Short communicationCrossover of formic acid through Nafion® membranes
Introduction
In previous work we found that formic acid fuel cells show interesting properties for micro power generation [1], [2]. In contrast to direct methanol fuel cells [3], [4], formic acid fuel cells run well at high formic acid concentrations and give reasonable power output at room temperature [2]. There was no evidence in our initial studies [1], [2] of significant formic acid crossover, but the crossover rate was not measured directly.
The purpose of this paper is to quantify the rate of formic acid permeation through Nafion® membranes. We chose to examine the properties of bare membranes so that we could tell if the permeability of formic acid through Nafion® was small, independent of the catalyst layer. Our procedure is to use a permeation cell similar to those used previously [5], [6], [8] to measure the flux of formic acid through the Nafion® in the absence of an electric field and then compare to the previous results to see if the permeation of formic acid is small. We chose to use a permeation cell, rather than an electrochemical measurement [3], [4], [9], [10], [11], [12], [13], [14], [15] so that we could avoid complications due to electric fields, reactions in the catalyst layer or CO2 crossover [16]. All of the work was done at room temperature since formic acid fuel cells are projected to run at room temperature.
Section snippets
Experimental
The permeation experiment involved putting a formic acid solution on one side of a membrane, putting distilled water on the other side of a membrane, and measuring the flux through the membrane as a function of time. The permeation measurement fixture was designed and built in house. The fixture has two glass compartments, whose volumes are approximately 40 ml. They are separated by a Nafion® membrane supported by two o-rings at both sides. Two teflon membrane holding structures containing the o
Results
Fig. 1 shows some typical data. In this experiment, a 1.131 cm2 Nafion® 117 membrane was loaded into the cell, one side of the cell was filled with various concentrations of formic acid, and the other side was filled with distilled water. Next, the concentration of formic acid in compartment A was measured as a function of time.
Generally, there is a slow buildup of formic acid in the water solution. The curves for 1 and 5 M fit straight lines through the origin with regression coefficients of
Discussion
The results here explain, in part, why formic acid fuel cells performed so well in our previous experiments [1], [2]. Notice that the formic acid fluxes in Table 1 are all relatively low. At a 1 M formic acid concentration, we observe a flux of only 2.03±0.07×10−8 mol/cm2 s. By comparison Jung et al. [6] report a methanol flux of 3.55×10−6 mol/cm2 s under similar conditions while Dimitrova et al. [7] report that with a 1.5 M methanol solution about 7×10−6 mol/cm2 s diffuse through a Nafion® 117
Conclusion
The permeation rate of formic acid through Nafion® membranes was directly measured in a house-built permeation measurement device. We find that the permeation rate of formic acid through Nafion® membranes ranged from 2.0×10−8 to 4.6×10−7 mol/cm2 s depending on the formic acid concentration and membrane thickness. These rates are much lower than that of methanol reported in the literature [6], [7], [8]. Further contimatory work is under consideration. The low permeation rate of formic acid
Acknowledgements
This material is based upon work supported by the Defense Advanced Research Projects Agency under US Air Force grant F33615-01-C-2172. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the US Air Force, or the Defense Advanced Research Projects Agency.
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Present address: Department of Chemical Engineering, Chungnam National University, Daejon, Korea.