Cosse, Carsten

The present work expands on the idea of the electric field modifier (EFM) as a control electrode in the membrane of a fuel cell (FC). Through electrochemical impedance spectroscopy (EIS) measurements it is shown experimentally that an EFM-electrode made of a gold mesh can be integrated into a fuel cell, without disrupting its function. Using the measured impedance spectra and a distribution of relaxation times (DRT)-approach a suitable equivalent circuit (EQC)-model is derived for the measured cell with the integrated EFM. Further measurements show that the EFM can mainly be regarded as a double layer (dl)-capacitor inside of the membrane. This capacitor, in conjunction with a specially designed H∞ -controller are used in a simulation to stabilise the cell voltage, while a square wave current is drawn from the cell, to represent a direct connection of the FC with an inverter. The system is shown to have excellent disturbance rejection.

Hydrogen fuel cell technology is one of the key focus areas to facilitate the transition from carbon-based fuels to more sustainable solutions in the transportation and mobile power sectors. Transient voltage fluctuations due to load changes and even operation of fuel cells with DC/DC and DC/AC converters are detrimental to the lifetime and this paper proposes a method to deal with these fluctuations. Adding electric field modifier (EFM) electrodes made of gold to the membrane of a fuel cell was proposed elsewhere as a way to influence the short term flux of charge carriers through the membrane. While electrochemical impedance spectroscopy shows a limited capacitance of such electrodes, experiments using square wave excitation of the system in the kHz frequency range show a promising reaction of the cell to this treatment. More in-depth analysis of the used electrode material reveals the need to insulate future EFM electrodes in order to prevent oxidative dissolution. However, this work shows that the principle of using EFM electrodes to manipulate transient oscillations is physically sound.

The dynamic controllability of the fuel cell could be improved by the addition of an electric field modifier (EFM), to selectively boost or attenuate the flux of protons through the membrane and, thereby, influence cell performance. This approach follows the commonly accepted idea of the potential gradient across the membrane being the main driving force behind the proton transport in the membrane. To evaluate the applicability of the idea, a simulation model for a membrane with an integrated EFM is developed to study the effects on the membrane behaviour. First, a modified Poisson-Boltzmann-Model (1D) is developed to characterise the capacitive behaviour of the double layer at the EFM. The approach considers steric restrictions in the membrane pores to estimate the double layer capacitance and the range of the effect at the EFM. Second, the characteristic behaviour of the capacitance is implemented in a secondary current distribution model (2D) as a variable capacitance. In transient simulations, boost of the cell current by up to 82% and attenuation up to a complete reversal of the direction compared to the stationary operation are achieved. Thus, it was possible to show the potential of EFMs to influence the characteristics of fuel cells and electrolysers during transient operation.

Optimized integration of fuel cells into grids or on-board power supplies is necessary to facilitate replacement of conventional energy producers by a reliable and plannable power generation technology. Due to the interdependency between fuel cell current and voltage, integration of fuel cells requires a power conditioning system, which increases integration weight and cost. For this reason, integration of electric field modifier electrodes into the setup of proton exchange membrane fuel cells is a new approach to control the output voltage in order to minimize the subsequent power conditioning system. This approach considers the physics of proton transport through the electrolyte membrane and could offer a lever to control the ohmic resistance. In this paper, a fuel cell model is implemented in MATLAB and extended by electric field modifier electrodes, allowing control of the ohmic resistance through an externally applied voltage. The concept of boosting and attenuating fuel cell voltage is presented along with different setups to enable this behavior. Furthermore, an electrical equivalent circuit for electrically controllable fuel cells is developed and implemented in MATLAB/Simulink. A method to parameterize the developed MATLAB and Simulink models by first experimental results is presented.

Optimized grid integration of proton exchange membrane fuel cells in various possible applications requires a suitable protection system. For this reason, this paper examines the transient behavior of a fuel cell stack after an external electrical shortening. In order to show the influence of operating parameters on the short-circuit behavior, various experiments with changed anode and cathode humidity, cell temperature and anode and cathode stoichiometry are carried out. With this, manufacturers can estimate the short-circuit magnitude of their stacks and recommend a suitable plant protection system. It could be shown that the peak short-circuit current depends on the operating point as well as the operating conditions. For the steady-state short-circuit current, the gas stoichiometry has an impact on the deliverable current. For all other operating conditions the steady-state short-circuit current is approximately twice the recommended maximum operating current. Furthermore, a method to estimate the effective fuel cell stack capacity out of the transient short-circuit current is presented.

The reduction of greenhouse gas and pollutant emissions is a major issue in modern society. Therefore, environmentally friendly technologies like fuel cells should replace conventional energy generation plants. Today, fuel cells are used in households for CHP (combined heat and power) applications, for emergency power supply in many stationary applications and for the power supply of cars, buses and ships and emergency power supply of aircrafts. A significant challenge is the optimal electrical grid integration and selection of the appropriate grid protection mechanism for fuel cell applications. For this, the short circuit capability and behavior needs to be known. This paper gives a mathematical estimation of the short circuit behavior of fuel cells. Five main transient and dynamic phenomena are investigated. The impact of the main transient effect for the provision of additional short circuit energy is simulated, and the simulation is experimentally validated. For this purpose, a 25 cm2 single cell consisting of a NafionTM 212 membrane and carbon cloth electrodes with a catalyst loading of 0.5 mg/cm2 Pt is analyzed. The magnitude of the transient short circuit current depends on the operating point right before the short circuit occurs, whereas the stationary short circuit current of fuel cells is invariably about twice the operational current. Based on these results, a novel fuel cell model for the estimation of the short circuit behavior is proposed.