Impedance Spectroscopy Characterization of Oxygen Transport in Low– and High–Pt Loaded PEM Fuel Cells

Abstract

We report fitting of the physics-based model for the cathode side impedance to the experimental spectra of low-Pt loaded (0.1/0.1 mg Pt cm −2 ) and high-Pt loaded (0.4/0.4 mg Pt cm −2 ) PEM fuel cells measured in the range of current densities from 50 to 400 mA cm −2 . Fitting allowed us to separate the oxygen diffusion coefficients in the catalyst layer D ox and in the gas-diffusion layer D b , and the respective mass transfer coefficients of the electrodes of both types. In the low-Pt electrode, D ox is an order of magnitude lower, than in the high-Pt electrode; however, due to 4-fold difference in the electrode thickness, the respective mass transfer coefficients are close to each other. In both the electrodes, the oxygen diffusion and the mass transfer coefficients in the GDL are nearly the same and they are much higher, than the respective coefficients in the CCLs. The ORR Tafel slope and D ox exhibit linear growth with the cell current density; both effects could be attributed to "cleaning" of Pt surface from oxides at lower cell potential.

Impedance Spectroscopy Study of the Dependence of High‐temperature Polymer Electrolyte Membrane Fuel Cell Parameters on Current

“…The Tafel slope exhibits low spread around the average value of 44 mV (Figure a). Unlike PEM fuel cell, where b linearly increases with the cell current density J , here, this parameter is independent of J . This is an indirect evidence that in low‐T cells, the growth of b with J is related to electrochemical “cleaning” of the Pt surface from oxides at lower cell potentials.…”

“…Further, in low‐T PEMFCs, the double layer capacitance decays with J which may be attributed to the dependence of specific capacitace of carbon black on the electrode potential. Here, C dl increases with the cell current density (Figure c).…”

“…The impedance model [28] utilizes a following simplification: the expression for the perturbation of the diffusive oxygen fluxÑ 1 b at the channel/GDL interface has been obtained assuming fast oxygen transport in the CCL. A more accurate approach is to use the exact expression forÑ 1 b , taking into account finite rate of the oxygen transport in the CCL.…”

“…(15) is used, which requires iterations: first, the factorD ox c 1 = x x ¼1 on the right side of Eq. (15) is replaced by the dimensionless perturbation of current density at the membrane interfacej 1 (the approximation used in [28]). Then, the system of equations for the perturbation amplitudes in the CCL is solved, the solution is used to calculate the fluxD ox c 1 = x x ¼1 , and to solve Eq.…”

“…A more accurate version of model Ref. [28] Eq. (5) is linear, and hence an equation for the small-amplitude perturbation of the oxygen concentration in the channel…”