Influence Of Metal Impurity Ions On Water Electrolysis

Hydrogen energy is a clean and efficient secondary energy. Water electrolyzed is an important method to achieve hydrogen industrialization. But energy consumption is the most important factor that hinders the development of water electrolysis for hydrogen energy, and the electrolyser, electrode materials and electrolyte are key points toward energy consumption. Recently, researches are concentrated in the electrolyser and electrode materials, less in the electrolyte. The thesis studied the influence of metal impurity ions in alkaline electrolyte (30% KOH solution) on water electrolysis efficiency and energy consumption. The highest possible concentration and the electrochemical behavior of metal impurity ions in alkaline electrolyte, and its impact on the efficiency and energy consumption of water electrolysis were investigated in the thesis.The experimental results on the highest possible concentration of metal impurity ions in alkaline electrolyte indicated that the solubility of Al³⁺, Cr³⁺, Zn²⁺ in 30% KOH solution is relatively large, and their highest possible concentration were separately 12304 mg/L, 11959 mg/L, 30542 mg/L; the solubility of Cu²⁺, Fe²⁺, Fe³⁺, Ni²⁺ in 30% KOH solution is small, their highest possible concentrations were separately 662.5, 056.91, 67.64, 1.614 mg/L.Study on metal impurity ions in the alkaline electrolyte in the electrochemical behavior found, Al³⁺, Ni²⁺, Cr³⁺, Mo6+, Ti³⁺ and other metal ions in 30% KOH solution can not be deposited individually, while Zn²⁺, Cu²⁺, Fe³⁺ in 30% KOH solution can. The deposition potential of Zn²⁺ is about -1.5V(vs Hg/HgO), and makes a positive shift with the increasing ion concentration and temperature. The reduction of Zn²⁺ is considered as a two-electron step, irreversible process. The deposition potential of Cu²⁺ is about -0.6V, and also makes a positive shift with the ion concentration and the rise of temperature. The reduction of Cu²⁺ is considered as two single-electron steps which are both irreversible process. The deposition potential of Fe³⁺ is -0.7V, shifting negatively with the increase of ion concentration. The reduction of Fe³⁺ is considered as a single-electron step and a two-electron step which are both irreversible process.The minimum concentrations of Zn²⁺, Cu²⁺, Fe³⁺ deposited in 30% KOH were 0.1268,1.249,0.273 mg/L, that is the lowest concentration of the three metal ions harmful to water electrolysis. Theη100 (100mA/cm2) of cathode which deposited Fe was about 500mV, which deposited Cu was about 600mV, and theη100 increased 300mV and 400mV compared with the high activity cathode such as Ni-Zn, Ni-Mo-Fe(η100=200mV), will greatly reduce the efficiency of water electrolysis, increase the energy consumption. The electrodeposited Zn can dissolve in 30% KOH solution, thence Zn²⁺will continue to reduce and oxidate in the process of water electrolysis. This process not only reduce the performance of cathodic for hydrogen, and continue to consume energy.