Heung Chan Lee

Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO4 Single Crystals: Intrinsic Behavior of a Complex Metal Oxide

Bismuth vanadate (BiVO4) is a promising photoelectrode material for the oxidation of water, but fundamental studies of this material are lacking. To address this, we report electrical and photoelectrochemical (PEC) properties of BiVO4 single crystals (undoped, 0.6% Mo, and 0.3% W:BiVO4) grown using the floating zone technique. We demonstrate that a small polaron hopping conduction mechanism dominates from 250 to 400 K, undergoing a transition to a variable-range hopping mechanism at lower temperatures. An anisotropy ratio of ~3 was observed along the c axis, attributed to the layered structure of BiVO4. Measurements of the ac field Hall effect yielded an electron mobility of ~0.2 cm(2) V(-1) s(-1) for Mo and W:BiVO4 at 300 K. By application of the Gärtner model, a hole diffusion length of ~100 nm was estimated. As a result of low carrier mobility, attempts to measure the dc Hall effect were unsuccessful. Analyses of the Raman spectra showed that Mo and W substituted for V and acted as donor impurities. Mott-Schottky analysis of electrodes with the (001) face exposed yielded a flat band potential of 0.03-0.08 V versus the reversible H2 electrode, while incident photon conversion efficiency tests showed that the dark coloration of the doped single crystals did not result in additional photocurrent. Comparison of these intrinsic properties to those of other metal oxides for PEC applications gives valuable insight into this material as a photoanode.

Unbiased Photoelectrochemical Water Splitting in Z-Scheme Device Using W/Mo-Doped BiVO4 and ZnxCd1−xSe

Photoelectrochemical water splitting to generate H2 and O2 using only photon energy (with no added electrical energy) has been demonstrated with dual n-type-semiconductor (or Z-scheme) systems. Here we investigated two different Z-scheme systems; one is comprised of two cells with the same metal-oxide semiconductor (W- and Mo-doped bismuth vanadate), that is, Pt-W/Mo-BiVO4, and the other is comprised of the metal oxide and a chalcogenide semiconductor, that is, Pt-W/Mo-BiVO4 and Zn(0.2)Cd(0.8)Se. The redox couples utilized in these Z-scheme configurations were I(-)/IO3(-) or S(2-)/S(n)(2-), respectively. An electrochemical analysis of the system in terms of cell components is shown to illustrate the behavior of the complete photoelectrochemical Z-scheme water-splitting system. H2 gas from the unbiased photolysis of water was detected using gas chromatography-mass spectroscopy and using a membrane-electrode assembly. The electrode configuration to achieve the maximum conversion efficiency from solar energy to chemical energy with the given materials and the Z-scheme is discussed. Here, the possibilities and challenges of Z-scheme unbiased photoelectrochemical water-splitting devices and the materials to achieve practical solar-fuel generation are discussed.

Screening of transition and post-transition metals to incorporate into copper oxide and copper bismuth oxide for photoelectrochemical hydrogen evolution

A new dispenser and scanner system is used to create and screen Bi-M-Cu oxide arrays for cathodic photoactivity, where M represents 1 of 22 different transition and post-transition metals. Over 3000 unique Bi : M : Cu atomic ratios are screened. Of the 22 metals tested, 10 show a M-Cu oxide with higher photoactivity than CuO and 10 show a Bi-M-Cu oxide with higher photoactivity than CuBi2O4. Cd, Zn, Sn, and Co produce the most photoactive M-Cu oxides, all showing a 200-300% improvement in photocurrent over CuO. Ag, Cd, and Zn produce the highest photoactivity Bi-M-Cu oxides with a 200-400% improvement over CuBi2O4. Most notable is a Bi-Ag-Cu oxide (Bi : Ag : Cu atomic ratio of 22 : 3 : 11) which shows 4 times higher photocurrent than CuBi2O4. This material is capable of evolving hydrogen under illumination in neutral electrolyte solutions at 0.6 V vs. RHE when Pt is added to the surface as an electrocatalyst.

Enhanced Electrochemical Stability of Quasi-Solid-State Electrolyte Containing SiO2 Nanoparticles for Li-O2 Battery Applications

A stable electrolyte is required for use in the open-packing environment of a Li-O2 battery system. Herein, a gelled quasi-solid-state electrolyte containing SiO2 nanoparticles was designed, in order to obtain a solidified electrolyte with a high discharge capacity and long cyclability. We successfully fabricated an organic-inorganic hybrid matrix with a gelled structure, which exhibited high ionic conductivity, thereby enhancing the discharge capacity of the Li-O2 battery. In particular, the improved electrochemical stability of the gelled cathode led to long-term cyclability. The organic-inorganic hybrid matrix with the gelled structure played a beneficial role in improving the ionic conductivity and long-term cyclability and diminished electrolyte evaporation. The experimental and theoretical findings both suggest that the preferential binding between amorphous SiO2 and polyethylene glycol dimethyl ether (PEGDME) solvent led to the formation of the solidified gelled electrolyte and improved electrochemical stability during cycling, while enhancing the stability of the quasi-solid state Li-O2 battery.

CHAPTER 6:Rapid Screening Methods in the Discovery and Investigation of New Photocatalyst Compositions

Discovery of an efficient, stable, and inexpensive photocatalyst is a key issue in the design of a practical photoelectrochemical (PEC) system for converting solar energy into chemical fuels, e.g. hydrogen production from water splitting. Despite over 40 years of enormous efforts in the area, no photocatalyst has yet been found as an optimized material for water photolysis. In this chapter, combinatorial rapid synthesis and screening of semiconducting materials to discover and improve photocatalysts for water photolysis are discussed. While several different techniques are briefly touched upon, the focus is on using modified scanning electrochemical microscopy (SECM) for combinatorial rapid screening. In this application of SECM, a piezo-dispensing tip and a fiber optic are placed in the SECM for rapid synthesis and fast scanning of the semiconductor spot arrays. The rapid synthesis and screening of electrocatalysts decorating the semiconducting photocatalysts, which is required to further enhance the water splitting reactions, i.e. hydrogen/oxygen evolution reactions (HER/OER), is also explained. In addition, several modes of SECM used to study material properties and chemical reactions on photocatalysts/electrocatalysts are generally introduced. Finally, factors affecting the activity of photocatalysts are briefly discussed to guide the rapid screening of materials in PEC systems.