In 2012, I published my first Ta₃N₅-related paper in Advanced Materials [1], reporting a Ta₃N₅ nanorod photoanode with a hall-cell solar-to-hydrogen efficiency (HC-STH) of 0.37%. Entering the photoelectrochemical (PEC) water splitting field with a background in nanomaterials, my motivation at that time was to utilize 1D nanostructure to improve the light absorption and charge collection in Ta₃N₅ photoanode. Nevertheless, the other efficiency determining factors, such as the surface defects and the oxygen evolution reaction (OER) co-catalyst, were far from optimized. By doping the Ta₃N₅ nanorod with Ba and modifying it with a Co-Pi OER co-catalyst, we significantly improved the HC-STH of the nanorod photoanode to 1.56% [2]. 

In 2016, Prof. Can Li's group at DICP proposed a novel hole-storage concept to mediate interfacial charge transfer from Ta₃N₅ to coupled molecular catalysts [3]. This integrated Ta₃N₅ photoanode achieved an impressive photocurrent of 12.1 mA cm^-2 at 1.23 V vs. RHE, approaching its theoretical photocurrent limit under sunlight (12.9 mA cm^-2). Owing to the high photocurrent, the HC-STH of the Ta₃N₅ photoanode was boosted to 2.5%. In 2020, Prof. Kazunari Domen's group further improved the HC-STH of Ta₃N₅ photoanode to 2.72% by designing a Ta₃N₅ nanorod structure to enhance light harvesting and charge collection [4]. 

Since starting my independent research in 2016, my group at UESTC has been devoted to improving the efficiency of Ta₃N₅ thin film photoanode via band structure engineering, defect engineering, and interface engineering. By constructing a gradient band structure in the bulk of Ta₃N₅ thin film through gradient Mg-doping, we achieved an HC-STH of 3.31% thanks to the improved bulk charge separation efficiency in the gradient Mg-doped Ta₃N₅ photoanode [5]. Alternatively, by modifying the interfaces of Ta₃N₅ thin film with an electron transport layer (ETL: In-doped GaN) and a hole transport layer (HTL: Mg-doped GaN), we realized a heterostructured Ta₃N₅ photoanode with an HC-STH of 3.46% [6]. Lately, we demonstrated a heterogeneous doping strategy that combined surface La doping with bulk gradient Mg doping in Ta₃N₅ thin film to decouple light absorption and carrier transport, resulting in a record-high HC-STH of 4.07% for Ta₃N₅ photoanode [7], ten times higher than what had been achieved a decade ago [1]. These results established Ta₃N₅ as a leading performer among visible‐light‐responsive photoanodes for PEC water splitting.

The next big challenge is to further improve the STH from the current 4% to a commercially viable efficiency of above 10%, which requires both material and device innovations. Material-wise, we believe the key is still to find effective ways to passivate the efficiency limiting deep-level defects in Ta₃N₅ [8]. Device-wise, constructing tandem devices with suitable photocathodes is probably a more practical way to realize high efficiency.

Nevertheless, there is still plenty of room for improvement - this shall be the correct mindset going forward. 

[1] Li, Y. et al. Vertically aligned Ta₃N₅ nanorod arrays for solar-driven photoelectrochemical water splitting. Adv. Mater. 25, 125-131 (2013).

[2] Li, Y. et al. Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency. Nat. Commun. 4, 2566 (2013).

[3] Liu, G. et al. Enabling an integrated tantalum nitride photoanode to approach the theoretical photocurrent limit for solar water splitting. Energy Environ. Sci. 9, 1327-1334 (2016).

[4] Pihosh, Y. et al. Ta₃N₅-Nanorods enabling highly efficient water oxidation via advantageous light harvesting and charge collection. Energy Environ. Sci. 13, 1519-1530 (2020). 

[5] Xiao, Y. et al. Band structure engineering and defect control of Ta₃N₅ for efficient photoelectrochemical water oxidation. Nat. Catal. 3, 932-940 (2020).

[6] Fu, J. et al. Interface engineering of Ta₃N₅ thin film photoanode for highly efficient photoelectrochemical water splitting. Nat. Commun. 13, 729 (2022).

[7] Xiao, Y. et al. Decoupling light absorption and carrier transport via heterogeneous doping in Ta₃N₅ thin film photoanode. Nat. Commun. 13, 7769 (2022).

[8] Fu, J. et al. Identifying performance-limiting deep traps in Ta₃N₅ for solar water splitting. ACS Catal. 10, 10316–10324 (2020).