Efficient photoelectrochemical water splitting electrodes using nanostructures and catalysts

Abstract

The Sustainable and efficient conversion of solar energy to transportable green energy and storable fuels, hydrogen, represents a solution to the energy crisis and can decreases the consumption of fossil fuels, which are mainly responsible for the rise of global temperature. Therefore, this thesis presents about photoelectrochemical water splitting electrodes, silicon based photocathode and titanium oxide based photoanode, in three main chapters. The second and third chapters are focus on silicon based photocathode. The solar to hydrogen conversion efficiency of a silicon photoelectrode is suppressed by overpotential, high reflectance and instability in the liquid electrolytes. These drawbacks were managed by synthesizing a multifunctional metal oxide and metal chalcogenides on the surface of silicon. First, the limitation of p-type silicon for solar water splitting coped using a solution processed TiO2 nanorods with controlled heights and diameters on a 4-inch p-type silicon wafer. The overpotential of bare p-type silicon photocathode was decreased from –0.75 V vs. RHE to 0.0 V vs RHE after catalytic TiO2 NRs were grown. The reflectance of silicon was decreased from about 37 % (arithmetic mean) to 1.4 %. The dramatic reduction of reflectance of silicon enhances the charge generation efficiency of the photocathode and resulted in increment of the saturated current density from 32 to 40 mA cm–2. After very small Pt nanoparticle, 1–2.5 nm diameter, were deposited on the surface of TiO2 NRs the photocathode (Pt/TiO2 NRs/p-Si) showed turn on potential of 440 mV and short circuit current density of 40 mA cm–2. The photocathode were generate hydrogen for 52 h without noticeable degradation and with 2.5 % ideal cell regenerative cell efficiency. Second, the limitation of p-type silicon photocathode for efficient PEC water splitting was managed by incorporation of a 3-dimenstional MoS2 thin film with high density of catalytic edge-sites on a TiO2 coated p-type silicon. The 3D MoS2 HER catalyst were synthesized directly on TiO2 coated 4-inch p-type silicon. Unlike the transferred MoS2 to p-type silicon, the direct growth brings many advantages such as, 1. Can decrease the lengthy transfer time, 2. The directly grown MoS2 catalysts can be immune from organic residue, 3. Better charge transfer between the MoS2/TiO2/p-Si interfaces and 4. Excellent adhesion of MoS2 on TiO2 coated p-type silicon. Therefore, the 3D MoS2/TiO2/p-Si photocathode were showed an onset potential of 0.35 V versus RHE at 1 mA cm–2 with a short circuit current density of 28 mA cm–2 and a saturation current density 37 mA cm–2. The optical reflectance of the 3D MoS2/TiO2/p-Si is 14% (arithmetic mean) lower than that of a TiO2/p-Si photocathode over the entire visible range. This antireflective 3D MoS2 layer enhanced the charge generation efficiency which result in higher saturation current density than TiO2/p-Si photocathode. Hydrogen generation in this photocathode lasts for more than 181 h without noticeable degradation. The third main chapter describes the studies of TiO2 NRs for solar water splitting. The drawback of TiO2 photoanode, which is transparent for visible light/no visible light absorption and low carrier kinetics, for solar water splitting were improved by incorporation of dual atoms sulfur and nitrogen. The codoped TiO2(S, N) NRs shows four times higher photocurrent density than pristine, specifically 2.82 mA cm–2 at 1.23 V vs. RHE and 0.7 mA cm–2 for codoped and pristine TiO2 NRs, respectively. The extracted photocurrent density from codoped TiO2 (S, N) NRs is found be the highest among the co-catalyst free TiO2 photoanodes reported up to date. Furthermore, the measured applied bias photon to-current conversion efficiency (ABPE) and incident photon-to-current conversion efficiency (IPCE) of the TiO2 (S, N) photoanode found to be 1.46 % and 97 % at wavelength of 360 nm, respectively. The partial density of states calculation shows all dopant configuration can induces defect/dopant energy states between the conduction and valance band of TiO2. The [NO-SO] configuration in the TiO2 shows higher optical absorption coefficient followed by SO and [STi-NO] configuration. Moreover, codoping of sulfur and nitrogen in the TiO2 results higher optical absorption coefficient than individual doping. However, the dopant formation energy of [STi-NO] in the TiO2 is lower than [NO-SO]. Therefore, the [STi-NO] configuration may exists more than the [SO-NO] configuration. The outperformance of the photoanode mainly resulted from the induced defect energy states between ECB and EVB. Codoping of sulfur and nitrogen in TiO2 photoanode cannot impair the stability of the TiO2, which is confirmed by measuring the chronoamperometry of TiO2 (S, N) NRs for ~58 hours at 1.23 V vs. RHE.