Energy-saving seawater electrolysis for hydrogen production
|Zenta Kato 1, Koichi Izumiya 2, Naokazu Kumagai 2, Koji Hashimoto 1|
1. Tohoku Institute of Technology, 6, Futatsuzawa,Taihakuku, Sendai 982-8588, Japan
For the use of renewable energy, fluctuating power generated in remote deserts or sea will be converted first to hydrogen by seawater electrolysis. The anodes for oxygen evolution without chlorine formation have been tailored for seawater electrolysis. In the electrolysis H2 and O2 formation induces high and low pH on the cathode and anode. The difference of pH 10 on the two electrodes leads to 0.6 V increase in the cell voltage. If seawater is flowed in a series first from the anode compartment and then to the cathode compartment, the pH of seawater decreases at the anode compartment inlet and becomes neutral at the cathode compartment outlet, and the pH in the most parts of the cell will be kept to the low value without neutralization because penetration of Na+ through the membrane prevents 100% H+ penetration. In the electrolysis flowing seawater from the bottom of the anode compartment of 1 m high cell in 1 h consuming 10 % of water from seawater, the cell voltage will be kept at the theoretical one in the most parts of the cell if the H+ penetration through the membrane is 98 %.
Necessary conditions for realization of this idea were examined. If pH of 0.5 M NaCl was 1.03, pH on the cathode surface did not increase by electrolysis at 1000 Am-2, but if pH was 1.34 and 1.98, pH on the cathode increased immediately to 10 by electrolysis at 400 and 30 Am-2, respectively.
When a proton selective membrane was used, the low cell voltage was kept at 1000 Am-2 from the bottom to 60 cm height in 0.25 M Na2SO4 due to 93-97% H+ penetration, although in 0.5 M NaCl the low voltage was kept till about 10 cm height due to 87-92% H+ penetration. This difference is based on higher degree of dissociation of NaCl than Na2SO4. When 0.025 M Na2SO4 was used, the H+ penetration was 96-97.5% and the cell voltage in the whole cell was 2.5-2.7 V in which 0.5 V were based on the membrane resistance. Thus the membrane through which the H+ penetration is 98% or higher is required.
Presentation: Oral at E-MRS Fall Meeting 2007, Symposium D, by Zenta Kato
See On-line Journal of E-MRS Fall Meeting 2007
Submitted: 2007-05-14 05:44 Revised: 2009-06-07 00:44