Elsevier

Journal of Power Sources

Volume 91, Issue 2, December 2000, Pages 202-209
Journal of Power Sources

Influence of flow field design on the performance of a direct methanol fuel cell

https://doi.org/10.1016/S0378-7753(00)00471-7Get rights and content

Abstract

Serpentine (SFF) and interdigitated (IFF) flow fields were investigated with regard to their use in a direct methanol fuel cell (DMFC). The DMFC equipped with SFFs showed lower methanol cross-over, higher fuel utilisation and slightly larger voltage efficiency at low current densities. IFFs enhanced mass transport and membrane humidification allowing to achieve high power densities of 450 and 290 mW cm−2 in the presence of oxygen and air feed, respectively, at 130°C. A fuel efficiency of 90% was obtained with the IFFs in the presence of 1 M methanol feed at 130°C and a current load of 500 mA cm−2.

Introduction

Solid polymer electrolyte direct methanol fuel cells (DMFCs) have achieved significant levels of performance in the last years [1], [2], [3], [4], [5], [6], [7]. Various technical approaches have contributed to the amelioration of the electrochemical characteristics. These mainly concern the choice of thin film, highly conductive, protonic membranes, which allow the operation of fuel cells at temperatures close or above 100°C and the development of highly active anodic catalysts [1], [7]. In this regard, an increase of fuel cell temperature produces a corresponding enhancement of the methanol reaction kinetics, which compensates for the electrochemical losses due to increased methanol cross-over across the membrane. Among the various catalysts investigated for the methanol electro-oxidation [8], [9], [10], [11], the Pt–Ru system shows significant advantages since the high intrinsic catalytic activity of such a bifunctional catalyst can be combined with a high active surface area [7]. Furthermore, unsupported or high metal concentration carbon-supported anode and cathode catalysts have been successfully used in DMFCs in recent years [1], [7]. These materials allow the fabrication of catalytic layers with a small thickness, thus, favouring the diffusion of reactants to the catalytic sites [1].

The reduction of kinetic and ohmic limitations at high operation temperatures determines an increase of the influence of mass-transport limitations to the polarisation characteristics. In this regard, some progress has been obtained by improving the characteristics of the backing layer in terms of composition and thickness. Nowadays, a few investigations have been extended to the reactant flow fields [12], [13], [14], [15], [16]. Actually, the most widely employed flow field in small fuel cell devices is based on the so-called serpentine configuration. In such configuration, the reactant is constrained to flow along parallel channels, which are machined in a graphite plate in contact with the electrode backing layer. Such design is often adopted for stacked cells. The reactant molecules have access to the catalytic sites through diffusion across the so-called diffusion layer, i.e. the backing layer, made of carbon cloth and carbon black and hydrophobized by appropriate addition of polytetrafluoroethylene (PTFE).

Recently, a different approach for the flow of reactants and products inside the electrode structure, i.e. an interdigitated design, was proposed by Nguyen [12] and Wilson et al. [13] for H2–O2 solid polymer electrolyte fuel cells (SPEFCs). In practice, the reactant gases are forced to enter into the electrode pores and exit from them under a gradient pressure by making the inlet and outlet channels dead-ended. As pointed out by Nguyen [12], the flow through the electrode, in the presence of the interdigitated design, is no more governed by diffusion but becomes convective in nature. This particular design was selected for H2–air SPEFCs in order to avoid the water flooding problem at the cathode and to facilitate the removal of inert nitrogen molecules, which accumulate in the catalyst pores producing a diffusion barrier [12]. The forced-flow-through characteristics created by the interdigitated flow fields (IFFs) in SPEFCs have been further investigated by various authors [14], [15], [16]. In general, it has been shown that enhanced mass transfer characteristics are achieved with the IFF.

Beside mass transport, the flow fields in a DMFC device play also a significant role on the characteristics of membrane hydration and cathode poisoning. These aspects greatly influence system efficiency. At low temperatures, although current densities in a DMFC are significantly lower than in a SPE fuel cell, the problem of water flooding at the cathode is still present. Whereas, cell operation at high temperatures imposes an efficient water transport from the cathode to the membrane. This phenomenon is less demanding at the anode, since the supply of a mixture of methanol and water allows appropriate membrane humidification on this side.

In the methanol fuel cell, an effective mass-transport of oxygen molecules to the cathode active sites has a two-fold role: (i) supply of reacting molecules at a rate compatible with the reaction kinetics, (ii) uniform supply of oxidant over reaction sites in order to counteract the effects of the electrochemical oxidation of methanol molecules crossing the membrane. When a significant oxygen depletion occurs in the cathodic layer, methanol molecules that have reached the cathode compartment readily chemisorb on the electrode surface with subsequent oxidation to CO2. This determines a “mixed potential”, which decreases the cell potential. The negative effects produced by the oxidation of such methanol molecules become less significant, if a fast and homogeneous distribution of oxygen molecules is present over the cathode surface. Accordingly, when air is supplied to the cathode, the cell performance significantly diminishes as a consequence of lower oxidant partial pressure and increased cathode polarisation due to the larger catalyst poisoning by methanol cross-over.

In the present work, the electrochemical behaviour of a DMFC in the presence of serpentine flow fields (SFFs) and IFFs at both cathode and anode was investigated. The cell was operated at temperatures of 100°C and 130°C in order to evaluate the influence of both kinetic and mass-transport limitations. In-house made unsupported Pt–Ru and 90% Pt/Vulcan XC catalysts were employed at the anode and cathode compartments, respectively, in conjunction with Nafion 112 membrane. The crystallographic structure and particle size of the catalysts employed in the present study were investigated by X-ray diffraction.

Section snippets

Experimental

Unsupported Pt–Ru (1:1) and 90% Pt/Vulcan XC 72 catalysts were in-house synthesised by using a colloidal procedure based on a modification of the “Prototech method” developed by Petrow and Allen [17]. Sulphite complexes of Pt or Pt and Ru were decomposed by hydrogen peroxide at 80–90°C to form aqueous colloidal solutions of Pt and Ru oxides. The colloidal Pt particles were adsorbed on a carbon black to form the Pt/Vulcan XC 72 catalyst, whereas, colloidal Pt–Ru particles were precipitated from

X-ray diffraction analysis

X-ray diffraction was carried out on both unsupported Pt–Ru (1:1) and 90% Pt/C catalysts. Both catalysts exhibited the diffraction peaks of Pt or Pt–Ru fcc structure (Fig. 2). The (002) reflection of the graphitic basal plane of carbon black is not observed in the 90% Pt/C catalyst due to the high Pt concentration (Fig. 2). This peak should occur at about 25° (2θ). A significant shift to higher 2θ values is observed for the diffraction peaks in the Pt–Ru catalyst with respect to the Pt/C

Conclusions

The IFFs significantly enhance mass-transport inside a DMFC allowing to achieve higher maximum power outputs compared to the classical serpentine geometry. Two aspects related to the use of IFFs in DMFC devices have been evidenced, i.e., a larger methanol cross-over through the polymer membrane and a better membrane humidification at high temperatures. Both aspects are strictly related with the forced-convection mechanism for the reactant flows inside the cell. The larger methanol permeation

Acknowledgements

The authors are grateful to Dr. K. M. El-Khatib (National Research Center, Dokki, Giza, Egypt) for the co-operation and helpful discussions.

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