Organic-inorganic hybrid-photoanode built from NiFe-MOF and TiO2 for efficient PEC water splitting

NiFe2O4 Material on Carbon Paper as an Electrocatalyst for Alkaline Water Electrolysis Module

NiFe2O4 material is grown on carbon paper (CP) with the hydrothermal method for use as electrocatalysts in an alkaline electrolyzer. NiFe2O4 material is used as the anode and cathode catalysts (named NiFe(+)/NiFe(-) hereafter). The results are compared with those obtained using CP/NiFe as the anode and CP/Ru as the cathode (named NiFe)(+)/Ru(-) hereafter). During cell operation with NiFe(+)/Ru(-), the current density reaches 500 mA/cm2 at a cell voltage of 1.79 V, with a specific energy consumption of 4.9 kWh/m3 and an energy efficiency of 66.2%. In comparison, for NiFe(+)/NiFe(-), the current density reaches 500 mA/cm2 at a cell voltage of 2.23 V, with a specific energy consumption of 5.7 kWh/m3 and an energy efficiency of 56.6%. The Faradaic efficiency is 96-99%. With the current density fixed at 400 mA/cm2, after performing a test for 150 h, the cell voltage with NiFe(+)/Ru(-) increases by 0.167 V, whereas that with NiFe(+)/NiFe(-) decreases by only 0.010 V. Good, long-term stability is demonstrated.

A TiO2 nanorod and perylene diimide based inorganic/organic nanoheterostructure photoanode for photoelectrochemical urea oxidation

Visible light-driven photoelectrochemical (PEC) urea oxidation using inorganic/organic nano-heterostructure (NH) photoanodes is an attractive method for hydrogen (H2) production. In this article, inorganic/organic NHs (TiO2/PDIEH) consisting of a N,N-bis(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic diimide (PDIEH) thin layer over TiO2 nanorods (NRs) were fabricated for the PEC urea oxidation reaction (UOR). In these NHs, a PDIEH layer was anchored on TiO2 NR arrays using the spin-coating technique, which is beneficial for the uniform deposition of PDIEH on TiO2 NRs. Uniform deposition facilitated adequate interface contact between PDIEH and TiO2 NRs. TiO2/PDIEH NHs achieved a high current density of 1.1 mA cm-2 at 1.96 VRHE compared to TiO2 NRs. TiO2/PDIEH offers long-term stability under light illumination with 90.21% faradaic efficiency. TiO2/PDIEH exhibits a solar-to-hydrogen efficiency of 0.52%. This outcome opens up new opportunities for inorganic/organic NHs for high-performance PEC urea oxidation.

Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p-n Heterostructures Derived from in Situ-Loaded ZIF-67

Exposing catalytically active metal sites in metal-organic frameworks (MOFs) while maintaining porosity is beneficial for increasing electron transport to achieve better electrochemical energy conversion performance. Herein, we propose an in situ method for MOF formation and loading onto TiO2 nanorods (NR) using a simple solution-processable method followed by annealing to obtain TiO2-Co3O4. The as-prepared TiO2-ZIF-67 based photoanodes were annealed at 350, 450, and 550 °C to study the effect of carbonization on photo-electrochemical water oxidation. The successful loading of ZIF-67 on TiO2 and the formation of TiO2-Co3O4 heterojunction were confirmed by XRD, XPS, FE-SEM, and HRTEM analyses. TiO2-Co3O4-450 (the sample annealed at 450 °C) showed an enhanced photocurrent of 2.4 mA/cm2, which was 2.6 times larger than that of pristine TiO2. The improved photocurrent might be ascribed to the prepared p-n heterostructures (Co3O4 and TiO2), which promote electron-hole separation and charge transfer within the system and improve the photoelectrochemical performance. Moreover, the preparation of Co3O4 from the MOF carbonization process improved the electrical conductivity and significantly increased the number of exposed active sites and enhanced the photoresponse performance. The as-prepared ZIF-67 derived TiO2-Co3O4 based photoanodes demonstrate high PEC water oxidation, and the controlled carbonization method paves the way toward the synthesis of low-cost and efficient electrocatalysts.

Photoelectrochemical water oxidation over TiO2 nanotubes modified with MoS2 and g-C3N4

TiO₂ nanotube arrays (TNAs) have been studied for photoelectrochemical (PEC) water splitting. However, there are two major barriers of TNAs, including a low photo-response and the fast charge carrier recombination in TNAs, leading to poor photocatalytic efficiency. Through a comparison of MoS₂/TNAs and g-C₃N₄/TNAs, it was found that TNAs modified with MoS₂ and g-C₃N₄ exhibited a current density of, respectively, 210.6 and 139.6 μA·cm^-2 at an overpotential of 1.23 V vs RHE, which is 18.2 and 12 times higher than that of pure TNAs under the same conditions. The stability of the MoS₂/TNAs heterojunction is higher than that of g-C₃N₄/TNAs.

CeO2/Ni-MOF with Synergistic Function of Enrichment and Activation: Efficient Reduction of 4-Nitrophenol Pollutant to 4-Aminophenol

The conversion of organic pollutants to value-added chemicals has been considered as a sustainable approach to solve environmental problems. However, it is still a challenge to construct a suitable heterogeneous catalyst that can synchronously achieve the enrichment and activation of organic pollutants (such as 4-nitrophenol, 4-NP). Here, an organic-inorganic hybrid catalyst (CeO₂/Ni-MOF) was successfully fabricated for efficiently reducing 4-NP to 4-aminophenol (4-AP) with water as the hydrogen source. Based on the synergistic effect of Ni-MOF (adsorption action) and CeO₂ (active sites), CeO₂/Ni-MOF could achieve a reaction rate of 1.102 μmol min^-1 mg^-1 with an ultrahigh Faraday efficiency (FE) (99.9%) and conversion (97.6%). In addition, the catalytic mechanism of 4-NP reduction over CeO₂/Ni-MOF was elaborated in depth. This work presents a new avenue for the effective reduction of pollutants and provides a new strategy for designing high-performance catalysts for rare-earth metals.

Facile preparation of tremella-like TiO2/Cd:ZnIn2S4 photoanode with enhanced photo-electro-chemical (PEC) performance for energy and environmental applications

The realization of solar-driven photoelectrochemical (PEC) process lies in the success development of materials with excellent photoelectric properties. Past reports identified TiO₂, upon modified with both Cd and ZnIn₂S₄ (ZIS), exhibits promising PEC performance; however, at the cost of tedious preparation. In view of this, our work proposes a facile one-step hydrothermal strategy to deposit both modifiers onto the pre-obtained TiO₂ nanotubes (NTs), realizing rose-like TiO₂/ZnIn₂S₄ (TiO₂/ZIS) or tremella-like TiO₂/Cd:ZnIn₂S₄ (TiO₂/Cd:ZIS) with improved PEC performances. Interestingly, the thickness of ZIS petals, which deposited on top of TiO₂ NTs, is positively-correlated to its Cd-composition, signifying the substitutional doping of Cd into the unit cell of ZIS. This renders robust ionic interactions between the constituents, prompting the enhanced optical properties of TiO₂ in conjecture to its reduced impedance. As a result, the recombination rate of photo-electric-derived carriers was drastically suppressed, with an improved photocurrent density of 606.2 μA cm^-2 recorded by TiO₂/Cd:ZIS photoanode under solar irradiation. Such performance is 80 times and 5.16 times higher than those of bare TiO₂ NTs and TiO₂/ZIS counterparts. The photoconversion efficiencies in terms of incident photon-to-current conversion efficiency (IPCE) and applied bias photon current efficiency (ABPE) for TiO₂/Cd:ZIS were significantly improved too, recorded at 1.12% and 0.38%, respectively, under standard evaluation condition. As summary, our work proposes a facile one-step hydrothermal approach that simultaneously-deposit both Cd and ZIS onto TiO₂ photoanode for an enhanced PEC performance. This opens up a wider horizon for PEC technology, further unlocking its potential in both energy and environmental applications.

Defect Engineering and Surface Polarization of TiO2 Nanorod Arrays toward Efficient Photoelectrochemical Oxygen Evolution

The relatively low photo-conversion efficiencies of semiconductors greatly restrict their real-world practices toward photoelectrochemical water splitting. In this work, we demonstrate the fabrication of TiO2-x nanorod arrays enriched with oxygen defects and surface-polarized hydroxyl groups by a facile surface reduction method. The oxygen defects located in the bulk/surface of TiO2-x enable fast charge transport and act as catalytically active sites to accelerate the water oxidation kinetics. Meanwhile, the hydroxyl groups could establish a surface electric field by polarization, for efficient charge separation. The as-optimized TiO2-x nanorod photoanode achieves a high photocurrent density of 2.62 mA cm−2 without any cocatalyst loading at 1.23 VRHE under 100 mW cm−2, which is almost double that of the bare TiO2 counterpart. Notably, the surface charge separation and injection efficiency of the TiO2-x photoanode reach as high as 80% and 97% at 1.23 VRHE, respectively, and the maximum incident photon-to-current efficiency reaches 90% at 400 nm. This work provides a new surface treatment strategy for the development of high-performance photoanodes in photoelectrochemical water splitting.

Recent Advances in Ferroelectric Materials-Based Photoelectrochemical Reaction

Inorganic perovskite ferroelectric-based nanomaterials as sustainable new energy materials, due to their intrinsic ferroelectricity and environmental compatibility, are intended to play a crucial role in photoelectrochemical field as major functional materials. Because of versatile physical properties and excellent optoelectronic properties, ferroelectric-based nanomaterials attract much attention in the field of photocatalysis, photoelectrochemical water splitting and photovoltaic. The aim of this review is to cover the recent advances by stating the different kinds of ferroelectrics separately in the photoelectrochemical field as well as discussing how ferroelectric polarization will impact functioning of photo-induced carrier separation and transportation in the interface of the compounded semiconductors. In addition, the future prospects of ferroelectric-based nanomaterials are also discussed.

Charge Separation by Creating Band Bending in Metal-Organic Frameworks for Improved Photocatalytic Hydrogen Evolution

Metal-organic frameworks (MOFs) have been intensively studied as a class of semiconductor-like materials in photocatalysis. However, band bending, which plays a crucial role in semiconductor photocatalysis, has not yet been demonstrated in MOF photocatalysts. Herein, a representative MOF, MIL-125-NH₂ , is integrated with the metal oxides (MoO₃ and V₂ O₅ ) that feature appropriate work functions and energy levels to afford the corresponding MOF composites. Surface photovoltage results demonstrate band bending in the MOF composites, which gives rise to the built-in electric field of MIL-125-NH₂ , boosting the charge separation. As a result, the MOF composites present 56 and 42 times higher activities, respectively, compared to the pristine MOF for photocatalytic H₂ production. Upon depositing Pt onto the MOF, ∼6 times higher activity is achieved. This work illustrates band bending of MOFs for the first time, supporting their semiconductor-like nature, which would greatly promote MOF photocatalysis.

Electrocatalytic reduction of 4-nitrophenol over Ni-MOF/NF: understanding the self-enrichment effect of H-bonds

The chemical adsorption and active sites play a key role in electrocatalysis, so Ni-MOF/nickel foam was fabricated for efficiently reducing 4-nitrophenol (4-NP) without any sacrificial agents. The coordinated water molecules induced the formation of hydrogen bonds (H-bonds) with the nitro group, contributing to the self-enrichment of 4-NP. The reaction rate reached 0.351 μmol min^-1 mg^-1. Therefore, this work provides a new insight into the H-bond effect in the field of electrocatalysis.

Boosting oxygen evolution of layered double hydroxide through electronic coupling with ultralow noble metal doping

Electrocatalytic water oxidation is a rate-determining step in the water splitting process; however, its efficiency is significantly hampered by the limitations of cost-effective electrocatalysts. Here, an advanced Co(OH)₂ electrocatalyst with ultralow iridium (Ir) doping is developed to enable outstanding oxygen evolution reaction (OER) properties; that is, in a 1 M KOH medium, an overpotential of only 262 mV is required to achieve a current density of 10 mA cm^-2, and a small Tafel slope of 66.9 mV dec^-1 is achieved, which is markedly superior to that of the commercial IrO₂ catalyst (301 mV@10 mA cm^-2; 66.9 mV dec^-1). Through the combination of experimental data and a mechanism study, it is disclosed that the high intrinsic OER activity results from the synergistic electron coupling of oxidized Ir and Co(OH)₂, which significantly moderate the adsorption energy of the intermediates. Moreover, we have also synthesized Ru-Co(OH)₂ nanosheets and demonstrated the universal syntheses of Ir-doped CoM (M = Ni, Fe, Mn, and Zn) layered double hydroxides (LDHs).

Bifunctional PDDA-stabilized β-Fe2O3 nanoclusters for improved photoelectrocatalytic and magnetic field enhanced photocatalytic applications

Bifunctional β-Fe2O3@PDDA nanoclusters applied for the efficient photoelectrocatalytic oxygen evolution reaction and magnetic field enhanced photocatalytic degradation of pollutants.

An iron based organic framework coated with nickel hydroxide for energy storage, conversion and detection

Although electrode materials based on metal organic frameworks (MOFs) were widely studied in the electrochemistry field, the origin of poor conductivity is still a bottleneck restricting their development. Herein, we constructed a conductive circuit by growing a layer of hydroxide on the surface of the Fe-MOF, and composite materials (Fe-MOF@Ni(OH)₂) are applied in the fields of supercapacitor, OER, and electrochemical sensing. Fe-MOF@Ni(OH)₂ not only maintains the intrinsic advantages of Fe-MOF, but also improves the electrical conductivity. Fe-MOF@Ni(OH)₂ exhibits a high specific capacity of 188 mAh g^-1 at 1 A g^-1 . The energy density of the asymmetric supercapacitor (Fe-MOF@Ni(OH)₂-20//AC) reaches 67.1 Wh kg^-1. During the oxygen evolution reaction, the overpotential of the material is 280 mV at 10 mA cm^-2, and the Tafel slope is 37.6 mV dec^-1. The electrochemical sensing tests showed the detection limit of BPA is 5 μM. Hence, these results provide key insights into the design of multifunctional electrode materials.

Fe-TiO2/AC and Co-TiO2/AC Composites: Novel Photocatalysts Prepared from Waste Streams for the Efficient Removal and Photocatalytic Degradation of Cibacron Yellow F-4G Dye

Fe-TiO2/AC and Co-TiO2/AC composites were prepared from activated carbon (AC) derived from residues of peanut hulls and TiO2 photocatalyst, electrochemically prepared from titanium scrap, and doped with Fe and Co, respectively. The adsorption capacity and photocatalytic activity of the Fe-TiO2/AC and Co-TiO2/AC composites were studied for removing and degrading Cibacron Yellow F-4G (CYF-4G) from wastewater. Doped ACs were characterized by thermogravimetry (TGA), scanning electron microscopy (SEM), X-ray diffraction (XRD), a new X-ray absorption technique (XRA), and elemental analysis (EA). Interesting relationships were found between SEM, XRA, and TGA data and the doped amount of catalyst on ACs. Optimal dye adsorption was found at a pH of 2.0. The CYF-4G adsorption kinetics are followed according to the pseudo-second order model. The experimental data revealed that the Langmuir model fits better than the Freundlich and Temkin models. A decrease in adsorption capacity was observed when the catalyst dope percentage increased. A removal and degradation efficiency of the dye close to 100% was achieved around 120 min. A synergistic adsorption and photocatalytic degradation effect of the Fe-TiO2/AC and Co-TiO2/AC composites could be observed when adsorption experiments were conducted under simulated visible radiation.

ZIF-8 derived ZnO/TiO2 heterostructure with rich oxygen vacancies for promoting photoelectrochemical water splitting

Due to the serious recombination of electron-hole and weak photoresponse ability, achieving highly efficient photoelectrochemical (PEC) water splitting activity for TiO₂ photoelectrode has become a key issue. In this paper, we reported a new method for preparing ZnO/TiO₂ photoelectrodes by electrostatic adsorption from zeolitic imidazolate framework-8 (ZIF-8) as the precursor. ZIF-8 was combined with TiO₂ nanorods (NRs) through electrostatic interaction and then calcined to obtain ZnO/TiO₂ heterojunction photoelectrodes with abundant oxygen vacancies (O_vac). The introduced ZnO with O_vac provides a large number of active sites which enhanced the electrical conductivity and charges separation of ZnO/TiO₂ photoelectrode. The optimal photocurrent density of ZnO/TiO₂ photoelectrodes at 1.23 V versus (vs.) reversible hydrogen electrode (RHE) under illumination (100 mW/cm²) has reached 1.76 mA/cm², almost 2.75 times that of the pure TiO₂. Meanwhile, the incident photon-to-electron conversion efficiency (IPCE) of the best photoelectrode has increased to 58.2% at 390 nm, the charge injection (η_injection) and separation (η_separation) efficiency have reached to 93.53% and 51.62% (1.23 V vs. RHE), respectively.

In situ assembly of bimetallic MOF composites on IF as efficient electrocatalysts for the oxygen evolution reaction

Since the complicated multiple electron transfer process and slow kinetics in the OER process seriously hinder the electrochemical decomposition of water, it is urgent to design and develop electrocatalysts with excellent performance and superior stability to reduce overpotential and accelerate the reaction dynamics of the OER. Herein, a unique ultra-thin nanosheet bimetal electrocatalyst NiFe-MOF/IF was synthesized by a one-step hydrothermal method, and characterized by SEM, XRD, TEM, and XPS. NiFe-MOF/IF shows superior OER electrocatalytic activity in 1 M KOH electrolyte solution, and an ultralow overpotential of only 230 and 262 mV was required to achieve a current density of 10 and 100 mA cm^-2, respectively, with a relatively small Tafel slope of 30.46 mV dec^-1 for the OER. No obvious degradation of the current density at 10 mA cm^-2 was observed over about 16 h, which indicates the excellent stability of the catalyst. Favourable activity and benign durability of NiFe-MOF/IF can be attributed to the three-dimensional high porosity conductive substrates, in situ growth of MOF nanosheets, bimetallic synergy, and unique layering. This research provides a promising strategy for the application of MOF materials in the field of electrocatalysis.

Fe/Ni Bimetallic Organic Framework Deposited on TiO2 Nanotube Array for Enhancing Higher and Stable Photoelectrochemical Activity of Oxygen Evaluation Reaction

Photoelectrochemical (PEC) water splitting is a promising strategy to improve the efficiency of oxygen evolution reactions (OERs). However, the efficient adsorption of visible light as well as long-term stability of light-harvesting electrocatalysis is the crucial issue in PEC cells. Metal–organic framework (MOF)-derived bimetallic electrocatalysis with its superior performance has wide application prospects in OER and PEC applications. Herein, we have fabricated a nickel and iron bimetallic organic framework (FeNi-MOF) deposited on top of anodized TiO₂ nanotube arrays (TNTA) for PEC and OER applications. The FeNi-MOF/TNTA was incorporated through the electrochemical deposition of Ni²⁺ and Fe³⁺ onto the surface of TNTA and then connected with organic ligands by the hydrothermal transformation. Therefore, FeNi-MOF/TNTA demonstrates abundant photoelectrocatalytic active sites that can enhance the photocurrent up to 1.91 mA/cm² under 100 mW/cm² and a negligible loss in activity after 180 min of photoreaction. The FeNi-MOF-doped photoanode shows predominant photoelectrochemical performance due to the boosted excellent light-harvesting ability, rapid photoresponse, and stimulated interfacial energy of charge separation under the UV-visible light irradiation conditions. The results of this study give deep insight into MOF-derived bimetallic nanomaterial synthesis for photoelectrochemical OER and provide guidance on future electrocatalysis design.

Adsorption and Photocatalytic Study of Phenol Using Composites of Activated Carbon Prepared from Onion Leaves (Allium fistulosum) and Metallic Oxides (ZnO and TiO2)

The aim of this study was to evaluate and compare the adsorption and photocatalytic activity of activated carbon-based photocatalysts. Titanium dioxide (TiO2) and zinc oxide (ZnO) were chosen as semiconductors to prepare composites with activated carbon by the wet impregnation method. Activated carbon was prepared using as starting material onion leaves (Allium fistulosum) and as activating agent phosphoric acid (H3PO4). Photooxidation and batch adsorption of phenol was studied to compare the efficiency of the materials prepared. The results showed that the composite AC–TiO2 has a greater photocatalytic activity and a better adsorption capacity compared to AC–ZnO composite.