arXiv subjects
Publications and source records attributed to Andrei Kulikovsky.
An impedance model for the cathode catalyst layer (CCL) in a PEM fuel cell demonstrates that in-phase harmonic perturbations to the cell current density and CCL temperature lower both CCL impedance and static resistivity. This mitigation is primarily driven by the oscillating exchange current density of the oxygen reduction reaction.
We have developed a non-isothermal analytical model for the impedance of the cathode catalyst layer (CCL) in a PEM fuel cell. In-phase harmonic perturbations to the current density and temperature reduce the impedance and the static polarisation resistivity of the CCL due to lowering proton transport losses. A special selection of the current and temperature perturbation amplitudes allows for complete elimination of these losses.
Effective water management is essential for the optimal performance of PEM fuel cells. We have developed an impedance model for liquid water transport through the membrane and coupled it with the two-phase model for cathode side impedance. The complete model was fitted to experimental spectra measured at anode/cathode relative humidities (RH) of 32/32\%, 50/50\% and 100/100\% within a current density range of 100 to 1000 mA cm$^{-2}$ and an air flow stoichiometry of 2. Cathode catalyst layer (CCL) saturation decreases with current density due to a growing liquid pressure gradient. For all RH values, the CCL oxygen diffusivity increases dramatically with cell current due to progressive involvement of larger pores into the proton current conversion. Higher RH leads to higher double layer capacitance, which indicates that liquid water increases the electrochemically active surface area.
Analytical low-current and numerical high--current models for the impedance of a hydrogen--fed anode of an anode--supported button SOFC are developed. The models use the dusty gas transport model for the binary H$_2$--H$_2$O mixture. We show that neglecting the pressure gradient may lead to a severe underestimation of the effective hydrogen diffusivity in the support layer. A least-squares fitting of the analytical model to a literature spectrum of a button cell is demonstrated. The analytical impedance allows to indicate traps when using equivalent circuits with the Warburg finite-length element for fitting experimental spectra. The model parameters include the Knudsen hydrogen diffusivity, hydraulic permeability, porosity/tortuosity ratio of the support layer and the ionic conductivity, double layer capacitance, and HOR Tafel slope of the active layer. All of the above parameters can be obtained by fitting the models to experimental spectra.
A complete Dusty-Gas Model for the H$_2$--H$_2$O mixture in the anode transport layer of the anode-supported SOFC is considered. An exact conservation law relating the total pressure and hydrogen molar fraction at any point inside the anode to their values in the anode channel is derived. Using this conservation law, approximate analytical solutions for the hydrogen molar fraction and total pressure in the anode transport layer are obtained. The solutions can be used to calculate concentration overpotential.
A model for impedance of a PEM fuel cell cathode catalyst layer under simultaneous application of potential and oxygen concentration perturbations is developed and solved. The resulting expression demonstrates dramatic lowering of the layer impedance under increase in the amplitude of the oxygen concentration perturbation. In--phase oscillations of the overpotential and oxygen concentration lead to formation of a fully transparent to oxygen sub--layer. This sub--layer works as an ideal non polarizable electrode, which strongly reduces the system impedance.
Transient Navies--Stokes equations for laminar flow of incompressible fluid in a channel with permeable wall are reduced to a single equation for the transversal profile of longitudinal flow velocity. Small--amplitude harmonic perturbation of injection velocity induces oscillations of longitudinal velocity with the peak at the walls. The oscillations amplitude in peaks dramatically increases with the distance along the channel; with the frequency growth peaks come closer to the walls.
Analytical model for impedance of oxygen transport in the gas--diffusion layer (GDL) and cathode channel of a PEM fuel cell is developed. The model is based on transient oxygen mass conservation equations coupled to the proton current conservation equation in the catalyst layer. Analytical formula for the "GDL+channel" impedance is derived assuming that the oxygen and proton transport in the cathode catalyst layer (CCL) are fast. In the Nyquist plot, the resulting impedance consists of two arcs describing oxygen transport in the air channel (low-frequency arc) and in the GDL. The characteristic frequency of GDL arc depends on the CCL thickness: large CCL thickness strongly lowers this frequency. At small CCL thickness, the high-frequency feature on the arc shape forms. This effect is important for identification of peaks in distribution of relaxation times spectra of low--Pt PEMFCs.
Impedance of all oxygen transport processes in PEM fuel cell has negative real part in some frequency domain. A model function (kernel) for calculation of distribution of relaxation times (DRT) of a PEM fuel cell is suggested. The kernel is designed for capturing impedance with negative real part and it stems from the equation for impedance of oxygen transport through the gas--diffusion transport layer (doi:10.1149/2.0911509jes). Using recent analytical solution for the cell impedance it is shown that DRT calculated with the novel $K_2$ kernel correctly captures the GDL transport peak, while the classic DRT based on the $RC$--circuit (Debye) kernel misses this peak. Employing $K_2$ kernel, analysis of DRT spectra of a real PEMFC is performed. The leftmost on the frequency scale DRT peak represents oxygen transport in the channel, and the rightmost peak is due to proton transport in the cathode catalyst layer. The second, third and fourth peaks exhibit oxygen transport in the GDL, faradaic reactions on the cathode side, and oxygen transport in the catalyst layer, respectively.
A model of PEM fuel cell impedance is developed taking into account imposed harmonic perturbation of the air flow velocity in the cathode channel. The flow velocity modulation with the amplitude proportional to AC amplitude of the cell potential lowers the resistivity $R_h$ due to oxygen transport in channel. When relative amplitudes of velocity and potential oscillations are equal, a complete compensation of $R_h$ occurs. This effect explains experimental findings of Kim et al. (doi:10.1016/j.jpowsour.2008.06.069) and Hwang et al.(doi:10.1016/j.ijhydene.2010.01.064), who demonstrated significant improvement of PEM fuel cell performance under oscillating air flow velocity.
A single--pore model for performance of the cathode catalyst layer (CCL) in a PEM fuel cell is developed. The model takes into account oxygen transport though the CCL depth and through the thin Nafion film, separating the pore from Pt/C species. Analytical solution to model equations reveals the limiting current density $j_N^{\rm lim}$ due to oxygen transport through the Nafion film. Further, $j_N^{\rm lim}$ linearly depends of the CCL thickness, i.e., the thinner the CCL, the lower $j_N^{\rm lim}$. This result may explain unexpected lowering of low--Pt loaded catalyst layers performance, which has been widely discussing in literature.