Evaluation of Copper Vanadate (β-Cu₂V₂O₇) as a Photoanode Material for Photoelectrochemical Water Oxidation

Abstract

Monoclinic copper vanadate (n-type Cu 2 V 2 O 7 ) thin film photoanodes were prepared for the first time by spray pyrolysis and evaluated for photoelectrochemical (PEC) water oxidation. The spray pyrolysis parameters were optimized to obtain phase pure photoanodes of -Cu 2 V 2 O 7 (ziesite) . The bandgap energy of -Cu 2 V 2 O 7 is ∼2.0 eV, which corresponds to a theoretical solar-to-hydrogen (STH) efficiency of 16% if it can be paired with an appropriate photocathode in a tandem device to drive overall water splitting. However, all Cu 2 V 2 O 7 photoanodes prepared so far have shown relatively low photoconversion efficiencies, and the properties that limit the efficiency have not yet been fully identified. In this work, many key physical and photoelectrochemical properties of -Cu 2 V 2 O 7 , such as optical band gap, doping type, flat-band potential, band positions, charge carrier dynamics, and chemical stability are reported. The photoelectrochemical performance of the -Cu 2 V 2 O 7 photoanodes is found to be limited by a short carrier diffusion length and slow water oxidation kinetics. Time-resolved microwave conductivity (TRMC) measurements reveal that the short carrier diffusion length (∼28 nm) is mainly due to a relatively low carrier mobility (∼3.5 × 10 -3 cm 2 V -1 s -1 ). The slow water oxidation kinetics can be improved by using cobalt phosphate (CoP i ) as a water oxidation co-catalyst, resulting in a doubling of the photocurrent.

Elucidating the optical, electronic, and photoelectrochemical properties of p-type copper vanadate (p-Cu5V2O10) photocathodes

“…Note that the b-Cu 2 V 2 O 7 phase has a similarly high dielectric constant. 26 Using this value, the N A of the lm is estimated to be about (2.0 AE 0.4) Â 10 17 cm À3 , which is relatively low but well within the range of values commonly observed for metal oxide semiconductors. 30,41 Since this low value results in a relatively wide space charge region even at moderate band bending, the space charge region will be insensitive to surface roughness effects and the lms can be considered as 'at' for the purpose of the Mott-Schottky analysis.…”

“…Tetrahedral VO 4 is the predominant coordination polyhedron in many metal oxides containing V 5+ . [24][25][26] No Raman peaks for CuO and V 2 O 5 are observed, again demonstrating that our synthesis method yields highly phase-pure thin lms of Cu 5 V 2 O 10 .…”

“…Although protection layers may still be required for practical applications, several reports have demonstrated stabilities of several hours for n-type copper vanadates. 12,26,57,58 To compare the oxidation states changes in Cu 5 V 2 O 10 electrode, XPS spectra of the sample aer 1 hour of PEC measurements (i.e., before the lms were completely dissolved) were recorded (Fig. S17 †).…”

Combining Electrosynthesis with Thermolysis: A Safe/Scalable Route to Multinary Oxide Semiconductor Films

“…The CuÀ VÀ O compound family has generated intense interest from the PEC solar fuel community. [3][4][5][6][13][14][15][16][17][18][19][20][21] [3][4][5][6][13][14][15][16][17][18][19][20][21] with significantly higher stability in a water splitting environment relative to their binary oxide (CuO and V 2 O 5 ) constituents. Notwithstanding the intrinsic advantages of electrosynthesis as noted earlier, the use of this approach for synthesizing copper vanadate films, however, has been conspicuous by its absence.…”

In situ characterizations of photoelectrochemical cells for solar fuels and chemicals

“…However, they suffer from similar bulk recombination losses as in hematite and as a result, efficiencies for state-of-the-art photoelectrodes are still far lower than the theoretical maximum. 41 Other materials systems of growing interest are the copper vanadates, 42,43 tin tungstate (α-SnWO 4 ), 44 and barium stannate (BaSnO 3 ). 45 Record photocurrent densities of 7.32 mA/cm 2 at a potential of 1.23 V RHE were recently reported for chemically reduced BaSnO 3 under AM 1.5 G solar simulated light.…”