A Generic Electrical Circuit for Performance Analysis of the Fuel Cell Cathode Catalyst Layer through Electrochemical Impedance SpectroscopyThe new electrical circuit is derived from fundamental electrochemical and diffusion theory. It consists of a transmission line in combination with distributed Warburg elements. The validation of this study is divided into a theoretical validation and an experimental validation. In the theoretical validation the impedance response of the CCL generated from three different circuits reported in the literature was compared with the simulated data from the generic electrical circuit. In the experimental validation, Electrochemical Impedance Spectroscopy (EIS) measurements were carried out in an H 2 /air PEFC and through a three-electrode configuration in the measurement system and were compared with the simulated data from the generic circuit. The results show that the generic circuit is able to accurately reproduce the measured data of the CCL at different current densities and is able to represent the electrochemical and diffusion mechanisms of the CCL in the frequency domain. It is possible to generate a deeper understanding of how and where the chemical energy that is released from the redox reaction is being dissipated and retained within the real physical system.
The high frequency electrochemical impedance measurements with positive imaginary components in the impedance complex plot of a polymer electrolyte fuel cell (PEFC) are attributable to the inductance of the electrical cables of the measurement system. This study demonstrates that the inductive effect of the electrical cables deforms the high frequency region of the cathode impedance spectrum and as such leads to an erroneous interpretation of the electrochemical mechanisms in the cathode catalyst layer (CCL). This study is divided into a theoretical analysis and an experimental analysis. In the theoretical analysis a validated model that accounts for the impedance spectrum of the CCL as reported in the authors’ previous study is applied with experimental impedance data reported in the literature. The results show that the ionic resistance of the CCL electrolyte which skews the oxygen reduction reaction (ORR) current distribution toward the membrane interface is masked in the cathode impedance spectrum by the inductive component. In the experimental analysis cathode experimental impedance spectra were obtained through a three-electrode configuration in the measurement system and with two different electrical cables connected between the electronic load and the PEFC. The results agree with the theoretical analysis and also show that the property of causality in the Kramers-Kronig mathematical relations for electrochemical impedance spectroscopy (EIS) measurements is violated by the external inductance of the measurement cables. Therefore the experimental data presenting inductance at high frequencies do not represent the physics and chemistry of the PEFC. The study demonstrates that a realistic understanding of factors governing EIS measurements can only be gained by applying fundamental modeling which accounts for underlying electrochemical phenomena and experimental observations in a complementary manner.
In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously-validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL-Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL.Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.
In this study, an impedance model based on electrochemical theory considering hydrogen peroxide formation during a two-step oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) has been developed. To validate the theoretical treatment, electrochemical impedance spectroscopy (EIS) measurements were carried out in an open-cathode 16 cm 2 H 2 /air PEFC stack. The results show that inductive loops at low frequencies of the impedance spectra are attributed to mechanisms related to hydrogen peroxide formation during ORR. The results also demonstrate that the mechanisms during consumption of hydrogen peroxide to form water (second-step in ORR) can be the dominating process for losses in the PEFC compared to the mechanisms during oxygen consumption to form hydrogen peroxide (first-step in ORR). Oxygen transport limitations can be a result of hydrogen peroxide adsorbed onto the surface of the electrode which reduces the number of active sites in the cathode catalyst layer for oxygen to react. This study could support results from other experimental techniques to identify hydrogen peroxide formation during the ORR that limit the performance of PEFCs.
In this study, an impedance model based on electrochemical theory of platinum oxide formation has been developed and combined with the impedance model based on hydrogen peroxide formation during the oxygen reduction reaction (ORR) and reported in a previous study to charaterise inductive loops in impedance spectra of polymer electrolyte fuel cells (PEFCs). To validate the theoretical treatment, the simulated frequency response predicted by the theoretical model is compared against electrochemical impedance spectroscopy (EIS) measurements carried out in an open-cathode 16 cm 2 H 2 /air PEFC stack at three different current densities. The results show that neither model in isolation (hydrogen peroxide nor platinum oxide models) can accurately reproduce the inductive loops in the EIS measurements at low frequencies. By deriving a model considering kinetics of hydrogen peroxide and platinum oxide formation, it is possible to reproduce the inductive loops at low frequencies and to estimate the DC polarisation resistance related to the slope of the polarisation curve as frequency reaches zero during EIS. This study demonstrates that different mechanisms that cause PEFC degradation and low performance could be manifested in EIS measurements simultaneously. The resulting model could support other electrochemical techniques to quantify the rates of hydrogen peroxide and platinum oxide formation during the ORR that limit the performance of PEFCs.
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