Membrane Transport in Cucl/hcl Electrolyzer

Open Access
Author:
Kim, Soohyun
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 03, 2013
Committee Members:
  • Serguei Lvov, Dissertation Advisor
  • Derek Elsworth, Committee Member
  • Yongsheng Chen, Committee Member
  • Michael Anthony Hickner, Committee Member
Keywords:
  • Ionic Transport
  • CuCl/HCl electrolyzer
  • Hydrogen
  • Membrane
  • CuCl thermochemical cycle
  • hot-pressing
  • Phenomenological coefficient
Abstract:
In the Cu-Cl thermochemical cycle, several sequential chemical processes are employed to split water into hydrogen and oxygen. This dissertation focuses on the CuCl/HCl electrolysis phase of the cycle, which produces hydrogen gas via oxidation of CuCl(s) dissolved in highly concentrated HCl(aq), and reduces aqueous protons to hydrogen gas. Transport properties in the electrolyzer membrane were experimentally and thermodynamically investigated. In particular, the membrane characteristics, hot-pressing effects on ionic transport, the effects of membrane characteristics on the performance of the electrolyzer system and phenomenological modeling of transport properties via irreversible thermodynamics were carried out. Experimental research was carried out in three phases: In the first phase (1), several types of proton conducting polymer membranes were characterized and evaluated using single conductivity and permeability cell under a variety of conditions, and the effects of hot-pressing membranes were recorded. Initial testing of proton conducting membranes showed that single and double layer Nafion 117 membranes demonstrate higher proton conductivity than other types of proton conductive membranes. Hot-pressed Nafion membranes, such as the pressed double layer Nafion 117 membrane, demonstrated the highest selectivity (proton conductivity/copper permeability) after a hot pressing procedure. The main conclusions were that hot-pressed double layer Nafion 117 membrane could be a promising membrane by reduced copper cross over through the membrane and maintained high conductivity for electrolyzer test. Significantly improved stability of electrolyzer system with hot-pressed Nafion membrane was expected based on the results of membrane conductivity and permeability tests. In the second phase (2), CuCl/HCl electrolyzer performance was investigated using linear sweep voltammetry, hydrogen production, electrochemical impedance spectroscopy, and analysis of copper ion flux through the membranes. These investigations show that two aspects -- lower copper permeability through membranes by hot-pressing, and high concentrated HCl(aq) catholyte and anolyte solutions -- can overcome a substantial electrolyzer obstacle: Cu(s) precipitation at the cathode. Higher concentrations of HCl(aq) and CuCl(aq) in the anolyte can significantly affect the electrolyzer performance, especially membrane conductivity. Through the concept of the copper ion dissolved in the solution, the copper deposition can be prevented by increasing the concentration of HCl(aq) catholyte solution. Such enhanced MEA and electrolyzer system allowed the electrolyzer performance to improve and extend the period time of electrolyzer operating. By optimization of test conditions in electrolyzer, a significantly improved current density of 0.456 A/cm2 was observed at 0.7 V, using 2 mol L-1 CuCl(aq) in 6 mol L-1 HCl(aq) anolyte solution when hydrogen production efficiency remained high, over 95 %. In the final phase, a simple phenomenological model based on irreversible thermodynamics was developed to describe the simultaneous transport of ions and electricity in the CuCl electrolyzer. Phenomenological coefficients were expressed in terms of gradients of electrical and chemical potentials, including conductivity, permeability and transport numbers determined by experiments. By implementing experiments, the phenomenological coefficients were obtained. Using developed single cell and electrolyzer cell designs, the phenomenological coefficients were estimated as a function of copper concentration.