A hygroscopic nano-membrane coating achieves efficient vapor-fed photocatalytic water splitting

Abstract

Efficient water vapor splitting opens a new strategy to develop scalable and corrosion-free solar-energy-harvesting systems. This study demonstrates highly efficient overall water splitting under vapor feeding using Al-doped SrTiO₃ (SrTiO₃:Al)-based photocatalyst decorated homogeneously with nano-membrane TiO_x or TaO_x thin layers (<3 nm). Here, we show the hygroscopic nature of the metal (hydr)oxide layer provides liquid water reaction environment under vapor, thus achieving an AQY of 54 ± 4%, which is comparable to a liquid reaction. TiO_x coated, CoOOH/Rh loaded SrTiO₃:Al photocatalyst works for over 100 h, under high pressure (0.3 MPa), and with no problems using simulated seawater as the water vapor supply source. This vapor feeding concept is innovative as a high-pressure-tolerant photoreactor and may have value for large-scale applications. It allows uniform distribution of the water reactant into the reactor system without the potential risk of removing photocatalyst powders and eluting some dissolved ions from the reactor.

Fig. 1. Schematic illustration of amorphous metal oxide coated, CoOOH/Rh loaded SrTiO₃:Al.

Red: O atom, blue: H atom.

Fig. 2. SrTiO₃:Al-based photocatalysts for overall water splitting under vapor feeding.

a Photographic images of a photocatalyst sheet (1 × 1 cm²), which consists of SrTiO₃:Al-based particulate photocatalysts immobilized on a frosted glass substrate. b Time course measurement of H₂ and O₂ gas evolution rate using CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x under various light intensity. The fed gas was Ar with saturated water vapor at 24 °C (10 mL min⁻¹, p_H2O = 2.9 kPa). c H₂ evolution rate of various cocatalysts loaded SrTiO₃:Al irradiated with 370 nm LED (5.1 mW cm⁻²) under saturated water vapor balanced with Ar at 24 °C (10 mL min⁻¹, p_H2O = 2.9 kPa). Error bars are standard error from other samples (n = 3–11). d Dependence of H₂ evolution rate of CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x (blue circle) and without TiO_x (green square) under various O₂ partial pressure ranged from 0 to 33 kPa under 370 nm LED (5.1 mW cm⁻²).

Fig. 3. The role of the coated TiO_x on CoOOH/Rh loaded SrTiO₃:Al.

a Bright field STEM image of CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x and b its corresponding overlayed EDS mapping of Ti and Sr. c Water adsorption isotherm of CoOOH/Rh loaded SrTiO₃:Al coated with or without TiO_x. d Dependence of H₂ evolution rate of CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x on relative humidity at 24 °C (blue circle), 50 °C (green diamond), and 80 °C (red square). Error bars are standard error from other samples (n = 3).

Fig. 4. Thermal deactivation of TaO_x coated SrTiO₃:Al photocatalysts.

a Dark field TEM image of CoOOH/Rh loaded SrTiO₃:Al coated with TaO_x and b its corresponding EDS mapping of Ta. c H₂ evolution rate of TaO_x coated SrTiO₃:Al type photocatalysts before and after thermal treatment and regeneration process. These demonstrations were under saturated water vapor balanced with Ar at 24 °C (10 mL min⁻¹, p_H2O = 2.9 kPa). Light source: 370 nm LED (5.1 mW cm⁻²). Error bars are standard error from other samples (n = 3). d Dependence of H₂ evolution rate of CoOOH/Rh loaded SrTiO₃:Al coated with TaO_x on relative humidity at 24 °C. Light source: 370 nm LED (5.1 mW cm⁻²). e Fourier transformed Ta L_III EXAFS patterns of CoOOH/Rh loaded SrTiO₃:Al coated with TaO_x recorded at 24 °C under dried condition (dashed) and (solid) after thermal treatment, and f corresponding XANES region of Ta L_III XAFS spectra of TaO_x coated, CoOOH/Rh loaded SrTiO₃:Al.

Fig. 5. Practical feasibility study of photocatalytic overall water splitting under vapor feeding.

a Durability test of CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x under saturated water vapor feeding balanced with Ar at 24 °C (10 mL min⁻¹, p_H2O = 2.9 kPa). The light source was a simulated sunlight (AM 1.5G). b The overall water splitting rate of CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x under water vapor feeding supplied from ultrapure water or brine (3 wt% NaCl_aq) balanced with Ar at 24 °C. c Photographic images of a vaper feeding photocatalytic overall water splitting system under pressurized condition. d H₂ evolution rate of CoOOH/Rh loaded SrTiO₃:Al coated with TiO_x under pressurized condition at total pressure (P_tot) of 0.1 or 0.3 MPa using the system shown in panel c. The light source was simulated sunlight (AM 1.5G).