Abstract
This thesis focuses on the development of sensors for the determination of aqueous methanol concentration in direct methanol fuel cell (DMFC) applications. The crossover of methanol from anodic compartment to the cathodic compartment and its subsequent oxidation in the cathodic compartment is the main reason for the low efficiency of DMFC, which is yet to be solved in the current DMFC technology. DMFC operated at controlled low concentration methanol feed is one of the approaches to avoid the cell voltage loss due to methanol crossover. Such an operation needs the development of a suitable methanol sensor which motivates the present study. The study consists of two major parts. In the first part, literature overview of DMFC, methanol crossover problem, methods to circumvent the methanol crossover are discussed. Apart from this, salient features of various methanol detection methods and the necessary features of a methanol sensor are discussed. Then, rationales for the selection of electrochemical sensor development as the subject matter are presented. Fabrication and operation of a fuel cell based sensor is presented in chapter 3. Even though, the sensitivity of the fuel cell based sensor is adequate for the methanol concentration in the range of 0.5 M to 1.5M, higher power consumption and bigger size are the problems with this design. Operating experience with the conventional methanol detection techniques like densitometer and refractive index detector showed the shortcomings of these techniques. An innovative concept of solid state planar structured methanol sensor design is introduced in chapter 4. Thick film screen printing process was used to print the gold working and counter electrodes on alumina substrate and process for the formation of the Ag/AgCl reference electrode was also developed. A thick recast Nafion film was the electrolyte. The sensor was characterized by Electrical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). The experimental results revealed that either hydrogen desorption (peak at ~0.22 VSHE) or methanol oxidation (~1.0 VSHE) could be used to evaluate the methanol concentration. The device should be used for the estimation of methanol concentration at 0.01mA/M by methanol oxidation current (or 0.1 mA/M by hydrogen desorption current) in the range of 0.5M to 2M. Silicon based microfabrication is introduced in chapter 5. This has the advantage of producing microsized structures in highly uniform and geometrically well defined manner. Platinum working and counter electrodes were prepared by sputtering. The process for the formation of recast nafion electrolyte film was the same as alumina-based device. The sensor could be used to monitor methanol concentration from the methanol oxidation current. The current linearly increased with methanol concentration in the range 0.5 to 1.5M (with 0.01mA/mole sensitivity). A summary of the work and recommendations for further activities were presented in chapter 6.