Exploration of the oxygen transport behavior in non-precious metal catalyst-based cathode catalyst layer for proton exchange membrane fuel cells

Abstract

High cost has undoubtedly become the biggest obstacle to the commercialization of proton exchange membrane fuel cells (PEMFCs), in which Pt-based catalysts employed in the cathodic catalyst layer (CCL) account for the major portion of the cost. Although non-precious metal catalysts (NPMCs) show appreciable activity and stability in the oxygen reduction reaction (ORR), the performance of fuel cells based on NPMCs remains unsatisfactory compared to those using Pt-based CCL. Therefore, most studies on NPMC-based fuel cells focus on developing highly active catalysts rather than facilitating oxygen transport. In this work, the oxygen transport behavior in CCLs based on highly active Fe-N-C catalysts is comprehensively explored through the elaborate design of two types of membrane electrode structures, one containing low-Pt-based CCL and NPMC-based dummy catalyst layer (DCL) and the other containing only the NPMC-based CCL. Using Zn-N-C based DCLs of different thickness, the bulk oxygen transport resistance at the unit thickness in NPMC-based CCL was quantified via the limiting current method combined with linear fitting analysis. Then, the local and bulk resistances in NPMC-based CCLs were quantified via the limiting current method and scanning electron microscopy, respectively. Results show that the ratios of local and bulk oxygen transport resistances in NPMC-based CCL are 80% and 20%, respectively, and that an enhancement of local oxygen transport is critical to greatly improve the performance of NPMC-based PEMFCs. Furthermore, the activity of active sites per unit in NPMC-based CCLs was determined to be lower than that in the Pt-based CCL, thus explaining worse cell performance of NPMC-based membrane electrode assemblys (MEAs). It is believed that the development of NPMC-based PEMFCs should proceed not only through the design of catalysts with higher activity but also through the improvement of oxygen transport in the CCL.

Graphical abstract

Keywords

proton exchange membrane fuel cells (PEMFCs) / non-precious metal catalyst (NPMC) / cathode catalyst layer (CCL) / local and bulk oxygen transport resistance

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Acknowledgments

The work was financially supported by the National Key R&D Program of China (Grant No. 2021YFB4001303) and the National Natural Science Foundation of China (Grant No. 21975157). We also want to thank Dr. Jiewei Yin for the guidance on fuel cell tests for this study.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11708-022-0849-1 and is accessible for authorized users.

Notations

A	Empirical parameter A
a	Half the width of the flow field
B	Empirical parameter B
c_{{\mathrm{O}}_2}^{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{n}\mathrm{e}\mathrm{l}}	Concentration of oxygen in flow channel
d	Depth of flow channel
217D_{{\mathrm{O}}_2}^{\mathrm{C}\mathrm{H}}	Oxygen diffusion coefficient in flow channel
F	Faraday’s constant
ilim	Limiting current density
L	Length of flow channel
N	Number of flow channel
P	Gas pressure in flow channel
P0	Atmospheric pressure
pchannel	Flow channel pressure
Pw	Water vapor partial pressure
Qdry	Total gas flow
R	gas constant
rbulk,DCL	Bulk oxygen transport resistance of DCL per unit thickness
Rlocal,CCL	Local oxygen transport resistance of CCL
Rchannel	Oxygen transport resistance of flow channel
RCCL	Oxygen transport resistance of CCL
RGDL	Oxygen transport resistance of gas diffusion layer
Rtotal	Total oxygen transport resistance
Slope	Fitted curve slope
T	Reaction temperature
x_{{\mathrm{O}}_2}	Mole fraction of oxygen
δCCL	Thickness of CCL
{\delta }_{\mathrm{C}\mathrm{C}\mathrm{L}}^{\mathrm{e}\mathrm{f}\mathrm{f}}	Effective thickness of CCL
δDCL	Thickness of DCL

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