Surface modification of photoelectrodes for photoelectrochemical water splitting

Ma, Zili; Dronskowski, Richard (Thesis advisor); Slabon, Adam (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Utilizing solar energy to produce clean and renewable energy is an attractive route to solve the two major restriction factors i.e. the growing pollution of environment and energy shortage. A photoelectrochemical (PEC) cell, which can absorb sufficiently solar energy to split abundant water into the energy carrier hydrogen, has proven to be an encouraging technology to address both the energy demand and environment problems of water and air pollution. The semiconducting photoelectrodes and the cocatalysts serving as the two main components of PEC devices are indispensable to achieve sufficiently water splitting efficiency. Keeping in line with these goals, different semiconducting photoabsorbers and cocatalysts have been investigated in this thesis.In chapter 2, the ternary metal oxide CuWO4 with a band gap of 2.2–2.4 eV has been prepared as a thin film photoanode for PEC water oxidation and the performance has been augmented by a facile post-annealing under nitrogen atmosphere at 623 K for 3.5 h. The post-treated CuWO4 exhibits a photocurrent of 80 μA cm–2 at 1.23 V versus reversible hydrogen electrode (RHE) under Air Mass 1.5 Global (AM 1.5G) illumination in a phosphate buffer electrolyte at pH 7, almost three times higher when comparing to the pristine photoanode. Physical characterization indicates that post-annealing of the CuWO4 thin films in a N2 atmosphere does not introduce nitrogen into the crystal structure, thus no bathochromic shift is observed. The greater concentration of oxygen vacancies, which can improve charge carrier separation and reduce the charge transfer resistance, has been proposed the reasons of superior PEC water oxidation performance. In chapter 3, a quaternary oxynitride nanowire SrTaO2N thin film has been used as the core photoabsorber to construct a core-shell structure with functional overlayers for PEC water oxidation. The perovskite-related oxynitride structure is obtained by converting the hydrothermally grown oxide precursor on tantalum substrate via nitridation. The performance of the as-prepared nanowire SrTaO2N has been enhanced by the deposition of three functional overlayers. The first TiOx layer can protect the oxynitride from photocorrosion and suppress the recombination of charge carrier at the surface. A hole-storage layer Ni(OH)x can decrease the dark-current, leading to a significantly improved extraction of photogenerated holes to the electrode-electrolyte surface. The cocatalyst cobalt phosphate layer can increase the photocurrent up to 0.27 mA cm–2 at 1.23 V versus RHE under AM 1.5G illumination. The common dark current in case of oxynitride photoanodes grown on metallic substrates has been minimized almost to zero. In chapter 4, the tailoring of the surface properties of quaternary tantalum-based oxynitrides ATa(O,N)3 is critical to obtain efficient hole extraction. A cubic CaTaO2N particle-based photoanode has been altered by acidic treatment for PEC water oxidation. The acidic etching effect has been addressed by means of complementary physical characterization techniques, such as X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, 1H and 14N solid-state nuclear magnetic resonance (NMR) spectroscopy, and electron microscopy. Combining with PEC measurements, solid-state NMR indicates that the restructured surface displays a meaningfully higher concentration of terminating OH groups. The deposition of the cocatalyst nickel borate on the etched surface yields a higher percentual upsurge of photocurrent in comparison to pristine CaTaO2N. The work in this chapter highlights the application of solid-state NMR spectroscopy for understanding the semiconductor-catalyst interface in photochemical devices. In chapter 5, a trivalent iron-only layered oxyhydroxide mössbauerite has been investigated as cocatalyst for PEC water oxidation by coupling with a WO3 semiconductor photoanode. By combining Mott-Schottky analysis and UV-Vis diffuse reflectance spectroscopy, the band edge positions of mössbauerite have been determined. Mössbauerite is identified to be a n-type semiconductor with a flat band potential of 0.34 V versus RHE. However, the bare mössbauerite does not produce noticeable photocurrent during water oxidation. By constructing a type-II heterojunction with WO3 thin films photoanode, the charge carrier separation is amended and a photocurrent of up to 1.22 mA cm–2 at 1.23 V versus RHE is achieved. In chapter 6, the monodisperse spherical alloy FePt and pure Pt nanocrystals as cocatalysts have been used to modify p-WSe2 single-crystal photocathodes. The photocurrents of – 0.27 and – 4.0 mA cm– 2 at 0 V versus RHE, which are 7.4 and 15 times higher compared to pristine WSe2 single crystal, are achieved for the hydrogen evolution reaction (HER) after the modification with Pt or FePt, respectively. The density functional theory computations reveal that the water adsorption and thus enhanced H2O dissociation are preferential on FePt in comparison to Pt. The edge sites of both Pt and FePt are the preferential sites for hydrogen production because of a more negative adsorption energy than on the (111) and (100) facets. The size of the Pt nanocrystal within the range of Pt55 (1.1 nm) and Pt147 (1.6 nm) does not significantly influence the mechanism for the HER, as reveled by computational results.

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