Materials For Photoelectrochemical Water Splitting Research Articles

Super-hydrophilic electrode encapsulated lead halide-perovskite photoanode toward stable and efficient photoelectrochemical water splitting

Lead halide perovskites with excellent optoelectronic properties have been recognized as one of the most promising photoelectrode materials for photoelectrochemical water splitting during the past decade. However, fabricating stable and efficient halide-perovskite photoanodes remains a considerable challenge due to the degradation of halide perovskites in electrolyte environment and the lack of effective encapsulating techniques. Herein, we report a unique halide-perovskite photoelectrochemical cell by using an inverted fluorine-doped tin oxide coated glass (FTO/glass) as the waterproof hole-transport layer for encapsulation, and inserting a two-dimensional (2D) perovskite layer for passivation. The halide-perovskite photoanode achieves high-efficiency oxygen evolution reaction, fast bubble releasing and an outstanding long-term stability of 30.9 h at 90 % of the initial photocurrent density (up to 16.55 mA cm−2 at 1.23 V versus the reversible hydrogen electrode) under solar illumination. Our unique and cost-effective encapsulation method could be a potential solution for realizing stable and efficient halide-perovskite photoelectrodes for solar-fuel energy conversion.

Supply risk considerations for photoelectrochemical water splitting materials

Absorber materials for photoelectrochemical water splitting have supply risks emerging from supply, demand, concentration, and political risks.

Strategies for Sustainable Fuel Production Via Photoelectrochemical Synthesis

Developing sustainable energy is highly imperative for humankind owing to the drastic rise in world population and the proportional rise in energy demand at the expense of energy consumption. Various efforts have been made to explore, generate, store, and utilize renewable energy to replace fossil fuels. Among them, solar energy is considered an alternative to fossil fuels because it is inexhaustible, clean, cost-effective, and versatile. Interestingly, this prime source provides massive energy that is high enough (1.9 × 108 TWh/yr) to counterattack global energy consumption (1.3 × 105 TWh/yr).Photoelectrochemistry has been considered a promising route to harness this incredible energy by producing sustainable energy fuels and carriers such as hydrogen, hydrocarbon, and ammonia. PEC systems have the great advantages of simple process steps and very low environmental burdens. However, this technology must overcome many challenges regarding commercial applications, particularly in fabricating electrode materials for PEC water splitting.One of the most important challenges to overcome this problem is discovering efficient catalysts for implementing photoelectrochemical (PEC) fuel production. A critical requirement for outstanding catalysts is an ability to boost the kinetics of a chemical reaction and durability against electrochemical and photo-induced degradation. Moreover, the charge-separation and transfer mechanism are the two paramount properties to consider while designing photoelectrodes. Briefly, the semiconductor materials to be used in PEC water splitting must possess a conduction band potential more negative than the reduction edge of hydrogen and a valence band potential positive to actuate water oxidation. In practice, the photocurrent density and solar-to-hydrogen (STH) efficiency of the n-type semiconductors are far behind the theoretical values.To address this critical and long-standing technical barrier, I have focused on an intense search for efficient, durable, and inexpensive alternative catalysts with moderate band gap, efficient charge excitation-separation property, and high solar energy conversion efficiency for photoelectrochemical systems of water splitting, hydrocarbon conversion, and ammonia production. Keywords : Photoelectrochemistry, Sustainable Energy, Water Splitting, Ammonia synthesis, Urea synthesis, Hydrocarbon

Development of octahedral shaped Zn2TiO4 loaded Ti3C2-TiO2 ternary composite with excellent photocatalytic efficiency

The development of highly efficient and stable earth-abundant photoanode materials for photoelectrochemical water splitting is crucial for a sustainable energy economy. Being earth-abundant, 2D Ti3C2 MXenes have recently emerged as promising candidate for efficient photocatalytic performances. However, pristine Ti3C2and its composites suffer from poor electron–hole separation and fail to prevent the spontaneous recombination process due to the poor conductivity derived from the serious agglomeration of MXene sheets during processing. Therefore, suitable heterojunction engineering of the MXene based composites is required for their efficient photocatalytic performances. Hence, in this work, we have developed a Ti3C2-TiO2 and octahedron-shaped, nanosized Zinc titanate (Zn2TiO4) based ternary nanocomposite with optimized composition via a simple process of alkalization followed by hydrothermal. As-synthesized Ti3C2-TiO2/Zn2TiO4 (1:0.5) nanocomposite shows a 3.7 fold augmentation in photocurrent density as compared to alkali treated Ti3C2-TiO2 at a potential of 0.9 V vs Ag/AgCl resulted due to the facile charge transfer evidenced from its impedance analysis having lowest charge transfer resistance. Furthermore, the Mott-Schottky measurements reveal that the as-synthesized nanocomposites possess n-type semiconductivity and the charge carrier concentration of Ti3C2-TiO2/Zn2TiO4 (1:0.5) is almost 5.2 times higher than that of alkali-treated Ti3C2-TiO2. This work may inspire more excellent work on developing MXenes-based photoanodes.

Improving the photovoltage of Cu2O photocathodes with dual buffer layers

Cuprous oxide (Cu2O) is a promising oxide material for photoelectrochemical water splitting (PEC), and increasing its photovoltage is the key to creating efficient overall PEC water-splitting devices. Previous reports are mostly focused on optimizing the energy band alignment between Cu2O and the n-type buffer layer to improve the photovoltage of Cu2O photocathodes. However, the band alignment between the n-type buffer layer and the protective layer is often ignored. In this work, Cu2O photocathodes with a single buffer layer (Ga2O3) and dual buffer layers (Ga2O3/ZnGeOx) are fabricated, and their PEC performances are compared. Results show that after inserting the second buffer layer (ZnGeOx), the onset potential of the Cu2O photocathode increases by 0.16 V. Operando electrochemical impedance spectroscopy measurements and analysis of the energy-level diagrams of each layer show that an energy level gradient between Ga2O3 and TiO2 is created when ZnGeOx is introduced, which eliminates the potential barrier at the interface of Ga2O3/TiO2 and improves the photovoltage of the Cu2O photocathode. Our work provides an effective approach to improve the photovoltage of photoelectrodes for solar water splitting by introducing dual buffer layers.

Mn-Modified ZnO Nanoflakes for Optimal Photoelectrochemical Performance Under Visible Light: Experimental Design and Theoretical Rationalization.

Doping of zinc oxide (ZnO) with manganese (Mn) tunes midbandgap states of ZnO to enhance its optical properties and makes it into an efficient photoactive material for photoelectrochemical water splitting, waste removal from water, and other applications. We demonstrate that ZnO modified with 1 at. % Mn exhibits the best performance, as rationalized by experimental, structural, and optical characterization and theoretical analysis. ZnO doped with the optimal Mn content possesses improved light absorption in the visible region and minimizes charge carrier recombination. The doping is substitutional and creates midgap states near the valence band. Mn atoms break localized charge traps at oxygen vacancy sites and eliminate photoluminescence peaks associated with oxygen vacancies. The optimal performance of Mn-modified ZnO is demonstrated with the photodegradation of Congo red and photoelectrochemical water splitting.

Cobalt Oxide-Coated Single Crystalline Bismuth Vanadate Photoanodes for Efficient Photoelectrochemical Chlorine Generation.

Bismuth vanadate (BiVO4) is an outstanding photoanode material for photoelectrochemical water splitting. In this work, a series of single crystalline BiVO4 photoanodes are synthesized by pulsed laser deposition (PLD). Once coated with a thin layer of cobalt oxide (CoOx) cocatalyst, also by PLD, the photoanodes support efficient photoelectrochemical generation of chlorine (Cl2) from brine under simulated solar light. The activity of the chlorine generation reaction (ClER) is optimized when the thickness of CoOx is about 3 nm, with the faradic efficiency of ClER exceeding 60%. Detailed studies show that the CoOx cocatalyst layer is amorphous, uniform in thickness, and chemically robust. As such, the cocatalyst also effectively protects the underlying BiVO4 photoanodes against chlorine corrosion. This work provides insights into using artificial photosynthesis for byproducts that carry significant economic value while avoiding the energetically expensive oxygen evolution reactions.

Visible light sensitized graphitic carbon nitride based semiconducting materials for photoelectrochemical water splitting

Semiconductor based photoelectrochemical water splitting technology directly harvests solar energy into fuels with environment friendly byproducts. The semiconducting material implemented in this technology should be highly photoactive, chemically stable, nontoxic, inexpensive and environmentally benevolent. Even though, various semiconducting materials have been implemented for this purpose, graphitic carbon nitride (GCN) has emerged as an efficient electrode material for water splitting through photoelectrochemical (PEC) route to generate sustainable hydrogen energy owing to its appropriate band position for water reduction reaction, low cost, visible light response, good chemical stability and non-toxicity. However, the practical application of GCN is hindered for PEC water splitting because of numerous challenges, such as its poor electrical conduction ability and high recombination rate. In this paper, we address the fundamental principles for PEC splitting of water with respect to GCN. The broad spectrum of modification strategies for the improvement of GCN based photoelectrodes including heteroelement doping, heterojunction construction, defect engineering and morphology control are also explored. These modification strategies made GCN an efficient material for photoelectrochemical water splitting. The maximum photocurrent density was reported as 320µA cm−2. Conclusively, the crucial challenges and probable solutions for forthcoming advancement in PEC water splitting by GCN based photoelectrodes are highlighted.

A novel α-Fe2O3 photoanode with multilayered In2O3/Co–Mn nanostructure for efficient photoelectrochemical water splitting

Hematite (α-Fe2O3) is considered one of the most promising materials for PEC water splitting under solar light. However, the drawbacks of lower charge transfer efficiency and slow oxygen evolution reaction (OER) kinetics seriously limited the practical application of α-Fe2O3 photoanodes. In this work, a multilayer structure modified α-Fe2O3 photoanode is proposed. Firstly, the In2O3 nanolayers were loaded onto the surface of α-Fe2O3 nanorods, significantly increasing the photoelectrochemical (PEC) water oxidation activity. The heterojunction form by the In2O3 passivation layer with α-Fe2O3 effectively promotes charge separation. Next, an electrodeposition method deposited a nanosheet combining ultrathin non-crystalline Co(OH)x and Mn3O4 nanocrystals (Co–Mn) on the α-Fe2O3 thin film. As an efficient co-catalyst, the Co–Mn nanosheets assisted the performance of α-Fe2O3 PEC water oxidation. The optimized α-Fe2O3–In2O3–Co(OH)x-Mn3O4 photoanode produces a high photocurrent density of 6.5 mA/cm2 at 1.23 V (vs. RHE), a maximum IPCE of 57.9% at 400 nm can reach.

Investigation of p-CuNb2O6 for use as photocathodes for photoelectrochemical water splitting

P-type copper niobate (CuNb2O6) was prepared for the first time by a two-step spray pyrolysis method on FTO glass substrates. Its feasibility in photoelectrochemical (PEC) decomposition water was systematically evaluated. The physical and photophysical properties of the thin film, including band edge position, bandgap energy, carrier lifetime, and incident photo-to-current efficiency (IPCE), were characterized and analyzed. The optical band gap of the prepared gray CuNb2O6 film is ∼1.41 eV, which indicates that it can absorb visible light and achieve sunlight-driven water splitting. The photocurrent density under simulated sunlight at AM1.5 was 0.25 mA/cm2, and the IPCE value could reach up to 3% at 400 nm in an aqueous solution (pH = 6.8) when applying a bias of 0.4 V vs. RHE. Despite exhibiting a good optical band gap and photocurrent density, the fabricated p-CuNb2O6 thin films remained unstable under illumination in aqueous solutions due to the reduction reaction of copper. Based on these findings, this study provides a new opinion on the applicability of CuNb2O6 as a photocathode material for PEC water splitting.

Enhanced photoelectrocatalytic performance of LaNiO3 sensitized with CdSe quantum dots for photoelectrochemical water splitting

CdSe Quantum Dots (QDs) are promising photocatalyst in the visible region and oxide perovskites like LaNiO3 (LNO) are one of the best materials for photoelectrochemical water splitting. In this work, we have successfully synthesized the LNO@CdSe hetero-junction by the impregnation method. Formation of LNO@CdSe hetero-junction was characterized by using physicochemical techniques such as XRD, UV-DRS, FE-SEM, XPS and HRTEM. These results confirmed the successful formation of the LNO@CdSe hetero-junction. Photoanodes were fabricated by the drop-casting method to perform their photoelectrochemical properties toward water splitting. LNO@CdSe hetero-junction exhibits photocurrent density with 1.11 mA/cm2 at 0.8 V vs. SCE, which is a 4.62-fold enhancement compared to bare LNO. The enhanced photoelectrochemical properties is due to the formation of hetero-junction with visible light absorbing CdSe QDs. Besides, LNO absorbs the light in the near infra-red region. Hence, the LNO@CdSe hereto-junction absorbs the light from the visible to near infra-red region. In addition to that, the band alignment of the hetero-junction is favourable for the transformation of photogenerated electrons from the conduction band of CdSe QDs to the conduction band of the LNO, which results in the lesser recombination of the electron/hole pairs. This work suggests, this material can be a potential candidate in near future for green hydrogen and oxygen production using solar energy.

Mn-doped ZnS nanoparticle photoanodes: Synthesis, structural, optical, and photoelectrochemical characteristics

In this work, Mn-doped zinc sulfide (Mn:ZnS) nanoparticles (NPs) have been synthesized as a promising material for photoelectrochemical water splitting (PEC), using the co-precipitation method. PEC properties of Mn-doped zinc sulfide NPs were considered under the correlation between Mn-doping level and their particle size. The highest photocurrent density (8.03 mA cm−2) and largest photoconversion efficiency (0.63%) (at 0.4 V vs. RHE) were reached at 6 mol% Mn. Based on the results of used various materials characterization techniques, including transmission electron microscopy (TEM), X-ray diffraction, photoluminescence spectrum, and absorbance spectrum, it can be assessed that the outstanding PEC characteristics of Mn:ZnS photoanode are attributed to the narrow bandgap of Mn:ZnS nanoparticles and their notably small particle size, which is originated from the Mn-doping. For application, the stability and the effect of various electrolytes were also investigated.

Charge Transport Enhancement in BiVO4 Photoanode for Efficient Solar Water Oxidation.

Photoelectrochemical (PEC) water splitting in a pH-neutral electrolyte has attracted more and more attention in the field of sustainable energy. Bismuth vanadate (BiVO4) is a highly promising photoanode material for PEC water splitting. Additionally, cobaltous phosphate (CoPi) is a material that can be synthesized from Earth's rich materials and operates stably in pH-neutral conditions. Herein, we propose a strategy to enhance the charge transport ability and improve PEC performance by electrodepositing the in situ synthesis of a CoPi layer on the BiVO4. With the CoPi co-catalyst, the water oxidation reaction can be accelerated and charge recombination centers are effectively passivated on BiVO4. The BiVO4/CoPi photoanode shows a significantly enhanced photocurrent density (Jph) and applied bias photon-to-current efficiency (ABPE), which are 1.8 and 3.2 times higher than those of a single BiVO4 layer, respectively. Finally, the FTO/BiVO4/CoPi photoanode displays a photocurrent density of 1.39 mA cm-2 at 1.23 VRHE, an onset potential (Von) of 0.30 VRHE, and an ABPE of 0.45%, paving a potential path for future hydrogen evolution by solar-driven water splitting.

Charge kinetics evaluation of Fe3O4/V2O5 nanowires under visible light for energy conversion applications

In this study, surface-modified bundled V2O5 nanobelts with Fe3O4 nanoparticles were synthesized and their photoelectrochemical properties were recorded under illumination by light. The effects of the electrolyte on the heterostructured photoanode, structural and optical properties, as well as charge kinetic behavior of the photoelectrode were investigated. A combination of V2O5 and Fe3O4 nanostructures exhibited better light absorption than pristine V2O5 and Fe3O4 samples. With V2O5–Fe3O4, an electron-hole transfer resistance of 24.13 Ω and ion-path resistance of 109.8 Ω were achieved in a 0.1 M Na2SO3 electrolyte under irradiation. These values were lower than those obtained with pure V2O5 and Fe3O4 nanostructures. The photocurrent density of the V2O5–Fe3O4 photoanode considerably increased in Na2SO3 electrolytes compared to those of the others due to greater active sites in the photoanode and considerable improvements in the charge transfer resistance, limiting current density, and Tafel slope. Thus, V2O5–Fe3O4 photoanodes were established to be durable and stable materials for photoelectrochemical water splitting.

Comprehensive evaluation of copper vanadate (α-CuV2O6) for use as a photoanode material for photoelectrochemical water splitting

This study is an initial account of solution-based spray pyrolysis-produced n-type copper vanadate (CuV2O6) thin film photoanodes. CuV2O6 photoelectrodes have been tested for their ability to oxidize water in aqueous (pH 6.8) solutions using photoelectrochemical (PEC) tests. The band edge positions, band gap energy, photocurrent generation, flat band potential, chemical make-up, chemical stability, charge carrier transit, and O2 production faradic efficiency are a few of the physical and photophysical features measured and described. According to the findings, CuV2O6 possesses a satisfactory bandgap of 1.9 eV and appropriate band locations that allow for the use of visible light for overall solar water splitting. Under AM1.5 simulated sunlight, the maximum photocurrent density recorded is 0.18 mA cm− 2 at 1.23 VRHE with Na2SO3 acting as a hole scavenger. As evidenced by film shape, surface compositions before and after the PEC measurements, and outstanding structural and compositional stability throughout the oxygen evolution reaction (OER), the CuV2O6 photoelectrodes performed well. CuV2O6 photoanodes also exhibit almost a unit faradic efficiency for the oxidation of water. The overall photoelectrochemical efficiency, however, is constrained by the photogenerated charge carriers' weak mobility (∼8 ×10−3 cm2 V−1 s−1), short lifetime (∼20 ns), and limited hole diffusion length (∼20.4 nm), according to time-resolved microwave conductivity (TRMC) tests. Based on the current findings, this study offers new insight into building enhanced high-quality CuV2O6 photoanodes by methodically clarifying numerous important physical and photoelectrochemical characteristics for the photooxidation of water.

Semiconductors-based Z-scheme materials for photoelectrochemical water splitting: A review

Photoelectrochemical (PEC) water splitting is among the most promising approaches for a potentially clean and renewable source for hydrogen fuel from the splitting of water into hydrogen and oxygen using light source and energy conversion due to its practical efficiency. The innovative approach is the construction of a two-step photoexcitation system, often called a Z-scheme photocatalysis system, which mimics natural photosynthesis. The PEC splitting of water using various Z-scheme materials is initiated by the direct absorption of a photon, which creates separated electrons and holes in the energy band gap of the material. A Z-scheme system employs two photocatalysts, one producing H2 and the other O2, usually with the aid of an electron shuttle. This review focuses on the current state of research on nanoscale-enhanced Z-scheme photocatalyst materials for the water splitting reaction. Additionally, the recent advances achieved in especially metal oxides, metal sulfides, g-C3N4, and semiconductors-based Z-scheme materials as photocatalyst systems for water-splitting, are also covered.

Photoluminescence and lifetime studies of C-dot decorated CdS/ZnFe2O4 composite designed for photoelectrochemical applications

Cadmium sulphide (CdS) is a well-known material for photoelectrochemical water splitting as a photoanode. The valence and conduction band edges are well suited for both hydrogen and oxygen evolution reactions. However, a major drawback in the case of cadmium sulphide is its photocorrosion. In this regard, formation of a Type II heterostructure helps in well separation of the photogenerated charge carriers and reduces photocorrosion. Herein, we have incorporated a carbon dot layer (high conductivity) in between a CdS/ZnFe2O4 Type II composite heterostructure to further enhance the photoelectrochemical performance of the material. Steady state photoluminescence (PL) study revealed that maximum PL intensity is observed in the case of bare CdS while minimum intensity in the case of ternary composite after the incorporation of carbon dots. The inflation of PL intensity revealed the overall increase in the conductivity and enhanced charge separation at the interface after composite formation. The time-resolved photoluminescence (TRPL) decay dynamics have also been studied. The lifetime of the charge carriers for the ternary composite was found to be 2.41 ns, which is much higher than the bare (0.78 ns) and binary composite (0.85 ns), indicating the improved charge separation and the stability after composite formation (CdS/C-dot/ZnFe2O4). This further aids in the enhancement of photocurrent generation. The photocurrent density is enhanced by approximately 50 times compared to the bare material (CdS) and 5 times compared to the binary composite (CdS/ZnFe2O4).

Spectroscopic Evidence of Intraband Gap States in α‐SnWO4 Photoanodes Introduced by Interface Oxidation

α‐SnWO4 is an emerging photoelectrode material for photoelectrochemical water splitting, with several promising properties such as the favorable bandgap of 1.9 eV and suitable positions of the valence and conduction band. However, a major challenge remains: unprotected α‐SnWO4 undergoes surface passivation that blocks further charge transfer, and α‐SnWO4 electrodes that are covered with a protection/catalytic overlayer (e.g., NiOx, CoOx) show limited photovoltage. Earlier studies reveal that interfacial oxidation occurs due to the deposition of the overlayer. This negatively impacts the photovoltage that can be extracted, which is attributed to Fermi‐level pinning at the interface. The exact origin of this Fermi‐level pinning mechanism, however, remains unclear. In the present study, a combination of surface photovoltage analysis and hard X‐ray photoelectron spectroscopy is used to elucidate the electronic structure of the α‐SnWO4/oxide interface. Both techniques offer compelling and consistent evidence for the presence of a defect state that is energetically located within the bandgap energy of α‐SnWO4 and is likely responsible for the Fermi‐level pinning.

Passivation of Hematite by a Semiconducting Overlayer Reduces Charge Recombination: An Insight from Nonadiabatic Molecular Dynamics.

Hematite (α-Fe2O3) is a promising photoanode material for photoelectrochemical water splitting. Surface-passivating layers are effective in improving water oxidation kinetics; however, the passivation mechanism is not fully understood due to the complexity of interfacial reactions. Focusing on the Fe-terminated Fe2O3 (0001) surface that exhibits surface states in the band gap, we perform ab initio quantum dynamics simulations to study the effect of an α-Ga2O3 overlayer on charge recombination. The overlayer eliminates surface states and suppresses charge recombination 4-fold. This explains in part the observed cathodic shift in the onset potential for water oxidation. The increased charge carrier lifetime is an outcome of two factors, energy gap and electron-vibrational coupling, with a positive contribution from the former but a negative contribution from the latter. This work presents an advance in the atomistic time-domain understanding of the influence of surface passivation on charge recombination dynamics and provides guidance for designing novel α-Fe2O3 photoanodes.

Two-dimensional materials for photoelectrochemical water splitting

This review summarizes the superiorities and utilizations of 2D materials for photoelectrochemical water splitting including transition metal dichalcogenides, graphene, graphdiyne, black phosphorus, layered double hydroxides, g-C3N4, and MXenes.