Efficient proton conduction in dry nanofilms of amorphous aluminosilicate

Aoki, Yoshitaka; Muto, Emi; Onoue, Shinya; Nakao, Aiko; Kunitake, Toyoki

doi:10.1039/b618920b

Cited by 16 publications

(27 citation statements)

References 24 publications

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“…Recently, we discovered that silica-based, nanometer-thick films gave high proton conductivity at 100-400°C under nonhumidified atmosphere due to formation of stable Brønsted acid sites when doped with some other metal ions. 4,5 Especially, the aluminosilicate, Al x Si 1−x O n , nanofilm showed the highest proton conductivity and revealed practically usable levels of ASR at around 300°C.5 Several other research groups reported that the ion conductivity of multilayered materials increased with decreasing layer thickness. 6,7 These studies prompted us to study the effect of thickness for our Al x Si 1−x O n film.…”

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“…These films are composed commonly of homogeneously mixed glasses without phase separation, as checked by transmission electron microscopy observation. 4,5 Al/Si atomic ratio of the film was determined by X-ray photoelectron spectroscopy ͑XPS͒. Before XPS measurement, the surface layer ͑a few tens of nanometers thick͒ of sample was sputtered by Ar + ion so as to form a clean surface and probe the inner area of the films.…”

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Thickness Dependence of Proton Conductivity of Amorphous Aluminosilicate Nanofilm

Aoki

Habazaki

Kunitake³

2008

Electrochem. Solid-State Lett.

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Al x Si 1−x O n films exhibit a drastic change of proton conductivity across the film by reducing their thickness to less than 100 nm. The temperature and humidity dependence of conductivity of the sub-100 nm films is quite different from those of the thicker films. Furthermore, in the former thickness range, the value of conductivity markedly increases with reducing the film thickness, and its thickness dependence follows a power law with a fixed index of −2.1. This size-scaling effect can be explained by the percolation conductivity model that the probability for percolating of the conductive moiety in Al x Si 1−x O n films increases with decreasing the thickness. Proton-and oxide ion-conducting metal oxides have attracted much attention as electrolyte material for solid oxide fuel cells. In particular, nanometer-thick membranes of these oxides are promising for developing next-generation intermediate-temperature fuel cells thanks to diminished area specific resistance ͑ASR͒ enabled by reduction in electrolyte thickness.1-3 Amorphous metal oxides have intrinsic advantages for their application to thin-film electrolyte due to their nongranular covalent networks. Recently, we discovered that silica-based, nanometer-thick films gave high proton conductivity at 100-400°C under nonhumidified atmosphere due to formation of stable Brønsted acid sites when doped with some other metal ions. 4,5 Especially, the aluminosilicate, Al x Si 1−x O n , nanofilm showed the highest proton conductivity and revealed practically usable levels of ASR at around 300°C.5 Several other research groups reported that the ion conductivity of multilayered materials increased with decreasing layer thickness. 6,7 These studies prompted us to study the effect of thickness for our Al x Si 1−x O n film. In fact, it is found that the proton conductivity of amorphous Al x Si 1−x O n films is exponentially enhanced by reduction of thickness into the sub-100 nm regime.The Al x Si 1−x O n film was prepared on an indium-tin-oxide ͑ITO͒ substrate ͑Aldrich͒ by multiple spin-coating of precursor solutions of tetraethoxysilane ͑TEOS͒ ͑Kanto͒ and aluminum sec-butoxide ͓Al͑O s Bu͒ 3 ͔ ͑Kanto͒ at the Al/Si atomic ratio of 5/95. The details of the procedure were described elsewhere. 6 The metal concentration ͑Al + Si͒ in the precursor mixture sol was adjusted in 30 mM for the film of Ͻ100 nm thickness and 100 mM for the film of Ͼ100 nm thickness. A film with thickness of 100 nm was prepared from both of these sols. The precursor sols were spin-coated onto the ITO substrate at 3000 rpm for 40 s by a Mikasa 1H-D7 spin coater. The deposited gel layer was hydrolyzed by blowing hot air for 30 s ͑Iuchi hot gun͒, and the substrate was cooled to room temperature by blowing cold air for 20 s. These cycles of spin-coating, hydrolysis, and cooling were repeated 10-20 times, and the gel film thus obtained was calcined at 400°C for 15 min. The combination of deposition and calcination was repeated more than three times, and the final calcination was performed at 450°C for...

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Thickness Dependence of Proton Conductivity of Amorphous Aluminosilicate Nanofilm

Aoki

Habazaki

Kunitake³

2008

Electrochem. Solid-State Lett.

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“…We recently applied this method to preparation of amorphous aluminosilicate nanofilm and found that this nanometer-thick film is a practically useful proton conductor with area-specific resistance of 0.2 V cm 2 at 400 8C in dry atmosphere. [9] In order to evaluate such unique proton conductivity, it is important to see if similar proton conductivity is observed with other silica-based metal oxides. These materials would be useful as electrolyte membranes if they could be fabricated into ultrathin membranes that are nonpermeable toward neutral gas molecules; hydrogen and oxygen, since silica is one of the most stable matrix under electrochemical conditions.…”

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“…In our preliminary experiments, an aluminosilicate film of 100-nm thickness prepared from a 100 mM mixed sol of AlCl 3 and tetraethoxysilane (TEOS) with atomic ratio of Al/Si ¼ 0.1/0.9 in ethanol showed a conductivity two orders of magnitude lower than that of a corresponding film (Al 0.18 Si 0.82 O 1.91 ), prepared from a 100 mM mixed sol of aluminum tri-sec-butoxide and TEOS in 1-propanol in a previous study. [9] Therefore, it was essential to employ identical conditions to compare ion conductivity of different silica-based oxide films. In this study, we decided to conduct a multiple spin-coating approach for very dilute precursor sols of TEOS and metal alkoxides [M(OR) n ] in 1-propanol, since this precursor preparation was applicable to a wide variety of silica-based, double-oxide films.…”

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Efficient Proton Conductivity of Gas‐Tight Nanomembranes of Silica‐Based Double Oxides

et al. 2008

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Fuel cells that operate at intermediate temperatures between 200 and 400 8C are of particular interest, since this temperature range allows for the use of less precious metal catalysts and alcohol fuel, enables in situ reforming of hydrocarbon fuels, and facilitates simpler module assembly. Unfortunately, these advantages are not available with electrolyte membranes of conventional organic polymers due to limited thermal stability. Although certain inorganic crystalline materials show satisfactory ion conductivity at high temperatures, only a few examples of proton-conductive inorganics that show promising conduction within this temperature range have been reported. [1][2][3][4] The advantage of using thermally stable inorganic electrolytes is not restricted to fuel cells; they should be valuable in the development of electrochemical pump devices, membrane reactors, and electrochemical sensors. Therefore, there is a strong motivation to develop suitable electrolyte materials that can operate at intermediate temperatures. [5] We discovered that silica-based, nanometer-thick films gave enhanced ionic conduction at 100-400 8C due to the formation of stable Brønsted acid sites in the silicate framework when doped with 13 other metal ions. Among these double oxides, Al, Zr, Ti, and Hf-doped membranes showed practically usable levels of area-specific resistance at 300-400 8C, and the determination of electromotive force (EMF) indicated that the Al-, Zr-, and Ce-doped membranes gave predominant proton conductivity without apparent permeation of H 2 gas. The development of membrane materials for fuel cells and related applications may be divided into the following three stages: i) development of efficient ion-conductive materials with negligible electron conduction, ii) fabrication of materials such as gas-tight (defect-free) membranes (the thinner the better), and iii) fabrication of working devices: membraneelectrode assembly in the case of fuel cells. Previously, we successfully fabricated void-free, uniform ultrathin films of various metal oxides by the surface sol-gel process. [6][7][8] This process involves stepwise film growth composed of the reaction of metal alkoxides on the surface hydroxyl group of solid substrates and subsequent hydrolysis of the ultrathin alkoxide layer. We recently applied this method to preparation of amorphous aluminosilicate nanofilm and found that this nanometer-thick film is a practically useful proton conductor with area-specific resistance of 0.2 V cm 2 at 400 8C in dry atmosphere. [9] In order to evaluate such unique proton conductivity, it is important to see if similar proton conductivity is observed with other silica-based metal oxides. These materials would be useful as electrolyte membranes if they could be fabricated into ultrathin membranes that are nonpermeable toward neutral gas molecules; hydrogen and oxygen, since silica is one of the most stable matrix under electrochemical conditions.[10] Here, we report that protonic conduction is, in fact, a common feature of amorphou...

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“…Recently, high density and defect-free thin oxide films free from organic species have been fabricated by a layer-by-layer sol-gel processing and the obtained thin films revealed interesting properties as dielectric 36,37 and proton-conducting materials. 38,39 In the layer-bylayer sol-gel processing, the thickness of the film developed by one processing cycle was less than 10 nm. Thus, layer-by-layer deposition of nm-thick oxide films is also of interest as a protective layer for corrosion protection.…”

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Highly Enhanced Corrosion Resistance of Stainless Steel by Sol-Gel Layer-by-Layer Aluminosilicate Thin Coatings

Habazaki

Kimura

Aoki

et al. 2013

J. Electrochem. Soc.

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In this study, aluminosilicate sol-gel coatings were deposited on Type 430 stainless steel by multiple spin casting cycles. Amorphous aluminosilicate coatings, 65 nm thick, were prepared from precursor solutions with 50, 100, and 500 mmol dm −3 total concentrations of aluminum and silicon species (molar ratio of Al/Si = 1/4) by 10, 5, and 1 spin casting cycles, respectively. Although the obtained coatings had a similar composition with a molar ratio of Al/Si = 25/75, the coatings with increased density were formed by reducing the concentration of precursor solution. The pitting potentials of the coated stainless steels, measured by potentiodynamic polarization in 3.5% NaCl solution, increased remarkably (to 1.1 V vs Ag/AgCl) with a decrease in the concentration of precursor solution. Cyclic corrosion tests, consisting of 30 cycles of spraying of 5% NaCl solution at 35 • C for 2 h, drying at 60 • C for 4 h, and wetting at 50 • C and >95% relative humidity for 2 h, revealed highly improved corrosion resistance. Layer-by-layer sol-gel deposition using diluted precursor solutions is an effective way to obtain highly protective coatings. Metallic materials have been widely used in structural, marine, automobile, and aircraft applications. They are highly susceptible to corrosion in aggressive environments, and corrosion is one of the main contributors to materials loss in our society. Protective coatings have often been applied for corrosion protection. It has been well accepted that chromate coatings are the most effective, and have been applied as a suitable surface treatment of aluminum, magnesium, and steels. However, recent tighter regulations restrict the use of chromates due to their toxicity. As a consequence, the development of novel coatings with low environmental impact has attracted attention in the last decade.Sol-gel processing is one of the prospective candidates for the production of corrosion protection coatings with low environment impact. Sol-gel films usually have good adhesion to both metallic substrates and organic top coatings. The low temperature processing of the solgel method, compared with usual ceramic coatings, is important for the substrates that show mechanical degradation when exposed to environments at elevated temperatures. Thus, sol-gel processing has been extensively studied for corrosion protection of metals, including aluminum, magnesium, and steels.1 Sol-gel coatings based on SiO 2 , 2-6 ZrO 2 , 7-9 and Al 2 O 3 10,11 have been reported to provide good corrosion protection on various metals. However, there are still some drawbacks of inorganic sol-gel coatings: (i) it is difficult to form thick coatings (>1 μm) without cracking; 12 (ii) porosity usually exists in the coatings because the removal of organic components during heattreatment (usually at 400-600• C) introduces micropores in the coatings, through which corrosive solutions can penetrate to substrate. 13In order to overcome these drawbacks, increased interest has been directed toward organic-inorganic hybrid sol-gel sys...

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Efficient proton conduction in dry nanofilms of amorphous aluminosilicate

Abstract: Amorphous aluminosilicate nanofilms as prepared by a sol-gel process and post-annealing exhibit proton conduction at a wide temperature range in dry air with a lowest area specific resistance of 0.24 omega cm2 at 400 degrees C.

Cited by 16 publications

References 24 publications

Thickness Dependence of Proton Conductivity of Amorphous Aluminosilicate Nanofilm

Thickness Dependence of Proton Conductivity of Amorphous Aluminosilicate Nanofilm

Efficient Proton Conductivity of Gas‐Tight Nanomembranes of Silica‐Based Double Oxides

Highly Enhanced Corrosion Resistance of Stainless Steel by Sol-Gel Layer-by-Layer Aluminosilicate Thin Coatings

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