Kinetic study of electrochemical reactions at catalyst-recast ionomer interfaces from thin active layer modelling

The objective of the following study was to test proton exchange membrane fuel cell catalysts. A mixture of supported catalyst and recast ionomer (Nafion®) was deposited on a rotating disc electrode (RDE). The resulting thin active layer was immersed in a dilute sulphuric acid solution. The RDE technique allows correction of mass transfer limitation in solution. To calculate the kinetic parameters from the current-potential relation, a mathematical model was written taking into account gas diffusion, ohmic drops and interfacial kinetics within the thin layer. Analytic and/or numerical expressions for the effectiveness factor and for the current-potential relation were obtained. The oxygen reduction reaction at various Pt/C-recast Nafion® interfaces demonstrates the validity of this test procedure.Nomenclature b Tafel slope (V dec^–1) - c Local gas concentration (mol cm^–3) - c_s gas concentration at the electrolyte side (mol cm^–3) - D gas diffusion coefficient within the layer (cm² s^–1) - F Faraday constant (96 500 C mol^–1) - i total current density based on the geometric area (A cm^–2) - i₀^* exchange current density per real catalyst area (A cm^–2) - I dimensionless total current density - j local ionic current density based on the geometric area (A cm^–2) - K ionic conductivity within the layer (S cm^–1) - K_e electronic conductivity within the layer (S cm^–1) - L layer thickness (cm) - m mass fraction of catalyst in the catalytic powder - n total number of electrons involved in reaction - R gas constant (8.31 J K^–1 mol^–1) - S specific catalyst area (m² g^–1) - T temperature (K) - u, v and w dimensionless parameters in Equations 8 and A4 - y dimensionless abscissa Greek symbols - cathodic transfer coefficient - effectiveness factor - local dimensionless overpotential - real catalyst area/geometric area ratio - local overpotential (V) - Nafion® volume fraction - tortuosity factor