A New Challeng for Hydrogen Isotope Electrolytic Separation System Combined with Fuel Cell

1. Introduction

    Hydrogen isotopes are important elements for nuclear and fusion reactors. The technique of hydrogen isotope separation has been investigated in several approaches [1-3]. However, the most methods are ineffective processes and shows low separation factor. That is, a large scale plant is generally necessary to deal with enormous water for the isotope enrichment. In Japan, a novel separation technology of tritium in Fukushima Daiichi nuclear power plant is urgently required.

    Although water electrolysis has been mainly used for heavy water production since the 1930s, it is no longer used except in the final separation stage. The reason is that water electrolysis consumes an enormous amount of the electric energy for a large-scale production. However, the electrolysis process is still attractive due to the highest separation factor and less polluted exhausts. In the conventional electrolysis process, no attempts have been made to recover energy from the gases evolved from the water electrolysis cell. The gases evolved from the electrolysis are recombined in a burner or catalytic exchange tower, and the left water vapor is condensed and fed to a lower stage. Supposed that the evolved hydrogen and oxygen gases are fed into a fuel cell as seen in Fig. 1, a large amount of energy will be recovered as electricity. For exploring this new concept [4], we have comprehensively investigated the electrochemical kinetics of hydrogen electrode reactions and separation factors in fuel cell and water electrolysis [5].  

2. Experimental

    The kinetic study was conducted by the rotating electrode with the rotation rate of 900, 1600, 2500 and 3600 rpm. The working electrode was a 0.785 cm^-2 polycrystalline Pt disk and the counter electrode was coiled Pt wire. The reference electrode was Hg/Hg₂SO₄ saturated with K₂SO₄. The polarization curves were recorded by linear sweep voltammetry at the scan rate of ±0.01 V s^-1. The electrolytes of 0.05 M D₂SO₄and 0.05 M KOD were maintained at 298 K.

3. Results and discussion

    Figure 2 shows Tafel plots of mass transfer corrected currents for the hydrogen oxidation reaction (HOR) and deuterium oxidation reaction (DOR) at low anodic overpotential (η ≤ 0.08 V). The Tafel region of HOR began to be observed at η ≈ 0.04 V. The slope value of HOR was 0.091 V dec^-1, which was larger than that of DOR (0.055 V dec^-1). Concerning on the exchange current density, the ratio of i_0(H2) / i_0(D2)was 0.46. The present kinetic comparison of deuterium with protium data supported the isotope effect by which the deuterium was more easily dissociated on platinum electrode, pointing out a new separation by fuel cells.

    We will calculate the total energy consumption in fuel cell and water electrolysis and discuss the possibility of new separation system for heavy water production.

ACKNOWLEGMENT

   The authors appreciate the financial aid given by the Ministry of Education,　Culture, Sports, Science and Technology (Project NO. 26870234). One of the authors, H. Matsushima, wishes to express his sincere gratitude to Inamori Foundation in Japan.

REFERENCES

[1] C. Banglin et al., J. Am. Chem. Soc., 130, 6411 (2008).

[2] I. Cristescu et al., Fusion Eng. Des., 82, 2126 (2007).

[3] I. A. Alekseev et al., Fusion Sci. Technol., 41, 1097 (2002).

[4] H. Matsushima, T. Nohira, T. Kitabata, Y. Ito, Energy, 30, 2413 (2005).

[5] S. Shibuya, H. Matsushima, M. Ueda, in preparation.