Methanol Sensor Operative in Wide Range of Fuel Concentration and Temperature with Fast Response

Abstract

An electrochemical methanol sensor for monitoring the fuel concentration in direct methanol fuel cells was constructed using a thin composite membrane as the electrolyte. The 6 m thick composite membranes were prepared by impregnating porous polyethylene-terephthalate film with Nafion ionomer. The resulting sensor is capable of monitoring the fuel in any concentration up to pure methanol. This can be accomplished by placing a palladium film at the interface between the anode and the membrane. The sensor is also operative even at ambient temperature and responds quickly to concentration changes, i.e., in less than 2 s. Direct methanol fuel cells ͑DMFCs͒ employing proton exchange membrane electrolytes have gained considerable interest because of their attractiveness as the power source for some applications. 1-5 The methanol fuel contacting the anode of the DMFC is provided as a dilute solution, usually less than 2 M for better fuel efficiency. 6,7 Because the anode consumes only a fraction of the methanol in the feed stream, the fuel solution needs to be recirculated with the controlled addition of concentrated methanol into the stream. Thus a methanol sensor is critical to the practical operation of the fuel cell system, both to monitor the inlet methanol concentration and provide feedback for the control of pure methanol injection.Methanol concentration can be determined by measuring some physical properties such as density, surface tension, refractive index, and light absorptivity that depend on the concentration. 8-11 But due to the narrowness of the range of detectable concentrations or lack of sensitivity or difficulty with miniaturization, the methods based on these principles have not been implemented as the operating mode of practical methanol sensors. On the other hand, methanol sensors based on electrochemical methods seem to alleviate those faults encountered by other methods. Electrochemical methanol sensors can be classified into two types depending on their mode of operation: galvanic or electrolytic modes. The galvanic mode sensor may be the simplest type because it does not need an independent sensing unit. Just by measuring some electrochemical properties of the cell such as cell potential, current or open cell potential, the concentration can be deduced. 12 But such measurements could lead to erroneous evaluation of the concentration since other factors such as methanol cross-over or temperature fluctuation could lead to incorrect signals. 13 The methanol sensor of electrolytic mode is based on the electrochemical reactionsAt the anode of the sensor the methanol fuel is oxidized to generate hydrogen ions that are eventually reduced to hydrogen gas at the cathode at the expense of electrical energy provided. 14-16 The amperometric sensor allows the feed-in of the fuel from either the cathode 14 or the anode side. 15 The cathode side feed-in has the advantage of determining the concentration more sensitively because the concentration difference is clearly developed into different limiting currents...

A strategy of estimating fuel concentration in a direct liquid-feed fuel cell system

“…Additional compensation is needed for situations that are varying. The electrolytic type sensing schemes measure the limiting current or the half-cell potential to establish the relationship with methanol concentration [11][12][13][14][15]. In these designs, a substantial sensor is required while the reactions wherein are not spontaneous.…”

A Miniature Electrochemical Methanol Concentration Sensor for Direct Methanol Fuel Cell Systems

“…For monitoring the methanol concentration electrochemical methods are known for their simplicity in design and operation and also for their highly sensitive response to the concentration. The previous work in this laboratory has demonstrated that the sensor is capable of monitoring the fuel in any concentration up to pure methanol 2) .…”

“…But it is also proposed that the fuel of very high concentration may render more efficient performance to the system. Our group has been engaged in developing the sensor with the aim of fast response, the capability of monitoring wide range of the concentration, and accuracy of the sensing [1][2] . For monitoring the methanol concentration electrochemical methods are known for their simplicity in design and operation and also for their highly sensitive response to the concentration.…”

Effects of gas diffusion backings on performance of low power methanol sensor

“…For most DMFC systems the required concentration is in the range of 0.1-2M. Our group has been engaged in developing electrochemical methanol sensor with the aim of fast response, the capability of monitoring wide range of the concentration, and accuracy of the sensing [1][2] . For monitoring the methanol concentration electrochemical methods are known for their simplicity in design and operation and also for their highly sensitive response to the concentration.…”