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Article

Photoelectrochemical Performance of a CuBi2O4 Photocathode with H2O2 as a Scavenger

by
Zohreh Masoumi
1,
Mahdi Tayebi
2,
S. Ahmad Masoumi Lari
3,
Bongkuk Seo
4,
Choong-Sun Lim
4,
Hyeon-Gook Kim
4,*,
Daeseung Kyung
1,* and
Meysam Tayebi
4,*
1
Department of Civil and Environment Engineering, University of Ulsan, Daehakro 93, Nam-gu, Ulsan 44610, Republic of Korea
2
Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran 15875-4413, Iran
3
Department of Biology, York University, Farquharson Life Sciences Building, Ottawa Rd, Toronto, ON M3J 1P3, Canada
4
Center for Advanced Specialty Chemical, Division of Specialty and Bio-Based Chemicals Technology, Korea Research Institute of Chemical Technology (KRICT), 45 Jonggaro, Ulsan 44412, Republic of Korea
*
Authors to whom correspondence should be addressed.
Inorganics 2023, 11(4), 147; https://doi.org/10.3390/inorganics11040147
Submission received: 2 March 2023 / Revised: 22 March 2023 / Accepted: 29 March 2023 / Published: 31 March 2023
(This article belongs to the Special Issue Bismuth Based Catalysts and Their Energy Application)
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Abstract

:
Photoelectrochemical (PEC) water splitting is an eco-friendly method for producing clean and sustainable hydrogen fuels. Compared with the fabrication of solar hydrogen using n-type metal oxide semiconductor photoanodes, that of solar hydrogen using p-type metal oxide semiconductor photocathodes has not been researched as thoroughly. Therefore, this study investigated the effect of drop casting time on the PEC performance of a prepared CuBi2O4 photocathode. XPS, HRTEM, UV-DRS, Raman spectroscopy, XRD, and SEM analyses were used to characterize the prepared CuBi2O4 photocathode. Owing to the high charge separation and transfer, the photocurrent density of the CuBi2O4 photocathode was ~0.6 mA cm−2 at 0.3 V vs. RHE. The nanoporous CuBi2O4 photocathode exhibited a high photocurrent density of up to 1.2 mA cm−2 at 0.3 V vs. RHE with H2O2 as a sacrificial agent. Mott–Schottky and impedance measurements were also performed on the CuBi2O4 photocathode to estimate its acceptor density and charge-transfer resistance.

Graphical Abstract

1. Introduction

PEC water splitting is an attractive renewable energy conversion and storage method that uses sunlight to transform water into hydrogen [1,2,3,4,5,6]. In this process, n-type semiconductors are used as photoanodes for water oxidation, whereas p-type semiconductors are used as photocathodes for water reduction. Numerous p-type semiconductors, such as Cu2O [7], GaN [8], NiO [9], and the ternary oxides CaFe2O4 [10], CuNb3O8 [11], CuFeO2 [12], LaFeO3 [13], and CuBi2O4 [14], can be applied as possible photocathode materials. Among the many p-type semiconductors, copper-based ternary oxides (CuBi2O4), which possess several attractive properties, are promising candidates for PEC water splitting. The favorable absorption energy bandgap of p-type CuBi2O4 (CBO) photocathodes, which are active in visible light, ranges between 1.5 and 1.8 eV and has received increasing attention [15,16,17,18,19,20].
CBO, a powder-type photocatalyst, has been synthesized via the hydrothermal method [21], the sol–gel method [22], solid-state synthesis [23], and metal–organic decomposition [24]. However, only a few studies have focused on fabricating a high-quality thin CBO layer with adequate electrical consistency, a high surface area, and consistent coating. Thus, developing synthesis techniques for preparing high-quality CBO electrodes would be advantageous for addressing this research gap and for enhancing their PEC water splitting. Herein, we present a facile and economical casting for fabricating CBO photocathodes with a high surface area.
Most reported porous CBO films have an unstable structure or only partially cover the substrate [25], which typically results in low photocurrent densities. CBO films have been fabricated for PEC water splitting reactions using various synthesis techniques. However, the PEC efficiency of such films may be reduced owing to their irregular nanoporous shape. Increasing the thickness of the nanoporous thin films can minimize the exposure of the substrate to the electrolyte; however, due to restricted charge transfer, this increase in thickness may have an inverse effect on the photocurrent. A simple process for fabricating high-quality nanoporous layers is critical for developing high-performance CBO photocathodes. Herein, a simple drop casting procedure is presented with the optimization thickness of CBO films for energy conversion into PEC photovoltaics.
We optimized the effect of drop casting time on the PEC performance of the prepared CuBi2O4/FTO thin film. The PEC performance was measured to comprehensively explore the influence of the drop casting time. Furthermore, the effect of H2O2 as a sacrificial agent on the CuBi2O4 photocathode was investigated. Mott–Schottky and electrochemical impedance (EIS) analyses were conducted to study the enhancement of the PEC performance of the CuBi2O4 photocathodes.

2. Experimental Section

2.1. Fabrication of CuBi2O4 Photocathodes’ Thin Films

The substrate, fluorine-doped tin oxide-coated glass (FTO, 25 mm × 25 mm × 2.2 mm, 7 O/sq), was cleaned with deionized water, ethanol, and acetone for 15 min. Initially, 0.058 g of Cu(NO3)2.3H2O was dissolved in ethanol, and 0.243 g of Bi(NO3)3.5H2O was dissolved in acetic acid. Precursor solution-based drop casting processes for fabricating CuBi2O4 photocathodes frequently use acetic acid and ethanol as solvents to dissolve Bi(NO3)3 and Cu(NO3)2, respectively. CBO photocathodes were fabricated using a low-cost and simple drop casting method. The precursor solution (100 μL) was dropped onto FTO before being placed in a furnace (denoted as #n-CuBi2O4) for 5 min at 550 °C. Drop casting was performed one, two, and four more times under identical conditions, and the resulting samples were designated as #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4, respectively. All CuBi2O4 photocathodes were calcined for 4 h at 550 °C in a furnace. Figure 1a shows a schematic of the CuBi2O4 photocathode synthesis process and the procedures involved.

2.2. Photoelectrochemical (PEC) Measurements

The synthesized CBO was utilized as the working electrode, Ag/AgCl was used as the reference electrode, and Pt wire was used as the counter electrode during the PEC measurements, which were performed in a standard three-electrode setup at a scan rate of 10 mV/s. The PEC performance was assessed in an electrolyte solution of 0.5 M Na2SO4 under 100 mW/cm2 irradiation (AM 1.5) from the front of the photocathode using a 300 W Xe lamp. H2O2 was used as the electron scavenger by adding an appropriate amount of H2O2 to 50 mL of a 0.5M Na2SO4 aqueous electrolyte. A VersaSTAT 3 potentiostat was used to record the PEC characteristics, including the photocurrent, electrochemical impedance, and Mott–Schottky plots. Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 0.02 Vs−1. A sinusoidal voltage pulse of an amplitude of 10 mV was applied on a bias potential, with frequencies ranging from 100 kHz to 0.1Hz for the EIS of the photocathodes. The Mott–Schottky measurement was performed in the potential range of 0.7−1.6 V vs. RHE, with frequencies of 1000 Hz. The Ag/AgCl reference potential was converted into RHE using Equation (1):
VRHE = VAg/AgCl + 0.197 + 0.059 pH

3. Results and discussion

3.1. Physiochemical and Morphological Properties

Figure 1b shows the SEM image of a CuBi2O4 photocathode prepared by one-time drop casting. After one-time drop casting of the CuBi2O4 precursor on FTO, the CuBi2O4 photocathode was almost transparent and uneven (inset of Figure 1b), indicating that it was not high-quality. As shown in the inset of Figure 1c, the #2-CuBi2O4 photocathode became dark brown after two-time drop casting, indicating the formation of uniform nanoporous structures. After four-time drop casting (#4-CuBi2O4), nanoparticles agglomerated on the photocathode surface (Figure 1d). This could be one of the reasons for the lower PEC performance of the #4-CuBi2O4 photocathode compared with that of #2-CuBi2O4 [26].
Figure 2a shows the SEM image of the CuBi2O4 photocathode with a nanoporous structure and a nanoparticle size of ~250 nm. The effect of thickness on the PEC performances of the #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 photocathodes was determined (Figure S5). The different drop casting processes conducted one, two, and four times (layers) led to the formation of a thin layer of CuBi2O4 on the substrate with thicknesses of ~450, 650, and 1000 nm, respectively. Increasing the thickness from 450 to 650 nm led to an increase in the photocurrent density. However, the photocurrent decreased when the drop casting process was repeated four times, resulting in a thickness of 1000 nm. Therefore, the best photocurrent density was obtained through the two-time drop casting processes (~650 nm) because the electron–hole pairs generated in the bulk of the films during PEC water splitting recombined before reaching the surface.
Furthermore, the HRTEM image (Figure 2b) shows CuBi2O4 with a nanoparticle size of ~250 nm. To illustrate the lattice spacing (the shortest distance between the planes of atoms in a crystal), HRTEM was performed using a fast Fourier transform (Figure 2c and Figure S4). The diffraction planes of the CuBi2O4 lattice included 0.35 nm, which matched well with the interplanar spacing of the (211) plane of CuBi2O4 [15]. TEM elemental mapping showed that Cu, Bi, and O were uniformly present in the CuBi2O4 photocathode (Figure 2d–g).
The XRD results confirmed the highly crystalline configuration of the photocathodes with a tetragonal structure of the CuBi2O4 photocathode (JCPDS No. 72–493) [27]. The Raman peaks (Figure S1) observed at 261, 404, and 587 cm−1 confirmed the presence of a tetragonal CuBi2O4 structure [21,28]. The small peak at 186 cm−1 was assigned to the Eg mode vibration of Cu–Cu [29]. The full-scan XPS profiles (Bi 4f, Cu 2p, and O 1s), shown in Figure 3b,c and Figure S2, reveal that the #2-CuBi2O4 shows all the components in the photocathode. The Cu 2p XPS profile of the #2-CuBi2O4 photocathode shown in Figure S2a has two main peaks at binding energies of 933.64 and 953.4 eV that are related to Cu 2p3/2 and Cu 2p1/2, respectively [30]. The Bi 4f XPS profile, shown in Figure 3c, has two main peaks at 158.6 and 163.9 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, respectively [31]. Furthermore, as shown in Figure S2b, the O 1s XPS profile has a main peak at 529.4 eV corresponding to lattice oxygen in metal oxides, whereas the smaller peak at 531.1 eV corresponds to the oxygen defect [32]. Figure 3d shows the UV-DRS profile of the CuBi2O4 photocathode. The bandgap was estimated to be 1.75 eV for the CuBi2O4 photocathode, consistent with the results of a previous study [17].

3.2. PEC Water Splitting Properties

All samples exhibited negligible dark current densities (Figure 4a,b). Increasing the drop casting time led to an improvement in the photocurrent density of CuBi2O4. However, the photocurrent decreased after four-time drop casting. Therefore, the highest photocurrent density was achieved via two-time drop casting (#2-CuBi2O4 photocathode), owing to the superior electron–hole separation in the bulk of the CuBi2O4 photocathode during PEC water splitting.
LSV scans of #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 are shown in Figure 4a. The LSV results indicate a p-type semiconductor in which a cathodic photocurrent was observed [33]. Figure 4a shows that the onset potentials of #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 were ~1.05 V vs. RHE under 100 mW cm2 illumination. At the optimal drop casting time (#2-CuBi2O4 photocathode), the photocurrent density was ~0.6 mA/cm2 at 0.3 V vs. RHE. The photocurrent responses (Figure 4c,d) were in good agreement with the LSV and chopped LSV results shown in Figure 4a,b, respectively. Figure 4d shows good photocurrent stability without any significant change in the initial photocurrent for 15 min after four cycles of on–off light under 100 mW/cm2 illumination.

3.3. Nyquist (Impedance Analysis) and Mott–Schottky Plots

EIS was performed to study the electron–hole transport of the CuBi2O4 photocathodes [34,35]. The resistance of the #2-CuBi2O4 photocathode (Rct) was determined from the radius of the semi-circle, which was smaller than that of the #1-CuBi2O4 and #4-CuBi2O4 photocathodes. The lower value of the Rct for the #2-CuBi2O4 photocathode suggests substantially higher charge transfer properties, as shown in the Nyquist plots (Figure 5a and Table S1) [36,37]. The inset in Figure 5a shows the equivalent circuit model used in the EIS profiles. The sheet resistance (RS), charge transfer resistance (Rct), and constant phase element (CPE) correspond to those at the interface between the electrode and electrolyte, as presented in Table S1. The charge-transfer resistances of the #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 photocathodes obtained from fitting were 1823.45, 850.37, and 1467.29 Ω/cm2, respectively (Table S1), indicating a decrease in Rct and the lowest arc diameter. Moreover, this suggests that the charge transport properties of these photocathodes were excellent.
Mott–Schottky measurements were performed to estimate the acceptor density (NA) and flat band potential (Vfb), which are two important factors for improving the PEC performance of the #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 photocathodes. The Mott–Schottky plots (Figure 5b) shows a negative slope for the CuBi2O4 photocathodes, suggesting the semiconductor’s p-type nature [38,39]. The slope of the Mott–Schottky plot was used to calculate the acceptor density (NA). We estimated the acceptor density (NA) and flat band potential values from the Mott–Schottky plot (see Table S1). Further, the flat band potentials were found to be 1.34, 1.27, and 1.29 V vs. RHE for the #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 photocathodes, respectively, which are close to the previously reported values [40,41,42]. The slope of #2-CuBi2O4 was smaller than that of the #1-CuBi2O4 and #4-CuBi2O4 photocathodes. Thus, #2-CuBi2O4 has a higher acceptor density, which is conducive to improving the PEC performance.

3.4. Effect of H2O2

The water splitting performance of the #2-CuBi2O4 photocathodes was investigated using H2O2 as an electron scavenger to assess their hydrogen generation capacity. H2O2 was added to the electrolyte as an electron scavenger to test the CuBi2O4 photocathodes without limitations on the reaction kinetics, which would be the case for proton reduction. In the presence of H2O2 as an electron scavenger, the photocurrent density can be increased to ~1.2 mA/cm2 at 0.3 V vs. RHE. As shown in Figure 6a, by adding H2O2 (1/12.5 in volume to the electrolyte), the photocurrent increases substantially. Figure 6b shows that the semicircle for the #2-CuBi2O4 photocathode with H2O2 as a scavenger is lower than that without a scavenger, suggesting substantially superior charge transfer properties. As shown in Figure 6c, the slope of #2-CuBi2O4 with H2O2 as an electron scavenger is lower than that without a scavenger, indicating that the addition of H2O2 enhances the acceptor density, resulting in improved PEC performance. As an effective electron scavenger, H2O2 is expected to eliminate surface recombination and overcome limitations in the reaction kinetics at the semiconductor–liquid interface. The electron scavenger (H2O2) leads to rapid electron transfer from the CuBi2O4 photocathodes to H2O2, thereby enhancing the PEC performance [28,41,43]. Open-circuit potential analysis was performed to investigate charge carrier separation in the #2-CuBi2O4 photocathode with and without H2O2 as a scavenger. ΔOCP was higher for the #2-CuBi2O4 photocathode with H2O2 than for the #2-CuBi2O4 photocathode without H2O2 (Figure 6d). This can be attributed to the improved reaction kinetics by the suppression of recombination after light irradiation [44,45].
Figure 7 shows the complete band diagram for the CuBi2O4 photocathode considering the experimentally established characteristics of the UV–vis and Mott–Schottky measurements. The band diagram of the drop-deposited #2-CuBi2O4 photocathode is shown in Figure 7. Clearly, the valence band (VB) of CuBi2O4 is close to the water oxidation potential. Based on an estimated bandgap of 1.75 eV, we estimated the location of the conduction band (CB) to be ~0.48 V vs. RHE, which is more negative than the water reduction potential for hydrogen generation. In other words, the photoexcited electrons in the CB of the CuBi2O4 thin films can thermodynamically convert protons to hydrogen. The efficient synthesis of H2 by PEC water splitting requires the device to use visible light. Moreover, water is reduced on the surface of p-type semiconductor materials under visible light, whereas it is oxidized at the counter electrode.

4. Conclusions

In this study, a CuBi2O4 photocathode was successfully prepared using a simple and low-cost drop casting method. The deposition layer was optimized using different drop casting cycles to obtain the highest photocurrent density in the CuBi2O4 photocathode. At the optimal time of the drop casting of the CuBi2O4 photocathode, the photocurrent density was ~0.6 mA/cm2 at 0.3 V vs. RHE under 100 mW/cm2 illumination. The CuBi2O4 photocathode exhibited a high photocurrent density of up to 1.2 mA/cm2 at 0.3 V vs. RHE with H2O2 as a sacrificial agent. Furthermore, Mott–Schottky and impedance measurements were performed to evaluate the acceptor density and charge transfer resistance of the CuBi2O4 photocathode. This study demonstrates that the semiconductors can significantly enhance the performance of metal oxide-based photocathodes toward an efficient PEC water splitting system.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11040147/s1. Figure S1: Raman spectra; Figure S2: High-resolution XPS of Cu2p and O1s; Figure S3: EDX spectra of the CuBi2O4 photocathode; Figure S4: Inverse fast Fourier transform (IFFT) of CuBi2O4; Figure S5: SEM cross-section; Table S1: PEC parameters of the CuBi2O4 photocathodes.

Author Contributions

Z.M.: Writing—review & editing, Writing—original draft, Formal analysis, Investigation. M.T. (Mahdi Tayebi): Writing—original draft, Investigation. S.A.M.L.: Writing—original draft, Investigation. B.S.: Conceptualization, Supervision. C.-S.L.: Conceptualization, Supervision. H.-G.K.: Writing—review & editing, Supervision. D.K.: Writing—review & editing, Supervision. M.T. (Meysam Tayebi): Writing—review & editing, Conceptualization, Supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Technology Innovation Program (20011124, TS227-28R) funded by the Ministry of Trade, Industry, & Energy (MOTIE, Korea) and the KRICT core project funded by the Korea Research Institute of Chemical Technology (SS2241-10). This work was also supported by “Region-al Innovation Strategy (RIS)” through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE)(2021RIS-003), and the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant 23UMRG-B158194-04).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Fabrication procedure of CuBi2O4 photocathodes. SEM images of CuBi2O4. Photocathodes by the drop casting method: (b) #1-CuBi2O4, (c) #2-CuBi2O4, and (d) #4-CuBi2O4.
Figure 1. (a) Fabrication procedure of CuBi2O4 photocathodes. SEM images of CuBi2O4. Photocathodes by the drop casting method: (b) #1-CuBi2O4, (c) #2-CuBi2O4, and (d) #4-CuBi2O4.
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Figure 2. (a) SEM result, (b) TEM results, (c) HRTEM image of #2-CuBi2O4, and (d–g) TEM–EDS analysis of the #2-CuBi2O4 photocathode.
Figure 2. (a) SEM result, (b) TEM results, (c) HRTEM image of #2-CuBi2O4, and (d–g) TEM–EDS analysis of the #2-CuBi2O4 photocathode.
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Figure 3. (a) XRD patterns, (b) XPS full-scan survey, (c) high-resolution XPS peaks for Bi 4f, and (d) band gap of the #2-CuBi2O4 photocathode.
Figure 3. (a) XRD patterns, (b) XPS full-scan survey, (c) high-resolution XPS peaks for Bi 4f, and (d) band gap of the #2-CuBi2O4 photocathode.
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Figure 4. (a) LSV, (b) chopped LSV, (c,d) photocurrent response of the #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 photocathodes. Under 100 mW/cm2 illumination, the electrolyte was a 0.5 M aqueous solution of Na2SO4.
Figure 4. (a) LSV, (b) chopped LSV, (c,d) photocurrent response of the #1-CuBi2O4, #2-CuBi2O4, and #4-CuBi2O4 photocathodes. Under 100 mW/cm2 illumination, the electrolyte was a 0.5 M aqueous solution of Na2SO4.
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Figure 5. (a) Nyquist and (b) Mott–Schottky plots of the #1–CuBi2O4, #2–CuBi2O4, and #4–CuBi2O4 photocathodes. Under 100 mW/cm2 illumination, the electrolyte was a 0.5 M aqueous solution of Na2SO4.
Figure 5. (a) Nyquist and (b) Mott–Schottky plots of the #1–CuBi2O4, #2–CuBi2O4, and #4–CuBi2O4 photocathodes. Under 100 mW/cm2 illumination, the electrolyte was a 0.5 M aqueous solution of Na2SO4.
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Figure 6. (a) Photocurrent response, (b) Nyquist, (c) Mott–Schottky plots, and (d) Open-circuit potential (OCP) measurements of the #2-CuBi2O4 photocathodes with and without H2O2. Under 100 mW/cm2 illumination, the electrolyte was a 0.5 M aqueous solution of Na2SO4.
Figure 6. (a) Photocurrent response, (b) Nyquist, (c) Mott–Schottky plots, and (d) Open-circuit potential (OCP) measurements of the #2-CuBi2O4 photocathodes with and without H2O2. Under 100 mW/cm2 illumination, the electrolyte was a 0.5 M aqueous solution of Na2SO4.
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Figure 7. The mechanism and band alignment of CuBi2O4 photocathodes.
Figure 7. The mechanism and band alignment of CuBi2O4 photocathodes.
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MDPI and ACS Style

Masoumi, Z.; Tayebi, M.; Lari, S.A.M.; Seo, B.; Lim, C.-S.; Kim, H.-G.; Kyung, D.; Tayebi, M. Photoelectrochemical Performance of a CuBi2O4 Photocathode with H2O2 as a Scavenger. Inorganics 2023, 11, 147. https://doi.org/10.3390/inorganics11040147

AMA Style

Masoumi Z, Tayebi M, Lari SAM, Seo B, Lim C-S, Kim H-G, Kyung D, Tayebi M. Photoelectrochemical Performance of a CuBi2O4 Photocathode with H2O2 as a Scavenger. Inorganics. 2023; 11(4):147. https://doi.org/10.3390/inorganics11040147

Chicago/Turabian Style

Masoumi, Zohreh, Mahdi Tayebi, S. Ahmad Masoumi Lari, Bongkuk Seo, Choong-Sun Lim, Hyeon-Gook Kim, Daeseung Kyung, and Meysam Tayebi. 2023. "Photoelectrochemical Performance of a CuBi2O4 Photocathode with H2O2 as a Scavenger" Inorganics 11, no. 4: 147. https://doi.org/10.3390/inorganics11040147

APA Style

Masoumi, Z., Tayebi, M., Lari, S. A. M., Seo, B., Lim, C. -S., Kim, H. -G., Kyung, D., & Tayebi, M. (2023). Photoelectrochemical Performance of a CuBi2O4 Photocathode with H2O2 as a Scavenger. Inorganics, 11(4), 147. https://doi.org/10.3390/inorganics11040147

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