Optimization of properties and operating parameters of a passive DMFC mini-stack at ambient temperature

Abstract

An investigation of properties and operating parameters of a passive DMFC monopolar mini-stack, such as catalyst loading and methanol concentration, was carried out. From this analysis, it was derived that a proper Pt loading is necessary to achieve the best compromise between electrode thickness and number of catalytic sites for the anode and cathode reactions to occur at suitable rates. Methanol concentrations ranging from 1 M up to 10 M and an air-breathing operation mode were investigated. A maximum power of 225 mW was obtained at ambient conditions for a three-cell stack, with an active single cell area of 4 cm², corresponding to a power density of about 20 mW cm⁻².

Introduction

Direct methanol fuel cells (DMFCs) are promising candidates for portable electric power sources because of their high energy density, lightweight, compactness, simplicity as well as easy and fast recharging [1], [2], [3], [4]. In the last decades, a large number of publications were focused on the development of DMFC components, such as catalysts, membranes, electrodes, etc. [5], [6], [7], [8], [9], [10], [11], [12], [13]. Initially, DMFC stacks have been developed for assisted power units (APUs) purpose or for electrotraction [14]. These stacks used a classical configuration where single cells are stacked in series through bipolar plates. More recently, the attention has been focused on portable applications and various stack configurations have been proposed [15], [16], [17], [18], [19], [20]. Thanks to integrated-circuit (IC) fabrication technology, micro-channel patterns can be featured on the silicon wafer with high resolution and good repeatability [19], [20], [21]. Moreover, in order to be commercially viable, it is necessary to eliminate power losses from auxiliaries, i.e. the pump and the fan that are usually used to feed methanol and air into the stack [22], [23]. To this scope, the concept of passive-feed DMFCs has been proposed [16], [24], [25], [26], [27], [28], [29]. Under this configuration, DMFCs operate without any external devices for feeding methanol and blowing air into the cells. Oxygen can diffuse into the cathode from the ambient due to an air-breathing action of the cell (partial pressure gradient), whereas methanol can reach the catalytic layer from a reservoir driven by a concentration gradient between the electrode and the reservoir and through capillary force action of electrode pores. The use of low cost miniaturised “step-up” DC/DC converters, recently commercialised [30], allows to suitably increase the stack potential with a very small dissipation of power (∼90% efficiency). This approach does not require extensive miniaturisation of the DMFC stack favouring the development of low cost DMFC stack architectures with practical electrode area. In the present paper, an optimization of properties and operating parameters, such as methanol concentration and catalyst loading, of a passive direct methanol fuel cell monopolar stack is presented. The influence of Pt loading, already investigated in a conventional forced-flow DMFC [31], was analysed taking into account the mass transfer constrains caused by high catalyst loading under passive operation.