Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

Functional oxide materials have become crucial in the continuous development of various fields, including those for energy applications. In this aspect, the synthesis of nanomaterials for low-cost green hydrogen production represents a huge challenge that needs to be overcome to move toward the next generation of efficient systems and devices. This perspective presents a critical assessment of hydrothermal and polymeric precursor methods as potential approaches to designing photoelectrodes for future industrial implementation. The main conditions that can affect the photoanode's physical and chemical characteristics, such as morphology, particle size, defects chemistry, dimensionality, and crystal orientation, and how they influence the photoelectrochemical performance are highlighted in this report. Strategies to tune and engineer photoelectrode and an outlook for developing efficient solar-to-hydrogen conversion using an inexpensive and stable material will also be addressed.

Ultrasound assisted growth of NiCo2O4@carbon cloth for high energy storage device application

The nanostructure of metal oxides attracts great attention in the field of supercapacitors because of fast charge transport process. The hydrothermal method is used for development of NiCo₂O₄nanostructure on carbon cloth as current collector backbone. The stepwise study of various structural, morphological and electrochemical properties of NiCo₂O₄electrode is studied. The ultrasonic treatment is used to obtain nanowire-like morphology of NiCo₂O₄ which exhibits hierarchical nanostructure, which provides surface properties such as high surface area and appropriate pore capacity. NiCo₂O₄ nanostructure with specific capacitance of 1460 F g^-1 with high electrochemical stability of 84% after 3000 cycles in 3 M KOH aqueous electrolyte at 100 mV s^-1. The electrochemical property shows NiCo₂O₄is one of the potential candidates for energy storage application. The specific capacitance and energy density for NiCo₂O₄@CC//NiCo₂O₄@CC symmetric supercapacitor device is of 124 F g^-1 and 16.18 Wh.kg^-1, respectively at current density of 5 mA.

Nature of Electronic Conduction in "Pseudocapacitive" Films: Transition from the Insulator State to Band-Conduction

The transition between the insulator state and the band-conducting state is investigated by means of cyclic voltammetry in cobalt oxide porous film electrodes in phosphate-buffered solutions. It is shown that a proton-coupled faradaic oxidative process starting in the insulator region eventually builds an ohmic conduction mode upon anodic polarization. This model allows one to understand the origin of the authentic capacitive behavior of conductive metal oxide films rather than the so-called "pseudocapacitive" behavior. The particular example of cobalt oxide serves to illustrate the way in which, more generally, the behavior of "pseudocapacitors", long ascribed to the superposition of faradaic reactions, is in fact that of true capacitors, once band-conduction has been established upon oxidation of the material.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode

Co-grafting of a cobalt diimine–dioxime catalyst and push–pull organic dye on NiO yields a photocathode evolving hydrogen from aqueous solution under sunlight, with equivalent performances compared to a dyad-based architecture using similar components.

Dye-sensitized photo-electrochemical cells (DS-PECs) form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and ease of processing of their constitutive photoelectrode materials. Preparing such molecular photocathodes requires a well-controlled co-immobilization of molecular dyes and catalysts onto transparent semiconducting materials. Here we used a series of surface analysis techniques to describe the molecular assembly of a push–pull organic dye and a cobalt diimine–dioxime catalyst co-grafted on a p-type NiO electrode substrate. (Photo)electrochemical measurements allowed characterization of electron transfer processes within such an assembly and to demonstrate for the first time that a Co^I species is formed as the entry into the light-driven H₂ evolution mechanism of a dye-sensitized photocathode. This co-grafted noble-metal free H₂-evolving photocathode architecture displays similar performances to its covalent dye–catalyst counterpart based on the same catalytic moiety. Post-operando time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of these photoelectrodes after extensive photoelectrochemical operation suggested decomposition pathways of the dye and triazole linkage used to graft the catalyst onto NiO, providing grounds for the design of optimized molecular DS-PEC components with increased robustness upon turnover.

A protocol for quantifying hydrogen evolution by dye-sensitized molecular photocathodes and its implementation for evaluating a new covalent architecture based on an optimized dye-catalyst dyad

A protocol that combines gas chromatography and a high-sensitivity micro Clark-type electrode is described to quantify hydrogen production across gas and solution phases for systems operating at very low currents such as dye-sensitized H₂-evolving photocathodes. Data indicate that a significant fraction of H₂ remains in aqueous solution even after several hours of experiments. Using this protocol, re-evaluation of a dye-sensitized H₂-evolving photocathode based on a dye-catalyst dyad showed a reproducible 66% increase of the faradaic efficiency compared with previously reported headspace GC measurements [Kaeffer et al., J. Am. Chem. Soc., 2016, 138, 12308-12311]. This dyad was based on an organic push-pull dye where donor and acceptor are separated by one thiophene group. Insertion of a second thiophene group between the donor and acceptor led to a more efficient system with 30% improved faradaic efficiency for H₂ evolution.

The effect of the synthetic route on the structural, textural, morphological and catalytic properties of iron(iii) oxides and oxyhydroxides

Precipitation and microemulsion methods allowed obtaining mesoporous and nanostructured materials, namely amorphous hematite and ferrihydrite, which exhibited great catalytic activity.

Synthesis of organized mesoporous metal oxide films templated by amphiphilic PVA–PMMA comb copolymer

We present a synthesis of organized mesoporous metal oxide films with high porosity and good interconnectivity using an amphiphilic PVA–PMMA comb copolymer which can be a promising template as an alternative to conventional block copolymers.

Structural, optical and photo-electrochemical properties of hydrothermally grown ZnO nanorods arrays covered with α-Fe2O3 nanoparticles

A low cost hydrothermal method and subsequent wet-chemical process has been used for the preparation of a ZnO nanorod (NR) array film grown on tin doped indium oxide (ITO) coated glass substrates, post decorated by α-Fe₂O₃ nanoparticles (NPs).

Templating Sol-Gel Hematite Films with Sacrificial Copper Oxide: Enhancing Photoanode Performance with Nanostructure and Oxygen Vacancies

Nanostructuring hematite films is a critical step for enhancing photoelectrochemical performance by circumventing the intrinsic limitations on minority carrier transport. Herein, we present a novel sol-gel approach that affords nanostructured hematite films by including CuO as sacrificial templating agent. First, by annealing in air at 450 °C a film comprising an intimate mixture of CuO and Fe2O3 nanoparticles is obtained. The subsequent treatment with NaCl and annealing at 700 °C under Argon reveals a nanostructured highly crystalline hematite film devoid of copper. Photoelectrochemical investigations reveal that the incorporation of CuO as templating agent and the inert conditions employed during the annealing play a crucial role in the performance of the hematite electrodes. Mott-Schottky analysis shows a higher donor concentration when annealing in inert conditions, and even higher when combined with the NaCl treatment. These findings agree well with the presence of an oxygen-deficient shell on the material's surface evidenced by FT-IR and XPS measurements. Likewise, the incorporation of the CuO enhances the photocurrent obtained at 1.23 V from 0.55 to 0.8 mA·cm(-2) because of an improved nanostructure. Optimized films demonstrate an incident photon-to-current efficiency (IPCE) of 52% at 380 nm when applying 1.23 V versus RHE, and a faradaic efficiency for water splitting close to unity.

Low-cost flexible supercapacitors with high-energy density based on nanostructured MnO2 and Fe2O3 thin films directly fabricated onto stainless steel

The facile and economical electrochemical and successive ionic layer adsorption and reaction (SILAR) methods have been employed in order to prepare manganese oxide (MnO2) and iron oxide (Fe2O3) thin films, respectively with the fine optimized nanostructures on highly flexible stainless steel sheet. The symmetric and asymmetric flexible-solid-state supercapacitors (FSS-SCs) of nanostructured (nanosheets for MnO2 and nanoparticles for Fe2O3) electrodes with Na2SO4/Carboxymethyl cellulose (CMC) gel as a separator and electrolyte were assembled. MnO2 as positive and negative electrodes were used to fabricate symmetric SC, while the asymmetric SC was assembled by employing MnO2 as positive and Fe2O3 as negative electrode. Furthermore, the electrochemical features of symmetric and asymmetric SCs are systematically investigated. The results verify that the fabricated symmetric and asymmetric FSS-SCs present excellent reversibility (within the voltage window of 0-1 V and 0-2 V, respectively) and good cycling stability (83 and 91%, respectively for 3000 of CV cycles). Additionally, the asymmetric SC shows maximum specific capacitance of 92 Fg(-1), about 2-fold of higher energy density (41.8 Wh kg(-1)) than symmetric SC and excellent mechanical flexibility. Furthermore, the "real-life" demonstration of fabricated SCs to the panel of SUK confirms that asymmetric SC has 2-fold higher energy density compare to symmetric SC.

Tin doping speeds up hole transfer during light-driven water oxidation at hematite photoanodes

Numerous studies have shown that the performance of hematite photoanodes for light-driven water splitting is improved substantially by doping with various metals, including tin. Although the enhanced performance has commonly been attributed to bulk effects such as increased conductivity, recent studies have noted an impact of doping on the efficiency of the interfacial transfer of holes involved in the oxygen evolution reaction. However, the methods used were not able to elucidate the origin of this improved efficiency, which could originate from passivation of surface electron-hole recombination or catalysis of the oxygen evolution reaction. The present study used intensity-modulated photocurrent spectroscopy (IMPS), which is a powerful small amplitude perturbation technique that can de-convolute the rate constants for charge transfer and recombination at illuminated semiconductor electrodes. The method was applied to examine the kinetics of water oxidation on thin solution-processed hematite model photoanodes, which can be Sn-doped without morphological change. We observed a significant increase in photocurrent upon Sn-doping, which is attributed to a higher transfer efficiency. The kinetic data obtained using IMPS show that Sn-doping brings about a more than tenfold increase in the rate constant for water oxidation by photogenerated holes. This result provides the first demonstration that Sn-doping speeds up water oxidation on hematite by increasing the rate constant for hole transfer.

Micro- and nanostructures of photoelectrodes for solar-driven water splitting

Artificial photosynthesis of clean fuels has aroused great interest to meet the great demand for clean and renewable energy. Great advances have recently been made in various photoelectrodes with their efficiencies and stabilities significantly improved by the design and implementation of novel structures, which are determinative for the optical absorption, charge-transport path, surface area, and electronic conductivity. This Research News article discusses perspectives of the synthetic methods and micro- and nanostructures (planar structures, 1D structures, and mesoporous structures) of photoelectrodes, and their relationships with the photo-electrochemical performance. Structural features, such as particle size, crystallinity, morphology, and film thickness, as well as the trade-offs among them are also evaluated and discussed for each category of structure.

Greenlighting photoelectrochemical oxidation of water by iron oxide

Hematite (α-Fe2O3) is one of just a few candidate electrode materials that possess all of the following photocatalyst-essential properties for scalable application to water oxidation: excellent stability, earth-abundance, suitability positive valence-band-edge energy, and significant visible light absorptivity. Despite these merits, hematite's modest oxygen evolution reaction kinetics and its poor efficiency in delivering photogenerated holes, especially holes generated by green photons, to the electrode/solution interface, render it ineffective as a practical water-splitting catalyst. Here we show that hole delivery and catalytic utilization can be substantially improved through Ti alloying, provided that the alloyed material is present in ultrathin-thin-film form. Notably, the effects are most pronounced for charges photogenerated by photons with energy comparable to the band gap for excitation of Fe(3d)→Fe(3d) transitions (i.e., green photons). Additionally, at the optimum Ti substitution level the lifetimes of surface-localized holes, competent for water oxidation, are extended. Together these changes explain an overall improvement in photoelectrochemical performance, especially enhanced internal quantum efficiencies, observed upon Ti(IV) incorporation.

Synergistic contributions by decreasing overpotential and enhancing charge-transfer in α-Fe2O3/Mn3O4/graphene catalysts with heterostructures for photocatalytic water oxidation

Proposed mechanism of oxygen evolution from α-Fe₂O₃/Mn₃O₄-1/rGO-3 photocatalyst.

Optimization of amorphous silicon double junction solar cells for an efficient photoelectrochemical water splitting device based on a bismuth vanadate photoanode

An effective photoelectrochemical water splitting device based on a bismuth vanadate photoanode and thin film silicon solar cells has been optimized.

Porous FeOx/BiVO4-deltaS0.08: highly efficient photocatalysts for the degradation of methylene blue under visible-light illumination

Porous S-doped bismuth vanadate with an olive-like morphology and its supported iron oxide (y wt.% FeOx/BiVO4-deltaS0.08, y = 0.06, 0.76, and 1.40) photocatalysts were fabricated using the dodecylamine-assisted alcohol-hydrothermal and incipient wetness impregnation methods, respectively. It is shown that the y wt.% FeOx/BiVO4-deltaS0.08 photocatalysts contained a monoclinic scheetlite BiVO4 phase with a porous olive-like morphology, a surface area of 8.8-9.2 m2/g, and a bandgap energy of 2.38-2.42 eV. There was co-presence of surface Bi5+, Bi3+, V5+, V3+, Fe3+, and Fe2+ species in y wt.% FeOx/BiVO4-deltaS0.08. The 1.40 wt.% FeOx/BiVO4-deltaS0.08 sample performed the best for Methylene Blue degradation under visible-light illumination. The photocatalytic mechanism was also discussed. We believe that the sulfur and FeOx co-doping, higher oxygen adspecies concentration, and lower bandgap energy were responsible for the excellent visible-light-driven catalytic activity of 1.40 wt.% FeOx/BiVO4-deltaS0.08.