Investigation of Charge-Transfer and Mass-Transport Resistances in PEMFCs with Microporous Layer Using Electrochemical Impedance Spectroscopy

Abstract

The influence of a microporous layer (MPL) on polarization and electrochemical impedance behavior of proton exchange membrane fuel cells (PEMFCs) was investigated. Commercial carbon backing electrode with MPL applied on one of its sides were employed for both the anode and cathode. The ohmic, charge-transfer, and mass-transport resistances at various current densities were obtained by deconvolution of electrochemical impedance spectroscopy data. PEMFCs with an MPL showed higher performance and lower variability in the charge-transfer and mass-transport regions of the polarization curve (current density above 300mAcm−2 ) within a batch of identically built cells. For cells with and without MPL, the charge-transfer resistance decreased while mass-transport resistance increased with an increase in current density. The difference in charge-transfer resistance for cells with and without MPLs was found to be statistically insignificant due to the large variability in data for cells without MPLs. Cells with MPLs demonstrated lower mass-transport resistance compared to cells without MPL. The time constant for the mass-transport process probed at a low-frequency regime of the impedance was obtained from the Warburg impedance. Among the various possible oxygen-transport processes, the estimated time constant for oxygen transport in the porous transport layer (PTL) was found to be within an order of magnitude of the Warburg-impedance-derived value. Accordingly, it was assessed that the presence of an MPL helped reduce the water saturation in the PTL, thereby improving the oxygen transport to the cathode catalyst layer.

An impedance model for analysis of EIS of polymer electrolyte fuel cells under hydrogen peroxide formation in the cathode

“…An increase in the number of parameters in an equation that represents the PEFC impedance response may lead to increased error in the resulting fitted values with experimental EIS. A Randles circuit as reported by Malevich et al [11] was first fitted to the measured EIS spectra using ZView 2.9 software (Scribner Associates, Inc), as shown in φ R in Eq. 38 is a frequency dependent parameter, therefore it cannot be expressed as a resistor in a commercial software (e.g.…”

“…This process can be represented through the Randles circuit where the faradaic impedance consists of the charge transfer resistance connected in series with the mass transport resistance (Warburg element) [11,12]. During a two-step electron transfer ORR the sum of the reactions, hydrogen peroxide formation and water formation, in the two-step ORR yields the reaction accounting for formation of water without any intermediates.…”

“…This ratio between oxygen concentrations is considered, as the EIS technique measures the change in oxygen concentration in the total CCL. This also allows the derivation of the Warburg Impedance which has been broadly used in the EIS area [11,12] defined as the time constant to diffuse oxygen through the CCL.…”

“…34 can be rearranged to express the electrical current that enters a circuit composed of resistive, and capacitive elements, as shown in Fig. 1 giving: The hydrogen oxidation reaction (HOR) mechanisms occurring at the anode have been proposed by Malevich et al [11] to be represented as an electrical circuit composed of a resistor (charge transfer resistance) in parallel with a capacitor (representing the double layer capacitance). HOR taking place in the anode is a faster and a less complicated reaction sequence than the ORR in the cathode and can be represented at the frequency domain as:…”

Electrochemical impedance study on estimating the mass transport resistance in the polymer electrolyte fuel cell cathode catalyst layer

“…One of the challenges in EIS research is to quantify the mass transport limitations between the CCL and the GDL. The Randles circuit [1] has been broadly used in impedance plots to estimate mass transport limitations in PEFC EIS measurements. However, it has not been clear how to define whether this effect of oxygen transport limitation is dominated by the CCL or the GDL.…”