Dual Shield: Bifurcated Coating Analysis of Multilayered WO3/BiVO4/TiO2/NiOOH Photoanodes for Sustainable Solar-to-Hydrogen Generation from Challenging Waters

The heterostructure WO3/BiVO4-based photoanodes have garnered significant interest for photoelectrochemical (PEC) solar-driven water splitting to produce hydrogen. However, challenges such as inadequate charge separation and photocorrosion significantly hinder their performance, limiting overall solar-to-hydrogen conversion efficiency. The incorporation of cocatalysts has shown promise in improving charge separation at the photoanode, yet mitigating photocorrosion remains a formidable challenge. Amorphous metal oxide-based passivation layers offer a potential solution to safeguard semiconductor catalysts. We examine the structural, surface morphological, and optical properties of two-step-integrated sputter and spray-coated TiO2 thin films and their integration onto WO3/BiVO4, both with and without NiOOH cocatalyst deposition. The J-V experiments reveal that the NiOOH cocatalyst enhances the photocurrent density of the WO3/BiVO4 photoanode in water splitting reactions from 2.81 to 3.87 mA/cm2. However, during prolonged operation, the photocurrent density degrades by 52%. In contrast, integrated sputter and spray-coated TiO2 passivation layer-coated WO3/BiVO4/NiOOH samples demonstrate a ∼88% enhancement in photocurrent density (5.3 mA/cm2) with minimal degradation, emphasizing the importance of a strategic coating protocol to sustain photocurrent generation. We further explore the feasibility of using natural mine wastewater as an electrolyte feedstock in PEC generation. Two-compartment PEC cells, utilizing both fresh water and metal mine wastewater feedstocks exhibit 66.6 and 74.2 μmol/h cm2 hydrogen generation, respectively. Intriguingly, the recovery of zinc (Zn2+) heavy metals on the cathode surface in the mine wastewater electrolyte is confirmed through surface morphology and elemental analysis. This work underscores the significance of passivation layer and cocatalyst coating methodologies in a sequential order to enhance charge separation and protect the photoanode from photocorrosion, contributing to sustainable hydrogen generation. Additionally, it suggests the potential of utilizing wastewater in electrolyzers as an alternative to freshwater resources.

Photoelectrocatalytic Surfactant Pollutant Degradation and Simultaneous Green Hydrogen Generation

For the first time, we demonstrate a photoelectrocatalysis technique for simultaneous surfactant pollutant degradation and green hydrogen generation using mesoporous WO3/BiVO4 photoanode under simulated sunlight irradiation. The materials properties such as morphology, crystallite structure, chemical environment, optical absorbance, and bandgap energy of the WO3/BiVO4 films are examined and discussed. We have tested the anionic type (sodium 2-naphthalenesulfonate (S2NS)) and cationic type surfactants (benzyl alkyl dimethylammonium compounds (BAC-C12)) as model pollutants. A complete removal of S2NS and BAC-C12 surfactants at 60 and 90 min, respectively, by applying 1.75 V applied potential vs RHE to the circuit, under 1 sun was achieved. An interesting competitive phenomenon for photohole utilization was observed between surfactants and adsorbed water. This led to the formation of H2O2 from water alongside surfactant degradation (anode) and hydrogen evolution (cathode). No byproducts were observed after the direct photohole mediated degradation of surfactants, implying its advantage over other AOPs and biological processes. In the cathode compartment, 82.51 μmol/cm2 and 71.81 μmol/cm2 of hydrogen gas were generated during the BAC-C12 and S2NS surfactant degradation process, respectively, at 1.75 V RHE applied potential.

Facile Synthesis of Gram-Scale Mesoporous Ag/TiO2 Photocatalysts for Pharmaceutical Water Pollutant Removal and Green Hydrogen Generation

This work demonstrates a two-step gram-scale synthesis of presynthesized silver (Ag) nanoparticles impregnated with mesoporous TiO₂ and evaluates their feasibility for wastewater treatment and hydrogen gas generation under natural sunlight. Paracetamol was chosen as the model pharmaceutical pollutant for evaluating photocatalytic performance. A systematic material analysis (morphology, chemical environment, optical bandgap energy) of the Ag/TiO₂ photocatalyst powder was carried out, and the influence of material properties on the performance is discussed in detail. The experimental results showed that the decoration of anatase TiO₂ nanoparticles (size between 80 and 100 nm) with 5 nm Ag nanoparticles (1 wt %) induced visible-light absorption and enhanced charge carrier separation. As a result, 0.01 g/L Ag/TiO₂ effectively removed 99% of 0.01 g/L paracetamol in 120 min and exhibited 60% higher photocatalytic removal than pristine TiO₂. Alongside paracetamol degradation, Ag/TiO₂ led to the generation of 1729 μmol H₂ g^-1 h^-1. This proof-of-concept approach for tandem pollutant degradation and hydrogen generation was further evaluated with rare earth metal (lanthanum)- and nonmetal (nitrogen)-doped TiO₂, which also showed a positive response. Using a combination of ab initio calculations and our new theory model, we revealed that the enhanced photocatalytic performance of Ag/TiO₂ was due to the surface Fermi-level change of TiO₂ and lowered surface reaction energy barrier for water pollutant oxidation. This work opens new opportunities for exploiting tandem photocatalytic routes beyond water splitting and understanding the simultaneous reactions in metal-doped metal oxide photocatalyst systems under natural sunlight.

MXene coupled graphitic carbon nitride nanosheets based plasmonic photocatalysts for removal of pharmaceutical pollutant

The continuous rise in the amount of industrial and pharmaceutical waste in water sources is an alarming concern. Effective strategies should be developed for the treatment of pharmaceutical industrial waste. Hence the alternative renewable source of energy, such as solar energy, should be utilized for a sustainable future. Herein, a series of Au plasmonic nanoparticle decorated ternary photocatalysts comprising graphitic carbon nitride and Ti₃C₂ MXene has been designed to degrade colourless pharmaceutical pollutants, cefixime under visible light irradiation. These photocatalysts were synthesized by varying the amount of Ti₃C₂ MXene, and their catalytic potential was explored. The optimized photocatalyst having 3 wt% Ti₃C₂ MXene achieved 64.69% removal of the pharmaceutical pollutant, cefixime within 105 min of exposure to visible light. The presence of the Au nanoparticles and MXene in the nanocomposite facilitates the excellent charge carrier separation and increased the number of active sites due to the formation of interfacial contact with graphitic carbon nitride nanosheets. Besides, the plasmonic effect of the Au nanoparticles improves the absorption of light causing enhanced photocatalytic performance of the nanocomposite. Based on the obtained results, a plausible mechanism has been formulated to understand the contribution of different components in photocatalytic activity. In addition, the optimized photocatalyst shows excellent activity and can be reused for up to three cycles without any significant loss in its photocatalytic performance. Overall, the current work provides deeper physical insight into the future development of MXene graphitic carbon nitride-based plasmonic ternary photocatalysts.

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO₂ reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic–inorganic model system that utilizes host–guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO₂ inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO₃ at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO₂ concentration close to the metal interface. Electrocatalytic CO₂ reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO₂ reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H₂ production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host–guest chemistry.

ZnSe Nanorods as Visible-Light Absorbers for Photocatalytic and Photoelectrochemical H2 Evolution in Water

A precious-metal- and Cd-free photocatalyst system for efficient H₂ evolution from aqueous protons with a performance comparable to Cd-based quantum dots is presented. Rod-shaped ZnSe nanocrystals (nanorods, NRs) with a Ni(BF₄ )₂ co-catalyst suspended in aqueous ascorbic acid evolve H₂ with an activity up to 54±2 mmol H 2  g_ZnSe ^-1  h^-1 and a quantum yield of 50±4 % (λ=400 nm) under visible light illumination (AM 1.5G, 100 mW cm^-2 , λ>400 nm). Under simulated full-spectrum solar irradiation (AM 1.5G, 100 mW cm^-2 ), up to 149±22 mmol H 2  g_ZnSe ^-1  h^-1 is generated. Significant photocorrosion was not noticeable within 40 h and activity was even observed without an added co-catalyst. The ZnSe NRs can also be used to construct an inexpensive delafossite CuCrO₂ photocathode, which does not rely on a sacrificial electron donor. Immobilized ZnSe NRs on CuCrO₂ generate photocurrents of around -10 μA cm^-2 in an aqueous electrolyte solution (pH 5.5) with a photocurrent onset potential of approximately +0.75 V vs. RHE. This work establishes ZnSe as a state-of-the-art light absorber for photocatalytic and photoelectrochemical H₂ generation.

Aerobic Conditions Enhance the Photocatalytic Stability of CdS/CdOx Quantum Dots

Photocatalytic H₂ production through water splitting represents an attractive route to generate a renewable fuel. These systems are typically limited to anaerobic conditions due to the inhibiting effects of O₂ . Here, we report that sacrificial H₂ evolution with CdS quantum dots does not necessarily suffer from O₂ inhibition and can even be stabilised under aerobic conditions. The introduction of O₂ prevents a key inactivation pathway of CdS (over-accumulation of metallic Cd and particle agglomeration) and thereby affords particles with higher stability. These findings represent a possibility to exploit the O₂ reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilise photocatalysts that suffer from similar degradation reactions.

Solar Hydrogen Generation from Lignocellulose

Photocatalytic reforming of lignocellulosic biomass is an emerging approach to produce renewable H2 . This process combines photo-oxidation of aqueous biomass with photocatalytic hydrogen evolution at ambient temperature and pressure. Biomass conversion is less energy demanding than water splitting and generates high-purity H2 without O2 production. Direct photoreforming of raw, unprocessed biomass has the potential to provide affordable and clean energy from locally sourced materials and waste.

ZnSe quantum dots modified with a Ni(cyclam) catalyst for efficient visible-light driven CO2 reduction in water

A precious metal and Cd-free photocatalyst system for efficient CO2 reduction in water is reported. The hybrid assembly consists of ligand-free ZnSe quantum dots (QDs) as a visible-light photosensitiser combined with a phosphonic acid-functionalised Ni(cyclam) catalyst, NiCycP. This precious metal-free photocatalyst system shows a high activity for aqueous CO2 reduction to CO (Ni-based TONCO > 120), whereas an anchor-free catalyst, Ni(cyclam)Cl2, produced three times less CO. Additional ZnSe surface modification with 2-(dimethylamino)ethanethiol (MEDA) partially suppresses H2 generation and enhances the CO production allowing for a Ni-based TONCO of > 280 and more than 33% selectivity for CO2 reduction over H2 evolution, after 20 h visible light irradiation (λ > 400 nm, AM 1.5G, 1 sun). The external quantum efficiency of 3.4 ± 0.3% at 400 nm is comparable to state-of-the-art precious metal photocatalysts. Transient absorption spectroscopy showed that band-gap excitation of ZnSe QDs is followed by rapid hole scavenging and very fast electron trapping in ZnSe. The trapped electrons transfer to NiCycP on the ps timescale, explaining the high performance for photocatalytic CO2 reduction. With this work we introduce ZnSe QDs as an inexpensive and efficient visible light-absorber for solar fuel generation.

Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode

The development of high-performance electrocatalytic systems for the controlled reduction of CO₂ to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2′-bipyridine)(CO)₃] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO₂ under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO₂ reduction product at low catalyst loadings. Using UV–vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn⁰ species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

Metal-encapsulated organolead halide perovskite photocathode for solar-driven hydrogen evolution in water

Lead-halide perovskites have triggered the latest breakthrough in photovoltaic technology. Despite the great promise shown by these materials, their instability towards water even in the presence of low amounts of moisture makes them, a priori, unsuitable for their direct use as light harvesters in aqueous solution for the production of hydrogen through water splitting. Here, we present a simple method that enables their use in photoelectrocatalytic hydrogen evolution while immersed in an aqueous solution. Field's metal, a fusible InBiSn alloy, is used to efficiently protect the perovskite from water while simultaneously allowing the photogenerated electrons to reach a Pt hydrogen evolution catalyst. A record photocurrent density of −9.8 mA cm⁻² at 0 V versus RHE with an onset potential as positive as 0.95±0.03 V versus RHE is obtained. The photoelectrodes show remarkable stability retaining more than 80% of their initial photocurrent for ∼1 h under continuous illumination.

Lead-halide perovskites are sensitive to humidity, which limits their use in water splitting applications. Here, the authors protect the perovskite layer with Field's metal, driving photoelectrocatalytic hydrogen evolution in an aqueous solution for approximately one hour under constant illumination.

Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO

Formic acid is considered a promising energy carrier and hydrogen storage material for a carbon-neutral economy. We present an inexpensive system for the selective room-temperature photocatalytic conversion of formic acid into either hydrogen or carbon monoxide. Under visible-light irradiation (λ>420 nm, 1 sun), suspensions of ligand-capped cadmium sulfide nanocrystals in formic acid/sodium formate release up to 116±14 mmol H2 g(cat)(-1) h(-1) with >99% selectivity when combined with a cobalt co-catalyst; the quantum yield at λ=460 nm was 21.2±2.7%. In the absence of capping ligands, suspensions of the same photocatalyst in aqueous sodium formate generate up to 102±13 mmol CO g(cat)(-1) h(-1) with >95% selectivity and 19.7±2.7% quantum yield. H2 and CO production was sustained for more than one week with turnover numbers greater than 6×10(5) and 3×10(6), respectively.