Catalyst layer oxygen reduction reaction

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

The most important electrokinetic data pertinent to fuel cell models are the specific interfacial area in the catalyst layer, a, the exchange current density of the oxygen reduction reaction (ORR), io, and Tafel slope of ORR. The specific interfacial area is proportional to the catalyst loading and inversely proportional to the catalyst layer thickness. It is also a strong function of the catalyst layer fabrication methods and procedures. The exchange current density and Tafel slope of ORR have been well documented in refs 28—31. 

These kinetic expressions represent the hydrogen oxidation reaction (HOR) in the anode catalyst layer and oxygen reduction reaction (ORR) in the cathode catalyst layer, respectively. These are simplified from the general Butler-Volmer kinetics, eq 5. The HOR... 

Figure 5.5. Experimental Nyquist diagrams for the ORR on a Pt nanoparticle catalyst (40% Pt/Pt+C) in alkaline solution (1 M NaOH) using an active layer on an RDE at 25°C (EIS frequency range 5 mHz-105 Hz). Electrode potentials (versus Hg/HgO electrode) ( ) -0.02 V ( ) -0.03 V (A) -0.06 V and ( ) -0.08 V [4], (Reprinted from Electrochimica Acta, 48(25-6), Genies L, Bultel Y, Faure R, Durand R. Impedance study of the oxygen reduction reaction on platinum nanoparticles in alkaline media, 3879-90, 2003, with permission from Elsevier.)...

Figure 6.9. Impedance spectra for the oxygen reduction reaction at an electrode potential of 0.6 V. The catalyst layer of the electrode contains various Nafion loadings ( ) 0.2 ( ) 0.8 (A) 2.0 mg/cm2 [5], (Reprinted from Journal of Power Sources, 94(1), Song JM, Cha SY, Lee WM. Optimal composition of polymer electrolyte fuel cell electrodes determined by the AC impedance method, 78-84, 2001, with permission from Elsevier and the authors.)...

Pt-doped carbon aerogels have been used successfully in the preparation of cathode catalyst layers for oxygen reduction reaction (ORR) in PEMFC systems [83-86]. Thus, different Pt-doped carbon aerogels with a Pt content of around 20 wt% were prepared by impregnation [83]. Results obtained with these Pt catalysts were compared with others supported on carbon blacks Vulcan XC-72 and BP2000, which are commonly used as electrocatalysts. The accessibility of the electrolyte to Pt surface atoms was lower than expected for high-surface-area... 

Many catalyst layer models have appeared in the literature during the last few years [15, 16, 17, 18, 19,20, 21]. This observation partly explains the complications associated with this topic. Still, much work remains to be completed since many effects have not yet been included, such as proton surface diffusion (outside the ionomer, [22,23]) and ionomer density (water content effect), which effectively and respectively increases/modifies the reactive surface area. The surface-sensitive nature of Pt catalysts on the oxygen reduction reaction rate [24] and electrochemical promotion (a catalytic effect, [25]) represent other examples which can also affect the reaction rate and surface area. All these effects are further compounded by the potential presence of hquid water which effectively modifies the reaction front, access to speeifie eatalyst particles and surface properties. 

Heat generated per unit area is fcj from ohmic losses through the membrane and Vtn — V — ico) i from electrochemical losses at the cathode catalyst layer, where Vtn is the thermoneutral voltage of the oxygen reduction reaction with product vapor. 

ELECTROCATALYSTS AND CATALYST LAYERS FOR OXYGEN REDUCTION REACTION... 

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