Mixed Proton/Electron Conductivity in Nano-Grained Bulk Body of TiO2
Authors: Honda Keiichiro , Yamaguchi Shu 
Published: February 2020

Background

Proton-conducting solid electrolytes are paid much attention from various applications such as SOFC and electrochemical catalyst. As a new class of materials other than bulk conductors, low temperature proton conductivity on the surface of nano-grained oxides like YSZ [1] and CeO2 is studied by many researchers in these days. The previous study by the authors has suggested that the dominant mechanism for proton conduction depends on the temperature, and three types of mechanism are suggested for corresponding temperature ranges; a proton hopping via hydroxyl(M-OH−) on the surface of metal oxide at the highest temperature range above 250, that via hydrogen-bonded H2O molecules by the Grotthuss mechanism at temperatures between 60 ºC and 250 ºC, and the Vehicle mechanism, using H3O+s as a vehicle. H3O+s are delocalized from the surface to diffuse into the hydrated multilayer above the surface below 60 ºC. Possible and unique applications of this surface proton conductivity are electrochemical catalysts and reactors by combining surface proton conductivity with variety of bulk functionalities like electronic conductivity and photocatalytic activity. To promote the catalytic reaction, i.e.COOH+3H++3e-→CH3OH+1/2O2, both protons and electrons are to be migrated to the active sites for anodic and cathodic reactions. As TiO2 in anatase form is a bulk n-type semiconductor, nano-grain anatase is a candidate to achieve both proton and electron conduction simultaneously. In this research, electrical conductivity of nano-grained TiO2 is analyzed to examine a possible application of TiO2 for such an electrochemical and photochemical catalysts.

Method

TiO2 nanoparticle in anatase form was obtained by hydrolysis of alkoxide (Ti(OC4H9)4). The obtained nano-grained powder with its grain size about ~20nm, estimated from XRD and SEM images, was pressed at room temperature (RT), using cubic anvil at 4GPa with pyrophylite as a pressure media, resulting in a compact body of anatase. After shaping into rectangular plate, Au electrodes on both sides of rectangular face ware fabricated, and the sample was subjected to subsequent electrical conductivity measurement. Two-probe ac impedance measurements ware conducted under controlled temperature and gas atmospheres. The gas mixtures employed are categorized as reducing (1%H2-99%Ar) and oxidizing atmosphere (100%O2) with and without CO2. All the gas mixtures are humidified by either H2O or D2O, to examine isotope effect. The measurements were made from RT to 400 ºC FT-IR is used to characterize surface adsorbates at the same atmosphere and temperatures with electrical measurements.

Result

Figure1 shows the electrical conductivity under oxidizing and reducing conditions with and without CO2. In oxidized atmosphere, the measured conductivity above 250 ºC increases with the increase of temperature, while the conductivity increases with the decrease of temperature below 250 ºC which is typical behavior of surface proton conductivity of nano-grained oxide materials. In addition, H-D isotope effect on the conductivity is observed in entire temperature range measured, suggesting the predominant proton conductivity. An inversion of the temperature dependence of the electrical conductivity around 250 ºC suggests the presence of two different proton conductivity mechanisms in the nano-grained TiO2 system.
In reducing atmosphere (1%H2−99%Ar), the conductivities above 150 ºC are higher than those in oxidized atmosphere by several orders of magnitude, in contrast to the similar values between oxidizing and reducing atmosphere below 150 ºC In addition, the conductivity is approximately proportional to one half power of P
H2(partial pressure of H2 gas). This dependency can be explained by the following formation reaction of interstitial Ti3+, which is considered to be the dominant defect in bulk TiO2.

TiTi
x+2OO
x+2H2→Tii
•••+3e’+2H2O
This consistency with the bulk defect chemical reaction suggests that the electronic conduction in bulk is dominant in reduced atmosphere above 150 ºC The influence of the CO2 adsorption is limited to the low temperature range below 175 ºC, probably because of desorption of CO2 above this temperature. The decrease of conductivity by CO2 adsorption observed agrees with the decrease observed in nano-grained YSZs reported earlier, suggesting similar mechanism is seemingly working behind.
In summary, we could successfully observe both electronic and ionic conductivities for nano-grained TiO2 bulk body fabricated by ultra-high pressure pressing at room temperature.

Reference

[1] S. Miyoshi et al. Chem. Mater., 26 (2014) 5194.
[2] R. Sato et al. J. Phys. Chem. C, 119 (2015) 28925.

Figure 1