Next Article in Journal
The Synthesis of a Core-Shell Photocatalyst Material YF3:Ho3+@TiO2 and Investigation of Its Photocatalytic Properties
Next Article in Special Issue
Towards Highly Performing and Stable PtNi Catalysts in Polymer Electrolyte Fuel Cells for Automotive Application
Previous Article in Journal
The Effect of Different Storage Media on Color Stability of Self-Adhesive Composite Resin Cements for up to One Year
Previous Article in Special Issue
Synthesis of 2D Nitrogen-Doped Mesoporous Carbon Catalyst for Oxygen Reduction Reaction
Open AccessArticle

Novel PEFC Application for Deuterium Isotope Separation

Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Sapporo, Hokkaido 060-8628, Japan
*
Author to whom correspondence should be addressed.
Academic Editors: Vincenzo Baglio and David Sebastián
Materials 2017, 10(3), 303; https://doi.org/10.3390/ma10030303
Received: 18 February 2017 / Revised: 14 March 2017 / Accepted: 15 March 2017 / Published: 16 March 2017
(This article belongs to the Special Issue Advanced Materials in Polymer Electrolyte Fuel Cells)

Abstract

The use of a polymer electrolyte fuel cell (PEFC) with a Nafion membrane for isotopic separation of deuterium (D) was investigated. Mass analysis at the cathode side indicated that D diffused through the membrane and participated in an isotope exchange reaction. The exchange of D with protium (H) in H2O was facilitated by a Pt catalyst. The anodic data showed that the separation efficiency was dependent on the D concentration in the source gas, whereby the water produced during the operation of the PEFC was more enriched in D as the D concentration of the source gas was increased.
Keywords: hydrogen isotope separation; fuel cell; CEFC; isotope exchange reaction hydrogen isotope separation; fuel cell; CEFC; isotope exchange reaction

1. Introduction

The heavy isotopes of hydrogen, deuterium (D) and tritium (T) play essential roles in nuclear energy production [1,2]. In current heavy-water nuclear fission reactors, D is used as a neutron-moderator. Similarly, in nuclear fusion reactors, which are expected to represent the next generation of nuclear power, the reaction of D and T is responsible for the energy-production stage.
Because D and T are not directly obtainable as pure isotopes, methods to separate them from the more common, lighter isotope, protium, are required. Many researchers have studied various isotope-separation methods, including water distillation [3], molecular sieving [4], water electrolysis [5,6,7,8] and combined electrolysis catalytic exchange [9,10]. The water electrolysis yields the most effective separation but consumes enormous amounts of electricity. Such large consumption has led to a search for other methods that are more energetically efficient. In particular, a new separation technology for tritium is urgently required at the Fukushima Daiichi Nuclear Power Plant in Japan.
We previously proposed a new hydrogen-separation system: the combined electrolysis fuel cell (CEFC) process [11]. Here, hydrogen and oxygen were produced by electrolysis and used for power generation in a fuel cell. By recycling the energy generated from the produced hydrogen, the electricity consumption of the isotope separation process was reduced. More recent work has reported D separation via the hydrogen isotope effect during the anodic reaction in polymer electrolyte fuel cells (PEFCs) [12,13,14] and alkaline membrane fuel cells [15]. We reported that the water produced by these power sources was enriched in D. This was caused entirely by the kinetic isotope effect during the hydrogen oxidation reaction (HOR) on a Pt catalyst. However, several other factors must be investigated to fully realize the potential of CEFC systems. The dependency of the separation efficiency on the isotope concentration is important from the practical viewpoint. The mass balance of the isotopes in fuel cells must be strictly controlled when radioactive species are involved. Therefore, this paper focuses on measuring D separation by a PEFC and investigates the factors influencing separation at both the cathode and anode, using isotopically mixed gases with several D concentrations.

2. Experimental

A JARI standard cell (FC Development Corp., Tsukuba, Japan) was employed as a PEFC. The membrane electrode assembly (50 × 50 mm) was composed of Nafion electrolyte (NRE-212) and two catalytic layers loaded with platinum catalyst (Pt, 0.52 mg·cm−2). The PEFC was operated at 298 K and the power generation was controlled under constant current mode (0.0–1.2 A) adjusted by a variable resistor unit (PLZ 164WA, Kikusui Electronics Corp., Yokohama, Japan).
The deuterium separation factor, α, of the PEFC was measured by quadrupole mass spectrometry (QMS) (Qulee-HGM 202, Ulvac Corp., Chigasaki, Japan). Figure 1 shows a schematic of the experimental setup. Humidified O2 gas was supplied into the cathode at 40 mL·min−1. Mixture gases of H2 and D2 were used at the anode. The mixing ratio was adjusted by a mass flow controller (MC-200SCCM-D, Alicat Scientific, Tucson, AZ, USA). The composition of the exhaust gas from each of Lines I–III was monitored by QMS. Ion currents of mass numbers (m) = 2, 3, 4 were recorded at Lines I–II and those of m = 18, 19 at Line III.

3. Results and Discussion

A mixture gas of H2 and D2 was supplied to the anode of the PEFC. The effect of the Pt catalyst on the mixture gas was investigated by comparing the gas component composition before and after introducing the anode. With the PEFC switched off, the effect of the catalyst could be studied in the absence of electrochemical kinetic factors arising from power generation by the cell. Figure 2 shows the QMS data of each mass number (m = 2–4) when the ratio of D/H was 10−2. With the PEFC switched off, the QMS data of Line I indicated no change in the isotopic composition of the gas over time. However, a trace amount of a species with m = 3 was detected. This can be attributed to HD, formed via the fragmentation of H2 and D2 during the ionizing process in the QMS chamber.
The mixture gas from the PEFC outlet side was monitored from Line II. The arrow in Figure 2 indicates the time when the gas line was switched from Line I to II. The ion current of m = 2, corresponding to H2, remained almost constant, and was independent of passing through the Pt catalytic layer. In contrast, the ratio of HD to D2 was inverted, showing a substantial increase of HD. From the ion currents of each mass number it was calculated that more than 95% of the D2 gas was converted to HD. As reported previously [12], the generation of HD gas is proceeded by an isotope exchange reaction, as expressed in Equation (1),
H2 + D2 → 2HD
The exchange reaction is reported to be enhanced on a Pt surface [16,17]. The high kinetic rate of exchange can be attributed to the well-developed catalyst structure in PEFCs [18]. The catalytic activity is also promoted by the use of nano-sized particles and the uniform distribution of these particles on the supporting materials. Additionally, the gas diffusion layer increases the degree of contact between the mixture gas and the catalyst.
Assuming that the ion currents of each mass number were proportional to the numbers of each molecular species, the effect of the membrane on separation was evaluated. The total amount of D in Line II was about 0.1% less than that of Line I. This loss may have occurred with the penetration of D into Line III or the uptake of D by the Nafion membrane.
The composition of the gas from the cathode was analyzed by the QMS connected to Line III (Figure 1). Pure O2 gas was supplied to the PEFC. This gas was fully humidified by protium water before being let in to the PEFC. The bubbler was maintained at 298 K. The two ion currents detected at this line had mass values of m = 18 and 19. The species with m = 18 was normal molecular water, H2O, while the other was H3O, produced by the fragmentation of H2O. It appears that H2O was easily decomposed by ionization in the QMS chamber, probably because of its large molecular size.
Figure 3 shows the variation of the ion currents of m = 18 and 19 when the isotopically mixed gas was supplied to the anode under the same conditions as in Figure 2. The ion current of m = 18 decreased over time, while that of m = 19 increased such that the ratio of the latter to the former was doubled. Assuming that the frequency of the fragmentation by QMS was independent of the anode condition, this result directly suggests that the increase of the D content resulted from the formation of HDO. The H2 and D2 species in the mixture gas were oxidized at the anode, resulting in their conversion into H+ and D+ ions, respectively. The dissociated ions diffused through the conducting polymer toward the cathode. The Pt catalyst facilitated the exchange of the deuterons, D+, for H in molecules of H2O. This isotope exchange reaction, expressed by Equation (2), was responsible for the increased content of HDO.
D+ + H2O → H+ + HDO
The D/H ratio at Line III was larger than that at the counter-anode. This difference indicates the difference between the diffusion rates of H+ and D+ in the membrane.
The PEFC was connected to the variable resistor and the power performance of the PEFC was measured under current control. Humidified O2 gas was used at a flow rate of 40 mL·min−1, while dry H2 was inlet at 20 mL·min−1. Figure 4 shows the current-voltage curves of the pure H2 gas and the mixture gases. The open circuit voltage of the pure H2 gas was smaller than that of the mixture gases. Since the cathode conditions were the same, the small difference probably corresponded to the isotope effect on the equilibrium potential of HOR [12,19].
When the PEFC was in operation, generating electric power, the current-voltage curves of both gases showed almost identical behavior. The cell voltage decreased abruptly at about 1.5 A. In such cells, the cathode potential dominates the cell voltage with an increasing output current, because oxygen reduction on Pt catalysts is inactive under these conditions and gas diffusion is slow.
The gas composition was monitored in situ. Several inlet H2/D2 gases with a range of D isotopic concentrations (D/H = 10−2–10−4) were compared. Figure 5 shows the mass analysis data of the gases from Line II. Before the power generation, the ion current of HD (m = 3) changed in accordance with the D concentration of the inlet gas. When the PEFC was operated at 1.2 A, the ion current of HD significantly decreased. The degree of this decrease was lessened with the decreasing D concentration. The contrast with the behavior of H2, which showed a constant ion current, is evidence that the D isotope reacted preferentially during HOR [12]. The D selectivity was clearly dependent on the isotope concentration.
The ion current of D2 was detectable only when the mixture gases with D/H = 10−2 were investigated. Detection was not possible with lower D concentrations because the current was below the detection limit of QMS. The ion current at D/H = 10−2 is also shown in Figure 5. The HOR selectively consumes D2 in preference to HD. This is a typical isotope effect, where the variation degree depends on the mass number.
The deuterium separation factor was calculated by the following equation,
α = ([H]/[D])a/([H]/[D])b
where [H] and [D] are the atomic fractions of protium and deuterium, and the subscripts (a) and (b) refer to after and before starting the power generation. The species D2 was not considered in the present study because of its low concentration, as shown in Figure 5. The very small ion current of m = 4 did not appreciably affect the α values.
The separation factors calculated at various D concentrations are shown in Figure 6. The error bars indicate the maximum and minimum values among several experiments. The α values exhibited concentration dependency. D was separated more effectively at higher D concentrations, as expected. However, it should be emphasized that α approached a certain limiting value at low concentrations (D/H < 10−5). The PEFC was able to produce water enriched in D. Even dilute mixture gases could be dispersed in the gas diffusion layer, resulting in extensive contact between the gas and the catalyst.

4. Conclusions

The D isotopic mass flow in a PEFC with a Nafion membrane was investigated by mass analysis of the mixture gases from both the anode and cathode. Before the power generation was switched on, the mixture gases of H2 and D2 were almost completely converted to HD at the anode side. A small amount of D could diffuse through the membrane as D+ ions and then form HDO at the cathode side by isotopic exchange with protium in H2O. The mass balance of D indicated the partial accumulation of D in the membrane.
The power generation of the PEFC was not affected by the introduction of D-containing mixture gases, while the open circuit potential was shifted to a more anodic potential than the equilibrium one of value for isotopically pure H2 gas. The D content from the anode side was significantly diluted by HOR. The value of α depended on the D concentration, decreasing from about 4 at D/H = 10−1 to about 2 at D/H = 10−5.

Acknowledgments

The authors gratefully acknowledge financial support from the Takahashi Industrial & Economic Research Foundation and the Inamori Foundation in Japan.

Author Contributions

Hisayoshi Matsushima conceived the experiments and wrote the paper; Ryota Ogawa and Shota Shibuya performed the experiments and analyzed the data; Mikito Ueda contributed the discussion of the data.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Xiang, X.; Wang, X.L.; Zhang, G.K.; Tang, T.; Lai, X.C. Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers. Int. J. Hydrogen Energy 2015, 40, 3697–3707. [Google Scholar] [CrossRef]
  2. Andreev, B.M.; Sakharovsky, Y.A.; Rozenkevich, M.B.; Magomedbekov, E.P.; Park, Y.S.; Uborskiy, V.V.; Trenin, V.D.; Alekseev, I.A.; Fedorchenko, O.A.; Karpov, S.P.; et al. Installations for Separation of Hydrogen Isotopes by The Method of Chemical Isotopic Exchange in The Water-Hydrogen System. Fusion Technol. 1995, 28, 515–518. [Google Scholar]
  3. Bhattacharyya, R.; Bhanja, K.; Mohan, S. Simulation studies of the characteristics of a cryogenic distillation column for hydrogen isotope separation. Int. J. Hydrogen Energy 2016, 41, 5003–5018. [Google Scholar] [CrossRef]
  4. Oh, H.; Savchenko, I.; Mavrandonakis, A.; Heine, T.; Hirscher, M. Highly Effective Hydrogen Isotope Separation in Nanoporous Metal-Organic Frameworks with Open Metal Sites. ACS Nano 2014, 8, 761–770. [Google Scholar] [CrossRef]
  5. Greenway, S.D.; Fox, E.B.; Ekechukwu, A.A. Proton exchange membrane electrolyzer operation under anode liquid and cathode vapor feed configurations. Int. J. Hydrogen Energy 2009, 34, 6603–6608. [Google Scholar] [CrossRef]
  6. Stojic, D.L.; Grozdic, T.D.; Kaninski, M.P.M.; Maksic, A.D.; Simic, N.D. Intermetallics as advanced cathode materials in hydrogen production via electrolysis. Int. J. Hydrogen Energy 2005, 34, 841–846. [Google Scholar] [CrossRef]
  7. Ogata, Y.; Sakuma, Y.; Ohtani, N.; Kotaka, M. Tritium separation by electrolysis using solid polymer electrolyte. Fusion Sci. Technol. 2005, 48, 136–139. [Google Scholar]
  8. Matsushima, H.; Nohira, T.; Ito, Y. Improved deuterium separation factor for the iron electrode prepared in a magnetic field. Electrochim. Acta 2004, 49, 4181–4187. [Google Scholar] [CrossRef]
  9. Huang, F.; Meng, C.G. Hydrophobic platinum-polytetrafluoroethylene catalyst for hydrogen isotope separation. Int. J. Hydrogen Energy 2010, 35, 6108–6112. [Google Scholar] [CrossRef]
  10. Alekseev, I.A.; Bondarenko, S.D.; Fedorchenko, O.A.; Grushko, A.I.; Karpov, S.P.; Konoplev, K.A.; Trenin, V.D.; Arkhipov, E.A.; Vasyanina, T.V.; Voronina, T.V.; et al. The CECE experimental industrial plant for reprocessing of tritiated water wastes. Fusion Sci. Technol. 2002, 41, 1097–1101. [Google Scholar]
  11. Matsushima, H.; Nohira, T.; Kitabata, T.; Ito, Y. A novel deuterium separation system by the combination of water electrolysis and fuel cell. Energy 2005, 30, 2413–2423. [Google Scholar] [CrossRef]
  12. Shibuya, S.; Matsushima, H.; Ueda, M. Study of Deuterium Isotope Separation by PEFC. J. Electrochem. Soc. 2016, 30, 2413–2423. [Google Scholar] [CrossRef]
  13. Yanase, S.; Oi, T. Observation of H/D isotope effects on polymer electrolyte membrane fuel cell operations. J. Nucl. Sci. Technol. 2013, 50, 808–812. [Google Scholar] [CrossRef]
  14. Kaninski, M.P.M.; Nikolic, V.M.; Maksic, A.D.; Tasic, G.S.; Miljanic, S.S. Electrochemical H/D isotope effects in PEM fuel cell. Electrochem. Commun. 2008, 10, 1463–1466. [Google Scholar] [CrossRef]
  15. Ogawa, R.; Matsushima, H.; Ueda, M. Hydrogen isotope separation with an alkaline membrane fuel cell. Electrochem. Commun. 2016, 70, 5–7. [Google Scholar] [CrossRef]
  16. Ye, L.S.; Luo, D.L.; Yang, W.; Guo, W.S.; Xu, Q.Y.; Jiang, C.L. Improved catalysts for hydrogen/deuterium exchange reactions. Int. J. Hydrogen Energy 2013, 38, 13596–13603. [Google Scholar] [CrossRef]
  17. Zhang, W.B.; Burgess, I.J. Kinetic isotope effects in proton coupled electron transfer. J. Electroanal. Chem. 2012, 668, 66–72. [Google Scholar] [CrossRef]
  18. Hiesgen, R.; Wehl, I.; Aleksandrova, E.; Roduner, E.; Bauder, A.; Friedrich, K.A. Nanoscale properties of polymer fuel cell materials. Int. J. Energy Res. 2010, 34, 1223–1238. [Google Scholar]
  19. Krishtalik, L.I. Kinetic isotope effect in the hydrogen evolution reaction. Electrochim. Acta 2001, 46, 2949–2960. [Google Scholar] [CrossRef]
Figure 1. Schematic illustration of experimental measurement of the deuterium separation factor of PEFC. 1. H2 gas; 2. D2 gas; 3. O2 gas; 4. Mass flow controller; 5. Gas mixture unit; 6. Bubbler; 7. Anode; 8. Electrolyte membrane assembly; 9. Cathode; 10. Variable resistor; 11. Q-mass.
Figure 1. Schematic illustration of experimental measurement of the deuterium separation factor of PEFC. 1. H2 gas; 2. D2 gas; 3. O2 gas; 4. Mass flow controller; 5. Gas mixture unit; 6. Bubbler; 7. Anode; 8. Electrolyte membrane assembly; 9. Cathode; 10. Variable resistor; 11. Q-mass.
Materials 10 00303 g001
Figure 2. Transient behavior of Q-Mass spectra of mass numbers m = 2 (H2, green line), 3 (HD, red line) and 4 (D2, blue line) at the anode side. A mixture of H2 (10.0 mL·min−1) and D2 (0.1 mL·min−1) was passed through the PEFC for 2 h and then passed directly to the Q-Mass for 3 h without power generation.
Figure 2. Transient behavior of Q-Mass spectra of mass numbers m = 2 (H2, green line), 3 (HD, red line) and 4 (D2, blue line) at the anode side. A mixture of H2 (10.0 mL·min−1) and D2 (0.1 mL·min−1) was passed through the PEFC for 2 h and then passed directly to the Q-Mass for 3 h without power generation.
Materials 10 00303 g002
Figure 3. Transient behavior of Q-Mass spectra of mass numbers m = 18 (H2O, blue line) and 19 (HDO, red line) at the cathode side. Arrow indicates the onset time, when the mixture of H2 (10.0 mL·min−1) and D2 (0.1 mL·min−1) was passed to the anode side.
Figure 3. Transient behavior of Q-Mass spectra of mass numbers m = 18 (H2O, blue line) and 19 (HDO, red line) at the cathode side. Arrow indicates the onset time, when the mixture of H2 (10.0 mL·min−1) and D2 (0.1 mL·min−1) was passed to the anode side.
Materials 10 00303 g003
Figure 4. Cell current-voltage curves when PEFC was in operation with pure H2 gas (black line) and mixture gases of H2 and D2 (red line).
Figure 4. Cell current-voltage curves when PEFC was in operation with pure H2 gas (black line) and mixture gases of H2 and D2 (red line).
Materials 10 00303 g004
Figure 5. Transient behavior of Q-Mass spectra of m = 3 at several D concentrations, D/H = 10−2 (blue line), D/H = 10−3 (green line) and D/H = 10−4 (red line). Arrow indicates the onset time, when the PEFC was switched on. The data of m = 2 (black) and m = 4 (orange) were measured when the mixture gas with D/H = 10−2 was supplied.
Figure 5. Transient behavior of Q-Mass spectra of m = 3 at several D concentrations, D/H = 10−2 (blue line), D/H = 10−3 (green line) and D/H = 10−4 (red line). Arrow indicates the onset time, when the PEFC was switched on. The data of m = 2 (black) and m = 4 (orange) were measured when the mixture gas with D/H = 10−2 was supplied.
Materials 10 00303 g005
Figure 6. Dependency of separation factor, α, on fuel gas concentration of D when PEFC was operated at 1.2 A.
Figure 6. Dependency of separation factor, α, on fuel gas concentration of D when PEFC was operated at 1.2 A.
Materials 10 00303 g006
Back to TopTop