The bismuth vanadate (BiVO4) photoanode receives extensive attention in photoelectrochemical (PEC) water splitting. However, the high charge recombination rate, low electronic conductivity, and sluggish electrode kinetics have inhibited the PEC performance. Increasing the reaction temperature for water oxidation is an effective way to enhance the carrier kinetics of BiVO4. Herein, a polypyrrole (PPy) layer was coated on the BiVO4 film. The PPy layer could harvest the near-infrared light to elevate the temperature of the BiVO4 photoelectrode and further improve charge separation and injection efficiencies. In addition, the conductive polymer PPy layer acted as an effective charge transfer channel to facilitate photogenerated holes moving from BiVO4 to the electrode/electrolyte interface. Therefore, PPy modification led to a significantly improved water oxidation property. After loading the cobalt–phosphate co-catalyst, the photocurrent density reached 3.64 mA cm−2 at 1.23 V vs the reversible hydrogen electrode, corresponding to an incident photon-to-current conversion efficiency of 63% at 430 nm. This work provided an effective strategy for designing a photothermal material assisted photoelectrode for efficient water splitting.

Photoelectrochemical (PEC) water splitting is a promising technology to obtain renewable hydrogen energy.1–5 Since the 1970s, various non-precious metal oxides such as titanium dioxide (TiO2), tungsten trioxide (WO3), hematite (α-Fe2O3), and bismuth vanadate (BiVO4) have been applied as photoanode materials.6–11 Among them, BiVO4 is widely studied due to its visible-light response (bandgap of ∼2.4 eV), a fitting photovoltage of ∼1.0 V, and earth-abundant elements.12,13 The theoretical photocurrent density of BiVO4 can reach 7.5 mA cm−2 under 1 sun irradiation.14 However, the high charge recombination rate, low electronic conductivity (<10−3 S cm−1), and poor surface water oxidation kinetics have limitations on the PEC performance.15 Multiple strategies, including morphology design and co-catalyst coating, have been used to modify the BiVO4 photoelectrodes to improve the photogenerated carrier mobility and surface reaction kinetics.16–18 

In addition to the material modification, it was reported that the BiVO4 photoanode exhibited higher water oxidation activity under increased electrode and reaction temperatures.19 The enhancement mainly arose from the small-polaron electron transport property of the BiVO4 semiconductor. The minority carrier (hole) hopping was intensified with the increased temperature, which promoted charge separation and transport inside the semiconductor.20 Besides, the oxygen evolution reaction (OER) kinetics were also improved under increased reaction temperature.21 Therefore, elevating the electrode temperature is a promising way to further enhance the PEC performance of BiVO4. Guided by this idea, utilizing photothermal material in converting solar irradiation to thermal energy and then the in situ heating of the BiVO4 photoelectrode represents an efficient and facile strategy for photothermal-assisted PEC water oxidation.22,23 Polypyrrole (PPy) is one of the low-cost polymers with excellent photothermal efficiency in the near-infrared (NIR) region (λ = 700–1100 nm),24,25 which has been demonstrated to be a promising photothermal agent for cancer therapy.26 Meanwhile, the electrochemically polymerized PPy has high electronic conductivity (>100 S cm−1) due to the free availability of π-electrons on the backbone of the polymeric chain.27 Given this, PPy has been applied in TiO2 and WO3 photocatalytic systems to improve the water splitting performance,28,29 and these modifications could promote charge transfer and lead to good activities.

Herein, we report a PPy photothermal layer modified BiVO4 photoelectrode for PEC water oxidation. The electrode temperature of PPy/BiVO4 was apparently raised with the assistance of NIR light irradiation. The photogenerated hole transfer was demonstrated to be accelerated with the coating of conductive polymer PPy. Cobalt–phosphate (CoPi) as a non-precious co-catalyst was coated on the PPy/BiVO4 photoelectrode to further promote the water oxidation activity. The improvement in PEC performance was mainly ascribed to the synergetic effect of increased electrode temperature and enhanced charge transfer capability.

A BiVO4 photoelectrode was prepared by the electrodeposition method.30 KI (0.4 mol l−1) and 0.125 ml of lactic acid were dissolved in 50 ml of deionized water, with HNO3 solution adjusting the pH to 1.9, and then Bi(NO3)3·5H2O (15 mmol l−1) was added with stirring to form solution A. p-Benzoquinone (46 mmol l−1) was dissolved in 20 ml of ethanol to form solution B. The solution B was added to the solution A drop by drop with vigorous stirring for 20 min. The pH of the mixed solution was adjusted to 3.4 by adding HNO3 solution. The three-electrode system was employed for the electrodeposition step. The Fluorine-doped Tin(IV) oxide (FTO) conductive glass, successively cleaned with deionized water, acetone, and ethanol, was applied as the working electrode. The Pt sheet and saturated Ag/AgCl electrode were employed as the counter electrode and the reference electrode, respectively. An initial potential of −0.35 V vs Ag/AgCl for 30 s and then another potential of −0.1 V vs Ag/AgCl for 3 min were applied to electrodeposit the BiOI precursor film. After that, 50 μmol of dimethyl sulfoxide containing VO(acac)2 (0.2 mol l−1) was dripped on the BiOI film. The precursor was calcined at 450 °C for 2 h with a heating rate of 2 °C min−1 to fabricate the BiVO4 photoelectrode. After cooling, the BiVO4 photoelectrode was soaked in the NaOH solution (0.5 mol l−1) for 30 min to remove the surface V2O5, and then rinsed with deionized water.

Monomeric pyrrole (0.01 mol l−1) was added into 50 ml of NaCl (0.1 mol l−1) solution with HCl solution adjusting the pH to 3. The BiVO4 photoelectrode was soaked in the above solution and applied to cyclic voltammetry (CV) electrodeposition (potential range: −1.1–1.1 V vs Ag/AgCl, scanning rate: 25 mV s−1), and then rinsed with deionized water.

CoPi was photo-electrodeposited on the PPy/BiVO4 film in a Co(NO3)2 (0.5 mmol l−1) solution, whose pH was adjusted to 7 by the phosphate buffer solution (0.1 mol l−1 of K2HPO4 and 0.1 mol l−1 of KH2PO4). The PPy/BiVO4 electrode was irradiated by a Xe lamp with an AM 1.5G filter (calibrating to 100 mW cm−2 on the photoelectrode surface) for 500 s with an electrodeposition potential of 0.4 V vs Ag/AgCl.

Photoelectrochemical measurements based on the three-electrode system were carried out on the electrochemical workstation (CHI 760E), with the modified BiVO4 photoelectrode, a Pt sheet, and a saturated Ag/AgCl electrode as the working, counter, and reference electrodes, respectively. The K2HPO4 (0.5 mol l−1) and KH2PO4 (0.5 mol l−1) buffer solution (pH = 7) was employed as the electrolyte. The simulated solar radiation during the measurements was provided by a Xe lamp (CHF-XM-500 W) coupled with an AM 1.5 G irradiation filter calibrated to 100 mW cm−2 (1 sun) on the photoelectrode surface. A NIR laser (MDL-H-808-5 W) was applied to investigate the photothermal performance of the modified BiVO4 photoelectrodes. Detailed test methods and characterization equipment were summarized in the supplementary material.

The scanning electron microscopy (SEM) images of the as-prepared samples are depicted in Fig. 1(a). From the top-view images, PPy/BiVO4 and CoPi/PPy/BiVO4 presented the identical wormlike morphology as BiVO4. The sheet-like morphology of PPy layers coated on the BiVO4 surface could be observed. According to the cross-sectional SEM images, the film thickness increased from ∼570 to ∼700 nm after PPy modification, further reaching ∼770 nm with the coated CoPi co-catalyst. The corresponding elemental mapping indicated that the C, N, Bi, V, O, Co, and P were uniformly dispersed in CoPi/PPy/BiVO4 (Fig. S1). A high-resolution TEM (HRTEM) image of the PPy/BiVO4 sample suggested tight contact between the crystalline BiVO4 and the amorphous PPy polymer [Fig. 1(b)]. The interplanar lattice spacing was 0.315 nm, which corresponded to the (112) crystallographic plane of monoclinic BiVO4. The X-ray diffraction (XRD) patterns of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 are shown in Fig. 1(c). The dominant diffraction peak was located at 28.91°, which corresponded to the (112) face of the monoclinic phase BiVO4 (PDF 01-075-1866). The XRD patterns of PPy/BiVO4 and CoPi/PPy/BiVO4 presented an extra broad peak at around 26°, which suggested that the PPy was deposited on the BiVO4 surface.31 The ultraviolet-visible (UV-Vis) absorption spectra are depicted in Fig. S2(a). The absorption onsets of all samples were located at ∼500 nm, and the deposition of PPy could dramatically improve the light absorption beyond 500 nm. The corresponding Tauc plots indicated that there was no significant change in the bandgap of the BiVO4 film after loading the PPy layer [Fig. S2(b)].

FIG. 1.

(a) Top-view and cross-sectional SEM images of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 films; (b) HRTEM image of a PPy/BiVO4 sample; and (c) XRD patterns of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 films.

FIG. 1.

(a) Top-view and cross-sectional SEM images of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 films; (b) HRTEM image of a PPy/BiVO4 sample; and (c) XRD patterns of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 films.

Close modal

The X-ray photoelectron spectroscopy (XPS) test of the as-prepared samples was carried out to determine the surface composition and valence states. As shown in Fig. 2(a), The Bi 4f7/2 peak shifted from 159.08 eV of the pristine BiVO4 to 159.24 eV of the PPy/BiVO4 and further to 159.36 eV of the CoPi/PPy/BiVO4. The accordingly enhanced binding energy suggested that the PPy and CoPi had close electronic interaction with BiVO4. According to Figs. 2(b) and 2(c), the C–N bond of PPy with C 1s peak at 286.4 eV and N 1s at 400.0 eV were observed. The C=O bond with C 1s peak at 288.4 eV further demonstrated the effective coating of PPy on BiVO4. For CoPi/PPy/BiVO4, the single P 2p peak was detected at 133.4 eV [Fig. S3(a)]. Co 2p3/2 consisted of a primary peak at 780.9 eV and a broad satellite peak at 785.6 eV, while Co 2p1/2 consisted of a primary peak at 796.2 eV and a broad satellite peak at 804 eV [Fig. S3(b)].32 

FIG. 2.

XPS spectra of (a) Bi 4f of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4, (b) C 1s of CoPi/PPy/BiVO4, and (c) N 1s of CoPi/PPy/BiVO4.

FIG. 2.

XPS spectra of (a) Bi 4f of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4, (b) C 1s of CoPi/PPy/BiVO4, and (c) N 1s of CoPi/PPy/BiVO4.

Close modal

The light absorption ability of PPy was confirmed according to the UV–Vis–NIR absorption spectrum (Fig. S4). The PPy film had a relatively high absorbance (∼0.6) in the NIR spectral region. Therefore, an 808 nm laser was applied to distinguish the photothermal performance of the PPy modified BiVO4 films. Infrared thermal images of BiVO4 and PPy/BiVO4 films irradiated by the Xe lamp and NIR laser in the air are depicted in Fig. 3(a). Samples without NIR irradiation had a nearly equal temperature elevation (from room temperature to ∼30 °C), which was caused by the direct Xe lamp illumination. After 120 s of NIR irradiation, the PPy/BiVO4 film surface temperature was increased to 44.6 °C, distinctly higher than that of the BiVO4 film (34.5 °C), which demonstrated that the PPy layer could effectively convert infrared light into thermal energy. The temperatures of the BiVO4 and PPy/BiVO4 electrodes in the electrolyte were detected by the thermocouple in the electrochemical measurement cell. Due to the light absorption and cooling effect of the electrolyte, the surface temperatures of BiVO4 and PPy/BiVO4 electrodes in the solution were decreased compared to those in the air (Fig. S5). However, the PPy/BiVO4 electrode, with a higher surface temperature, had a less rapid decline (from 44.6 to 41.6 °C) compared to the BiVO4 electrode (from 35.4 to 27.6 °C). This phenomenon suggested that the PPy layer maintained good photothermal conversion ability in the electrolyte.

FIG. 3.

(a) Infrared thermal images of BiVO4 and PPy/BiVO4 films under an intensity of 100 mW cm−2 Xe lamp illumination (with and without 120 s of NIR irradiation before photographing). (b) Photocurrent density-applied potential curves and (c) IPCE curves of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 photoelectrodes (measured at 1.23 V vs RHE). Reaction conditions: 100 ml of an aqueous solution containing 0.5 mol l−1 of K2HPO4 and 0.5 mol l−1 of KH2PO4; a Xe lamp with an AM 1.5 G irradiation filter calibrated to an intensity of 100 mW cm−2 on the photoelectrode surface; 120 s of NIR irradiation for the “NIR-” samples.

FIG. 3.

(a) Infrared thermal images of BiVO4 and PPy/BiVO4 films under an intensity of 100 mW cm−2 Xe lamp illumination (with and without 120 s of NIR irradiation before photographing). (b) Photocurrent density-applied potential curves and (c) IPCE curves of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 photoelectrodes (measured at 1.23 V vs RHE). Reaction conditions: 100 ml of an aqueous solution containing 0.5 mol l−1 of K2HPO4 and 0.5 mol l−1 of KH2PO4; a Xe lamp with an AM 1.5 G irradiation filter calibrated to an intensity of 100 mW cm−2 on the photoelectrode surface; 120 s of NIR irradiation for the “NIR-” samples.

Close modal

The photocurrent density-applied potential curves were obtained to investigate the PEC performance [Fig. 3(b)]. The pristine BiVO4 exhibited a photocurrent density of 1.01 mA cm−2 at 1.23 V vs the reversible hydrogen electrode (RHE). After coating the PPy layer, the photocurrent density at 1.23 V vs RHE increased to 1.73 mA cm−2 and was further raised to 2.31 mA cm−2 with the irradiation of NIR light. For comparison, the photocurrent density of a pristine BiVO4 photoanode reached 1.57 mA cm−2 at 45 °C (Fig. S6), which revealed that the PPy layer acted as more than an IR-induced “heater” in promoting the PEC performance. With the assistance of the CoPi co-catalyst, the photocurrent density at 1.23 V vs RHE was further improved to 3.64 and 3.22 mA cm−2 with and without NIR irradiation, respectively [Fig. 3(b)].

Based on Fig. S7, the Faraday efficiencies of H2 and O2 evolution during PEC water electrolysis were calculated as 97.5% and 96.2%, respectively. Moreover, the onset potential had a negative shift of 0.2 V after loading CoPi, indicating improved surface reaction kinetics. Under the NIR irradiation, the current density of the CoPi/PPy/BiVO4 photoanode had a 24% reduction after 2400 s of measurement (Fig. S8), which was slightly better than the bare BiVO4 photoanode (30% of decrease). The long-term test demonstrated that the stability of the as-prepared photoelectrode was mainly determined by the pristine BiVO4. As a result, the incident photon-to-current conversion efficiency (IPCE) had a significant improvement with the modification of PPy and CoPi [Fig. 3(c)]. The IPCE of CoPi/PPy/BiVO4 was 46% at 430 nm and was further increased to 63% with the assistance of NIR light irradiation.

Photocurrent density-applied potential curves using Na2SO3 as the hole scavenger in the electrolyte were measured to calculate the charge separation efficiency ηsep and charge injection efficiency ηinj (Fig. S9). As shown in Figs. 4(a) and 4(b), with the coating of PPy, the ηsep increased slightly, while the ηinj had a 41% enhancement, which suggested that the promoted PEC performance was mainly ascribed to the faster charge transfer capability in the conducting polymer layer. The ηinj further increased with the loading of the CoPi co-catalyst, indicating a significantly enhanced level of water oxidation activity, especially at low applied potential. Moreover, the increased ηinj inhibited surface recombination caused by surface hole accumulation, which in turn had a further promotion on the ηsep.33,34 After adding the NIR light irradiation, the generated photothermal effect of PPy enhanced both ηsep and ηinj. NIR irradiation slightly enhanced the ηinj of the CoPi/PPy/BiVO4 photoanode, which was ascribed to the improved OER kinetics under increased reaction temperature.

FIG. 4.

(a) Charge separation efficiencies, (b) charge injection efficiencies, and (c) EIS curves of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 photoelectrodes. (d) Schematic diagram of charge carrier transfer between BiVO4 and PPy under simulated solar and NIR light irradiation.

FIG. 4.

(a) Charge separation efficiencies, (b) charge injection efficiencies, and (c) EIS curves of BiVO4, PPy/BiVO4, and CoPi/PPy/BiVO4 photoelectrodes. (d) Schematic diagram of charge carrier transfer between BiVO4 and PPy under simulated solar and NIR light irradiation.

Close modal

The electrochemical impedance spectroscopy (EIS) analysis suggested that, with coating PPy, the electrode bulk resistance (Rbulk) and interfacial charge transfer resistance (Rct) had a 36% and 68% reduction, respectively [Fig. 4(c) and Table S1]. Therefore, the conductive polymer layer could facilitate the hole transfer from the BiVO4 bulk to the photoanode/electrolyte interface, which corresponded to the increased ηsep and ηinj of the PPy/BiVO4 photoelectrode. The negative shift in the onset potential of the CoPi/PPy/BiVO4 photoanode should be ascribed to the enhanced water oxidation kinetics promoted by the CoPi cocatalyst, which was reflected in the greatly reduced Rct. Adding NIR irradiation to the CoPi/PPy/BiVO4 photoanode mainly reduced the Rbulk, suggesting that increased temperature enhanced the carrier transfer capability in the photoelectrode. Based on the above analysis, the charge carrier transfer in the PPy/BiVO4 photoanode was obtained [Fig. 4(d)]. The photogenerated holes moving from the BiVO4 to the PPy layer were dramatically accelerated due to the high conductivity of the PPy conducting polymer. The loading of CoPi co-catalyst accelerated the water oxidation kinetics, which mitigated the surface accumulation of holes. Combined with enhanced charge separation and injection efficiencies from the increased electrode temperature induced by photothermal conversion, the PPy modification could dramatically improve the PEC performance of the BiVO4 photoelectrode.

In this work, we developed a PPy photothermal layer modified BiVO4 photoelectrode for water oxidation. Compared to the pristine BiVO4, the PPy/BiVO4 photoanode exhibited remarkable improvements in PEC performance. The PPy layer could effectively convert IR irradiation into thermal energy to elevate the photoelectrode temperature, which could enhance charge separation and water oxidation properties. In addition, the conductive polymer PPy significantly accelerated the hole transfer from BiVO4 to the electrode/electrolyte interface. The PEC performance had a further improvement with the loaded CoPi co-catalyst, realizing an IPCE of 63% at 430 nm. This work can provide some fundamental understanding of the role of photothermal materials in PEC water splitting and offer a new horizon into the design of efficient photoelectrodes.

See the supplementary material for sample characterization equipment, test methods, and additional experimental data.

The authors would like to thank the financial support from the National Natural Science Foundation of China (Grant No. 52076177), the China National Key Research and Development Plan Project (Grant No. 2021YFF0500503), the National Natural Science Foundation of China (Grant No. 52106259), the Postdoctoral Science Foundation (Grant No. 2021M692005), the Sichuan Science and Technology Program (Grant No. 2021YFSY0047), and the China Fundamental Research Funds for the Central Universities.

The authors have no conflicts to disclose.

J.W. and X.X. contributed equally to this work.

Jiazhe Wu: Data curation (lead); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Xiaoya Xu: Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Xu Guo: Investigation (equal). Wensheng Xie: Investigation (equal). Lixia Pan: Investigation (equal). Yubin Chen: Data curation (lead); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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