Insights Into Electrochemical And Photoelectrochemical Water-splitting

Open Access
Author:
Vargas-barbosa, Nella Marie
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 27, 2015
Committee Members:
  • Thomas E Mallouk, Dissertation Advisor
  • Thomas E Mallouk, Committee Chair
  • Benjamin James Lear, Committee Member
  • Lasse Jensen, Committee Member
  • Darrell Velegol, Committee Member
  • Barbara Jane Garrison, Special Member
Keywords:
  • electrochemistry
  • iridium oxide
  • solar fuels
  • catalysis
  • membrane
  • buffer
  • photo-electrochemistry
  • heterogeneous kintetics
Abstract:
The water-splitting reaction has been known for over a century, yet its efficient execution remains to be one of the “holy grails” for current researchers. Here, molecular water is converted to oxygen and hydrogen gas via multiple proton- and electron-transfer steps. Although the product of interest is high-purity hydrogen gas fuel, the thermodynamic and kinetic requirements of the oxygen evolution reaction (OER) are the main limiting factor. The goal of this dissertation was to develop and understand model electro- and photoelectro-catalytic systems that can address the kinetic limitations of the OER, as well as guidelines for the future development of water-splitting devices. Chapter 1 introduces the kinetic theory of heterogeneous electron-transfer reactions and how it is applied to the understanding of the water-splitting reaction. The chemical properties that make iridium oxide an ideal model electrocatalyst for the OER are discussed, as well as an overview of previous work on this material. Furthermore, the fundamentals of photo-electrochemical water-splitting are presented. Here, sunlight is used as the main driving force for producing oxygen and hydrogen. It has been previously demonstrated that the synthesis of IrOx∙nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through iridium hydroxide intermediates. Chapter 2 is a detailed spectro-electrochemical and DFT study of such intermediates and their effect in photoelectrochemical water-splitting cells. Primarily, we have identified the monomeric nature of this hydroxide intermediates as well as their most likely chemical composition and their relative ratio between Ir(III) and Ir(IV). The results from this study address a very important, current dilemma in IrOx∙nH2O–based photoelectrochemical water-splitting cells: how does the chemistry of the catalyst and its interface with the semiconductor influence the photoresponse of the cell? The careful preparation and characterization of both IrOx∙nH2O catalysts and photoanodes provide reasonable explanations to previously observed discrepancies in photoelectrochemical cells that utilize IrOx∙nH2O colloids as co-catalysts on the photoanode electrode. The recent development of inexpensive electrocatalysts for the OER has suggested that efficient photoelectrochemical cells (PECs) might be constructed from terrestrially abundant materials, such as cobalt, nickel and iron. However, since these catalysts operate in aqueous buffer solutions at neutral to slightly basic pH, it is important to consider whether electrolytic cells can have low series loss under these conditions. Membranes are important components of water-splitting PECs because they prevent crossover of the cathode (e.g. hydrogen gas) products from oxygen produced at the anode. Chapter 3 is a detailed study of the resistive series losses in model electrolytic cells that utilize buffers and commercial monopolar and bipolar ion exchange membranes. Here, we have identified that long-term electrolysis of water (for periods greater than 24 hours) cannot be sustained efficiently in buffered-monopolar membrane systems due to a migration-induced concentration polarization of the electrolytic cells. In the case of buffered–bipolar membrane systems, we have determined that there is a significant trade off between the resistivity of the membrane and the losses due to concentration gradients. However, under reverse bias conditions and in a pH-polarized electrolyte, bipolar membranes minimize the series losses of the membrane resistivity as well as those due to pH gradients. This configuration is particularly promising because it allows the de-coupling of the optimization of anode and cathode materials for overall water-splitting. In Chapter 4 we focus again on the IrOx∙nH2O electrocatalyst and focus on a fundamental and thorough kinetic characterization of the OER on IrOx∙nH2O films. In a solely electrochemical study, we study the effects of temperature, pH and hydrogen/deuterium isotope on the overall OER kinetics. Preliminary results suggest that the rate of the OER in these films does not depend on the proton concentration. Interestingly, however, measurements of the hydrogen/deuterium kinetic isotope effect suggest that the mechanism in the films is rate-dependent, i.e. the kinetically-relevant surface intermediates of the OER change as a function of overpotential on IrOx∙nH2O films. Chapter 5 is a brief compilation of Chapters 1–4 and their implications for future research in water-splitting systems. Finally, Appendix A focuses on introducing the reader to hydrodynamic electrochemical techniques, namely rotating ring-disk electrochemistry (RRDE). Here, the characterization of N(5)-ethylflavinium ion, a fully-organic homogeneous OER catalyst, is presented and the RRDE results confirm its catalytic performance.