Ákos Kriston

Simulation of the transient behavior of fuel cells by using operator splitting techniques for real-time applications

The functioning of fuel cells, in which simultaneous processes having different kinetics and different time constants occur, can be simulated by applying rather complex models. For the sake of better modeling larger numbers of sub-processes and their couplings have to be considered, which leads to complex and multi-step simulation frameworks. In this work new methods are introduced for the simulation of the behavior of fuel cells, which are based on operator splitting techniques. These methods can be applied for the simulation of rather complex problems, consequently they open up new vistas in respect to the real-time simulation. The errors of the schemes are analyzed while applying different kinetic approaches. The effects of constant current, current sweep and pulsed current are calculated. The qualitative and quantitative errors are analyzed and compared with measured data. It is proven that the method developed is suitable for describing the fast transient behavior, therefore it makes the real-time monitoring and controlling of the functioning of fuel cells possible.

Electrochemical nanogravimetric studies of adsorption, deposition, and dissolution processes occurring at platinum electrodes in acid media*

Polycrystalline smooth and platinized platinum electrodes have been extensively employed in electrochemistry. It is of utmost importance to gain a deeper insight into the processes occurring during their electrochemical transformations. Piezoelectric nanogravimetry by using electrochemical quartz crystal nanobalance (EQCN) is one of the most powerful tools for obtaining information on the events occurring at the electrode surface. This method has been exploited to monitor the surface mass changes as a function of the electrode potential varying the experimental conditions (time scale, solution composition, temperature), which allows one to draw conclusions in respect of the formation and removal of adsorbed and deposited species as well as changes in the electrochemical double layer. Furthermore, platinum dissolution processes, which are of importance (e.g., regarding the long-term stability of proton exchange fuel cells), are also discussed.

An IMEX scheme for reaction-diffusion equations: application for a PEM fuel cell model

An implicit-explicit (IMEX) method is developed for the numerical solution of reaction-diffusion equations with pure Neumann boundary conditions. The corresponding method of lines scheme with finite differences is analyzed: explicit conditions are given for its convergence in the ‖·‖∞ norm. The results are applied to a model for determining the overpotential in a proton exchange membrane (PEM) fuel cell.

Unusual surface mass changes in the course of the oxygen reduction reaction on platinum and their explanation by using a kinetic model

An unusual change of the surface mass with time has been observed during the oxygen reduction reaction on Pt using chronopotentiometry and simultaneous electrochemical quartz crystal nanobalance measurements. A simplified kinetic model of Damjanovic and Brusic, which involves two electrochemical and a chemical step, was analyzed using phase plane analysis. The theoretical analysis predicted that bistability might occur in this system at a certain set of parameter values. The mathematical simulation of the different trajectories explained well the strong influence of the starting potential and the current density on the change of the surface mass observed. Evidence was found that the surface coverage can increase at lower potentials, which can lead to the formation of hydrogen peroxide even if it is energetically unfavorable.

Testing and analyzing metastable pitting corrosion

Abstract In this study a novel characterization and an evaluation of the metastable pitting is introduced. The lifetime of the pits is described as a time period during which the alloy is dissolved. It was proved by theoretical calculation that the lifetime of pits could be unambiguously determined from a very important stochastical characteristic of the system, from the autocorrelation. The power spectrum density (PSD) of the process was also calculated and the effect of the event density, the propagation kinetic and the lifetime on the PSD was analyzed. The influence of the potential on the metastable pitting was investigated, and it was shown that mainly the lifetime of the pits increases with increasing potential.

Two Types of Platinum Dissolution in Acid Media: An Electrochemical Nanogravimetric Study

Smooth and platinized platinum electrodes in contact with sulfuric acid solutions were studied using electrochemical quartz crystal nanobalance (EQCN) technique at different temperatures. Two types of dissolution processes have been observed. A platinum loss was detected during the reduction of platinum oxide, the extent of which depends on the positive potential limit and the scan rate, and to a lesser extent on the temperature. The platinum dissolution during the electroreduction of oxide is related to the interfacial place exchange of the oxygen and platinum atoms in the oxide region. At elevated temperatures two competitive processes take place at high positive potentials: a dissolution of platinum and platinum oxide formation. These phenomena are of importance regarding the long-term stability of proton exchange fuel cells.

Improvement of accuracy of multi-scale models of Li-ion batteries by applying operator splitting techniques

In this work operator splitting techniques have been applied successfully to improve the accuracy of multi-scale Lithium-ion (Li-ion) battery models. A slightly simplified Li-ion battery model is derived, which can be solved on one time scale and multiple time scales. Different operator splitting schemes combined with different approximations are compared with the non-splitted reference solution in terms of stability, accuracy and processor cost. It is shown, that the reverse Strang-Marchuk splitting combined with the implicit scheme to solve the diffusion operator and Newton method to approximate the non-linear source term can improve the accuracy of the commonly applied vertical (sequential) multi-scale models by almost 3 times without considerably increasing the processor cost.

Stabilization of a Numerical Model Through the Boundary Conditions for the Real-Time Simulation of Fuel Cells

The fuel cells are to convert chemical energy directly to electricity with high efficiency. From an environmental point of view, it is worth emphasizing that the fuel cells are not emitting any greenhouse gases or other pollutants. To increase the efficiency of the fuel cells their properties must be analyzed deeply. To this aim numerical modeling is a very useful tool. The fuel cell system models are usually solved by applying constant currents with long time step. For our purposes high amplitude sine perturbation and real-time simulation are needed, because during the control of a fuel cell the information needs to be available in real-time. In this article our method for approaching the real time simulation is formulated including the so-called scaling method. Using this method, the real-time simulation of fuel cells is getting easier and more accurate.

The Effect of Charging and Discharging Lithium Iron Phosphate-graphite Cells at Different Temperatures on Degradation

The effect of charging and discharging lithium iron phosphate-graphite cells at different temperatures on their degradation is evaluated systematically. The degradation of the cells is assessed by using 10 charging and discharging temperature permutations ranging from -20 °C to 30 °C. This allows an analysis of the effect of charge and discharge temperatures on aging, and their associations. A total of 100 charge/discharge cycles were carried out. Every 25 cycles a reference cycle was performed to assess the reversible and irreversible capacity degradation. A multi-factor analysis of variance was used, and the experimental results were fitted showing: i) a quadratic relationship between the rate of degradation and the temperature of charge, ii) a linear relationship with the temperature of discharge, and iii) a correlation between the temperature of charge and discharge. It was found that the temperature combination for charging at +30 °C and discharging at -5 °C led to the highest rate of degradation. On the other hand, the cycling in a temperature range from -20 °C to 15 °C (with various combinations of temperatures of charge and discharge), led to a much lower degradation. Additionally, when the temperature of charge is 15 °C, it was found that the degradation rate is nondependent on the temperature of discharge.

Safety of rechargeable energy storage systems with a focus on Li-ion technology

In this chapter the safety of rechargeable energy storage systems is discussed with a focus on Li-ion batteries. The main hazards, such as fire, explosion, direct electrical hazards (electrical shock and arcing), indirect electrical hazards, and chemical hazards are reviewed. Relevant failure scenarios—overheating, mechanical deformation, external short circuit, and overcharge—are presented together with the main approaches for risk mitigation. Potential safety implications of the application of nanomaterials in rechargeable energy storage systems are discussed. Finally, a comprehensive summary of the most common tests for assessing safety under thermal, electrical, and mechanical abusive conditions as described in relevant standards and regulations is given.