Advanced photoanodes for photoassisted water electrolysis

Abstract

With continuously growing energy demands, alternative, emission-free solar energy solutions become ever more attractive. However, to achieve sustainability, efficient conversion and storage of solar energy is imperative. Photoelectrolysis harnesses solar energy to evolve hydrogen and oxygen from water, thereby enabling energy storage via chemical means. Hematite or [alpha]-Fe₂O₃ has emerged as a highly promising photoanode candidate for photoelectrochemical cells. While significant improvements in its performance have recently been achieved, it remains unclear why the maximum photocurrents still remain well below their theoretical predictions. This study investigates the defect chemistry and conduction mechanism of hematite in order to understand and improve this material's shortcomings. A defect model for donor doped hematite was derived and its predictions conformed by the electrical conductivity of ilmenite hematite solid solution bulk samples as a function of temperature and oxygen partial pressure. The enthalpies of the Schottky defect formation and the reduction reaction for hematite were determined as 13.4 eV and 5.4 eV, respectively. In addition, a temperature independent value for the electron mobility of 0.10 cm2/Vs for 1% Ti donor doped hematite was derived. Furthermore, the electrical conductivity of nanometer scale, epitaxially grown thin films of the ilmenite hematite solid solution system was characterized by electrical impedance spectroscopy. This work reports a detailed correlation between the electrical conductivity of the undoped hematite, the 1 atom% Ti doped hematite and the thin films with higher ilmenite content and the conditions under which they were annealed (20° C=/< T =/< 800° c and 10-4 atm =/< po2 =/< atm). Hematite's room temperature conductivity can be increased from ~10-11 S/cm for undoped hematite films by as much as nine orders of magnitude by doping with the Ti donor. Furthermore, by controlling the non-stoichiometry of Ti-doped hematite, one can tune its conductivity by up to five orders of magnitude. Depending on processing conditions, donor dopants in hematite may be compensated largely by electrons or by ionic defects (Fe vacancies). The electron mobility of the film was determined to be temperature independent at 0.01 cm2/Vs for the < 0001 > epitaxial film containing a Ti donor density of 4.0 x 1020 cm-3. Finally, the photoelectrochemical performance of these materials was tested by cyclic voltammetry and measurements of their quantum efficiencies. The 1% Ti doped hematite thin film exhibited the highest photocurrent density of these dense, thin films at 0.9mA/cm2 with an applied bias of 1.5V vs. RHE. The IPCE of this sample reached 15% at wavelengths between 300nm and 350nm after an annealing treatment at 580° for 36 h. The solid solution containing 33% ilmenite preformed nearly as well as the doped hematite. The performance decreased with higher ilmenite concentrations in the solid solution. For all samples containing any ilmenite, the onset potential shifted to lower values by ~200mV after the annealing treatment. The increase in charge carrier density upon reduction of Ti doped hematite was conformed by a Mott-Schottky analysis of the hematite/electrolyte interface. In contrast, only minor changes in the carrier density were observed when reducing an undoped hematite photoanode. Changes in slope of the Mott-Schottky plots revealed the presence of deep trap states in the hematite films. In-situ UV-vis spectroscopy displayed a pronounced optical signature corresponding to the existence of such deep levels. These results highlight the importance of carefully controlling photoanode processing conditions, even when operating within the material's extrinsic dopant regime, and more generally, provide a model for the electronic properties of semiconducting metal oxide photoanodes.