Powerhouses of PEM Fuel Cells

The catalyst layers represent the major competitive ground in view of the multivariate and hierarchical design challenge of PEFCs. All species, structural components, and processes that appear somewhere in the cell show up as well in CLs at the anode and, first and foremost, the cathode. 

The main processes are electrochemical reactions at electrified metal-electrolyte interfaces reactant diffusion through porous networks proton transport in water and at aggregates of ionomer molecules electron transport in electronic support materials water transport by gasous diffusion, hydraulic permeation, and electro-osmotic drag in partially saturated porous media and vaporization/condensation of water at interfaces between liquid water and gas phase in pores. 

Key structural characteristics determining these processes are atomistic surface structure and electronic structure of the catalyst, morphology of the pore network, surface structure and wettability of the support, catalyst nanoparticle shape and size, ionomer structure, mixed wettability of the composite layer, and, last but not least, the electrode thickness, Icl- 

This section gives an overview of the main principles of CL structure and operation. 

High-performance electrodes for PEFCs are evaluated by (i) their Pt mass loading per unit electrode surface area, mpt (in mg cm ), (ii) voltage efficiency, = Eceii/Ef ji, (iii) (volumetric) energy density, Wceii = FEceii/lcL (in J L ), (iv) (volumetric) power density, Pceii = joEceii/lcL (in W L ), and (v) specific power, = joEceii/mpt (in W gp/). These parameters are related, as is obvious from their definitions. Yet, it is possible to adjust their values independently to specific requirements of energy-consuming appliances. 

---