Impedance studies on direct methanol fuel cell anodes

Abstract

The processes taking place in direct methanol fuel cell anodes are characterized by ac impedance spectroscopy. Under conditions of practical interest, i.e., low methanol stoichiometry factors, the kinetic and the mass-transport resistance give rise to two well-resolved semicircles in the Nyquist plot. When mass-transport limitations are excluded, inductive loops occur in the complex plane which are interpreted in terms of the most widely accepted reaction mechanism for methanol electrooxidation. A simple equivalent circuit is used to model this impedance behaviour.

Introduction

Direct methanol fuel cells (DMFC) receive increasing attention. This is mainly due to their characteristics such as simple construction, reduced system weight, size and complexity, high energy efficiency and low emissions. Recently reported advances in DMFC power densities are very promising; however, they have been attributed to improvements in the electrode structure and optimization of operating conditions rather than to progress in electrocatalysis 1, 2. In a previous paper [3], we reported on our results of DMFC impedance measurements carried out under conditions of practical interest, i.e., low methanol stoichiometry factors were used in order to reduce the unwanted effect of fuel crossover. It was shown that anode kinetics, anode mass transport, cathode kinetics, cathode mass transport, and membrane resistance can be studied separately in situ. Here, we focus on the faradaic impedance of DMFC anodes operating without mass-transport limitations in order to investigate the kinetics and the mechanism of methanol electrooxidation.