Research Papers

J. Electrochem. En. Conv. Stor.. 2018;15(2):021001-021001-14. doi:10.1115/1.4038626.

We present a macrohomogeneous two-phase model of a proton exchange membrane fuel cell (PEMFC). The model takes into account the mechanical compression of the gas diffusion layer (GDL), the two-phase flow of water, the transport of the gas species, and the electrochemical reaction of the reactant gases. The model was used to simulate the behavior of a PEMFC with a patterned GDL. The results of the reduced model, which considers only the mechanical compression and the two-phase flow, are compared to the experimental ex-situ imbibition data obtained by neutron radiography imaging. The results are in good agreement. Additionally, by using all model features, a simulation of an operating fuel cell has been performed to study the intricate couplings in an operating fuel cell and to examine the patterned GDL effects. The model confirms that the patterned GDL design liberates the predefined domains from liquid water and thus locally increases the oxygen diffusivity.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(2):021002-021002-10. doi:10.1115/1.4038632.

Electrochemical impedance spectroscopy is used during operation of different polymer electrolyte membrane fuel cell (PEMFC) stack assemblies at various conditions with special interest given to the characteristic time constant τlow-f derived from the low-frequency arc of the spectra which is typically in the range of approximately 15–0.5 Hz. This was done by fitting an equivalent electrical circuit (EEC) consisting of one resistor and two RC-elements to the data. Parameter variation performed on a 90-cell stack assembly suggests that conditions leading to different air flow velocities in the flow channels affect τlow-f while other parameters like humidity influence the impedance spectrum, but not τlow-f. Comparison of the stoichiometry variation between short stack and locally resolved single cell shows similar results with the stack's time constant matching that of the cell's segments which are located off-center toward the outlet. However, a nonlinear dependency between gas flow velocity and τlow-f especially at low stoichiometric values is obvious. Results from stoichiometry variations at different pressure levels suggest that this could be attributed to the different steady-state oxygen partial pressures during the experiments. Comparison of the stoichiometry variation between different stack platforms result in similar dependencies of τlow-f on air flow rate with respect to a reference oxygen partial pressure regardless of size, flow field, geometry, or cell count of the stack. The time constant caused by oxygen diffusion through the gas diffusion layer (GDL), τGDL, was approximated and compared to τlow-f. While it was found that τlow-f ≫ τGDL at low stoichiometric values, τlow-f decreases toward τGDL at very high gas flow rates, suggesting that τGDL offsets τlow-f and becomes dominating if no oxygen concentration variation along the flow channel was present.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(2):021003-021003-19. doi:10.1115/1.4038628.

Water management is critical for the operation of a polymer electrolyte membrane fuel cell (PEMFC). For the purposes of high power and long working-lifetime of PEMFCs, external humidifiers are always utilized as a necessary part of balance of plants to keep the imported air and fuel wet. However, they have several disadvantages, and it is beneficial to remove them so as to reduce system volume and to enhance the cold-starting capability. In this paper, a self-humidified PEMFC of an active area 250 cm2 and cell number 320 is proposed and investigated. The imported dry air on the cathode side is mixed with moisty exhaust gas by using a recirculation valve, and the dry hydrogen on the anode side is humidified by back-diffusion water through the membrane. A nonlinear model is set up based on mass transport and energy conservation equations to capture dynamics of gases in the supply and exhaust manifolds, the gas diffusion layers (GDLs), and the membrane. An analysis is conducted to investigate the influences of parameters on dynamic and stable performances. Simulation results show that system performances can be greatly affected by parameters such as air stoichiometry, current density, exhaust gas recirculation (EGR) ratio, and membrane thickness. By accurately controlling the EGR ratio and carefully selecting design and operation parameters, it is probably for a PEMFC without an external humidifier to have similar system efficiency compared to a traditional system.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(2):021004-021004-12. doi:10.1115/1.4038631.

This study combined a simple two-dimensional (2D) finite volume model (Kim model), which employs Ohm's law along with charge conservation over the electrodes and Butler–Volmer charge transfer kinetics for prismatic battery cells coupled with the single particle model (SPM) in order to model the thermal state of automotive battery packs. The objective here was to determine the effects of liquid cooling applied to the packs under standard driving cycles. A model developed by Kim provided a means for determining a nonuniform current distribution over the surface of the current collectors. The Kim model is based on the application of Ohm's law over a conducting medium, with empirical source terms representing current flowing into or out of an adjacent electrode layer. Here, a modeling advance is presented where empirical source terms in the Kim model were replaced with ones based on the chemistry and physics occurring inside the battery. As such, fundamental battery function was imparted to the model by integrating the SPM into the 2D finite volume Kim model. The 2D procedure described above was carried out on electrode sheets at different positions inside the cell, and determined thermal generation values that were mapped volumetrically into a heat transfer simulation, which, in turn, updated the electrochemical simulation. Capacity fade kinetics were determined by fitting experimental data to simulated results. With time-temperature profiles produced as described above for different pack cooling levels and varying degrees of cell degradation, a basic SPM simulation was then used with thermal overlays to estimate automotive cell life under various driving scenarios and various cooling levels. With these simulations, scenarios representing different thermal management regimes along with driving behavior were able to show the combined impact on automotive battery pack lifetimes.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(2):021005-021005-9. doi:10.1115/1.4039043.

This paper presents the design and part-load operation of a molten carbonate-micro gas turbine (MCFC/MGT) hybrid system (HS), and proposes a multiloop control strategy for the HS. A mathematical model of the system is introduced. Then, the structure of process is changed and the performance of HSs at part-load operation is studied. The novelty includes utilizing some part of the main fuel instead of auxiliary fuel in the combustion stage. The results show that the new configuration has more efficiency (about 63%). In order to keep the operating system within safe limits, variables of the control system are determined. Those controlled variables are as follows: stack temperature, fuel utilization (FU), turbine inlet temperature (TIT), and output power of HS. Based on relative gain array (RGA) analysis, control structures are suggested for two HS. Investigations on results of RGA analysis indicate that the new configuration has more interactions between inputs and outputs and so has different control structure. The dynamic simulation results show that the proposed control structure is achievable for MCFC/MGT HSs.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(2):021006-021006-11. doi:10.1115/1.4038635.

The performance of a 300 kW solid oxide fuel cell gas turbine (SOFC-GT) pilot power plant simulator is evaluated by applying a set of robust proportional-integral-derivative (PID) controllers that satisfy time delay and gain uncertainties of the SOFC-GT system. The actuators are a fuel valve (FV) that models the fuel cell (FC) thermal exhaust, and a cold-air (CA) valve which bypasses airflow rate from the FC cathode. The robust PID controller results for the upper and lower boundary of uncertain gains are presented first, followed by a design for the upper and lower boundary of uncertain time delays process for both, FV and CA bypass valves. The final design incorporates the combined uncertain gain and the time delay modeling for the upper and lower boundary of both actuators. This multiple-input multiple-output technique is beneficial to plants having a wide range of operation and a strong parameter interaction. The practical implementation of the PID controllers and the set point responses are presented through simulation in the matlab/simulink environment.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;15(2):021007-021007-7. doi:10.1115/1.4038799.

Nanomicro spheres of LiNi0.5Mn1.5O4 materials are prepared by carbonate coprecipitation method. The effect of calcination temperatures on morphology and electrochemical property is explored. Results show that the structure of the material becomes more compact with the increase of the temperature, which is propitious to the improvement of electrical conductivity and activation level of the material. The charge–discharge tests show that the sample obtained at 850 °C (LNMO850) exhibits optimal rate capability and cyclic stability, due to the fact that LNMO850 has a high diffusion coefficient, which is propitious to the improvement of electrical conductivity and activation level of the material.

Commentary by Dr. Valentin Fuster

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