Nuclear Hydrogen and Heat Application Research9-3

Increased Energy Efficiency for Hydrogen Production－ Study of Ion Permeation Mechanism in Membrane of HI Concentrator －Hydrogeniodidedecomposition

Heat
H2+I2I2

Production ofHI and H2SO4(Bunsen reaction)

2HI

Iodinecirculation

H 2 SO42HI + H2SO40.9

1/2O2+
SO2 +H2O

Sulfurcirculation SO

I2 + SO2 + 2H2O

1.0O2

Heat

HTGR+
H2O

Sulfuric aciddecomposition

2H2O
Fig.9-5 Thermochemical water-splitting IS process
Hydrogen and oxygen are produced from water by acombination of three chemical reactions. The process canharness the heat generated by HTGRs.

H+ selectivity (–)

H20.80.7
0.60.5: Experimental value: Theoretical formula

0.4

2040

Rich HIIon exchange membrane

t+ =I

Anode

H+H+I

Selective H+ permeation

Cathode

I2

Reduction of I2I2

60

80

100120Temperature (°C)

HI-I2 -H 2 O solution
Electron

We have been investigating a hydrogen production method
known as the iodine-sulfur (IS) process for heat applicationof high-temperature gas-cooled reactors (HTGRs). The IS
process is a water-splitting thermochemical cycle withoutcarbon dioxide emission that proceeds by chemical reactions ofiodine and sulfur compounds (Fig.9-5). One of the important
R&D goals for industrialization of the process is to improve thehydrogen production efficiency.
A task to improve efficiency is to concentrate the circulating
material, hydriodic acid (HI solution, an iodine compound),with low energy consumption; therefore, we have been studying
an electro-electrodialysis method. This method can enrich HI in
the HI solution with a HI concentrator employing ion-exchange
membranes that selectively permeate protons (H+) (Fig.9-6). Thusfar, we successfully prepared novel ion-exchange membranes
using a radiation-induced graft polymerization technique(radiation-grafted membranes), which are a key device for low
energy consumption in the HI concentrator.

D H + nH +

 D: diffusion coefficient of ions
D H + nH + + D I - nI -  n: ions content of membrane

Fig.9-7 Temperature dependence of H+ selectivity: comparisonof experimental results and theoretical formula
Good agreement is observed between the experimental results
and the theoretical formula based on the mathematical model ofpermeation phenomena in the radiation-grafted membrane. Thissuggests that a high-performance membrane can be prepared byadjusting the membrane properties.

Fig.9-6 Hydriodic acid concentration using HI concentrator
Enriched HI solution can be obtained by reducing iodine (I2) andpermeating protons (H+) through the ion-exchange membrane.

We are currently pursuing studies to improve the performanceof the radiation-grafted membrane. For this purpose, wehave tried to reveal the ion permeation mechanism using amathematical model to characterize the performance.
In the model, the driving force of ion movement is assumed
to be the electric potential gradient. A theoretical formula for H+selectivity, which is an essential performance, was derived asa function of the H+ content, iodide ion content, and diffusion
coefficient. The good agreement between the experimental
results and the theoretical formula (Fig.9-7) shows that themodel we employed is appropriate.
Next, we plan to modify the model regarding the effectsof HI concentration. In addition, we will work on thedevelopment of higher-performance membranes by adjusting
the membrane properties. One way to do this is by varying themembrane’s affinity to HI using grafted polymers to introduce
new functional groups.

ReferenceTanaka, N. et al., Effect of Temperature on Electro-Electrodialysis of HI-I2-H2O Mixture using Ion Exchange Membranes, Journal of Membrane Science,
vols.411-412, 2012, p.99-108.

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JAEA R&D Review 2013