Progress in Ternary Metal Oxides as Photocathodes for Water Splitting Cells: Optimization Strategies

Abstract

The conversion of solar energy into fuels and, specifically hydrogen, is a critical process for ensuring the sustainability of the future energy system. Among the different ways of converting solar energy into fuels and chemicals, photoelectrochemical tandem cells, containing two photoactive materials, stand out as they have the potential for direct solar‐to‐fuel conversion, thus minimizing cost. The implementation of photoelectrolysis for hydrogen generation is hindered by the lack of suitable electrode materials, particularly photocathodes. Among the candidates for photocathodes, ternary (and multinary) transition metal oxides have the advantage of lower cost and, potentially, higher stability than other metal compounds. Herein, the aim is to provide an overview of the current state of the art for ternary oxides (spinels, delafossites, perovskites, etc.), with a focus on the modification strategies that can optimize their behavior and applicability. Both copper‐based and iron‐based ternary oxides are the most studied and, currently, the most promising. Among the strategies being used for their optimization, doping, the deposition of underlayers and overlayers, and the use of cocatalysts are the most popular. However, it is apparent that both solar‐to‐hydrogen efficiencies and stability will need to be addressed in the future. Some guidelines in this respect are also provided.

Photoelectrochemical properties of copper pyrovanadate (Cu2V2O7) thin films synthesized by pulsed laser deposition

“…Among these ternary metal oxides, there are main families such as spinels (CuFe 2 O 4 , 8 CaFe 2 O 4 , 9 …), delafossite-type oxides (CuFeO 2 , 10 AgRhO 2 , 11 …), perovskites (LaFeO 3 , 12 BiFeO 3 , 13 …) and many others. 14 Copper pyrovanadate (Cu 2 V 2 O 7 ) oxides is also promising candidates for the design of photo-electrodes which can be used in PEC cells. For example, Guo et al 15 obtained a current density of 12 μA cm −2 (at 1.23 V vs. NHE) on thick films of β-Cu 2 V 2 O 7 produced by the drop-casting technique in 0.1 M sodium borate buffer solution.…”

High-throughput exploration of activity and stability for identifying photoelectrochemical water splitting materials

“…In fact, almost all possible binary oxides have been explored already with only limited success. [5][6][7] Thus, venturing into more complex multinary oxide systems presents a major opportunity to balance the deleterious properties of one material with the benecial characteristics of another. 7 The combinatorial possibilities offered by ternary (>19 000) or quaternary (>220 000) oxides, considering about 50 possible candidate metals in the periodic table, give a high dimensional search space from which the chances to identify a candidate with all desired properties of an ideal photoanode material are higher.…”

“…5,8 Many ternary systems have been explored in the past, but the inherent limitations of charge transfer and separation efficiency, PEC kinetics, and durability still persist. 6,9 Quaternary oxides are just recently gaining interest and several reports have already demonstrated viable combinations for PEC water splitting. [10][11][12][13] An interesting quaternary oxide is the system Fe-Ti-W-O.…”

Evaluation of the Efficiency of Photoelectrochemical Activity Enhancement for the Nanostructured LaFeO3 Photocathode by Surface Passivation and Co-Catalyst Deposition

“…Among TMO materials, perovskites are particularly interesting due to the flexibility of their electronic and crystal structure and their chemical versatility [ 7 , 8 ]. Most of the TMO-based materials, which were extensively studied in photoelectrochemical processes, are photoanodes [ 1 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ] (TiO 2 , Fe 2 O 3 , BiVO 4 , SrTiO 3 ), while much less attention has been paid to metal oxide photocathodes [ 17 , 18 , 19 ], except for copper-based materials. Copper-based photocathodes [ 20 ], such as Cu 2 O and CuBi 2 O 4 , show attractive activity in photoelectrochemical processes, yet their chemical and electrochemical stability under operating conditions remains a significant challenge [ 21 ] since successful application requires surface modification with protective coatings [ 22 ].…”

“…However, these materials show quite low external quantum efficiency (EQE) for hydrogen evolution reaction (HER), and that motivates further research aimed at enhancing their PEC performance. p -type LaFeO 3 (LFO) is an attractive perovskite-structured photocathode material, which was reported to be sufficiently stable under the photoelectrolysis conditions in alkaline media, yet shows very modest EQE [ 17 , 23 , 24 , 25 , 26 , 29 , 30 , 31 , 32 , 33 ]. Various methods of improving EQE have been proposed, such as doping to tune the electronic structure and affect the charge carrier mobilities [ 31 , 33 , 34 ], nanostructuring to reduce the diffusion length [ 23 , 25 ], and adding a buffer gold layer to improve charge collection efficiency and charge separation [ 32 ].…”