Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

Fabrication of platinum coated nanoporous gold film electrode: A nanostructured ultra low-platinum loading electrocatalyst for hydrogen evolution reaction

The electrolytic hydrogen evolution reaction (HER) on platinum coated nanoporous gold film (PtNPGF) electrode is demonstrated. The deposition of platinum occurred as a spontaneous redox process in which a copper layer, obtained by underpotential deposition, was oxidized by platinum ions, which were reduced and simultaneously deposited. The present method could provide a very low Pt-loading electrode and the results demonstrated that ultra thin Pt coating effected efficiently and behaved as the nanostructured Pt for electrocatalytic hydrogen evolution reaction. The loading of Pt was calculated as 4.2 × 10⁻³ μg cm⁻² for PtNPGF electrode. The current density at −0.4 V and −0.8 V vs. Ag/AgCl was as high as 0.66 A μg⁻¹ Pt and 3 A μg⁻¹ Pt, respectively and the j₀ was evaluated as 0.03 mA cm⁻² or 8 mA μg⁻¹ Pt. The results indicated that increasing electrode area had no catalytic effect, but the nanostructure nature of as-fabricated electrode and submonolayer deposition of copper resulted in electrocatalytic activity for PtNPGF electrode.     

Comparison of the electrochemical oxidation of borohydride and dimethylamine borane on platinum electrodes: Implication for direct fuel cells

The electrochemical behaviour of dimethylamine borane and borohydride on platinum electrodes was investigated by cyclic voltammetry and polarization curves in discharges processes. Several overlapping peaks appear in the domain of hydrogen oxidation, i.e., in the potential range of −1.25 V to −0.50 V versus Ag/AgCl, mainly with the borohydride. This behaviour is associated with the hydrolysis of BH₄⁻ or (CH₃)₂NHBH₃. As a consequence of secondary reactions the borohydride and dimethylamine borane oxidation in 3 M NaOH solution shows, respectively, a four- to six-electron process and a four- to five-electron process in direct fuel cells. The direct oxidation of the borohydride exhibits a peak at about −0.07 V versus Ag/AgCl, while the dimethylamine borane peak is at about −0.03 V versus Ag/AgCl. For the 0.04 M concentration the borohydride displays a power density of 31 W m⁻² which is 16% higher than that of the dimethylamine borane.     

Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting

Highly efficient water splitting electrode based on uniform platinum (Pt) nanoparticles on self-organized TiO₂ nanotube arrays (TNTAs) was prepared by a combination of multi-step electrochemical anodization with facile photoreduction process. Uniform platinum (Pt) nanoparticles with an average diameter of 8 nm are distributed homogeneously on nanoporous top layer and underneath TiO₂ nanotube wall. In comparison to pristine TNTAs, Pt@TNTAs show substantially enhanced photocurrent density and the incident photon-to-current conversion efficiency (IPCE) in the entire wavelength window. The maximum photocurrent density and IPCE from the optimized Pt@TNTAs photoelectrode (Pt, ～1.57 wt%) were about 24.2 mA cm⁻² and 87.9% at 350 nm, which is much higher than that of the pure nanotubes sample (16.3 mA cm⁻² and 67.3%). The resultant Pt@TNTAs architecture exhibited significantly enhanced photoelectrochemical activities for solar water splitting with hydrogen evolution rate up to 495 μmol h⁻¹ cm⁻² in 2 M Na₂CO₃ + 0.5 M ethylene glycol under the optimal external bias of −0.3 V_SCE.     

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.     

Large photocurrent generation of an ITO electrode modified with a red copper(II) complex

A film of copper complex [CuL](NO₃)₂ (L = 2,3,8,9-tetraphenyl-1,4,7,10-tetraazacyclododeca-1,3,7,9-tetraene) formed on indium-tin oxide (ITO) coated glass by the solvent evaporation of the acetonitrile solution of the complex onto the ITO substrate, was characterized by ultraviolet-visible absorption spectroscopy, cyclic voltammetry, scanning electron microscopy, and photoelectrochemistry. The photoelectrochemical cell by using [CuL](NO₃)₂ modified ITO, a saturated calomel electrode, and platinum wire as working, reference and counter electrodes respectively in 1 M Na₂SO₄, was found to exhibit a large prompt and reproducible cathodic photocurrent density of 71 μA/cm² under white light irradiation of 70 mW/cm² at an applied potential of −0.4 V, and an incident photon to current efficiency (IPCE) of 1.1% at λ = 660 nm. This Cu(II) complex photosensitizer has advantages of simple synthesis, low-cost, environmentally benign and good photoelectrochemical performance.     

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.     

Synthesis of reduced graphene oxide–TiO2 nanoparticle composite systems and its application in hydrogen production

The utilization of solar energy for the conversion of water to hydrogen and oxygen has been considered to be an efficient strategy to solve crisis of energy and environment. Here, we report the synthesis of reduced graphene oxide–TiO₂ nanoparticle composite system through the photocatalytic reduction of graphite oxide using TiO₂ nanoparticles. Photoelectrochemical characterizations and hydrogen evolution measurements of these nanocomposites reveal that the presence of graphene enhances the photocurrent density and hydrogen generation rate. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm⁻² and 127.5 μmole cm⁻²h⁻¹ in 0.5 M Na₂SO₄ electrolyte solution under 1.5AM solar irradiance of white light with illumination intensity of 100 mW cm⁻². In graphene–TiO₂ nanocomposite, photogenerated electrons in TiO₂ are scavenged by graphene sheets and percolate to counter electrode to reduce H⁺ to molecular hydrogen thus increasing the performance of water-splitting reaction.     

Performance of a new polymer electrolyte incorporated with diphenylamine in nanocrystalline dye-sensitized solar cell

A new solvent-free composite polymer electrolyte consisting of poly(ethylene oxide) (PEO) incorporated into diphenyl amine (DPA) along with KI and I₂ has been developed. The current-voltage characteristics of this nanocrystalline dye-sensitized solar cell measured under simulated sunlight with 1.5 AM at 60 mW/cm² have indicated that this cell generates a photocurrent of 10.2 mA/cm², together with a photovoltage of 810 mV and fill factor of 0.47 yielding an overall energy conversion efficiency of 6.5%. This result suggests that the electron donicity of DPA influences the interaction of nanocrystalline TiO₂ electrode and I⁻/I₃⁻ electrolyte, leading to a high performance of the fabricated solar cell.     

Photoelectrochemical performances of AgInS2 film electrodes fabricated using the sulfurization of Ag-In metal precursors

Ternary silver-indium-sulfide samples were deposited on various substrates using the sulfurization of Ag-In metal precursors. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in metal precursors on the structural, morphological, and photoelectrochemical properties of the samples was examined. X-ray diffraction patterns of samples show that the films are in the polycrystalline AgInS₂ phase. The thickness and direct band gap of the films were in the ranges of 1.1-1.2 μm and 1.92-1.94 eV, respectively. The conduction type of all samples was n-type. The carrier concentration, mobility, and resistivity of samples were in the ranges of 1.5×10¹³-7.0×10¹³ cm⁻³, 2.6-14.8 cm²V⁻¹s⁻¹, and 2.6×10⁴-3.5×10⁴ Ωcm, respectively. It was found that the samples with an [Ag]/[In] molar ratio of 0.89 in Ag-In metal precursors had a maximum photo-enhancement current density of 2.43 mAcm⁻² at an applied bias of +0.5 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.5 M K₂SO₄. The results show that high-quality AgInS₂ films can be obtained using the sulfurization of Ag-In metal precursors for photoelectrochemical (PEC) applications.     

Preparation of a microalgal photoanode for hydrogen production by photo-bioelectrochemical water-splitting

In this study, a microalga Tetraselmis subcordiformis (synonym: Platymonas subcordiformis)-based photoanode was prepared by a novel method developed in our lab. The optimal photocurrent density of microalgae photoanode, 37 μA/cm², was achieved under illumination of 145 μmol s⁻¹ m⁻² at anode potential of 0.5 V vs Ag|AgCl|sat. KCl, immobilized cell density of 2.08 × 10⁶/cm² and BQ concentration of 300 μmol/L. The results of measurements showed that oxygen evolution peak, hydrogen evolution peak and photocurrent response were all synchronous to light impulse in a three-electrode system. It revealed that there occurred a process of photo-bioelectrochemical water-splitting. Hydrogen can be produced by the method. The investigation for whole photo-bioelectrochemical process also indicated that the electrons for hydrogen evolution had two sources, microalgal metabolic process in dark condition and photosynthetic water oxidation. The photo-hydrogen evolution was twice more than hydrogen evolution in dark condition.     

Electrochemical and fuel cell evaluation of Co based catalyst for oxygen reduction in anion exchange polymer membrane fuel cells

Co based catalyst were evaluated for oxygen reduction (ORR) in liquid KOH and alkaline anion exchange membrane fuel cells (AAEMFCs). In liquid KOH solution the catalyst exhibited good performance with an onset potential 120 mV more negative than platinum and a Tafel slope of ca. 120 mV dec⁻¹. The hydrogen peroxide generated, increased from 5 to 50% as the electrode potential decreased from 175 to −300 mV vs. SHE.In an AAEMFC environment, one catalyst (GP2) showed promising performance for ORR, i.e. at 50 mA cm⁻² the differences in cell potential between the stable performance for platinum (more positive) and cobalt cathodes with air and oxygen, were only 45 and 67 mV respectively. The second catalyst (GP4) achieved the same stable power density as with platinum, of 200 and 145 mW cm⁻², with air at 1 bar (gauge) pressure and air (atm) cathode feed (60 °C), respectively. However the efficiency was lower (i.e. cell voltage was lower) i.e. 40% in comparison to platinum 47.5%.     

Application of mesoporous carbon to counter electrode for dye-sensitized solar cells

The mesoporous carbons were prepared by the carbonation of the triblock copolymer F127/phloroglucinol-formaldehyde composite self-assembled in an acid medium and employed as the catalyst for triiodide reduction in dye-sensitized solar cells (DSCs). The characteristics of mesoporous carbon were analyzed by scanning electron microscopy, transmission electron microscopy, N₂ sorption measurement and X-ray diffraction. The mesoporous carbon with low crystallinity exhibited Brunauer-Emmett-Teller surface area of 400 m² g⁻¹, pore diameter of 6.8 nm and pore volume of 0.63 cm³ g⁻¹. The photovoltaic performances of DSCs with mesoporous carbon counter electrode were improved by increasing the carbon loading on counter electrode due to the charge-transfer resistance of mesoporous carbon counter electrode decreasing with the increase of the carbon loading. However, further carbon loading increase has no obvious effect on the photovoltaic performance of DSCs with carbon electrode when carbon loading exceeds 300 μg cm⁻². The overall conversion efficiency of 6.18% was obtained by DSCs composed of mesoporous carbon counter electrode with the carbon loading of 339 μg cm⁻². This value is comparable to that of DSCs with conventional platinum counter electrode.     

Supercapacitive properties of polyaniline/Nafion/hydrous RuO2 composite electrodes

Chemically prepared polyaniline is tested for its supercapacitive behaviour in an aqueous electrolyte of 1.0 M H₂SO₄. In order to improve the cycleability of the polyaniline electrode, it is made into a composite with Nafion. This composite electrode shows improved cycleability and higher specific capacitance compared with a pure polyaniline electrode. It is therefore used as a matrix for the electrochemical deposition of hydrous RuO₂. The resulting ternary composite electrode has a high specific capacitance of 475 F g⁻¹ at 100 mV s⁻¹ and 375 F g⁻¹ at 1000 mV s⁻¹ in the voltage range of −0.2 to 0.8 V versus Ag/AgCl. All three types of electrode are characterized by cyclic voltammetry and impedance anaylsis.     

Effect of length of anodized TiO2 tubes on photoreactivity: Photocurrent, Cr(VI) reduction and H2 evolution

Anodized tubular TiO₂ electrodes (ATTEs) are prepared using an organic additive consisting of either (i) ethylene glycol (EG) or (ii) glycerol (Gly) to make various photoanodes with different length of TiO₂ tubes and thereby to investigate the effect of their length on the photo-driven activity for hydrogen evolution and Cr(VI) reduction, as well as on the photocurrent. The ATTEs with EG have longer TiO₂ tubes (3.42-15.6 μm) than those with Gly (0.26-1.95, 6.82 μm). The former samples exhibit higher photocurrent densities (22.8-32.8 mA cm⁻²) than the latter (8.0-19.4, 20.3 mA cm⁻²). The latter samples (tube length of less than 7 μm) clearly exhibit a change of the rate-determining step from electron migration to photohole capture as the scanned applied bias increases, since the photocurrent shows a plateau for tube lengths above 2 μm. Meanwhile, the samples with EG remain in the electron migration step up to a tube length of 16 μm and is due to the difference of the morphology, crystal phase and crystallinity. This favourable characteristic is also applied to and well matched with the results from the reactions of Cr(VI) reduction and hydrogen evolution (up to ca. 250 μmol h⁻¹).     

Impact of SO2 poisoning of platinum nanoparticles modified glassy carbon electrode on oxygen reduction

An extraordinary recovery characteristic of Pt-nanoparticles from SO₂ poisoning is introduced in this study. Platinum nanoparticles (nano-Pt) modified glassy carbon electrode (nano-Pt/GC) has been compared with polycrystalline platinum (poly-Pt) electrode towards SO₂ poisoning. Two procedures of recovery of the poisoned electrodes were achieved by cycling the potential in the narrow potential range (NPR, 0-0.8 V vs. Ag/AgCl/KCl (sat.)) and wide potential range (WPR, −0.2 to 1.3 V). The extent of recovery was marked using oxygen reduction reaction (ORR) as a probing reaction. SO₂ poisoning of the electrodes changed the mechanism of the oxygen reduction from the direct reduction to water to the stepwise reduction involving the formation of H₂O₂ as an intermediate, as indicated by the rotating ring-disk voltammetry. Using the WPR recovery procedure, it was found that two potential cycles were enough to recover 100% of the activity of the ORR on the nano-Pt/GC electrode. At the poly-Pt electrode, however, four potential cycles of the WPR caused only 79% in the current recovery, while the peak potential of the ORR was 130 mV negatively shifted as compared with the fresh poly-Pt electrode. Interestingly, the NPR procedure at the nano-Pt/GC electrode was even more efficient in the recovery than the WPR procedure at the poly-Pt electrode.     

Correlation of electrical and physical properties of photoanode with hydrogen evolution in enzymatic photo-electrochemical cell

In this study, the electrical and physical properties, including the current density, open-circuit voltage, morphology and crystalline structure, of an anodized TiO₂ electrode on a titanium foil are correlated with the hydrogen production rate in an enzymatic photo-electrochemical system. The effect of light intensity at ca. 74 and ca. 146 mW cm⁻² on the properties is also examined. Anodizing (20 V; bath temperature 5 °C; anodizing time 45 min) and subsequent annealing (350–850 °C for 5 h) of the Ti foils in an O₂ atmosphere led to the formation of a tube-shaped, or a compact layered, TiO₂ film on the Ti substrate depending on the annealing temperature. The annealing temperature has a similar effect on the properties of the sample and the hydrogen evolution rate. The generated electrical value, the chronoamperometry (CA), is +13 to −229 and +13 to −247 μA for light intensities of ca. 74 and ca. 146 mW cm⁻², while the corresponding open-circuit voltage (OCV) is in the range of −41 to −687 and −144 to 738 mV, respectively. In the absence of light (dark), the CA is 13–29 μA and the OCV is +258 to −126 mW cm⁻². The trend in the electrical properties for the different samples is well matched with the rate of hydrogen evolution. The samples with higher activities (450, 550, and 650 °C) have similar X-ray diffraction (XRD) patterns, which clearly indicates that the samples showing the highest evolution rate are composed of both anatase and rutile, while those showing a lower evolution rate are made of either anatase or rutile. Increasing the intensity of the irradiated light causes a remarkable enhancement in the rate of hydrogen production from 71 to 153 μmol h⁻¹ cm⁻².     

Photoelectrochemical study of ZnIn2Se4 electrodes fabricated using selenization of RF magnetron sputtered Zn–In metal precursors

Polycrystalline ZnIn₂Se₄ samples are grown on glass substrates and fluorine-doped tin oxide coated glass substrates using the selenization of radio-frequency magnetron sputtered Zn–In metal alloys. The effect of the [Zn]/[Zn + In] molar ratio in the metal alloys on the physical and photoelectrochemical properties of the samples is investigated. X-ray diffraction patterns of samples reveal that the samples are polycrystalline tetragonal ZnIn₂Se₄. The thicknesses and direct band gaps of the samples are in the ranges of 1.15–1.44 μm and 1.68–1.81 eV, as obtained from surface profile measurements and transmittance/reflectance spectra, respectively. The flat-band potentials of the samples in 0.6 M K₂SO₃ electrolyte are in the range of −0.41 to −0.95 V vs. an Ag/AgCl reference electrode. The highest photoelectrochemical response of samples was 1.84 mA/cm² at an external potential of +1.0 V vs. an Ag/AgCl electrode in 0.6 M K₂SO₃ solution under illumination from a 300 W Xe lamp system with the light intensity set at 100 mW/cm².     

Iron oxide (n-Fe2O3) nanowire films and carbon modified (CM)-n-Fe2O3 thin films for hydrogen production by photosplitting of water

Iron oxide n-Fe₂O₃ nanowire photoelectrodes were synthesized by thermal oxidation of Fe metal sheet (Alfa Co. 0.25 mm thick) in an electric oven then tested for their photoactivity. The photoresponse of the n-Fe₂O₃ nanowires was evaluated by measuring the rate of water splitting reaction to hydrogen and oxygen, which is proportional to photocurrent density, J_p. The optimized electric oven-made n-Fe₂O₃ nanowire photoelectrodes showed photocurrent densities of 1.46 mA cm⁻² at measured potential of 0.1 V/SCE at illumination intensity of 100 mW cm⁻² from a Solar simulator with a global AM 1.5 filter. For the optimized carbon modified (CM)-n-TiO₂ synthesized by thermal flame oxidation the photocurrent density for water splitting was found to increase by two fold to 3.0 mA cm⁻² measured at the same measured potential and the illumination intensity. The carbon modified (CM)-n-Fe₂O₃ electrode showed a shift of the open circuit potential by −100 mV/SCE compared to undoped n-Fe₂O₃ nanowires. A maximum photoconversion efficiency of 2.3% at applied potential of 0.5 V/E_aoc was found for CM-n-Fe₂O₃ compared to 1.69% for n-Fe₂O₃ nanowires at higher applied potential of 0.7 V/E_aoc. These CM-n- Fe₂O₃ and n- Fe₂O₃ nanowires thin films were characterized using photocurrent density measurements under monochromatic light illumination, UV-Vis spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM).     

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.