Plasmonic nickel nanoparticles decorated on to LaFeO3 photocathode for enhanced solar hydrogen generation

Plasmonic Ni nanoparticles were incorporated into LaFeO₃ photocathode (LFO-Ni) to excite the surface plasmon resonances (SPR) for enhanced light harvesting for enhancing the photoelectrochemical (PEC) hydrogen evolution reaction. The nanostructured LFO photocathode was prepared by spray pyrolysis method and Ni nanoparticles were incorporated on to the photocathode by spin coating technique. The LFO-Ni photocathode demonstrated strong optical absorption and higher current density where the untreated LFO film exhibited a maximum photocurrent of 0.036 mA/cm² at 0.6 V vs RHE, and when incorporating 2.84 mmol Ni nanoparticles the photocurrent density reached a maximum of 0.066 mA/cm² at 0.6 V vs RHE due to the SPR effect. This subsequently led to enhanced hydrogen production, where more than double (2.64 times) the amount of hydrogen was generated compared to the untreated LFO photocathode. Ni nanoparticles were modelled using Finite Difference Time Domain (FDTD) analysis and the results showed optimal particle size in the range of 70–100 nm for Surface Plasmon Resonance (SPR) enhancement.     

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

A ternary Ag/TiO₂/CNT photoanode was prepared by grafting Ag nanoparticles on the surface of as-synthesized TiO₂/CNT nanocomposite for the photoelectrochemical (PEC) water splitting under visible light irradiance. The ternary composite photoanode was observed to generate four times higher photocurrent density compared to binary TiO₂/CNT nanocomposite under visible light irradiance. The Ag nanoparticles on the surface of nanocomposite act as a surface plasmon resonance (SPR) photosensitizer under visible light. The enhanced photocurrent density of Ag/TiO₂/CNT ternary photoanode is attributed to the increased light absorption in the visible region, decrease in band-bending and effective interfacial electron transfer due to the synergetic effect of Ag nanoparticles and CNTs. The enhanced charge transfer within the Ag/TiO₂/CNT was also confirmed by the electrochemical impedance spectroscopy. This work demonstrates a feasible route to improve the PEC performance of TiO₂ towards water splitting under sunlight irradiation.     

Investigation of plasmonic Cu with controlled diameter over TiO2 photoelectrode for solar-to-hydrogen conversion

Plasmonic metal nanoparticles (NPs) have been used to improve the solar-to-hydrogen conversion efficiency. Relative to Au and Ag, Cu is cheaper and more abundant. In the present work, Cu NPs with the controlled diameter were deposited on TiO₂ nanotube arrays (TNTAs) by using a pulse electrochemical deposition method. When the deposition was cycled 3600 times, the size of Cu NPs can be tuned to approximately 30 nm with the most uniform distribution, resulting in the remarkable characteristic peak of surface plasmon resonance and higher photocurrent density. The hydrogen production rates remained unchanged during irradiation (AM 1.5, 100 mW/cm²) of 2 h, indicating a good stability of the resultant Cu/TNTAs electrode. The photoelectrochemical performances of as-prepared Cu/TNTAs can also be comparable to those of Ag/TNTAs electrode fabricated by the same method.     

Facile SILAR preparation of Fe(OH)3/Ag/TiO2 nanotube arrays ternary hybrid for supercapacitor negative electrode

The ternary hybrid composite electrode of Fe(OH)₃/Ag/TNTA (where TNTA stands for TiO₂ nanotube arrays) was prepared by a simple successive ionic layer adsorption and reaction method. The effects of calcination temperature of Ag/TNTA, drying temperature of Fe(OH)₃/Ag/TNTA, and deposition amount of Ag and Fe(OH)₃ on the supercapacitor performance of the composite electrode were investigated, and the related reasons were discussed in detail. The results show that Ag modification can obviously improve the performance of Fe(OH)₃/TNTA composite electrode. Both the calcination temperature of Ag/TNTA and the deposition amount of Ag affect the particle size of Ag and the reaction resistance of the electrode. The deposition amount of Fe(OH)₃ also has influence on the reaction resistance of the electrode. Under the optimized conditions, the capacitance value of the Fe(OH)₃/Ag/TNTA composite electrode is as high as 84.67 mF cm^?2@5 mV s^?1（596.30 F g^?1@5 mV s^?1）, and the electrode has high rate performance and good cycle stability. The asymmetric supercapacitor assembled with Fe(OH)₃/Ag/TNTA as the negative electrode and activated carbon as the positive electrode can store energy stably under the potential window of 0–1.5 V. When the power density is 2.77 kW kg^?1 (50 mW cm^?3), the energy density can reach 18.34 Wh kg^?1 (0.33 mWh cm^?3).     

Plasmon-enhanced photocurrent in dye-sensitized solar cells

Plasmonic structures of FTO/TiO₂/NPs-Ag and FTO/NPs-Ag/TiO₂ electrodes were fabricated by sputter technology and the sol–gel & spin coating procedure. These electrodes with similar optical absorptions in the visible region enhanced by the surface plasmon resonance of silver nanoparticles have different photovoltaic properties, revealing that the significant design can be used to identify the favorably enhanced direction of plasmonic structure resulting from plasmonic scattering to trap light which confines light within the active TiO₂ layer to promote dye absorption in dye-sensitized solar cells (DSSCs). In the FTO/TiO₂/NPs-Ag, a 60% enhancement in photocurrent and an improvement in photovoltage were observed and the increased incident photon-to-photocurrent efficiency (IPCE) was consistent with the enhanced absorption spectrum. However, the photovoltaic properties of the FTO/NPs-Ag/TiO₂ were similar to those of the standard electrode. This concept is potentially applicable to new kinds of solar cells.     

Surface plasmon enhanced GaAs thin film solar cells

As a new method to improve the light trapping in solar cells, surface plasmon resonance (SPR) has attracted considerable attention because of its unique characteristics. Several studies have been reported on the photocurrent improvement of Si solar cells by surface plasmons, while little research has been done on III-V solar cells. In this work, we performed a systematic study of SPR on GaAs thin film solar cells with different sizes of Ag nanoparticles on the surface. The nanoparticles were fabricated by annealing E-beam evaporated Ag films in a N₂ atmosphere. It was found that the surface plasmon resonance wavelength does not undergo a simple red-shift with increasing metal thickness. It depends on the shape of the metal nanoparticles and the interparticle spacing. It is necessary to optimize the particle size to obtain an optimum enhancement throughout the visible spectrum for solar cells. We found that the optimum thickness of the Ag film was 6 nm under our experimental conditions. Furthermore, from the calculation based on the external quantum efficiency data, the short circuit current density of a GaAs solar cell with 6 nm Ag film after annealing was increased by 14.2% over that of the untreated solar cell.     

Fabrication and photoelectrochemical study of Ag@TiO2 nanoparticle thin film electrode

Ag@TiO₂ nanoparticle thin film was fabricated for photoelectrochemical water splitting in the visible light region. Under the irradiation of UV light, positive photocurrent was enhanced in both electrolytes of 0.1 M HNO₃ and 0.1 M NaOH owing to the excitation of photoelectrons within the TiO₂ shells. However, under the irradiation of visible light, the enhancement of positive photocurrent was observed only in 0.1 M HNO₃ because of the formation of a Schottky barrier band bending at the Ag-TiO₂ core-shell interface and the generation of photoelectrons resulted from the surface plasmon resonance of Ag cores. In 0.1 M NaOH, significant negative photocurrent was enhanced due to the influences of higher pH on the surface state and energy level of TiO₂ shells. Such a visible light-induced photoresponse enhancement and photocurrent direction switching made the Ag@TiO₂ nanoparticle thin film useful not only as a photoelectrode for water splitting but also as a photo-switch in a basic electrolyte.     

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.     

Reorganization of a photosensitive carbo-benzene layer in a triptych nanocatalyst with enhancement of the photocatalytic hydrogen production from water

The preparation of a triptych nanomaterial made of TiO₂ nanoparticles as semiconductor, Ag plasmonic nanoparticles and a carbo-benzene macrocyclic molecule as photosensitizer is described, and used to produce hydrogen by photo-reduction of pure deionized water under 2.2 bar argon pressure without any electrical input. Silver nanoparticles (~5 nm) are grafted onto the surface of commercial TiO₂ nanoparticles (~23 nm) by a photo-deposition process using an original silver amidinate precursor. The thickness of the photosensitive layer (2 nm), which completes the assembly, plays a crucial role in the efficiency and robustness of the triptych nanocatalyst. Thanks to the organic layer reorganization during the first ~24 h of irradiation, it leads to an enhancement of the hydrogen production rate up to 5 times. The amount of silver and carbo-benzene are optimized, along with the mass concentration of nanocatalyst in water and the pH of the aqueous medium, to allow reaching a hydrogen production rate of 22.1 μmol·h⁻¹·g_{photocatalyst}⁻¹.     

Photocurrent and photoelectrochemical hydrogen production with tin porphyrin and platinum nanowires immobilized with nafion on glassy carbon electrode

Platinum nanowires mixed with Tin meso-tetra (4-pyridyl) porphine dichloride and nafion solution was used to modify the surface of glassy carbon electrode for photocurrent generation and photo-electrochemical hydrogen production. Different concentrations of porphyrin (50 μM, 100 μM, 300 μM and 500 μM) and platinum loading (200 μg/cm², 400 μg/cm², 600 μg/cm² and 800 μg/cm²) were tested at −150 mV Vs Ag/AgCl in reaction cell containing the modified glassy carbon electrode as working electrode, platinum wire as counter electrode and Ag/AgCl as reference electrode, under illumination to determine the optimum, based on photocurrent production in 50 mM potassium hydrogen phthalate buffer (pH 3) containing 0.1Na₂SO₄ as supporting electrolyte. Optimum photocurrent was obtained at 100 μM tin porphyrin and 600 μg/cm² platinum loading. Detectable amount of hydrogen was produced at −350 mV Vs Ag/AgCl under irradiation with visible light.     

Bias-free visible light-driven photoelectrochemical water splitting of type II ZnO/CuS core/shell heterojunction nanotube arrays

Solar-driven water splitting of semiconductor photoelectrodes via photoelectrochemical (PEC) cell has been regarded as the most promising approach to mitigate the energy crisis and environmental issues in the future. In this work, CuS nanoparticles (NPs) are deposited on ZnO nanotube arrays (ZnO/CuS NTAs) via successive ion layer absorption and reaction method for PEC water splitting under visible light irradiation without applying bias. The excellent light harvesting capacity of CuS NPs from visible to near infrared region not only expands the light harvesting of ZnO NTAs into near infrared region, but also substantially boosts light absorption ranging from 300 to 800 nm. Moreover, CuS NPs coupled on ZnO NTAs can establish a type-II band alignment between ZnO and CuS. Consequently, the ZnO/CuS NTAs photoanode exhibits the significantly boosted PEC water splitting performance under visible light illumination (λ > 420 nm) without applying bias. The photocurrent density of the ZnO/CuS NTAs photoanode is 21.2 μA/cm², which is increased by 9 times compared to that of the pure ZnO NTAs photoanode. The enhancement in PEC water splitting performance for ZnO/CuS NTAs is attributed to (i) the cooperative actions of ZnO and CuS; (ii) significant enhancement in light absorption from the visible to near infrared region achieved by CuS NPs and (iii) efficient charge carrier separation achieved by type-II band alignment.     

Hydrogen production over metal-loaded mesoporous-assembled SrTiO3 nanocrystal photocatalysts: Effects of metal type and loading

Mesoporous-assembled SrTiO₃ photocatalysts with different loaded metal co-catalysts (Au,Pt, Ag, Ni, Ce, and Fe) synthesized by the single-step sol–gel method with the aid of a structure-directing surfactant were tested for the photocatalytic activity of hydrogen production from a methanol aqueous solution under both UV and visible light irradiation. The Au, Pt, Ag, and Ni loadings had a positive effect on the photocatalytic activity enhancement, whereas the Ce and Fe loadings did not. The best loaded metal was found to be Au due to its electrochemical properties compatible with the SrTiO₃-based photocatalyst and its visible light harvesting enhancement. A 1 wt.% Au-loaded SrTiO₃ photocatalyst exhibited the highest photocatalytic hydrogen production activity with a hydrogen production rate of 337 and 200 μmol h⁻¹ g_cat⁻¹ under UV and visible light irradiation, respectively. The hydrogen diffusivity from the liquid phase to the gas phase also significantly affected the photocatalytic hydrogen production efficiency. An increase in the hydrogen diffusability led to an increase in the photocatalytic hydrogen production efficiency.     

Activation of inert Ag by nanoplasmonic synergy for enhanced hydrogen evolution reaction

The application of Ag as an electrocatalyst for hydrogen evolution reaction (HER), which holds promise to quench the worldwide thirst for clean energy source, is severely limited by its poor intrinsic activity. To address this issue, in the present contribution the Ag electrode is anodically etched, giving rise to the Ag nanocorals (NCs) consisted of closely interconnected Ag nanoparticles (NPs), between which the grain boundaries are flooded with coordinately unsaturated Ag atoms. Electrokinetic studies reveal that those under–coordinated Ag atoms stabilize the hydrogen intermediates bound to Ag NCs to facilitate the subsequent transfer of the hot electrons stemmed from the relaxation of the localized surface plasmon resonance (LSPR) of Ag NCs under visible and near–infrared (NIR) light illumination. As a result of such synergistic effect is HER over Ag NCs largely accelerated, resulting in the cathodic current density of 10 mA cm^?2 readily turned on at an early overpotential η = 156 mV with respect to those of additional Ag-based electrocatalysts reported in the literature. Such outperformance unambiguously highlights the strong prospect of Ag NCs as an alternative photoelectrocatalyst, which additionally takes advantage of the incident light to boost HER, to the state–of–the–art Pt electrocatalytic counterpart for solar fuel production.     

1D ZnFe2O4 nanorods coupled with plasmonic Ag,Ag2S nanoparticles and Co-Pi cocatalysts for efficient photoelectrochemical water splitting

Improving the absorption of visible light, accelerating the separation of carries and reducing the recombination of electron-hole pairs are critical to enhance photoelectrochemical (PEC) performance of ZnFe₂O₄. Herein, the ZnFe₂O₄/Ag/Ag₂S films are firstly prepared with a photocurrent density of 0.91 mA/cm² at 1.23 V vs. RHE, which is 9.10 times higher than that of pristine ZnFe₂O₄ (0.10 mA/cm² at 1.23 V vs. RHE). On the basis, Co-Pi cocatalyst is deposited on ZnFe₂O₄/Ag/Ag₂S to further optimize PEC performance of ZnFe₂O₄, the photocurrent density of ZnFe₂O₄/Ag/Ag₂S/Co-Pi is 1.18 mA/cm² at 1.23 V vs. RHE. The improved PEC performance of ZnFe₂O₄/Ag/Ag₂S/Co-Pi films could be attributed to: (i) fast transmission of electron-hole pairs owing to 1D ZnFe₂O₄ NRs; (ii) surface plasmon resonance (SPR) effect of Ag nanoparticles; (ⅲ) visible light absorption is improved by sensitization of Ag₂S nanoparticles; (ⅳ) Co-Pi cocatalyst decreases the recombination of electron-hole pairs by capturing holes. This work provides new insights for metal plasmas, sensitizers and cocatalysts synergistically modify photoanodes for efficient PEC water splitting.     

Shape-dependent localized surface plasmon enhanced photocatalytic effect of ZnO nanorods decorated with Ag

We report shape-dependent localized surface plasmon enhanced photocatalytic effect of ZnO nanorods decorated with Ag nanostructures. The plasmonic ZnO photoelectrode modified by Ag nanoprisms exhibits a significant enhancement in photoelectric conversion with a maximum photoconversion efficiency of 1.45%. The photocurrent intensity (at 0.5 V vs. Ag/AgCl) of ZnO–Ag nanoprisms is 3.1 and 10 times greater than that of ZnO–Ag nanoparticles and as-grown ZnO nanorods, respectively. Moreover, ZnO–Ag nanoprisms showed a fast photoresponse due to the fast transport of photogenerated charge carriers in ZnO nanorods with a low recombination rate. It is suggested that the mechanism of photocatalytic enhancement by Ag nanoprisms is mainly ascribed to the significantly enhanced plasmonic ‘hotspots’ in sharp tips of nanoprisms. Such shape-dependent localized surface plasmon effect is further confirmed by FDTD simulation, which revealed that Ag nanoprisms showed stronger electromagnetic field intensity than that of Ag nanoparticles.     

Efficient photon-to-electron conversion with rhodamine 6G-sensitized nanocrystalline n-ZnO thin film electrodes in acetonitrile solution

This work encompasses a study of the photosensitizing action of the commercially available laser dye rhodamine 6G (Rh 6G) on nanocrystalline n-ZnO thin film electrodes prepared by sol–gel technique. This dye, having strong absorption in the visible range with a pronounced absorption peak at 525 nm, was found to convert into electrical energy the visible light in the range of 450–560 nm. The electron injection by photo-excited dye molecules into the conduction band of ZnO was evidenced by the matching of the action spectrum of dye-capped ZnO electrode with the absorption spectrum of the dye in solution. The maximum incident photon-to-current conversion efficiency (IPCE) of Rh 6G-sensitized ZnO based cell was found to be 2.3% at 520 nm under short-circuit condition, which is nearly 1.5 times the IPCE value reported for Rh 6G-sensitized SnO₂ based cell. Dependence of the photocurrent on light intensity and the stability of the photocurrent obtainable from the Rh 6G-sensitized ZnO based photoelectrochemical cell on its prolong operation were also determined. On irradiation of the semiconductor electrode with monochromatic light (λ=520 nm), the power conversion efficiency (η) of the (ZnO electrode/ Rh 6G—containing electrolyte/carbon electrode) cell was found to be 0.2% with fill factor value of 0.44. Open-circuit photovoltage up to 500 mV could be obtained with this cell under visible (λ>420 nm) and white lights illumination.     

Photocurrent and hydrogen production by overall water splitting based on polymeric composite Calix[n]arene/Cyanin Dye/IrO2 nanoparticle

In this study, a new photoelectrochemical cell based on overall splitting of water into oxygen and hydrogen is constructed to obtain an improved photocurrent under a visible range of light. The photoanode was obtained by a gold electrode (GE) modified with carboxylic acid functionalized SH-Calix-4-arene-COOH and IrO₂ nanoparticles attached light absorbing cyanine dye via polymeric oligoaniline linkages. The conductive polymer, 4- (4H-Dithieno [3,2-b: 2 ′, 3′-d] pyrrol-4-yl)aniline, was coated on GE using electropolymerization and used as a photocathode after platinum nanoparticles (Pt) were attached on the surface. The system was illuminated under the visible light, and the water was oxidized via IrO₂ catalyst to produce hydrogen on the photocathode side while oxygen on the photoanode. A photocurrent density of 182.03 μA cm⁻² was obtained by direct transfer of electrons without using a mediator. The bilirubin oxidase (BOx) enzyme was successfully used to remove excess oxygen from the reaction chamber and a further increase in photocurrent was reached up to 272.44 μA cm⁻². Hydrogen production in the reaction chamber was measured by gas chromatography at different time intervals and a maximum of 1.25 × 10⁻⁸ mol cm⁻² was obtained.     

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.     

Rational design of ZnO–CuO–Au S-scheme heterojunctions for photocatalytic hydrogen production under visible light

An integration of S-scheme heterojunction catalyst with surface plasmon resonance effect is the prime focus of current research activites in the field of visible light driven photocatalytic hydrogen (H₂) evolution. Herein, a sol-gel route is used to design a heterojunction of ZnO–CuO–Au. The effect of process parameters, including irradiation time, catalyst dose, and sacrificial reagents on the hydrogen evolution is studied. The S-scheme ZnO–CuO–Au heterojunction catalyst demonstrated high surface area, better optical absorption response in the visible part of light spectrum, and improved separation and transportion of charge carriers as verified by DRS, PL, and photoelectrochemical studies. The maximum H₂ evolution rateof ZnO–CuO–Au reaches 4655 μmolh⁻¹g⁻¹, which is 5 and 3.2 times higher than ZnO–CuO and Au–ZnO catalysts, respectively. A possible reason of this increase in H₂ evolution rate is inhibited recombination of charge carriers because of the S-scheme design to increase electrons with strong reduction potential and prolong lifetime, Au serves as an SPR source and conductive channel to swift the transfer of electrons and high density of active sites. This work offers innovative insight into designing plasmonic metals-modified S-scheme systems for solar fuel production.     

Functionalized conjugated polymer with plasmonic Au nanoalloy for photocatalytic hydrogen generation under visible-NIR

Gold based multimetallic nanoalloys decorated on conjugated polymer nanofibers have been prepared using a greener approach without using any reducing agent. The as-prepared nanohybrids exhibited superior catalytic activity for solar hydrogen production under visible light (λ > 420 nm) irradiation and near infrared light irradiation. The UV–Visible diffuse reflectance spectra displayed strong absorption in the visible region which significantly favours the photocatalytic performance. Furthermore, the efficient charge separation suggested by electrochemical impedance measurement and photocurrent response curves for Au₅₀Pt₂₄Pd₂₆/PPy nanohybrids which exhibited significant enhancement in H₂ generation rate compared to Au/PPy nanohybrids. The strong interface contact between Au nanoalloys and PPy nanofibers play an important role for the migration of electron during catalysis. The Mott–Schottky plot revealed that photo generated charge carrier concentration has been increased for Au₅₀Pt₂₄Pd₂₆/PPy nanohybrids (7.93 × 10¹¹cm⁻³) compare to pure PPy (1.43 × 10¹¹ cm⁻³). The present study provides a new prospect for using conducting polymer based hybrid as photocatalysts for efficient solar hydrogen production.