Intermediate temperature fuel cells based on doped ceria–LiCl–SrCl2 composite electrolyte

A new type of oxide-salt composite electrolyte, gadolinium-doped ceria (GDC)–LiCl–SrCl₂, was developed and demonstrated its promising use for intermediate temperature (400–700 °C) fuel cells (ITFCs). The dc electrical conductivity of this composite electrolyte (0.09–0.13 S cm⁻¹ at 500–650 °C) was 3–10 times higher than that of the pure GDC electrolyte, indicating remarkable proton or oxygen ion conduction existing in the LiCl–SrCl₂ chloride salts or at the interface between GDC and the chloride salts. Using this composite electrolyte, peak power densities of 260 and 510 mW cm⁻², with current densities of 650 and 1250 mA cm⁻² were achieved at 550 and 625 °C, respectively. This makes the new material a good candidate electrolyte for future low-cost ITFCs.     

Oxide ion and proton conduction in doped ceria–carbonate composite materials

The direct current four-probe method has been employed to investigate the conduction of oxide ion and proton in a doped ceria–carbonate composite electrolyte for fuel cells. The measurements are conducted in oxygen and in hydrogen atmospheres in the temperature range of 425–650 °C. The conductivities of both of O²⁻ and H⁺ increase with the increase of carbonate content above the melting point of the carbonate. The ionic conductivities of the composite electrolytes have also been simulated using the effective medium percolation theory. The deviations between experimental results and simulated values of O²⁻ conductivity are caused by the associating effect of ceramic and carbonate phases, which leads to a higher O²⁻ migration energy through the phase interface. According to the comparison of experimental data and simulated values, the conduction mechanisms of O²⁻ and H⁺ have been proposed.     

The effect of ceria content in nickel–ceria composite anode catalysts on the discharge performance for solid oxide fuel cells

Optimum ceria content in nickel–ceria composite anode catalyst from the point of discharge performance is discussed. The ohmic loss increased when the ceria content was higher than 30 mol%. Even though the electrical conductivity of the anode decreased with increasing ceria content in the anode catalyst in association with decreasing nickel content, the ohmic loss was kept low until the ceria content was ≤30 mol% because the semiconducting ceria compensated for the decreased current path owing to the decreasing nickel content. The lowest activation loss was observed when the ceria content in the nickel anode catalyst was 30 mol% and the maximum activation loss was obtained for ceria content of 2 mol%. Ceria content in nickel anode influenced microstructure of the anode matrix. When the CeO₂ content was 2 mol%, sintering of anode catalyst was evident and the porosity of anode matrix was almost 57% - highest in this study. Whereas sintering of anode catalyst was not evident and the porosity of anode matrix was 46% when the ceria content in the nickel anode catalyst was 30 mol%. Activation loss was strongly influenced by microstructure of anode matrix, and highest activation loss when the CeO₂ content was 2 mol% was owing to the inappropriate microstructure for electrochemical reaction: sintering of the anode catalyst and excessive porosity of the anode.     

Validation of H/O conduction in doped ceria–carbonate composite material using an electrochemical pumping method

A hydrogen and oxygen electrochemical pump technique has been employed to elucidate the conduction of proton and oxygen ion in a doped ceria–carbonate composite electrolyte for intermediate temperature solid oxide fuel cells. The composite material shows efficient conductivities of both of the two ions at 650 °C. The molten carbonate phase is important for the migration of both of the two ions. The mechanism of the conduction of proton and oxygen ion is also discussed.     

Semiconductor ionic Ce0.8Sm0.2O2-δ-Na2CO3 – LiCo0.225Cu0.075Ni0.7O 3-δ composite material as electrolyte for low temperature solid oxide fuel cells

Semiconductor ionic electrolytes have obtained much attention because of good ionic conductivity and electrochemical performance. Novel semiconductor ionic NSDC (Ce_0.8Sm_0.2O_2-δ-Na₂CO₃)-LCCN (LiCo_0·225Cu_0·075Ni_0·7O_3-δ) composite materials have been adopted as electrolyte membrane for the first time, in which symmetrical cell composed of NSDC-LCCN membrane is constructed with Ni_0·8Co_0·15Al_0·05LiO₂ (NCAl)-pasted Ni foam electrodes. An open circuit voltage (OCV) above 1 V and improved power density are obtained in the NSDC-LCCN cells, which confirms the functionality of the proposed semiconductor ionic materials. Meanwhile, X-ray diffractometer (XRD) and Scanning electron microscope (SEM) analyses identify the phase purity and homogenous nanocomposite morphology of all the NSDC-LCCN materials samples with various mass ratios. The performance illustrated by much more steady instead of transient state evaluation reveals that 3NSDC-LCCN composite electrolyte is most optimum, and the corresponding cell exhibits a considerable maximum power density of 598 mW cm⁻² at 550 °C, over five times of that of pure NSDC electrolyte cells. Short-term duration test of 3NSDC-LCCN cell at 550 °C shows that the cell could steadily operate up to ~9 h without obvious degradation at a remarkable current density of 469 mA cm⁻², which indicates that NSDC-LCCN composite electrolyte is a promising material for low temperature solid oxide fuel cells.     

The effect of a perforated plate on the propagation of laminar hydrogen flames in a channel – A numerical study

Laminar hydrogen flame propagation in a channel with a perforated plate is investigated using 2D reactive Navies-Stokes simulations. The effect of the perforated plate on flame propagation is treated with a porous media model. A one step chemistry model is used for the combustion of the stoichiometric H₂–air mixture. Numerical simulations show that the perforated plate has considerable effect on the flame propagation in the region downstream from the perforated plate and marginal effect on the upstream region. It is found to squeeze the flame front and result in a ring of unburned gas pocket around the flame neck. The resulting abrupt change in flow directions leads to the formation of some vortices. Downstream of the perforated plate, a wrinkled “M”-shape flame is observed with “W” shape flame speed evolution, which lastly turns back to a convex curved flame front. Parametric studies have also been carried out on the inertial resistance factor, porosity, perforated plate length and its location to investigate their effects on flame evolution. Overall, for parameter range studied, the perforated plate has an effect of reducing the flame speed downstream of it.     

The effect of concentration gradients on deflagration-to-detonation transition in a rectangular channel with and without obstructions – A numerical study

Explosions in homogeneous reactive mixtures have been widely studied both experimentally and numerically. However, in accident scenarios, mixtures are usually inhomogeneous due to the localized nature of most fuel releases, buoyancy effects and the finite time between release and ignition. It is imperative to determine whether mixture inhomogeneity can increase the explosion hazard beyond what is known for homogeneous mixtures. The present numerical investigation aims to study flame acceleration and transition to detonation in homogeneous and inhomogeneous hydrogen-air mixtures with two different average hydrogen concentrations in a horizontal rectangular channel. A density-based solver was implemented within the OpenFOAM CFD toolbox. The Harten–Lax–van Leer–Contact (HLLC) scheme was used for accurate shock capturing. A high-resolution grid is provided by using adaptive mesh refinement, which leads to 30 grid points per half reaction length (HRL). In agreement with previous experimental results, it is found that transverse concentration gradients can either strengthen or weaken flame acceleration, depending on average hydrogen concentration and channel obstruction. Comparing experiments and simulations, the paper analyses flame speed and pressure histories, identifies locations of detonation onset, and interprets the effects of concentration gradients.     

Energy loss in electrochemical diaphragm process of chlorine and alkali industry – A collateral effect of the undesirable generation of chlorate

Contamination of NaOH with chlorate constitutes a major problem for the chlorine–alkali industry, particularly when electrolytic cells based on the diaphragm process are employed. In this paper, pilot and laboratory cell experiments revealed that chlorate contamination in diaphragm cells also inhibits hydrogen evolution and gives rise to a significant increase in electrical energy consumption. Electrolysis carried out under conditions that simulated the industrial process (current density 240 mA cm⁻²; temperature 90 °C; brine flux 23 L cm⁻² h⁻¹) revealed that chlorate formation depends on brine flux and NaOH production. The inhibitory effect of chlorate on the main cathodic reaction was demonstrated in bench cell experiments, with cathodic displacement of the hydrogen evolution reaction by more than 100 mV in the presence of 0.4% chlorate compared with ideal conditions in which chlorate formation was absent. This hydrogen generation overpotential can charge the total electric energy balance in more than 5% of the total value, consisting of a critical loss for this process.     

SUNRISE – A caesura after nuclear has gone: Energy in Germany(and in other potentially de-nuclearizing countries)

The German Bundestag decided June 30, 2011 to shut down by 2022 stepwise the complete national nuclear power plant capacity which at the time of decision generated some 22% of the nation’s electricity demand. This presentation tries to present a technology forecast of three potential compensations 1) energy and exergy efficiency gains, 2) renewable energies, and 3) hydrogen energy, thereby bearing in mind that fossil fuels such as coal, mineral oil and natural gas will by no means be gone after that short 10 year transition time. Consequently, not only the three compensations, but also fossil fuels – now efficient to the technological utmost – have to meet the obligation of reducing anthropogenic environmental and climate changing influences, and, in Germany’s case with 75% of its energy demand covered by imports of great importance, try to decrease the almost life risking high import rate by distributing suppliers all over the world and start introducing global clean renewable energies and trade in renewable hydrogen energy. Whether SUNRISE will evolve into a paragon for all those nations thinking of, planning for, or already taking the first steps towards saying farewell to nuclear is too early to determine. The four components of energy sustainability compensating for nuclear – energy and exergy efficiency gains, clean fossil, solar and hydrogen – pluck up courage, make headway and leave nuclear behind. And, in particular, hydrogen energy is and will increasingly become humankind’s common cause!     

Analysis of chemical,electrochemical reactions and thermo-fluid flow in methane-feed internal reforming SOFCs: Part I – Modeling and effect of gas concentrations

Direct internal reforming solid oxide fuel cells (DIR SOFCs) have complicated distributions of temperature and species concentrations due to various chemical and electrochemical reactions. The details of these properties are studied by a 3-D numerical simulation in this work. The simulation modeling used governing equations (mass, momentum, energy and species balance equations) generally suitable to porous medium with porosity variable of zero (solid), 0.3 (porous medium) and 1.0 (fluid). Chemical kinetics equations for the internal reforming and shift reactions based on the Langmuir–Hinshelwood model were incorporated. Hydrogen and carbon monoxide oxidations were considered both participating in electrochemical reactions. The experimentally measured current density–potential curves were compared with the simulation data to validate the code, which revealed that the simulation model was able to predict the dilution effect of nitrogen and the mass transfer under high current densities. It is found that the temperature dramatically declined near the fuel inlet with strong endothermic reactions, but it increased along the fluid flow with electrochemically exothermic reactions. A low steam-to-carbon ratio (SCR) led to high steam reforming and water gas shift reaction rates, which generated a greater amount of hydrogen. Therefore, current density increased with low SCR. The average current density due to carbon monoxide electrochemical oxidation varies from 205.3 A/m² under an SCR of 2.0 to 47.6 A/m² under an SCR of 4.0. The average current density due to hydrogen electrochemical oxidation was 5535.4 A/m² under an SCR of 2.0, which was 27 times higher than that of carbon monoxide. The total current density ranged from 5740.8 A/m² under an SCR of 2.0 to 2268.9 A/m² under an SCR of 4.0.     

D'Alembert's Solution in Thermoelasticity – Impact of a Rod Against a Heated Barrier: Part II. A Case of Coupled Strain and Temperature Fields

The problem of impact of a thermoelastic rod against a heated rigid barrier is considered, in so doing lateral surfaces and free end of the rod are heat insulated, and free heat exchange between the rod and the rigid obstacle or ideal thermal contact occurs within contacting end. The rod's thermoelastic behavior is described by the Green–Naghdi theory of thermoelasticity. D'Alembert's method, which is based on the analytical solution of equations of the hyperbolic type describing the dynamic behavior of the thermoelastic rod, is used as the method of solution. This solution involves four arbitrary functions which are determined from the initial and boundary conditions and are piecewise constant functions. The procedure developed enables one to analyze the influence of thermoelastic parameters on the values to be found and to investigate numerically the longitudinal coordinate dependence of the desired functions at each fixed instant of the time beginning from the moment of the rod's collision with the barrier up to the moment of its rebound both without account for the stress and temperature fields coupling (in the companion paper, Part I) and in the case of coupling thermoelasticity (in this paper). As a numerical example, the impact of a thermoelastic rod against a heated barrier is considered with a small parameter of coupling between the strain and temperature fields. It has been shown that the presence of small coupling results in the generation of a new shock wave of small amplitude, namely: the reflected thermal wave from the incident elastic wave at the free rod's end. The rod's rebound may occur either at the moment of simultaneous arrival at the contact place of two reflected waves: elastic wave from the incident thermal wave and thermal wave from the incident elastic wave—or at the time when the reflected elastic wave from the incident elastic wave reaches the contact point.     

Mitigating ethanol crossover in DEFC: A composite anode with a thin layer of Pt50–Sn50 nanoparticles directly deposited into Nafion membrane surface

A composite anode comprising an outer and an inner catalyst layer is proposed to 1) suppress the ethanol crossover in direct ethanol fuel cell (DEFC), and 2) improve the cell performance as well as the utilization efficiency of ethanol fuel. The inner catalyst layer contains a thin layer of Pt₅₀–Sn₅₀ nanoparticles directly deposited on the Nafion® membrane surface through impregnation-reduction (IR) method, and acts as the reactive ethanol filter. In this paper, several aspects of the research are reported. First, the mitigation of ethanol crossover and the performance of membrane electrode assembly (MEA) of the proposed structure are compared to those with normal structure. Next, a candidate mechanism of the mitigation of ethanol crossover and the improvement of MEA performance is investigated. Third, SEM, X-ray, EDS and EPMA analysis are used to characterize microstructures, phases, chemical composition and distributions of the obtained Pt₅₀–Sn₅₀ layer. Finally, the ethanol crossover rate in a DEFC is determined through measuring the CO₂ concentration at the cathode exhaust in real time. Experimental results demonstrate that the composite anode with an inner layer of Pt₅₀–Sn₅₀ nano-catalyst particles on Nafion membrane surface suppresses ethanol crossover up to 17% more than the anode without the inner layer, and yield a 6% better MEA performance than the normal-MEA. The inner Pt₅₀–Sn₅₀ catalyst layer serves both as an ethanol filter and an electrode. Its dual-role contributes to the suppression of ethanol crossover, and improvement of both cell performance and the utilization efficiency of ethanol fuel, both of which are dependent on the catalyst activity of the ethanol electro-oxidation over the thin catalyst layer directly deposited on Nafion membrane surface.