Spinel-Oxide-Integrated BiVO4 Photoanodes with Photothermal Effect for Efficient Solar Water Oxidation

MnCo2O4 as a Photothermal Modifier for BiVO4 Photoanode to Achieve Efficient Photoelectrochemical Water Oxidation

Spinel oxides are widely used to enhance photoanodes' photoelectrochemical (PEC) water oxidation performance. In this paper, we demonstrate that a p-type photothermal MnCo2O4 layer, inserted between BiVO4 and FeOOH cocatalysts, can significantly enhance PEC performance. On the one hand, the charge recombination is greatly suppressed since p-type MnCo2O4 can form a p-n heterojunction with n-type BiVO4. On the other hand, upon near-infrared (NIR) light irradiation, the deposited MnCo2O4 shows excellent photothermal conversion performance that can further facilitate charge transfer and accelerate the water oxidation process by elevating the operating temperature. In addition, the FeOOH cocatalyst is introduced to fully utilize the holes reaching the surface and further enhance the surface oxygen evolution kinetics. Consequently, the carefully constructed BiVO4/MnCo2O4/FeOOH photoanode shows excellent photocurrent (4.71 mA cm-2) at 1.23 V vs RHE (VRHE) and superior applied bias photon-to-current efficiency (1.62%) at 0.6 VRHE. The photocurrent density maintains more than 90% of the initial photocurrent after 4 h of combined solar and NIR light irradiation. Enhancing the PEC catalytic activity of photoelectrodes using photothermal materials is a simple and effective strategy, which can be used to explore PEC activity enhancement in other photoelectrodes.

Photothermal effect and hole transport properties of polyaniline for enhanced photoelectrochemical water splitting of BiVO4 photoanode

As modification strategies are actively developed, the photothermal effect is expected to be a viable way to enhance the PEC water splitting performance. Herein, we demonstrate that the photothermal polyaniline (PANI) layer inserted between CoF2 cocatalyst and BiVO4 can enhance the photocurrent density of pure BiVO4 by 3.50 times. The coated PANI layer exhibits excellent photothermal conversion and hole transport properties. Under near-infrared (NIR) light irradiation at 808 nm, PANI can raise the temperature of the photoanode in situ, thus promoting bulk charge transfer and broadening the light absorption range. After the CoF2 cocatalyst is deposited on the BiVO4/PANI surface, the water oxidation activity of the composite photoanode is also significantly enhanced due to the temperature elevation. In addition, density-functional theory (DFT) simulations demonstrate that BiVO4/PANI/CoF2 can dramatically reduce the energy barrier required for oxygen evolution reaction, accelerating the oxygen evolution kinetics. Under NIR light irradiation, the meticulously designed BiVO4/PANI/CoF2 (BPC) photoanode displays a photocurrent density of 4.34 mA cm-2 at 1.23 V vs. RHE (VRHE) with an excellent charge injection efficiency of 88.14 %. In addition, at 350 nm, the incident photon-to-current efficiency (IPCE) of the BPC photoanode reaches up to 60.45 %, which is much higher than that of pure BiVO4 (7.75 %). At 0.66 VRHE, the applied bias photon-to-current efficiency (ABPE) value of BPC photoanode can reach 1.37 %, which is 12.5 times higher than that of pure BiVO4. This simple and robust strategy provides a pathway to employ photothermal materials to design PEC water splitting photoanodes with excellent overall performance.

BiVO4 Film Coupling with CoAl2O4 Nanoparticles for Photoelectrochemical Water Splitting Utilizing Broad Solar Spectrum through p-n Heterojunction, Photothermal, and Cocatalytic Synergism

Water oxidation is an endothermic and kinetics-sluggish reaction; the research of photoanodes with photothermal and cocatalytic properties is of great significance. Herein, BiVO4/CoAl2O4 film photoanodes were studied for solar water splitting through coupling spinel p-type CoAl2O4 nanoparticles on n-type BiVO4 films. Compared to the BiVO4 photoanode, better performance was observed on the BiVO4/CoAl2O4 photoanode during water oxidation. A photocurrent of 3.47 mA/cm2 was produced on the BiVO4/CoAl2O4 photoanode at 1.23 V vs RHE, which is two-fold to the BiVO4 photoanode (1.70 mA/cm2). Additionally, the BiVO4/CoAl2O4 photoanodes showed an acceptable stability for water oxidation. The BiVO4/CoAl2O4 photoanode being of higher water oxidation performance could be attributed to the presence of p-n heterojunction, cocatalytic, and photothermal effects. In specific, under the excitation of λ < 520 nm light, the holes produced in/on BiVO4 can be transferred to CoAl2O4 owing to the p-n heterojunctions of BiVO4/CoAl2O4. Meanwhile, the temperature on the BiVO4/CoAl2O4 photoanode rises quickly up to ∼53 °C under AM 1.5 G irradiation due to the photothermal property of CoAl2O4 through capturing the 520 < λ < 720 nm light. The temperature rising on the BiVO4/CoAl2O4 photoanode improves the cocatalytic activity of CoAl2O4 and modifies the wettability of BiVO4/CoAl2O4 for effective water oxidation.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Promoting the electrocatalytic oxygen evolution reaction on NiCo2O4 with infrared-thermal effect: A strategy to utilize the infrared solar energy to reduce activation energy during water splitting

Utilization of the infrared (IR) solar energy remains a challenging task for traditional photo(electro)catalysis. Taking advantage of the IR-thermal effect to facilitate sluggish electrocatalytic reactions emerges as a promising way to utilize the IR band of the solar spectrum. In this work, nickel foam (NF) supported NiCo₂O₄ nanoneedles (NF/NiCo₂O₄ NNs) were prepared to promote the oxygen evolution reaction (OER) via the IR-thermal effect, with the NF/NiCo₂O₄ NNs acting as both the IR absorbing antennae and the OER active anode. The potential required to deliver a current density of 200 mA cm^-2 is negatively shifted from 1.618 V in the dark to 1.578 V under IR irradiation, and the Tafel slope is also decreased from 106 to 89 mV dec^-1. We demonstrate that the enhancement of OER activity is due to the localized temperature rise under IR irradiation. We measured the electrochemical activation energy of OER on NF/NiCo₂O₄ with and without IR irradiation, and the results reveal that IR irradiation reduces the kinetic energy barrier of the OER by IR-thermal effect and then facilitates OER kinetics. This work highlights a new approach to utilizing the IR portion of the sunlight to produce renewable hydrogen energy via water splitting.

Evaluating high temperature photoelectrocatalysis of TiO2 model photoanode

Sunlight concentration has been demonstrated as one promising strategy for practically photoelectrochemical (PEC) water splitting with exceeding 10% solar-to-hydrogen efficiency. However, the operating temperature of PEC devices, including the electrolyte and photoelectrodes, can be elevated to 65 ℃ naturally due to the concentrated sunlight and the thermal effect of near-infrared light. In this work, high temperature photoelectrocatalysis is evaluated using titanium dioxide (TiO₂) photoanode as a model system, which is believed to be one of the most stable semiconductors. During the studied temperature range of 25-65 ℃, a linear increment of photocurrent density with a positive coefficient of 5.02 μA cm^-2 K^-1 can be observed. The onset potential for water electrolysis shows a significant negative shift by 200 mV. An amorphous titanium hydroxide layer and a number of oxygen vacancies generate on the surface of TiO₂ nanorods, promoting the water oxidation kinetics. During long-term stability testing, the NaOH electrolyte degradation and TiO₂ photocorrosion at high temperatures could cause the decaying photocurrent. This work evaluates the high temperature photoelectrocatalysis of TiO₂ photoanode and reveals the mechanism of temperature effects on TiO₂ model photoanode.

Photoelectrocatalytic peroxymonosulfate activation over CoFe2O4-BiVO4 photoanode for environmental purification: Unveiling of multi-active sites, interfacial engineering and degradation pathways

This work reported on the development of CoFe₂O₄-BiVO₄ photoanode based photoelectrocatalytic system collaborating with peroxymonosulfate activation for organic contaminants removal. CoFe₂O₄ layer not only provided active sites for direct peroxymonosulfate activation but also accelerated charge separation process for the enhancement of photocurrent density and photoelectrocatalytic performance. Junction of CoFe₂O₄ layer on BiVO₄ photoanode led to the improvement of photocurrent density to 4.43 mA/cm² at 1.23 V_RHE, which was approximately 4.06 times higher than that of pure BiVO₄. Subsequently, the corresponding optimal degradation efficiency toward the tetracycline model contaminant achieved to be 89.1% with total organic carbon removal value of about 43.7% within 60 min. Moreover, the degradation rate constant of CoFe₂O₄-BiVO₄ photoanode in photoelectrocatalytic system was 0.037 min^-1, which was about 1.23, 2.64 and 3.70 times higher than the values in photocatalysis, electrocatalysis and PMS only based systems, respectively. In addition, radical scavenging experiments and electron spin resonance spectra indicated a synergy of radical and nonradical coupling process where •OH and ¹O₂ played vital roles during tetracycline degradation. Plausible photoelectrocatalytic mechanism and degradation pathway were proposed. This work provided an effective strategy to construct peroxymonosulfate assisted photoelectrocatalytic system toward green environmental applications.

Cobalt-plasma treatment enables structural reconstruction of a CoOx/BiVO4 composite for efficient photoelectrochemical water splitting

Cobalt-plasma sparked by a pulsed laser was utilized for the first time to modify a BiVO₄ photoanode, inducing surface structural reconstruction and CoO_x/BiVO₄ composite construction. Combining charge redistribution and cobalt, the center of hole accumulation and catalytic activity, the photoanode with optimized charge transfer capacity exhibits a 3 times improvement in photoelectrochemical water oxidation performance.

Junction of ZnmIn2S3+m and bismuth vanadate as Z-scheme photocatalyst for enhanced hydrogen evolution activity: The role of interfacial interactions

A series of Zn_mIn₂S_3+m photocatalysts were synthesized to show tunable band gap energy with the variation of Zn/S atomic ratio. The junction of Zn_mIn₂S_3+m and BiVO₄ led to intimate interfacial contacts. Both experimental and theoretical results implied that electrons flowed from Zn_mIn₂S_3+m to BiVO₄ at the Zn_mIn₂S_3+m/BiVO₄ interface to form built-in electric field due to the variation of Fermi level, which promised Z scheme charge transfer feature for improving separation of charge carriers for enhanced photocatalytic performance. A higher degree of charge transfer process occurred for Zn₂In₂S₅/BiVO₄ heterostructure promised stronger built-in electric field, higher charge separation efficiency and improved photocatalytic activity in comparison to ZnIn₂S₄/BiVO₄ and Zn₃In₂S₆/BiVO₄ heterojunctions. The optimal hydrogen production rate of Zn₂In₂S₅/BiVO₄ photocatalyst is 8.42 mmol•g^-1•h^-1 with apparent quantum yield of 22.32 % at 435 nm, which is about 2.2 and 1.5 times higher than that of ZnIn₂S₄/BiVO₄ and Zn₃In₂S₆/BiVO₄, respectively.