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Renewable electrochemical fuel production: Fundamental principles and materials development

  • Author(s): Resasco, Joaquin
  • Advisor(s): Bell, Alexis
  • et al.
Abstract

Direct solar to fuel conversions offer an attractive and sustainable alternative to the use of fossil fuels an energy source. Electrochemical reactions can produce fuels that act as an energy storage medium for intermittent solar energy. However, current limitations in both materials development and fundamental understanding of electrochemical processes has limited the efficiency of photoelectrochemical systems. In the first portion of this dissertation, we discuss the simplest electrochemical process for producing a fuel using solar energy: water splitting to form hydrogen and oxygen. For this reaction, we discuss how materials development strategies can be developed to maximize the performance of the catalytic and light absorbing components of a photoelectrode. In particular we explore how one dimensional nanostructures can be utilized to improve performance for water oxidation. Although these nanostructures have several beneficial properties, their applicability can be hindered by difficulty in tuning the material composition without sacrificing morphology and material quality. In the first study, we present a method for controlling the composition of metal oxide nanowires without changing their morphology or crystallinity. In particular using this method based on solid state diffusion, and a novel process for manganese oxide atomic layer deposition, we produced manganese doped rutile TiO2 nanowires. Using a variety of physical characterization techniques, it was determined that Mn could be incorporated as a substitutional dopant for Ti in the rutile lattice. We investigated how this compositional control allows for modification of the optical, electronic, and electrochemical properties of the semiconductor nanowire. The doping process resulted in an enhancement in the electrocatalytic activity for water oxidation, consistent with theoretical predictions. This demonstrated that this simple and general method could be used to control the properties of one-dimensional nanostructures.

We then utilized this technique to improve upon the efficiency of light absorbing photoandoes for water oxidation. Metal oxides that absorb visible light such as monoclinic BiVO4 are attractive for use in this application. However, their performance is often limited by poor charge carrier transport. We investigated the possibility of addressing this issue by using separate materials for light absorption and carrier transport. As a carrier transport material, we used TiO2 nanowires, modified with a Ta dopant to improve the conductivity. The doping process was accomplished using the previously mentioned solid state diffusion technique. BiVO4 was added to these conductive nanowire arrays as a visible light sensitizer. Electrochemical and spectroscopic measurements were used to provide experimental evidence for the correct band alignment needed for favorable electron transfer from BiVO4 to TiO2. This host−guest nanowire architecture allows for the benefits of both materials to be taken advantage of, resulting in both high light absorption and carrier collection efficiency. This system resulted in an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent density of 2.1 mA/cm2 at 1.23 V vs RHE.

In the second portion of this work, we discuss the more complex reaction of carbon dioxide reduction. This reaction is appealing because it can produce more energy dense carbonaceous fuels, rather than hydrogen. While this is an appealing prospect, the efficiency of CO2 reduction systems is currently lower than those for water splitting. One reason for this is that the fundamental understanding of electrochemical processes for this reaction are still lacking, particularly in how the conditions used for testing catalysts for this reaction can significantly impact the measured performance. In particular, it has been shown that the choice of both electrolyte cation and anion has an impact on the measured electrocatalyst performance. In the final works we seek to develop a deeper understanding of these effects using a combination of experimental and theoretical methods.

The first study focuses on the effect of alkali metal cations on the on the intrinsic activity and selectivity of metal catalysts for the reduction of CO2. Experiments were conducted under conditions where mass transport limitations were minimal to show that cation size affects the intrinsic rates of formation of certain reaction products. Over Cu oriented thin films, increasing cation size increased rates of production of HCOO-, C2H4, and C2H5OH, while CO and HCOO- rates increased over polycrystalline Ag and Sn. Reduction of key reaction intermediates identified the elementary reaction steps affected by cation size, namely the activation of CO2 and the formation of carbon carbon bonds. Density functional theory calculations demonstrated that the alkali metal cations influence the CO2 reduction reaction due to electrostatic interactions between solvated cations present at the outer Helmholtz plane and polarizable adsorbed species. The observed trends in activity with cation size are attributed to an increase in the concentration of cations at the outer Helmholtz plane with increasing cation size.

The next study presents a combination of experimental and computational studies aimed at understanding the role of electrolyte anions on the reduction of CO2 over Cu surfaces. The effects of bicarbonate buffer concentration and identity on the rates of formation of the major products formed by reduction of CO2 over Cu was investigated experimentally. It was demonstrated that the composition and concentration of electrolyte anions has relatively little effect on the formation of CO, HCOO-, C2H4, and CH3CH2OH, but significantly affects the formation of H2 and CH4. Numerical simulations were used to assess the magnitude of changes in pH at the electrode surface for different electrolytes. The influence of pH on the activity of Cu for producing H2 and CH4 was also considered. It was determined that these differences in pH at electrode surface were insufficient to explain the trends in activity and selectivity observed with changes in anion buffering capacity observed for the formation of H2 and CH4. We therefore proposed that these differences are the result of the ability of buffering anions to donate hydrogen directly to the electrode surface and in competition with water. The effectiveness of buffering anions to serve as hydrogen donors is found to increase with decreasing pKa of the buffering anion. This understanding of how electrochemical conditions affect measured activity can lead to insights into how to maximize efficiency of CO2 reduction systems.

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