Electrochemical Impedance Analysis for Fuel Cell (& economy a bit).

I. PEFC (Polymer Electrolyte Fuel Cell)

(1) PEFC Equivalent Circuit Model

According to Matsumoto, the equivalent circuit model of a polymer electrolyte fuel cell is composed of an Ra-Cd high-frequency component and an Rr low-frequency component. Here, Ra, Cd, and Rr denote the activation polarization resistance, electric double layer capacitance, and ionic/electronic resistance, respectively.

Ra－Cd high-frequency component is mainly attributed from the water generation at the cathode since the activation energy of that reaction is much larger than that of H2 dissociation at the anode. The former is composed of (i) the H+ diffusion through the electrolyte and ionomers from the anode to the cathode, (ii) O2 permeation from the air to Pt catalysts at the cathode through the ionomers, and (iii) the water generation from O2. This process takes place at the solid-liquid-gas three-phase interface, thus, an electric double layer capacitance must be included in the model.

Rr low-frequency component is composed of (i) the resistance of H+ conduction through the electrolyte and the resistance of electron conduction through the separator, the gas-diffusion layer, and the catalyst layer, and (ii) the resistance of the gas (mainly O2) diffusion. Note that the flooding mainly occurs at the cathode gas-diffusion layer and the cathode catalyst layer.

(2) PEFC Durability

According to Osaka Gas, the main deterioration factors are

Electrolyte/ionomers oxidation/decomposition resulting from the reaction with hydroxy radicals, HO・ and H2O・ which are the products of the reaction between the peroxide, H2O2, and the impurities such as Fe2+ and Cu2+, thus usually the radical quencher, Ce3+ or Mn2+, is added to the layer (Ra increases),

Anode catalyst deactivation (when the fuel does not contain CO, this does not take place.),

Cathode catalyst oxidation/resintering resulting in the decrease in the specific surface area and carbon oxidation (= oxidation) resulting in the cathode catalyst loss (Ra increases and Cd decreases), and

Cathode gas-diffusion deterioration mainly resulting from the water-repellent material loss (see the electrolyte oxidation) (Rr increases).

In 2015, Toyota reported that the worst case scenario is 15% power decline per 15 years.

The durability has been improved year by year because of new composition/structure catalysts, the new catalyst support, the operation scheme etc.

II. FCV

(cf.) Hydrogen Roadmap Europe. Exhibit 10: Comparison of Range, Payroad, and Preferred Technology

III. SOFC (Solid Oxide Fuel Cell)

(1) SOFC Equivalent Circuit Model

According to Kishimoto,

Ohmic resistance, Rhf (hf denotes high-frequency), is attributed to electron/hole conduction and oxide ion conduction,

At the Ni-YSZ interface, the electron deficiency in Ni and oxide-ion richness in YSZ causes the electric double layer.

The electrode reaction takes place as the charge transfer through the electric double layer, causing the charge transfer resistance, Rct.

Rdif and Cdif is attributed to gas diffusion.

According to Hashimoto and Mori, the Cole-Cole plot becomes two semicircles shown in the left below. Note that one semicircle can be shown in the lowest frequency region because of the Pt blocking electrode.

(2) SOFC Durability

Hotta et al. has reported on the deterioration mechanism, such as the impurities pile-up at the electrode-electrolyte interface and the electrode composition change. The decrease in the operation temperature can improve the durability.

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Electrochemical Impedance Analysis for Fuel Cell (& economy a bit).