Transparent mesoporous WO₃ film enhances solar water splitting efficiency and stability

A team of scientists have unveiled a breakthrough in the field of renewable energy materials. They have developed a transparent, crystalline mesoporous tungsten trioxide (WO₃) film that exhibited exceptional efficiency and stability for photoelectrochemical (PEC) water splitting.

The research is published in the journal Applied Catalysis B: Environment and Energy.

The innovation can accelerate the transition toward sustainable solar-to-hydrogen technologies, a clean energy pathway with far-reaching implications for global decarbonization.

The research developed a transparent film of tungsten trioxide (WO₃) with a highly ordered mesoporous structure and tailored crystal orientation and achieved unprecedented efficiency and long-term stability, particularly in neutral pH conditions. This discovery offers a promising path for developing tandem photoelectrochemical (PEC) devices for sustainable solar-to-hydrogen conversion.

The team fabricated the film directly on a conductive glass substrate (fluorine-doped tin oxide, FTO) using a surfactant-template method coupled with an in-situ template-carbonization technique.

This process employed the triblock copolymer Pluronic F127 and enabled the formation of a crystalline mesoporous network with ultrathin pore walls (~10 nm) and high surface area (124 m²/g). The design ensures shorter migration paths for charge carriers, abundant active sites for water oxidation, and efficient electron transport within the transparent film.

The lead author, Dr. Debraj Chandra highlighted the significance of the synthesis method and stated, “in situ template-carbonization technique preserves the uniquely crystalline organized-mesoporous structure of WO₃ film. The materials fabrication technique is promising for development of other unattained crystalline mesoporous metal oxide films.”

The mesoporous WO₃ photoanode demonstrated incident photon-to-current conversion efficiencies (IPCE) of 49% in acidic conditions and 41% under neutral pH at 420 nm and 1.23 V in comparison to the reversible hydrogen electrode.

The IPCE value of 49% was approximately three times higher than those of conventional, untemplated WO₃ films. In comparison with standard WO₃, mechanistic investigations demonstrated a 3.6-fold increase in water oxidation rate constants.

The introduction of cobalt oxide (CoO_x) nanoparticles as co-catalysts within the mesoporous channels resulted in a further improvement, as it accelerated surface reactions and increased the oxygen evolution rate constant to 5.7 × 10² s^-1.

The faradaic efficiency for oxygen evolution of 93% was a remarkable accomplishment for WO₃ photoanodes, and the mesoporous WO₃ electrode also demonstrated remarkable durability, retaining 98% of its initial photocurrent after 30 h of continuous operation under neutral conditions.

The optical transparency of a material is one of its most notable features that contributes to its role as a front light-harvesting layer when combined with PEC devices. In these devices, the overall efficiency is enhanced by stacking multiple photoabsorbers to capture different regions of the solar spectrum.

According to the corresponding author Dr. Masayuki Yagi, “The high optical transparency and exceptional long-term stability under neutral pH conditions of the mesoporous WO₃ electrode provides a scalable strategy for tandem photoelectrochemical water splitting devices by using it as a front light-harvested layer, thereby advancing the prospects of sustainable solar-driven water splitting.”

Although hydrogen is considered a sustainable energy carrier that can decarbonize a variety of sectors, including transportation and heavy industry, the instability and inefficiency of photoactive materials hinder the sustainable production of hydrogen through sunlight-driven water splitting.

Thus, this study offers a blueprint for the development of next-generation photoanodes that combine long-term stability, transparency, and high efficiency by addressing these fundamental challenges in WO₃.

Additionally, the fabrication strategy, which is based on scalable templating and carbonization methods, can be expanded to other metal oxide semiconductors, thereby further increasing its impact.

The study emphasizes that this innovation paves the way for the development of practical solar water-splitting systems that are capable of producing renewable hydrogen on a large scale.

Transparent mesoporous WO₃ films have the potential to herald in a new era of sustainable, efficient solar fuels with the addition of further optimization and integration into tandem device architectures.