Numerical simulation of the ordered catalyst layer in cathode of proton exchange membrane fuel cells

https://doi.org/10.1016/j.elecom.2005.09.022Get rights and content

Abstract

A steady-state, one-dimensional numerical model based on cylindrical electrode structure is presented to analyze the performance of the ordered cathode catalyst layer in Proton Exchange Membrane Fuel Cells. The model equations account for the Tafel kinetics of oxygen reduction reaction, proton migration, oxygen diffusion in the cylindrical electrolyte and the gas pores, oxygen distribution at the gas/electrolyte interface. The simulation results reveal that ordered catalyst layers have better performance than conventional catalyst layers due to the improvements of mass transport and the uniformity of the electrochemical reaction rate across the whole width of the catalyst layer. The influences of oxygen diffusivity in gas phase and electrolyte, and the proton conductivity have been shown. The limitation by oxygen diffusion in gas phase drives the active region of the catalyst layer to the catalyst layer/gas diffuser interface. The limitation by proton migration confines the active region of the catalyst layer to the membrane/catalyst layer interface. The limitation due to oxygen diffusion in electrolyte film maintains the uniform distribution of the active region throughout the ordered catalyst layer.

Introduction

Despite the dramatic performance improvement, proton exchange membrane (PEM) fuel cells still have to overcome some technical obstacles before they become commercially viable. Among these obstacles, the slow kinetics and mass transport limitations of the oxygen reduction reaction (ORR) in the cathode catalyst layer of PEM fuel cells are critical [1]. The most commonly used cathode catalyst layer in PEM fuel cells, named conventional catalyst layer (CCL) in this paper, consists of a porous solid matrix of Pt/C particles and an ionomer electrolyte (usually Nafion) network that penetrates into the pores. The CCL is usually prepared by random mixing of Pt/C catalysts, Nafion and/or Teflon. In such CCL significant amount of expensive Pt is only surrounded either by gas or by Nafion and, therefore, is wasted since it cannot particpate in the electrochemical reaction [2], [3]. Recently, a novel nanostructured catalyst layer which can be called ordered catalyst layer (OCL) has become available [4]. In this OCL, the transport limitations are greatly reduced by orientation of the proton conductor, the electron conductor and pores in the direction perpendicular to the membrane. By this means, the OCL performance can be improved dramatically, and 100% utilization of catalyst can be achieved.

Mathematical models are useful for the study of PEM fuel cells. In the past decade, a lot of simulation researches were conducted to analyze and optimize the structure and performance of the catalyst layer. Homogeneous models have been presented in [5], [6], [7], in which the catalyst layer is treated as a uniform blend of carbon supported catalysts and proton conducting polymer. Agglomerate models have been developed by Siegel et al. [8], Perry et al. [9], Jaoueu et al. [10] and Song et al. [11]. In these agglomerate models, the catalyst layer is assumed to contain small agglomerates consisting of carbon, platinum and Nafion, which are separated by gas pores. These homogeneous and agglomerate models have been presented to study different aspects of the catalyst layer and provided deep insight into the catalyst layer operation. However, most of these models focus on the CCL, and to the best of our knowledge, few papers have been published to date that study the OCL behavior in PEM fuel cells.

In this paper, an one-dimensional, steady-state, isothermal, and cylindrical model for the cathode OCL of PEM fuel cells is presented. The model accounts for the kinetics of oxygen reduction at the catalyst/membrane interface, the proton transport through the polymer electrolyte, the oxygen diffusion through gas pores, and the dissolved oxygen diffusion through the electrolyte. The effects of proton migration and oxygen diffusion in the gas pore and the electrolyte on the performance of the OCL are studied in detail. The results will be helpful in the optimization design of the OCL in PEM fuel cells.

Section snippets

Mathematical model

A schematic diagram of the cathode OCL is shown in Fig. 1. The OCL is assumed to be constituted of oriented carbon nano-threads which are covered by electrolyte films and then separated by parallel gas micropores. The catalysts are uniformly distributed on the surface of the carbon nano-threads. The oxygen diffusion coefficient in gas pores and the electrolyte film, and the proton conductivity are assumed to be constant. Furthermore, the diameter of carbon nano-threads, the thickness of the

Results and discussion

The set of governing differential equations (Eqs. (10), (12), (13)) with the given boundary conditions is solved by using Newton–Raphson method and the program was written in C language. For simplicity, a dimensionless proton current density J = j/jtot (jtot is the total current density of the OCL), a dimensionless concentration C = cO,g/c0 and a dimensionless coordinate X = x/L (L is the OCL thickness) are defined. The base-case conditions and physical properties for the simulation are specified in

Conclusions

A numerical model for the ordered cathode catalyst layer of PEM fuel cells is presented. Simulation results indicate that the obvious performance enhancement of the OCL can be due to the oriented mass transports which also result in the uniform distribution of the ORR rate across the catalyst layer. The OCL performance may be limited by decreasing the proton conductivity, and the oxygen diffusion coefficients in gas phase and electrolyte film for higher current densities. The limitation of

References (14)

  • S. Gamburzev et al.

    J. Power Sources

    (2002)
  • S. Litster et al.

    J. Power Sources

    (2004)
  • G.S. Kumar et al.

    Electrochim. Acta

    (1995)
  • E. Middelman

    Fuel Cells Bulletin

    (2002)
  • C. Marr et al.

    J. Power Sources

    (1999)
  • M. Eikerling et al.

    J. Electroanal. Chem.

    (1998)
  • D. Song et al.

    Electrochim. Acta

    (2005)
There are more references available in the full text version of this article.

Cited by (45)

  • Modeling of high-efficient direct methanol fuel cells with order-structured catalyst layer

    2019, Applied Energy
    Citation Excerpt :

    To insight into the electrode structure so as to improve cell performance and increase energy conversion efficiency, tentative mathematical models of DMFC with OCCL have been developed. Du et al. [20] presented a one-dimensional steady state cylindrical model with ordered catalyst layer. This model studied the effect of proton conductivity on cell performance.

  • A new discovery of the active impact of Pt/C particles aggregation on electrode performance in PEMFC

    2017, International Journal of Hydrogen Energy
    Citation Excerpt :

    For now, there have been two main kinds of PEMFC catalyst layer models which are classified into microstructure models, and macrostructure models. The macrostructure models included interface model [1–4], homogeneous model [5–10], aggregation model [11–15] and orientated structure Catalyst layer model [16–19]. Interface model referred to only regarding the catalyst layer as an Interface without considering the thickness and the inner structure of catalyst layer itself.

  • A cost-effective nanoporous ultrathin film electrode based on nanoporous gold/IrO<inf>2</inf> composite for proton exchange membrane water electrolysis

    2017, Journal of Power Sources
    Citation Excerpt :

    The impact of MEA structure on single cell performance has been intensively studied in the field of proton exchange membrane fuel cell. Mathematical model [17,18] and experimental work [19–22] have revealed that ordered MEA with rationally designed catalyst layer structure would enhance the single cell performance and reduce the noble metal consumption. Unlike the situation in PEM fuel cell, the application of ordered MEA in PEM water electrolysis has been rare [23,24].

View all citing articles on Scopus
View full text