Stable Solar Water Splitting Enabled in Anodic W/WO3 Nanorod Based Electrodes by Hydrothermal Engineering

A ternary Ag/TiO2/CNT photoanode for efficient photoelectrochemical water splitting under visible light irradiation

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

The ability of photoanodes to simultaneously tailor light absorption, charge separation, and water oxidation processes represents an important endeavor toward highly efficient photoelectrochemical (PEC) water splitting. Here, a robust strategy is reported to render markedly improved PEC water splitting via sandwiching a photothermal Co3 O4 layer between a BiVO4 photoanode film and an FeOOH/NiOOH electrocatalyst sheet. The deposited Co3 O4 layer manifests compelling photothermal effect upon near-infrared irradiation and raises the temperature of the photoanodes in situ, leading to extended light absorption, enhanced charge transfer, and accelerated water oxidation kinetics simultaneously. The judiciously designed NiOOH/FeOOH/Co3 O4 /BiVO4 photoanode renders a superior photocurrent density of 6.34mA cm-2 at 1.23V versus a reversible reference electrode (VRHE ) with outstanding applied bias photon-to-current efficiency of 2.72% at 0.6VRHE . In addition to the metal oxide, a wide variety of metal sulfides, nitrides, and phosphides (e.g., CoS, CoN, and CoP) can be exploited as the heaters to yield high-performance BiVO4 -based photoanodes. Apart from BiVO4 , other metal oxides (e.g., Fe2 O3 and TiO2 ) can also be covered by photothermal materials to impart significantly promoted water splitting. This simple yet general strategy provides a unique platform to capitalize on their photothermal characteristics to engineer high-performing energy conversion and storage materials and devices.

Controlling shape anisotropy of hexagonal CdS for highly stable and efficient photocatalytic H2 evolution and photoelectrochemical water splitting

g-C3N4:Sn-doped In2O3 (ITO) nanocomposite for photoelectrochemical reduction of water using solar light

Remarkable improvement of photoelectrochemical water splitting in pristine and black anodic TiO2 nanotubes by enhancing microstructural ordering and uniformity

Exploiting the complete efficacy of 3D-nitrogen-doped ZnO nanowires photoanode via type-II ZnS core-shell formation toward highly stable photoelectrochemical water splitting

Integration of surficial oxygen vacancies and interfacial two-dimensional NiFe-layered double hydroxide nanosheets onto bismuth vanadate photoanode for boosted photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting using semiconductors has been a promising means to directly produce hydrogen gas from water. Semiconductors capable of absorbing a wide wavelength range of visible light are in the limelight at the moment, because the favourable optical property causes thermodynamically high solar-to-hydrogen (STH) conversion efficiency. The n-type perovskite oxynitrides AB(O,N)3 (A=Ca, Sr, Ba and La, B=Ti, Ta and Nb) have intensive absorption bands in visible light above 600 nm so that they are one of the leading candidates for the solar water splitting.1 For instance, LaNbON2 has an Eg of 1.7 eV (wavelength λ = 750 nm) and thus potential for the STH conversion efficiency of 29.5% in reference to an incident photon-to-current efficiency of unity. However, the water splitting activity using the oxynitrides is relatively low and unstable under long-term illumination.1-2 The synthesis of oxynitrides, namely, the nitridation of starting oxides to the corresponding oxynitrides, is performed mostly in harsh conditions including a high temperature, long nitridation time, and a high flow rate of NH3. Such the severe nitridation obviously increases a bulk crystallinity of oxynitrides, which it permits efficient light absorption and charge separation under irradiation. In particular, it is necessary to prepare high-crystalline BaBO2N because the oxynitrides have the absence of stoichiometric, crystalline oxide precursors (with Ba/B ratio of unity) for nitridation.3-4 Unfortunately, the severe nitridation enhances reductions of B-site cations to a lower oxidation state (e.g., Nb5+ to Nb4+ or Nb3+) leading to anion vacancy and/or surface impurity phases such as NbO x N y , owing to the high electronegativity of the B-site cations. The surface defect sites, where the photoreaction takes place, promotes recombination of photoexcited holes and electrons during the PEC water oxidation, thus resulting in low photocurrent density. There were several reports on how to decrease defect density of oxynitrides during the nitridation, including flux-assisted nitridation and etching treatment by an aqua regia.4-5 However, these methods should be positively considered the suitable flux media and unintended doping depending on each oxynitride, and the partial loss of oxynitride particles. There was also a limitation in improving the photocurrent density.Herein we present the active and stable solar water oxidation over oxynitride photoanodes, which was remarkably improved by surface annealing in an inert Ar flow to increase the degree of crystallinity and as well to suppress the generation of surface defects.6 Also, we report the quantitative relationship between the surface defect density of the oxynitride and its photocurrent density, demonstrating that surface properties of oxynitrides greatly influence the solar water splitting performance.7 Oxides (or carbonates) contains A-site cations and B2O5 (B=Ta, Nb) were blended and calcined in air at different temperatures from 1273 to 1573 K for 30 h to obtain crystalline oxides such as Ba5B4O15. The starting oxides were nitrided at different temperatures and durations under a NH3 flow. The products were washed by distilled water and dried naturally. Subsequently, the as-prepared oxynitrides were annealed under an Ar flow at an appropriate temperature depending on oxynitride. For PEC measurements, particulate oxynitride photoanodes were prepared by a particle transfer method or an electrophoretic deposition.The nitridation of Ba5B4O15 caused highly-defective BaBO2N surface due to Lewis base and Ba-rich conditions of the starting oxide. The annealing in Ar both enhances the crystallinity of amorphous surface and effectively suppresses the formation of reduced defects. As a result, the annealed BaNbO2N photoanode exhibited greatly enhanced photocurrent of 5.2 mA cm-2 at 1.23 VRHE during sunlight-driven water oxidation, which is approximately twenty times higher than that of as-prepared BaNbO2N. BaTaO2N is thermally more stable in an Ar flow as compared with BaNbO2N, so that the high-temperature annealing at 1073 K was found to improve both the bulk and surface properties of inactive as-prepared BaTaO2N. Consequently, the particulate BaTaO2N photoanode followed by the Ar annealing produced a half-cell STH energy conversion efficiency of 1.4% at 0.88 VRHE. In addition, the solar water oxidation retained 79% of the initial photocurrent over 24 h. Surface and bulk characterizations of perovskite oxynitrides annealed in Ar and the corresponding photoresponse will be discussed in detail in the presentation.References J. Seo et al., Angew. Chem. Int. Ed., 2018, 57, 2.T. Hisatomi et al., Energy Environ. Sci., 2013, 6, 3595.J. Seo et al., Adv. Energy Mater., 2018, 1800094.S. Jadhav et al., J. Mater. Chem. A, 2020, 8, 1127.M. Matsukawa et al., Nano Lett., 2014, 14, 1038.J. Seo et al., ACS Appl. Energy Mater., 2019, 2, 5777.J. Seo et al., J. Mater. Chem. A, 2019, 7, 493.

Enhanced charge transfer with tuning surface state in hematite photoanode integrated by niobium and zirconium co-doping for efficient photoelectrochemical water splitting

Hydrogen generation via photoelectrochemical (PEC) overall water splitting is an attractive means of renewable energy production so developing and designing the cost-effective and high-activity bifunctional PEC catalysts both for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) has been focused on. Based on first-principles calculations, we propose a feasible strategy to enhance either HER or OER performance in the monoclinic exposed BiVO4 (110) facet by the introduction of oxygen vacancies (Ovacs). Our results show that oxygen vacancies induce charge rearrangements, which enhances charge transfer between active sites and adatoms. Furthermore, the incorporation of oxygen vacancies reduces the work function of the system, which makes charge transfer from the inner to the surface more easily; thus, the charges possess stronger redox capacity. As a result, the Ovac reduces both the hydrogen adsorption-free energy (ΔGH*) for the HER and the overpotential for the OER, facilitating the PEC activity of overall water splitting. The findings provide not only a method to develop bifunctional PEC catalysts based on BiVO4 but also insight into the mechanism of enhanced catalytic performance.

Copper chalcopyrite semiconductors of Cu(In,Ga)(S,Se)2 include a wide range of compounds that are of interest for photoelectrochemical (PEC) water splitting due to their band gaps in the range of 1.0 eV to 2.5 eV by varying the alloy ratio. The conduction band of the compounds is also well positioned above the reduction potential of water, enabling them to be used as photocathodes for H2 generation. Two of important factors required for photocathodes of PEC water splitting are photocurrent (photocurrent density) and onset potential (photovoltage), i.e., photocathodes having positive onset potentials coupled with high photocurrents are promised in order to induce water splitting by using a tandem system without applied bias energy. For chalcopyrite materials applied to the photocathode part, this can be achieved through band engineering by altering the composition of the alloy. In this study, wide-gap Cu(In,Ga)S2 (CIGS) films with various amounts of Ga were fabricated using spray pyrolysis followed by sulfurization. Photoelectrochemical water splitting properties of photocathodes based on these films were evaluated after modification with a CdS layer and Pt nanoparticles. Both photocurrent densities and onset potentials of the photocathodes were gradually improved by increase in the amount of Ga in the CIGS films up to a Ga/(In+Ga) ratio of 0.25 using 0.1 M Na2SO4 (pH 9) as an electrolyte under illumination of simulated sunlight (AM 1.5G). Further inclusion of Ga in the CIGS film was detrimental for both photocurrent density and onset potential. The maximum photocurrent density of 6.8 mA cm-2 (at 0 V vs. RHE) and the highest photocurrent onset potential of 0.84 V vs. RHE were obtained by using such photocathode. Achievements of a relatively wide interface band gap of the CIGS/CdS heterointerface and formation of relatively large grains in the CIGS with Ga/(In+Ga) ratio of 0.25 were likely to be responsible for such superior water reduction properties. The preliminary result of band engineering via replacement of Cu with Ag in the Cu(In,Ga)(S,Se)2 system in relation with its application as photocathode also will be presented.

<p indent=0mm>With increasing energy demands and ever-growing environmental concerns, solar energy and hydrogen energy have attracted worldwide attention. In particular, hydrogen energy not only has a high energy density, but also is clean, renewable, and carbon-free, when compared with primary energy sources such as coal, oil, and natural gas and secondary energy sources such as coal gas, petrol, and diesel. Photoelectrocatalytic (PEC) water splitting for hydrogen generation is a process in which a PEC cell containing photoelectrodes and electrolyte is used to split water into hydrogen and oxygen by solar energy. Therefore, PEC water splitting is one of the ideal ways to covert and store solar energy to hydrogen energy in terms of chemical bond energy. In a PEC cell, the photoanode is commonly based on n-type semiconductors and the photocathode based on p-type semiconductors. The efficiency of a PEC cell is determined by performance of these photoelectrodes interfaced with the electrolyte. However, because the oxygen evolution reaction on the photoanode is kinetically sluggish involving four electrons and the valance band maximum of photoanodes must be more positive than <sc>1.23 V</sc> versus reversible hydrogen electrode, suitable n-type semiconductors are quite few for this purpose, limiting the common photoanodes to low efficiencies for PEC water splitting. In recent years, the bismuth vanadate photoanode has attracted great attention due to its relatively high theoretical maximum photocurrent density <sc>(~7 mA cm^–2)</sc> and suitable band structure for water splitting, compared with other traditional photoanodes such as titanium dioxide, tungsten oxide, and zinc oxide. Extensive efforts have been made to unleash the full potential of the bismuth vanadate photoanode for PEC water splitting. In this mini review, we survey and analyze the design ideas and synthesis methods of high-performance bismuth vanadate photoanodes by looking back at the research progress made over the past few years on improving the light harvesting efficiency, photo-generated carrier separation efficiency and surface oxygen evolution efficiency of bismuth vanadate photoanodes. The strategies for improving the efficiencies of the bismuth vanadate photoanodes include defect state introduction, crystal facet and morphology control, and heterojunction engineering. Among the strategies, a single one, such as the defect state introduction, may enhance efficiencies of several processes (e.g., photo-generated carrier separation efficiency and surface oxygen evolution efficiency) of bismuth vanadate at the same time, but sometimes, it may enhance the efficiency of one process but degrade the efficiencies of others for the bismuth vanadate photoanode. Thus, how to comprehensively consider the cooperative mechanism to enhance the efficiencies of all the processes involved in PEC water splitting is the key to obtaining high performance bismuth vanadate photoanodes. At present, bismuth vanadate-based photoanodes have exhibited an extremely high photocurrent density and photo-generated carrier separation efficiency at higher bias voltage (over <sc>5 mA cm^–2</sc> at <sc>1.23 V</sc> versus reversible hydrogen electrode with over 90% photo-generated carrier separation efficiency), but the light reflection of bismuth vanadate-based photoanodes makes it unable to reach the theoretical maximum photocurrent density <sc>(~7 mA cm^–2).</sc> Moreover, the efficiencies of bismuth vanadate-based photoanodes at low bias voltages are still too low. Therefore, the future development direction should be to obtain higher photocurrent density at a lower voltage, and increase the absorption efficiency and wavelength range of bismuth vanadate to reach the theoretical photocurrent density and beyond. Although bismuth vanadate photoanodes are not necessarily the final large-scale application scheme of PEC water splitting in the future, their studies will help to provide guidelines for searching new high-performance photoanode materials.

The strategic construction of bifunctional electrocatalytic electrodes integrating high activity and exceptional durability is critical for sustainable hydrogen generation through water and seawater splitting. Addressing challenges including sluggish reaction kinetics and chloride-induced corrosion in marine electrolyzers remains imperative. Mixed transition metal oxides/sulfides, particularly cobalt–vanadium-based composites, demonstrate superior electrocatalytic properties owing to their tunable electronic configurations, multivalent redox states, enhanced charge transfer capabilities, and abundant exposed active sites. Here, we have prepared CuCo2S4@Co–V–O–F. The electrode material is then calcined under argon protection, and a synergistic structural engineering and surface treatment adjustment strategy is adopted to construct nanostructures. The optimized catalyst exhibits remarkable bifunctional performance: low HER overpotentials of 87.8 mV (1 M KOH) and 95.5 mV (alkaline seawater) at −10 mA cm−2, coupled with OER overpotentials of 227.3 mV and 213.5 mV, respectively. Notably, the symmetric electrolyzer assembled with these nanowire arrays achieves an ultralow cell voltage of 1.796 V at 50 mA cm−2, demonstrating exceptional efficiency for overall water splitting while maintaining robust stability in corrosive saline media.

Porous WO3 monolith-based photoanodes for high-efficient photoelectrochemical water splitting