The Effect on In-Cell Measurements from Water and Methanol Transport in Direct Methanol Proton Exchange Membrane Fuel Cells

Direct methanol fuel cells are attracting much attention as they are becoming poised to enter the portable power marketplace, offering significant advantages in potential device runtime over present technologies. The heart of a fuel cell is the anode-electrolyte-cathode assembly, where acidic solid polymer membranes are the electrolytes of choice. Fundamental technical issues in this area have been focused upon increasing reaction kinetics and controlling species transport through the cell. Obvious consumer device concerns about, for instance, fuel efficiency, product or fuel-diluting water management must be addressed at the electrolyte membrane design level. Polymer electrolyte membranes are designed to conduct ions, most specifically protons, from anode to cathode. Membrane and system design becomes an optimization problem requiring maximization of proton conductivity while minimizing methanol and water transport from fuel anode to oxidant cathode. Parasitic methanol transport is commonly called methanol crossover (MeOH XO) and water transport arises passively, from diffusion, and actively as it is dragged along from anode to cathode with the proton in the fuel cell reaction, the so-called electroosmotic drag (EOD). For membrane or system screening, an area of interest for us has been rapid and reliable property measurement methods. The most widely adopted method for measuring MeOH XO in fuel cells is the diffusion limited plateau current from electrochemical MeOH oxidation at the fuel cell cathode. In our efforts to expedite our in-cell screening and qualification testing we adopted the use of dry nitrogen flow on the cathode during the MeOH XO measurement. This method obviated the purging of humidifier bottles that helped shave tens of minutes off this measurement. What we have found, however, was that an increase of the flow rate of N2 on the cathode during this measurement would increase in turn the MeOH XO values. This trend for one of our DMFC samples operating in 1.0M MeOH and at 60C is shown in Figure 1. The effect of increased MeOH XO with increased dry nitrogen flow arises because of enhanced water transport at higher nitrogen flow rates on the cathode. When measuring the MeOH XO of an operating fuel cell, it is usual for the amount of MeOH XO to drop with increasing operating current. This observation is well known and is the result of a decreasing MeOH concentration immediately adjacent to the membrane on the anode side. Under high MeOH flow or when using high MeOH concentrations, however, it can happen that MeOH XO can increase over a small range of operating current density as is demonstrated in Figure 2 for low and high (1.0M) MeOH flow conditions. In this contribution we will discuss our experimental observations on the interrelationship of methanol and water transport as it affects commonly used in-cell membrane mass transport characterization measurements, specifically, MeOH XO and EOD. Cognizance of contributing effects can help understand the limits of these experimental measurements.