Thermodynamics and Kinetics of the CuCl(aq)/HCl(aq) Electrolyzer for Hydrogen Production

Open Access
Author:
Hall, Derek M
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 25, 2015
Committee Members:
  • Serguei Lvov, Dissertation Advisor
  • Serguei Lvov, Committee Chair
  • Ljubisa R Radovic, Committee Member
  • Michael John Janik, Committee Member
  • Derek Elsworth, Committee Member
Keywords:
  • CuCl HCl Electrolyzer
  • Thermodynamics
  • Kinetics
  • Efficiency
Abstract:
The CuCl(aq)/HCl(aq) electrolyzer is an important component in the Cu-Cl hybrid thermochemical cycle. Here we intend to provide information on how this electrolytic cell impacts the cycle’s efficiency and electric energy requirements. Through a better understanding the thermodynamics and kinetics of this electrochemical cell, the electric energy requirements needed for hydrogen production with this cycle can become available. Chapter 1 focuses on the relationship between equilibrium thermodynamics of the electrochemical reactions and the cycle’s efficiency. Using Gibbs energy minimization (GEM), thermodynamic speciation diagrams of CuCl(aq) and CuCl2(aq) were generated to provide insights into the electrochemically active species. Results from GEM were used to quantify the Gibbs energy, Enthalpy and entropy of the electrochemical reactions. Additionally, Gibbs energy values theoretically calculated were compared to those experimentally measured. Thermodynamic, voltage, current and overall efficiencies of the electrolyzer were quantified to include speciation effects and activity coefficients. Chapter 2 explores the electrochemical kinetics of the positive electrode using a rotating disc electrode (RDE) and the effectiveness of catalyst application techniques from scanning electron microscopy (SEM) images and full cell polarization curves. With the RDE, the positive electrode overpotential-current density relationship was defined. It was found that electrochemical kinetic parameters could be obtained for the positive electrode reaction on different catalyst materials with electrochemical impedance spectroscopy (EIS) and polarization curves. On both platinum and glassy carbon surfaces, the positive electrode reaction was very fast relative to other electrochemical reactions. Furthermore, removing platinum completely from the positive electrode had little effect on the polarization curves obtained from the full cell, whereas improving spray application technique significantly improved the performance relative to the painting technique. Chapter 3 investigates the effects of concentrated HCl(aq) on the electrochemical kinetics of the hydrogen evolution reaction. EIS and LSV were used to define kinetic parameters of the electrochemical reaction for polycrystalline platinum. It was found that the overpotential – current density behavior of the reaction on platinum followed the generalized Butler-Volmer equation. Chapter 4 presents a model to simulate the applied potential for the CuCl(aq)/HCl(aq) electrolyzer over a range of experimental conditions. The model presented here separates the potential contributions of the positive electrode, negative electrode and the membrane. Total applied potential was described using non-equilibrium thermodynamics, equilibrium thermodynamics and electrochemical kinetics. Using the information collected in Chapters 1-3 and some literature data, it was found that model simulations could match experimental data with only one adjustable parameter. Simulations of different values of active electrode area, ohmic resistance, and extent of CuCl(aq) conversion were performed. It was found that the extent of CuCl(aq) conversion and ohmic resistance, Rohm, strongly impacted the simulated Ecell value at high cell currents. Significant improvements to the electrolyzer performance could be obtained if Rohm was decreased relative to similar improvements in electrochemical kinetics or active electrode area.