Taehee Han

Equivalent Electric Circuit Modeling and Performance Analysis of a PEM Fuel Cell Stack Using Impedance Spectroscopy

In this paper, equivalent electric circuit models of a commercial 1.2-kW proton exchange membrane (PEM) fuel cell stack are proposed based on AC impedance studies. The PEM fuel cell stack was operated using room air and pure hydrogen (99.995%). Using electrochemical impedance spectroscopy (EIS) technique, impedance data were collected in the laboratory under various loading conditions. Impedance data were analyzed and circuit models developed using basic circuit elements like resistors and inductors, and distributed elements such as Warburg and constant-phase elements. A nonlinear least-square fitting technique is employed to obtain the circuit parameters by fitting a curve to the experimental impedance data. Two circuit models of the fuel cell, one for low and one for high currents are proposed. The average ohmic resistance for the whole stack is estimated to be 41 mΩ. Double-layer capacitances are determined at anode and cathode at various current densities. As expected, cathode charge transfer resistance turns out to be much higher than the anode charge transfer resistance because of slower kinetics of the oxygen reduction reaction. At higher load currents, a significant increase in mass transfer resistance as well as low-frequency inductive effects is observed. These low-frequency inductive effects are recognized and modeled in the fuel cell models of this work. Finally, a semiquantitative analysis was used to determine the contribution of individual performance factors to the overall fuel cell voltage drop. The transient response of the fuel cell circuit models is simulated using MATLAB/Simulink and their performance is validated by comparison with experimental data.

Platinum supported on titanium–ruthenium oxide is a remarkably stable electrocatayst for hydrogen fuel cell vehicles

Significance In this study, we showcase titanium–ruthenium oxide (TRO)—a remarkably stable support material, and Pt/TRO, a derivative electrocatalyst that is also extremely stable. These materials have been tested to establish their catalytic activity and stability using accelerated tests that mimic conditions and degradation modes encountered during long-term fuel cell operation. We have evaluated Pt/TRO using these protocols, and provide concrete evidence that this material is far more stable than Pt/C and, would meet the requirements for use in an automotive fuel cell stack. Our cost analysis indicates the use of ruthenium is not a factor given that >90% of catalyst cost resides in the platinum metal; moreover, the exceptional stability of Pt/TRO removes the needs for overdesign or replacement. We report a unique and highly stable electrocatalyst—platinum (Pt) supported on titanium–ruthenium oxide (TRO)—for hydrogen fuel cell vehicles. The Pt/TRO electrocatalyst was exposed to stringent accelerated test protocols designed to induce degradation and failure mechanisms identical to those seen during extended normal operation of a fuel cell automobile—namely, support corrosion during vehicle startup and shutdown, and platinum dissolution during vehicle acceleration and deceleration. These experiments were performed both ex situ (on supports and catalysts deposited onto a glassy carbon rotating disk electrode) and in situ (in a membrane electrode assembly). The Pt/TRO was compared against a state-of-the-art benchmark catalyst—Pt supported on high surface-area carbon (Pt/HSAC). In ex situ tests, Pt/TRO lost only 18% of its initial oxygen reduction reaction mass activity and 3% of its oxygen reduction reaction-specific activity, whereas the corresponding losses for Pt/HSAC were 52% and 22%. In in situ-accelerated degradation tests performed on membrane electrode assemblies, the loss in cell voltage at 1 A · cm−2 at 100% RH was a negligible 15 mV for Pt/TRO, whereas the loss was too high to permit operation at 1 A · cm−2 for Pt/HSAC. We clearly show that electrocatalyst support corrosion induced during fuel cell startup and shutdown is a far more potent failure mode than platinum dissolution during fuel cell operation. Hence, we posit that the need for a highly stable support (such as TRO) is paramount. Finally, we demonstrate that the corrosion of carbon present in the gas diffusion layer of the fuel cell is only of minor concern.

Hydrogen dew point control in renewable energy systems using thermoelectric coolers

This paper describes a system, utilizing the Peltier effect, to reduce and control the dew point of hydrogen gas by water condensation and desublimation using thermoelectric coolers and water cooled heat sinks. The design is compared to a two-tube desiccant-drying system used in some commercial proton exchange membrane electrolyzer systems. The desiccant system in the water electrolyzer consumes roughly 0.2 kg per day of hydrogen product gas (corresponding to 3.4 kWh per kg of hydrogen based on the higher heating value) to maintain the two desiccant beds. Thermodynamic modeling was performed to determine the appropriate sizing for the thermoelectric coolers and water-cooled heat sinks for a 1 Nm3 hr -1 hydrogen flow rate to obtain a theoretical dew point of -35 degC. The potential benefits and energy consumed by the thermoelectric approach (3.05 kWh per kg of hydrogen) is compared to the hydrogen loss of the desiccant system. The thermoelectric cooler-based system has the ability to control the dew point to match the variable flow rate of hydrogen in a renewable electrolysis system.