High Proton Conductivity in Anodic ZrO₂/WO₃ Nanofilms

Abstract

Anodization of the valve metals Al, Ti, Zr, Nb, Hf, Ta, and W leads to formation of barrier-type (compact) or self-organized porous-type anodic oxide films, depending on the electrolytes used. The oxide films formed are dielectrics or semiconductors with useful properties that make them of great interest for many applications, including solid electrolytic capacitors, solar cells, photocatalysis, electrochromic devices, and selfcleaning materials.[1-2] Here we report the first example of an anodic oxide with high proton conductivity below 200 8C. Proton-conducting thin films are promising electrolytes for solid-oxide fuel cells that operate at intermediate temperatures (100-400 8C) and other electrochemical devices. [3] Intermediate-temperature fuel cells have several advantages over polymer electrolyte fuel cells, which operate below 100 8C, under fully hydrated conditions.[4] The former type allows the use of nonprecious-metal electrocatalysts [5] and hydrocarbon fuels [6] and facilitates simpler module assembly by means of anhydrous proton conductors.Inorganic solid acids such as CsHSO 4 , Rb 3 H(SeO 4 ) 2 , and CsH 2 PO 4 exhibit sufficient proton conductivities fuel-cell operation, but their poor processability and low stability limit their practical application.[7] ZrO 2 /WO 3 is a very attractive acid catalyst that displays high catalytic activity and stability in demanding reactions that require strong Brønsted acidity. [8] The acid strength of this material has been reported to be H o À14.5 and is comparable with that of fully anhydrous hydrofluoric acid.[9] The proton-transport properties of this material are therefore of great interest.For practical use of a solid electrolyte in fuel-cell technology, the area-specific resistivity (ASR) of the electrolyte should be below 0.2 W cm 2 .[10] To obtain a sufficiently low ASR value at temperatures below 400 8C, one approach is reducing the thickness of the solid electrolyte to the nanometer scale. Shim et al. reported atomic-layer deposition of yttria-stabilized zirconia nanofilms of 60 nm thickness, which showed high ion conductivity as an electrolyte for solid-oxide fuel cells operated at and below 350 8C.[11] The high proton conductivity of nanofilms below 400 8C has also been reported for amorphous metallosilicate [12] and zirconium pyrophosphate [13] nanofilms prepared by a layer-by-layer sol-gel process. Native protons in these amorphous nanofilms contribute to efficient ionic conduction even in a dry atmosphere. Anodic oxide films formed on valve metals usually contain hydrogen species. [14] It is likely that high proton conductivity occurs in anodic oxide if the oxide is strongly acidic. In addition, the anodizing process is suitable for fabricating nanofilms of a desired thickness, since the thickness of barrier-type anodic oxide films changes linearly with the formation voltage. In this work, we prepared ZrO 2 /WO 3 nanofilms by anodizing magnetron-sputtered Zr/50 atom % W alloy films with flat surfaces. The obtained anodic oxide films showed high pr...

Mixed layered WO3/ZrO2 films (with and without rhodium) as active supports for PtRu nanoparticles: enhancement of oxidation of ethanol

“…3B) does imply fast charge (proton, electron) transport in WO 3 / ZrO 2 film. This result is not surprising because high proton conductivity was postulated for the films of mixed (W, Zr) oxide films described elsewhere [49][50][51][52].…”

“…The present approach to electrochemical generation of WO 3 / ZrO 2 films seems to resemble a concept of the formation of the efficient proton-conducting mixed oxide films by anodizing the zirconium/tungsten alloys [51]. Comparison of the cyclic voltammetric response of the mixed WO 3 /ZrO 2 film (Fig.…”

“…Vulcan, carbon nanotubes) were also incorporated [3,22]. Recently, mixed systems of WO 3 and ZrO 2 have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions that include the hydrocarbon conversion reactions or selective catalytic reduction of NO x to N 2 [49][50][51]. In addition to high population of acid sites and their strength, it has been postulated that, at ZrO 2 domains stabilized through ZrÀ ÀOÀ ÀW bonds and surrounded by partially reduced tungsten oxide nanostructures, hydrogen molecules have been found to undergo dissociation to yield H d+ ionic species stabilized at electronegative tungsten oxide domains.…”

Gas sensing properties of the nanostructured anodic Zr–W oxide film

“…Partly this may be due to increasing the numbers of Brоnsted acid sites, which originate from the conversion of spillover H-atoms to acidic hydrogen species (H + or H ı+ ) as reported by Shimizu and co-workers for the case of a Pt-WO 3 /ZrO 2 -based sensing device [4]. In the Zr-W mixed oxide nanostructures prepared here, proton conduction may similarly contribute to the measured conductivity value without yet the presence of hydrogen gas [19]; the effect being enhanced in an H 2 atmosphere at elevated temperature and humidity. However, we will refrain from speculating on the matter until a systematic electrochemical impedance spectroscopy characterization of the film is done and adequately interpreted (to be presented in a future work).…”

“…acid or base. Since ZrO 2 -WO 3 is a strong acidic metal oxide [19], the reaction that takes place when the sensor is exposed to ethanol is as follows:…”