a mechanism for water splitting and hydrogen absorbing functions of metal-oxide layered hydrogen...

Review

Received: 6 September 2013 Revised: 15 November 2013 Accepted: 17 November 2013 Published online in Wiley Online Library: 16 January 2014

(wileyonlinelibrary.com) DOI 10.1002/sia.5359

A mechanism for water splitting and hydrogenabsorbing functions of metal–oxide layeredhydrogen storage materials studied by meansof ion beam analysisKenji Moritaa* and Bun Tsuchiyab

This review describes a study of catalytic functions of water splitting at the surface and hydrogen gas emitting from thebulk of metal–oxide layered materials as well as hydrogen storage materials as its application by means of the ion beamanalysis techniques. First are described a microscopic model for water splitting at the oxide surface and mass balanceequations for hydrogen atoms in the bulk. The latter is a mathematical expression of a one-way diffusion model proposedfor an anomalous isotope effect in D–H and H–D replacements of both deuterium (D) implanted into perovskite oxideceramics by protium (H) in H2O vapour and the vise versa. The latter model brings about finding of catalytic functionsof water splitting at the surface and hydrogen gas emitting from the bulk. Second, experimental results on the anomalousisotope effect are presented and the D–H replacement rates are described in detail. Subsequently are shown results on H2

gas emission measured with a Bach method, which give a clear evidence for the water splitting and hydrogen gas emittingcatalytic functions of the oxide surface. Finally, we present experimental data on the hydrogen absorption and emissioncharacteristics of the metal–oxide layered hydrogen storage materials as an application of the water splitting and hydrogenabsorbing catalysts. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: D–H replacement; dipole-induced water splitting; hydrogen production; hydrogen storage; metal–oxide–metal sandwichmaterials; elastic recoil detection analysis technique; Rutherford backscattering spectroscopy; isotope effects; complex metal oxides;oxygen vacancy

* Correspondence to: Kenji Morita, Department of Research, Nagoya IndustrialResearch Institute, Noa Yotsuya Bld. 2F, 1–13 Yotsuya-tori, Chikusa-ku, Nagoya464–8019, Japan.E-mail: [email protected]

a Department of Research, Nagoya Industrial Research Institute, Noa YotsuyaBld. 2F, 1-13 Yotsuya-tori, Chikusa-ku, Nagoya, 464-8019, Japan

b Department of General Education, Faculty of Science and Technology, MeijoUniversity, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya, 468-8052, Japan

11

Introduction

Hydrogen isotopes have been receiving an intensive attention asfuel in nuclear fusion reactors and hydrogen fuel cells.[1–3] Under-standing of their dynamic behaviour in materials is of primaryimportance for their practical applications. Since tritium (T) is aradioisotope, its data have been extrapolated from the isotopeeffect between data on protium (H) and deuterium (D), becauseonly T diluted in H and D at tracer levels has been usually han-dled and the T data obtained include inherent isotope effects.For fusion applications, the present author group has beenstudying the isotope effects in reaction rates of H and Dimplanted into fusion reactor materials by means of the ion beamanalysis technique.[4,5] Especially, an elastic recoil detection (ERD)technique is one of the most powerful tools to obtain quantita-tive data (depth profile of concentration) on the dynamic behav-iour of H and D in materials at a comparable sensitivity.[6]

Recently, we have found that D implanted into perovskite protonconducting oxide ceramics (SrCe0.95Yb0.05O3� δ) is almost com-pletely replaced by H, for a short time, when the specimen isexposed to atmospheric air, whereas H implanted into the samespecimen is hardly replaced by D, when exposed to D2O vapourin air.[7] The similar D–H replacements have been also observedfor a single crystal of SrTiO3 and tritium breeding ceramicsmaterials of Li2TiO3 and Li2ZrO3.

[8] In the latter ceramics, T–Hreplacement is predicted to take place. The T–H replacement isvery effective for both recovery and removal of tritium from the

Surf. Interface Anal. 2014, 46, 113–127

breeding material used (burnt) in the fusion reactor. The strongisotope effect in the rates of D–H and H–D replacements ob-served is reasonably explained, macroscopically, only in termsof a one-way diffusion model.[9] In the D–H replacement case,H+ is absorbed with adsorption of OH� at the surface due to split-ting of H2O vapour from air, diffuses into the bulk andrecombines with D implanted (trapped) into a HD moleculewhich is rapidly emitted out from the specimen and finallyanother H+ absorbed is subsequently trapped into the vacanttrap, as shown schematically in Fig. 1, where an atomistic modelis also included. According to the model, the strong isotopeeffect is attributed to isotope effects both in splitting rates ofH2O and D2O at the surface and diffusion constants of H+ andD+ in the oxide specimen. The latter isotope effect has beenexperimentally confirmed. In the later stage of the D–H replace-ment, an emission of H2molecule takes place due to recombination

Copyright © 2014 John Wiley & Sons, Ltd.

3

Figure 1. A schematic diagram showing water splitting due to attractive coulomb interaction of water dipole(H2O*) with charges of O vacancy (VO2+)

and trivalent impurity (YbCe�) leading to adsorption of OH� and absorption of H+ desorption of H2O and O atom(1/2O2) at the surface. Is also shown

emission of hydrogen gases (H2 and HD) due to molecular recombination of H+ with H+ replaced and D+ trapped. Diffusion of H+ is enhanced dueto potential gradient induced by electric dipole of H+ (or D+) and its donor electron.

K. Morita and B. Tsuchiya

114

of H+ absorbed with H replaced (trapped) in the traps.[10] Theemission of H2 gas means production of H2 gas from water vapour.The emission of mixed molecule of TH has been experimentallyconfirmed by use of implantation of T into SrCe0.95Yb0.05O3� δ witha pure T ion beam and activity detection of TH gas.[11]

The hydrogen isotope replacement model of the one-waydiffusion[9] has been mathematically expressed in the form ofmass balance equations of three kinds of particles: D implantedinitially, H+ absorbed at the surface and freely migrating in thebulk and H trapped (exchanged for D). They are described inthree terms of diffusion of H+, molecular recombination of H+

with both D implanted and H trapped and subsequent trappingof H+ in their vacant traps under two boundary conditionsthat H+ is absorbed at the surface and HD and H2 moleculesare rapidly emitted from the bulk. The solution of the massbalance equations has been shown to reproduce reasonablyboth the decay curve of D implanted and the uptake curveof H exchanged for D. Furthermore, the model has predictedthat after complete decay of D due to the D–H replacement,H2 molecule is also continuously emitted due to molecularrecombination of H+ with H replaced. The prediction hasbeen experimentally confirmed from a measurement ofenrichment in the hydrogen gas contents in normal airsealed with a specimen in a vacuum vessel of stainless steel(25 cm3 in volume) at room temperature.[12] The experimen-tal confirmation gives also a clear evidence that complexmetal (two metals) oxide ceramics implanted energeticallywith protons provide a catalytic function for productionand emission of H2 due to splitting of H2O vapour in air atroom temperature.

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Various technologies have been developed for production,storage and usage of hydrogen gas without CO2 emission athigher energy-efficient rate, at lower cost and in less loadedway to protect the global environment. Many efforts for hydro-gen production from water have been invested in developingphoto-catalysts capable of using the less energetic but moreabundant visible light compared with ultraviolet light.[13,14] Thephoto-catalytic splitting of water using oxide ceramics semicon-ductors is based on the concept that the direct absorption ofphoton creates separated electrons and holes in the energygap of the material. These charges can move to the surface ofthe semiconductor and interact with water adsorbed at the sur-face and both H2 and 1/2O2 are released, provided the potentialdifference of this reaction exceeds 1.23 eV.[15] Here, we describethe microscopic mechanism for the water splitting and hydrogengas emitting functions and the investigation of more activematerials in the hydrogen gas emission from measurements ofthe D–H replacement rates by means of the ERD technique. Atfirst, an atomistic model for interaction of water molecule withthe catalytic surface of SrCe0.95Yb0.05O3� δ is proposed, as shownin Fig. 1, that the continuous production of H2 gas from H2O atroom temperature without light irradiation is energeticallysustained by attractive coulomb interaction of the electric dipolewith charges of lattice defects at the specimen surface (so calleddipole-induced water splitting);[12] positively double-chargedoxygen vacancies both produced by energetic deuterium ionimplantation and formed due to the charge-compensation todoping of Yb and negatively charged Yb3+ ions substituted atthe Ce4+ site of SrCeO3 lattice. Oxygen vacancy and trivalentimpurity doped introduce two kinds of donor levels just below

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Mechanism for water splitting and hydrogen absorbing functions

the edge of the conduction band and also two acceptor levelsjust above the valence band. Even at room temperature, thetrivalent ion is negatively charged in defect of hole and theoxygen vacancy is positively double-charged due to occupationof two holes via charge transfer. Based on these facts, it has beenproposed that the water splitting at room temperature isenergetically sustained by the attractive coulomb interaction ofthe water dipole with charges of the surface defects, which isdescribed below with experimental results on the D–H replace-ment and the H2 gas emission.

Various technologies for storage of H2 gas have also beendeveloped in different ways. For hydrogen storage materialssupplying on board H2 gas to a hydrogen fuel cell providedwith vehicles, absorption of H2 gas at room temperature andre-emission of it at temperatures lower than 150 °C, as wellas storage capacity higher than 5.5 wt.%, are required withrecycled usage for long term. So far, many metallic alloysand compounds have been designed by combination ofdifferent metals, in which hydrogen atoms are both stableand unstable. However, metallic alloys and compounds havebeen never found to be fitted to the fuel cell. To store directlydilute H2 gas produced from air vapour at room temperature,we have tested metal–oxide bi-layer composites as a hydrogenstorage material, on the basis of the concept of the Pd–WO3

bi-layer composites developed as a hydrogen sensor.[18] Here,as one of applications of the splitting catalysts of air vapourat ambient temperature, described are the experimental re-sults on Pt/Li2ZrO3 (or, Li4SiO4)/Pt composite materials[16,17]

which have been created by combination of a water splittingand hydrogen absorbing metal with oxide ceramics.

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Water splitting model and mass balanceequations on the one-way diffusion model

Water splitting model

Here, a microscopic model on coulomb interactions of the elec-tric dipole of water with charges of lattice defects (negativelycharged Yb3+ ions at Ce4+ site and positively double-chargedoxygen vacancies) of SrCe0.95Yb0.05O3� δ, proposed for the D–Hreplacement,[12] is described in detail, on which the watersplitting even at room temperature is energetically sustained.The coulomb interaction of theYb3+ ions with the positive chargeof water dipole induces the uptake of H+ and the adsorption ofOH� at the surface, as shown schematically in Fig. 1. The uptakenH+ diffuses in the bulk along the negative potential gradientinduced by the local alignment of dipoles (composed of im-planted protons and their donor electrons) parallel to the surfacenormal that is initiated by the growth of OH� layers of adsorbedat the surface. The uptaken H+ recombines with H+ implanted intraps into H2 which is emitted and subsequently the anotheruptaken H+ is trapped into the vacant traps. On the other hand,the coulomb interaction of the oxygen vacancy at the surfacewith negative charge of water dipole creates simultaneously freetwo holes, which mediate production of O and H2O from twoOH� near the vacancy. This interaction removes the two OH� fromthe surface, which means the creation of a new space (site) foradsorption of OH� to absorb H+ due to further water splitting.Since splitting of water vapour stops, if the OH� layers wouldcompletely cover the surface specimen, the interaction is a key

Surf. Interface Anal. 2014, 46, 113–127 Copyright © 2014 John

reaction for continuation of the D–H replacement, namely thesustainable emission of H2 gas.

The reaction kinetics for absorption of H+ due to splitting ofH2O* at the YbCe

� site of the surface is expressed as thefollowing equation:[12]

YbCe� þ H2O*þ E1–Ed → Hþ þ OH� (1)

where YbCe� represents Yb3+ ion substituted for Ce4+ ion of

SrCeO3 lattice, E1 is the attractive coulomb energy betweenYbCe

� and effective positive charge of proton due to polariza-tion of H2O* and Ed is the dissociation energy of H2O*physisorbed at the surface and is evaluated to be 1.23 eV.[15]

The reaction kinetics, on the other hand, for production of1/2O2 through mediation of two holes created due to splittingof H2O* at the VO

2+ site of the surface is also given by thefollowing equation:

Vo2þ þ H2O*þ OH� þ E2–Ed→ Hþ þ OH� þ 2 holesþOH�→ Hþ þ H2Oþ 1=2O2

(2)

where Vo2+ is the vacancy at the O site of SrCeO3 lattice, E2 isthe attractive coulomb energy between Vo2+ and effectivenegative charge of O ion due to polarization of H2O*. If E1� Ed and E2 � Ed are positive, H+ is absorbed and OH� isadsorbed due to splitting of H2O* at the YbCe

� site and also1/2O2 is produced and one OH� is removed from the surface,thus a new site being created for continuous absorption of H+

at the surface, as shown in Fig. 1. The attractive coulombinteractions with H2O* neutralize negative charged YbCe

� ionby uptake of H+ and positively double-charged O vacanciesby occupation of O atom, respectively. The coulomb interac-tions are sustained by recharging of both YbCe ion and Ovacancy due to diffusion of H+ into the bulk and emissionof O atom into the vacuum, respectively, thus the rechargingrates being dependent on the energy levels of the acceptorholes introduced by YbCe ion and the donor electrons intro-duced by O vacancy, i.e. their thermal activation rate. Thefact indicates that surface vacancies of O become oxidizationcatalyst in aid of water vapour.

The value of E1 is evaluated from relation e e γ/rc = 27.2 eV γ a0/rc,where a0 is the Bohr radius, rc is the characteristic distance forattractive coulomb force between YbCe

� ion and H2O* to becreated and γ is the effective fraction of dipole charge ofH2O and is roughly evaluated to be ~0.3 using that its dipolemoment is 1.85 Debye and H–O distance is 0.096 nm. On theother hand, the value of E2 is evaluated from the relation 2e2e γ/rc. When rc is assumed to be a half of the lattice constant(0.43 nm) of SrCeO3 unit cell, coulomb energies of E1 and E2are roughly computed to be ~2 eV and ~8 eV, respectively.This consideration suggests that the continuous emission ofH2 gas by coulomb interaction of defect charges with dipolecharge of water at the surface is energetically sustainable, ifthe atomic structure and the electronic structure of thesurface are kept stable. In order that the ~1017/cm2 D atomsimplanted into the oxide ceramics is almost completelyreplaced by H for a short time, the lattice defects at the sur-face should be almost always charged. Although the attractivecoulomb interactions neutralize the negative charge of YbCe

�

ion by H+ and the positively double charge of Vo2+ by twoelectrons from two OH�, the YbCe ion and Vo can be rapidlyrecharged by thermal reionization of the hole and donor elec-trons without any artificial energy input, respectively, namely,

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116

thermally H+ diffuses and O atom is ejected. The fact is con-sistent with the calculation that their energy levels are closeto the top of the valence band and the bottom of conductionband[19] and the D–H replacement rate is strongly dependenton the energy level of donor electron, as shown below. Thisargument predicts that the anomalous isotope effect in theD–H and H–D replacement rates described below may bepartly ascribed to the smaller diffusion rate of D+.

Mass balance equations on the one-way diffusion model

Here, we describe the one-way diffusion model in the form of themass balance equations which has been proposed for explana-tion of the anomalous isotope effect in rates of D–H and H–Dreplacements, addressed in the introduction.[9] The time evolu-tion of the concentrations of free proton nH(r,t) absorbed at thesurface due to splitting of H2O, deuteron, nDT r; tð Þ implanted intothe traps and proton nHT r; tð Þ replaced in the traps during the D–Hreplacement is described by the following equations in a spheri-cal co-ordinates,

dnH r; tð Þdt

¼ D

r2d

drr2dnH r; tð Þ

dr

� �� ∑T

�C0 � nHT r; tð Þ

�nDT r; tð ÞÞ:nH r; tð Þ � KHHnHT r; tð Þ : nH r; tð Þ

�KHDnDT r; tð Þ : nH r; tð Þ

(3)

dnH r; tð Þdt

¼ ∑T C0 � nHT r; tð Þ � nDT r; tð Þ� �: nH r; tð Þ

�KHHnHT r; tð Þ : nH r; tð Þ

(4)

dnDT r; tð Þdt

¼ �KHDnDT r; tð Þ:nH r; tð Þ (5)

DdnH r; tð Þ

drr ¼ R ¼ αΦ (6)

nH r; 0ð Þ ¼ 0 and nDT r; 0ð Þ ¼ nD0 (7)

where r is the distance from the center of spherical crystalliteswith a radius R, D is the diffusion constants of free proton, ∑ T isthe trapping rate, KHH and KHD are the local recombination ratesof free proton with proton and deuteron in the traps, respec-tively, C0 is the trap density, α is the effective absorption coeffi-cient of proton due to water splitting at the surface exposedto air vapour and Φ is the correction factor of the dose rateof water vapour in Ar gas to that of air vapour. Since the ab-sorption process of H+ at the surface is rather complicate, asdescribed below, the correction factor is introduced insteadof the dose rate. The initial implantation concentration nD0 isassumed to be constant and we do not consider the depthdependence of the concentration, because it has been exper-imentally observed that the D–H replacement takes placeuniformly over the whole depth.Eqn (3) has been solved analytically by linearization,[9] based

on both the experimental results that the sum of the concentra-tions of deuteron retained and proton replaced in the traps isconstant, as shown below, and the assumption that nHT r; tð Þ þnDT r; tð Þ ¼ ξC0 ξ < 1ð Þ:


nH r; tð Þ ¼ αΦRD

1

π1

x∑∞

n¼1�1ð Þ

n sinnπ xn

e� π2π2D=R2ð Þt= ∑∞

n¼1e� π2π2D=R2ð Þt

(8)

where x = r/R. In the derivation of Eqn (8), it was assumed thatKHH= KHD. Since the D–H replacement takes place uniformly overthe implanted depth of ~30 nm, it indicates that the higher orderterms of n≥ 2, which decay much faster than the first term maybe neglected. Substituting Eqn (8) into Eqns (4) and (5) andaveraging these equations over the spherical crystallites, theaverage concentration of deuteron ND

T tð Þ retained and proton

NHT tð Þ replaced in the traps of the specimen is expressed as

following equations:

NDT tð Þ ¼ ND

0 e�K:t (9)

NHT tð Þ ¼ ∑T 1� ξð ÞC0

KHD 1� e�K :t� �

(10)

where ND0 is the average initial implantation concentration of

deuteron in the specimen. The decay constant K is expressedas αΦRKHD/D.

The emission rate of H2 is analytically calculated by integrationof KHH n(x, t) nT(x, t) over the whole depth x of the specimen. Theemission rate of H2 at steady state, after the concentration ofdeuterons implanted has been completely decayed out, isapproximately expressed in the following form:[12]

η ¼ 3KHHΣ TC0ns02 KHH þ Σ T

� � 1� 1

3

ffiffiffiα

pR

� �ΔV (11)

where n0s is the concentration of protons uptaken at the surface

and ΔV is the ion implantation volume expressed as the spotarea of D2

+ ion beam multiplied by their projected range inthe specimen.

It is clearly seen from Eqn (9) that the decay constant of D dueto the D–H replacement is apparently proportional to α, Φ, KHD

and 1/D. Based on the diffusion limited reaction model, KHD isexpressed as 4πrRDβ, where rR is the effective radius for molecularrecombination and β the molecular formation coefficient of H+

with D in the trap.[5] Thus, it is seen that the key parameters forthe D–H replacement are the absorption coefficient α of H+ dueto splitting of H2O* at the surface, the molecular formationcoefficient β of H+ with D in the trap and the correction factorfor the dose rate Φ of water vapour. Eqn (9) was fitted to theexperimental data and the decay constant K was determined.The values of KHH (= KHD), ΣT and n0

s were determined by fittingthe theoretical curves of decay of D and uptake of H to the exper-imental ones measured in the D–H replacement.

Experimental

Experiments on D–H and H–D replacements

The specimens of BaCe1� XYXO3� δ and SrCe1� XYbXO3� δ used(X = 0.05, 0.10, 0.15, 0.20) were discs of 15mm in diameter and2mm in thickness, which were prepared by a solid-state reactionand sintering processes.[20] The SEM observation of the specimensurfaces showed that the grain sizes of crystallites in each speci-men were around 1μm and not so different from each other. Thethin film specimens of BaCeO3 and BaZrO3 used were preparedby annealing the ceramics substrate of Ce0.8Sm0.2O2� δ of 8mmin diameter and of 1mm in thickness and 8YSZ of 14 × 14 mm2



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and 1mm in thickness, spread by Ba(NO3)2 saturated watersolution, at 1573 K for 10 h, respectively, namely so-called POC(proton-oxide-conductor) devices method.[21] The XRD analysisshowed that the BaCeO3 and BaZrO3 crystallites were grown inthe surface layers of each substrate and the PIXE analysis withMeV proton beam indicated the traces of Sm and Y in surfacelayers of each substrate. The concentrations of Sm and Y in bothfilms, however, could not be quantitatively determined.Therefore, they were assumed to keep the composition of thesubstrates, namely BaCe0.8Sm0.2O3� δ and BaZr0.84Y0.16O3� δ wereassumed to be grown. The specimen was placed on a mani-pulator in contact with a ceramic heater in a conventional UHVchamber. The details of the experimental arrangement weredescribed elsewhere.[22] Prior to deuterium implantation, thespecimen was heated at 973 K for several tens min in order toremove residual hydrogen (protium). The specimen wasimplanted with 5 keV, or 10 keV D2

+ ions and 5 keVH2+ ions, of

which the projected range around 30–50 nm, at room tempera-ture up to the saturation concentration. After the deuteriumimplantation, the specimens of SrCe1� XYbXO3� δ were exposedto atmospheric air at room temperature. The specimens of BaCe1XYXO3� δ were exposed to water vapour carried by Ar gas of47–54% humidity at the dew point �60 °C passing througha water bubbler in order to reduce the dose rate of watervapour. The dose rate of water vapour was calibrated by useof the experimental data obtained by exposure to atmo-spheric air. After the exposure, the concentrations of D andH in the specimen were measured by means of the ERD tech-nique. The saturation concentration of D implanted was foundto be about one D atom per unit cell of BaCeO3 and SrCeO3

lattice, using the standard analysis technique described else-where.[23] The exposure of the specimen to water vapourand the ERD measurement of the specimen were repeatedat several times, and the decay curve of D and the uptakecurve of H in the specimen were obtained as a function ofthe exposure time of water vapour.

Experiments on hydrogen gas emission

The specimen used was thin films of BaCeO3: Sm and BaZrO3: Yand bulk SrCe0.95Yb0.05O3� δ which were used in the experimenton the D–H replacement. The bulk SrCe0.95Yb0.05O3� δ specimenwas a disc of 6mm in diameter and of 4mm in thickness andwas irradiated with RF (13.56MHz) H2 plasma at a pressure of100 mTorr and at a bias voltage of �470 V after removal out ofresidual hydrogen at 400 °C for 30min. The concentration of Himplanted was not determined by means of the ERD technique,because of too large size. The thin film specimens of BaCeO3

and BaZrO3 were implanted with 10 keV D2+ ions up to saturation

concentration after removal out of residual hydrogen. Thesespecimens, a piece of bulk SrCe0.95Yb0.05O3� δ, four pieces ofBaCeO3 thin film and three pieces of BaZrO3 thin film, wereexposed to normal air of 50% in humidity at 40 °C that was tightlysealed with oxygen-free copper gaskets in a stainless steel vesselwith a cylindrical shape of 7 cm in inner diameter and of 20 cm inlength (750 cm3 in volume), which is so-called Bach method. Thevessel was laid horizontally, at the bottom of which the specimenwas laid. The contents of hydrogen emitted from each specimenin air sealed in the vessel were measured as a function of theexposure time by sampling air of 0.5 cm3 in volume by astandard injector and using a gas chromatography (ShimazuCo. Ltd., GC-14B-type).


Experiments on hydrogen storage

Specimens used were sandwiches of Pt/Li2ZrO3/Pt and Pt/Li4SiO4/Pt, where 1 nm and 10 nm Pt films were deposited onthe front and back surfaces of sintered Li2ZrO3 and Li4SiO4 discsof 1mm in thickness and 8mm in diameter. For Pt/Li2ZrO3/Pt,the specimen with 100 nm Pt films was also prepared. Residualhydrogen atoms in the as-received specimen were removed byisochronal heating for 10min in the vacuum before exposure toair at room temperature. At each stage of the annealing, thespecimen was monitored by means of RBS and ERD techniques.The concentration of hydrogen atoms absorbed (stored) in thespecimen was measured as a function of exposure time by meansof the ERD technique. Since the amounts of hydrogen absorbedwere found to be considerably large by the ERD measurement,the weight gain measurement (WGM) of 1 nm and 10 nm Pt filmspecimens was performed by means of a commercial chemical-microbalance (Shimazu Co. Ltd., AUW220D-type). The gas emit-ted from the two 100 nm Pt specimens exposed to air at roomtemperature for 3500 h was analysed by means of the thermaldesorption spectroscopy (TDS) technique.

Experimental results and discussion

D–H and H–D replacements in oxide ceramics

Here, for the first place, the experimental result on the anom-alous isotope difference between replacement rates of D–Hand H–D,[24] addressed in the introduction is shown, as an ex-ample of data obtained by means of the ERD technique, inFig. 2, where the two sets of ERD spectra were obtained withthe specimens of SrCe0.95Yb0.05O3� δ implanted with 5 keV D2

+

ions (a) and 5 keV H2+ ions (b) up to saturation concentration

which were soaked in pure H2O solution and D2O solutionfor 10min at room temperature, respectively. From Fig. 2(a),it is seen that the D-peak (closed dots) at channel 600 arisingfrom D in the as-implanted specimen is completely reducedby soaking in H2O and the H-peak (open dots) at channel380 increases. From Fig. 2(b), on the other hand, it is alsofound that the H-peak (closed dots) at channel 380 from H inthe as-implanted one is slightly reduced by soaking in D2O, whilethe D-peak (open dots) is only slightly visible at channel 600. Thisresult indicates clearly that there exists a strong isotope effect:for the oxide ceramics of SrCe0.95Yb0.05O3� δ, D-implant wascompletely exchanged for H by soakage in H2O, but H-implantwas hardly exchanged for D by soakage in D2O. The ratio ofthe D–H replacement rate to the H–D replacement rate obtainedquantitatively from the analysis of the decay and uptake curveswas found to be 100,[24] which is attributed to both the isotopeeffects in the absorption rates due to splitting of H2O and D2Oat the surface and the diffusion rates of H+ and D+ in the bulk.The former isotope effect may be discussed from the point ofview of thermodynamic quantities[25] for the ionization ofH2O→H+ + OH� and D2O→D+ + OD�. When the experimentaldata on the ionization quotient of H2O and D2O are applicable tothe water solution in which the specimens are soaked, theionization ratio of H+/D+ is estimated to be about 10. This consid-eration indicates that the absorption rate due to splitting ofwater molecules partly contributes to the strong isotope effectobserved. The latter isotope effect was also estimated from boththe decay curves of D implanted with 5 keV D2

+ ions by bom-bardment of 0.5 keV H2

+ ions and H implanted with 5 keVH2+ ions


Figure 2. ERD spectra obtained for specimens of SrCe0.95Yb0.05O3� δ implanted with 5 keV D2+ ions and subsequently soaked in H2O solution for 10min

(a) and also implanted with H2+ ions and soaked D2O solution for 10min (b) measured by the ERD technique.


118

by bombardment of 0.5 keV D2+ ions to be 2–4.[10] The isotope

effect is considerably large compared with the classical isotopeeffect of the root of the mass ratio, which might suggest thatthere exists a quantum effect in the diffusion of hydrogenisotopes in proton-conducting oxide ceramics. Further detailedmeasurement on the latter point is required. Hereafter, theD–H replacement rates investigated in detail are described.Second, the temperature dependence of the D–H replacement

rate is described. Typical ERD spectra for the specimen measuredat several stages of the exposure to water vapour at 313 K[26] areshown in Fig. 3, where the spectra as-implanted and at the totalexposure time of 5min, 10min, 40min and 70min are shown. InFig. 3, the channel number in the horizontal axis represents theenergies of H and D atoms recoiled from the specimen by theincident He+ ions, thus the peaks between 550 and 360 channelscorresponding to D, the ones between 150 and 360 channelsdoing to H absorbed by splitting of water vapour. The smallH peak at 300 channel number for the as-implanted specimenrepresents the H atoms are uptaken from residual H2O duringthe D-implantation. It is clearly seen from Fig. 3 that as theexposure time increases, the peak height of D implantedbecomes lower and that of H absorbed does higher and thensaturates. The vertical axis represents the intensities of H andD atoms recoiled, from which the concentrations of H and Dwere determined using the standard technique with the dataon elastic recoil cross sections and nuclear reaction crosssections for MeV He ions.[23] The saturation concentration of

Figure 3. ERD spectra measured for BaCe0.9Y0.1O3� δ as-implanted with10 keV D2

+ ions at room temperature and subsequently exposed to Argas containing water vapour for the total exposure time of 5min,10min, 40min and 70min at 313 K.


H absorbed was also found to be almost equal to that of Das-implanted from this figure.

The decay curve of D in the specimen exposed to water vapourat 313 K, which was obtained from the area of D peaks in Fig. 3, inorder to reduce the statistical error in the experimental data, isshown as a function of the exposure time in Fig. 4, where thedecay curves at the other temperatures are also shown and thehorizontal axis represents the exposure time for atmospheric airwhich was corrected for water vapour carried by Ar gas throughthe bubbler in the present experiment and the vertical axis repre-sents the retained fraction of D normalized by the initial concen-tration in a logarithmic scale. The dotted lines in Fig. 4 representthe decay curves of the exponential function best fitted to theexperimental ones, in order to determine the decay constant ofD in the D–H replacement. The decay curve of D at room (implan-tation) temperature and at the lower temperatures has beenanalysed using the solution of the mass balance equationsdescribed in the preceding section. The solution, which can beapproximately expressed as an exponential function, has beenfound to reproduce the experimental decay curves of D and thedecay constants determined have also been found to be propor-tional to both the absorption coefficient and the molecularformation coefficient. At elevated temperatures, however, weshould take into account thermal detrapping of D and H in themass balance equations. In such a case, the equations becamenon-linear, thus it not being analytically solved. Therefore, we

Figure 4. Decay curves of D implanted in BaCe0.9Y0.1O3� δ exposed towater vapour at temperatures of 295 K, 305 K, 313 K and 323 K. The dottedlines represent the best fitting curves of the exponential function approx-imated on the one-way diffusion model.[9]



assumed the decay curve to be an exponential function in Fig. 4,in order to discuss the effect of thermal detrapping. It can beseen from Fig. 4 that the experimental decay curves at tempera-tures except at 305 K are excellently well reproduced by theexponential function.

The temperature dependence of the decay constants of D inthe D–H replacement obtained from the decay curves in Fig. 4is summarized as a function of 1000/T (K) in Fig. 5, where for com-parison the temperature dependence for SrCe0.95Yb0.05O3� δ,

[9]

measured at cooling temperatures lower than the implantationtemperature is also shown. The apparent activation energy ofthe decay constant of D in the D–H replacement inBaCe0.9Yb0.1O3� δ is estimated to be 0.80 eV from Fig. 5, whichis rather higher than that (0.49 eV) in SrCe0.95Yb0.05O3� δ. Theactivation energy of the D–H replacement rate is expressed asthe summation of those of the absorption rate of H+ due to watersplitting at the surface and of the molecular recombination rate,as seen from Eqn (9). The activation energy of the absorption rateis directly correlative to the energy levels of donor electrons andacceptor holes. The energy levels of donor electrons and acceptorholes introduced by oxygen vacancy and trivalent (Y) atom dopedBaCeO3, SrCeO3 and BaZrO3 lattices were numerically obtained asEf and Ef

Y in the present study,[27] using the first-principles calcula-tion,[28] based on the generalized gradient approximation (GGA),[29]

where Ef, so-called Fermi level, represents the energies at whichthe oxygen vacancy is transfered from neutral (or positively singlecharged) to positively single charged (or positively double charged)and Ef

Y also represents the energies at which Y and Yb atoms aretransfered from neutral to negatively single charged. The valuesof Eg � Ef and Ef

Y are shown with the band gap energies, Eg, in

cooling

E=0.41 eV

heating

Figure 5. Temperature dependence of the decay constant of Dimplanted in BaCe0.9Y0.1O3� δ exposed to water vapour (●) at elevatedtemperatures above the implantation temperature and inSrCe0.95Yb0.05O3� δ (▲) at cooling temperatures below the one.

Table 1. D-H replacement rates in bulk BaCe0.95Y0.05O3-δ and SrCe0.95Yb0.05Eg and values of Eg - Ef

Y calucalated using the first principles method

Samples BaCe0.95Y0.05O3-δ SrCe0.95Yb

Rates (s-1) 2.0 x 10-2 6.9 x 1

Eg (eV) 2.59 2.74

Eg – Ef (eV) 0.29 (0.14) 0.39 (0

EfY (eV) -0.04 -0.02

Ef and EfY represents energies at which oxygen vacancy is transferred from

single charge ) and at which Y and Yb atoms are transferred from neutr


Table 1, from which the values of Eg are found to be rathersmaller than the experimental ones. The values of Eg � Ef,however, are generally accepted to be almost the same asthe experimental ones. In Table 1 also are shown the valuesof the D–H replacement rates for SrCe0.95Yb0.05O3� δ (bulk),BaCe0.95Y0.05O3� δ (bulk), BaCeO3: Sm (thin film) and BaZrO3:Y (thin film) which are discussed later. From Table 1, thevalues of Ef

Y are seen to be by an order of magnitude smallerthan Eg � Ef. Its negative value also means that trivalent ionsare always negatively charged, thus the thermal activationenergy of the acceptor hole being negligibly small, comparedwith that of donor electrons, Eg � Ef, which mainly contributes tothe total activation energy of the absorption rate of H+. Accordingto this argument, the activation energy of the molecular recombi-nation rate in SrCe0.9Yb0.1O3� δ may be estimated to be 0.12 eV(0.49 – 0.37). When the activation energy of the molecular recom-bination rate in SrCe0.95Yb0.05O3� δ is assumed to be the same asthat in BaCe0.9Y0.1O3� δ, the activation energy of D–H replacementrates is estimated to be 0.41 eV. The difference (0.08 eV) in thevalues of Eg � Ef for SrCeO3 and BaCeO3 in Table 1 correspondswell to that in D–H replacement rates of SrCe0.95Yb0.05O3� δ andBaCe0.9Y0.1O3� δ at 1000/T= 3.4 in Fig. 5.

In order to compare the activation energies at the elevatedand cooling temperature ranges, we should consider the dynamicbehaviour of H and D at the both temperature ranges relative tothe implantation (room) temperature. The saturation concentra-tion of D during the D implantation was established at a steadystate, under which the concentration of D in the traps is balancedby ion impact induced and thermally activated detrapping of Dfrom the traps, trapping of the free D and emission of D from trapsas D2 molecule due to local molecular recombination with free D.After stopping of the implantation at room temperature, theconcentration of D in the traps has been experimentally foundto hardly decrease, when the specimen has been kept in thevacuum for a long time, which indicates that thermally activateddetrapping of D as well as H from traps hardly takes place. Thus,the difference between activation energies at elevated andcooling temperature ranges is ascribed to increase in the decreaserate of D due to thermal detrapping of D implanted and Hexchanged for D at elevated temperatures.

Thermal release curves of both D as-implanted and H absorbedin the specimen by subsequent exposure to water vapour for anenough time at room temperature, which were measured by iso-chronal annealing for 10min in the vacuum, are shown in Fig. 6,where the data for H in the as-received specimen is shown forcomparison. It is clearly seen from Fig. 6 that both D implantedand H absorbed are almost released by annealing up to 423 Kfor 10min in the vacuum. It is also seen that the fraction ofretained H is somewhat larger than that of D. This fact is ascribed

O3-δ and thin films of BaCeO3:Sm and BaZrO3:Y, their band gap energies

0.05O3-δ BaCeO3:Sm BaZrO3:Y

0-4 1.0 x 10-1 3.0 x 10-3

2.59 3.10

.19) 0.29 (0.14) 0.03 (0.13)

………. 0.04

single to double charge ( values in ( ) correspond to from neutral toal to negative charge


119

Figure 6. Thermal release curves of D implanted, H absorbed inBaCe0.9Y0.1O3� δ by exposure to water vapour at room temperature andH contained in as-received BaCe0.9Y0.1O3� δ. The vertical axis representsthe retained fraction normalized by the initial one, respectively.


120

to absorption of H due to splitting of H2O adsorbed at the surfacefrom residual gases, since the thermal release of D and H atelevated temperatures was explained in terms of thermaldetrapping, local molecular recombination and rapid diffusionof hydrogen molecule. The result also supports the validity ofthe one-way diffusion model. Moreover, it has also been shownabove that the release rate of H from the as-received specimenis smaller than that of H absorbed by the D–H replacement.The origin is attributed to lattice defects introduced by theimplantation of D that enhance the migration rate of H detrappedthermally from traps which recombines with H in the traps into H2

molecule, namely the effective recombination rate.It is seen from Fig. 5 that the D–H replacement rates in

BaCe0.9Y0.1O3� δ at an elevated temperature of 323 K are ratherlarger than the dotted line of those at cooling temperatures,

Figure 7. Decay curves of D and uptake curves of H in D-implanted BaCe1�replacement by exposure to water vapour at room temperature. The solid andusing the solutions of the mass balance equations.


drawn so as for the activation energy to be 0.41 eV. The enhance-ment in the D–H replacement rates (decay constants of D) atelevated temperatures is explained in terms of thermaldetrapping of D and H in traps, which hardly take place at coolingtemperatures lower than the implantation temperature, andenough fast absorption of H due to splitting of H2O at the surfacewhich compensates the enhanced emission of D2 and DH gasesdue to molecular recombination of free D and H activated ther-mally from the traps. This fact indicates that the emission rateof H2 gas is enhanced by exposure of the specimen to watervapour at elevated temperatures, as long as the concentrationof H retained in the specimen is not reduced by enough fastsupply of H due to splitting of H2O at the surface.

Third, the dependence of the D–H replacement rate on theconcentration of trivalent impurities doped[30] is described.Typical experimental results on the decay curve of D and theuptake curve of H in the specimens of BaCe1� XYXO3� δ areshown for X = 0.05, 0.10, 0.15 and 0.20 as a function of exposuretime in Fig. 7, where the solid and dotted lines represent the bestfitting curves to the experimental ones which were theoreticallycalculated from the solutions of the mass balance equationsdescribed above. Similar decay and uptake curves for SrCe1XYbXO3� δ were also obtained. The values of the effective D–Hreplacement rate (the decay constant of D) obtained by bestfitting of the decay curves of D for both specimens are shownas a function of the impurity concentration (X) of BaCe1XYXO3� δ and SrCe1� XYbXO3� δ in Figs. 8 and 9, respectively. Itis clearly seen from both figures that the D–H replacement ratesin both specimens, similarly, increase and after a maximum atX = 0.05–0.10 decreases as X increases and the absolute valuesfor BaCe1� XYXO3� δ are larger by an order of magnitude thanthose for SrCe1� XYbXO3� δ. The results on the concentration de-pendence observed in Figs. 8 and 9 indicate that the concentra-tion of oxygen vacancies at the surface, which controls the D–Hreplacement rate, is maximized at the X = 0.05–0.10 by D-

XYXO3� δ which were obtained by means of the ERD technique in the D–Hdotted lines represent the best fitting of the theoretical curves calculated


D-H Exchangein BaCe Y O

Dec

ay C

on

stan

t (1

/s)

Y Fraction (X)

Figure 8. Decay constants of D (D–H replacement rates) as a function ofY fraction (X) which were obtained in the D–H replacement by exposureof D implanted BaCe1�XYXO3� δ to water vapour at room temperature.

0.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.00 0.05 0.10 0.15 0.20 0.25

D-H Exchangein SrCe Yb O

Dec

ay C

on

stan

t (1

/s)

Yb Fraction (X)

Figure 9. Decay constants of D (D–H replacement speeds) as a function ofYb fraction (X) which were obtained in the D–H replacement by exposure ofD implanted SrCe1�XYbXO3� δ to water vapour at room temperature.

Figure 10. The contents of H2, enriched due to emission from thespecimens, SrCe0.95Yb0.05O3� δ and BaCeO3: Sm and BaZrO3:Y thin films,in air sealed in the stainless steel vessel at 40 °C as function of theexposure time, which was measured by means of the gas chroma-tography technique.


121

implantation and it may be ascribed to small negative values ofEfY in both specimens, as shown in Table 1. The latter large differ-ence in the absolute values for two ceramic specimens is notattributed to the kind of trivalent impurity, but that of bulk lattice,namely the difference in the values of Eg � Ef, as seen from theresult in Table 1.

Finally are discussed the results on the D–H replacement ratesshown in Table 1, from which it is seen that the D–H replacementrate in the thin film BaCe0.8Sm0.2O3� δ is by a factor of 5 largerthan that in BaCe0.95Y0.05O3� δ. Since the optimum fraction forthe rate is around 0.05–0.10, as seen in Figs. 8 and 9, the differ-ence may be ascribed to the concentration of trivalent impurityions doped and the molecular recombination rate. From thethermal release curves of H in Fig. 6, it is seen that the releasetemperature from the thin film BaCe0.8Sm0.2O3� δ is a little lowerthan that from BaCe0.9Y0.1O3� δ. This result indicates that themolecular recombination coefficient in BaCe0.8Sm0.2O3� δ is alittle larger than that in BaCe0.9Y0.1O3� δ, since the thermal re-lease rates of H in both specimens may be regarded to be almostthe same because of its high concentration (H/unit cell = ~1),based on the fact that the thermal release rate of H as H2 isproportional to the molecular recombination rate. It is seen fromTable 1 that the value of Eg � Ef for BaZrO3 is considerably lowerthan the value for BaCeO3. Nevertheless, the D–H replacementrate in BaZr0.84Y0.16O3� δ is by a factor of 7 lower than that inBaCe0.95Y0.05O3� δ. The fractional concentration of Y may be theone of the origins for the discrepancy, since the optimum


concentration is expected to be around 0.05–0.10.[30] Not onlythe energy level, 0.04 eV of the acceptor hole but also theactivation energy of the molecular recombination coefficientare considered to partly contribute to the discrepancy. The sizeof crystallites in BaZr0.95Y0.05O3� δ is also considerably smallerthan that in BaZr0.84Y0.16O3� δ.

[21] This fact may be also ascribedto the discrepancy. Further systemic studies including thetemperature dependence are required to clarify the problem.

Hydrogen gas emission rates in d-implanted oxide ceramics

Here, the H2 gas emission, predicted on the one-way diffusionmodel and already demonstrated,[12] was measured again usinga larger scale of a vacuum vessel and a gas chromatography.The experimental hydrogen emission curves for three specimens,SrCe0.95Yb0.05O3� δ bulk specimen and both BaCeO3: Sm andBaZrO3: Y thin films are shown in Fig. 10, where the hydrogencontents are plotted as a function of the exposure time. Althoughsome data points for the BaCeO3:Sm thin film are lack because ofsome mechanical defect of the gas chromatography during theexperiment, the hydrogen contents are seen to increase linearlyin the beginning and hereafter saturate with increasing the expo-sure time. The pictures of two specimens, bulk SrCe0.95Yb0.05O3

δ specimen both as-prepared and exposed to air after irradiationwith RF H2 plasma and BaCeO3: Sm thin film on Ce0.8Sm0.2O2� δ

substrate both as-prepared and exposed to air after implantationwith 10 keV D2

+ ions up to saturation concentration are shown inFig. 11. It is clearly seen from Fig. 11 that the color of the bulkSrCe0.95Yb0.05O3� δ specimen surface was changed by the plasmairradiation and air exposure and the specimen itself was crackedand that the BaCeO3: Sm thin films were peeled off from the sub-strates by the air exposure. The BaZrO3: Y thin films on 8YSZ sub-strates were not apparently changed except of cracking of thesubstrates into two pieces. These defects of the specimens areregarded as the effects induced by accumulation of hydrogengas in the grain boundaries and in the interface of film/substrate.It was also clearly found that the surface of oxygen free coppergaskets, which were used for sealing the cylindrical stainless steelvessel from atmospheric air, was black-colored. It is ascribed tooxidization by active oxygen atom created by water splitting.These facts are concluded to give the clear evidence that the


Figure 12. ERD energy spectra of 2.8MeV He2+ ions from a sandwichspecimen of Pt/Li2ZrO3/Pt with 100 nm Pt films, measured at severalstages of isochronal annealing for 10min up to 423 K.

As PreparedAfter Exposure to Air

After Exposure to Air As Prepared

Figure 11. Photo-pictures of two specimens, SrCe0.95Yb0.05O3-d andBaCeO3: Sm thin films as-prepared and exposed to air at 40 °C for3200h and for 4000 h, respectively. Both specimens were irradiated withRF H2 plasma and 10 keV D2

+ ion beam, respectively.


122

hydrogen implanted oxide ceramics provide sustainable watersplitting and hydrogen emitting catalytic function.The saturation curves of H2 emission in Fig. 10 may be

analysed in terms of the emission rate of H2 from the bulk andits recombination with O atom at the exit surface and in thevacuum (air) near the surface where it is ejected. In such a case,the decrease rate in the contents of H2 gas is roughly approxi-mated to be the second-order reaction, although some O atomsare recombined into a molecule and react with other substances.The time variation of the H2 gas contents in the vessel was easilyobtained in an analytical form and both the H2 emission rate andrecombination rate were estimated from the initial slope and theratio of the saturation value to the initial slope in Fig. 10. Theexperimental values for each specimen are tabulated in Table 2,where the emission efficiency is the ratio of the experimentalvalue to the one calculated from Eqn (11). The emission efficiencyfor SrCe0.95Yb0.05O3� δ was obtained as the ratio of emission ratesof HT to HTO which were measured with 1 keV pure T2

+ ions in theprevious work.[11] It is clearly seen from Table 2 that the emissionefficiency decreases as the emission rate decreases. The decreasein the H2 gas emission rates is ascribed to back reaction with Oatom into H2O at the exit surface, because the specimen surfaceis almost completely covered with OH� layers and H2 can escapefrom the surface only through atomically open space: openedges of grain boundaries and oxygen vacancy sites at the sur-face. According to the coulomb interaction model, the result isexplained, as follows: as the absorption rate is larger, the timefor oxygen vacancy not to be occupied by O atom is longer, thusthe back reaction rate decreases and the emission efficiency

Table 2. D-H replacement rates, the measured and estimated rates of H2

SrCe0.95Yb0.05O3-δ and thin films of BaCeO3:Sm and BaZrO3:Y

Specimen SrCe0.95Yb0.05O3-δ (thi

D-H Replacement Rates (s-1) 6.9 x 10-4

H2 Emission Rates (H2 cm-3s-1)

(Estimated) 2.8 x 1012

(Measured) 1.1 x 1010 (Tritium) 1.1

Emission Efficiencies 0.04

Recombination Rates (cm-3s-1)


increases. It is also seen from Table 2 that the recombination ratefor SrCe0.95Yb0.05O3� δ with large surface area is considerablysmaller than those for the other small specimens, although therates are expected to be almost equal with each other as thevolume of specimens is much smaller than the vessel volume.The back reaction due to the recombination is considered to takeplace mainly near the specimen surface, because the localdensity of H2 gas emitted near the surface becomes lower forthe specimen with larger area.

Hydrogen storage and emission characteristics of metal–oxideceramics composites

Here, as one of applications of the water splitting catalysts foundin the present study, the hydrogen absorption (storage) andemission characteristics of metal–oxide composite hydrogenstorage materials of Pt/Li2ZrO3/Pt and Pt/Li4SiO4/Pt sandwichesare described.

For the first place, the thermal emission characteristics of resid-ual hydrogen in the as-received specimen of Pt/Li2ZrO3/Pt with100 nm Pt films on isothermal annealing for 10min were mea-sured by the ERD technique, while the change in the composi-tions of the specimen was monitored by the RBS technique. Onthe annealing of the first specimen, the heating above 423 Kwas found to change the peak height (composition) of Pt filmfrom the RBS spectra monitored. Therefore, the thermal emissioncharacteristics were again measured with the second specimen.The ERD spectra measured for the second specimen at theseveral stages of the annealing are shown in Fig. 12, from whichit is clearly seen that the height of the spectra decreases, as thetemperature increases and the ERD spectrum at 423 K was still

gas emission, the emission efficiencies and recombination rates for bulk

n film) BaCeO3:Sm (thin film) BaZrO3:Y (bulk)

1.0 x 10-1 3.0 x 10-3

……… 4.5 x 1014 1.5 x 1012

x 109 (RF plasma) 1.4 x 1014 1.3 x 1012

………… 0.35 0.08

0.15 x 1021 0.56 x 1021 0.52 x 1021



observed. The thermal emission curves for the two specimensobtained by integration of the peak area of each ERD spectrumin Fig. 12 are shown as a function of temperature in Fig. 13. FromFig. 13, it is also seen that the fraction of H retained in the twospecimens is almost the same and that H in the first specimenis completely reduced at 573 K. Moreover, it is seen that 80% ofresidual hydrogen was emitted by heating at 373 K. In general,residual hydrogen in the as-received specimen is more stablethan hydrogen re-absorbed after the heating. Thus, the re-emission temperature of hydrogen absorbed and stored in thespecimen becomes lower than 373 K estimated from Fig. 13.

The change in the RBS spectra from Pt/Li2ZrO3/Pt with 100 nmPt films induced due to the pre-heating is shown in Fig. 14, whereRBS spectra for the specimen as-prepared before annealing (red)and heated at 573 K (blue) and for Li2ZrO3 without Pt layer(green) are compared. The large peak at 500 channels in Fig. 14represents the Pt film on Li2ZrO3 substrate. From the figure, it isclearly seen that the peak height is reduced and the widthbecomes larger, which indicates that the reaction between Ptand Li2ZrO3 took place. From the standard analysis,[23] it wasfound that the 100 nm Pt film was transformed into Pt3.3Zr2.7Oby heating at 573 K, where the Li concentration will be measuredby means of the ERD technique with 9MeV O ion beam. There-fore, in further experiments, both the first and the second speci-mens heated up to 573 K and 423 K, respectively, were exposed

Figure 14. RBS spectra of 2.8MeV He ion beam from a Pt–Li2ZrO3 (Ptthickness: 100 nm) bi-layer specimen as-prepared before annealing(red), after heated at 573 K (blue) and for Li2ZrO3 without Pt layer (green).

Figure 13. Thermal release curves of H from two sandwich specimens ofPt/Li2ZrO3/Pt with 100 nm Pt films by isochronal annealing for 10min upto 573 K (first) (●) and 423 K (second) (○), which were measured by meansof the ERD technique.


to normal air of ~760 mTorr at room (ambient) temperature forabout 3500 h. The ERD spectra which were measured at severalstages of the exposure are shown in Fig. 15, where ERD spectrafor the specimen non-annealed (as-received) (●), annealed upto 573 K (○) and exposed to normal air at room temperature for40 days (x) and 140 days (▴), are compared with each others.From Fig. 15, it is seen that residual hydrogen was almostcompletely removed by heating up to 573 K and the ERD spectrafrom the specimens exposed for 40 days and 140 days are flat,which indicates that the distribution of H absorbed is uniformover the depth at least within the detection limits of 1μm evenat room temperature, thus its diffusion constant being very large.The saturation concentration was estimated to be a half of atomin unit cell of a Li2ZrO3 lattice from the height of the spectrum for140 days in Fig. 15, which corresponds to 0.82 × 1021 H/cm3. Theabsorption curves of H obtained for three specimens with 1 nm,10 nm and 100 nm Pt films have shown that the absorption ratesof H for 1 nm and 10 nm Pt specimens estimated from each initialslope are by a factor of about 5 larger than that for 100 nm one.

Gas emitted by thermal heating of specimens exposed to airfor hydrogen storage was analysed by means of the TDS tech-nique. The six kinds of gases with the mass numbers of 1, 2, 14,15, 16 and 18 were analysed as their spectra. The TDS spectraof main gases (Mass 1, Mass 2 and Mass 18) emitted from thetwo specimens of Pt/Li2ZrO3/Pt with 100 nm Pt films preheatedup to 573 K and 423 K and exposed for 3500 h are shown inFig. 16, where the estimated amounts of the three species emit-ted in unit of H/m2 are shown by red words. It is seen from thespectra that the amounts of H2 and H2O are roughly 6 to 4 witheach other and Mass 1 is considered to be produced by crackingof H2 and H2O within the detection system. The total amount ofgases emitted from the specimen preheated up to 573K was foundto be by a factor of ~5 larger than that from the one preheated upto 423K, although almost no difference was observed by the ERDmeasurement which detected hydrogen in the specimens within1μm (the detection limit in depth) from the surface.

The absorption curves of hydrogen for the Pt/Li2ZrO3/Pt spec-imens with 1 nm and 10 nm Pt films exposed to air at room tem-perature, which were obtained by means of the WGM, are shownas a function of the exposure time in Fig. 17, where the verticalaxis represents the weight gain of specimens (0.19 g) and the

Figure 15. ERD spectra from a sandwich specimen Pt/Li2ZrO3/Pt with100 nm Pt films before annealed (as-received) (●), annealed up to 573 K(○) and exposed to normal air at room temperature for 40 days (x) and140 days (▴).


123

Figure 16. Thermal desorption spectra of Mass1 (H), Mass2 (H2) and Mass (H2O) emitted from the two specimens Pt/Li2Zr3/Pt with 100 nm Pt films pre-heated up to 573 K and 423 K, which were exposed to normal air at room temperature for 3500 h. Data were obtained by TDS technique.

Figure 17. Weight gain curves of the sandwich specimens Pt/Li2ZrO3/Ptwith 1 nm and 10 nm Pt films pre-heated up to 423 K to remove residualhydrogen (first run:●, ○), heated at 773 K to remove hydrogen stored inthe first run (second run:▲, △) and furthermore heated at 773 K to re-move hydrogen stored in the second run (third run: ♦, ◊).


124

data obtained at the same time for the two specimens preheatedat 423 K for about 2400 h (first run), heated at 773 K to removehydrogen absorbed at the first run and subsequently exposedfor about 2200 h (second run) and further heated at 773 K toremove hydrogen absorbed at the second run and subsequentlyexposed for about 1400 h (third run) are shown. It is clearly seenfrom Fig. 17 that the initial slope of the absorption curves at thefirst run is by a factor of 5 smaller than that of at the second run


and the initial slope at the second run is somewhat larger thanthat at third run. The former large difference between the twoinitial slopes is ascribed to the difference in heating temperaturesof 423 K at which Pt films were not changed, and 773 K at whichPt films were modified into Pt3.3Zr2.7O. The latter difference isascribed to change in the exposure conditions of temperatureand humidity in the room without air conditioner where thespecimens were exposed to air, since the second exposure wasdone from 1 August to the end of October, whereas the thirdrun was done from the beginning of December to the middleof February in Tohoku University. Moreover, in the fourth runfrom the beginning of the March to the end of May, the satura-tion level of the weight gain was observed to attain to ~20wt.%. These absorption curves indicate that the Pt/Li2ZrO3/Pt speci-men can be repeatedly used. One can see in Fig. 17 that theweight gain of the specimen with 1 nm Pt films preheated upto 423 K is almost the same as that with 10 nm Pt films at the firstrun, but in the second run and in the third run the former is a lit-tle bit larger than the latter, which is attributed to difference inthe concentrations of hydrogen absorbed at the interface be-tween Pt film and Li2ZrO3 substrate during the air exposure, thusthe specimen surface being covered completely with the Pt filmsso that the film thickness became effectively larger due to thepreheating induced reaction of Pt–Li2ZrO3. One can also see inFig. 17 that the saturation level of the weight gain at secondrun is 21wt.%, from which the amount of H2 gas is estimatedto be 3wt.% for Li2ZrO3 using the TDS data in Fig. 16. This storageamount of H2 gas Li2ZrO3 obtained by the WGM technique isseen to be extremely larger than that by the ERD technique. Such


Figure 18. Weight gain curves (green and blue) at first run of Pt/Li4SiO4/Ptspecimens with 1 nm and 10nm Pt films, which were pre-annealed at 423Kto remove residual hydrogen, exposed to air at 40 °C and at humidityof 49–57%, in comparison with that (red) of the Pt/Li2ZrO3/Pt refer-ence specimen. Is also shown a weight gain curve (black) at secondrun for the Pt/Li4SiO4/Pt specimen with 10 nm Pt films, which wasannealed at 773 K to remove hydrogen absorbed at first run andexposed to air at 40 °C and at humidity of 20–24%.


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a large difference is ascribed to the following fact: the WGM mea-sures hydrogen in the whole specimen of 1mm in thickness,whereas the ERD detects hydrogen within the depth of 1μm(the detection limit in depth), from which hydrogen is stored asH2 gas in closed micro-pores in the bulk, because hydrogen isable to diffuse in the whole specimen because of its large diffu-sivity, hydrogen molecule is produced due to their molecular re-combination with each other. This reason for the difference isalso very consistent with the large amount of H2 gas emission.The experimental results on the emission temperature of 100 °Cand the storage amount of results on the emission temperatureof 100 °C and the storage amount of 3wt.% and the possible re-peated use indicate that the Pt/Li2ZrO3/Pt specimen is very prom-ising as the hydrogen storage material for the practical use. Aconcept of the sandwich structure is very useful to developnew materials, because it separates the storage function fromthe water splitting and hydrogen absorbing (storing) function,thus both functions individually being able to be modified.

Finally, are described the absorption characteristics of thePt/Li4SiO4/Pt specimen (0.11 g), exposed to air at 314 K andat a humidity of 49–57% in a chamber where the humiditycould not be kept constant, in comparison with those of thePt/Li2ZrO3/Pt specimen. The hydrogen emission temperaturefor Pt/Li4SiO4/Pt, measured during pre-heating to remove residualhydrogen, was found to be by 20K lower than that for Pt/Li2ZrO3/Pt. The weight gain curves of the sandwich specimens of Pt/Li4SiO4/Pt with 1 nm and 10nm Pt films, measured by means ofthe chemical microbalance, are shown in Fig 18, where as the refer-ence data of Pt/Li2ZrO3/Pt with 10 nm Pt layers measured at thesame time are plotted for comparison and the marks of A and Brepresent the times at which the exposure condition in the experi-ments was accidentally varied to 304K and humidity of 75% be-cause of stopping of the electricity and to 313 K at humidityof~15% because of switch-on of the air conditioner. The blue andgreen data are for the specimens with 1 and 10nm Pt layers pre-annealed up to 423K. The black data (second run) for the specimenwith 10 nm Pt layers annealed up to 773K after the air exposure at


the first run was obtained under the air exposure condition of 313Kand humidity of 20–24%.

One can see from Fig. 18 that the saturation weight gain of thespecimens with 1 and 10 nm Pt layers, pre-heated at 423 K are14wt.% and 12wt.%, respectively. These values are seen to beconsiderably lower than the reference value of 20wt.% for Pt/Li2ZrO3/Pt, from where ~60% of it was emitted as H2 and ~40%was emitted as H2O, as described above. The small differencebetween the saturation values of the weight gain of bothspecimens with 1 and 10 nm Pt layers is attributed to the smalldifference in the hydrogen concentrations at the interfaceinduced by diffusion of hydrogen in both Pt layers which isabsorbed from the surface, as well as small thickness of both Ptlayers. The difference in the saturation values of two differenthydrogen storage ceramics is also mainly ascribed to the differ-ence in the absorption rate and the desorption rate, becausethe saturation level of hydrogen stored is determined by abalance between the absorption rate of hydrogen at the surfaceand the desorption rate of hydrogen via molecular recombina-tion in the storage ceramics.

It is also clearly seen that at the points A and B, the weight gainjumps up and decreases due to change in the exposure condi-tion, respectively. The former is ascribed to increase in thehumidity in air and decrease in the temperature, namely increasein the absorption rate of hydrogen at the surface and decrease inthe thermal desorption rate, because the storage amount isdetermined by the ratio of the absorption rate at the surface tothe emission rate due to molecular recombination in the bulk.The latter is also ascribed to decrease in the absorption ratedue to decrease in the humidity. On the other hand, the changein the weight gain of Pt/Li2ZrO3/Pt specimen measured at thesame time is hardly observed at the points of A and B. This resultis also ascribed to the high saturation level of the storage amountat the point A and its higher thermal desorption temperature.The fact seems to indicate that Li2ZrO3 is more stable than Li4SiO4

as the hydrogen storage ceramics.It is also seen from Fig. 18 that the initial slope of the weight

gain (black points) of Pt/Li4SiO4/Pt with 10 nm Pt layers obtainedat the second run is rather larger than that at the first run andalmost as the same as that of the reference data of Pt/Li2ZrO3/Pt,whereas the weight gain itself is also smaller. The former fact indi-cates that the annealing of the specimen at higher temperature of773 K to remove hydrogen absorbed at the first run brought aboutthe solid-state reaction between Pt and Li4SiO4 which modified Ptlayers into Pt–Si–Li–O complex oxides, as similar to Pt/Li2ZrO3/Pt.[16] The latter fact is attributed to lower humidity of 20–24% inair exposed and to lower thermal desorption temperature. Thus,the saturation level is expected to become higher at higher humid-ity and at lower exposure temperature than 313K.

Summaries

In this review, it have been described the water splitting andhydrogen gas emitting catalytic function invented from the com-plete replacement of D implanted into oxide ceramics by H in airvapour for short time, the experimental results on D–H replace-ment and theoretical calculations supporting the microscopicmodel for the catalytic function and the hydrogen absorptionand emission characteristics of the metal/complex metal (twometals) oxide ceramics/metal sandwich specimens applicable tothe hydrogen storage materials.



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For the water splitting and hydrogen gas emitting catalytic func-tion, an atomistic coulomb interaction model of the electric dipoleof water with charges of lattice defects at the surface and one-waydiffusion model have been proposed: According to the coulombinteraction water splitting model, the oxygen vacancy at thesurface of oxides plays important roles in not only water splitting,itself, but also its sustainability, namely thermal ionization of donorelectrons introduced in the energy gap by oxygen vacancy to theconduction band, controls the water splitting rate (speed) andbrings about production and ejection of oxygen atom. This issupported by the experimental data on the D–H replacement rateand on the H2 gas emission as well as the theoretical calculationsof electronic structures on the basis of the first principles and theGGA. This fact means that oxide ceramics also provide theoxidization catalysis function by help of water vapour.The anomalous isotope effect in the exchange rates of D–H

and H–D has been explained in terms of the difference insplitting rates of H2O and D2O at the surface and in diffusion ratesof H and D in oxides. The latter difference has been evaluatedfrom the amounts of exchanges of D and H implanted intodeeper layers with 5 keV ions for H and D implanted near thesurface with 0.5 keV ions. In such a case, the ion irradiation effecton diffusion might reduce the difference; thus, the evaluation ofthe diffusion rates of H and D without irradiation is required. Theclarification of the anomalous isotope effect is not only ofscientific interest, but also of importance for future applications.The decay curves of D and uptake curves of H in the D–H

replacement experiments have been demonstrated to beexcellently well reproduced by the solutions of the mass balanceequations expressed mathematically on the one-way diffusionmodel. The experimental results on the D–H replacement ratesin dependence of the crystal lattice, the concentration of trivalentimpurities and the temperature have been shown to be reason-ably explained by the dipole-induced water splitting model withthe correlative theoretical calculations. The measurement of H2

gas emission by means of a gas chromatography with the Bachmethod has been shown to give a clear evidence for the predic-tion of the one-way diffusion model which has been proposedfor explanation of the anomalous isotope effect in the D–H andH–D replacement. A dynamic measurement of H2 content in acarrier gas emitted from the specimen due to water splittingmay demonstrate the enhancement in the H2 gas emissionexpected from the temperature dependence of the D–H replace-ment rates. A measurement of life time of catalysts is also impor-tant for the practical use.Finally, the hydrogen absorption and emission characteristics

of Pt/Li2ZrO3/Pt and Pt/Li4SiO4/Pt sandwich specimens, whichhave been created to store dilute H2 gas produced from airvapour, have been described. For the Pt/Li2ZrO3/Pt specimens,it has been shown that hydrogen is absorbed from the sur-face up to 5wt.% due to water splitting at room temperaturefor 1000 h and 80% of it were emitted by heating at 100 °C asH2 gas: 60% and H2O: 40%; thus, the storage amount as H2

gas being 3wt.%. It has been also found that the preheatingof specimens at temperatures 573 K enhances the absorptionrate owing to modification of Pt films into complex oxidefilms of Pt–Zr–Li–O and the specimens are used repeatedlyof storage and emission. The enhancement in the storage rateis ascribed to increase in the absorption rate due to the mod-ification of Pt films. Furthermore, it has been found that thehydrogen storage amount in Pt/Li2ZrO3/Pt is somewhat largerthan that in Pt/Li4SiO4/Pt, whereas the absorption (storage)


rate of Pt/Li4SiO4/Pt is contrarily somewhat larger than thatof Pt/Li2ZrO3/Pt and the thermal emission temperature of Pt/Li4SiO4/Pt is by 20 K lower than that that of Pt/Li2ZrO3/Pt.Since the storage amount of hydrogen is determined by abalance between the absorption rate and the emission rate,the difference in the storage amount is ascribed to the ratioof the activation energies of the absorption rate to the emis-sion rate. In order to select the optimum materials, furtherfundamental and systematic researches are required. For de-velopment of Pt-alternative metals for water splitting at thesurface, the dipole-induced model proposed in this work isvery effective for predicting higher materials in rates of watersplitting. Furthermore, it is noted that the ion beam analysistechnique is also a very powerful tool for new finding ofhigher materials in diffusion rates.

Acknowledgements

These works were partly supported by the Industrial TechnologyResearch Grant Program in 05A50002c from the New EnergyDevelopment Organization (NEDO) and both the Feasibility StudyProgram and the Seed Development Testing Program of JSTInnovation Plaza Tokai of Japan. These works also wereperformed under valuable co-operations at Advanced ResearchCenter of Metallic Glasses, Institute for Materials Research,Tohoku University. Furthermore, the TDS measurement was donein valuable co-operation with Profs. K. Okuno and Y. Oya, Radia-tion Laboratory, Shizuoka University. The authors are indebtedto Mr. Y. Yamaguchi, Graduate School of Engineering, NagoyaUniversity for the H2 emission measurement. The authors aregrateful to Leader (Professor Y. Arita) and Members (ProfessorH. Sugai and Drs. K. Katahira, M. Yoshino, J. Yuhara and T. Ishijma)of the Industrial Technology Research Grant Program forvaluable discussions.

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a mechanism for water splitting and hydrogen absorbing functions of metal-oxide layered hydrogen...

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