Formation of ultrafast-switching viologen-anchored TiO2 electrochromic device by introducing Sb-doped SnO2 nanoparticles

Ultrafast-switching viologen-anchored TiO₂ electrochromic device (ECD) was developed by introducing Sb-doped SnO₂ (Sb_xSn_1−xO₂, ATO) as counter electrode (CE), and the switching behavior of the fabricated ECD was investigated as a function of Sb-doping concentration. About 9-nm-sized Sb_xSn_1−xO₂ (x=0–0.3) nanoparticles were synthesized by a solvothermal reaction of tin (IV) chloride and antimony (III) chloride at 240 °C, and employed to fabricate 2.4-μm-thick transparent CE. Working electrode (WE) was formed from the 7-nm-sized TiO₂ nanoparticle by a doctor blade method, and the thickness of the nanoporous TiO₂ electrode was 4.5 μm. The phosphonated viologen, bis(2-phosphonylethyl)-4,4′-bipyridinium dibromide, was then adsorbed on the prepared films for the construction of the ECD. The response time was strongly dependent on the doping concentration of Sb in ATO, and the fastest switching response was observed at 3 mol%. At this composition, the coloration time was 5.7 ms, and the bleaching time was 14.4 ms, which is regarded as one of the best results so far reported.     

A hybridized electron-selective layer using Sb-doped SnO2 nanowires for efficient inverted polymer solar cells

We developed a novel hybridized electron-selective layer comprised of Sb-doped SnO₂ nanowires for efficient inverted polymer solar cells. A device containing Sb-doped SnO₂ nanowires with 0.1 mg/ml concentration showed a significant increase in power conversion efficiency to 3.23% with an enhanced fill factor, compared to a reference device without the nanowires (2.89%). Such improvement is attributed to the high electrical conductivity of one-dimensional Sb-doped SnO₂ nanowires and to the good light transmittance through the wide band gap of tin oxide. Also the surface morphology of the hybridized electron-selective layer is made denser and improved by incorporating one-dimensional Sb-doped SnO₂ nanowires, resulting in the enhancement of the photovoltaic performance.     

RuO2 nanorod coated cathode for the electrolysis of water

In this investigation crystalline ruthenium dioxide square nanorods have been grown on a variety of substrate surfaces and were used as the cathodes in a system to electrolyze an aqueous solution of KOH. Gaseous hydrogen was produced at the cathode and oxygen at the anode. The voltage measured at the cathode relative to the solution, and that across both electrodes was found to be dependent on the cathode material used, specifically the substrate material and the density and nature of a RuO₂ nanorod coating. The formation of a double layer on the nanorod surface was proposed to render the production rate of hydrogen, dependent on total interfacial area alone and not on the geometric shape of the nanorods themselves. At a current density of 20 mA/cm², the operating efficiency of the RuO₂ nanorod cathode cell was found to be 1.7 times that obtained for a cathode electrode fabricated with a simple thin film of RuO₂.     

A Li-doped Co3O4 oxygen evolution catalyst for non-precious metal alkaline anion exchange membrane water electrolysers

Li-doped Co₃O₄ (Li_xCo_3−xO₄, x = 0, 0.07, 0.21, 0.35, 0.49) spinel powders were prepared with a thermal decomposition method and characterized by XRD, SEM, TEM, and XPS. The Li_xCo_3−xO₄ samples were formed as tetragonal powders with a simple spinel structure and with particle sizes about 30–40 nm. All Li_xCo_3−xO₄ samples exhibited a 50 mV more negative onset potential for oxygen evolution reaction (OER) than Co₃O₄. The influence of Li-doping is discussed regarding cation distribution, electronic conductivity and oxygen binding energy. Li_0.21Co_2.79O₄ exhibited the highest OER activity amongst the five samples. A single cell, non-precious metal alkaline anion exchange membrane water electrolysers (AAEMWE) with Li_0.21Co_2.79O₄ anode exhibited a current density of 300 mA cm⁻² at a voltage 2.2 to 2.05 V at temperatures of 20–45 °C and the stability was examined with a continuous operation for 10 h at 300 mA cm⁻² and at 30 °C.     

Effect of treatment temperature on the performance of RuO2 anode electrocatalyst for high temperature proton exchange membrane water electrolysers

Ruthenium oxide catalysts were prepared by a sol–gel technique and calcined at different temperatures e.g., 400 °C, 500 °C and 600 °C. The catalysts performance for the oxygen evolution reaction was studied using cyclic voltammetry and their performance in a high temperature proton exchange membrane water electrolyser (PEMWE) examined. Physio-chemical characterization was carried out to study the thermal stability, oxygen-metal bond formation, crystallinity phase and crystallite size, particle size and elemental analysis by TGA, FTIR, XRD, TEM and EDX respectively. The electrolyte used for electrochemical characterisation was 1.0 M H₃PO₄ and 0.5 M H₂SO₄. Additionally, the effect of calcination and electrolyte temperature on oxygen evolution reaction of RuO₂ catalysts was studied and the apparent activation energy was determined using chronoamperometry. The prepared RuO₂ were tested as anode catalyst in PEMWE in the temperature range of 120–150 °C using phosphoric acid doped polybenzimidazole membrane electrolyte. The physio-chemical and electrochemical characterization results indicate that RuO₂ calcined at 500 °C gave the best performance with a current density of 0.875 A cm⁻² at 1.8 V in a PEMWE operated at 150 °C.     

Investigation of IrO2 electrocatalysts prepared by a sulfite-couplex route for the O2 evolution reaction in solid polymer electrolyte water electrolyzers

IrO₂ electrocatalysts were prepared and electrochemically characterized for the oxygen evolution reaction in a Solid Polymer Electrolyte (SPE) electrolyzer. By using a sulfite complex-based preparation procedure, an amorphous iridium oxide precursor was obtained at 80 °C, which was, successively, calcined at different temperatures: 350 °C, 400 °C and 450 °C. A physico-chemical characterization was carried out by X Ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and X-ray-photoelectron spectroscopy (XPS). The various IrO₂ catalysts were sprayed onto a Nafion 115 membrane with a loading of 2.5 mg cm⁻² to form the anode. A Pt/C catalyst (Pt loading 0.5 mg cm⁻²) was used as cathode. The best electrochemical performance was obtained for the cell based on the IrO₂ calcined at 350 °C. The maximum current density at high potentials (1.8  V) was about 1.75 A cm⁻². Accelerated time-tests at 2 A cm⁻² demonstrated a suitable stability of the IrO₂ calcined at 350 °C; however, the intrinsic stability appeared to increase with the calcination temperature. The sample calcined at 400 °C could represent a good compromise between performance and intrinsic stability.     

Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis

In proton exchange membrane (PEM) water electrolysis, iridium oxide (IrO₂) has often been utilized as a main catalyst for the oxygen evolution reaction (OER) as a rate-determining step. In general, the performance of PEM water electrolysis is dominantly affected by the specific surface area and the porous structure of the IrO₂ catalyst. Thus, in this study, IrO₂ and antimony-doped tin oxide (ATO) nanostructures with high specific surface areas were synthesized through the Adams fusion method. The as-prepared samples showed well-defined porous high-crystalline nanostructures. The ATO nanoparticles as a support were surrounded by IrO₂ nanoparticles as a catalyst without serious agglomeration, indicating that the IrO₂ catalyst was uniformly distributed on the ATO support. Compared to pure IrO₂, the IrO₂/ATO mixture electrodes showed superior OER properties because of their increased electrochemical active sites.     

Structure and electro-catalytic properties of electrode materials consisting of Pt nanorod decorated/RuO2 nanorod coated substrates

Nanorod coated materials have the potential of being exceptional electro-catalysts. In this investigation Pt nanorod decorated ruthenium dioxide square nanorods have been grown on aluminized Si substrates and used as the cathode in a system to electrolyze a [2M] aqueous solution of KOH. Gaseous hydrogen was produced at the cathode. The voltage measured at the cathode relative to the solution, and that across both electrodes was found to be dependent on the cathode material used. At a current density of 30 mA/cm², the potential drop from the cathode electrode to the liquid electrolyte was measured to be −0.72 V for a solid Pt electrode and −0.69 V for the best Pt nanorod coated electrode.     

Water splitting for hydrogen production using a high surface area RuO2 electrocatalyst synthesized in supercritical water

In water electrolysis, a major obstacle is the anodic reaction for water oxidation where substantial energy loss occurs mainly due to the large overpotential. Therefore, the electrocatalytic material of an anode is of importance to achieve a highly efficient water splitting reaction. Among the various requirements, the surface area of electrocatalysts has been considered a key factor in electrocatalytic reactions. In this study, to obtain a high surface area RuO₂ electrocatalyst, a supercritical hydrothermal synthetic method was applied. Indeed, RuO₂ particles with the surface area of 78.2 m²/g were successfully synthesized, which is 7 times higher compared with the commercial version. We also investigated the electrocatalytic activity of the synthesized high surface area RuO₂ by evaluating the number of active sites and hydrogen production rates and compared them with the commercial one as a reference. Based on the cyclic voltammetric measurements, the number of active sites was estimated to be 152.1 mC cm⁻² and 17.0 mC cm⁻² for the synthetic and the commercial RuO₂, respectively. More importantly, the hydrogen production rates measured by the water splitting device with RuO₂ films for the anode showed 4 times higher value for the synthetic RuO₂ compared with the commercial one.     

Nano-grain SnO2 electrodes for high conversion efficiency SnO2-DSSC

The nano-grain ZnO/SnO₂ composite electrode was prepared by adding 5 w% of the 200-250 nm ZnO particles to the 5 nm SnO₂ colloid in the presence of hydroxypropylcellulose (M.W.=80,000). The nano-grain SnO₂ electrode was obtained by removing the ZnO particles from the composite electrode using acetic acid. The FE-SEM micrographs revealed that both electrodes consisted of interconnected nano-grains that were ca. 800 nm in size, and the large pores between the grains furnished the wide electrolyte diffusion channels within the electrodes. The photovoltaic properties of the nano-grain electrodes were investigated by measuring the I-V behaviors, the IPCE spectra and the ac-impedance spectra. The nano-grain electrodes exhibited remarkably improved conversion efficiencies of 3.96% for the composite and 2.98% for the SnO₂ electrode compared to the value of 1.66% for the usual nano-particle SnO₂ electrode. The improvement conversion efficiencies were mainly attributed to the formation of nano-grains, which facilitated the electron diffusion within the grains. The improved electrolyte diffusion as well as the light-scattering effects enhanced the photovoltaic performance of the SnO₂ electrode.     

Activity of dealloyed PtCo3 and PtCu3 nanoparticle electrocatalyst for oxygen reduction reaction in polymer electrolyte membrane fuel cell

We report a comparative study of the alloy formation and electrochemical activity of dealloyed PtCo₃ and PtCu₃ nanoparticle electrocatalysts for the oxygen reduction reaction (ORR). For the Pt-Co system the maximum annealing temperatures were 650 °C, 800 °C and 900 °C for 7 h to drive the Pt-Co alloy formation and the particle growth. EDS and XRD were employed for the characterization of catalyst powders. The RDE and RRDE experiments were conducted in 0.1 M HClO₄ at room temperature.We demonstrate that the mass and surface area specific ORR activities of Pt-Co and Pt-Cu alloys after voltammetric activation exhibit a considerable improvement compared to those of pure Pt/C. The dealloyed PtCo₃ (800 °C/7 h) electrocatalyst performs 3 times higher in terms of Pt-based mass activity and 4-5 times higher in terms of ECSA-based specific activity than a 28.2 wt.% Pt/C. Dealloyed Pt-Co catalysts (800 °C/7 h) show the most favorable balance between mass and specific ORR activity with a particle size of 2.2 ± 0.1 nm. We hypothesize that geometric strain effects of the dealloyed Pt-Co nanoparticles, similar to those found in dealloyed PtCu₃ nanoparticles, are responsible for the improvement in ORR activity [1].     

Electrospun TiO2/SnO2 nanofibers with innovative structure and chemical properties for highly efficient photocatalytic H2 generation

Innovative TiO₂/SnO₂ nanofibers were fabricated via electrospinning an innovated precursor solution and used for photocatalytic H₂ generation. The nanofibers exhibited greatly enhanced H₂ evolution rate compared to bare TiO₂ nanofiber and P25. The enhanced efficiency of the TiO₂/SnO₂ nanofibers was attributed to its excellent synergistic properties: (1) its good mesoporosity; (2) the red-shift of absorbance spectra to enhance light absorbance capability; (3) its long nanofibrous structure and (4) anatase TiO₂ – rutile TiO₂ – rutile SnO₂ ternary junctions favorable for the separation of electrons and holes. Based on our experimental results, the optimum ratio of TiO₂/SnO₂ nanofibers with 3% Sn demonstrated the highest efficiency in H₂ generation.     

Influence of CeO2 nanoparticles in the stability of electrodeposited Ni anodes for alkaline electrolysers

In Mexico, the National Electric System Development Program (PRODESEN 2022–2036) establishes the use of green hydrogen to be supplied in Combined Cycle Power Plants in a mixture of 70% CH₄ and 30% H₂ for electric energy generation. For those places where natural resources such as sun, wind, and water are available, the best option is to use alkaline electrolysers due to cost and lifetime. The oxygen evolution reaction (OER), which takes place at the anode, is the limiting factor in the performance of an alkaline electrolyser because large overpotentials are required to break the (O–H)^- bond needed to form the double bond of the gaseous oxygen molecule (OO). Many studies under the initial research stage report using Ni–CeO₂ as a promising catalyst toward OER, observing good catalytic activity and stability at low overpotentials. This work deposited electrodes with Ni and Ni–CeO₂ films of 80 μm on AISI 304 Stainless steel substrates.A kinetic study was performed using linear sweep voltammetry to determine the Tafel slope of the OER. An electrolysis system was integrated with these anodic electrodes and tested for 500 h to determine the stability of films at real operating conditions using 15 wt% NaOH as electrolyte @ 0.5 A cm⁻². A better OER activity of the modified Ni–CeO₂ electrodes than Ni electrodes was obtained. Ni–CeO₂ electrode shows an onset potential of 1.48 V, a potential of 1.56 V at 5 mA cm⁻², and a Tafel slope of 75.71 mV dec⁻¹, which is related to a reaction determining step in an oxidation process of (OH)⁻ via the one-electron transfer to the surface which is partial cover by OH species adsorbed. In the studies with the electrolyser test system, the Ni electrodes with an electrodeposited film of 80 μm showed good stability, and the film remained on the surface electrode.On the other hand, the electrode modified with Ni–CeO₂ showed instability and high overpotentials as 500 h were completed. The last is attributed to the low conductivity of CeO₂ and the formation of the passive NiO/NiOH₂ layer on the electrode surface, which is not easily detached due to the presence of CeO₂. These results confirm the importance of conducting tests in an electrolyser under real conditions.     

Platinum nanoparticles embedded in layer-by-layer films from SnO2/polyallylamine for ethanol electrooxidation

Self-assembled films from SnO₂ and polyallylamine (PAH) were deposited on gold via ionic attraction by the layer-by-layer (LbL) method. The modified electrodes were immersed into a H₂PtCl₆ solution, a current of 100 μA was applied, and different electrodeposition times were used. The SnO₂/PAH layers served as templates to yield metallic platinum with different particle sizes. The scanning tunnel microscopy images show that the particle size increases as a function of electrodeposition time. The potentiodynamic profile of the electrodes changes as a function of the electrodeposition time in 0.5 mol L⁻¹ H₂SO₄, at a sweeping rate of 50 mV s⁻¹. Oxygen-like species are formed at less positive potentials for the Pt–SnO₂/PAH film in the case of the smallest platinum particles. Electrochemical impedance spectroscopy measurements in acid medium at 0.7 V show that the charge transfer resistance normalized by the exposed platinum area is 750 times greater for platinum electrode (300 kΩ cm²) compared with the Pt–SnO₂/PAH film with 1 min of electrodeposition (0.4 kΩ cm²). According to the Langmuir–Hinshelwood bifunctional mechanism, the high degree of coverage with oxygen-like species on the platinum nanoparticles is responsible for the electrocatalytic activity of the Pt–SnO₂/PAH concerning ethanol electrooxidation. With these features, this Pt–SnO₂/PAH film may be grown on a proton exchange membrane (PEM) in direct ethanol fuel cells (DEFC).     

Organic–inorganic hybrid solar cells based on conducting polymer and SnO2 nanoparticles chemically modified with a fullerene derivative

Organic–inorganic hybrid solid solar cells were fabricated by using a conjugated polymer (MDMO-PPV) and SnO₂ nanoparticles chemically modified with C₆₀C(COOH)₂. The cell performance was improved by the chemical modification, suggesting that the modification with photoelectrochemically active organic materials is useful for establishing good electronic junction at the organic–inorganic interface. The short-circuit current density J_SC increased with increasing thickness of MDMO-PPV up to 40 nm, and then decreased gradually. This thickness dependence was explained by the fluorescence quenching of MDMO-PPV by Au electrode and the film resistance of MDMO-PPV.     

Characterization of new heterosystem CuFeO2/SnO2 application to visible-light induced hydrogen evolution

Photo-assisted H₂ evolution has been realized over the new heterosystem CuFeO₂/SnO₂ without any noble metal and was studied in connection with some physical parameters. The delafossite CuFeO₂ has been prepared by thermal decomposition from various salts. The polarity of generated voltage is positive indicating that the materials exhibit p-type conductivity whereas the electroneutrality is achieved by oxygen insertion. The plot of the logarithm (conductivity) vs. T⁻¹ gives average activation energy of 0.12 eV. CuFeO₂ is a narrow band gap semiconductor with an optical gap of 1.32 eV. The oxide was characterized photoelectrochemically; its conduction band (−1.09 V_RHE) is located below that of SnO₂ (−0.86 V_RHE) at pH ∼13.5 itself more negative than the H₂O/H₂ level leading to a thermodynamically favorable H₂ evolution under visible irradiation. The sensitizer CuFeO₂, working as an electron pump, is stable towards photocorrosion by hole consumption reactions involving the reducing agents X²⁻ (=S₂O₃²⁻ and SO₃²⁻). The photoactivity was dependent on the precursor and the best performance (0.026 ml h⁻¹ mg⁻¹) was obtained in S₂O₃²⁻ (pH ∼13.5) over CuFeO₂ synthesized from nitrate with a mass ratio (CuFeO₂/SnO₂) equal to unity. A quantum yield of 0.5% was obtained under polychromatic light. H₂ liberation occurs concomitantly with the oxidation of S₂O₃²⁻ to dithionate and sulfate. The tendency towards saturation, in a closed system, is mainly ascribed to the competitive reduction of the end product S₂O₆²⁻.     

Nanoporous SnO2 electrodes for dye-sensitized solar cells: improved cell performance by the synthesis of 18 nm SnO2 colloids

This paper reports on the synthesis of SnO₂ 18 nm diameter colloidal suspension for the fabrication of nanoporous electrodes. The new suspension allows the fabrication of thick and homogeneous electrodes by simple one layer spreading; in contrast to the successive spin coating of the commonly used commercial suspension that results in a thin inhomogeneous electrode. When used in dye-sensitized solar cells, the new electrodes increase the light-to-energy conversion efficiency by a factor of 2.1 in comparison with standard commercial suspension based electrodes. The improvement is mostly the result of an increase of the photocurrent. This increase is attributed to the better electrolyte migration, and presumably also to an increase of the photoinjected electron diffusion rate in the electrode.     

Tailoring of RuO2 nanoparticles by microwave assisted “Instant method” for energy storage applications

RuO₂ nanoparticles are synthesized by Instant method using Li₂CO₃ as stabilizing agent, under microwave irradiation at 60 °C and investigated for the anodic oxygen evolution reaction (OER) and for their supercapacitance properties in 0.5 M H₂SO₄ medium. Structural and morphological characterizations of RuO₂ are investigated by in situ X-ray diffraction (XRD), thermogravimetric analysis (TG-DTA), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDS) and Raman spectroscopy. The TEM images of as prepared material show the uniform distribution of RuO₂ nanoparticles with mean diameter of ca. 1.5 nm. Analysis on as prepared material indicates the structural formula as [RuO₂·2.6H₂O] 0.7H₂O with low crystallinity. The influence of annealing temperature on RuO₂ is studied in light of electrocatalytic activity for oxygen evolution reaction (OER) and capacitance. Electrochemical performances of RuO₂ electrodes are followed by current-potential curves, galvanostatic charge-discharge cycles and evolved oxygen measurements. The amount of oxygen gas evolved during the OER by the crystalline RuO₂ is found to be consistent with the electrical energy supplied to the catalyst. The cyclic voltammogram of RuO₂ exhibits the typical capacitance behavior with highly reversible nature. The specific capacitance of hydrous RuO₂ is found to be 737 F g⁻¹ at the scan rate of 2 mV s⁻¹, by the balanced transport of proton through the structural water and electron transport along dioxo bridges, which makes a suitable material for energy storage. The specific capacitance decreases with increase in the crystallinity of RuO₂. The present study shows the potential method to synthesize rapid and uniform nano particles of RuO₂ for water electrolysis and supercapacitors.     

Redox cycling of CuFe2O4 supported on ZrO2 and CeO2 for two-step methane reforming/water splitting

CuFe₂O₄ supported on ZrO₂ and CeO₂ for two-step methane reforming was evaluated to determine if it could enhance the reactivity, CO selectivity and thermal stability of CuFe₂O₄. Two-step methane reforming consists of a syngas production step and a water splitting step. CuFe₂O₄ supported on ZrO₂ and CeO₂ was prepared using an aerial oxidation method. Non-isothermal methane reduction was carried out on TGA to compare the reactivity of CuFe₂O₄/ZrO₂ and CuFe₂O₄/CeO₂. In addition, a syngas production step was performed at 900 °C and water splitting was conducted at 800 °C alternatively five times to compare the methane conversion, CO selectivity, cycle ability and hydrogen production by water splitting in a fixed bed reactor. If the 1st syngas production step results are excluded due to over-oxidation, CuFe₂O₄/ZrO₂ and CuFe₂O₄/CeO₂ showed approximately 74.0–82.8% and 60.3–87.5% methane conversion, respectively, and 44.0–47.8% and 65.2–81.5% CO selectivity, respectively. Using CeO₂ and ZrO₂ as supports effectively improved the reactivity and methane conversion compared to CuFe₂O₄. CuFe₂O₄/ZrO₂ showed high methane conversion due to the high phase stability and thermal stability of ZrO₂ but the selectivity was not improved. After 5 successive cycles, the CeFeO₃ phase was found on CuFe₂O₄/CeO₂. Furthermore, methane conversion, CO selectivity and the amounts of hydrogen production of CuFe₂O4/CeO₂ increased with increasing number of cycles. Additional test up to the 11th cycle on CuFe₂O₄/CeO₂ revealed that CeO₂ is a better support that ZnO₂ in terms of the reactivity and CO selectivity.     

Ethanol oxidation reaction activity of highly dispersed Pt/SnO2 double nanoparticles on carbon black

Highly dispersed Pt and SnO₂ double nanoparticles containing different Pt/Sn ratios (denoted as Pt/SnO₂/CB) were prepared on carbon black (CB) by the modified Bönnemann method. The average size of Pt and SnO₂ nanoparticles was 3.1 ± 0.5 nm and 2.5 ± 0.3 nm, respectively, in Pt/SnO₂(3:1)/CB, 3.0 ± 0.5 nm and 2.6 ± 0.3 nm, respectively, in Pt/SnO₂(1:1)/CB, and 2.8 ± 0.5 nm and 2.5 ± 0.3 nm, respectively, in Pt/SnO₂(1:3)/CB. The Pt/SnO₂(3:1)/CB electrode showed the highest specific activity and lowest overpotential for ethanol oxidation reaction (EOR), and was superior to a Pt/CB electrode. Current density for EOR at 0.40 and 0.60 V vs. reversible hydrogen electrode for the Pt/SnO₂(3:1)/CB electrode decayed more slowly than that for the Pt/CB electrode because of a synergistic effect between Pt and SnO₂ nanoparticles. The predominant reaction product was acetic acid, and its current efficiency was about 70%, while that for CO₂ production was about 30%.