Evaluation Criterion of Proton Exchange Membrane (ECPEM) fuel cells considering inserted porous media inside the gas flow channel

The focus of this study is to evaluate the effects of porous media inside the gas flow channel of Proton Exchange Membrane Fuel Cells (PEMFC) on four different output parameters of voltage, power density, pressure drop, and Nusselt number (Nu) considering the impacts of its thickness, viscous resistance, and current density. Although it is proved that the new design will improve the convective heat transfer, there have not been studies to evaluate the effects of this porous layer on the electrochemical performance and heat transfer simultaneously. The results showed that viscous resistance has by far the highest impact on the power densities in high current densities while thicker inserted porous layer improves the performance. Results also demonstrate that a parameter is needed to consider all these output parameters at the same time, hence the Evaluation Criterion of Proton Exchange Membrane (ECPEM) is defined using artificial neural network (ANN) modeling. Single-objective optimization of the ECPEM is developed using the ANN models to produce 250,000 data. The optimum value of ECPEM was obtained 78.88 in the thickness of 500 mu m and the viscous resistance of 2,111,000 (1/m(2)) while the current density is equal to 0.65 (A/cm(2)).

Stability and performance of tape cast anode supported electrolyte (ASE) cells.

Raphaël Ihringer, Jan Van Herle

Recent progress in our development of anode supported electrolyte (ASE) SOFcells is reported. The cells are fabricated by cocasting of aqueous slurries of NiO/YSZ and 8YSZ, followed by cofiring. A gradient cathode is deposited by screenprinting successive layers of LSM/YSZ composite and lanthanum cobaltite, followed by a 2nd firing step. Full cells show a final thickness of ? 0.25 mm, high flexibility and reasonable warpage. Proper anode current collection is crucial and obtained by cofiring an additional outer layer together with the substrate. Power density can reach 0.2 W/cm2 at 600°C with active cathodes (LSC, short-lived) and >1 W/cm2 at 810°C with the LSM cathode. Stability was tested during 2900 h, giving an average degradation of 3%/1000 h at 0.5 A/cm2 current density. Degradation can be delayed by short-time polarisation at higher current density: e.g. “reactivation” at 0.85 A/cm2 (30 min.) delayed degradation by 300 h.

European Solid Oxide Fuel Cell Forum2002

Reversible solid-oxide cell stack based power-to-x-to-power systems: Comparison of thermodynamic performance

Philippe Aubin, Tzu-En Lin, François Maréchal, Maria del Mar Perez Fortes, Jan Van Herle, Ligang Wang, Yumeng Zhang

The increasing penetration of variable renewable energies poses new challenges for grid management. The economic feasibility of grid-balancing plants may be limited by low annual operating hours if they work either only for power generation or only for power storage. This issue might be addressed by a dual-function power plant with power-to-x capability, which can produce electricity or store excess renewable electricity into chemicals at different periods. Such a plant can be uniquely enabled by a solid-oxide cell stack, which can switch between fuel cell and electrolysis with the same stack. This paper investigates the optimal conceptual design of this type of plant, represented by power-to-x-to-power process chains with x being hydrogen, syngas, methane, methanol and ammonia, concerning the efficiency (on a lower heating value) and power densities. The results show that an increase in current density leads to an increased oxygen flow rate and a decreased reactant utilization at the stack level for its thermal management, and an increased power density and a decreased efficiency at the system level. The power-generation efficiency is ranked as methane (65.9%), methanol (60.2%), ammonia (58.2%), hydrogen (58.3%), syngas (53.3%) at 0.4 A/cm2, due to the benefit of heat-to-chemical-energy conversion by chemical reformulating and the deterioration of electrochemical performance by the dilution of hydrogen. The power-storage efficiency is ranked as syngas (80%), hydrogen (74%), methane (72%), methanol (68%), ammonia (66%) at 0.7 A/cm2, mainly due to the benefit of co-electrolysis and the chemical energy loss occurring in the chemical synthesis reactions. The lost chemical energy improves plant-wise heat integration and compensates for its adverse effect on power-storage efficiency. Combining these efficiency numbers of the two modes results in a rank of round-trip efficiency: methane (47.5%) > syngas (43.3%) ≈ hydrogen (42.6%) > methanol (40.7%) > ammonia (38.6%). The pool of plant designs obtained lays the basis for the optimal deployment of this balancing technology for specific applications.

2020

Molecular Engineering of Electrode Surfaces for Electrocatalysis

Patrick Eduard Alexa

Electrocatalysts play an important part on the route towards environmental friendly energy sources. They support modern technologies like fuel cells to move further away from the utilization of fossil fuels and the resulting CO2 emission into our atmosphere. The main principle of such energy conversion technologies is to store chemical energy within chemical bonds and to release it as electric energy to fulfill the energy demand of today's society. However energy conversion chemistry in such technologies requires catalysts in order to be effective by providing an alternative reaction pathway that lowers activation energies and maximizes the energy output. Providing suitable catalyst surfaces requires a fundamental knowledge of the interaction of reactants and reaction intermediates with the catalyst surfaces. This concern is addressed in this thesis by fabricating specifically tailored molecular components on metal surfaces, which act as organic/inorganic hybrid electrodes for various energy conversion reactions in electrocatalysis. Characterizing the electrode surface is crucial in order to identify active sites of the catalyst. The molecular structure is investigated by surface sensitive techniques, such as STM and XPS in combination with a home-build transfer system, which enables electrode fabrication in UHV and electrocatalytic experiments in a liquid environment. A detailed characterization of organic polymer components on electrode surfaces is obtained by combining experimental data with MD simulations. The morphology of polymer networks can be successfully mimicked and depending on the chemical composition and the structure, spatial correlations on short length scales are reported. Hybrid electrodes with organic polymer co-catalysts reveal an increased catalytic activity for the HER by providing suitable docking sites that act as catalytic active sites. Reactants and reaction intermediates are stabilized via hydrogen bonding, which results in a higher catalytic turnover. The specific designed structure of the organic co-catalyst helps to identify a fraction of the reaction mechanism and highlights the importance of molecular components in electrocatalysis. The utilization of organic units on electrocatalysts provides also an excellent opportunity in order to use single metal atoms as catalysts. Engineering large pi-systems has no significant impact on the ORR activity, while the distance between active sites and the underlying electrode are crucial for enhancing the catalytic activity. A significant factor in using molecular components in electrocatalysis is the stability of the organics. Using electrografting allows to design organic/inorganic hybrid electrodes with covalent bonds between the molecular component and the metal substrate. Attaching empty porphyrin molecules to substrates introduces a powerful technique in molecular engineering, since empty porphyrin cycles can be metallized with various metal centers that can act as electrocatalysts for different electrochemical reactions. In summary, this thesis provides multiple engineering aspects, which are useful in designing molecular components on metal electrodes for energy conversion. Such hybrid catalyst surfaces are capable of improving the catalytic activity and allow to study the mechanism of catalytic reactions. Implementing organic components provides new routes for tailoring novel materials, which deliver new insight into the complex field of electrocatalysis.