טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAmikam Gidon
SubjectChlorine-Free Seawater Electrolysis for Hydrogen Production
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Dr. Youri Gendel


Abstract

In 2016 the global energy consumption was ≈154.4 PWh with a growth of 1% from 2015. Electricity is the major form of energy used by mankind and today more than 70% of electrical energy is generated from fossil fuels. Concerns with global CO2 emissions and increasing costs of fossil fuels motivate an intensive research for the development of technologies aimed at energy production from renewable resources such as solar and wind energy. The time-and weather-dependent nature of solar and wind resources causes fluctuations in electricity production. For these reason "buffers" for energy storage such as electrical energy conversion and storage systems (EECSSs) are required. Hydrogen gas produced by water electrolysis is very attractive for energy conversion and storage applications due to its high gravimetric energy density and because it is a "clean" fuel with water as the major product of its reaction with oxygen in internal combustion engines and fuel cells. Seawater is potentially an endless source of water for electrochemical generation of hydrogen. Unfortunately, well-established technologies of water electrolysis cannot be applied directly on seawater due to the anodic chlorine evolution and deposition of Mg2 and Ca2 species on a cathode and in a membrane of electrochemical reactors.

A new process for chlorine-free seawater electrolysis for hydrogen production is proposed in this study. At the first step of the process nano-filtration (NF) is applied to separate Mg2 and Ca2 ions from seawater. Next, the NF permeate is dosed into the electrochemical system where it is completely split into hydrogen and oxygen gases and NaCl precipitate. Electrochemical system, that was the main object of this research, comprises electrochemical cell operated at elevated temperature (e.g.  ≥ 50⁰C) and a settling tank filled with aqueous NaOH solution (20-40 %wt) operated at lower temperature (e.g. 20-30⁰C). High concentration of hydroxide ions in the electrolyzed solution prevents anodic chlorine evolution, while the accumulated NaCl precipitates in the settling tank. Batch electrolysis tests performed in NaCl-saturated NaOH solutions showed absolutely no chlorine formation on Ni200 and Ti/IrO2-RuO2-TiO2 anodes at [NaOH] > 100 g/kgH2O. Three long-term operations (9, 30 and 12 days) of the electrochemical system showed no Cl2 or chlorate (ClO3-) production on both electrodes operated at current densities of 93- 467 mA/cm2. Ni200 anode was corroded in the continuous operation that resulted in formation of nickel oxide on the anode surface. On the other hand, the system was successfully operated with Ti/IrO2-RuO2-TiO2 electrodes in NaCl-saturated solution of NaOH (30 %wt) for 12 days without clogging of the equipment, Cl2 and ClO3- production. The performance of the system was stable as indicated by insignificant fluctuations in the applied cell potential and constant concentrations of NaOH(aq) and NaCl(aq) in the electrolyte solution. During 12 days of operation at ≈ 470 mA/cm2 about 1.2 m3 of H2 and ≈ 150 grams of solid NaCl were produced in the system. Electrical energy demand of the electrolysis cell was 5.6-6.7 kWh/m3H2 for the current density range of 187-467 mA/cm2.