Demonstration of a Reactive Hydrogen Pump Using Methanol

Polymer electrolyte membrane (PEM) fuel cells are an attractive energy conversion device because of the potential for extremely high efficiency and low emissions. Hydrogen is an ideal fuel for PEM fuel cells but it is energy intensive to produce.

Currently the main method of producing hydrogen is steam reforming of natural gas. This is a break from the renewable energy future promised by hydrogen fuel cells. The most obvious method of producing hydrogen is water electrolysis but that is highly energy intensive since the electrolysis requires greater than the theoretical voltage of 1.23 V and thus is somewhat inefficient. To fulfill the promise of fuel cells, less energy intensive methods of producing hydrogen must be found.

Here we describe some initial exploration of the concept of replacing water in the anode electrolyzer feed with a feedstock from which hydrogen can be stripped and purified in a combined electrochemical reactor. As a first step in testing this concept, we use methanol as the feed.  Oxidation of methanol to hydrogen takes place at a theoretical voltage of .03 V. This is a significantly lower over-potential than water electrolysis and since most of the operating cost is energy, it has the potential to be significantly cheaper. Of course, the methanol oxidation reaction requires substantial overpotential because of sluggish kinetics but this is only an initial study.

We constructed a methanol water electrolyzer based the conventional direct methanol fuel cell architecture as shown in Figure 1. The electrolysis cell uses a Pt-Ru black catalyst in the anode and a Pt cathode with a Nafion 117 membrane. We examined the effects of several different catalyst loadings, back-pressures, temperatures, and humidity levels. Finally, we utilized electrochemical impedance spectroscopy (EIS) and a dynamic hydrogen electrode (DHE) to more accurately determine the effectiveness of the catalysis and sources of voltage loss. 

Figure 2 shows an initial polarization curve obtained with this cell.  Hydrogen was chosen for the working electrode and methanol for the counter electrode. The counter electrode potential stays fairly steady as does the ASR, while the working and cell potentials depend somewhat on the current being drawn. Increasing polarization of the working electrode begins to have a negative effect on cell current after .6V vs DHE.

Acknowledgements

We gratefully acknowledge the support of the Office of Naval Research for this activity.

 References

C. Cloutier and D. Wilkinson, International Journal of Hydrogen Energy., 35,9,(2010)

S. Thomas, X. Ren, S. Gottesfeld and P. Zelenay, Electrochimica Acta., 47 (2002)