Low-Temperature Protonic Conduction Based on Surface Protonics

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Low-temperature protonic conduction based on surface protonics: an example of nano-structured yttria-doped zirconia Shogo Miyoshi, Yasuaki Akao, Naoaki Kuwata, Junichi Kawamura, Yukiko Oyama, Takehiko Yagi, and Shu Yamaguchi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5012923 • Publication Date (Web): 08 Aug 2014 Downloaded from http://pubs.acs.org on September 2, 2014

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Chemistry of Materials

Low-temperature protonic conduction based on surface protonics: an example of nano-structured yttria-doped zirconia Shogo Miyoshi a, *, Yasuaki Akao a, Naoaki Kuwata b, Junichi Kawamura b, Yukiko Oyama a, Takehiko Yagi c, Shu Yamaguchi a a: Dep. of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, JAPAN b: Inst. of Multidisciplinary Res. for Adv. Mater., Tohoku Univ., 2-1-1 Katahira, Aoba-ku, 980-8577 Sendai, JAPAN c: The Institute for Solid State Physics, The University of Tokyo, Kashiwanoha 5-1-5, 277-8581 Kashiwa, JAPAN KEYWORDS. proton conductivity; yttria-stabilized zirconia; nano-grained oxides; surface protonics; interfacial hydrated layer.

ABSTRACT: In contrast to conventional ceramic ionic conductors relying on bulk ionic transport, making use of interfaces such as grain boundary and surface may provide various new possibilities to develop novel ionic conductors. Here we demonstrate that nano-grained structures of yttria-doped zirconia (YSZ), of which the bulk property involves negligible proton solubility or conductivity, are endowed with appreciable proton conductivity via interfacial hydrated layers. A combination of nano-powder synthesis and ultra high-pressure compaction (4GPa) at room temperature enables us to fabricate the nano-grained specimens. The material thus prepared can retain appreciable amount of protons and water within the grain-boundary or “internal surface”, resulting in a hierarchical structure of hydroxyl groups and water molecules with different thermal stability and thereby mobility. The physico-chemical properties of those protonic species have been investigated by means of in-situ FT-IR, 1H MAS-NMR and thermal desorption spectroscopy. At lower temperatures, proton conductivity prevails over normally-observed oxide-ion conductivity, which is facilitated by interplay of those protonic species at the interfaces. The present study provides a new prospect for developing proton-conducting materials which are based on “surface protonics” of nano-grained oxides.

1. INTRODUCTION Solid-state protonic conductors are of prime interest, being expected as electrolytes of sensors, fuel cells, electrolyzers and so on. While there have been numerous proton-conducting materials reported, known materials in solid state do not cover the intermediate temperature range (200-500°C) with satisfactory proton conductivity 1. Low-temperature proton conductors such as polymerbased materials (e.g., imidazole intercalated sulfonated polyaromatic polymer, IISPAP 2), hydrogen-phosphates 3 and -sulfates 4, can not endure as high temperatures as 200°C. On the other hand, high-temperature proton conductors, which include perovskite-based oxides 5 and phosphates 6, can work satisfactorily only at high temperatures like 500°C or above. In order to develop protonconducting materials working in the intermediate temperature region, where many of the application devices are expected to operate, it would be necessary to design materials beyond bulk properties of substances. Whereas most of the known proton-conductors rely on protons within crystalline structure, hydroxyls and water molecules on oxide surface may serve as a source of protons which act as ionic charge carrier. On this point,

ZrO2-based oxides, which have been recognized as hightemperature oxide-ion conductors, are of prime interest with respect to its surface chemistry 7-16. For alleviating substantial instability of bare surface, cation and anion on the surface, which act as Lewis acid and base sites, adsorb water molecules to form hydroxyl groups. Due to the strong interaction between the bare surface and water 8,9, the Brønsted acidity of the surface hydroxyls is quite low, resulting in the strongly basic nature of the surface 10-12. In addition, some layers of water molecules are adsorbed on the hydroxyl groups with weaker interaction, which are essentially instable and readily desorbed upon heating and/or evacuation 13-16. Thus the surface of ZrO2-based oxides shows characteristic water adsorption properties. Here we propose a design of proton-conducting materials based on “surface protonics”. The design is based on the features of nanograined oxides. That is, in contrast to stable surface of well-grown grains, the surface of the nano-grains involves disordered structure and resultant dangling bonds, which is considered to lead to increased instability of the bare surface. This surface instability as well as elevated surface energy of nano grains are expected to make dissociative water adsorption energetically favorable, which results in

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increased concentration of surface hydroxyls and overlaying water molecules. It will be further suggested that the interplay of those protonic species takes the important part in the proton transport properties. In order to realize the material design based on “surface protonics”, we employ an ultra high-pressure compaction (4 GPa) technique at room temperature to fabricate the nano-structured YSZ from the nano-grain powder synthesized at a low temperature. Since the technique requires no heating for fabricating densely-compacted materials, it is possible to retain the hydrated surface and nano-sized grains of the as-synthesized powder by avoiding thermal dehydration and grain growth 17,18. In our previous study 19, it has been found that the nano-grained YSZ specimens thus fabricated can retain considerable amount of proton-containing species within the materials. Appreciable protonic conductivity has been observed under humidified atmospheres at intermediate temperatures (300~500°C), which is ascribed to transport along inter-grain hydrated layers. In addition, it has been inferred that the interfacial hydrated layer consists of several kinds of surface hydroxyls and water molecules with significantly different character and stability. In the present study, with the temperature of interest extended to the lower temperature, the physicochemical properties of those protonic species and resulting protonic transport are investigated to give insight into design of proton-conducting materials based on the “surface protonics”.

2. EXPERIMENTAL SECTION The nano-sized powder of xYSZ (Zr1-x/100Yx/100O2-x/200, x=0, 1, 4) was synthesized via the sol-gel route, which follows the process previously developed 17,18. The appropriate amounts of Zr(O-n-C4H9)4 and Y(O-n-C3H7)3 (Kojundo Chemical Laboratory Co.,Ltd.) were dissolved into 2methoxyethanol, and refluxed for over 1hour. The solution was heated up to ca. 120°C in an Ar atmosphere to be dried, and then hydrolyzed by supplying water vapor with Ar gas as carrier. After careful hydrolysis with several intermittent grindings, the product was finally treated at 350°C in O2 flow to remove any remaining organic components, and then lightly ground. The synthesized powders were analyzed with X-ray diffractometry (Ultima III / Rigaku Corporation) and Raman spectroscopy (NRS-5200 / JASCO Corporation) (Fig. S1, S2). The obtained powder was put into a mold made of pyrophyllite, which serves as a pressure medium in the subsequent high-pressure compaction. The pyrophyllite mold was pressurized in a 700 ton multi-anvil hydraulic pressure device, where the effective pressure applied to the sample is estimated as ca. 4 GPa, resulting in a hard compact of YSZ. The compacted specimens have enough strength to be handled similarly to dense materials. For example, the specimens can be cut by using a low-speed wheel saw without crumbling or cracking. In addition, hardness indentation tests of the as-compacted 1YSZ specimen were performed by applying a 1.96 N force with a micro hardness tester (HMV-2 / Shimadzu Corpora-

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tion), showing well-formed imprints (Fig. S4) and the Vickers hardness of ca. 3.5 GPa. While the value of hardness is of course lower than normal ZrO2 ceramics (around 12 GPa 20), the specimen is still considered as of acceptable mechanical property. The microstructure of the specimen was observed with FE-SEM (JSM-7000F / JEOL Ltd.). The in-situ FT-IR analyses were carried out using a FTIR spectrometer (FTIR 670 Plus / JASCO Corporation) equipped with an atmosphere-controlled hightemperature diffuse reflection accessory. The specimen temperature were controlled at room temperature to 600°C, and an oxygen gas with or without H2O (or D2O) vapor, of which the partial pressure was regulated with a temperature-controlled water-saturator, was supplied to the specimen chamber. The diffuse reflectance spectra were collected under equilibrium or in the course of H2OD2O exchange reaction. 1

H MAS-NMR (Avance 600 / Bruker Biospin) measurements were performed with a MAS frequency of 30 kHz. The powdered samples were heat-treated under flow of Ar gas including 2 vol% H2O or D2O vapor, with the temperature stepwise decreased from 600°C to room temperature, so as to achieve full protonation or deuteration. A part of the samples heat-treated in the D2O atmosphere were heat-treated again in ambient air (including H2O vapor) at 160°C for partial exchange of deuterons for protons. Each sample thus prepared was put into a ZrO2 rotor, and served for measurements. The chemical shift is referred to TMS. The gaseous desorption behavior was observed with a TDS apparatus (TDS W1000S, ESCO Ltd.). Prior to the measurements, the specimen was equilibrated at designated temperatures under the D2O-O2 atmosphere, and then stepwise cooled down or quenched to room temperature. A small portion of the specimen (~10 mg) was set to the TDS apparatus, and transferred to the analyzer chamber in vacuum. After the pressure of the chamber was stabilized, the specimen was heated up to 1000°C at a constant heating rate of 1 deg/min, and the desorbed gas was analyzed with a mass spectrometer. For the electrical conductivity measurements, the specimen was cut into a disk, and the electrodes of Au were fabricated onto the both surfaces. The specimen was set to a specimen holder equipped with Au leads and a Ktype thermocouple, which was then inserted to an atmosphere-controlled tube furnace. The temperature and atmosphere of the specimen were regulated as in the in-situ FT-IR analyses. The electrical conductivity under equilibrium was evaluated by electrical impedance measurements using a frequency response analyzer (SI 1260 / Solartron) in combination with a dielectric interface (SI 1296 / Solartron).

3. RESULTS AND DISCUSSION The results of XRD and Raman analyses (Fig. S1) consistently indicate that the as-synthesized powders consist mainly of the tetragonal phase with a small amount of the


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monoclinic phase coexisting. The crystalline size has been evaluated as ca. 5 nm from the Scherrer’s equation on the FWHM of the XRD peaks. As previously reported 19 , TG-DTA analysis on the as-synthesized powders show essentially no exothermic response upon heating in oxygen atmosphere, suggesting that any remaining organics in the powders are negligible. The compacted specimens were successfully fabricated with the ultra-high-pressure (4 GPa) compaction. XRD and Raman analyses indicate that compared with the assynthesized powders, the portion of the monoclinic phase is significantly increased in the as-compacted specimen. Evolution of crystallographic structure upon heating as well as compaction is further discussed in the supporting information.

Figure 1. FE-SEM micrographs of the fractured surface of the as-compacted 1YSZ specimen, (a) low magnification (x500), (b) high magnification (x300,000).

Fig. 1 shows the FE-SEM micrographs of fractured surface of the as-compacted 1YSZ (Zr0.99Y0.01O1.995) specimen, which is characterized by closely packed nano-grains (~10 nm). While no significant macroscopic porosity is acknowledged, the density of the specimens is ca. 70% of the crystallographic one. The low relative density can be ascribed to the nano-pores and loose structure of grain boundary, which is considered to result from the following scheme. The as-synthesized YSZ powders are loaded with an appreciable amount of surface-adsorbed water molecules and hydroxyls due to the basic nature of zirconia as well as the increased surface instability characteristic of nano-grains. Upon ultra-high-pressure compaction at room temperature, those protonic species on the surface are compacted together with the nano-grains to form interfacial hydrated layer as well as nano-pores as illustrated in the lower part of Fig. 2.

Figure 2. Schematic of the surface protonic species and ultra-high-pressure compaction of nano-grains. The upper part shows that the nano-grain surface is terminated with hydroxyls (-OH), to which water molecules are hydrogenbonded. Free water molecules are further adsorbed. Upon ultra-high pressure compaction at room temperature, those protonic species are compacted together with the nanograins to form interfacial hydrated layer as well as nanopores, which is illustrated in the lower part.

The N2 adsorption isotherm and its t-plot 21 of the ascompacted 1YSZ specimen is of typical micro-porous structure (Fig. S3). The specific surface area of micropores (excluding outer surface) and average pore diameter are evaluated to be 71 m2/g and 1.1 nm, respectively. It is expected that this small pore size of 1.1 nm makes gaseous adsorption energetically favorable, and induces capillary condensation of water at lower temperatures. On the other hand, it can be acknowledged in Fig. 1(b) that there sparsely exist relatively large pores of several nanometers in diameter as marked with dotted circles, which are clearly distinguished from the micro-pores (~1 nm diameter) suggested by the N2 desorption analysis. It is considered that as mentioned above, this relatively large pores are considered to originate from condensation of surface water during the high-pressure compaction, and mainly account for the low relative density (~70%). From the implication of the previous study 19 as well as the basic picture mentioned above, the microscopic situation of the surface can be described as follows and illustrated in the upper part of Fig. 2. For chemical relaxation of the surface instability, the nano-grain surface is terminated with hydroxyl groups (-OH). To the surfaceterminating hydroxyl, water molecules are bonded via hydrogen bond (H-bond). And a few layers of “free” water molecules are further adsorbed on the hydrogenbonded water molecules. The surface protonic species are characterized by different thermal stability, which is consistent to the fact that the adsorption enthalpy of water is dispersed in dependence on the amount of adsorbed water 22. A hierarchical structure of those surface protonic species constitutes the surface and interfacial hydrated layer. The properties of those protonic species have been well elucidated by in-situ FT-IR analysis under controlled



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temperature and atmosphere. Shown in Fig. 3(a) is the evolution of IR relative reflectance spectra of the 1YSZ specimen upon heating in H2O-O2 atmosphere. An intensive and broad absorption (A) is observed between 3800 and 1800 cm-1, which can be attributed to O-H stretching vibration 13. Since the broad absorption rapidly reduces with temperature elevating from room temperature, the “free” H2O molecules, which are the most unstable, should take part in the broad absorption. However, the broad absorption is still somewhat remaining in H2O-O2 atmosphere even at 600°C, which is too high a temperature to retain the free H2O molecules. Here we assume that the hydrogen-bonded H2O molecules as well as the free H2O also contribute to the broad IR absorption, and the rather high stability of the former originates from the strong interaction via hydrogen bond with the surfaceterminating OH. After switching the atmosphere from humidified O2 to dry one at 600°C, the broad absorption finally disappears, while two sharp absorption peaks at 3770 and 3670 cm-1 ((B) and (C)) still remain, which can be also attributed to O-H stretching. The species causing the two sharp absorption bands should be thermally stable ones, surface-terminating OH groups.

Figure 3. In-situ FT-IR spectra of 1YSZ. (a) Dehydration behavior upon heating in H2O-O2 atmosphere. Spectrum in the most-dehydrated state (600°C, dry O2 atmosphere) is also presented. (b)-(d) Time-dependent spectra after switching the atmosphere from H2O-O2 to D2O-O2 at 400, 200 and 50°C. The insets are the close-ups of the O-D absorption band in steady state under the D2O condition.

Exchange reaction of H2O-D2O has also been studied with the in-situ FT-IR technique. Figure 3(b-d) shows the isothermal transition of IR spectra after switching the atmosphere from H2O-O2 to D2O-O2 at 400°C, 200°C and 50°C. At 400°C (Fig. 3(b)), the absorption bands of O-H stretching vibration immediately start attenuating after switching the atmosphere, and almost disappear after 120 minutes. At the same time, the absorption bands of O-D stretching vibration including the broad and two sharp features (B’ and C’) start growing, which correspond to the above-mentioned O-H absorption bands in the isotopic relation of O-H/O-D stretching vibration. At 400°C,

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the proton-containing species can thus undergo almost full H/D isotope exchange. At a lower temperature (200°C), on the other hand, the H/D exchange behavior is somewhat different (Fig. 3(c)). While the broad O-H absorption from the hydrogen-bonded H2O molecules gradually reduces after switching the atmosphere, the two sharp absorption peaks from the surface-terminating OH groups exhibit only a small reduction in intensity. It is noted that a significant portion of the hydrogen-bonded H2O also remains unexchanged. The unfavorable nature of H/D exchange in the hydroxyl groups is rather reasonable, because the dissociation of protons from hydroxyl groups seems generally difficult at low temperatures. However, Fig. 3(c) shows that in the O-D stretching absorption region, one of two peaks from the surface-terminating hydroxyl group appears at 2700 cm-1 (peak C’), which corresponds to the OH group appearing at 3670 cm-1 (peak C) in the isotopic relation. The surface-terminating hydroxyl groups corresponding to peak C and C’ are thus referred to as dissociative hydroxyl groups. These hydroxyl groups are involved with a mobile sort of protons, which allow the H/D exchange even at this low temperature. We propose that the mobile nature of this sort of protons is assisted by a strong interaction with hydrogen-bonded water molecules. In the H/D exchange measurements at both 400°C and 200°C (Fig. 3(b, c)), the growth rates of the absorption bands from the hydrogen-bonded D2O (broad peak) and surface-terminating OD groups (sharp peaks) appear almost comparable with each other, although one may expect that the hydrogen-bonded water molecules are less strongly attracted to the underlayer and thereby more mobile than the surface-terminating hydroxyls. This observation signifies the strong interaction between the hydrogen-bonded water molecules and surface-terminating hydroxyls. The relatively slow exchange kinetics of the hydrogen-bonded water molecules is contrasted by the fast exchange kinetics of the free water molecules observed at 50°C (Fig. 3(d)). At this low temperature, only the free water molecules are supposed to undergo isotopic exchange, and the exchange reaction is completed within 10min to signify that the “free” water molecules are most weakly interacted to the surface and rather mobile even at room temperature. The similar distinction of proton-containing species and their thermal stability has also been inferred from the results of 1H MAS-NMR analyses on 4YSZ. Although the yttrium concentration of the sample is slightly different from the samples served for the other analyses (1YSZ), the compacted specimens of the both compositions consist of the monoclinic and tetragonal phases with the former being predominant (Fig. S2). Considering the equivalence in the phase constitution between 1YSZ and 4YSZ, the both compositions can be treated as practically comparable to each other in discussion of qualitative properties and relative stability of the surface OH and H2O. As shown in Fig. 4(a), the spectrum (i), which was taken from the specimen equilibrated in a H2O-O2 atmosphere,


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include a sharp peak (α) at 5.2 ppm in chemical shift and a set of sharp peaks (β) at 0.9-2.1 ppm. On the other hand, after the specimen is fully saturated with D2O and OD (spectrum (iii)), the sharp peaks (β) are totally absent. Since deuterium is not detectable in 1H NMR, the results signify that the protonic species corresponding to sharp peaks (β) have been fully exchanged for D2O and/or OD, and then those are well retained. In contrast, the peak (α) is still prominent in the spectrum of the D2Otreated specimen, as in the H2O-treated specimen. It is asserted that the “free” H2O molecules contribute to the peak (α), and the “free” D2O molecules in the D2O-treated specimen had been readily exchanged for H2O molecules during handling in ambient atmosphere prior to the 1H NMR measurement. This room-temperature isotope exchange is consistent with the fast exchange kinetics of the “free” water molecules observed at 50°C with the in-situ FT-IR (Fig. 3(d)). (a)

(b)

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the peaks (β3-β5). Consequently, the peaks (β1-β2) come from the non-dissociative surface-terminating OH. In the fully protonated sample (i), the intensity of β3 relative to β4 and β5 is significantly larger than that in the sample (ii), which does not contain non-dissociative surfaceterminating OH. Therefore, the peak β3 is contributed to also by the non-dissociative surface-terminating OH. It is interesting to note that the peaks of the dissociative surface-terminating OH (β3-β5) appear at smaller chemical shifts than the non-dissociative OH, while acidic protons generally tends to appear at larger chemical shifts. This point will be discussed later together with the proton conduction behavior. Each of the dissociative and non-dissociative surfaceterminating hydroxyls gives peaks further split into three. It is supposed that the splitting of the response from the surface-terminating hydroxyls, which signifies the difference in acidity of the concerned protons, originates from variation in the surface structure, i.e., geometric arrangement of the cations and anions around the hydroxyls, because the local instability and resulting basicity of a surface site should be affected by such surface structure. While the specific structure corresponding to each hydroxyls peak is not clear at present, the distinction between dissociative and non-dissociative hydroxyls can be also associated with the surface structure. While the hydrogen-bonded water molecules have not been identified in the NMR spectra, it is plausible that its response overlaps the peak (α), to which the “free” H2O molecules contribute, considering that the comparable extent of the shielding effect can be expected for the “free” and hydrogen-bonded H2O molecules. This assignment can be supported by the fact that the peak (α) of the D2O-treated sample (iii), which contains essentially no hydrogen-bonded H2O molecules, is clearly sharp compared with the other samples fully and partially containing hydrogen-bonded H2O ((i) and (ii), respectively).

1

Figure 4. H MAS-NMR spectra of 4YSZ with MAS frequency of 30 kHz, (i) saturated with H2O, (ii) heat-treated at 160°C in H2O atmosphere after saturation with D2O, (iii) saturated with D2O. (a) The spectra in the overall chemical shift range of interest, (b) close up of the small chemical shift range with the results of peak deconvolution.

Accordingly, the sharp peaks (β) are considered to originate from more stable protonic species. After observing none of the peaks (β) present in the D2O-treated specimen, the identical specimen was further heat-treated at 160°C in a H2O-atmosphere for 180 min, which results in some of the peaks (β) grown (spectrum (ii)). As shown in Fig. 4(b), deconvoluting the peak group (β) in the spectrum (i) reveals the five peaks (β1-β5) present, and the peaks grown after re-protonation at 160°C correspond to the three low frequency responses, (β3-β5). Since the results of the in-situ FT-IR suggest that the temperature (160°C) allows only the dissociative surface-terminating hydroxyls to undergo isotope exchange, it is deduced that the dissociative surface-terminating OH contributes to


Chemistry of Materials Thermal desorption

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Figure 5. Thermal dehydration behavior of 1YSZ. (a) The 1YSZ sample fully deuterated by annealing in D2O-O2 atmosphere at 600°C and then stepwise-cooling. (b) D2O desorption of the 1YSZ samples which have been equilibrated in H2OO2 atmosphere at 600°C, and then annealed in D2O-O2 atmosphere at the designated temperature, followed by quenching to room temperature. (c) Schematic of the dehydration behavior.

On the other hand, the desorption at higher temperatures is predominated by heavy water (D2O). The low and high temperature part of the D2O desorption curve, which are characterized by broad peaks centered at 250°C and 550°C, are considered to originate from hydrogen-bonded water molecules and surface-terminating hydroxyls, respectively, indicating that those protonic species have been fully dueterated and well quenched to room temperature. The energetics associated with those protonic species can be further inferred from the following observations. After equilibration in H2O-O2 atmosphere at 600°C, the 1YSZ specimens have been equilibrated under a D2O-O2 atmosphere at various temperatures to have hydrogenbonded D2O and surface-terminating OD, and then quenched to room temperature, followed by the TDS measurements for analyzing D2O desorption behavior upon heating. As found in Fig. 5(b), with the temperature of the D2O-exchange increasing, the rising edge of the D2O desorption shifts to high-temperature side, which is reasonable even if a single hydration Gibbs energy is assumed. On the other hand, the tail at the hightemperature side also shifts to high temperature with the D2O-exchange temperature, resulting in a relatively narrow desorption peak centered at the almost identical temperature to the D2O equilibration treatment. This observation suggests that hydration free energy of each protonic species (terminating hydroxyls and hydrogenbonded water molecules) is further widely dispersed, which originates from variety of surface structure and instability of the nano-structured specimen. This point is indicated also by the results of in-situ FT-IR during isotope exchange reaction at 200°C (Fig. 3(c)), that is, only a part of hydrogen-bonded water molecules undergoes isotope exchange at the temperature. Thus the partial dehy-

dration curves from those species significantly overlap each other as schematically shown in Fig. 5(c).

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The dehydration behavior was analyzed with thermal desorption spectroscopy (TDS). Figure 5(a) shows the thermal desorption behavior of the 1YSZ specimen after thorough deuteration treatment. At low temperature (below 150°C), the gas desorption is dominated by light water (H2O) in spite of the deuteration treatment, which indicates that the “free” D2O molecules in the D2Otreated specimen were easily exchanged for H2O molecules during handling in ambient atmosphere as in the 1H MAS-NMR experiments (Fig.4).

log (σT / Scm K)

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1YSZ H O (0.023 atm) H2O (0.0232 atm) D O (0.023 atm) D2O (0.0232 atm)

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Figure 6. The conductivity Arrhenius plot of 1YSZ. (a) The conductivity in H2O-O2 and D2O-O2 atmosphere, and the ratio (σ(H2O)/ σ(D2O)) is plotted. (b) Comparison with the other nano-structured fluorite-type oxides: 4YSZ and 16YSZ 19 23 , ZrO2 (this work), SDC (Sm-doped CeO2) and CeO2 , GDC 25 23 (Gd-doped CeO2) , YSZ . Also included are the conductiv3 2 ities of IISPAP and CsHSO4 as representative of polymers and solid acids, respectively.

Figure 6(a) shows the electric conductivity of the ascompacted 1YSZ specimen in humidified atmospheres, which was measured starting from 275°C. With decreasing temperature, the apparent activation energy gradually decreases, and the conductivity finally shows a thermally de-activated nature. Above 60°C, the conductivity in a


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D2O-O2 atmosphere (σ(D2O)) is clearly lower than that in a H2O-O2 atmosphere (σ(H2O)) with the equivalent water partial pressure. As shown in the upper panel of Fig. 6(a), the extent of the isotope effect (σ(H2O)/σ(D2O)) is around 2 to approximate the theoretical value of 1.4, which signifies hopping-based conduction of protons. Below 60°C, on the other hand, the isotope effect almost disappears to suggest the emergence of a vehicle transport, probably of H3O+. The transition of the conduction mechanism around 50°C should be related to the capillary condensation of water at lower temperature, and the temperature is roughly consistent to the Kelvin equation of saturation vapor pressure as described in the Supporting Information. While protonic conduction in nano-structured fluoritetype oxides has been reported 23-27, which is also considered to take place along interal surface 28, the protonic conductivity found in the present study is 1~3 orders of magnitude higher than those in the literature (Fig. 6(b)). The specific features in the present material are the microstructure with nano-sized grains (~10 nm) as well as the interfacial hydrated layer, which is attained by the low-temperature fabrication process. It is also noted that the protonic conductivity of the present material is superior to perovskite-based oxides suffering from severe grain-boundary resistance especially at low temperatures 29,30 . The results show that Y2O3-doping is not essential for the surface protonic conductivity as found in Fig. 6(b). That is, undoped ZrO2 also shows the similar protonic conductivity at low temperatures. In addition, Y2O3 concentration does not seem to have a significant impact on the protonic conductivity. Concentration of oxygen vacancy, crystal structure and particle morphology can be affected by Y2O3-doping, which are all related to the surface protonics behavior. Considering that instability of bare surface should greatly depends on the arrangements of cations and anions at surface, we expect that the crystal structure in particular has a significant impact on the stability and amount of surface protonic species and resulting protonic conductivity, which is now under investigation. It is rather surprising that protons on the highly basic surface of oxide materials dissociate at the low temperatures to contribute to electric conduction via a hoppingbased mechanism. Here we assert that the dissociative nature of the protons is attributed to the strong interaction between the surface-terminating hydroxyls and hydrogen-bonded water molecules, which has been suggested by their correlated isotopic exchange kinetics observed in the in-situ FT-IR analyses. At this point, it is interesting to note that in the 1H MAS-NMR spectra (Fig. 4), the peaks of the dissociative surface-terminating hydroxyls appear at smaller chemical shifts (showing nominally lower acidity) than the non-dissociative hydroxyls. This observation implies that the hopping-like protonic transport above 60°C is not of a simple hopping of single proton, and that a mechanism involving species other

than the surface-terminating hydroxyls is working, which will be studied in future works. It is also emphasized that the ability of retaining the protonic species and their transport property are particular to its microstructure that consists of nano-grains connected with the interfacial hydrated layer. That is, the bare surface of nano-sized grains is highly unstable due to its disordered structure and dangling bonds, which promotes dissociative adsorption of water molecules. This hydrative nature as well as the small grain size is preserved upon fabrication of the nano-grained specimen via the ultra high-pressure compaction at room temperature. The interfacial hydrated layer thus formed serves as the pathway of protonic conduction via hopping, which is facilitated by the interplay between surface-terminating hydroxyls and hydrogen-bonded water molecules. The present study proposes a design of proton-conducting materials based on the “surface protonics” of nanograined oxides.

4. CONCLUSIONS The nano-structured YSZ specimen fabricated by ultrahigh-pressure compaction of nano-grained powder shows high protonic conductivity based on a hopping-based mechanism and vehicle mechanism at intermediate and low temperature region, respectively. The surfaceterminating hydroxyl groups, hydrogen-bonded water molecules and free water molecules, which constitute the hierarchical interfacial hydrated layer within the nanostructured oxide, are characterized by thermal stability and isotopic exchange kinetics. Dissociation of protons, which contributes to the protonic conduction at the intermediate temperature region, is facilitated by interaction with the overlaying hydrogen-bonded water molecules.

ASSOCIATED CONTENT Supporting Information XRD and Raman analysis, evaluation of specific surface area and average pore size. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The present work has been supported by the Grant-in-Aid for Scientific Research (A) (23246112) from the MEXT Japan. The ultra-high pressure experiments have been done with facilities of the Institute for Solid State Physics, the University of Tokyo. The FE-SEM observation have been conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the MEXT. The authors are grateful to Prof. J. Inoue and Dr. M. Ojima of the Department of Materi-



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als Engineering, the University of Tokyo, for their help in the hardness indentation test.

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Low-Temperature Protonic Conduction Based on Surface Protonics

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