that has reduced capital expenses and energy requirements as there is no need for hydrogen’s transportation and delivery. RSOCs, can store energy for micro- grid or large context, as unique standalone solution or hybridised with other storage systems. RSOC system has a variety of applications. Great interest presents the possibility of direct electro-lyzing CO2, or even co-electrolyzing of CO2 and H2O simultaneously. The product of this process is syngas (CO and H2) which is a widely used fuel. Syngas is traditionally derived from fossil fuels, con-tributing to the greenhouse effect. Syngas from co-electrolysis in a SOEC system can be feedstock to produce every hydrocarbon that can be used in the chemical industry, refineries via Fischer-Tropsch synthesis or for the production of synthetic natural gas (SNG) via Sabatier process with zero environ-mental impact [11]. SOFC operation can distribute power as CHP unit (Combined Heat and Power), can generate power using natural gas or LPG (remote system), and backup power [12]. Fuel cell mode, with methane as fuel, generates high purity hydrogen through steam-reforming process and dry re-forming process. Additionally, surplus heat from coupling processes can be used or exported from the RSOC system, achieving surplus energy. SOCs due to the innovative functions, have been acknowl-edged appropriate for earthy and space applications. Hydrogen produced in RSOC system can be potential useful in the chemical, metallurgical and glass industry, directly as fuel for cars, and public transport generally, in refineries to replace fossil-based feedstocks, or for electricity and heat pro-duction [12]. So far SOCs technology is an innovative proposal, operating still at the lab-scale due to stability problems that prevent the widespread use and commercialization. Limited long-term durability and high capital costs are the key challenges to implement large scale power production. For large scale applications, the good scalability allows construction of stacks by assembling individual SOCs cells. By increasing the size of the cell and the stack, the cost per unit of product (power, fuel) minimizes, but the internal stack temperature increases causing degradation to the cells. Furthermore, an issue that has been noticed is the deactivation of the Ni/GDC (or Ni/YSZ) hydrogen/steam electrode, which is usually ascribed to nickel’s re-oxidation, coarsening, evaporation and agglomeration during H2O elec-trolysis, or/and carbon deposition during H2O/CO2 co-electrolysis. Oxidization of nickel by the for-mation of volatile compounds, such as nickel hydroxides results to decreased electronic conductivity and causes mechanical stress to the electrode [13]. Another disadvantage is the delamination of the oxygen electrode. In conclusion, it is believed that material’s failure accelerates cell’s degradation in long term operation [14], [15]. Fuel cells are an alternative to heat engines for electricity generation. The coefficient of perfor-mance of heat engines, where the produced heat by the combustion of a fuel is converted into me-chanical energy, it is not possible to exceed the Carnot coefficient of efficiency. Fuel cells are not subjected to this limitation and therefore their efficiency is usually greater than a thermal engine’s. Nevertheless, the overall disadvantage of RSOCs is the low Technology Readiness Level. More re-search and development must be carried out before rSOCs technology is ready for the general power and fuel market. Constantly efforts are conducting to optimize materials to give high performance and durability, with low-cost and long-term stable cells. Investigations are conducting of new and more tolerant fuel and oxygen electrodes and of optimization of operating conditions and adequate utilization of external heat sources [15]. This diploma thesis is an introduction to the key features and characteristics of RSOCs’ working principles and optimization of the fuel electrode, by means of chemical modification with transition metal elements, for water electrolysis and power production. Specifically, the presented assignment is part of a wider research study, which aims to elucidate the modifying effect from different loadings of Mo and Au in Ni/GDC, with the objective to find their optimum content, especially limiting Au, for stable and enhanced operation under RSOC mode.
κελία καυσίμου, Fuel cells, Green energy