Modeling of Low-Temperature Fuel Cell Electrodes Using Non-Precious Metal Catalysts

Abstract

An electrode-scale, transport model for a proton-exchange-membrane fuel cell (PEMFC) cathode is presented. The model describes the performance of non-precious metal catalysts for the oxygen reduction reaction in a fuel cell context. Because of its relatively high thickness, emphasis is placed on phenomena occurring in the cathode layer. Water flooding is studied in terms of its impact on gas-phase transport and on electrochemically accessible surface area (ECSA). Although cathode performance in both air and oxygen are susceptible to ECSA loss, gas diffusion limitations at high current density in air are more significant. In oxygen, catalyst utilization at high current density is primarily limited by conductivity. For this reason, air fuel cell data is recommended over oxygen data for characterizing catalyst performance. Due to both ohmic and mass transport limitations, increased loading of low-cost catalysts does not necessarily lead to higher performance. Therefore, careful optimization of catalyst layer thickness is required. The high environmental costs of current energy systems drives a search for commercializable energy technologies with low carbon footprints. Proton exchange membrane fuel cells present one energy option for transportation applications. A significant impediment to commercialization has been the cost and availability of catalysts for the oxygen reduction reaction (ORR) due to prevalent use of platinum group metals (PGM). Metal-Nitrogen-Carbon (MNC) catalysts are a potential solution to the cost and availability challenges that come with using PGMs. MNC catalysts are synthesized by mixing metal, carbon, and nitrogen precursors followed by one or more pyrolysis steps at 700-900 C.1-8 Such MNC catalysts are generally washed in acid to remove excess metal precursors.1-5 These catalysts generally involve lower volume-specific activity than platinum, which results in thicker electrodes. It has been previously proposed that a low-cost catalyst may be allowed to have 10-fold lower activity than platinum, as long as the catalyst layer was 10-fold thicker.9 This proposal was based on the assumption that transport loses would not be significant in this thickness regime. The purpose of this paper is to understand quantitatively how the thickness of these electrodes will impact membrane electrode assembly (MEA) performance for MNC catalyst.A number of models have addressed electrode scale transport issues.10-16 These models generally concentrate on water flooding in the gas diffusion layer (GDL) and consider a relatively thin catalyst layer. Here, gas and liquid transport in the catalyst layer are considered in a way that is similar to treatment of gas diffusion layers in previous models. We also treat the GDL consistently with previous models.Multiple works have addressed gas diffusion in the cathode catalyst layers, 17-23 with a subset considering flooding impacts on gas diffusion. 17,19,21,23 The works that do consider flooding are primarily concerned with thin, precious-metal catalyzed layers of ∼10 ...

Non–Pt Catalyst Layer Operation in a PEM Fuel Cell: A Variable‐thickness Regime

“…It is well known that in the region of small cell currents, the potential of a cell with the standard Pt-based catalyst layer decreases logarithmically with the current density j 0 (Tafel law). However, the curves [6] are almost linear in j 0 at small cell currents. This quasi-linearity is supported in [6] by impedance measurements, which show that the cell differential resistivities at 10 mA cm -2 and 50 mA cm -2 are nearly the same.…”

“…The experimental polarization curves measured in [6] exhibit an interesting and unusual feature. It is well known that in the region of small cell currents, the potential of a cell with the standard Pt-based catalyst layer decreases logarithmically with the current density j 0 (Tafel law).…”

“…One of the key problems in emerging PEMFC technology is development of cheap non-Pt catalysts for the ORR [2][3][4][5]. Recently, Leonard et al [6] reported experimental and modeling study of a membrane electrode assembly (MEA) with the Fe-N/C catalyst for a PEMFC. The numerical model [6] includes the standard macro-homogeneous model of the cathode catalyst layer (CCL) with the detailed account of water transport in the CCL and membrane [7,8].…”

Resolving Challenges of Mass Transport in Non Pt-Group Metal Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells

“…These can be distinguished from hydrophilic pores of low contact angle that are water-filled. 39 Dashed lines indicate deconvolution of distributions into three (NPC-2000) 33 and two (MOF SR) log-normal distributions with mean radius, r 0 , and volume fractions indicated in the table.…”

“…35,36 With the exception of the adjustable parameters noted above, all other parameters are taken from the original one-dimensional model. 33 For a first order process of rate kC (mol L −1 s −1 ) in an ideal plug flow reactor, the integral form of the material balance can be expressed as 37,38 …”