Short communication
Crossover of formic acid through Nafion® membranes

https://doi.org/10.1016/S0378-7753(03)00352-5Get rights and content

Abstract

Formic acid has been proposed as a possible fuel for miniature fuel cells, because formic acid is expected to show low crossover and easy water management. In this paper, the permeation of formic acid through Nafion® membranes is investigated at room temperature. It is found that the permeation of formic acid through Nafion® 112 and 117 is much lower than that of methanol. For example, at a 1 M concentration, the steady state flux of formic acid through Nafion® 117 is only 2.03±0.07×10−8 mol/cm2 s. By comparison, previous workers have observed a methanol flux of 3 to 6×10−6 mol/cm2 s through Nafion® 117 under similar conditions. The flux through Nafion® 117 increases with increasing formic acid concentration, reaching a maximum of 1.86±0.11×10−7 mol/cm2 s at a formic acid concentration of 10 M. The flux of formic acid is about a factor of two higher through Nafion® 112 than through Nafion® 117 but still low. These results show that the permeation of formic acid through Nafion® is much slower than the permeation of methanol through the same membrane. Consequently, formic acid is an attractive alternative fuel for small polymer electrolyte membrane (PEM) fuel cells.

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.

References (22)

  • C Rice et al.

    J. Power Sources

    (2002)
  • S.C Thomas et al.

    Electrochim. Acta

    (2002)
  • Z.G Qi et al.

    J. Power Sources

    (2002)
  • S.R Yoon et al.

    J. Power Sources

    (2002)
  • D.H Jung et al.

    J. Power Sources

    (2002)
  • P Dimitrova et al.

    J. Electroanalyt. Chem.

    (2002)
  • J Cruickshank et al.

    J. Power Sources

    (1998)
  • A Heinzel et al.

    J. Power Sources

    (1999)
  • J.-T Wang et al.

    J. Appl. Electrochem.

    (1996)
  • S Ha et al.

    Power Sources

    (2002)
  • C. Rice, S. Ha, R.I. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, in: Proceedings of the Conference of the 40th Power...
  • Cited by (293)

    • Nafion/functionalized metal–organic framework composite membrane for vanadium redox flow battery

      2022, Microporous and Mesoporous Materials
      Citation Excerpt :

      Because VRFBs are operated in a sulfuric acid solution (strong acid), Nafion is the most widely used perfluorinated membrane because of its high electrical conductivity and excellent chemical stability [8,9]. However, Nafion suffers from a high cost and the crossover phenomenon, in which vanadium ions in one cell permeate to the other half cell, which in turn leads to a reduction in battery capacity that lowers the battery performance [10–16]. To solve the problems associated with applying Nafion to VRFBs, various non-perfluorinated membranes such as sulfonated poly(ether ketone), sulfonated poly(arylene ether ketone), and sulfonated poly(imide) have been studied, but they failed to achieve a chemical stability surpassing that of Nafion [17–19].

    View all citing articles on Scopus
    1

    Present address: Department of Chemical Engineering, Chungnam National University, Daejon, Korea.

    View full text