An efficient and stable heterojunction photoanode for solar water oxidation was fabricated by hybridization of WO and conducting polymers (CPs). Organic/inorganic hybrid photoanodes were readily prepared by the electropolymerization of various CPs and the codeposition of tetraruthenium polyoxometalate (RuPOM) water-oxidation catalysts (WOCs) on the surface of WO. The deposition of CPs, especially polypyrrole (PPy) doped with RuPOM (PPy:RuPOM), resulted in a remarkably improved photoelectrochemical performance by the formation of a WO/PPy p-n heterojunction and the incorporation of efficient RuPOM WOCs. In addition, there was also a significant improvement in the photostability of the WO-based photoanode after the deposition of the PPy:RuPOM layer due to the suppression of the formation of hydrogen peroxide, which was responsible for corrosion. This study provides insight into the design and fabrication of novel photosynthetic and photocatalytic systems with excellent performance and stability through the hybridization of organic and inorganic materials.

An efficient and stable bias-free photoelectrochemical cell was readily fabricated using only simple solution processes.

We studied the kinetics of photoelectrochemical (PEC) water oxidation using a model photoanode BiVO4 modified with various water oxidation catalysts (WOCs) by electrochemical impedance spectroscopy. In particular, we prepared BiVO4 photoanodes with catalytic multilayers (CMs), where cationic polyelectrolytes and anionic polyoxometalate (POM) WOCs were assembled in a desired amount at a nanoscale precision, and compared their performance with those with well-known WOCs such as cobalt phosphate (CoPi) and NiOOH. Our comparative kinetics analysis suggested that the deposition of the CMs improved the kinetics of both the photogenerated charge carrier separation/transport in bulk BiVO4 due to passivation of surface recombination centers and water oxidation at the electrode/electrolyte interface due to deposition of efficient molecular WOCs. On the contrary, the conventional WOCs were mostly effective in the former and less effective in the latter, which is consistent with previous reports. These findings explain why the CMs exhibit an outstanding performance. We also found that separated charge carriers can be efficiently transported to POM WOCs via a hopping mechanism due to the delicate architecture of the CMs, which is reminiscent of natural photosynthetic systems. We believe that this study can not only broaden our understanding on the underlying mechanism of PEC water oxidation but also provide insights for the design and fabrication of novel electrochemical and PEC devices, including efficient water oxidation photoanodes.

An efficient water oxidation photoanode based on hematite has been designed and fabricated by tailored assembly of graphene oxide (GO) nanosheets and cobalt polyoxometalate (Co-POM) water oxidation catalysts into a nacre-like multilayer architecture on a hematite photoanode. The deposition of catalytic multilayers provides a high photocatalytic efficiency and photoelectrochemical stability to underlying hematite photoanodes. Compared to the bare counterpart, the catalytic multilayer electrode exhibits a significantly higher photocurrent density and large cathodic shift in onset potential (∼369 mV) even at neutral pH conditions due to the improved charge transport and catalytic efficiency from the rational and precise assembly of GO and Co-POM. Unexpectedly, the polymeric base layer deposited prior to the catalytic multilayers improves the performance even more by facilitating the transfer of photogenerated holes for water oxidation through modification of the flat band potential of the underlying photoelectrode. This approach utilizing polymeric base and catalytic multilayers provides an insight into the design of highly efficient photoelectrodes and devices for artificial photosynthesis.

Solar‐to‐chemical energy conversion, or so‐called artificial photosynthesis, is a promising technology enabling sustainable production and use of various chemical compounds such as H2, CO, CH4, HCOOH, CH3OH, and NH3. For practical applications, it is necessary to improve the interfacial properties of light‐harvesting semiconductors through modification with proper electrocatalysts, by trying to overcome their intrinsic limitations such as rapid recombination, sluggish reaction kinetics, and photocorrosion. Compared to their heterogeneous counterparts, molecular electrocatalysts have a higher catalytic activity and more flexibility in their design/synthesis and integration with semiconducting materials. In this article, we review recent efforts on the tailored assembly of molecular electrocatalysts to address the above issues for artificial photosynthesis, especially those for oxygen evolution reactions on semiconductor photoelectrodes for photoelectrochemical water oxidation. One can expect that the strategies and methods developed for the tailored assembly and integration of molecular electrocatalysts on water oxidation photoanodes can provide insights for the design and fabrication of various forms of photosynthetic devices due to the similarity between their underlying principles.