Earth-abundant, visible-light activated photoelectrodes for photoelectrochemical water splitting
Date of Issue2017-06-30
School of Materials Science and Engineering
Water splitting utilizing solar energy is an attractive way to produce clean and sustainable energy. Among various approaches, photoelectrochemical cell (PEC) has proven to be a promising route to achieve highly efficient artificial photosynthesis. In general, there are three types ofPEC: single absorber PEC, dualabsorber PEC and Photovoltaic (PV)-assisted PEC. Between these three types, dual-absorber PEC has the most advantages as it has higher photon absorption efficiency compared to single absorber PEC. Meanwhile, it also has simpler configuration and lower manufacture cost compared with PV -assisted PEC. The performance of a PEC depends on the photocatalytic performance of its photoelectrodes. Hence, fabricating photoelectrodes with high photocatalytic efficiencies is the most important objective. In this work, high performance W03 photoanode was achieved through optimizing the nanostructure. A triple layered W03 was prepared using a one-step controllable anodizing method. The anodization was carried out in a heated acidic solution containing NH4F as the surfactant for morphology control. Results show that with this unique triple layer structure, light generated charge carriers can be efficiently separated and migrated to the electrode surface for the chemical reactions. This triple layered W03 is able to generate photocurrent as high as 0.9 mA.cm-2 at 1.2 V vs RHE. By the time we reported this work, this was the highest photocurrent achieved by pure W03 photo anode without any cocatalyst deposition. Moreover, it has been greatly reported that the stability of W03 is a main limiting factor in its photoelectrochemical application, even in neutral electrolyte. Thus, an ultrathin (10 nm) Ti02 protective overlayer was added on top of triple layered W03 enhanced the stability of the triple-layer W03 photoanodes considerably. Meanwhile, efficient CU20 photocathodes have been prepared through depositing a thin layer of NiFe layered double hydroxide (LDH) co-catalyst on top of CU20. Both materials were prepared using simple electrodeposition methods without any post treatment. Through modifying CU20 with NiFe-LDH co-catalyst,the photocurrent of CU20 photocathode has been greatly improved, especially in the low bias range. It is observed that under as low as -0.2 V vs Ag/ AgCl applied voltage, photocurrent increases almost seven times with the assist of NiFe-LDH cocatalyst. The origin of such a pronounced effect is the improved electron transportation towards the electrolyte as the NiFe-LDH overlayer induces an appropriate energy level alignment. LDH are known to be used as oxygen evolution cocatalyst, and it was the first time for NiFe-LDH to be reported as a photocathode cocatalyst. Moreover, It is well known that the electrochemical instability of CU20 poses a serious challenge to its PEC application. The stability of CU20 has shown great improvement after coating with NiFe-LDH. Results show that there is almost zero photocurrent loss after 40 hours continuous illumination at -0.2 V vs Ag/ AgCI condition. To explore other semiconductor oxide systems, BiV04 was also prepared as an alternative photoanode in the current work. Results indicate that an ultra-thin layer of NiCo-LDH improves the photocurrent by three-folds. Finally, by combining the synthesized photoelectrodes (i.e. W03-based photoanode, CU20-based photocathode and BiV04-based photoanode) with commercially available semiconductors (i.e. p-type Si, and p-type GaP) and another commonly used photoanode (a-Fe203), the mechanisms behind the voltage generation of a dual-absorber PEC was investigated. It is clear that the important factors for the overall voltage generation are the photoanode and photocathode photocurrent, especially under low bias conditions. In conclusion, modification of the nanostructure and deposition of a cocatalyst are two efficient approaches in improving the photo electrode activities. In general, the nanostructure increases the light harvesting efficiency and reduces the charge recombination rate; the co-catalyst improves the reaction kinetics and increases the charge transportation and inject rate towards the electrolyte. Meanwhile, in order to achieve high efficiency dual-absorber PEC, the voltage generated within the integrated dual-absorber PEC upon illumination is very important. This can be improved by increasing the efficiencies of the photoelectrodes especially their charge separation and injection rate.