Water Splitting Hydrogen Generation Research Articles

Electrocatalytic Activities of High-Entropy Oxides for the Oxygen Evolution Reaction

Electrocatalytic water-splitting hydrogen generation consists of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), where the four-electron-relevant OER is the rate-determining step. So far, there have been many efforts to substitute for the highly expensive noble-metal electrocatalysts (platinum, ruthenium or rhodium oxides, etc.). Transition-metal oxides based on Co, Ni, Mn, and V have been suggested as such alternatives, due to their low cost, high efficiency, and high stability. Recently, since the compositional diversity can provide a new breakthrough in that area, a high-entropy oxide (HEO) with five transition-metal cations has been suggested as a promising electrocatalyst toward the OER. In our studies, two kinds of HEOs were prepared and their OER activities were investigated. To begin with, for the (Mg0.2Fe0.2Co0.2Ni0.2Cu0.2)O, the effect of constituent cations on the OER activity was unveiled. Furthermore, a core cation driving the high OER activity was found. For it, the medium-entropy oxides (MEOs) with four cations are prepared by subtracting each cation (Mg, Fe, Co, Ni, or Cu) from the HEO, exhibiting homogeneous morphology, equiatomic composition, and single-phase rocksalt structure. As a result, it is found that the highest concentration of Co3+ in the MEO (w/o Cu) leads to the best OER activity, and thus Co3+ is the core ion driving the high OER activity. Furthermore, it is regarded that Cu2+ ions prevent the conversion of Co or Fe cations from 2+ to 3+ in the HEO and MEOs. Accordingly, maximizing the concentration of Co3+ within electrocatalysts is suggested as an effective design strategy for the high-efficiency electrocatalysts based on high or medium entropy materials. Secondly, the relationship between structure and OER activity was elucidated for the (Mg0.2Fe0.2Co0.2Zn0.2Cu0.2)O with a temperature-dependent rocksalt-to-spinel transition.

Photodegradation of CdTe thin film via PVD for water splitting to generate hydrogen energy

The cost-effective catalysts are attracting attention in the field of water-splitting hydrogen generation for their effective light utilization and charge separation activity. In this work, CdTe has introduced this promising photocathodic material among the few photocatalysts with a low band gap. This photocatalyst has a high activity for the PEC hydrogen evolution under simulated sunlight irradiation. XRD study reveals the confirmation of the prepared CdTe thin film. SEM study provided the morphology and surface structure for the absorption of the solar spectra, the optical properties of CdTe thin film confirmed the absorption of solar spectra for the hydrogen production. FTIR provided the functional group of the CdTe thin film. Electrochemical impedance spectroscopy showed that the charge transfer at the electrode–electrolyte interface for CdTe thin film is much better than another semiconductor. The LSV for STH% was noted to be 1.07% for CdTe thin film and the production of hydrogen generation of CdTe thin film was observed at 4953.21 mol.g−1.h−1.

High efficiency Bi2S3/Bi2MoO6/TiO2 photoanode for photoelectrochemical hydrogen generation

The ternary Bi2S3/Bi2MoO6/TiO2 photoanode with waterfall-type energy level structure was engineered firstly for photoelectrochemical (PEC) water-splitting hydrogen generation. The results of XRD, SEM, EDS and TEM illustrated that Bi2MoO6 flakes were self-assembled in flower-ball shape on TiO2 nanoparticles via solvothermal reaction and the “green” Bi2S3 quantum dots (QDs) distributed in Bi2MoO6 flake edge through in-situ anion exchange process. UV-Vis. DRS and PL spectra showed that Bi2S3 QDs broadened the response range in visible and near-infrared region and waterfall-type energy level structure is helpful to inhibit the recombination of photo-generated electrons and holes. EIS spectra proved that Bi2S3 QDs amount and size can affect significantly photoanode/electrolyte interface transfer resistance and the minimum is down to 14.20 kΩ/cm2. The PEC device comprised with Bi2S3/Bi2MoO6/TiO2 photoanode, Ag/AgCl reference electrode, Pt counter electrode and Na2SO4 solution electrolyte exhibited high performance for water splitting: the photocurrent density maximum is 21.42 mA/cm2 and the applied bias photon-to-current efficiency (ABPE) maximum is 20.8%. However, the stability of PEC cell samples is a little poor and ABPE declines to 7.28% after 12 h. The ternary Bi2S3/Bi2MoO6/TiO2 photoanode with waterfall-type energy level structure can be also employed in other photovoltaic devices besides PEC cells.

Engineering two-dimensional SnC/HfSSe heterojunction as a direct Z-scheme photocatalyst for water splitting hydrogen evolution

Developing robust water-splitting photocatalyst remains a pivotal challenge for the conversion of solar energy to hydrogen fuel. Herein, we report SnC/HfSSe heterojunctions to be a competitive direct Z-scheme photocatalysts with HfSSe acting as an oxidant and SnC as a reductant for water-splitting hydrogen generation by employing the first-principles calculations. The photogenerated electron–hole migration logically follows the Z-scheme path due to the large interlayer built-in electric field, small interlayer bandgap, proper band edge alignment, and high interlayer carrier recombination. The overpotential is appropriate to the oxygen evolution reaction (OER), while the overpotential is slightly large to the hydrogen envolution reaction (HER) so that the incident photon energy above the band gap of the components is required for the HER. The heterojunctions have excellent visible light absorption, exhibiting the remarkable red shift and stronger light absorption with respect to the two components of the heterojunction. Moreover, the heterojunction exhibits an outstanding solar-to-hydrogen (STH) efficiency as high as 27% in extremely acidic solution.

Improving Electrocatalytic Activity of MoO3 for the Oxygen Evolution Reaction by Incorporation of Li Ions

Several polymorphs of MoO3 with different crystal and electronic structures provide many opportunities to find suitable electrocatalysts for water splitting hydrogen generation. So far, promising electrocatalytic activity for hydrogen evolution reaction (HER) has been found for α-MoO3 (orthorhombic), while the polymorph has shown a poor oxygen evolution reaction (OER) activity. In this work, it is found that the intrinsic OER activity of α-MoO3 is remarkably improved by incorporating guest ions of lithium-ion (Li+). α-MoO3 nanoparticles were deposited on a carbon fiber paper (α-MoO3/CFP) by chemical vapor transport and a subsequent lithiation process was applied to prepare lithiated α-MoO3/CFP (Li-α-MoO3/CFP), showing the robust η10 of 458 mV. The improved OER activity in the lithiated electrode is attributed to the incorporation of Li+ into the MoO3, which increases the electron carrier density and the corresponding electrical conductivity, thereby improving the interfacial charge transfer kinetics as well as activating a large number of ECSA.

Development of Aldehyde Functionalized Iridium(III) Complexes Photosensitizers with Strong Visible-Light Absorption for Photocatalytic Hydrogen Generation from Water

Four iridium(III) dyes functionalized with aldehyde functional group in the cyclometalating (C^N) ligands, bearing either diethyl [2,2′-bipyridine]-4,4′-dicarboxylate or tetraethyl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) anchoring groups, coded as Ir1–Ir4, are synthesized and explored as photosensitizers. The synthetic route is described and all of the complexes are characterized with respect to their electrochemical and photophysical properties. Density functional theory (DFT) calculation was used to gain insight into the factors responsible for the photocatalytic properties of Ir1–Ir4 as effective photosensitizers for photocatalytic hydrogen generation. Relative to common iridium(III) dyes, such as [Ir(ppy)2(dcbpy)]+ (ppy = 2-phenylpyridine), the absorption spectra of our dyes are broader, which is attributed to the extended π-conjugation in their C^N ligands. All of the new iridium(III) dyes were used as photosensitizers for visible-light driven hydrogen production by attaching to platinized TiO2 nanoparticles (Pt–TiO2) in the presence of sacrificial electron donor (SED) of ascorbic acid (AA) in a purely aqueous solution. A H2 turnover number (TON) up to 5809 was demonstrated for 280 h irradiation. Complexes with tetraethyl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) anchoring groups were found to outperform those with classical diethyl [2,2′-bipyridine]-4,4′-dicarboxylate, which may be one of the important steps in developing high-efficiency iridium(III) photosensitizers in water splitting hydrogen generation.

Selective electro-oxidation of glycerol to dihydroxyacetone by a non-precious electrocatalyst – CuO

An earth abundant and non-precious electrocatalyst, CuO, is developed for the high selectivity (∼60 %) towards the glycerol electro-oxidation to dihydroxyacetone (DHA) at high current density (3 mA/cm2) under mild basic condition, pH 9. CuO demonstrates the catalytic ability towards the secondary hydroxyl group oxidation of glycerol. However, under strong basic condition, pH 13, DHA would transform to glyceraldehyde (GLAD) spontaneously without applying potential. Thus, under strong basic condition, the glycerol oxidation usually results with other two-carbon and one-carbon products deriving from GLAD oxidation. Based on HPLC, in-situ Raman spectra, and electrochemical studies, the glycerol electro-oxidation pathway was proposed. With this study, the waste by-product from biodiesel plant, glycerol, can be converted to the valuable DHA and formate at the anode while water is split to hydrogen at the cathode. As a result, both biodiesel and water splitting hydrogen generation industries can be beneficial and the system can be more sustainable.

Facile preparation of nanocrystalline TiO2 thin films using electrophoretic deposition for enhancing photoelectrochemical water splitting response

Titanium oxide (TiO2) nanocrystalline particles have been successfully deposited on fluorine-doped tin oxide (FTO) glass using an electrophoretic deposition (EPD) with a good uniformity. This study aims to optimize the heat treatment temperature, applied electric field, and deposition time to produce a good uniformity nanocrystalline TiO2 layer coating on FTO glass for better photoelectrochemical water splitting hydrogen generation performance under UV irradiation. In the present study, EPD technique allows controlling of TiO2 nanocrystalline layer characteristics on FTO glass, including smoothness and thickness. Characterisation like FE-SEM revealed that nanocrystalline TiO2 with a 14.6 μm thickness with good uniformity and minimal cracking surface exhibited the highest photocurrent density (2.12 mA/cm2) among samples. Besides, Raman and XRD analyses showed that nanocrystalline TiO2 thin coating form successfully. Based on our experiments, we proposed that the optimum condition to form nanocrystalline TiO2 thin film layer using EPD technique is 30 V, 60 s of deposition and post-annealing of the thin film at 400 °C based on the jp–V characteristic. Under illumination of 100 W UV light, the photoconversion efficiency can be up to 2.49%. In addition, hydrogen production was tested with the production rate is up to 0.45 mL/cm2/h and the photoelectrode can be re-used and remained stable for five cycles continuously.

Influence of Pt deposition on water-splitting hydrogen generation by highly-ordered TiO2 nanotube arrays

Highly-ordered TiO2 nanotube arrays (TNTAs) were fabricated on Ti sheets by electrochemical anodization. Uniform Pt nanoparticles with an average diameter of 3 nm could be successfully located on the TiO2 nanotubes on only one side (Pt/TNTAs) or both sides of the Ti sheet (Pt/TNTAs/Pt). Pt/TNTAs, the single-sided Pt deposited TNTAs, could be directly used to split water without a counter electrode. The hydrogen evolution rate can reach 120 μmol h−1 cm−2 in a mixed solution of 0.5 M Na2SO4 and 0.5 M ethylene glycol without any applied bias, which is six times of that by the pure TNTAs. In comparison to the traditional three electrode system, this single-sided Pt deposited TNTAs is a much more simple and efficient water splitting system. Meanwhile, the photoelectrical conversion mechanism has been investigated in detail.

Fabrication and optical properties of silicon nanowire/Cu2O nano-heterojunctions by electroless deposition technique

Si NWs/Cu2O nano-heterojunctions were prepared by adopting electroless deposition technique to deposit Cu2O nanocrystallines on the surface of silicon nanowire arrays (Si NWs). Raman spectra of the composite structures demonstrate that the infrared (IR) active mode originated from the defects in the crystal lattice and is activated in Cu2O nanocrystallines. Photoluminescence (PL) spectra of the nano-heterojunctions suggest that the as-prepared Si NWs/Cu2O composite structures present a new luminescence wavelength after the formation of nano-heterojunctions. In addition, the PL peak of Si NWs in the composite structure exhibits a little red-shift and becomes broad in comparison with that of the pure Si NWs. The radial Si NWs/Cu2O nano-heterojunction structure would have promising applications in many fields such as heterojuncion solar cell, photocatalysis, and water splitting hydrogen generation.

Photocatalytic water splitting of surfactant-free fabricated high surface area NaTaO3 nanocrystals

Uniform NaTaO3 ca. 15 nm and ca. 70 nm nanocrystals were synthesized via a novel surfactant-free solvothermal reaction. The acidic alkoxide hydrolyzation leads to small size and high surface area particles. The different size of the nanocrystals was controlled by altering concentration. The alkoxide-based synthetic route leads to small particle size and high surface area, which improved the charge separation and migration of photogenerated carriers and benefited the surface chemical reaction of catalysts. As a result, the high total yield of photocatalytic water splitting hydrogen generation was obtained, which was as high as 3.106 mmol h−1 g−1.

Preparation of hybrid WO3–TiO2 nanotube photoelectrodes using anodization and wet impregnation: Improved water-splitting hydrogen generation performance

Hybrid tungsten trioxide-titanium dioxide (WO3–TiO2) nanotube photoelectrodes were prepared using simple anodization and wet impregnation. These hybrid nanotube photoelectrodes significantly enhanced their photoelectrochemical (PEC) water-splitting performances compared with pure TiO2 nanotube photoelectrodes. This study aims to determine the optimum soaking time in ammonium paratungstate used as the tungsten precursor for incorporating WO3 species into TiO2 nanotube photoelectrodes. A low content of WO3 species successfully diffused into the TiO2 lattice and formed W–O–Ti bonds, which significantly promoted effective charge separation by trapping photo-induced electrons from TiO2. Thus, the photocurrent density, photoconversion efficiency, STH efficiency, and H2 generation of the resultant hybrid nanotubes were increased. However, excess WO3 species in the TiO2 nanotubes resulted in poor PEC water-splitting performance. This behavior was attributed to the large agglomerates of WO3 species were covered on the surface nanotubes that formed undesirable layers. Consequently, these undesirable layers would act as recombination sites for photo-induced electrons and holes.

Ab initio modeling of sulphur doped TiO2 nanotubular photocatalyst for water-splitting hydrogen generation

In order to construct an efficient visible-light-driven TiO2 photocatalyst for water splitting applications, one has to perform improvements of its electronic structure. In this theoretical study we consider single-walled anatase TiO2 nanotubes having following morphologies: (101) 3-layered wall with chirality indexes (n,0) and (n,n), (101) 6-layered wall with (n,0) and (0,n), (001) 6-layered wall with (n,0) and (0,n), and (001) 9-layered wall with (n,0) and (0,n). The latter configuration occurs to be the most energetically stable, due to possessing negative strain energy. In our study the most stable 9-layered anatase (001) (0,n) nanotube has been doped with sulphur. According to obtained results sulphur dopant creates the mid-gap states making the TiO2 nanotube to be a good candidate for efficient photocatalyst working under day light irradiation.

Higher water splitting hydrogen generation rate for single crystalline anatase phase of TiO2nanotube arrays

This paper presents a detailed investigation on the effect of heat-treatment process on the highly ordered titanium dioxide (TiO 2 ) nanotube arrays in connection with the photoelectrochemical (PEC) response and hydrogen evolution rate. TiO 2 nanotube arrays have been systematically heat-treated to control the transformation of as-anodized TiO 2 amorphous structure to crystalline anatase and rutile phases. In this study, single crystalline TiO 2 anatase phase exhibited a higher PEC response and hydrogen evolution rate at 400 °C heat treatment. The photocurrent density increase was mainly attributed to the effective transport of photo-induced electrons within the single crystal anatase phase. However, polycrystalline anatase and rutile phases showed the fluctuation in lower photocurrent density upon heat treatment above 500 °C. The mobility of photo-induced electrons was obviously hindered due to the recombination losses in defect sites between the anatase and rutile phase.

Size-dependent electrocatalytic activities of printed Co 3O 4 films for a monolithic photovoltaic-electrolytic hydrogen generation system

As an electrocatalyst in a monolithic PV-electrolytic cell for water splitting hydrogen generation Co 3O 4 films were prepared by a paste coating method using Co 3O 4 particles. Different sized Co 3O 4 particles with average diameters of 145.9, 63.3 and 36.5 nm were prepared using a supercritical hydrothermal synthesis method. Electrochemical properties with respect to the particle size in the film were investigated by evaluating overpotential, charge transfer resistance (R ct), and number of active sites (q∗). The relation between overpotential in water oxidation at 5 mA/cm 2 and BET surface area showed a slope of −73.8 ± 6.6, implying strong particle size dependence on electrocatalytic activity. Moreover, the R ct and q ∗ values and the actual hydrogen evolution rate indicated that the electrocatalytic activity of Co 3O 4 is attributed to physical properties (e.g. particle size) of the film. Whereas, intrinsic single site activity of the film was not significantly changed with respect to the particle size in the film.