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DEVELOPMENT OF CuCl-HCl ELECTROLYSIS FOR HYDROGEN PRODUCTION VIA Cu-Cl THERMOCHEMICAL CYCLE
A new thermodynamic model for the Cu(I,II)-HCl-H 2 O system was developed on the basis of the
representative data on CuCl(s) solubility in aqueous solutions of HCl in a concentration interval from
1 to 6 mol kg –1 HCl (Akinfiev, 2009). The model takes into account a number of aqueous Cu(I) species
[Cu + , CuOH o , Cu(OH) 2– , CuCl o , CuCl 2– , HCuCl 2o ], aqueous Cu(II) species [Cu 2+ , CuOH + , CuO o , HCuO 2– ,
CuO 2
2–
, CuCl + , CuCl 2o , CuCl 3– , CuCl 4
2–
)] and a mixed Cu(I)/Cu(II) chloride aqueous complex, Cu 2 Cl 3o . The
thermodynamic approach used a modelling approach based on: i) the standard thermodynamic
properties of the listed above species; ii) a model for the activity coefficients; iii) use of HCh software
(Shvarov, 1999).
The equilibrium Cu(I) → Cu(II) conversion (electrolysis) calculations were carried out for two
anolyte solutions of CuCl in HCl(aq): (a) 0.2 M CuCl in 1 M HCl(aq), and (b) 1 M CuCl in 6 M HCl(aq),
which are two compositions used in our electrolysis tests.
The calculated distribution of the main Cu(I) and Cu(II) species versus redox potential (E h ) during
the equilibrium electrolysis for solutions (a) and (b) is shown in Figure 3. The main Cu(I) species for
the 0.2099 mol kg –1 CuCl(aq) solution in 2.099 mol kg –1 HCl are HCuClo 2 and CuCl– 2 while the main Cu(II)
species are Cu 2+ and CuCl + . In the more concentrated anolyte solution (b) (1.172 mol kg –1 CuCl in
7.031 mol kg –1 HCl), there are the same Cu(I) species, but the speciation of Cu(II) becomes more
versatile with the input of CuCl 2o , CuCl– 3 and CuCl2– 4 .
Figure 3: Speciation diagrams for the Cu(I) → Cu(II) conversion in equilibrium
electrolysis in (a) 0.2 M CuCl + 2 M HCl(aq) and (b) 1 M CuCl + 6 M HCl(aq)
Case (a): m(Cl – ) = 1.889 mol.kg –1 and m(H + ) = 1.974 mol.kg –1 at Eh = 0.2 V
and m(Cl – ) = 2.173 mol.kg –1 and m(H + ) = 1.884 mol.kg –1 at Eh = 0.8 V.
Case (b): m(Cl – ) = 5.859 mol.kg –1 and m(H + ) = 6.014 mol.kg –1 at Eh = 0.2 V
and m(Cl – ) = 5.473 mol.kg –1 and m(H + ) = 5.798 mol.kg –1 at Eh = 0.8 V.
The listed molalities are in accordance with electroneutrality of the aqueous solutions.
(a)
(b)
The results of our thermodynamic calculations show that the theoretical OCP for both solutions
is close to the experimentally observed values, which are 0.421 and 0.466 V for solutions (a) and (b)
respectively. These calculations take into account the influence of a small amount of atmospheric
oxygen which is normally dissolved in water. Note that based on our calculations the virtually full
conversion of Cu(I) to Cu(II) takes place at ~0.7 V, while in the experiment reported by Stolberg (2008),
this value is apparently close to 0.9 V.
Effect of operating temperature and flow rate on electrolysis kinetics
Linear sweep voltammetry measurements with a scanning range from 0.3 to 0.9 V and a sweep rate of
1 mV/s were used to evaluate the process kinetics. The set of voltammetric polarisation curves for the
CuCl(aq)/HCl(aq) electrolysis obtained at 24, 45, and 65°C in the cell with the AHA anion-exchange
membrane (Gong, 2009) show that the electrolysis process is prompted by the temperature increase.
A similar effect was observed with the Nafion-115 membrane. The apparent enhancement of the
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