Materials For Solar Hydrogen Production Research Articles

On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications.

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.

(Digital Presentation) Surface Photovoltage Provides Quasi Fermi Level Splitting and Minority Carrier Electrochemical Potential of BiVO4 /Liquid Junctions Under Illumination

The solar energy conversion efficiency of semiconductor-liquid junctions is determined by their photovoltage and by their photocurrent under illumination. Photoelectrochemistry (PEC) gives information on the current, but not on the photovoltage or the quasi Fermi level splitting (qFLS), which for slow redox couples is obscured by kinetic overpotentials and surface states. Here we show that surface photovoltage spectroscopy (SPS) provides direct measurement of the qFLS potential in BiVO4-electrolyte contacts. Measurements were done with a vibrating Kelvin probe on FTO deposited BiVO4 films in contact with aqueous electrolytes containing triiodide/iodide, oxygen/ hydrogen peroxide, or sulfate/sulfite redox couples and for iron-oxyhydroxide modified BiVO4 films. The triiodide/iodide couple, for example, yields qFLS=0.78 V under 49 mW/cm-2 (400 nm LED) illumination, comparable to the photovoltage from PEC (0.79 V). Furthermore, the reversibility of the redox reactions determines the reversibility of the SPS transients. For example, the triiodide/iodide couple gives nearly complete SPS reversibility on the 20 sec timescale, while the sulfate/sulfite couple gives an irreversible photovoltage signal. Lastly, by combining the SPS data with the majority carrier potential from open circuit potential measurements, the minority carrier electrochemical potentials, Efp can be obtained for all systems. For instance, Efp = 1.0 V vs RHE for the BiVO4 - triiodide/iodide junction and Efp = 0.26 V vs RHE for the sulfate/sulfite junction. These shows that BiVO4 can photoelectrochemically oxidize both redox couples, without applied bias. These results shed new light on the photoelectrochemistry of BiVO4, an important photoanode material for solar hydrogen production from water. Figure 1

III–V Semiconductor Materials for Solar Hydrogen Production: Status and Prospects

Following recent developments in photoelectrochemical and photovoltaic–electrosynthetic systems, we present the benefits of III–V semiconductors for solar water splitting. In addition to their inte...

Flame synthesized MoO3 nanobelts and nanoparticles coated with BiVO4 for photoelectrochemical hydrogen production

MoO3 nanoparticles and nanobelts are synthesized by flame synthesis. Flame synthesis is used in combination with spin coating as the main method to prepare the BiVO4/MoO3 heterojunction photoelectrode. Compared with the use of MoO3 and BiVO4 as a single material, the BiVO4/MoO3 heterojunction broadens the range of light absorption and improves charge transport and separation performance, overcoming the constraints of low visible light utilization and slow water oxidation kinetics of single material. The experiment shows that the increase of BiVO4/MoO3 nanobelts composite structure in photocurrent production is three times more than that of BiVO4/MoO3 nanoparticles under the same condition. When the applied external bias is 1.23 V vs. RHE, BiVO4/MoO3 nanoparticles will produce a photocurrent density of 0.18 mA/cm2, while the optimal BiVO4/MoO3 nanobelts heterojunction produces a photocurrent density of 0.54 mA/cm2. In the process of testing, the BiVO4/MoO3 nanobelts have more stability within the same deteriorating time. The photocurrent decay rate of BiVO4/MoO3 nanoparticles heterojunction is 27.8% while that of the BiVO4/MoO3 nanobelts is only 18.4%. In addition, abundant oxygen vacancies or defects are found in the BiVO4/MoO3 nanobelts composite structure, which accelerate the transfer and separation of charge carriers, resulting in higher photocatalytic activity. Thus, the results show that the composite photoelectrode of BiVO4 and one-dimensional MoO3 is a promising material for solar hydrogen production.

Two-Dimensional CuTe2X (X = Cl, Br, and I): Potential Photocatalysts for Water Splitting under the Visible/Infrared Light

Exploring new two-dimensional (2D) materials with excellent light absorption ability is important for efficient photocatalytic water splitting. In this work, we systematically studied the mechanical, electronic, and optical properties of an unexplored class of 2D material (CuTe2Cl, CuTe2Br, and CuTe2I monolayers) by density functional theory (DFT) calculations. We find that CuTe2X monolayers with great thermal and dynamic stability can be easily exfoliated from their bulk phases. CuTe2X monolayers possess band gaps of 1.7–1.8 eV and their band edge positions bestride the water oxidation and reduction potential. Moreover, they exhibit extraordinary light harvesting ability in both visible and near-infrared regions. Thus, CuTe2X monolayers would be promising photocatalysts for water splitting. In addition, their strain-tunable electronic structure can further improve their photocatalytic capability. Our work not only expands the family of 2D materials but also highlights new interesting materials for solar hydrogen production.

Mechanistic Study of Monolayer NiP2 (100) toward Solar Hydrogen Production

Understanding the catalytic mechanism at the molecular level is critical to the design and exploration of new materials for solar hydrogen production. Herein, the catalytic activity toward hydrogen evolution reaction (HER) is surveyed for all low‐index surfaces of different nickel phosphides (Ni3P, Ni12P5, Ni2P, Ni5P4, NiP, NiP2, and NiP3). It is found that the surface P atoms with only one single PP bond possess excellent HER activity. Accordingly, a monolayer NiP2 (100) is explored as the cathode material. Comprehensive density functional theory study is used to verify the HER activity of this material, including the effect of vacancy and doping. Defect‐free and Zn‐doped monolayer NiP2 (100) are found to possess excellent HER activity and thermodynamic stability. This crystal facet also has a high density of active sites (0.126 N Å−2 under an applied over‐potential of 200 mV), which is very close to the Pt (111) surface (0.132 N Å−2). The low surface energy and strengthened bonding imply that the monolayer NiP2 (100) can be stable and easy to synthesize. This finding not only promotes a comprehensive understanding of NixPy material toward its catalytic activity for HER, but also suggests a possible new material for the experimentalist in the field.

Oxysulfide photocatalyst for visible-light-driven overall water splitting.

Oxysulfide semiconductors have narrow bandgaps suitable for water splitting under visible-light irradiation, because the electronegative sulfide ions negatively shift the valence band edges of the corresponding oxides1,2. However, the instability of sulfide ions during the water oxidation is a critical obstacle to simultaneous evolution of hydrogen and oxygen3. Here, we demonstrate the activation and stabilization of Y2Ti2O5S2, with a bandgap of 1.9 eV, as a photocatalyst for overall water splitting. On loading of IrO2 and Rh/Cr2O3 as oxygen and hydrogen evolution co-catalysts, respectively, and fine-tuning of the reaction conditions, simultaneous production of stoichiometric amounts of hydrogen and oxygen was achieved on Y2Ti2O5S2 during a 20 h reaction. The discovery of the overall water splitting capabilities of Y2Ti2O5S2 extends the range of promising materials for solar hydrogen production.

Enabling Pt-free photocatalytic hydrogen evolution on polymeric melon: Role of amorphization for overcoming the limiting factors

Polymeric melon shows real promise to be a next generation photocatalyst material for solar hydrogen production. Because it is earth abundant, and highly stable in photoredox reactions. However, a long-standing problem with polymeric melon is that it is inactive for photocatalytic hydrogen evolution without a metal cocatalyst, such as Pt. Sluggish charge transfer kinetics and a high activation energy for proton reduction makes pristine melon overly dependent on a high loading of precious Pt-cocatalyst for hydrogen evolution. While Pt-assisted hydrogen evolution is not considered economic for scale-up, a modulation of intrinsic photophysical and physicochemical properties of melon is therefore crucial to overcome the Pt dependency. However, to our knowledge, no systematic study has been reported to date to make melon independent of Pt for solar hydrogen evolution. Here we show that the limiting factors of Pt-dependent hydrogen production can be overcome by amorphization. The amorphization lowers the band gap, shifts the proton reduction potential close to the origin, and enhances the light harvesting and charge transport. Therefore, it results in a Pt-free hydrogen evolution under visible light irradiation. We here also present a mechanistic understanding of the origin of this cocatalyst-free hydrogen evolution.

Predicting New Two-Dimensional Pd3(PS4)2 as an Efficient Photocatalyst for Water Splitting

Photocatalytic water splitting is a potential solution for energy and environmental problems. By using density functional theory method, here, we present an experimentally unexplored two-dimensional (2D) material, palladium thiophosphate (Pd3(PS4)2), as a promising photocatalyst for water splitting. 2D Pd3(PS4)2 is predicted to be exfoliated mechanically from the bulk phase and is dynamically stable. The calculated band gaps for mono-, bi-, and trilayered Pd3(PS4)2 are 2.81 eV (direct), 2.79 eV (indirect), and 2.70 eV (indirect), respectively. Moreover, their band edges straddle between the redox potentials of water. Thus, 2D Pd3(PS4)2 would be a promising photocatalyst for water splitting. Additionally, the strain has significant impact on light absorption and can further improve the photocatalytic capability. Our work expands the family of 2D materials and highlights a new interesting material for solar hydrogen production.

The “Rust” Challenge: On the Correlations between Electronic Structure, Excited State Dynamics, and Photoelectrochemical Performance of Hematite Photoanodes for Solar Water Splitting

In recent years, hematite's potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics, and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics.

Oxygen-Intercalated CuFeO2 Photocathode Fabricated by Hybrid Microwave Annealing for Efficient Solar Hydrogen Production

Delafossite CuFeO2 is a promising photocathode material for solar hydrogen production, but its performance is low because of poor charge transport properties. When the prepared CuFeO2 electrode is annealed by hybrid microwave annealing (HMA), its photoelectrochemical water reduction activity increases by more than 4 times (−1.3 mA cm–2 @ 0.4 VRHE), while the conventional thermal annealing (CTA) improves the performance by only 2 times (−0.62 mA cm–2 @0.4 VRHE). The postannealing of the electrode intercalates extra oxygen into the CuFeO2 lattice to form CuFeO2+1.5δ, which increases the charge carrier density and thus improves charge transport properties. The oxygen intercalation with HMA takes place more uniformly over the whole solid and is more effective than CTA. In addition, HMA post-treated CuFeO2 is modified with a NiFe-layered double hydroxide/reduced graphene oxide electrocatalyst, which exhibits a high photoactivity of −2.4 mA cm–2 @ 0.4 VRHE, unprecedented for CuFeO2-based photocathodes.

Boosting up the Low Catalytic Activity of Silver for H2 Production on Ag/TiO2 Photocatalyst: Thiocyanate as a Selective Modifier

Noble metal cocatalysts like Pt have been widely employed as an essential ingredient in many kinds of photocatalytic materials for solar hydrogen production. The high material cost of Pt is the biggest limitation. Silver is far less expensive but much less active than Pt and Au as a hydrogen evolving catalyst. Here we demonstrate a new strategy to boost up the activity of silver in Ag/TiO2 for photocatalytic H2 production via forming a simple surface complexation of thiocyanate (SCN–) on silver. The addition of thiocyanate in the suspension of Ag/TiO2 markedly enhanced the photocatalytic production of H2 by about 4 times. Thiocyanate was not consumed at all during the photoreaction, which ruled out the role of thiocyanate as an electron donor. Such a positive role of thiocyanate was not observed with bare TiO2, Pt/TiO2, and Au/TiO2. The selective chemisorption of thiocyanate on silver was confirmed by the analyses of Raman spectroscopy and spot-profile energy-dispersive spectroscopy. In the presence of th...

Robust and effective Ru-bipyridyl dye sensitized Ti-MCM-48 cubic mesoporous materials for photocatalytic hydrogen evolution under visible light illumination

A visible light absorbing solar hydrogen evolution photocatalyst was developed by adsorbing [Ru(bpy)2(dcbpy)][PF6]2 (Ru-4,4′ dye) to Ti-MCM-48 cubic mesoporous materials. The Ru-4,4′ dye was found to bind strongly with Ti-MCM-48 without sacrificing the periodic mesoporous MCM-48 cubic structure. The photocatalytic activities of Ru-4,4′ sensitized Ti-MCM-48 materials for solar hydrogen production under visible light illumination was examined. The recycling studies indicate long-term stability of the Ru-4,4′-Ti-MCM-48 photocatalysts. The results indicate that cubic MCM-48 mesoporous materials with spatially isolated titania moieties sensitized by [Ru(bpy)2(dcbpy)][PF6]2 are robust photocatalysts for solar hydrogen production.

Enhancing the Performance of a robust sol-gel-processed p-type delafossite CuFeO2 photocathode for solar water reduction.

Delafossite CuFeO2 is a promising material for solar hydrogen production, but is limited by poor photocurrent. Strategies are demonstrated herein to improve the performance of CuFeO2 electrodes prepared directly on transparent conductive substrates by using a simple sol-gel technique. Optimizing the delafossite layer thickness and increasing the majority carrier concentration (through the thermal intercalation of oxygen) give insights into the limitations of photogenerated charge extraction and enable performance improvements. In oxygen-saturated electrolyte, (sacrificial) photocurrents (1 sun illumination) up to 1.51 mA cm(-2) at +0.35 V versus a reversible hydrogen electrode (RHE) are observed. Water photoreduction with bare delafossite is limited by poor hydrogen evolution catalysis, but employing methyl viologen as an electron acceptor verifies that photogenerated electrons can be extracted from the conduction band before recombination into mid-gap trap states identified by electrochemical impedance spectroscopy. Through the use of suitable oxide overlayers and a platinum catalyst, sustained solar hydrogen production photocurrents of 0.4 mA cm(-2) at 0 V versus RHE (0.8 mA cm(-2) at -0.2 V) are demonstrated. Importantly, bare CuFeO2 is highly stable at potentials at which photocurrent is generated. No degradation is observed after 40 h under operating conditions in oxygen-saturated electrolyte.

Nanostructured materials for solar hydrogen production

Abstract One of the main requirements for a future Hydrogen Economy is a clean and efficient process for producing hydrogen using renewable energy sources. Hydrogen is a promising energy carrier because of its high energy content and clean combustion. In particular, the production of hydrogen from water and solar energy, i.e., photocatalysis and photoelectrolysis, represent methods for both renewable and sustainable energy production. Here, we will present the principles of photocatalysis and the PhotoElectroChemical cell (PEC cell) for water splitting, along with functional materials. Defect chemical aspects will be high-lighted. To date, the decreasing length scale to the nanoscale of the functional materials attracts widespread attention. The nanostructure is beneficial in case diffusion lengths of the photo-generated charge carriers are substantially different.

Birnessite-type manganese oxides nanosheets with hole acceptor assisted photoelectrochemical activity in response to visible light

A cost-effective and simple electroplating technique has been developed to prepare layered manganese oxide (MnO2) arrays as a promising material for solar hydrogen production and waste-water cleaning through photoelectrochemical (PEC) process. The microstructure of these MnO2 nanosheets can be referenced to Birnessite-type, as characterized by Raman spectra and transmission electron microscopy. The bandgap energy of the as-grown nanosheets determined from UV-vis spectroscopy is about 2.1 eV. Mott–Schottky plots show the flat band potential of the MnO2 nanosheets to be −0.01 V and a donor concentration of 4.68 × 1020 cm−3. Remarkable photocurrent in response to visible light is observed in the presence of hole acceptors, such as sodium formate and methanol, which efficiently suppress the recombination loss of electron–hole pairs from localized d–d transitions within manganese ion. Meanwhile, the transient photocurrent–time responses and the effect of different hole acceptors on PEC activity are studied with an increase of respective hole acceptor concentration, and the results reveal the critical role in the process of absorption and decomposition of the hole acceptor. Significantly, the MnO2 nanosheets exhibit an incident photon-to-electron conversion efficiency of 7% in response to the monochromatic wavelength of 400 nm, which is comparable to that from hematite (α-Fe2O3). These results demonstrate the nanoporous MnO2 nanosheets have great potential in solar hydrogen applications and organic pollutant cleaning.

Engineered Nanorod Perovskite Film Photocatalysts to Harvest Visible Light

The recent fl ourishing of nanostructured materials has widened its potential applications in the much-desired effi cient energy materials. Specifi cally, materials for solar hydrogen production by water splitting should possess superior optoelectric properties in addition to suitable band energetics and durability in aqueous solutions. The tunability of the physicochemical properties of nanostructred materials by virtue of their size and shape renders a wider applicability. Since the critical limitation of popular TiO 2 photocatalyst that absorbs only UV light, [ 1 , 2 ] the fi eld of visible light water splitting photocatalyst is currently being nurtured by various kinds of conventional and new single component materials (CdS, WO 3 , Fe 2 O 3 , TaON, etc.), as well as composite materials. [ 3–20 ] However, there is still much potential for nanostructured materials in this area. There have been several reports on the binary [ 3–6 ] and ternary metal oxides, [ 7–13 ]