A mixed-reactants solid-polymer-electrolyte direct methanol fuel cell

Abstract

Mixed-reactants solid-polymer-electrolyte direct methanol fuel cells (SPE-DMFCs) with a PtRu/C anode and a methanol-tolerant oxygen-reduction cathode catalyst have been assembled and have been subjected to galvanostatic polarisation studies. The oxygen-reduction cathode was either of the FeTMPP/C, CoTMPP/C, FeCoTMPP/C and RuSe/C. It was found that the SPE-DMFC with the RuSe/C cathode yielded the best performance. It has been possible to achieve power densities of approximately 50 and 20 mW/cm² while operating a mixed-reactants SPE-DMFC at 90 °C with oxygen and air fed cathodes, respectively. Interestingly, these SPE-DMFCs exhibit no parasitic oxidation of methanol with oxygen.

Introduction

Solid-polymer-electrolyte direct methanol fuel cells (SPE-DMFCs) are presently under active development particularly for applications in transport and various portable electronic devices [1]. In a SPE-DMFC, methanol is oxidised at the anode according to the reaction

$$\text{CH}{}_3\text{OH}\text{+}\text{H}{}_2\text{O}\text{→}\text{CO}{}_2\text{+6}\text{H}{}^{\mathrm{+}}\text{+6}\text{e}{}^{\mathrm{-}}\text{(E}{}_{\mathrm{<mtext>a</mtext>}}\text{=0.02}\text{V}\text{versus}\text{SHE}\text{)}$$

Protons generated at the anode pass through the solid-polymer-electrolyte membrane, usually Nafion^®-117, to combine with electrons and the oxidant air or oxygen simultaneously reducing to water as,

$$\text{3}\text{2}\text{O}{}_2\text{+6}\text{H}{}^{\mathrm{+}}\text{+6}\text{e}{}^{\mathrm{-}}\text{→3}\text{H}{}_2\text{O}\text{(E}{}_{\mathrm{<mtext>c</mtext>}}\text{=1.23}\text{V}\text{versus}\text{SHE}\text{)}$$

Accordingly, the net cell reaction in a SPE-DMFC is the production of CO₂ and H₂O as,

$$\text{CH}{}_3\text{OH}\text{+}\text{3}\text{2}\text{O}{}_2\text{→}\text{CO}{}_2\text{+2}\text{H}{}_2\text{O}\text{(E}{}_{\mathrm{<mtext>cell</mtext>}}\text{=1.21}\text{V}\text{)}$$

During the last decade, significant advances have been made in the development of the DMFC. Peak power densities of 450 and 300 mW/cm² under oxygen and air-feed operation, respectively, and a power density of 200 mW/cm² at a cell potential of 0.5 V have been reported for cells operating at temperatures around 100 °C under pressurised condition with platinum loadings of 1–2 mg/cm² [2]. Besides, the development of DMFC stacks for both transportation and portable applications has gained momentum in the last 2–3 years, and stack power densities of 1 kW/dm³ and an overall efficiency of 37% at a design point of 0.5 V per cell have been accomplished [3]. The performance of DMFCs is thus competitive with respect to the reformer-based hydrogen/air PEFCs, especially if one considers the complexity of the latter as a whole system [4]. However, further innovations in the DMFCs would be mandatory for their commercial realisation [5]. A step in this direction appears to be the development of mixed-reactants direct methanol fuel cells (MRDMFCs) which rely upon the selectivity of anode and cathode electrocatalysts to separate the electrochemical oxidation and reduction of the oxidant without the need for physical separation of fuel and oxidant [6], [7]. Accordingly, in a MRDMFC, the methanol fuel and oxidant oxygen (or air) are mixed together before feeding to the fuel cell. It is noteworthy that the selective-electrode fuel cells with mixed-reactants feed has been considered both for solid-oxide fuel cells [8] as well as polymer-electrolyte fuel cells [9], [10]. In the MRDMFCs, there would be no need for gas-tight structures within the stack providing relaxation for sealing and reactants/products delivery structures [7].

In a MRDMFC, cathode selectivity is paramount and is accomplished by using an oxygen-reduction catalyst which besides being tolerant to methanol does not oxidise it. Such catalysts of current interest are: (a) macrocyclic complexes, namely transition-metal tetra-methyl phenylporphyrins (TMPPs) such as FeTMPP, CoTMPP, FeCoTMPP [11], [12], [13] and transition-metal tetra-azaanulenes such as CoTAA [14], (b) transition-metal chalcogenides based on Chevrel phases such as Mo₄Ru₂Se₈ [15], [16], [17], [18], [19], [20], [21], and (c) transitional-metal clusters with sulphur or selenium such as RuS and RuSe [22]. Among these, the latter class of catalyst materials and, in particular, ruthenium-based cluster catalyst with selenium (RuSe) has been reported to be attractive for its selective catalytic activity towards oxygen-reduction reaction in the presence of methanol [22]. Anodic selectivity is typically achieved with the PtRu bimetal catalyst [6].

During the present study, we have assembled and tested MRDMFCs with membrane-electrode assemblies (MEAs) comprising a carbon-supported PtRu anode, a cathode containing either of the above mentioned carbon-supported transition-metal TMPPs or a carbon-supported selenium containing ruthenium-based cluster catalyst (RuSe/C), and Nafion^®-117 or Nafion^®-112 membrane as the electrolyte. The findings of the present study could also be significant to the development of conventional SPE-DMFCs where methanol crossover through the Nafion^® membrane electrolyte from anode to the cathode results in a poor catalytic activity at the cathode [3], [23], [24].