1D PEM FUEL CELL MODEL for NPMC CATHODE Catalyst LAYER

Fuel cells represents one of the most promising ways to obtain sustainable energy because they provide power and heat cleanly and efficiently, using diverse domestic fuels like hydrogen produced from renewable resources, biomass-based fuels, and natural gas. Fuel cells open the way to integrated “open energy systems” and they have the potential to reduce greenhouse gas emissions in many applications. For instance, applications have been demonstrated in combined heat and power (CHP) systems, light-duty highway vehicles, fuel cell electric vehicles (FCEVs) and Auxiliary power units (APUs) [1].

Transportation is one of the wider areas of application for fuel cells.  While Internal Combustion Engines (ICEs) generate power without any catalysts, needing precious metals only for catalytic converters in order to eliminate noxious gases, the catalysts found in electrodes are key to electric power generation by Polymer Electrolyte Membrane Fuel Cells (PEMFCs). Moreover, efforts in the catalysis of the PEMFC reactions (1) and (2) are the key to enhance sustainable power generation. The cathode is the most studied electrode as the major source of losses in efficiency and power density due to the five-orders-of-magnitude slower kinetics of the oxygen reduction reaction (ORR) on Pt, Reaction 2 [2].

2H₂→4H⁺                           Anode Reaction   (1)

O₂+4H⁺+ 4e^-→2H₂O         Cathode Reaction (2)

Pt is expensive and of limited availability. This greatly impacts the cost of PEMFCs, and consequently limits their potential for mass commercialization. In order to reduce the Pt loading in cathodes without decreasing their performance there are three options: (1) enhancing Pt mass activity for ORR via alloying or core-shell nanostructuring, (2) improving mass-transport properties of Pt-based cathodes, and (3) developing well-performing non-precious metal catalysts (NPMCs) for ORR [2, 3].

The aim of this study is to estimate the voltage drop due to transport processes inside a non-precious metal loaded cathode catalyst layer, namely, electron, proton and oxygen transport. There are many problems with obtaining accurate fits to polarization data, both in the kinetic region of the polarization curve as Figure 1 shows, and in mass-transport-controlled regions.

In an NPM catalyst layer, we need to identify and classify the main parameters to describe catalyst layers such as  catalyst loading and thickness, diffusion coefficients, active catalyst surface area, volume fraction of polymer in the agglomerate. For this we will use the Jaouen model [4] as a starting point and then validate the result with our catalyst characterization and structure measurements. It is also critical to use adequately complex experimental data sets to be analyzed and to include as many independently and experimentally determined parameters as are available to reduce the degrees of freedom for fitting.  Only then will a modeling study serve to reveal the physical basis of the important performance differences resulting from the use of given catalyst layer structures, on one hand, and different non-precious reaction pathways on the other.

Figure 1.Kinetic model vs. experimental results for NPMC.

Acknowledgement

 

We gratefully acknowledge the support of the NSF EPSCoR program and Colciencias for support of this work.

 

References

 

 

1.             Program, U.S.D.o.E.-F.C.T., The Department of Energy Hydrogen and Fuel Cells Program Plan, U.S.D.o. Energy, Editor. 2011. p. 92.

2.             Jaouen, F., et al., Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells. Energy & Environmental Science, 2011. 4(1): p. 114-130.

3.             Goenaga, G.A., et al., Synthesis and Electrochemical Characterization of Co, Cu, Fe, Ni and Mn-Based Catalysts for ORR in PEM Fuel Cells. ECS Transactions, 2013. 50(2): p. 1749-1757.

4.             Jaouen, F., Electrochemical Characterization of Porous Cathodes in the Polymer Electrolyte Fuel Cell, in Department of Chemical Engineering and Technology. 2003, Kungl Tekniska Hogskolan. p. 68.