Nanostructured Ti-doped hematite (α-Fe2O3) photoanodes for efficient photoelectrochemical water oxidation

We present a form of hematite (α-Fe₂O₃) nanostructured architecture suitable for photoelectrochemical water oxidation that is easily synthesized by a pulsed laser deposition (PLD) method. The architecture is a column-like porous nanostructure consisting of nanoparticles 30–50 nm in size with open channels of pores between the columns. This nanostructured film is generated by controlling the kinetic energy of the ablated species during the pulsed laser deposition process. In a comparison with the nanostructured film, hematite thin film was also synthesized by PLD. All of the developed films were successfully doped with 1.0 at% of titanium. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy were used to characterize the films. To fabricate the photoelectrochemical (PEC) cell, Ti-doped hematite films were used as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode and an aqueous solution of 1 M NaOH as the electrolyte. The photovoltaic characteristics of all cells were investigated under AM 1.5G sunlight illumination of 100 mW/cm². The photocurrent density was enhanced by approximately 220% using nanostructured film at 0.7 V versus Ag/AgCl compared to hematite thin film, and the highest photocurrent density of 2.1 mA/cm² at 0.7 V/Ag/AgCl was obtained from the 1.0 at% Ti-doped hematite nanostructured film. The enhanced photocurrent density is attributed to its effective charge collection due to its unique column-like architecture with a large surface area.     

Fabricating CdS/BiVO4 and BiVO4/CdS heterostructured film photoelectrodes for photoelectrochemical applications

The photoelectrochemical (PEC) properties of heterostructured CdS/BiVO₄ and BiVO₄/CdS film electrodes on conducting glass for hydrogen production under visible light were investigated. These two types heterostructured film electrodes were prepared using spin coating method and ultrasonic spray pyrolysis method. The structural analyses of the prepared films were determined by using XRD, SEM, EDX and UV–vis. Photoelectrochemical measurements were carried out in a convenient three electrodes cell with 0.5 M Na₂SO₃ aqueous solution. In order to investigate band gap influence of electrode PEC property, a series ITO/Cd_1−xZn_xS/BiVO₄ and ITO/BiVO₄/Cd_1−xZn_xS (x = 0 ∼ 1) film electrodes were also synthesized. After PEC test, a maximum photocurrent density from ITO/CdS/BiVO₄ film electrode was confirmed. The maximum photocurrent density, 3 times and 113 times as that of single CdS film electrode and single BiVO₄ film electrode, respectively. Incident photon to current conversion (IPCE) of as prepared film electrodes were measured and the value were 65% (ITO/CdS/BiVO₄), 22% (single CdS film) and 10% (ITO/BiVO₄/CdS) at 480 nm with 0.3 V external bias. Comparison with ITO/BiVO₄/CdS electrode and single Cd_1−xZn_xS electrodes, the heterostructured ITO/CdS/BiVO₄ electrode can effectively suppress photogenerated electron-hole recombination and enhance light harvesting. Therefore, the ITO/CdS/BiVO₄ electrode gave the maximum photocurrent density and IPCE value.     

Fabrication of large area nanorod like structured CdS photoanode for solar H2 generation using spray pyrolysis technique

Large area nanorod like structured CdS films (9 × 9 cm²) were deposited on the FTO glass substrate using simple and economic spray pyrolysis deposition technique for photoelectrochemical (PEC) hydrogen production. With an intention of electrode scaling-up, the deposition area of photoanode was varied to evaluate its effect on the PEC hydrogen generation capability. High photocurrent of 5 mA has been achieved from the PEC active area of 37.5 cm². Its unit area (1 cm²) counterpart yielded Solar-to-Hydrogen (STH) conversion efficiency of 0.20% at a bias of 0.2 V vs Ag/AgCl using sacrificial reagents under solar simulator (AM1.5) with 80 mW/cm² irradiance. The 500 nm thick film exhibiting uniformly distributed nano-rod features yielded 3-times more photocurrent, as well as hydrogen evolution than other films. It exhibited an enhanced photo-activity as indicated by the higher IPCE values (5–9%) in the wavelength range of 450–550 nm. It exhibited superior optical properties (E_g ∼2.4 eV), and formation of high crystallinity hexagonal CdS phase with space group P63MC. The superior performance of the photoanode is attributed to the nanostructured morphology acquired under optimized spray pyrolysis conditions. Large area photoanodes showed unaltered photo-activity indicating the homogeneity in the film properties even in scaled-up version.     

3D-nano-hetero-structured n/n junction,CuO/Ru–ZnO thin films,for hydrogen generation with enhanced photoelectrochemical performances

Hydrogen, derived from solar-water splitting, is a clean and renewable fuel for which per gram energy storage capacity is even higher than fossil fuels. Towards the development of a viable technology for above conversion, this report describes enhanced performance in photoelectrochemical water splitting using uniquely evolved nano-hetero-structured bilayered thin films, CuO/Ru–ZnO as photoanode. Grown over ITO (In:SnO₂) glass substrates by using low-cost and easily up-scalable wet chemical methods, films were characterized for microstructure, optical behaviour and surface characteristics, using XRD and other spectral measurements viz. FESEM, AFM, TEM, UV–Visible Spectroscopy, EDX and XPS. Against monolayered pristine films of CuO and ZnO, bilayered films yielded a major gain in PEC water splitting photocurrent, on being used as working electrode in PEC cell, in conjunction with platinum counter electrode and saturated calomel reference electrode (electrolyte solution 0.1 M NaOH solution, pH 13, temperature 30 ± 3.6 °C). Films with 1% Ru-incorporation yielded highest photocurrent (2.04 mA/cm²). Enhanced photoactivity of bilayered films was found correlated with increments in light absorption, charge carrier density and film surface area, coupled with reduced electrical resistivity. The study highlights an important role played by Ru added in ZnO overlayer, apparently existing as RuO₂ nanoparticles dispersed in ZnO lattice, in hole-transfer from valence band of CuO underlayer to electrolyte, thereby imparting a significant boost on photocurrent generation.     

Nanostructured Zn-Fe2O3 thin film modified by Fe-TiO2 for photoelectrochemical generation of hydrogen

Nanostructured semiconductor thin films of Zn-Fe₂O₃ modified with underlying layer of Fe-TiO₂ have been synthesized and studied as photoelectrode in photoelectrochemical (PEC) cell for generation of hydrogen through water splitting. The Zn-Fe₂O₃ thin film photoelectrodes were designed for best performance by tailoring thickness of the Fe-TiO₂ film. A maximum photocurrent density of 748 μA/cm² at 0.95 V/SCE and solar to hydrogen conversion efficiency of 0.47% was observed for 0.89 μm thick modified photoelectrode in 1 M NaOH as electrolyte and under 1.5 AM solar simulator. To analyse the PEC results the films were characterized for various physical and semiconducting properties using XRD, SEM, EDX and UV–Visible spectrophotometer. Zn-Fe₂O₃ thin films modified with Fe-TiO₂ exhibited improved visible light absorption. A noticeable change in surface morphology of the modified Zn-Fe₂O₃ film was observed as compared to the pristine Zn-Fe₂O₃ film. Flatband potential values calculated from Mott–Schottky curves also supported the PEC response.     

Enhancing photoelectrochemical activity of nanocrystalline WO3 electrodes by surface tuning with Fe(Ⅲ)

Tungsten oxide (WO₃) photoelectrodes with the surface tuned by Fe(Ⅲ) for photoelectrochemical water splitting were successfully synthesized. Nanostructured WO₃ films were prepared using doctor blade method, then a facile and economical deposition-annealing process was employed to fabricate Fe(Ⅲ) modified WO₃ films. The resulting composite's structural and optical properties were analyzed by SEM, EDX, XRD, UV–Vis spectrometry and XPS. The photoelectrochemical properties were evaluated by photocurrent density under 500 W Xe lamp with an intensity of 100 mW/cm². The Fe(Ⅲ) modified WO₃ electrode exhibited a larger photocurrent than the pure WO₃ electrode. Significantly, the optimized Fe(Ⅲ) modified WO₃ film achieved the maximum photocurrent density of 1.18 mA/cm² at 0.8 V vs. Ag/AgCl in the 0.2 M Na₂SO₄. The enhanced photocurrent was attributed to the extension of the light response and the electron hole separation at the interface Fe(Ⅲ)/WO₃ which was confirmed by Mott–Schottky and electrochemical impedance spectroscopy.     

Hierarchical mesoporous SnO2/BiVO4 photoanode decorated with Ag nanorods for efficient photoelectrochemical water splitting

Bismuth vanadate has been extensively investigated as a potential visible light photoanode for PEC water splitting. The performance of BiVO₄ is restricted by fast charge recombination and slow oxygen evolution reaction kinetic. To address these issues, hierarchical SnO₂ (HSN) mesoporous support is developed via a novel sol-electrophoretic approach, and BiVO₄ film is decorated with silver nanorods (Ag NRs). The photocurrent density of HSN/BiVO₄ photoanode is 3.98 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and onset potential (V_onset) of 0.5 V vs. RHE. The PEC performance is attributed to the appropriate band alignment between SnO₂ and BiVO₄, as well as the hierarchical structure of SnO₂. Ag-HSN/BiVO₄ photoanode shows photocurrent density of 4.30 mA/cm² at 1.23 V vs. RHE and V_onset of 0.28 V vs. RHE. The enhanced photocurrent and negatively shifted V_onset can be attributed to radiative localized surface plasmon resonance decay and catalytic effect of Ag NRs, respectively.     

Nano-wall like visible-light active carbon modified n-TiO2 thin films for efficient photoelectrochemical oxygen separation from water

Efficient photoelectrochemical oxygen separation from water was demonstrated using a nano-wall like carbon modified n-type titanium oxide (CM-n-TiO₂) electrode during water splitting reaction. The CM-n-TiO₂ electrode was synthesized by flame-oxidation of Ti metal sample. The combustion products of natural gas flame acted as the carbon source. The oxygen separation rate during water splitting was evaluated in terms of anodic photocurrent density, J_p, under solar simulated light illumination of 1 sun. Upon incorporation of carbon within the titanium oxide, the photocurrent density was enhanced to 4.97 mA cm⁻² at CM-n-TiO₂ electrode compared to 0.66 mA cm⁻² at regular of n-TiO₂ both at the same measured potential of - 0.6 V/SCE. Such a multiple-fold increase in photocurrent density at CM-n-TiO₂ thin film electrode was attributed to its enhanced absorption in the UV region, red-shift to visible region due to carbon incorporation and as well as due to pronounced nano-wall like surface morphology generated under the harsh conditions of flame oxidation. CM-n-TiO₂ photoelectrodes were characterized in terms of photocurrent measurements under white light and as well as under monochromatic light illuminations, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), the valence band X-ray photoelectron spectroscopy (XPS) and the AC-impedance measurements.     

Polydivinylferrocene surface modified electrode for measuring state-of-charge of lead–acid battery

This paper outlines an investigation of the electrochemical behaviour of polymeric divinylferrocene (PDVF) produced by direct polymerisation of divinylferrocene (DVF) monomer on a glassy carbon substrate. The findings indicate that PDVF undergoes reversible reduction/oxidation in neutral and acidic aqueous media containing perchlorate (ClO₄⁻) and sulfhate (SO₄²⁻). The anodic peak potential of the PDVF shifts linearly to less positive potentials as the sulfuric acid (H₂SO₄) concentration is increased from 1 to 5 M. The polymer film strongly adheres to the glassy carbon surface and is electrochemically stable when subjected to repeated voltammetric cycling in the potential range of −0.2 to +0.8 V vs. Ag|AgCl. The potential of the partially oxidized film of PVDF on a glassy carbon substrate against a Ag|AgCl/KCl reference electrode in sulfuric acid solution is stable, reproducible and varies linearly with the acid concentration in the range of 1–5 M. This observation may be suitable for potentiometrically measuring the state-of-charge of lead–acid batteries.     

Dynamic behavior of surface film on LiCoO2 thin film electrode

Electrochemical oxidation behavior of non-aqueous electrolytes on LiCoO₂ thin film electrodes were investigated by in situ polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). LiCoO₂ thin film electrode on gold substrate was prepared by rf-sputtering method. In situ PM-FTIR spectra were obtained at various electrode potentials during cyclic voltammetry measurement between 3.5 V vs. Li/Li⁺ and 4.2 V vs. Li/Li⁺. During anodic polarization, oxidation of non-aqueous electrolyte was observed, and oxidized products remained on the electrode at the potential higher than 3.75 V vs. Li/Li⁺ as a surface film. During cathodic polarization, the stripping of the surface film was observed at the potential lower than 3.9 V vs. Li/Li⁺. Depth profile of XPS also showed that more organic surface film remained on charged LiCoO₂ than that on discharged one. AFM images of charged and discharged electrodes showed that some decomposed products deposited on charged electrode and disappeared from the surface of discharged one. These results indicate that the surface film on LiCoO₂ is not so stable.     

Impedance characterization of dye-sensitized solar cells in a tandem arrangement for hydrogen production by water splitting

The present work reports the photoelectrochemical characterization of a dye-sensitized solar cell (DSC) to assist water split in a photoelectrochemical (PEC) cell. Performance parameters were extracted from standard current–voltage characteristic (I–V) and the charge transfer phenomena occurring at different interfaces of the DSC were evaluated by electrochemical impedance spectroscopy (EIS). The DSC comprised the N719 dye and a robust electrolyte (1-propyl-3-methylimidazolium iodide in guanidinium thiocyanate additive). At 1 sun illumination the DSC yielded a short-circuit photocurrent density of 14.9 mA cm⁻², an open-circuit voltage of 0.797 V, a fill factor of 0.712 and an overall efficiency of 8.5%. Different PEC systems based on silicon-doped and undoped hematite photoelectrodes were considered. The required additional anodic bias necessary for actual water cleavage was supplied by two DSCs in series operating just under open-circuit voltage (1.56 V), allowing a conversion efficiency of about 1.12% for the silicon-doped hematite deposited by APCVD, 0.51% for the silicon-doped prepared by USP and 0.12% for the undoped hematite sample.     

Ultrathin hematite films deposited layer-by-layer on a TiO2 underlayer for efficient water splitting under visible light

Ultrathin hematite (α-Fe₂O₃) film deposited on a TiO₂ underlayer as a photoanode for photoelectrochemical water splitting was described. The TiO₂ underlayer was coated on conductive fluorine-doped tin oxide (FTO) glass by spin coating. The hematite films were formed layer-by-layer by repeating the separated two-phase hydrolysis-solvothermal reaction of iron(III) acetylacetonate and aqueous ammonia. A photocurrent density of 0.683 mA cm⁻² at +1.5 V vs. RHE (reversible hydrogen electrode) was obtained under visible light (>420 nm, 100 mW cm⁻²) illumination. The TiO₂ underlayer plays an important role in the formation of hematite film, acting as an intermediary to alleviate the dead layer effect and as a support of large surface areas to coat greater amounts of Fe₂O₃. The as-prepared photoanodes are notably stable and highly efficient for photoelectrochemical water splitting under visible light. This study provides a facile synthesis process for the controlled production of highly active ultrathin hematite film and a simple route for photocurrent enhancement using several photoanodes in tandem.     

CdSe quantum dots sensitized nanoporous hematite for photoelectrochemical generation of hydrogen

A novel system of CdSe quantum dots (QDs) sensitized porous hematite (α-Fe₂O₃) films has been investigated as a potential photoelectrode for hydrogen generation via photoelectrochemical (PEC) splitting of water. Before sensitization, nanoporous hematite thin films were prepared by spray pyrolysis. Characterizations for crystalline phase formation, crystallite size, absorption spectra, and flatband potential were carried out to analyze PEC data. Loading time of sensitizer to hematite thin films was found to be crucial in affecting its PEC properties. Film having sensitizer loading time as 42 h exhibited best photocurrent density of 550 μA cm⁻² at 1.0 V versus SCE. Current study, for the first time, explores the possibility of using low band gap QDs sensitization on a low band gap film, hematite in PEC splitting of water.     

Electrodeposited zirconium-doped α-Fe2O3 thin film for photoelectrochemical water splitting

Nanostructured hematite thin films were doped with zirconium successfully using electrodeposition method for their implementation as photoanode in photoelectrochemical (PEC) cell for hydrogen generation. XRD, Raman, XPS, SEM and UV-visible spectroscopy techniques were used to characterize the thin films. Highest photocurrent density of 2.1 mA/cm² at 0.6 V/SCE was observed for 2.0 at.% Zr⁴⁺ doped α-Fe₂O₃ sample with solar to hydrogen conversion efficiency of 1.43%. Flatband potential (−0.74 V/SCE) and donor density (2.6 × 10²¹ cm⁻³) were found to be maximum for the same sample. These results suggest substantial potential of hematite thin films with controlled doping of zirconium in PEC water splitting applications.     

Copper oxide photocathodes prepared by a solution based process

Solution based processes are well known by their low-cost trait to fabricate semiconductor devices. In this study, we devised an economical solution based route to photoelectrochemical (PEC) cells, taking copper nitrate as the copper ion source and adding an alkali hydroxide, here NaOH, to produce high aspect ratio (3.1–9.7) CuO nanoparticles. These CuO particles were used for splitting water and generation of hydrogen via a PEC cell. CuO nanoparticle morphology, i.e. rod-like, spindle-like, and needle-like, was dependent on the processing temperature. Sintering the spin coated CuO films, improved crystallinity. The bandgaps for these films were estimated to be 1.35 eV and 1.64 eV for sintering temperatures of 600 °C and 400 °C for 1 h, respectively. The porous structure of the nano-sized CuO films increased surface area and thus led to a high photocurrent, i.e. 1.20 mA/cm², for powder prepared at 60 °C and sintered at 600 °C for 1 h. These films demonstrated 0.91% solar conversion efficiency at an applied voltage of −0.55 V vs. Ag/AgCl in 1 M KOH electrolyte with 1 sun (AM1.5G) illumination. The charge carrier density was estimated to be 6.1 × 10²⁰ cm⁻³. This relatively high charge carrier density may be due to the high surface area and short transport distance to the electrode/electrolyte interface in the porous nanostructure.     

Photoelectrochemical water splitting at nanostructured α-Fe2O3 electrodes

In this study, nanostructured α-Fe₂O₃ thin films were deposited by simple electrodeposition for photoelectrochemical water splitting. Post-annealing temperature was found to have drastic effect on photoactivity of these films. SEM analysis illustrated that size of nanoparticles increases with annealing temperature. The current–potential characteristics showed that the water-splitting photocurrent strongly depends on post-annealing temperature. A maximum photocurrent density of 0.67 mA/cm² was observed at 1.23 V versus reversible hydrogen electrode (RHE) under standard illumination conditions (AM 1.5 G 100 mW/cm²), and the water-splitting current was over 1.0 mA/cm² before the dark current flow starts (at 1.55 V versus RHE). The electrode shows an onset potential as low as 0.8 V (versus RHE) for water photooxidation, which is one of the best results reported for hematite photoanodes. This high photoactivity of electrodes is attributed to the preferential growth of hematite nanostructures along the most conductive plane (001) and incorporation of Sn in film from the substrate at high annealing temperature. The best-performing electrode shows an incident photon conversion efficiency (IPCE) of 12% at 400 nm (in 1 M NaOH at 1.23 V versus RHE), which indicate the improved light-harvesting properties of these nanostructures.     

Photoanode of LDH catalyst decorated semiconductor heterojunction of BiVO4/CdS to enhance PEC water splitting efficiency

BiVO₄ is an ideal photoanode material for solar-driven photoelectrochemical (PEC) water splitting but it easily suffers from the recombination of photogenerated electrons and holes due to its low carrier mobility thus cause low efficiency of PEC water splitting. Herein, the BiVO₄/CdS/NiCo-LDH photoanode was prepared by combining methods of metal organic decomposition, chemical and electrodeposition. The photoanode photocurrent density reaches 2.72 mA cm⁻² at 1.23 V (vs. RHE), which is 3.6 folds of pure BiVO₄ photoanode and onset potential shifts 450 mV toward cathodic. The incident photon-to-electron conversion efficiency (IPCE) value is 2.86 folds of BiVO₄, the calculated photon–to–current efficiency (ABPE) is 1.24% at 0.62 V (vs. RHE). The obtained results are higher than that of most BiVO₄ based photoanodes published so far. The enhancement benefits from increase of visible light absorption capacity, enhancement of separation efficiency of photoexcited electron-hole and fast transfer of holes accumulated on electrode/electrolyte surface for water oxidation, which has been confirmed by calculating carrier density and carrier transport rate.     

α-Fe2O3 nanoarrays photoanodes decorated with Ni-MOFs for enhancing photoelectrochemical water oxidation

Owing to severe recombination of photogenerated charges, sluggish kinetics of oxygen evolution reaction (OER) and high overpotential, the efficiency of photoelectrocatalytic (PEC) water splitting is severely restricted currently. Herein, a metal-organic framework (Ni-MOF) as cocatalyst has been introduced onto Fe₂O₃ nanoarrays for PEC water oxidation. The new Ni-MOFs/Fe₂O₃ photoanode obviously improves the PEC water oxidation performance with respect to the Fe₂O₃. Specifically, a high photocurrent density is achieved on the Ni-MOF/Fe₂O₃ film, which corresponds to two-fold over the pristine Fe₂O₃ film at 1.23 V vs. RHE. Moreover, the photoanode also exhibits a significant cathodic shift of the onset potential (~240 mV) relative to the bare Fe₂O₃. The enhanced PEC performance is attributed to effective utilization the surface-reaching holes and reduction of the surface charge recombination, which are confirmed by electrochemical impedance spectroscopy and the derived Bode analysis. This study brings new insight into the development of MOF-based materials in the field of PEC water splitting.     

Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering

Polycrystalline WO₃ thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O₂ gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO₃, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm⁻² at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO₃ films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm.     

Cr–N–C multilayer film on 316L stainless steel as bipolar plates for proton exchange membrane fuel cells using closed field unbalanced magnetron sputter ion plating

To combine the advantages of chromium nitride (CrN) and amorphous carbon (a-C) film, this study proposes a novel Cr–N–C multilayer film on 316L stainless steel (SS316L) as bipolar plates for proton exchange membrane fuel cells (PEMFCs) using closed field unbalanced magnetron sputter ion plating (CFUBMSIP) method. The characterizations of Cr–N–C film are analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), and scanning electron microscopy (SEM). Scratch tests indicate that the adhesion strength between the film and SS316L substrate has been greatly improved which is beneficial to prevent the multilayer film from spalling. Interfacial contact resistance (ICR) between coated SS316L sheets and simulated gas diffusion layer (GDL) decreases to 2.64 mΩ cm² at 1.4 MPa. Potentiodynamic results reveal that the anodic corrosion potential of coated samples is more positive than the operation potential and the cathodic passivation current density is only 0.61 μA cm⁻² at 0.6 V. Potentiostatic test, contamination analysis and surface morphology results reveal that the substrate is well protected by the Cr–N–C film. This research demonstrates that the novel Cr–N–C film exhibits excellent ex-situ performance including strong adhesion strength, high corrosion resistance and low ICR.