Towards Highly Efficient Water Photoelectrolysis

Open Access
Author:
Elavambedu Prakasam, Haripriya
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 27, 2008
Committee Members:
  • Craig A Grimes, Committee Chair
  • Thomas E Mallouk, Committee Member
  • Jerzy Ruzyllo, Committee Member
  • Qiming Zhang, Committee Member
Keywords:
  • Anodization
  • Iron (III) Oxide
  • Hematite
  • Ethylene Glycol
  • Water Photolysis
  • Metal oxide semiconductors
Abstract:
The motivation for this work was to develop an efficient and relatively inexpensive material architecture suitable for solar water splitting by photoelectrolysis. Iron (III) Oxide (hematite), has bandgap energy (~ 2.2 eV) well suited for capturing solar spectrum, is abundant and non-toxic. However, it suffers from recombination losses due to low electron mobility and a minority carrier diffusion length of only 2- 4 nm. The primary focus of this dissertation was to synthesize thin walled, self-aligned, vertically oriented nanotubular/nanoporous iron (III) oxide structures through electrochemical oxidation. The underlying hypothesis was that thin walled nanotubes would allow charge separation prior to recombination, resulting in a significant increase in the photoelectrochemical properties. Both aqueous and non-aqueous electrolytes were explored as an electrochemical oxidation solvent. Iron oxide film topologies achieved include nanopillar, nanoporous and nanoplatelet structures from aqueous electrolytes, and nanoporous and nanochannel architectures from non-aqueous electrolytes. This dissertation encompasses the first report on synthesis of nanoporous/nanochannel iron (III) oxide structures through potentiostatic anodization, as well as the use of ethylene glycol for the electrochemical oxidation of both iron and titanium. Through control of anodization parameters, including potential and anodization bath composition, excellent control over the morphology and dimensions of the synthesized iron (III) nanostructures have been achieved. As dependent upon the applied potential and electrolytic composition, diameters of the self-aligned nanopores range from 30 nm to 250 nm. The synthesized structures were crystallized in nitrogen ambient to form hematite photoanodes; a maximum photocurrent efficiency of 0.73 % was obtained from nanoporous iron (III) oxide synthesized using a glycerol anodization bath. The electrochemical oxidation of titanium in fluoride ion containing ethylene glycol resulted in remarkable growth characteristics of titania nanotube arrays, hexagonal closed packed up to 1 mm in length, with tube aspect ratios of approximately 10,000. For the first time, complete anodization of the starting titanium foil has been demonstrated resulting in back to back nanotube array membranes ranging from 360 µm – 1 mm in length. The nanotubes exhibited growth rates of up to 15 µm/hr. A detailed study on the factors affecting the growth rate and nanotube dimensions is presented. It is suggested that faster high field ionic conduction through a thinner barrier layer is responsible for the higher growth rates observed in electrolytes containing ethylene glycol. Methods to fabricate free standing, titania nanotube array membranes ranging in thickness from 50 µm – 1000 µm has also been an outcome of this dissertation. In an effort to combine the charge transport properties of titania with the light absorption properties of iron (III) oxide, films comprised of vertically oriented Ti-Fe-O nanotube arrays on FTO coated glass substrates have been successfully synthesized in ethylene glycol electrolytes. Depending upon the Fe content the bandgap of the resulting films varied from about 3.26 to 2.17 eV. The Ti-Fe oxide nanotube array films demonstrated a photocurrent of 2 mA/cm2 under global AM 1.5 illumination with a 1.2% (two-electrode) photoconversion efficiency, demonstrating a sustained, time-energy normalized hydrogen evolution rate by water splitting of 7.1 mL/W•hr in a 1 M KOH solution with a platinum counter electrode under an applied bias of 0.7 V. The Ti-Fe-O material architecture demonstrates properties useful for hydrogen generation by water photoelectrolysis and, more importantly, this dissertation demonstrates that the general nanotube-array synthesis technique can be extended to other ternary oxide compositions of interest for water photoelectrolysis.